浙江大学精品课程
浙江省精品课程
浙江省教学成果一等奖
国内首创的基础医学整合课程
申报信息 课程学习
首 页 | 课程导学 | 教学内容 | 案例教学 | 练习与作业 | 扩展学习 | 学生信息 | 教学队伍 | 课程互动 | 双语园地
信息申报
教学队伍
教学内容
教学条件
教学方法与手段
教学效果
特色、政策及辐射
申报表下载
站内搜索
当前位置:首页 > 双语园地

Molecular Basis of Cancer

Molecular Basis of Cancer

The literature on the molecular basis of cancer continues to proliferate at such a rapid pace that it is easy to get lost in the growing forest of information. We list some fundamental principles before delving into the details of the molecular basis of cancer.

  

?   

Nonlethal genetic damage lies at the heart of carcinogenesis. Such genetic damage (or mutation) may be acquired by the action of environmental agents, such as chemicals, radiation, or viruses, or it may be inherited in the germ line. The term "environmental," used in this context, involves any acquired defect caused by exogenous agents or endogenous products of cell metabolism. Not all mutations, however, are "environmentally" induced. Some may be spontaneous and stochastic.

  

?   

A tumor is formed by the clonal expansion of a single precursor cell that has incurred the genetic damage (i.e., tumors are monoclonal). Clonality of tumors can be assessed in women who are heterozygous for polymorphic X-linked markers, such as the enzymes glucose-6-phosphate dehydrogenase (G6PD), iduronate-2-sulfatase and phosphoglycerate kinase. The principle underlying such an analysis is illustrated in Figure 7-26 . The most commonly used method to determine tumor clonality involves the analysis of methylation patterns adjacent to the highly polymorphic locus of the human androgen receptor gene (HUMARA).[33] The frequency of HUMARA polymorphism in the general population is more than 90%, so it is easy to establish clonality by showing that all the cells in a tumor express the same allele. For tumors with a specific translocation, such as in myeloid leukemias, the presence of the translocation can be used to assess clonality. Immunoglobulin receptor and T-cell receptor gene rearrangements serve as markers of clonality in B- and T-cell lymphomas, respectively.

  

?   

Four classes of normal regulatory genes—the growth-promoting protooncogenes, the growth-inhibiting tumor suppressor genes, genes that regulate programmed cell death (apoptosis), and genes involved in DNA repair—are the principal targets of genetic damage. Mutant alleles of protooncogenes are considered dominant because they transform cells despite the presence of a normal counterpart. In contrast, both normal alleles of the tumor suppressor genes must be damaged for transformation to occur, so this family of genes is sometimes referred to as recessive oncogenes. However, there are exceptions to this rule, and some tumor suppressor genes lose their suppressor activity when a single allele is lost or inactivated.[34] This loss of function of a recessive gene caused by damage of a single allele is called haploinsufficiency. Genes that regulate apoptosis may be dominant, as are protooncogenes, or they may behave as tumor suppressor genes.

  

?   

DNA repair genes affect cell proliferation or survival indirectly by influencing the ability of the organism to repair nonlethal damage in other genes, including protooncogenes, tumor suppressor genes, and genes that regulate apoptosis. A disability in the DNA repair genes can predispose to mutations in the genome and hence to neoplastic transformation. Such propensity to mutations is called a mutator phenotype.[35] With some exceptions, both alleles of DNA repair genes must be inactivated to induce such genomic instability; in this sense, DNA repair genes may also be considered as tumor suppressor genes.

  

?   

Carcinogenesis is a multistep process at both the phenotypic and the genetic levels. A malignant neoplasm has several phenotypic attributes, such as excessive growth, local invasiveness, and the ability to form distant metastases. These characteristics are acquired in a stepwise fashion, a phenomenon called tumor progression. At the molecular level, progression results from accumulation of genetic lesions that in some instances are favored by defects in DNA repair.

 

Figure 7-26  Diagram depicting the use of X-linked isoenzyme cell markers as evidence of the monoclonality of neoplasms. Because of random X inactivation, all females are mosaics with two cell populations (with G6PD isoenzyme A or B in this case). When neoplasms that arise in women who are heterozygous for X-linked markers are analyzed, they are made up of cells that contain the active maternal (XA) or the paternal (XB) X chromosome but not both.

http://www.mdconsult.com/images/blank.gif


ESSENTIAL ALTERATIONS FOR MALIGNANT TRANSFORMATION

With this overview we can now address in some detail the molecular pathogenesis of cancer and then discuss the carcinogenic agents that inflict genetic damage. Over the past two decades, hundreds of cancer-associated genes have been discovered. Some, such as p53, are commonly mutated; others, such as c-ABL, are affected only in certain leukemias. Each of the cancer genes has a specific function, the dysregulation of which contributes to the origin or progression of malignancy. It is traditional to describe cancer-causing genes on the basis of their presumed function. It is beneficial, however, to consider cancer-related genes in the context of seven fundamental changes in cell physiology that together determine malignant phenotype.[36] (Another important change for tumor development is the escape from immunity and rejection. This property is discussed later in this chapter.)

  

?   

Self-sufficiency in growth signals: Tumors have the capacity to proliferate without external stimuli, usually as a consequence of oncogene activation.

  

?   

Insensitivity to growth-inhibitory signals: Tumors may not respond to molecules that are inhibitory to the proliferation of normal cells such as transforming growth factor-β (TGF-β), and direct inhibitors of cyclin-dependent kinases.

  

?   

Evasion of apoptosis: Tumors may be resistant to programmed cell death, as a consequence of inactivation of p53 or other changes.

  

?   

Defects in DNA repair: Tumors may fail to repair DNA damage caused by carcinogens or unregulated cellular proliferation.

  

?   

Limitless replicative potential: Tumor cells have unrestricted proliferative capacity, associated with maintenance of telomere length and function.

  

?   

Sustained angiogenesis: Tumors are not able to grow without formation of a vascular supply, which is induced by various factors, the most important being vascular endothelial growth factor (VEGF).

  

?   

Ability to invade and metastasize: Tumor metastases are the cause of the vast majority of cancer deaths and depend on processes that are intrinsic to the cell or are initiated by signals from the tissue environment.

Mutations in genes that regulate these cellular traits are seen in every cancer. However, the precise genetic pathways that give rise to these attributes differ between cancers, even within the same organ. It is widely believed that the occurrence of mutations in cancer-causing genes is conditioned by the robustness of the DNA repair machinery of the cell. When genes that normally sense and repair DNA damage are impaired or lost, the resultant genomic instability favors mutations in genes that regulate the other acquired capabilities of cancer cells. The main principles of the molecular basis of cancer are summarized in simplified form in Fig. 7-27 .

Figure 7-27  Flow chart depicting a simplified scheme of the molecular basis of cancer.

http://www.mdconsult.com/images/blank.gif


In the following sections, we first discuss the molecular regulation of the normal cell cycle, since cell-cycle abnormalities are fundamental to cancer growth and many of the genes that cause cancer perturb the cell cycle. This is followed by discussion of the genes involved in each of the seven biologic alterations listed earlier. We end with a discussion of epigenetic changes and chromosomal abnormalities in cancer.

THE NORMAL CELL CYCLE

As discussed in Chapter 3 , resting (nondividing) cells are in the G0 stage of the cell cycle and need to be recruited into the G1 stage and beyond in order to undergo replication ( Fig. 3-3 ). The orderly progression of cells through the various phases of cell cycle is orchestrated by cyclins and cyclin-dependent kinases (CDKs), and by their inhibitors.[37][38][39][40] CDKs drive the cell cycle by phosphorylating critical target proteins that are required for progression of the cells to the next phase of the cell cycle. CDKs are expressed constitutively during the cell cycle but in an inactive form. They are activated by phosphorylation after binding to the family of proteins called cyclins.[37] By contrast with CDKs, cyclins are synthesized during specific phases of the cell cycle, and their function is to activate the CDKs. On completion of this task, cyclin levels decline rapidly ( Fig. 7-28 ). More than 15 cyclins have been identified; cyclins D, E, A, and B appear sequentially during the cell cycle and bind to one or more CDKs.

Figure 7-28  Expression of cyclin-cyclin-dependent kinase (CDK) complexes during the cell cycle. The phases of the cycle are indicated inside the arrows.  (Modified from Pollard TD, Earnshaw WC: Cell Biology. Philadelphia, WB Saunders, 2002.)

http://www.mdconsult.com/images/blank.gif



Cyclin D and RB Phosphorylation.

Cyclin D, the first cyclin to increase in the cell cycle, appears in mid G1 but is no longer detectable in the S phase (see Fig. 7-28 ). There are three forms of cyclin D, named D1, D2, and D3, but to simplify matters, we will use the general term "cyclin D." Cyclin D, like other cyclins, is unstable and is degraded through the ubiquitin-proteasome pathway ( Chapter 1 ). During the G1 phase of the cell cycle, cyclin D binds to and activates CDK4, forming a cyclin D-CDK4 complex ( Fig. 7-29 ). This complex has a critical role in the cell cycle by phosphorylating the retinoblastoma susceptibility protein (RB). The phosphorylation of RB is a molecular ON-OFF switch for the cell cycle.[38] In its hypophosphorylated state, RB prevents cells from replicating by forming a tight, inactive complex with the transcription factor E2F. (E2F is a family of transcription factors, referred to here as "E2F.") Phosphorylation of RB dissociates the complex and releases the inhibition on E2F transcriptional activity (see below). Thus, phosphorylation of RB eliminates the main barrier to cell-cycle progression and promotes cell replication.

Figure 7-29  Schematic illustration of the role of cyclins, CDKs, and cyclin-dependent kinase inhibitors in regulating the G1/S cell-cycle transition. External signals activate multiple signal transduction pathways, including those involving the MYC and RAS genes, which lead to synthesis and stabilization of cyclin D (there are several D cyclins, but, for simplification, we refer to them as "cyclin D"). Cyclin D binds to CDK4, forming a complex with enzymatic activity (cyclin D can also bind to CDK6, which appears to have a similar role as CDK4). The cyclin D-CDK4 complex phosphorylates RB, located in the E2F/DP1/RB complex in the nucleus, activating the transcriptional activity of E2F (E2F is a family of transcription factors, which we refer to as "E2F"), which leads to transcription of cyclin E, cyclin A and other proteins needed for the cell to go through the late G1 restriction point. The cell cycle can be blocked by the Cip/Kip inhibitors p21 and p27 (red boxes) and the INK4A/ARF inhibitors p16INK4A and p14ARF (green boxes). Cell-cycle arrest in response to DNA damage and other cellular stresses is mediated through p53. The levels of p53 are under negative regulation by MDM2, through a feedback loop that is inhibited by p14ARF.

http://www.mdconsult.com/images/blank.gif


The molecular basis of the RB braking action on the cell cycle has been unraveled in elegant detail.[38][39] Hypophosphorylated RB, present in quiescent cells (in G0 or early G1), binds to a protein complex that contains E2F and a subunit called DP1. The E2F/DP1/RB complex binds to promoters of E2F-responsive genes. Bound to the E2F/DP1/RB complex, such genes are silent because RB recruits histone deacetylase, an enzyme that causes compaction of chromatin and inhibition of transcription ( Fig. 7-29 and Fig. 7-30 ). When quiescent cells are stimulated by growth factors, the concentrations of cyclins D and E go up, resulting in the activation of cyclin D-CDK4 and cyclin E-CDK2 at the G1/S restriction point and causing phosphorylation of RB. Hyperphosphorylated RB dissociates from the complex, activating the transcription of E2F target genes that are essential for progression through the S phase. These include cyclin E, DNA polymerases, thymidine kinase, dihydrofolate reductase, and several others. During the M phase, the phosphate groups are removed from RB by cellular phosphatases, thus regenerating the hypophosphorylated form of RB.

Figure 7-30  Mechanism of cell-cycle regulation by RB. In a resting cell, RB is a component of the E2F/DP1/RB complex, which represses gene transcription through the recruitment of histone deacetylase, an enzyme that alters the conformation of chromatin, making it more compact. Phosphorylation of RB by cyclin D-CDK4 removes histone deacetylase from chromatin, allowing the activation of E2F transcriptional activity (RB can also be phosphorylated by cyclin E-CDK2). E2F-mediated transcription of cyclins E and A, and of genes required for DNA replication, permit the passage through the G1 restriction point.  (Adapted from Pollard TD, Earnshaw WC: Cell Biology. Philadelphia, WB Saunders, 2002, p. 689.)

http://www.mdconsult.com/images/blank.gif



Cell-Cycle Progression Beyond the G1/S Restriction Point.

Further progression through the S phase and the initiation of DNA replication involve the formation of an active complex between cyclin E and CDK2 (see Fig. 7-29 ). Activated E2F increases the transcription of cyclin E and of polymerases needed for DNA replication, thus stimulating DNA synthesis. The next decision point in the cell cycle is the G2/M transition. This transition is initiated by the E2F-mediated transcription of cyclin A, which forms the cyclin A-CDK2 complex that regulates events at the mitotic prophase. The main mediator that propels the cell beyond prophase is the cyclin B-CDK1 complex, which is activated by a protein phosphatase (Cdc 25) and begins to accumulate in the nucleus in early prophase. Cyclin B-CDK1 activation causes the breakdown of the nuclear envelope and initiates mitosis. Complexes of CDKs with cyclins A (there are two cyclin A isoforms, A1 and A2; A2 is essential for the cell cycle) and B regulate some critical events at the G2/M transition, such as the decrease in microtubule stability, the separation of centrosomes, and chromosome condensation. Exit from mitosis requires the inactivation of cyclin B-CDK1. Newly divided cells can then return to G1 and initiate a new replicative cycle or go into quiescence. Recent data show that, in proliferating cells, cyclin E-CDK2 may be replaced in some of its functions by a complex between cyclin A2 and CDK1. However, the absence of both isoforms of cyclin E (E1 and E2) prevent quiescent cells from entering the cell cycle.[41]

Cell-Cycle Inhibitors.

The activity of cyclin-CDK complexes is tightly regulated by inhibitors, called CDK inhibitors.[38][39][40] There are two main classes of CDK inhibitors: the Cip/Kip and the INK4/ARF families (see Fig. 7-29 and Table 7-7 ). These inhibitors function as tumor suppressors and are frequently altered in tumors (discussed below). The Cip/Kip family has three components, p21, p27, and p57, which bind to and inactivate the complexes formed between cyclins and CDKs. Transcriptional activation of p21 is under the control of p53, a tumor suppressor gene that is mutated in a large proportion of human cancers. The main role of p53 in the cell cycle is one of surveillance, triggering checkpoint controls that slow down or stop cell-cycle progression of damaged cells, or causes apoptosis. The human INK4a/ARF locus (a notation for "inhibitor of kinase 4/alternative reading frame") encodes two proteins, p16INK4a and p14ARF, which block the cell cycle and act as tumor suppressors. p16INK4a competes with cyclin D for binding to CDK4 and inhibits the ability of the cyclin D-CDK4 complex to phosphorylate RB, thus causing cell-cycle arrest at late G1. It is frequently mutated or inactivated by hypermethylation (discussed later) in human cancers. The INK4a locus encodes a second gene product, p14ARF (p19ARF in mice), which acts on p53. p14ARF arises from an alternative reading of the INK4a gene, providing for an "economical" way to utilize gene-coding sequences.[42] Although both p16INK4a and p14ARF block the cell cycle, their targets are different; p16INK4a acts on cyclin D-CDK4, whereas p14ARF prevents p53 degradation.


Table 7-7   -- Main Cell-Cycle Components and Their Inhibitors

Cell-Cycle Component

Main Function

Cyclin-Dependent Kinases

? CDK4

Forms a complex with cyclin D. The complex phosphorylates RB, allowing the cell to progress through the G1 restriction point.

? CDK2

Forms a complex with cyclin E in late G1, which is involved in the G1/S transition. Forms a complex with cyclin A at the S phase that facilitates the G2/M transition.

? CDK1

Forms a complex with cyclin B, which acts on the G2/M transition.

Inhibitors

? Cip/Kip family: p21, p27

Block the cell cycle by binding to cyclin-CDK complexes. p21 is induced by the tumor suppressor p53. p27 responds to growth suppressors such as transforming growth factor-β.

? 1NK4/ARF family: p16INK4A, p14ARF

p16INK4a binds to cyclin D-CDK4 and promotes the inhibitory effects of RB. p14ARF increases p53 levels by inhibiting MDM2 activity.

Checkpoint Components

? p53

Tumor suppressor altered in the majority of cancers; causes cell-cycle arrest and apoptosis. Acts mainly through p21 to cause cell-cycle arrest. Causes apoptosis by inducing the transcription of pro-apoptotic genes such as BAX. Levels of p53 are negatively regulated by MDM2 through a feedback loop. p53 is required for the G1/S checkpoint and is a main component of the G2/M checkpoint.

? Ataxia-telangiectasia mutated (ATM)

Activated by mechanisms that sense double stranded DNA breaks. Transmits signals to arrest the cell cycle after DNA damage. Acts through p53 in the G1/S checkpoint. At the G2/M checkpoint, it acts both through p53-dependent mechanisms and through the inactivation of CDC25 phosphatase, which disrupts the cyclin B-CDK1 complex. Component of a network of genes that include BRCA1 and BRCA2, which link DNA damage with cell-cycle arrest and apoptosis.

 

Cell-Cycle Checkpoints.

The cell cycle has its own internal controls, called checkpoints. There are two main checkpoints, one at the G1/S transition and another at G2/M.[43][44] The S phase is the point of no return in the cell cycle, and before a cell makes the final commitment to replicate, the G1/S checkpoint checks for DNA damage. If DNA damage is present, the DNA repair machinery and mechanisms that arrest the cell cycle are put in motion. The delay in cell-cycle progression provides the time needed for DNA repair; if the damage is not repairable, apoptotic pathways are activated to kill the cell. Thus, the G1/S checkpoint prevents the replication of cells that have defects in DNA, which would be perpetuated as mutations or chromosomal breaks in the progeny of the cell. DNA damaged after its replication can still be repaired as long as the chromatids have not separated. The G2/M checkpoint monitors the completion of DNA replication and checks whether the cell can safely initiate mitosis and separate sister chromatids. This checkpoint is particularly important in cells exposed to ionizing radiation. Cells damaged by ionizing radiation activate the G2/M checkpoint and arrest in G2; defects in this checkpoint give rise to chromosomal abnormalities. To function properly, cell-cycle checkpoints require sensors of DNA damage, signal transducers, and effector molecules.[44] The sensors and transducers of DNA damage appear to be similar for the G1/S and G2/M checkpoints. They include, as sensors, proteins of the RAD family and ataxia telangiectasia mutated (ATM) and as transducers, the CHK kinase families. The checkpoint effector molecules differ, depending on the cell-cycle stage at which they act. In the G1/S checkpoint, cell-cycle arrest is mostly mediated through p53, which induces the cell-cycle inhibitor p21. Arrest of the cell cycle by the G2/M checkpoint involves both p53-dependent and independent mechanisms. Defect in cell-cycle checkpoint components is a major cause of genetic instability in cancer cells.

With this background on the cell cycle and its control, we now proceed to discuss the genes that determine the malignant phenotype. This discussion will take place in the context of the seven fundamental changes in cell physiology (listed earlier) that are the hallmarks of malignant cells.

SELF-SUFFICIENCY IN GROWTH SIGNALS: ONCOGENES

Genes that promote autonomous cell growth in cancer cells are called oncogenes, and their normal cellular counterparts are called protooncogenes. Protooncogenes are physiologic regulators of cell proliferation and differentiation; oncogenes are characterized by the ability to promote cell growth in the absence of normal mitogenic signals. Their products, called oncoproteins, resemble the normal products of protooncogenes with the exception that oncoproteins are devoid of important regulatory elements. Their production in the transformed cells becomes constitutive, that is, not dependent on growth factors or other external signals. To aid in the understanding of the nature and functions of oncoproteins, and their role in cancer, it is necessary to briefly mention the sequential steps that characterize normal cell proliferation. Under physiologic conditions, cell proliferation can be readily resolved into the following steps:

  

?   

The binding of a growth factor to its specific receptor generally located on the cell membrane

  

?   

Transient and limited activation of the growth factor receptor, which, in turn, activates several signal-transducing proteins on the inner leaflet of the plasma membrane

  

?   

Transmission of the transduced signal across the cytosol to the nucleus via second messengers or by signal transduction molecules that directly activate transcription

  

?   

Induction and activation of nuclear regulatory factors that initiate DNA transcription

  

?   

Entry and progression of the cell into the cell cycle, ultimately resulting in cell division

With this background, we can readily identify the strategies used by cancer cells to acquire self-sufficiency in growth signals. They can be grouped on the basis of their role in growth factor-mediated signal transduction cascades and cell-cycle regulation. We start with a description of oncogenes and their protein products, and how these were discovered.

Protooncogenes, Oncogenes, and Oncoproteins

As often happens in science, the discovery of protooncogenes was not straightforward. These cellular genes were first discovered in their mutated or "oncogenic" forms as "passengers" within the genome of acute transforming retroviruses by the 1989 Nobel laureates Harold Varmus and Michael Bishop. These retroviruses cause rapid induction of tumors in animals and can also transform animal cells in vitro. Molecular dissection of their genomes revealed the presence of unique transforming sequences (viral oncogenes [v-onc]) not found in the genomes of nontransforming retroviruses. Most surprisingly, molecular hybridization revealed that the v-onc sequences were almost identical to sequences found in normal cellular DNA. From this evolved the concept that during evolution, cellular oncogenes were transduced (captured) by the virus through a chance recombination with the DNA of a (normal) host cell that had been infected by the virus. Because they were discovered initially as viral genes, these protooncogenes were named after their viral homologues. Each v-onc is designated by a three-letter word that relates the oncogene to the virus from which it was isolated. Thus, the v-onc contained in feline sarcoma virus is referred to as v-FES, whereas the oncogene in simian sarcoma virus is called v-SIS. The corresponding protooncogenes are referred to as FES and SIS, dropping the prefix.

The viral oncogenes are not present in several cancer-causing RNA viruses. One such example is a group of so-called slow transforming viruses that cause leukemias in rodents after a long latent period. The mechanism by which they cause neoplastic transformation implicates protooncogenes. Molecular dissection of the cells transformed by these leukemia viruses revealed that the proviral DNA is always integrated (inserted) near a protooncogene. One consequence of proviral insertion near a protooncogene is to induce a structural change in the cellular gene, thus converting it into a cellular oncogene (c-onc, or onc). This mode of protooncogene activation is called insertional mutagenesis. Alternatively, strong retroviral promoters inserted in the vicinity of the protooncogenes lead to dysregulated expression of the cellular gene.

Although the study of transforming animal retroviruses provided the first glimpse of protooncogenes, these investigations did not explain the origin of human tumors, which (with rare exceptions) are not caused by infection with retroviruses. Hence the question was raised: Do nonviral tumors contain oncogenic DNA sequences? The answer was provided by experiments involving DNA-mediated gene transfer (DNA transfection). When DNA extracted from several different human tumors was transfected into mouse fibroblast cell lines in vitro, the recipient cells acquired some properties of neoplastic cells. The conclusion from such experiments was inescapable: DNA of spontaneously arising cancers contains oncogenic sequences, or oncogenes. One of the first oncogenic sequences detected in cancers was a mutated form of the RAS protooncogene. This protooncogene is the forbear of v-oncs contained in Harvey (H) and Kirsten (K) sarcoma viruses.

A large number of protooncogenes have been identified during the past 20 years, most of which do not have a viral counterpart. Protooncogenes have multiple roles, participating in cellular functions related to growth and proliferation. Proteins encoded by protooncogenes may function as growth factor ligands and receptors, signal transducers, transcription factors, and cell-cycle components ( Fig. 7-31 ). Oncoproteins encoded by oncogenes generally serve similar functions as their normal counterparts ( Table 7-8 ). However, because they are constitutively expressed, oncoproteins endow the cell with self-sufficiency in growth.[45]

Figure 7-31  Subcellular localization and functions of major classes of cancer-associated genes. The protooncogenes are colored red, cancer suppressor genes blue, DNA repair genes green, and genes that regulate apoptosis purple.

http://www.mdconsult.com/images/blank.gif



Table 7-8   -- Selected Oncogenes, Their Mode of Activation, and Associated Human Tumors

Category

Protooncogene

Mode of Activation

Associated Human Tumor

Growth Factors

 

 

 

PDGF-β chain

SIS

Overexpression

Astrocytome

 

 

 

Osteosarcoma

Fibroblast growth factors

HST-1

Overexpression

Stomach cancer

 

INT-2

Amplification

Bladder cancer

 

 

 

Breast cancer

 

 

 

Melanoma

TGFα

TGFα

Overexpression

Astrocytomas

 

 

 

Hepatocellular carcinomas

HGF

HGF

Overexpression

Thyroid cancer

Growth Factor Receptors

 

 

 

EGF-receptor family

ERB-B1 (ECFR)

Overexpression

Squamous cell carcinomas of lung, gliomas

 

ERB-B2

Amplification

Breast and ovarian cancers

CSF-1 receptor

FMS

Point mutation

Leukemia

Receptor for neurotrophic factors

RET

Point mutation

Multiple endocrine neoplasia 2A and B, familial medullary thyroid carcinomas

PDGF receptor

PDGF-R

Overexpression

Gliomas

Receptor for stem cell (steel) factor

KIT

Point mutation

Gastrointestinal stromal tumors and other soft tissue tumors

Proteins Involved in Signal Transduction

 

 

 

GTP-binding

K-RAS

Point mutation

Colon, lung, and pancreatic tumors

 

H-RAS

Point mutation

Bladder and kidney tumors

 

N-RAS

Point mutation

Melanomas, hematologic malignancies

Nonreceptor tyrosine kinase

ABL

Translocation

Chronic myeloid leukemia

 

 

 

Acute lymphoblastic leukemia

RAS signal transduction

BRAF

Point mutation

Melanomas

WNT signal transduction

β-catenin

Point mutation

Hepatoblastomas, hepatocellular carcinoma

 

 

Overexpression

 

Nuclear Regulatory Proteins

 

 

 

Transcriptional activators

C-MYC

Translocation

Burkitt lymphoma

 

N-MYC

Amplification

Neuroblastoma, small cell carcinoma of lung

 

L-MYC

Amplification

Small cell carcinoma of lung

Cell-Cycle Regulators

 

 

 

Cyclins

CYCLIN D

Translocation

Mantle cell lymphoma

 

 

Amplification

Breast and esophageal cancers

 

CYCLIN E

Overexpression

Breast cancer

Cyclin-dependent kinase

CDK4

Amplification or point mutation

Glioblastoma, melanoma, sarcoma

 

To summarize, protooncogenes may be converted into cellular oncogenes (c-oncs) that are involved in tumor development. Two questions follow: (1) What are the functions of oncogene products, the oncoproteins? (2) How do the normally "civilized" protooncogenes turn into "enemies within"? These issues are discussed below.

Growth Factors.

Many cancer cells develop growth self-sufficiency by acquiring the ability to synthesize the same growth factors to which they are responsive. The protooncogene SIS, which encodes the β chain of platelet-derived growth factor (PDGF), is overproduced in many tumors, especially low-grade astrocytomas and osteosarcomas. Furthermore, it appears that the same tumors also express receptors for PDGF and are hence responsive to autocrine stimulation. Although an autocrine loop is considered to be an important element in the pathogenesis of several tumors, in most instances the growth factor gene itself is not altered or mutated. More commonly, products of other oncogenes such as RAS (that lie along many signal transduction pathways) cause overexpression of growth factor genes, thus forcing the cells to secrete large amounts of growth factors, such as transforming growth factor-α (TGF-α). This growth factor is related to epidermal growth factor (EGF) and induces proliferation by binding to the EGF receptor. TGF-α is often detected in carcinomas such as astrocytomas that express high levels of EGF receptors.

In addition to SIS, a group of related oncogenes that encode homologues of fibroblast growth factors (FGFs) (e.g., HST-1 and INT-2) is activated in several gastrointestinal and breast tumors; bFGF, a member of the fibroblast growth factor family, is expressed in human melanomas but not in normal melanocytes. Hepatocyte growth factor and its receptor c-MET are overexpressed in follicular carcinomas of the thyroid, constituting a growth-stimulatory autocrine loop. Small cell lung carcinomas produce bombesin-like peptides that stimulate their proliferation.

Despite extensive documentation of growth factor-mediated autocrine stimulation of transformed cells, increased growth factor production by itself is not sufficient for neoplastic transformation. Extensive cell proliferation, in all likelihood, contributes to the malignant phenotype by increasing the risk of spontaneous or induced mutations in the cell population.

Growth Factor Receptors.

Several oncogenes that encode growth factor receptors have been found. To understand how mutations affect the function of these receptors, it should be recalled that several growth factor receptors are transmembrane proteins with an external ligand-binding domain and a cytoplasmic tyrosine kinase domain ( Chapter 3 ). In the normal forms of these receptors, the kinase is transiently activated by binding of the specific growth factors, followed rapidly by receptor dimerization and tyrosine phosphorylation of several substrates that are a part of the signaling cascade. The oncogenic versions of these receptors are associated with constitutive dimerization and activation without binding to the growth factor. Hence, the mutant receptors deliver continuous mitogenic signals to the cell.

Growth factor receptors are activated in human tumors by several mechanisms. These include mutations, gene rearrangements, and overexpression. The RET protooncogene, a receptor tyrosine kinase, exemplifies oncogenic conversion via mutations and gene rearrangements.[46] The RET protein is a receptor for the glial cell line-derived neurotrophic factor and structurally related proteins that promote cell survival during neural development. RET is normally expressed in neuroendocrine cells, such as parafollicular C cells of the thyroid, adrenal medulla, and parathyroid cell precursors. Point mutations in the RET protooncogene are associated with dominantly inherited MEN types 2A and 2B and familial medullary thyroid carcinoma ( Chapter 24 ). In MEN 2A, point mutations in the RET extracellular domain cause constitutive dimerization and activation, leading to medullary thyroid carcinomas and also adrenal and parathyroid tumors. In MEN 2B, point mutations in the RET cytoplasmic catalytic domain alter the substrate specificity of the tyrosine kinase and lead to thyroid and adrenal tumors but no involvement of the parathyroid. Complete loss of RET function results in Hirschsprung disease ( Chapter 17 ), in which there is lack of development of intestinal nerve plexuses. In all these familial conditions, the affected individuals inherit the RET mutation in the germ line. Sporadic medullary carcinomas of the thyroid are associated with somatic rearrangements of the RET gene, generally similar to those found in MEN 2B.[46][47]

Oncogenic conversions by mutations and rearrangements have been found in other growth factor receptor genes. Point mutations that activate c-FMS, the gene encoding the colony-stimulating factor 1 (CSF-1) receptor, have been detected in myeloid leukemias. In certain chronic myelomonocytic leukemias with the t(12;9) translocation, the entire cytoplasmic domain of the PDGF receptor is fused with a segment of the ETS family transcription factor, resulting in permanent dimerization of the PDGF receptor.

Far more common than mutations of these protooncogenes is overexpression of normal forms of growth factor receptors. In sporadic papillary thyroid carcinomas, c-MET is overexpressed in almost every case.[48] In these tumors, increased expression of c-MET is not caused by gene mutation but results from enhanced transcription of the gene. In some tumors, increased receptor expression results from gene amplification, but in many cases, the molecular basis of increased receptor expression is not fully known. Two members of the EGF receptor family are most commonly involved. The normal form of ERB B1, the EGF receptor gene, usually referred to as EGFR, is overexpressed in up to 80% of squamous cell carcinomas of the lung, in 50% or more of high-grade astrocytomas called glioblastomas ( Chapter 28 ), in 80% to 100% of head and neck tumors, and less commonly, in carcinomas of the urinary bladder and the gastrointestinal tract.[49][50] In contrast, the ERB B2 gene (also called HER 2/Neu), the second member of the EGF receptor family, is amplified in approximately 25% of breast cancers and in human adenocarcinomas arising within the ovary, lung, stomach, and salivary glands.[51] Because the molecular alteration in ERB B2 is specific for the cancer cells, new therapeutic agents consisting of monoclonal antibodies against ERB B2 have been developed and are currently in use clinically.[49][51] This type of therapy, directed to a specific alteration in the cancer cell, is called targeted therapy.[52] Another example of very successful targeted cancer therapy is the blockage of receptor tyrosine kinase activity of c-KIT in stromal tumors of the gastrointestinal tract.[53] In these tumors, a mutation in c-KIT, the gene encoding the receptor for stem cell factor (also known as steel factor), constitutively activates the receptor tyrosine kinase, independent of ligand binding.

Signal-Transducing Proteins.

Several examples of oncoproteins that mimic the function of normal cytoplasmic signal-transducing proteins have been found. Most such proteins are strategically located on the inner leaflet of the plasma membrane, where they receive signals from outside the cell (e.g., by activation of growth factor receptors) and transmit them to the cell's nucleus. Biochemically, the signal-transducing proteins are heterogeneous. The best and most well studied example of a signal-transducing oncoprotein is the RAS family of guanine triphosphate (GTP)-binding proteins (G proteins).

The RAS Oncogene.

The RAS proteins were discovered as products of viral oncogenes. Point mutation of RAS family genes is the single most common abnormality of dominant oncogenes in human tumors. Approximately 15% to 20% of all human tumors contain mutated versions of RAS proteins.[54] Several distinct mutations of RAS have been identified in cancer cells, all of which dramatically reduce the GTPase activity of the RAS proteins. The mutations generally involve codons 12, 59, or 61 of HRAS, KRAS, and NRAS. The frequency of such mutations varies with different tumors, but in some types it is very high. For example, 90% of pancreatic adenocarcinomas and cholangiocarcinomas contain a RAS point mutation, as do about 50% of colon, endometrial, and thyroid cancers and 30% of lung adenocarcinomas and myeloid leukemias.[55][56][57] In general, carcinomas (particularly from colon and pancreas) have mutations of KRAS, bladder tumors have HRAS mutations, and hematopoietic tumors bear NRAS mutations. RAS mutations are infrequent in certain other cancers, particularly those arising in the uterine cervix or breast.

Several studies indicate that RAS plays an important role in mitogenesis induced by growth factors. For example, blockade of RAS function by microinjection of specific antibodies blocks the proliferative response to EGF, PDGF, and CSF-1. Normal RAS proteins are tethered to the cytoplasmic aspect of the plasma membrane, and they flip back and forth between an activated, signal-transmitting form and an inactive, quiescent state. Recently it was found that these proteins may also be found in the endoplasmic reticulum and Golgi membranes, where they can be activated by growth factor binding to the plasma membrane, through a still-uncertain mechanism.[58] In the inactive state, RAS proteins bind guanosine diphosphate (GDP); when cells are stimulated by growth factors or other receptor-ligand interactions, RAS becomes activated by exchanging GDP for GTP ( Fig. 7-32 ). Activated RAS, in turn, acts on the MAP kinase pathway by recruiting the cytosolic protein RAF-1. The MAP kinases so activated target nuclear transcription factors and thus promote mitogenesis. In normal cells, the activated signal-transmitting stage of the RAS protein is transient because its intrinsic GTPase activity hydrolyzes GTP to GDP, thereby returning RAS to its quiescent ground state (described below).

Figure 7-32  Model for action of RAS genes. When a normal cell is stimulated through a growth factor receptor, inactive (GDP-bound) RAS is activated to a GTP-bound state. Activated RAS recruits RAF and stimulates the MAP-kinase pathway to transmit growth-promoting signals to the nucleus. The mutant RAS protein is permanently activated because of inability to hydrolyze GTP, leading to continuous stimulation of cells without any external trigger. The anchoring of RAS to the cell membrane by the farnesyl moiety is essential for its action.

http://www.mdconsult.com/images/blank.gif


The orderly cycling of the RAS protein depends on two reactions: (1) nucleotide exchange (GDP by GTP), which activates RAS protein, and (2) GTP hydrolysis, which converts the GTP-bound, active RAS to the GDP-bound, inactive form. Both these processes are enzymatically regulated. The removal of GDP and its replacement by GTP during RAS activation are catalyzed by a family of guanine nucleotide-releasing proteins that are recruited to the cytosolic domain of activated growth factor receptors by adapter proteins. More importantly, the GTPase activity intrinsic to normal RAS proteins is dramatically accelerated by GTPase-activating proteins (GAPs). These widely distributed proteins bind to the active RAS and augment its GTPase activity by more than 1000-fold, leading to rapid hydrolysis of GTP to GDP and termination of signal transduction. Thus, GAPs function as "brakes" that prevent uncontrolled RAS activity. The response to this braking action of GAPs seems to falter when mutations affect the RAS gene. Mutant RAS proteins bind GAP, but their GTPase activity fails to be augmented. Hence the mutant proteins are "trapped" in their excited GTP-bound form, causing, in turn, a pathologic activation of the mitogenic signaling pathway. The importance of GTPase activation in normal growth control is underscored by the fact that a disabling mutation of neurofibromin (NF-1), a GTPase-activating protein, is also associated with neoplasia (see discussion of tumor suppressor genes below).

In addition to RAS, other members of the RAS signaling cascade (RAS/RAF/MAP kinase) may also be altered in cancer cells. Thus, mutations in BRAF, one of the members of the RAF family, have been detected in more than 60% of melanomas and in more than 80% of benign nevi.[59][60] This suggests that dysregulation of the RAS/RAF/MAP kinase pathway may be one of the initiating events in the development of melanomas, although it is not sufficient by itself to cause tumorigenesis.

Recent studies have revealed that, in addition to its role in transducing growth factor signals, RAS is also involved in regulation of the cell cycle. As described above, the passage of cells from G1 to the S phase is modulated by cyclins and CDKs. RAS proteins can indirectly regulate the levels of cyclins by activating the MAP kinase pathway and the AP-1 transcription factor.

Because RAS is so frequently mutated in human cancers, much effort has been spent to develop anti-RAS modalities of targeted therapy. Several such strategies for cancer treatment are being evaluated. The specific targets include blockade of the association of RAS with the cell membrane, using inhibitors of farnesyl transferase (an enzyme that provides the bridge between RAS and the lipid components of the plasma membrane); blockade of downstream components of RAS signaling pathways (i.e., RAF and MAP kinase inhibitors); direct blockade of RAS; and blockade of signaling from the EGF receptor, to prevent the activation of RAS pathways. Unfortunately, none of these strategies has so far proven to be successful for clinical use.[61] Nevertheless, given the frequency of RAS mutations in human cancer and the importance of RAS in cell proliferation, efforts to disable this signaling pathway continue as a potential modality of cancer therapy.

Alterations in Nonreceptor Tyrosine Kinases.

As discussed in Chapter 3 , several nonreceptor-associated tyrosine kinases function in the signal transduction pathways that regulate cell growth. With the notable exception of c-ABL, however, they are rarely activated in human tumors. The ABL protooncogene product has tyrosine kinase activity, which is dampened by negative regulatory domains. In chronic myeloid leukemia and some acute lymphoblastic leukemias, however, this activity is unleashed because the c-ABL gene is translocated from its normal abode on chromosome 9 to chromosome 22 ( Fig. 7-33 ), where it fuses with the BCR gene (see discussion of chromosomal translocations, later in this chapter). As a consequence of the fusion, c-ABL loses a region that controls tyrosine kinase activity.[62][63] Thus, the BCR-ABL protein, the product of the fusion gene, has potent and constitutive tyrosine kinase activity, which is critical to the oncogenic capacity of the gene. Treatment of chronic myeloid leukemia has been revolutionized by the development of imatinib mesylate, a "designer" drug that targets the BCR-ABL tyrosine kinase. It has low toxicity and high therapeutic efficacy.[52][62][63]

Figure 7-33  The chromosomal translocation and associated oncogenes in Burkitt lymphoma and chronic myelogenous leukemia.

http://www.mdconsult.com/images/blank.gif


Transcription Factors.

Signal transduction pathways generate transcriptional regulators that enter the nucleus and act on a large bank of responder genes. These genes orchestrate the cells' orderly entry and progression through the cell cycle, leading to DNA replication and cell division. Transcription factors contain specific amino acid sequences or motifs that allow them to bind DNA or to dimerize for DNA binding. Examples of such motifs include helix-loop-helix, leucine zipper, zinc-finger, and homeodomains. Many of these proteins bind DNA at specific sites from which they can activate or inhibit transcription of adjacent genes. Not surprisingly, therefore, mutations affecting genes that encode nuclear transcription factors are associated with malignant transformation.

A whole host of oncoproteins, including products of the MYC, MYB, the JUN family, and FOS oncogenes, is found in the nuclei of transformed cells. Of these, MYC is most commonly involved in human tumors, and hence a brief overview of its function is warranted.

The MYC Oncogene.

The MYC protooncogene is expressed in virtually all eukaryotic cells and belongs to the immediate early response genes, which are rapidly induced when quiescent cells receive a signal to divide (see discussion of liver regeneration in Chapter 3 ). After a transient increase of MYC mRNA, the expression declines to a basal level. The molecular basis of MYC function in cell replication is not entirely clear, but some general principles have emerged.[64][65] The MYC protein is rapidly translocated to the nucleus, sometimes as a dimer with another protein, called MAX. This heterodimer binds to DNA sequences in target genes and is a potent transcriptional activator. Some of its target genes, such as ornithine decarboxylase and cyclin D2, are known to be associated with cell proliferation. However, the range of activities attributed to MYC is very broad and includes histone acetylation, reduced cell adhesion and increased cell motility, increased protein synthesis, and decreased proteinase activity.[66]

While on one hand MYC activation is linked to proliferation, on the other hand, cells in culture undergo apoptosis if MYC activation occurs in the absence of survival signals (growth factors). The MYC protooncogene contains separate sequences that encode the growth promoting and apoptotic activities, but it is not clear whether MYC-induced apoptosis occurs in vivo.

In contrast to the regulated expression of MYC during normal cell proliferation, persistent expression, and in some cases overexpression, of the MYC protein are commonly found in tumors. This may lead to sustained transcription of critical target genes and subsequent neoplastic transformation. Dysregulation of MYC expression resulting from translocation of the gene occurs in Burkitt lymphoma, a B-cell tumor (see Fig. 7-33 ). MYC is amplified in some cases of breast, colon, lung, and many other carcinomas. The related N-MYC and L-MYC genes are amplified in neuroblastomas ( Fig. 7-34 ) and small cell cancers of the lung, respectively.

Figure 7-34  Amplification of the N-MYC gene in human neuroblastomas. The N-MYC gene, normally present on chromosome 2p, becomes amplified and is seen either as extra chromosomal double minutes or as a chromosomally integrated, homogeneous staining region. The integration involves other autosomes, such as 4, 9, or 13.  (Modified from Brodeur GM: Molecular correlates of cytogenetic abnormalities in human cancer cells: implications for oncogene activation. In Brown EB (ed): Progress in Hematology, Vol 14. Orlando, FL, Grune & Stratton, 1986, pp. 229–256.)

http://www.mdconsult.com/images/blank.gif



Cyclins and Cyclin-Dependent Kinases.

Based on our earlier discussion of the normal functions of cyclins and CDKs in cell-cycle control, it is easy to appreciate that dysregulation of the activity of these proteins might favor cell proliferation. Abnormalities in the expression of cyclins and CDKs are present in several human cancers. Indeed, mishaps affecting the expression of cyclin D or CDK4 seem to be a common event in neoplastic transformation. The cyclin D genes are overexpressed in many cancers, including those affecting the breast, esophagus, head and neck, and liver, and in a subset of lymphomas (mantle cell lymphomas), in which the CYCLIN D1 gene is a component of a fusion gene created by chromosomal translocation (see below). Amplification of the CDK4 gene occurs in sarcomas and glioblastomas. Cyclin E and its low-molecular-weight forms are overexpressed in breast cancers, and the level of expression correlates with disease progression and survival.[67]

INSENSITIVITY TO GROWTH INHIBITORY SIGNALS: TUMOR SUPPRESSOR GENES

The growth of cells has to be controlled by many external signals to maintain a steady state (homeostasis). Failure of growth inhibition is one of the fundamental alterations in the process of carcinogenesis. The proteins that apply brakes to cell proliferation are the products of tumor suppressor genes ( Table 7-9 ). In a sense, the term "tumor suppressor genes" is a misnomer because the physiologic function of these genes is to regulate cell growth, not to prevent tumor formation.[68] Because the loss of function of these genes is a key event in many, possibly all, human tumors and because their discovery resulted from the study of tumors, the name tumor suppressor persists.


Table 7-9   -- Selected Tumor Suppressor Genes Involved in Human Neoplasms

Subcellular Location

Gene

Function

Tumors Associated with Somatic Mutations

Tumors Associated with Inherited Mutations

Cell surface

TGF-β receptor

Growth inhibition

Carcinomas of colon

Unknown

 

E-cadherin

Cell adhesion

Carcinoma of stomach

Familial gastric cancer

Inner aspect of plasma membrane

NF-1

Inhibition of RAS signal transduction and of p21 cell-cycle inhibitor

Neuroblastomas

Neurofibromatosis type 1 and sarcomas

Cytoskeleton

NF-2

Cytoskeletal stability

Schwannomas and meningiomas

Neurofibromatosis type 2, acoustic schwannomas and meningiomas

Cytosol

APC/β-catenin

Inhibition of signal transduction

Carcinomas of stomach, colon, pancreas; melanoma

Familial adenomatous polyposis coli/colon cancer

 

PTEN

PI-3 kinase signal transduction

Endometrial and prostate cancers

Unknown

 

SMAD 2 and SMAD 4

TGF-β signal transduction

Colon, pancreas tumors

Unknown

Nucleus

RB

Regulation of cell cycle

Retinoblastoma; osteosarcoma carcinomas of breast, colon, lung

Retinoblastomas, osteosarcoma

 

p53

Cell-cycle arrest and apoptosis in response to DNA damage

Most human cancers

Li-Fraumeni syndrome; multiple carcinomas and sarcomas

 

WT-1

Nuclear transcription

Wilms tumor

Wilms tumor

 

p16 (INK4a)

Regulation of cell cycle by inhibition of cyclin-dependent kinases

Pancreatic, breast, and esophageal cancers

Malignant melanoma

 

BRCA-1 and BRCA-2

DNA repair

Unknown

Carcinomas of female breast and ovary; carcinomas of male breast

 

KLF6

Transcription factor

Prostate

Unknown

 

Similar to many discoveries in medicine, the tumor suppressor genes were discovered by studying rare diseases, in this case retinoblastoma, a tumor that affects about 1 in 20,000 infants and children. Approximately 60% of retinoblastomas are sporadic, and the remaining 40% are inherited, with the predisposition to develop the tumor being transmitted as an autosomal dominant trait. To explain the inherited and sporadic occurrence of an apparently identical tumor, Knudson proposed his now famous "two-hit" hypothesis of oncogenesis.[21][69] He suggested that in hereditary cases, one genetic change ("first hit") is inherited from an affected parent and is therefore present in all somatic cells of the body, whereas the second mutation ("second hit") occurs in one of the many retinal cells (which already carry the first mutation). In sporadic cases, however, both mutations (hits) occur somatically within a single retinal cell, whose progeny then form the tumor.

Retinoblastoma as a Paradigm for the Two-Hit Hypothesis of Oncogenesis.

Knudson's hypothesis has been amply substantiated by cytogenetic and molecular studies with other tumor suppressor genes and can now be formulated in more precise terms, using retinoblastoma as a paradigm:

  

?   

The mutations required to produce retinoblastoma involve the RB gene, located on chromosome 13q14. In some cases, the genetic damage is large enough to be visible in the form of a deletion of 13q14.

  

?   

Both normal alleles of the RB locus must be inactivated (two hits) for the development of retinoblastoma ( Fig. 7-35 ). In familial cases, children are born with one normal and one defective copy of the RB gene. They lose the intact copy in the retinoblasts through some form of somatic mutation (point mutation, interstitial deletion of 13q14, or even complete loss of the normal chromosome 13). In sporadic cases, both normal RB alleles are lost by somatic mutation in one of the retinoblasts. The end result is the same: A retinal cell that has lost both normal copies of the RB gene gives rise to cancer.

  

?   

Patients with familial retinoblastoma are also at greatly increased risk of developing osteosarcoma and some other soft tissue sarcomas. Furthermore, inactivation of the RB locus has been noted in several other tumors, including adenocarcinoma of the breast, small cell carcinoma of the lung, and bladder carcinoma. Most importantly, alterations in the "RB pathway," involving INK4a proteins, cyclin D-dependent kinases, and RB family proteins, are almost always present in cancer cells.[36]

 

Figure 7-35  Pathogenesis of retinoblastoma. Two mutations of the RB locus on chromosome 13q14 lead to neoplastic proliferation of the retinal cells. In the familial form, all somatic cells inherit one mutant RB gene from a carrier parent. The second mutation affects the Rb locus in one of the retinal cells after birth. In the sporadic form, on the other hand, both mutations at the RB locus are acquired by the retinal cells after birth.

http://www.mdconsult.com/images/blank.gif


At this point, we should clarify some terminology. A child carrying an inherited mutant RB allele in all somatic cells is perfectly normal (except for the increased risk of developing cancer). Because such a child is heterozygous at the RB locus, it implies that heterozygosity for the RB gene does not affect cell behavior. Cancer develops when the cell becomes homozygous for the mutant allele or, put another way, when the cell loses heterozygosity for the normal RB gene (a condition known as LOH, for loss of heterozygosity). Because the RB gene is associated with cancer when both normal copies are lost, it is sometimes referred to as a recessive cancer gene.

The RB gene stands as a paradigm for several other genes that act similarly. For example, one or more genes on the short arm of chromosome 11 play a role in the formation of Wilms tumor, hepatoblastoma, and rhabdomyosarcoma. The von Hippel Lindau (VHL) gene is a tumor suppressor gene that causes familial clear-cell renal carcinomas and is also involved in sporadic forms of the same tumor.[70] Consistent and nonrandom LOH has provided important clues to the location of several tumor suppressor genes.

The protein products of tumor suppressor genes are involved in cell-cycle control, the regulation of apoptosis, and many other activities critical for cell survival and growth. They may function as transcription factors, cell-cycle inhibitors, signal transduction molecules, cell surface receptors, and regulators of cellular responses to DNA damage. A list of selected tumor suppressor genes is provided in Table 7-9 . In the following section we discuss the functions of the most important tumor suppressor genes, and how their defects contribute to carcinogenesis.

RB Gene.

Much is known about the RB gene because this was the first tumor suppressor gene discovered.[68] RB protein, the product of the RB gene, is a nuclear phosphoprotein that plays a key role in regulating the cell cycle. It is expressed in every cell type examined; as we have already seen, RB exists in an active hypophosphorylated state in quiescent cells and an inactive hyperphosphorylated state in the G1/S cell-cycle transition ( Fig. 7-36 ). When cells enter the S phase, they can continue to cell division independent of growth factors. It should be obvious from this discussion that if RB is absent (owing to gene deletions) or its ability to regulate E2F transcription factors is derailed, the molecular brakes on the cell cycle are released, and the cells move into the S phase followed by cell replication. The mutations of RB genes found in tumors are localized to a region of the RB protein, called the "RB pocket," that is involved in binding to E2F.

Figure 7-36  Role of RB as a cell-cycle regulator. Various growth factors promote the formation of the cyclin D-CDK4 complex. This complex (and to some extent cyclin E-CDK2) phosphorylates RB, changing it from an active (hypophosphorylated) to an inactive state (hyperphosphorylation). RB inactivation allows the cell to pass the G1/S restriction point. Growth inhibitors such as TGF-β and p53 and the Cip/Kip (e.g., p21, p57) and INK4a (p161NK4a and p19ARF) cell-cycle inhibitors prevent RB activation. Transforming proteins of oncogenic viruses bind hypophosphorylated RB and cause its functional inactivation. Virtually all cancers show dysregulation of the cell cycle by affecting the four genes marked by an asterisk.

http://www.mdconsult.com/images/blank.gif


It was mentioned previously that germ-line loss or mutations of the RB gene predispose to occurrence of retinoblastomas and to a lesser extent osteosarcomas. Furthermore, somatically acquired mutations have been described in glioblastomas, small cell carcinomas of lung, breast cancers, and bladder carcinomas. Given the presence of RB in every cell and its importance in cell-cycle control, two questions arise: (1) Why do patients with germ line mutation of the RB locus develop mainly retinoblastomas? (2) Why are inactivating mutations of RB not much more common in human cancer? The basis for the occurrence of tumors restricted to the retina in patients who inherit one defective allele of RB is not fully understood, but some possible explanations have emerged from the study of mice with targeted disruption of the RB locus. For instance, RB mutation may be a critical initiating event for retinoblastomas but may be only an accessory factor for malignancies at other sites.

With respect to the second question (i.e., why the loss of RB is not more common in human tumors), the answer is much simpler: Mutations in other genes that control RB phosphorylation can mimic the effect of RB loss, and such genes are mutated in many cancers that may have normal RB genes. Thus, for example, mutational activation of cyclin D or CDK4 would favor cell proliferation by facilitating RB phosphorylation. As previously discussed, cyclin D is overexpressed in many tumors because of gene amplification or translocation. Mutational inactivation of CDK inhibitors would also drive the cell cycle by unregulated activation of cyclins and CDKs. Thus, the emerging paradigm is that loss of normal cell-cycle control is central to malignant transformation and that at least one of four key regulators of the cell cycle (p16INK4a, CYCLIN D, CDK4, RB) is dysregulated in the vast majority of human cancers.[38] In cells that harbor mutations in any one of these other genes, the function of RB is disrupted even if the RB gene itself is not mutated.[45]

Several other pathways of cell growth regulation, some to be discussed in more detail later, also converge on RB ( Fig. 7-36 ):

  

?   

TGF-β induces inhibition of cellular proliferation. This effect of TGF-β is mediated, at least in part, by up-regulation of the CDK inhibitor p27.

  

?   

The transforming proteins of several oncogenic animal and human DNA viruses seem to act, in part, by neutralizing the growth inhibitory activities of RB. In these cases, RB protein is functionally deleted by the binding of a viral protein and no longer acts as a cell-cycle inhibitor. Simian virus 40 and polyomavirus large T antigens, adenoviruses EIA protein, and human papillomavirus (HPV) E7 protein, all bind to the hypophosphorylated form of RB. The binding occurs in the same RB pocket that normally sequesters E2F transcription factors; in the case of HPV, the binding is particularly strong for viral types, such as HPV 16, which confer high risk for the development of cervical carcinomas. Thus, the RB protein, unable to bind the E2F transcription factors, is functionally deleted, and the transcription factors are free to cause cell-cycle progression.

  

?   

The p53 tumor suppressor gene exerts its growth-inhibiting effects at least in part by up-regulating the synthesis of the CDK inhibitor p21 (see Fig. 7-29 and Fig. 7-36 ).

p53: Guardian of the Genome.

The p53 gene is located on chromosome 17p13.1, and it is the most common target for genetic alteration in human tumors.[71] A little over 50% of human tumors contain mutations in this gene. Homozygous loss of p53 gene activity can occur in virtually every type of cancer, including carcinomas of the lung, colon, and breast—the three leading causes of cancer death.[72] In most cases, the inactivating mutations affect both p53 alleles and are acquired in somatic cells (not inherited in the germ line). Less commonly, some individuals inherit one mutant p53 allele. As with the RB gene, inheritance of one mutant allele predisposes individuals to develop malignant tumors because only one additional "hit" is needed to inactivate the second, normal allele. Such individuals, said to have the Li-Fraumeni syndrome, have a 25-fold greater chance of developing a malignant tumor by age 50 than the general population.[73] In contrast to patients who inherit a mutant RB allele, the spectrum of tumors that develop in patients with the Li-Fraumeni syndrome is quite varied; the most common types of tumors are sarcomas, breast cancer, leukemia, brain tumors, and carcinomas of the adrenal cortex. As compared with sporadic tumors, those that afflict patients with the Li-Fraumeni syndrome occur at a younger age, and a given individual may develop multiple primary tumors.[74]

The fact that p53 mutations are common in a variety of human tumors suggests that the p53 protein functions as a critical gatekeeper against the formation of cancer. Indeed, it is evident that p53 acts as a "molecular policeman" that prevents the propagation of genetically damaged cells. The p53 protein is a DNA-binding protein localized to the nucleus; when called into action, it functions primarily by controlling the transcription of several other genes. Approximately 80% of the p53 point mutations present in human cancers are located in the DNA-binding domain of the protein. Mutated p53 that does not bind to DNA, produces a defective protein (missense mutation) that blocks the activity of the normal protein. In addition to somatic and inherited mutations, p53 functions can be inactivated by other mechanisms. As with RB, the transforming proteins of several DNA viruses, including the E6 protein of HPV, can bind to and promote the degradation of p53. Another mechanism of p53 neutralization is via MDM2, a protein that normally inhibits the function of p53 by causing its degradation. MDM2 levels are increased in 33% of human sarcomas and in 50% of leukemias, thereby causing functional loss of p53 in these tumors.[75][76]

The major functional activities of the p53 protein are cell-cycle arrest and initiation of apoptosis in response to DNA damage. p53 is called in to apply emergency brakes when DNA is damaged by irradiation, UV light, or mutagenic chemicals and also in response to changes in cellular redox potential, hypoxia, senescence, and other stress conditions that may not directly damage DNA.[71] Following DNA damage, there is a rapid increase in p53 levels. At the same time, kinases such as DNA-dependent protein kinase and ATM (ataxia-telangectasia mutated) are activated in response to DNA damage. These enzymes phosphorylate p53, and the protein then unfolds, is able to bind to DNA, and becomes an active transcription factor ( Fig. 7-37 ). p53 stimulates transcription of several genes that mediate cell-cycle arrest and apoptosis. p53-induced cell-cycle arrest occurs late in the G1 phase and is caused by the p53-dependent transcription of the CDK inhibitor p21. Such a pause in cell cycling is welcome because it allows the cells enough time to repair the DNA damage inflicted by the mutagenic agent. Under physiologic conditions, p53 has a short half-life (about 20 minutes) because of ubiquitin-mediated proteolysis; hence, in contrast to RB, it does not police the normal cell cycle. p53 also helps in the repair process directly by inducing the transcription of GADD45 (growth arrest and DNA damage), which encodes a protein involved in DNA repair. If the DNA damage is repaired successfully, quite ingeniously, p53 activates MDM2, whose product binds to and degrades p53, thus relieving the cell-cycle block (see Fig. 7-29 ). If during the pause in cell division the DNA damage cannot be successfully repaired, normal p53, perhaps as a last-ditch effort, sends the cell to the graveyard by inducing the activation of apoptosis-inducing genes, such as BAX. BAX, as we discuss later, binds to and antagonizes the apoptosis-inhibiting protein BCL-2; thus, BAX promotes cell death.

Figure 7-37  The role of p53 in maintaining the integrity of the genome. Activation of normal p53 by DNA-damaging agents or by hypoxia leads to cell-cycle arrest in G1 and induction of DNA repair, by transcriptional up-regulation of the cyclin-dependent kinase inhibitor p21, and the GADD45 genes, respectively. Successful repair of DNA allows cells to proceed with the cell cycle; if DNA repair fails, p53-induced activation of the BAX gene promotes apoptosis. In cells with loss or mutations of p53, DNA damage does not induce cell-cycle arrest or DNA repair, and hence genetically damaged cells proliferate, giving rise eventually to malignant neoplasms.

http://www.mdconsult.com/images/blank.gif


To summarize, p53 links cell damage with DNA repair, cell-cycle arrest, and apoptosis. In response to DNA damage, it is phosphorylated by genes that sense the damage and are involved in DNA repair. p53 assists in DNA repair by causing G1 arrest and inducing DNA repair genes. A cell with damaged DNA that cannot be repaired is directed by p53 to undergo apoptosis (see Fig. 7-37 ). In view of these activities, p53 has been rightfully called a "guardian of the genome." With homozygous loss of p53, DNA damage goes unrepaired, mutations become fixed in dividing cells, and the cell turns onto a one-way street leading to malignant transformation.

The ability of p53 to control apoptosis in response to DNA damage has important practical therapeutic implications. Radiation and chemotherapy, the two common modalities of cancer treatment, mediate their effects by inducing DNA damage and subsequent apoptosis. Tumors that retain normal p53 are more likely to respond to such therapy than tumors that carry mutant alleles of the gene. Such is the case with testicular teratocarcinomas[77] and childhood acute lymphoblastic leukemias. By contrast, tumors such as lung cancers and colorectal cancers, which frequently carry p53 mutations, are relatively resistant to chemotherapy and radiotherapy. Various therapeutic strategies aimed at increasing normal p53 activity in tumor cells that retain this type of activity or selectively killing cells defective in p53 function are being investigated. One type of strategy relies mostly on the modulation of MDM2 activity; a second uses modified adenoviruses that lyse cancer cells that lack p53 or p14ARF function.[38][78]

In closing this discussion of the p53 gene, it should be pointed out that p53 is actually a member of a multigene family.[79] The p73 gene (dubbed the big brother of p53), another member of this family, is located on 1p36 and encodes a protein that has 60% homology to p53. It can cause cell-cycle arrest as well as apoptosis under appropriate conditions.[80] The newest member of the family is p63. It is likely that at least in some tissues, p53 deficiency may be compensated by expression of the other genes of the same family.

APC/β-Catenin Pathway.

Down-regulation of growth-promoting signals is another potential area in which products of tumor suppressor genes may be operative. The products of the APC and NF-1 genes fall into this category. Germ line mutations at the APC (5q21) and NF-1 (17q11.2) loci are associated with benign tumors that are precursors of carcinomas that develop later.

In the case of the APC gene, all individuals born with one mutant allele develop thousands of adenomatous polyps in the colon during their teens or twenties (familial adenomatous polyposis; Chapter 17 ). Almost invariably, one or more of these polyps undergoes malignant transformation, giving rise to colon cancer. As with other tumor suppressor genes, both copies of the APC gene must be lost for tumor development. When this occurs, adenomas form. This conclusion is supported by the development of colon adenomas in mice with targeted disruption of APC genes in the colonic mucosa.[81] As discussed later, several additional mutations must occur for cancers to develop in adenomas. In addition to these tumors, which have a strong hereditary predisposition, 70% to 80% of nonfamilial colorectal carcinomas and sporadic adenomas also show homozygous loss of the APC gene, thus firmly implicating APC loss in the pathogenesis of colonic tumors.[56]

The molecular basis of APC action and the basis of its tumor suppressor activity have been learned by the study of homologous genes in the fruitfly Drosophila and the amphibian Xenopus ( Fig. 7-38 ). APC is a component of the WNT signaling pathway, which has a major role in controlling cell fate, adhesion, and cell polarity during embryonic development. WNT signaling is also required for self-renewal of hematopoietic stem cells.[82] WNT signals through a family of cell-surface receptors called frizzled (FRZ), and stimulates several pathways, the central one involving β-catenin and APC.[83]

Figure 7-38  A, The role of APC in regulating the stability and function of β-catenin. APC and β-catenin are components of the WNT signaling pathway. In resting cells (not exposed to WNT), β-catenin forms a macromolecular complex containing the APC protein. This complex leads to the destruction of β-catenin, and intracellular levels of β-catenin are low. B, When cells are stimulated by secreted WNT molecules, the destruction complex is deactivated, β-catenin degradation does not occur, and cytoplasmic levels increase. β-catenin translocates to the nucleus, where it binds to TCF, a transcription factor that activates several genes involved in the cell cycle. C, When APC is mutated or absent, the destruction of β-catenin cannot occur. β-Catenin translocates to the nucleus and coactivates genes that promote the cell cycle, and cells behave as if they are under constant stimulation by the WNT pathway.

http://www.mdconsult.com/images/blank.gif


An important function of the APC protein is to down-regulate β-catenin. In the absence of WNT-signaling APC causes degradation of β-catenin, preventing its accumulation in the cytoplasm. It does so by forming a macromolecular complex with β-catenin, which results in the degradation of β-catenin. Inactivation of the APC gene disrupts the complex and increases the cellular levels of β-catenin, which, in turn, translocates to the nucleus.[84] Thus, with loss of APC the cell behaves as if it is under continuous WNT-signaling and there is an excess of free β-catenin. In the cell nucleus, β-catenin forms a complex with TCF, a transcription factor that up-regulates cellular proliferation by increasing the transcription of c-MYC, CYCLIN D1, and other genes. The importance of the APC/β-catenin signaling pathway in tumorigenesis is attested to by the fact that colon tumors may have normal APC genes but have mutations in β-catenin. Mutated β-catenin is not inhibited by APC and migrates into the nucleus. Dysregulation of the APC/β-catenin pathway is not restricted to colon cancers; mutations in β-catenin are present in more than 50% of hepatoblastomas and in approximately 20% of hepatocellular carcinomas.[85] As mentioned in Chapter 3 , β-catenin binds to cytoplasmic E-cadherin, a cell-surface protein that maintains intercellular adhesiveness. The reduced adhesiveness of cancer cells may result from defects in the cadherin-catenin axis. The cell adhesiveness effects of β-catenin are independent of its role as a transcription factor.

Other Genes That Function as Tumor Suppressors.

There is little doubt that many more tumor suppressor genes remain to be discovered. Often, their location is suspected by the detection of consistent sites of chromosomal deletions or by analysis of LOH. Some of the tumor suppressor genes that are associated with well-defined clinical syndromes are briefly described below (see Table 7-9 ):

  

?   

The INK4a/ARF locus. Mutations of this locus have been found in about 20% of familial melanomas.[26] Among sporadic tumors, p16INK4a mutations are present in up to 50% of pancreatic adenocarcinomas and squamous cell carcinomas of the esophagus, and have also been detected in bladder, head, and neck tumors and in cholangiocarcinomas. Mutated alleles of p16INK4a present in these tumors have lost their capacity to block cyclin D-CDK4 activity and to prevent RB phosphorylation during the cell cycle. In some tumors, such as cervical cancer, p16INK4a is frequently inactivated by hypermethylation of the gene, without the presence of a mutation (see discussion of epigenetic changes).

  

?   

The TGF-β pathway. The gene encoding the type II TGF-β receptor is inactivated in 70% or more of colon cancers that develop in patients with HNPCC, in sporadic colon cancers with microsatellite instability (discussed in conjunction with DNA repair genes), and in gastric cancers that develop in HNPCC patients.[86] SMAD4, which encodes a component of the TGF-β growth-inhibitory signal transduction pathway, is inactivated in approximately 50% of pancreatic cancers, while mutations in SMAD2, another component of the pathway, are present in some colorectal tumors. Because of its association with pancreatic cancers, SMAD4 was originally designated DPC4, deleted in pancreatic cancer.[57][87]

  

?   

NF-1 gene. Individuals who inherit one mutant allele of the NF-1 gene develop numerous benign neurofibromas as a result of inactivation of the second copy of the gene.[88] This condition is called neurofibromatosis type 1 ( Chapter 5 ). Some of the neurofibromas later develop into neurofibrosarcomas. Children with neurofibromatosis type 1 also are at increased risk of developing gliomas of the optic nerve. Neurofibromin, the protein product of the NF-1 gene, regulates signal transduction through a RAS protein. Recall that RAS transmits growth-promoting signals and flips back and forth between GDP-binding (inactive) and GTP-binding (active) states. Neurofibromin is a member of a family of GTPase-activating proteins, which facilitate conversion of RAS from an active to an inactive state. With loss of NF-1 function, RAS is trapped in an active, signal-emitting state.

  

?   

NF-2 gene. Germ line mutations in the NF-2 gene predispose to the development of neurofibromatosis type 2.[89] As discussed in Chapter 5 , patients with NF-2 deficiency develop benign bilateral schwannomas of the acoustic nerve. In addition, somatic mutations affecting both alleles of NF-2 have also been found in sporadic meningiomas and ependymomas. The product of the NF-2 gene, called merlin, shows a great deal of homology with the red cell membrane cytoskeletal protein 4.1 ( Chapter 13 ), and is related to the ERM (ezrin, radixin, and moesin) family of membrane cytoskeleton-associated proteins. Merlin binds, on one hand, to actin and, on the other hand, to CD44, a transmembrane protein that is involved in cell-matrix interactions ( Chapter 3 ). Although the mechanism by which NF2 deficiency leads to carcinogenesis is not known, cells lacking merlin are not capable of establishing stable cell-to-cell junctions and are insensitive to normal growth arrest signals generated by cell-to-cell contact.

  

?   

VHL. Germ line mutations of the von Hippel Lindau (VHL) gene on chromosome 3p are associated with hereditary renal cell cancers, pheochromocytomas, hemangioblastomas of the central nervous system, retinal angiomas, and renal cysts.[71] Mutations of the VHL gene have also been noted in sporadic renal cell cancers ( Chapter 20 ). The VHL protein forms a complex that function as ubiquitin ligases. A main substrate for this activity is HIF-1 (hypoxia inducible transcription factor 1), which regulates several genes, including VEGF and PDGF. Lack of VHL activity prevents ubiquitination and degradation of HIF-1 and is associated with increased levels of angiogenic growth factors.

  

?   

PTEN. Phosphatase and tensin homologue, deleted on chromosome 10 (PTEN) gene, mapped on chromosome 10q23, is frequently deleted in many human cancers but at particularly high frequency in endometrial carcinomas and glioblastomas.[90] PTEN activity causes cell-cycle arrest and apoptosis as well as inhibition of cell motility. It has been proposed that PTEN blocks the cell cycle by increasing the transcription of the p27 Cip/Kip cell-cycle inhibitor and stabilizing the protein.[90][91] With loss of PTEN, therefore, cells are released into the cell cycle.

  

?   

WT-1. The WT-1 gene, located on chromosome 11p13, is associated with the development of Wilms tumor, a pediatric kidney cancer.[92] Both inherited and sporadic forms of Wilms tumor occur, and mutational inactivation of the WT-1 locus has been seen in both forms. The WT-1 protein is a transcriptional activator of genes involved in renal and gonadal differentiation. It regulates the mesenchymal to epithelial transition that occurs in kidney development. Although not precisely known, it is likely that the tumorigenic effect of WT-1 deficiency is intimately connected with the role of the gene in the differentiation of genitourinary tissues. Another Wilms gene, WT-2, located on 11p15, is associated with the Beckwith-Wiedeman syndrome ( Chapter 10 ).

  

?   

Cadherins. These are a family of glycoproteins that act as glues between epithelial cells ( Chapter 3 ). Loss of cadherins can favor the malignant phenotype by allowing easy disaggregation of cells, which can then invade locally or metastasize. Reduced cell-surface expression of E-cadherin has been noted in many types of cancers, including those that arise in the esophagus, colon, breast, ovary, and prostate.[93] Germ line mutations of the E-cadherin gene can predispose to familial gastric carcinoma, and mutation of the gene and decreased E-cadherin expression are present in a variable proportion of gastric cancers of the diffuse type. The molecular basis of reduced E-cadherin expression is varied. In a small proportion of cases, there are mutations in the E-cadherin gene (located on 16q); in other cancers, E-cadherin expression is reduced as a secondary effect of mutations in β-catenin genes. β-catenins, as discussed earlier, bind to the intracellular portion of cadherins and stabilize their expression.

  

?   

KLF6. KLF6 encodes a transcription factor that has many target genes, including TGF-β and TGF-β receptors. KLF6 is mutated in more than 70% of primary prostate cancers. It has been proposed that KLF6 inhibits cell proliferation by increasing the transcription of the Cip/Kip cell-cycle inhibitor p21, independent of p53. Mutation of the gene in tumor cells eliminates the cell-cycle-blocking activity of p21.[94]

  

?   

Patched (PTCH). PTCH is a tumor suppressor gene that encodes a cell-membrane protein (PATCHED), which functions as a receptor for a family of proteins called Hedgehog.[95] The Hedgehog/PATCHED pathway regulates several genes, including TGF-β and PDGF-R. Mutations in PTCH are responsible for Gorlin syndrome, an inherited condition also known as nevoid basal cell carcinoma syndrome (see Chapter 26 ). PTCH mutations are present in 20% to 50% of sporadic cases of basal cell carcinoma. About one half of such mutations are of the type caused by UV exposure.

EVASION OF APOPTOSIS

Just as cell growth is regulated by growth-promoting and growth-inhibiting genes, cell survival is conditioned by genes that promote and inhibit apoptosis. Therefore, the accumulation of neoplastic cells may occur not only by the activation of oncogenes or inactivation of tumor suppressor genes, but also by mutations in the genes that regulate apoptosis.[96][97][98] A large family of genes that regulate apoptosis has been identified in both normal and cancer cells. The main pathways of apoptosis were described in Chapter 1 . Here we discuss the role of BCL-2 in protecting tumor cells from apoptosis.

The discovery of BCL-2, the prototypic gene in this category, began with the observation that approximately 85% of B-cell lymphomas of the follicular type ( Chapter 14 ) carry a characteristic t(14;18)(q32;q21) translocation, in which the BCL-2 gene from 18q21 is translocated to the immunoglobulin heavy-chain locus on 14q32. (Recall that the immunoglobulin heavy-chain locus is also involved in translocation—of the MYC gene—in Burkitt lymphoma.) Removal of BCL-2 from its normal controls leads to increased transcription and overexpression of the BCL-2 protein. As mentioned above and discussed in Chapter 1 , BCL-2 protects cells from apoptosis by the mitochondrial pathway. Thus, there is a steady accumulation of B lymphocytes (the cells in which the translocation typically occurs as the immunoglobulin locus is open), resulting in lymphadenopathy and marrow infiltration. Because lymphomas that overexpress BCL-2 arise in large part from reduced cell death rather than explosive cell proliferation, they tend to be indolent (slow growing) compared with many other lymphomas. Supporting the role of BCL-2 in lymphomagenesis is the observation that mice transgenic for BCL-2 develop B-cell lymphomas. Not only is the function of BCL-2 unusual among cancer-associated genes, but its location in the outer mitochondrial membrane is also different from that of most such genes.[99][100]

At least two other cancer-associated genes are also intimately connected with apoptosis: p53 and MYC. The molecular mechanisms of cell death induced by these two intersect with the BCL-2 pathways. As discussed, p53 increases the transcription of pro-apoptotic genes such as BAX. Lack of p53 activity, caused by mutations in p53 or alterations in INK4a and MDM2, decreases transcription of the pro-apoptotic gene BAX, reduces apoptotic activity, and reduces the response to chemotherapy. Studies in mice show that BAX expression is required for the p53-induced apoptotic response. BID, another pro-apoptotic member of the BCL-2 family, is also regulated by p53 and might enhance cell death in response to chemotherapy.[101] MYC and BCL-2 may collaborate in tumorigenesis: MYC triggers proliferation, and BCL-2 prevents cell death, even if growth factors become limiting. This is one of many examples in which two or more genes cooperate in giving rise to cancer. It should also be noted that normal cells require continuous survival signals as, for instance, signaling through the PI-3 kinase/AKT pathway, which prevents the activity of the apoptotic machinery. Lack of these signals can cause apoptosis, a condition known as "death by neglect."[98] AKT expression in cancer cells is often increased as a consequence of mutations in AKT or inactivating mutations in the PTEN tumor suppressor gene. These alterations increase the resistance of the cancer cell to apoptotic cell death.[102]

DNA REPAIR DEFECTS AND GENOMIC INSTABILITY IN CANCER CELLS

Humans literally swim in a sea of environmental carcinogens. Although exposure to naturally occurring DNA-damaging agents, such as ionizing radiation, sunlight, dietary carcinogens, and ROS generated by cell metabolism and oxidative stress, is common, cancer is a relatively rare outcome of such encounters. This fortunate state of affairs results from the ability of normal cells to repair DNA damage and thus prevent mutations in genes that regulate cell growth and apoptosis.[103] In addition to possible DNA damage from environmental agents, the DNA of normal dividing cells is susceptible to alterations resulting from errors that occur spontaneously during DNA replication. Such mistakes, if not repaired promptly, can also push the cells along the slippery slope of neoplastic transformation. The importance of DNA repair in maintaining the integrity of the genome is highlighted by several inherited disorders in which genes that encode proteins involved in DNA repair are defective. Those born with such inherited mutations of DNA repair proteins are at a greatly increased risk of developing cancer. These conditions are known as genomic instability syndromes. Moreover, defects in repair mechanisms are present in sporadic human cancers. DNA repair genes themselves are not oncogenic, but their abnormalities allow mutations in other genes during the process of normal cell division. Typically, genomic instability occurs when both copies of these genes are lost. Thus, in this respect they resemble tumor suppressor genes. Defects in three types of DNA repair systems, namely, mismatch repair, nucleotide excision repair, and recombination repair, are presented next.

Hereditary Nonpolyposis Cancer Syndrome.

The role of DNA mismatch repair genes in predisposition to cancer is illustrated dramatically by the HNPCC syndrome.[104] This disorder is characterized by familial carcinomas of the colon affecting predominantly the cecum and proximal colon ( Chapter 17 ). Although mismatch errors in DNA replication can occur in any dividing cell, carcinomas occur mainly in the proximal colon in those with HNPCC. In some families, there is also an associated increase in endometrial and ovarian cancers, but mysteriously, most other tissues are spared. In contrast to the carcinomas in patients with germ line APC mutations discussed earlier, the cancers in the HNPCC syndrome do not arise in adenomatous polyps.

When a strand of DNA is replicating, mismatch repair genes act as "spell checkers." Thus, for example, if there is an erroneous pairing of G with T, rather than the normal A with T, the mismatch repair proteins correct the defect. Without these proofreaders, errors slowly accumulate in several genes, including protooncogenes and tumor suppressor genes. Cells with such defects in DNA repair are said to have the replication error phenotype, which can be readily documented by examination of microsatellite sequences in the tumor cell DNA.[105] Microsatellites are tandem repeats of one to six nucleotides scattered throughout the genome ( Chapter 5 ). Microsatellite sequences of an individual are fixed for life and are the same in every tissue. With errors in mismatch repair, there are expansions and contractions of these repeats in tumor cells, creating alleles not found in normal cells of the same patient. Such microsatellite instability is a hallmark of defective mismatch repair.[24] Of the various DNA mismatch repair genes, at least four are involved in the pathogenesis of HNPCC, but germ line mutations in the MSH2 (2p16) and MLH1 (3p21) genes each account for approximately 30% of cases. The remaining cases have mutations in PMS, PMS2, and other mismatch repair genes. Each affected individual inherits one defective copy of one of the several DNA mismatch repair genes and acquires the "second hit" in the colonic epithelial cells. Thus, DNA repair genes behave similarly to tumor suppressor genes in their mode of inheritance, but, in contrast to the classic tumor suppressor genes, they do not affect cell growth directly. Because mutations occur more readily and more rapidly in patients with HNPCC, the evolution of tumors occurs more rapidly, and hence patients develop colon cancers at a much younger age (<50 years) than those who do not have any defects in DNA repair.

Although HNPCC accounts for only 2% to 4% of all colonic cancers, microsatellite instability can be detected in about 15% of sporadic colon cancers. The growth-regulating genes that are mutated in patients with HNPCC have not yet been completely characterized but include the genes encoding TGF-β receptor II, the TCF component of the β-catenin pathway, BAX, and other oncogenes and tumor suppressor genes.[106] As discussed earlier, loss of TGF-β receptors nullifies the growth-inhibiting action of TGF-β, and mutations in the BAX gene dysregulate apoptosis.

Xeroderma Pigmentosum.

Patients with xeroderma pigmentosum, another inherited disorder involving defective DNA repair genes, are at increased risk for the development of cancers of the skin when exposed to the UV rays contained in sunlight.[107] UV light causes cross-linking of pyrimidine residues, thus preventing normal DNA replication. Such DNA damage is repaired by the nucleotide excision repair (NER) pathway, which is discussed later in conjunction with the induction of carcinogenesis by UV rays. Several proteins and genes are involved in NER, and an inherited loss of any one can give rise to xeroderma pigmentosum.

Inherited Diseases with Defects in DNA Repair by Homologous Recombination.

A group of autosomal recessive disorders, including ataxia-telangiectasia, Bloom syndrome, and Fanconi anemia, are characterized by hypersensitivity to other DNA-damaging agents, such as ionizing radiation (ataxia-telangiectasia and Bloom syndrome), or DNA cross-linking agents (Fanconi anemia). These syndromes include, in addition to predisposition to cancer, other features such as neural symptoms (ataxia-telangiectasia), anemia (Fanconi anemia), and developmental defects (Bloom syndrome).[108] Patients with Bloom syndrome have a predisposition to a very broad spectrum of tumors. The defective gene is located on chromosome 15 and encodes a helicase (BLM helicase), which participates in DNA repair by homologous recombination.[109]

Patients with ataxia-telangiectasia have a complex phenotype, characterized by gradual loss of Purkinje cells in the cerebellum that causes cerebellar ataxia, defective lymphocyte maturation and proliferation. These defects lead to immunodeficiency, acute sensitivity to ionizing radiation, and profound susceptibility to lymphoid malignancies. The disease is caused by mutation of the ATM gene, resulting in absence or almost complete loss of function of the protein. ATM encodes a protein kinase that senses DNA double-strand breaks, a type of damage caused by ionizing radiation and oxygen free radicals.[110] After such damage, the kinase activity of ATM is rapidly increased. ATM phosphorylates p53, leading to cell-cycle arrest in G1 or apoptosis. In cells lacking the normal ATM genes, the p53-induced delay in the cell cycle does not occur, and hence the DNA-damaged cells continue to proliferate and are prone to transformation. There is much current interest in the ATM gene because it is estimated that approximately 1% of the population is heterozygous for this gene, and hence carriers. Although heterozygotes do not develop cancers, they are presumed to be at increased risk for radiation-induced DNA damage. It is therefore speculated that they may be at risk of developing cancers after exposure to doses of irradiation used in common radiologic procedures such as mammography.

BRCA-1 and BRCA-2 Genes.

BRCA-1, on chromosome 17q21, and BRCA-2, on chromosome 13q12-13, are two genes associated with the occurrence of breast and several other cancers[12]( Chapter 24 ). As with tumor suppressor genes, individuals who inherit mutations of BRCA-1 or BRCA-2 are highly susceptible to the development of breast cancer. With germ line mutations of the BRCA-1 gene, there is, in addition, a substantially higher risk of epithelial ovarian cancers and a slightly increased risk of prostate and colon cancers. Likewise, mutations in the BRCA-2 gene increase the risk of developing cancers of the ovary (although less than for BRCA-1) and of the male breast as well as other cancers such as melanomas and pancreatic tumors. Approximately 10% to 20% of breast cancers are familial; mutations in BRCA-1 and BRCA-2 account for 80% of the familial cases in families with multiple affected members but are present in less than 3% of all breast cancers.[25] Thus, in contrast to other tumor suppressor genes (RB, p53, NF-1, VHL) that are associated with heritable cancer syndromes, mutations in neither of the two BRCA genes are associated with the development of nonfamilial (sporadic) forms of breast cancer. Nevertheless, it is possible that alterations in the expression of genes related to BRCA-1 and BRCA-2 may be involved in non-hereditary breast cancer.[109a]

The functions of BRCA-1 and BRCA-2 are not completely defined. Protein products of both genes are localized to the nucleus and are believed to be involved in transcription regulation. BRCA-1 is involved in the regulation of estrogen receptor activity and is also a co-activator of the androgen receptor.[22] Both BRCA-1 and BRCA-2 participate in the process of homologous recombination of DNA repair; they bind to RAD51, a gene involved in the repair of double-strand DNA breaks, and are also involved in chromatin remodeling (see below and Chapter 23 ). BRCA-1 and BRCA-2 have a close connection with components of the G1/S checkpoint that delays the cell cycle to allow for repair of DNA damage. Given the many activities of BRCA-1 and BRCA-2, there are multiple ways in which defects in these genes can increase the risk of development of breast and ovarian tumors, but a precise explanation of their role is not yet available.

There is now much evidence that cancer susceptibility genes, including those involved in breast cancer, are linked to a network of genes that participate in the repair of double-strand DNA breaks by homologous recombination[111][112] ( Fig. 7-39 ). ATM and CHEK2 (a protein kinase activated by DNA damage) phosphorylate BRCA1 and RAD51, which colocalize at sites of DNA damage.[113] A Fanconi anemia complex of proteins also colocalizes at damage sites, and BRCA2 was recently identified as the FANCD1 gene, one of several genes involved in Fanconi anemia.[114] Thus BRCA genes may be mutated in Fanconi anemia and breast cancers, entirely different diseases that may have in common the genetic instability produced by deficiencies in homologous recombination DNA repair genes.[112] Another example of the linkages that exist between cancer susceptibility genes and DNA repair genes is the recent demonstration that ATM mutations are present at high frequency in individuals with a familial pattern of breast and ovarian tumors.[115]

Figure 7-39  Interaction between cancer susceptibility genes and DNA repair. ATM (ataxia-telangiectasia mutated) senses a double-strand break in DNA, induced by agents such as ionizing radiation. ATM and CHEK2 phosphorylate BRCA1, promoting its migration to the break site. The Fanconi's anemia protein complex (proteins A, C, E, F, G) triggers the ubiquitination and colocalization of the Fanconi protein D2 with BRCA1 at the break site. BRCA2 carries RAD51, an enzyme involved in DNA recombination repair, to the same site. BRCA1, BRCA2, and RAD51 repair the DNA break by an error-free recombination mechanism. RAD51 is a component of cell cycle check points.  (Redrawn from Venkitaraman AR: A growing network of cancer-susceptibility genes. N Engl J Med 348:1917, 2003.)

http://www.mdconsult.com/images/blank.gif



LIMITLESS REPLICATIVE POTENTIAL: TELOMERASE

In the discussion of cellular aging ( Chapter 1 ), it was pointed out that after a fixed number of divisions, normal cells become arrested in a terminally nondividing state known as replicative senescence. How normal cells can "count" their divisions is not known, but it has been noted that with each cell division there is some shortening of specialized structures, called telomeres, at the ends of chromosomes.[116] Once the telomeres are shortened beyond a certain point, the loss of telomere function leads to activation of p53-dependent cell-cycle checkpoints, causing proliferative arrest or apoptosis ( Fig. 7-40 ). Thus, telomere shortening functions as a clock that counts cell divisions. In germ cells, telomere shortening is prevented by the sustained function of the enzyme telomerase, thus explaining the ability of these cells to self-replicate extensively. This enzyme is absent from most somatic cells, and hence they suffer progressive loss of telomeres. Introduction of telomerase into normal human cells causes considerable extension of their life span,[117] thus supporting the hypothesis that telomerase loss is causally associated with loss of replication ability. If loss of telomerase is the basis of the finite life span of cells, how do cancer cells continue to divide indefinitely? Cancer cells must find a way to prevent telomere shortening, and a mechanism that accomplishes this is the reactivation of telomerase activity. Indeed, telomerase activity has been detected in more than 90% of human tumors.[118] Telomerase may also act to promote tumorigenesis by mechanisms that do not depend on telomere length.[119] Thus, telomerase activity and maintenance of telomere length are essential for the maintenance of replicative potential in cancer cells. As mentioned above, in normal cells, short telomeres activate cell-cycle checkpoints that lead to cell-cycle arrest or apoptosis. However, transformed cells may have defects in cell-cycle checkpoints, allowing for critical telomere shortening in dividing cells. These cells may die by apoptosis or survive with chromosome defects that cause genomic instability (see Fig. 7-40 ). Reactivation of telomerase in cells with abnormal genomes confers an unlimited proliferative capacity to cells that have tumorigenic potential.

Figure 7-40  Cellular responses to telomere shortening. The figures show the responses of normal cells, which have intact cell-cycle checkpoints and of cells with checkpoint defects.  (From Wong JMY, Collins K: Telomere maintenance and disease. Lancet 362:983, 2003.)

http://www.mdconsult.com/images/blank.gif



DEVELOPMENT OF SUSTAINED ANGIOGENESIS

Tumors stimulate the growth of host blood vessels, a process called angiogenesis, which is essential for supplying nutrients to the tumor. Even with genetic abnormalities that dysregulate growth and survival of individual cells, tumors cannot enlarge beyond 1 to 2 mm in diameter or thickness unless they are vascularized.[120] Presumably the 1- to 2-mm zone represents the maximal distance across which oxygen and nutrients can diffuse from blood vessels. Beyond this size, the tumor fails to enlarge without vascularization because of hypoxia-induced cell death. Neovascularization has a dual effect on tumor growth: perfusion supplies nutrients and oxygen, and newly formed endothelial cells stimulate the growth of adjacent tumor cells by secreting polypeptide growth factors such as insulin-like growth factors and PDGF.[121] Angiogenesis is a requisite not only for continued tumor growth, but also for metastasis. Without access to the vasculature, the tumor cells cannot readily spread to distant sites.

How do growing tumors develop a blood supply? Several studies indicate that tumors produce factors that are capable of triggering the entire series of events involved in the formation of new capillaries ( Chapter 3 ). Tumor angiogenesis can occur by recruitment of endothelial cell precursors or by sprouting of existing capillaries, as in physiologic angiogenesis. However, tumor blood vessels differ from the normal vasculature by being tortuous and irregularly shaped ( Fig. 7-41 ) and by being leaky. The leakiness is attributed largely to the increased production of VEGF.[122] In contrast to normal mature vessels, which are quiescent structures, tumor vessels may grow continuously. Tumor cells may, in some special cases, line structures that resemble capillaries, a phenomenon called vasculogenic mimicry.[123]

Click to view full size figure

Figure 7-41  Tumor angiogenesis. Compared to normal blood vessels (left panels), tumor vessels are tortuous and irregularly shaped. The tumor vasculature (upper right) is formed from circulating endothelial precursor cells and existing host vessels ( Chapter 3 ); myofibroblasts give rise to pericytes cells at the periphery of the vessels. By contrast to the stable vessel network of normal tissue, the networks formed by tumor vessels are unstable and leaky. Arterioles, capillaries, and veins are clearly distinguishable in the normal vasculature (lower left); in the tumor the vessels are disorganized and not identifiable (lower right).  (Redrawn from Jain RK: Molecular regulation of vessel maturation. Nature Med 9:685, 2003; and McDonald DM, Choyke PL: Imaging of angiogenesis from microscope to clinic. Nature Med 9:713, 2003.)

http://www.mdconsult.com/images/blank.gif



Tumor-associated angiogenic factors are produced by tumor cells or may be derived from inflammatory cells (e.g., macrophages) that infiltrate tumors. Of the dozen or so known tumor-associated angiogenic factors, the two most important are VEGF[124] and basic fibroblast growth factor (bFGF). VEGF is mostly produced by tumor cells but may also be made by cells of the tumor stroma. The mechanisms whereby bFGF and VEGF cause angiogenesis were discussed in Chapter 3 . These two factors are commonly expressed in a wide variety of tumor cells, and elevated levels can be detected in the serum and urine of a significant fraction of cancer patients.

Experimental and clinical data indicate that early in their growth most human tumors do not induce angiogenesis. They exist in situ without developing a blood supply for months to years; then, some cells within the small tumor change to an angiogenic phenotype. This change is known as the angiogenic switch.[125] The molecular basis of the angiogenic switch is not entirely clear but may involve increased production of angiogenic factors or loss of angiogenesis inhibitors. In some cases, wild-type p53 inhibits angiogenesis by inducing the synthesis of the antiangiogenic molecule thrombospondin-1 and down-regulating the production of angiogenic factors such as VEGF and HIF-1, the hypoxia-inducible factor that stimulates VEGF transcription. With mutational inactivation of both p53 alleles (common in many cancers), the levels of thrombospondin-1 drop precipitously, VEGF levels increase, and HIF-1 production is enhanced by tumor hypoxia, thus tilting the balance in favor of angiogenic factors.

Tumor cells not only produce angiogenic factors, but also induce anti-angiogenesis molecules. Tumor growth is thus controlled by the balance between angiogenic factors and those that inhibit angiogenesis.[126] Some anti-angiogenesis factors, such as thrombospondin-1, may be produced by the tumor cells themselves, whereas others, such as angiostatin, endostatin, and tumstatin, are produced in response to the tumor. These latter three potent angiogenesis inhibitors are derived by proteolytic cleavage of plasminogen (angiostatin) and of collagens (endostatin, tumstatin).

Because angiogenesis is critical for the growth and spread of tumors, much attention is focused on the use of angiogenesis inhibitors as adjuncts to other forms of therapy. Success has been achieved in treating several tumors in mice by administration of endostatin[120][126] and tumstatin. Endostatin is being tested for its effects on human tumors. Trials are also being conducted to test the antitumor effects of antibodies to VEGF and VEGF-R2, and of small molecules that inhibit signal transduction through VEGF-R2.[124][125]

INVASION AND METASTASIS

Invasion and metastasis are biologic hallmarks of malignant tumors. They are the major cause of cancer-related morbidity and mortality and hence are the subjects of intense scrutiny. For tumor cells to break loose from a primary mass, enter blood vessels or lymphatics, and produce a secondary growth at a distant site, they must go through a series of steps (summarized in Fig. 7-42 ). Each step in this sequence is subject to a multitude of influences; hence, at any point in the sequence the breakaway cell may not survive.[127]

Figure 7-42  The metastatic cascade. Schematic illustration of the sequential steps involved in the hematogenous spread of a tumor.

http://www.mdconsult.com/images/blank.gif


Studies in mice reveal that although millions of cells are released into the circulation each day from a primary tumor, only a few metastases are produced. What then is the basis of the apparent inefficiency of this process? One prevalent view ( Fig. 7-43A ) is that certain tumor cell subclones possess the right combination of gene products to complete all the steps involved in metastasis. An alternative hypothesis is that metastasis is the result of multiple abnormalities that occur in many, perhaps most, cells of a primary tumor ( Fig. 7-43B and C ). Such abnormalities give the tumor a general predisposition for metastasis, which has been called a "metastasis signature."[128][129] This signature may involve not only properties intrinsic to the cancer cells, but also the characteristics of the stroma, such as the components of the stroma, the presence of infiltrating immune cells, and angiogenesis ( Fig. 7-43D ). A clear understanding of the origin of metastasis is of major importance for the management of cancer patients and the development of effective therapies to prevent tumor spread. For the purpose of this discussion, the metastatic cascade will be divided into two phases: (1) invasion of the extracellular matrix and (2) vascular dissemination and homing of tumor cells.

Figure 7-43  Mechanisms of metastasis development within a primary tumor. A nonmetastatic primary tumor is shown (light blue) on the left side of all diagrams. Four models are presented: A, Metastasis is caused by rare variant clones that develop in the primary tumor; B, Metastasis is caused by the gene expression pattern of most cells of the primary tumor, referred to as a metastatic signature; C, A combination of A and B, in which metastatic variants appear in a tumor with a metastatic gene signature; D, Metastasis development is greatly influenced by the tumor stroma, which may regulate angiogenesis, local invasiveness and resistance to immune elimination, allowing cells of the primary tumor, as in C, to become metastatic.

http://www.mdconsult.com/images/blank.gif


Invasion of Extracellular Matrix

The structural organization and function of normal tissues is to a great extent determined by interactions between cells and the extracellular matrix (ECM).[130] As we discussed in Chapter 3 , tissues are organized into compartments separated from each other by two types of ECM: basement membrane and interstitial connective tissue. Although organized differently, each of these components of ECM is made up of collagens, glycoproteins, and proteoglycans. As shown in Figure 7-42 , tumor cells must interact with the ECM at several stages in the metastatic cascade. A carcinoma must first breach the underlying basement membrane, then traverse the interstitial connective tissue, and ultimately gain access to the circulation by penetrating the vascular basement membrane. This cycle is repeated when tumor cell emboli extravasate at a distant site. Invasion ofthe ECM is an active process that can be resolved into several steps ( Fig. 7-44 ):

  

?   

Detachment ("loosening up") of the tumor cells from each other

  

?   

Attachment to matrix components

  

?   

Degradation of ECM

  

?   

Migration of tumor cells

 

Figure 7-44  A–D, Schematic illustration of the sequence of events in the invasion of epithelial basement membranes by tumor cells. Tumor cells detach from each other because of reduced adhesiveness, and cells then attach to the basement membrane via the laminin receptors and secrete proteolytic enzymes, including type IV collagenase and plasminogen activator. Degradation of the basement membrane and tumor cell migration follow.

http://www.mdconsult.com/images/blank.gif


Normal cells are neatly glued to each other and their surroundings by a variety of adhesion molecules.[131] Of these, the cadherin family of transmembrane glycoproteins is of particular importance. E-cadherins mediate homotypic adhesions in epithelial tissue, thus serving to keep the epithelial cells together and to relay signals between the cells. In several epithelial tumors, including adenocarcinomas of the colon and breast, there is a down-regulation of E-cadherin expression. Presumably, this down-regulation reduces the ability of cells to adhere to each other and facilitates their detachment from the primary tumor and their advance into the surrounding tissues. E-cadherins are linked to the cytoskeleton by the catenins, proteins that lie under the plasma membrane ( Fig. 7-38 ). The normal function of E-cadherin is dependent on its linkage to catenins. In some tumors, E-cadherin is normal, but its expression is reduced because of mutations in the gene for a catenin.

To penetrate the surrounding ECM, the tumor cells must first adhere to the matrix components. Epithelial cells of a tumor are separated from the stroma by a basement membrane. Thus, for tumor cells to penetrate the basement membrane, the membrane must be degraded and remodeled.[124] As this process occurs, components of the basement membrane send both positive and negative growth signals to tumor cells and have a major role in regulating angiogenesis. There is substantial evidence that receptor-mediated attachment of tumor cells to laminin and fibronectin is important for invasion and metastasis. Normal epithelial cells express high-affinity receptors (typically members of the integrin and immunoglobulin families of proteins) for basement membrane laminin that are polarized to their basal surface. In contrast, some carcinoma cells have many more receptors, and they are distributed all around the cell membrane. Moreover, there seems to be a correlation between the density of laminin receptors and invasiveness in cancers of the breast and colon. Tumor cells, like normal cells, also express integrins that serve as receptors for many components of the ECM, including fibronectin, laminin, collagen, and vitronectin. Neoplastic epithelial cells may express a higher amount of integrins and produce integrins that are not present in the corresponding normal tissue. As with laminin receptors, there seems to be a correlation between the expression of certain integrins (e.g., α4β1 integrin on melanoma cells) and their ability to metastasize.

After attachment to the components of the basement membrane or interstitial ECM, tumor cells must create passageways for migration ( Fig. 7-44 ). Invasion of the ECM is not merely due to passive growth pressure but requires active enzymatic degradation of the ECM components.[131] Tumor cells secrete proteolytic enzymes themselves or induce host cells (e.g., stromal fibroblasts and infiltrating macrophages) to elaborate proteases. The activity of these proteases is tightly regulated by antiproteases. At the invading edge of tumors, the balance between proteases and antiproteases is titled in favor of proteases. Three classes of proteases have been identified: the serine, cysteine, and matrix metalloproteinases (MMPs). MMP9 and MMP2 are collagenases that cleave type IV collagen of epithelial and vascular basement membranes. There is compelling evidence[132] supporting the role of MMPs that degrade type IV collagen in tumor cell invasion:

  

?   

Several invasive carcinomas, melanomas, and sarcomas produce high levels of these collagenases.

  

?   

In situ lesions and adenomas of breast and colon express much less collagen IV-degrading collagenases than do invasive lesions. MMP expression is higher as tumors enlarge.

  

?   

Inhibition of collagenase activity by transfection with the gene for tissue inhibitors of metalloproteinases greatly reduces metastases in experimental animals. Thus, metalloproteinase inhibitors could be of value in the treatment of cancer. Synthetic compounds with such activity are being tested as therapeutic agents in certain forms of cancer.

While the most obvious effect of matrix destruction is to create a path for invasion by tumor cells, cleavage products of matrix components, derived from collagen and proteoglycans, also have growth-promoting, angiogenic, and chemotactic activities. The latter may promote the migration of tumor cells into the loosened ECM. MMPs are produced mostly by cells located in the tumor stroma, including cells of the immune system that migrate into this area. MMP9, and to a lesser extent MMP2, degrades collagen type IV and mobilizes VEGF that is sequestered in the basement membrane. Degradation of collagen IV also exposes normally cryptic domains of the protein, which serve as important signals for angiogenesis and cell interactions. Interestingly, collagen IV degradation in the basement membrane not only produces angiogenic stimuli, but also generates collagen fragments, such as endostatin and tumstatin, which are antiangiogenic. Thus, an important role of MMPs is to generate from the ECM, factors that promote angiogenesis, tumor growth, and tumor cell motility.[126] These factors counteract substances that inhibit angiogenesis, which are also produced by the partial digestion of basement membrane components.

Vascular Dissemination and Homing of Tumor Cells

Once in the circulation, tumor cells are particularly vulnerable to destruction by innate and adaptive immune defenses. The details of tumor immunity are considered later.

Within the circulation, tumor cells tend to aggregate in clumps. This is favored by homotypic adhesions among tumor cells as well as heterotypic adhesion between tumor cells and blood cells, particularly platelets ( Fig. 7-42 ). Formation of platelet-tumor aggregates may enhance tumor cell survival and implantability. Arrest and extravasation of tumor emboli at distant sites involve adhesion to the endothelium, followed by egress through the basement membrane. Involved in these processes are adhesion molecules (integrins, laminin receptors) and proteolytic enzymes, discussed earlier. Of particular interest is the CD44 adhesion molecule, which is expressed on normal T lymphocytes and is used by these cells to migrate to selective sites in the lymphoid tissue. Such migration is accomplished by the binding of CD44 to hyaluronate on high endothelial venules, and overexpression of this molecule may favor metastatic spread. At the new site, tumor cells need to proliferate, develop a vascular supply, and evade the host defenses.[127]

The site at which circulating tumor cells leave the capillaries to form secondary deposits is related, in part, to the anatomic location of the primary tumor. Many observations, however, suggest that natural pathways of drainage do not wholly explain the distribution of metastases. For example, prostatic carcinoma preferentially spreads to bone, bronchogenic carcinomas tend to involve the adrenals and the brain, and neuroblastomas spread to the liver and bones. Such organ tropism may be related to the following mechanisms:

  

?   

Because the first step in extravasation is adhesion to the endothelium, tumor cells may have adhesion molecules whose ligands are expressed preferentially on the endothelial cells of the target organ. Indeed, it has been shown that the endothelial cells of the vascular beds of various tissues differ in their expression of ligands for adhesion molecules.[133]

  

?   

Chemokines have a very important role in determining the target tissues for metastasis. For instance, some breast cancer cells express the chemokine receptors CXCR4 and CCR7.[134] The chemokines that bind to these receptors are highly expressed in tissues to which breast cancers commonly metastasize. Blockage of the interaction between CXCR4 and its receptor decreases breast cancer metastasis to lymph nodes and lungs. Some target organs may liberate chemoattractants that tend to recruit tumor cells to the site. Examples include insulin-like growth factors I and II.

  

?   

In some cases, the target tissue may be an unpermissive environment—unfavorable soil, so to speak, for the growth of tumor seedlings. For example, although well vascularized, skeletal muscles are rarely the site of metastases.

While primary tumors at various sites have a preferential type of metastatic spread, the precise localization of metastases cannot be predicted with certainty for any form of cancer.

Molecular Genetics of Metastasis Development

Are there oncogenes or tumor suppressor genes that elicit metastases as their principal or sole contribution to tumorigenesis? This question is of more than academic interest because if altered forms of certain genes promote or suppress the metastatic phenotype, their detection in a primary tumor may have prognostic as well as therapeutic implications. Comparisons between genetic profiles of metastatic and non-metastatic tumors have been used to search for candidate metastasis suppressor genes. At present, no single "metastasis gene" has been identified, with the exception of the membrane-cytoskeleton component ezrin, which appears to be necessary for metastases in rhabdomyoscarcoma and osteosarcoma.[135] Several genes have been proposed as suppressors of metastasis. They include NM23 and the KAI-1 and KiSS genes.[136] Much remains to be known.

STROMAL MICROENVIRONMENT AND CARCINOGENESIS

In the preceding sections, several examples of cross talk between the extracellular matrix and tumor cells were described. For example, cleavage of matrix components such as type IV collagen releases angiogenic factors (VEGF), and enzymatic degradation of laminin-5 by MMP-2 generates a proteolytic fragment that favors cancer cell motility. The ECM also stores growth factors in inactive forms, which are released by active matrix proteases. Such factors include PDGF, TGFβ, and b-FGF, which in turn affect the growth of tumor cells in a paracrine manner. In addition to these well-established interactions of tumor cells with stroma, there is emerging, tantalizing, evidence that stromal cells within the extracellular matrix can transmit oncogenic signals to tumor cells.[136a] This has been documented most extensively in experimental models of prostate and breast cancers. In prostate cancer, smooth muscle cells that normally lie adjacent to benign prostatic epithelium transform themselves into so called "carcinoma-associated fibroblasts," perhaps under the inductive influence of the tumor cells. These stromal cells acquire several altered properties such as enhanced collagen production and hyaluronate synthesis. But more interestingly, when isolated carcinoma-associated fibroblasts were recombined with immortalized, but non-tumorigenic, human prostate epithelial cells, the latter gave rise to poorly differentiated carcinomas in athymic mice. These carcinomas had multiple genetic abnormalities not present in the parent cell line, suggesting that the stroma can drive genetic changes that promote carcinogenesis. How such changes come about remains mysterious, as does their relevance to carcinogenesis in vivo. However, the results are sufficiently intriguing to merit attention since they suggest a novel form of cancer therapy that could be targeted to stromal cells.

DYSREGULATION OF CANCER-ASSOCIATED GENES

The genetic damage that activates oncogenes or inactivates tumor suppressor genes may be subtle (e.g., point mutations) or may involve segments of chromosomes and be large enough to be detected in a karyotype. Activation of oncogenes and loss of function of tumor suppressor genes by mutations were discussed earlier in this chapter. Here we discuss chromosomal abnormalities. We end this section by discussing the epigenic changes in cancer cells and global patterns of gene expression by cancer cells, known as genetic profile or "signature."

Chromosomal Changes

In certain neoplasms, karyotypic abnormalities are non-random and common. Specific chromosomal abnormalities have been identified in most leukemias and lymphomas and in an increasing number of nonhematopoietic tumors. In addition, whole chromosomes may be gained or lost. Although changes in chromosome number (aneuploidy) and structure are generally considered to be late phenomena in cancer progression, it has been suggested that aneuploidy and chromosomal instability may be initiating events in tumor growth.

The study of chromosomal changes in tumor cells is important on two accounts. First, molecular cloning of genes in the vicinity of chromosomal breakpoints or deletions has been extremely useful in identification of oncogenes (e.g., BCL-2, ABL) and tumor suppressor genes (e.g., APC, RB). Second, certain karyotypic abnormalities are specific enough to be of diagnostic value, and in some cases they are predictive of clinical course. The translocations associated with the ABL oncogene in chronic myeloid leukemia and with c-MYC in Burkitt lymphoma have been mentioned earlier, in conjunction with the discussion of molecular defects in cancer cells (see Fig. 7-32 ). Several other karyotype alterations in cancer cells are presented in the discussion of specific forms of neoplasia.

Two types of chromosomal rearrangements can activate protooncogenes—translocations and inversions. Chromosomal translocations are much more common ( Table 7-10 ) and are discussed here. Translocations can activate protooncogenes in two ways:

  

?   

In lymphoid tumors, specific translocations result in overexpression of protooncogenes by removing them from their regulatory elements.

  

?   

In many hematopoietic tumors, the translocations allow normally unrelated sequences from two different chromosomes to recombine and form hybrid genes that encode growth-promoting chimeric proteins.


Table 7-10   -- Selected Examples of Oncogenes Activated by Translocation

Malignancy

Translocation

Affected Genes

Chronic myeloid leukemia

(9;22)(q34;q11)

Ab1 9q34

 

 

bcr 22q11

Acute leukemias (AML and ALL)

(4;11)(q21;q23)

AF4 4q21

 

 

MLL 11q23

 

(6;11)(q27;q23)

AF6 6q27

 

 

MLL 11q23

Burkitt lymphoma

(8;14)(q24;q32)

c-myc 8q24

 

 

IgH 14q32

Mantle cell lymphoma

(11;14)(q13;q32)

Cyclin D 11q13

 

 

IgH 14q32

Follicular lymphoma

(14;18)(q32;q21)

IgH 14q32

 

 

bcl-2 18q21

T-cell acute lymphoblastic leukemia

(8;14)(q24;q11)

c-myc 8q24

 

 

TCR-α 14q11

 

(10;14)(q24;q11)

Hox 11 10q24

 

 

TCR-α 14q11

Ewing sarcoma

(11;22)(q24;q12)

Fl-1 11q24

 

 

EWS 22q12

Underlined genes are involved in multiple translocations.

AML, acute myeloid leukemia; ALL, acute lymphoblastic leukemia.

 

 

 

Overexpression of a protooncogene caused by translocation is best exemplified by Burkitt lymphoma. All such tumors carry one of three translocations, each involving chromosome 8q24, where the MYC gene has been mapped, as well as one of the three immunoglobulin gene-carrying chromosomes. At its normal locus, the expression of the MYC gene is tightly controlled; it is expressed only during certain stages of the cell cycle. In Burkitt lymphoma, the most common form of translocation results in the movement of the MYC-containing segment of chromosome 8 to chromosome 14q band 32 ( Fig. 7-33 ), placing it close to the immunoglobulin heavy-chain (IgH) gene. The genetic notation for the translocation is t(8:14)(q24;q32). The molecular mechanisms of the translocation-associated activation of MYC are variable, as are the precise breakpoints within the gene. In most cases, the translocation causes mutations or loss of the regulatory sequences of the MYC gene. As the coding sequences remain intact, the gene is constitutively expressed at high levels. The gene may be translocated to the antigen receptor loci simply because these loci are accessible (i.e. in "open" chromatin) and active in developing lymphocytes. The invariable presence of the translocated MYC gene in Burkitt lymphomas attests to the importance of MYC overexpression in the pathogenesis of this tumor.

There are other examples of oncogenes translocated to antigen receptor loci in lymphoid tumors. As mentioned earlier, in mantle cell lymphoma, the CYCLIN D1 gene on chromosome 11q13 is overexpressed by juxtaposition to the IgH locus on 14q32. In follicular lymphomas, a t(14;18)(q32;q21) translocation, the most common translocation in lymphoid malignancies, causes activation of the BCL-2 gene. Not unexpectedly, all these tumors in which the immunoglobulin gene is involved are of B-cell origin. In an analogous situation, overexpression of several protooncogenes in T-cell tumors results from translocations of oncogenes into the T-cell antigen receptor locus. The affected oncogenes are diverse, but in most cases, as with MYC, they encode nuclear transcription factors.

The Philadelphia chromosome, characteristic of chronic myeloid leukemia and a subset of acute lymphoblastic leukemias, provides the prototypic example of an oncogene formed by fusion of two separate genes. In these cases, a reciprocal translocation between chromosomes 9 and 22 relocates a truncated portion of the protooncogene c-ABL (from chromosome 9) to the BCR (break point cluster region) on chromosome 22 ( Fig. 7-33 ). The hybrid fusion gene BCR-ABL encodes a chimeric protein that has constitutive tyrosine kinase activity. As mentioned, BCR-ABL tyrosine kinase has served as a target for leukemia therapy, with remarkable success so far. Although the translocations are cytogenetically identical in chronic myeloid leukemia and acute lymphoblastic leukemias, they differ at the molecular level. In chronic myeloid leukemia, the chimeric protein has a molecular weight of 210 kD, whereas in the more aggressive acute leukemias, a 190-kD BCR-ABL fusion protein is formed.[62][63] The molecular pathways activated by the BCR-ABL protein are complex and not completely understood. It inhibits apoptosis, decreases the requirement for growth factors, binds to cytoskeleton components, decreases cell adhesion, and activates multiple pathways, including those of RAS, PI-3 kinase, and STATs ( Chapter 3 ). BCR-ABL also acts on DNA repair and may cause genomic instability that contributes to the progression of the disease.

Transcription factors are often the partners in gene fusions occurring in cancer cells. For instance, the MLL (myeloid, lymphoid leukemia) gene on 11q23 is known to be involved in 25 different translocations with several different partner genes, some of which encode transcription factors (see Table 7-10 ). The Ewing Sarcoma (EWS) gene at 22q12 was first described in the t(11;22)(q24;12) reciprocal translocation present in Ewing sarcoma (a highly malignant tumor of children; Chapter 26 ) but may be translocated in other types of sarcomas. EWS is itself a transcription factor, and all of its partner genes analyzed so far also encode a transcription factor. In Ewing tumor, for example, the EWS gene fuses with the FLI 1 gene; the resultant chimeric EWS-FLI 1 protein is a member of the ETS transcription factor family, which has transforming ability.

Gene Amplification

Activation of protooncogenes associated with overexpression of their products may result from reduplication and amplification of their DNA sequences. Such amplification may produce several hundred copies of the protooncogene in the tumor cell.[137] The amplified genes can be readily detected by molecular hybridization with appropriate DNA probes. In some cases, the amplified genes produce chromosomal changes that can be identified microscopically. Two mutually exclusive patterns are seen: multiple small, chromosome-like structures called double minutes (dms), and homogeneous staining regions (HSRs). The latter derive from the assembly of amplified genes into new chromosomes; because the regions containing amplified genes lack a normal banding pattern, they appear homogeneous in a G-banded karyotype (see Fig. 7-34 ). The most interesting cases of amplification involve N-MYC in neuroblastoma and ERB B2 in breast cancers. N-MYC is amplified in 25% to 30% of neuroblastomas, and the amplification is associated with poor prognosis. In neuroblastomas with N-MYC amplification, the gene is present both in dms and HSRs. ERB B2 amplification occurs in about 20% of breast cancers and may represent a distinct tumor phenotype. Amplification of C-MYC, L-MYC, and N-MYC correlates with disease progression in small cell cancer of the lung. Another gene frequently amplified is CYCLIN D1 (breast carcinomas, head and neck carcinomas, and other squamous cell carcinomas).

Epigenetic Changes

It has become evident during the past few years that certain tumor suppressor genes may be inactivated not because of structural changes but because the gene is silenced by hypermethylation of promoter sequences without a change in DNA base sequence.[138] Such changes appear to be stably maintained through multiple rounds of cell division. Methylation takes place in CpG islands in DNA, but de novo methylation rarely occurs in normal tissues. However, methylation has been detected in various tumor suppressor genes in human cancers. They include p14ARF in colon and stomach cancers, p16INK4a in various types of cancers, BRCA1 in breast cancer, VHL in renal cell carcinomas, and the MLH1 mismatch repair gene in colorectal cancer.[139] Methylation also participates in the phenomenon called genomic imprinting, in which the maternal or paternal allele of a gene or chromosome is modified by methylation and is inactivated. The reverse phenomenon, that is, demethylation of an imprinted gene leading to its biallelic expression (loss of imprinting) can also occur in tumor cells.[140] Although the discussion of whether methylation of tumor suppressor genes has a causal role in cancer development continues, there has been great interest in developing potential therapeutic agents that act to demethylate DNA sequences in tumor suppressor genes. Recent data demonstrating that genomic hypomethylation causes chromosomal instability and induces tumors in mice greatly strengthens the notion that epigenetic changes may directly contribute to tumor development.[141]

Molecular Profiles of Cancer Cells

A new era in cancer research was initiated with the development of methods to measure the expression of thousands of genes in tumors and normal tissues. Among these new methods, the determination of RNA levels by microarray analysis has found wide application ( Box 7-1 ). Currently, this method can measure RNA expression from virtually all known genes ( Fig. 7-45 ). The expression profiles obtained from DNA microarray analysis are known as gene expression signatures or molecular profiles. The application of this technique to the study of breast cancers and leukemias has been particularly rewarding (see Box 7-1 ). It was recently found that there are breast cancer subtypes that can be identified by their molecular profiles and that the molecular signatures of some of these subtypes can help predict the course of the disease (see Box 23-1 , Chapter 23).[142] Analysis of acute lymphoblastic leukemia by DNA microarrays has established the molecular signatures of prognostic subtypes and uncovered novel markers associated with these subtypes.[143]

Box 7-1

Gene Expression Profiles of Human Cancers. Microarrays and Proteomics

Until recently, studies of gene expression in tumors involved the analysis of individual genes. These studies have been revolutionized by the introduction of methods that can measure the expression of thousands of genes simultaneously.[208][209] The most common method for large-scale analysis of gene expression in use today is based on DNA microarray technology. In this method, DNA fragments, either cDNAs or oligonucleotides, are spotted on a glass slide or on some other solid support. As the techniques used for the spotting are similar to those employed to produce semiconductor chips for electronic products, the arrays are known as "gene chips." Chips can be purchased from commercial suppliers or produced in-house, and can contain more than 20,000 gene fragments. The fragments are typically obtained from complementary DNA (cDNA) libraries or sets of nucleotides from known and uncharacterized genes. The gene chip is then hybridized to "probes" prepared from tumor and control samples (the probes are usually cDNA copies of RNAs extracted from tumor and uninvolved tissues). Before hybridization to the chip, the probes are labeled with fluorochromes that emit different colors (e.g. red color for tumor RNA and green color for control RNA). After hybridization the chip is read using a laser scanner ( Fig. 7-45 ); each spot on the array will be red (increased expression of a gene in the tumor), green (decreased expression in the tumor) or, if there is no difference in gene expression between the tumor and control sample, the spots will be either black or yellow (depending on the type of fluorescent scanning). Sophisticated software has been developed to measure the intensity of the fluorescence for each spot and produce data sets in which genes with similar expression patterns are clustered.[210] This method of analysis, called hierarchical clustering, groups together genes according to the similarity of their gene expression patterns. The software can be linked to large sequencing and array databases available through the Web. This allows appropriate gene identification and comparison between expression profiles from various sources. A major problem in the analysis of gene expression in tumors is the heterogeneity of the tissue. In addition to the heterogeneity between tumor cells, samples may contain variable amounts of stromal connective tissue, inflammatory infiltrates, and normal tissue cells. One way to overcome this problem is to obtain nearly pure tumor cells or small tumors free from associated tissues using laser capture microdissection. In this technique, the dissection of the tumor or cells is made under a microscope through a focused laser. The dissected material is then captured or "catapulted" into a small cap and processed for RNA and DNA isolation.

Gene expression profiling of tumors has multiple uses, and the number of publications using this technique has grown enormously during the past few years. Much of the work performed is not directed toward proving or disproving a proposed hypothesis. Gene expression analysis can be used to classify tumors; to predict metastatic potential, prognosis, and response to therapy; to reveal gene expression patterns that are dependent on the mutation of a single oncogene; and to analyze the effects of hormones and environmental agents on cancer development.[209] The applications of this technology keep expanding and being refined, but much has already been accomplished.[141][209][210][211] We mention only a few interesting examples. Profiling of cells from adult and pediatric T-cell acute lymphoblastic leukemia has identified the patterns of gene expression in leukemic blast cells and has accurately classified each prognostic subtype.[211] The work that has received the highest publicity involves gene expression profiling of breast cancers. In addition to identifying new subtypes of breast cancers, a 70-gene prognosis profile was established. Using this type of profile, it has been reported that: (1) the profile was a powerful predictor of disease prognosis for young patients; (2) it was particularly accurate for predicting metastasis during the first 5 years after diagnosis; and (3) prognosis determined by gene expression profiles correlated highly with histologic grade and estrogen receptor status but not with lymphatic spread of the tumor.[211] A more recent analysis has pooled together data gathered by different laboratories and has confirmed the identification of distinct subtypes of breast cancer.[142] Given all of these remarkable results, it is time to ask whether this technology is "ready for prime time"; that is, ready for day to day clinical applications. Things are moving very fast in this area, but before clinical applications are considered, many issues need to settled. Not only do larger trials need to be conducted to prove the reliability and accuracy of the analysis but also, just as important, the procedures for handling samples, performing the analyses, and reporting the data need to be standardized, so that data obtained in various laboratories can be compared.

Next on the horizon of molecular techniques for the global analysis of gene expression in cancers is proteomics, a technique used to obtain expression profiles of proteins contained in tissues, serum, or other body fluids. The original method consisted of the separation of proteins by 2-dimensional gel electrophoresis, followed by identification of individual proteins by mass spectrometry. A more recent technique, called ICAT (isotope-coding affinity tags) does not rely on electrophoresis for protein separation. In ICAT, proteins in the test and control samples are labeled with light or heavy isotopes. The differentially labeled proteins are then identified and quantified by mass spectrometry. A variation of proteomic analysis has been used to obtain protein profiles in the blood of cancer patients without identification of individual proteins.[215]

The excitement created by the development of new techniques for the global molecular analysis of tumors has led some scientists to predict that the end of histopathology is in sight, and to consider existing approaches to tumor diagnosis as the equivalent of magical methods of divination. Indeed, it is hard to escape the excitement generated by the development of entirely new and powerful methods of molecular analysis. However, what lies ahead is not the replacement of one set of techniques by another. On the contrary, the most accurate diagnosis and prognosis of cancer will be arrived at by a combination of morphologic and molecular techniques.[208]

 

Figure 7-45  Schematic representation of the steps required for the analysis of global gene expression by DNA microarray. RNA is extracted from tumor and normal tissue. cDNA synthesized from each preparation is labeled with fluorescent dyes (in the example shown, normal tissue cDNA is labeled with a green dye; tumor cDNA is labeled with a red dye). The array consists of a solid support in which DNA fragments from many thousands of genes are spotted. The labeled cDNAs from tumor and normal tissue are combined and hybridized to the genes contained in the array. Hybridization signals are detected using a confocal laser scanner and downloaded to a computer for analysis (red squares, expression of the gene is higher in tumor; green square, expression of the gene is higher in normal tissue; black squares, no difference in the expression of the gene between tumor and normal tissue). In the display, the horizontal rows correspond to each gene contained in the array; each ventrical row corresponds to single samples.

(From: http://www.mdconsult.com/das/book/body/111818921-4/0/1249/64.html?tocnode=51154820&fromURL=64.html)



访问次数:0