Causes of Cancer: Immunology (PDF)

16.2 The Causes of Cancer 677 cancer cells activate a transcription factor called HIF that induces the virus will never develop this malignancy. HPV is also linked the formation of new blood vessels and promotes the migratory as a primary...

16.2 The Causes of Cancer 677 cancer cells activate a transcription factor called HIF that induces the virus will never develop this malignancy. HPV is also linked the formation of new blood vessels and promotes the migratory as a primary causative agent of cancers of the mouth and tongue properties of the cells, which may contribute to the spread of the in both men and women. Effective vaccines against this virus are tumor. However, even when oxygen is plentiful, many tumor now available. Other viruses linked to human cancers include cells continue to generate much of their ATP by glycolysis (called hepatitis B virus, which is associated with liver cancer; Epstein‐ aerobic glycolysis). Even though glycolysis generates much less Barr virus, which is associated with Burkitt’s lymphoma in areas ATP per glucose than does oxidative phosphorylation in the where malaria is common; and a herpes virus (HHV‐8), which is mitochondrion, it produces ATP at a more rapid rate. The associated with Kaposi’s sarcoma. increased uptake of glucose by tumor cells compared to normal Certain gastric lymphomas are associated with chronic cells can be used as a means to locate metastatic tumors within infection by the stomach‐dwelling bacterium Helicobacter pylori, the body using PET scans (see Figure 16.24). which can also cause ulcers. Recent evidence suggests that many It is these properties, which can be demonstrated in culture, of these cancers linked to persistent viral and bacterial infections together with their tendency to spread to distant sites within the are actually caused by the chronic inflammation that is triggered body, that make cancer cells such a threat to the well‐being of the by the presence of the pathogen. Inflammatory bowel disease entire organism. (IBD), which is also characterized by chronic inflammation, has been associated with an increased risk of colon cancer. These findings have caused researchers to look more closely at the gen- eral process of inflammation as a previously unexplored factor in 16.2 The Causes of Cancer the development of many types of cancers. Determining the causes of different types of cancer is an In 1775, Percivall Pott, a British surgeon, made the first known cor- endeavor carried out by epidemiologists, researchers who study relation between an environmental agent and the development of disease patterns in populations. The causes of certain cancers are cancer. Pott concluded that the high incidence of cancer of the nasal obvious: Smoking causes lung cancer, exposure to ultraviolet cavity and the skin of the scrotum in chimney sweeps was due to radiation causes skin cancer, and inhaling asbestos fibers causes their chronic exposure to soot. Within the past several decades, the mesothelioma. But despite a large number of studies, we are still carcinogenic chemicals in soot have been isolated, along with uncertain as to the causes of most types of human cancer. hundreds of other compounds shown to cause cancer in laboratory Humans live in complex environments and are exposed to many animals. In addition to a diverse array of chemicals, a number of potential carcinogens in a changing pattern over a period of dec- other types of agents are also carcinogenic, including ionizing radi- ades. Attempting to determine the causes of cancer from a moun- ation and a variety of DNA‐ and RNA‐containing viruses. All of tain of statistical data obtained from the answers to questionnaires these agents have one property in common: They alter the genome. about individual lifestyles has proven very difficult. The impor- Carcinogenic chemicals, such as those present in soot or cigarette tance of environmental factors (e.g., diet) is seen most clearly in smoke, can almost always be shown either to be directly mutagenic studies of the children of couples that have moved from Asia to or to be converted to mutagenic compounds by cellular enzymes. the United States or Europe. These individuals no longer exhibit Similarly, ultraviolet radiation, which is the leading cause of skin a high rate of gastric cancer, as occurs in Asia, but instead are cancer, is also strongly mutagenic. subject to an elevated risk of colon and breast cancer, which is A number of viruses can infect mammalian cells growing in characteristic of Western countries (FIGURE 16.6). cell culture, transforming them into cancer cells. These viruses There is a general consensus among epidemiologists that are broadly divided into two large groups: DNA tumor viruses diet can play a major role in the risk of developing cancer. Cancer and RNA tumor viruses, depending on the type of nucleic acid rates are higher among obese individuals than the non‐obese found within the mature virus particle. Among the DNA viruses population and studies in primates suggest that a calorie‐ capable of transforming cells are polyoma virus, simian virus 40 restricted diet (Human Perspective, Chapter 3) protects against (SV40), adenovirus, and herpes‐like viruses. RNA tumor viruses, cancer. Recent attention has focused on elevated levels of insulin or retroviruses, are similar in structure to HIV (see Figure 1.22b) and insulin‐like growth factor (IGF‐1) that are found in obese and are the subject of the Experimental Pathways at the end of the individuals as being a primary cause of the increased cancer inci- chapter. Tumor viruses can transform cells because they carry dence in this group. There is also evidence that some ingredients genes whose products interfere with the cell’s normal growth‐ in the diet, such as animal fat and alcohol, can increase the risk regulating activities. Although tumor viruses were an invaluable of developing cancer, whereas certain compounds found in food tool for researchers in identifying numerous genes involved in cell items may reduce that risk. Examples of the latter include isofla- transformation, they are associated with only a small number of vones found in soy, sulforaphanes found in broccoli, and EGCG human cancers. Other types of viruses are, however, linked to as found in tea. Several widely prescribed drugs also have a preven- many as 20 percent of cancers worldwide. In most cases, these tive effect. Drugs that interfere with the action of estrogen (e.g., viruses greatly increase a person’s risk of developing the cancer, tamoxifen or raloxifene) or the metabolism of testosterone (e.g., rather than being the sole determinant responsible for the disease. finasteride) can reduce the incidence of breast cancer or prostate This relationship between viral infection and cancer is illustrated cancer, respectively. Long‐term use of nonsteroidal anti‐inflam- by human papilloma virus (HPV), which can be transmitted matory drugs (NSAIDs) such as aspirin and indomethacin has through sexual activity and is increasing in frequency in the pop- been shown to markedly decrease the risk of colon cancer. They ulation. Although the virus is present in about 90 percent of cervi- are thought to have this effect by inhibiting cyclooxygenase‐2, an cal cancers, indicating its importance in development of the enzyme that catalyzes the synthesis of hormone‐like prostaglan- disease, the vast majority of women who have been infected with dins, which promote the growth of intestinal polyps. The karp_c16.indd 677 10/14/2017 5:25:07 PM  678 CHAPTER 16 Cancer 100 100 100 during our own lifetime (somatic mutations). There are a few types of mutations that we can inherit that make us much more likely to 80 80 80 develop cancer. The study of these mutations has taught us a great Incidence per 100,000 Incidence per 100,000 Incidence per 100,000 deal about how malfunctioning genes can lead to the development 60 60 60 of cancer; some of these inherited cancer syndromes will be dis- cussed later in this section. However, for the most part, inherited 40 40 40 mutations are not a major factor in the occurrence of most cases of the disease. One way to determine an overall estimate of the impact 20 20 20 of inheritance in tumor formation is to ascertain the likelihood that two identical twins will develop the same type of cancer by the time the individuals reach a certain age. Studies of this type suggest 0 0 0 Stomach (male) Breast (female) Colon (male) that the likelihood two 75‐year‐old identical twins will share a par- ticular cancer, such as breast cancer or prostate cancer, is generally between 10 and 15 percent, depending on the type of cancer. Japanese Second-generation migrants Clearly, the genes that we inherit have a significant influence on First-generation migrants Caucasian Hawaiians our risks of developing cancer, but the greatest impact comes from genes that are altered during our lifetime. FIGURE 16.6 Changing cancer incidence in persons of Japanese The development of a malignant tumor (tumorigenesis) is a descent following migration to Hawaii. The incidence of stomach multistep process characterized by a progression of permanent cancer declines, whereas that of breast and colon cancer rises. genetic alterations in a single line of cells, which may occur over However, of the three types of cancer, only colon cancer has reached the course of many successive cell divisions and take decades to rates equivalent to Caucasian Hawaiians by the second generation. complete. Each genetic change may elicit a particular feature of SOURCE: From L. N. Kolonel et al., reprinted with permission from Nature the malignant state, such as protection from apoptosis, as dis- Revs. Cancer 4:3, 2004; copyright 2004. Nature Reviews Cancer by cussed in Section 16.1. As these genetic changes gradually occur, Nature Publishing Group. Reproduced with permission of Nature the cells in the line become increasingly less responsive to the Publishing Group in the format reuse in a book/textbook via Copyright body’s normal regulatory machinery and better able to invade Clearance Center. normal tissues. According to this concept, tumorigenesis requires that the cell responsible for initiating the cancer be capable of a cancer‐suppressing action of NSAIDs supports the idea that large number of cell divisions. This requirement has focused a inflammation plays a major role in the development of various great deal of attention on the types of cells that are present in a cancers. Persons who have taken the antidiabetes drug met- tissue that might have the potential to develop into a tumor. formin also appear to have a significantly reduced risk of devel- The most common solid tumors—such as those of the oping cancer. In this case, the benefit may be a result of the drug’s breast, colon, prostate, and lung—arise in epithelial tissues that action in lowering the circulating levels of insulin and IGF‐1. are normally engaged in a relatively high level of cell division. The same is true of leukemias, which develop in rapidly dividing blood‐forming tissues. The cells of most tissues can be roughly 16.3 The Genetics of Cancer divided into three groups: (1) stem cells, which possess unlim- ited proliferation potential, have the capacity to produce more of Cancer is one of the two leading causes of death in Western themselves, and can give rise to all of the cells of the tissue countries, afflicting approximately one in every three individu- (Human Perspective, Chapter 1); (2) progenitor cells, which are als. Viewed in this way, cancer is a very common disease. But at derived from stem cells and possess a limited ability to prolifer- the cellular level, the development of a cancer is a remarkably ate; and (3) the differentiated end products of the tissue, which rare event. Whenever the cells of a cancerous tumor are geneti- generally lack the capability to divide. Examples of these three cally scrutinized, they are invariably found to have arisen from a groups of cells are illustrated in Figure 17.6. single cell. Thus, unlike other diseases that require modification Given the fact that tumor formation requires that a cell be of a large number of cells, cancer results from the uncontrolled capable of extensive division, two general scenarios have been con- proliferation of a single wayward cell. (Cancer is said to be mono- sidered for the origin of tumors. According to one scenario, cancer clonal.) Consider for a moment that the human body contains arises from within the relatively small population of stem cells that trillions of cells, billions of which undergo cell division on any inhabit each adult tissue. Given their long life and unlimited divi- given day. Although almost any one of these dividing cells may sion potential, stem cells have the opportunity to accumulate the have the potential to change in genetic composition and grow mutations required for malignant transformation. According to into a malignant tumor, this only occurs in about one‐third of another scenario, progenitor cells can give rise to malignant tumors the human population during an entire lifetime. by acquiring certain properties, such as the capacity for unlimited One of the primary reasons why a greater number of cells do proliferation, as part of the process of tumor progression. As illus- not give rise to cancerous tumors is that malignant transformation trated in FIGURE 16.7, these two scenarios are not mutually exclu- requires more than a single genetic alteration. We can distinguish sive, in that some tumors are thought to arise from stem cells and between two types of genetic alterations that might make us more others from the progenitor cell population. likely to develop a particular type of cancer—those that we inherit As a cancer grows, the cells in the tumor mass are subjected from our parents (germ‐line mutations) and those that occur to a type of natural selection that drives the accumulation of cells karp_c16.indd 678 10/14/2017 5:25:08 PM  16.3 The Genetics of Cancer 679 cell and, consequently, represents a permanent, inheritable alter- ation. Even after they have become malignant, cancer cells con- Oncogenic tinue to accumulate mutations and epigenetic changes that make event A them increasingly abnormal (as is evident in Figure 16.5). This Tissue genetic instability makes the disease difficult to treat by conven- stem cell Tumor subtype x tional chemotherapy because cells that are resistant to the drug often arise within the tumor mass. The genetic changes that occur during tumor progression are Oncogenic often accompanied by histological changes, that is, changes in the event A appearance of the cells. The initial changes often produce cells that Pluripotent can be identified as “precancerous,” which indicates that they have progenitor cell Oncogenic Tumor subtype y gained some of the properties of a cancer cell, such as loss of certain event A growth controls, but lack the capability to invade normal tissues or metastasize to distant sites. The Pap smear is a test for detecting precancerous cells in the epithelial lining of the cervix. The devel- opment of cervical cancer typically progresses over a period of Committed Committed more than 10 years and is characterized by cells that appear increas- progenitor cell progenitor cell ingly abnormal (less well differentiated than normal cells, with Tumor subtype z larger nuclei, as in FIGURE 16.8). When cells having an abnormal appearance are detected, the precancerous lesion in the cervix can Mature cells FIGURE 16.7 The proposed cells of origin of malignant tumors. Tissues contain cells in various stages of commitment and differentia- tion. These include stem cells, multipotent progenitor cells that can give rise to a variety of types of differentiated cells, committed progenitor cells that can give rise to only one type of differentiated cell, and the differentiated cells themselves (see Figure 17.6 for examples). According to the model depicted here, tumors can arise from either tissue stem cells or progenitor cells, although in some cases at least, these different cells of origin give rise to different types of cancers (a) (indicated by the three different colors of the tumors. SOURCE: J. E. Visvader, Nature 469:316, 2011 Figure 2b. Nature by Nature Publishing Group. Reproduced with permission of Nature Publishing Group in the format reuse in a book/textbook via Copyright Clearance Center. with properties that are most favorable for tumor growth. For example, only those tumors containing cells that maintain the length of their telomeres will be capable of unlimited growth (page 240). Any cell that appears within a tumor that happens to express telomerase will have a tremendous growth advantage over other cells that fail to express the enzyme. Over time, the telomerase‐expressing cells will flourish while the nonexpressing cells will die off, and all of the cells in the tumor will contain tel- omerase. Expression of telomerase illustrates another important feature of tumor progression: Not all of these changes result from (b) genetic mutation. The activation of telomerase expression can be FIGURE 16.8 Detection of abnormal (premalignant) cells in a Pap considered an epigenetic change, one that results from the acti- smear. (a) Normal squamous epithelial cells of the cervix. The cells have a vation of a gene that is normally repressed. As discussed in uniform shape with a small centrally located nucleus. (b) Abnormal cells Chapter 6, this type of activation process likely involves a change from a case of carcinoma in situ, which is a preinvasive cancer of the in the structure of chromatin in and around the gene and/or a cervix. The cells have heterogeneous shapes and large nuclei. change in the state of DNA methylation. Once the epigenetic SOURCE: (a) Dr. E. Walker/Photo Researchers, Inc.; (b) SPL /Photo change has occurred, it is transmitted to all of the progeny of that Researchers, Inc. karp_c16.indd 679 10/14/2017 5:25:10 PM  680 CHAPTER 16 Cancer be located and destroyed by laser treatment, freezing, or surgery. of cancer. If the absence of such genes is correlated with the Some tissues often generate benign tumors, which contain cells that development of a tumor, then it follows that the presence of these have proliferated to form a mass that poses little threat of becoming genes normally suppresses the formation of the tumor. malignant. The moles that we all possess are an example of benign Oncogenes, on the other hand, encode proteins that pro- tumors. Studies indicate that the pigment cells that compose a mole mote the loss of growth control and the conversion of a cell to a have undergone a response that causes them to enter a permanent malignant state (Figure 16.9b). Most oncogenes act as accelera- state of growth arrest, referred to as senescence. Senescence is tors of cell proliferation, but they have other roles as well. apparently triggered in these pigment cells after they have under- Oncogenes may lead to genetic instability, prevent a cell from gone certain of the genetic changes that would have otherwise set becoming a victim of apoptosis, or promote metastasis. The them on a course to becoming a malignant cancer. This process of existence of oncogenes was discovered through a series of inves- “forced senescence” represents another pathway that has evolved to tigations on RNA tumor viruses that is documented in the restrict the development of cancers in higher organisms. The Experimental Pathways. These viruses transform a normal cell molecular basis of senescence is discussed further in Section 16.6. into a malignant cell because they carry an oncogene that encodes a protein that interferes with the cell’s normal activities. The turning point in these studies came in 1976, when it was discovered that an oncogene called src, carried by an RNA tumor 16.4 An Overview of Tumor‐ virus called avian sarcoma virus, was actually present in the Suppressor Genes and genome of uninfected cells. The oncogene, in fact, was not a viral gene, but a cellular gene that had become incorporated into the Oncogenes viral genome during a previous infection. It soon became evident that cells possess a variety of genes, now referred to as proto‐ The genes that have been implicated in carcinogenesis are divided oncogenes, that have the potential to subvert the cell’s own into two broad categories: tumor‐suppressor genes and onco- activities and push the cell toward the malignant state. genes. Tumor‐suppressor genes act as a cell’s brakes; they As discussed below, proto‐oncogenes encode proteins that encode proteins that restrain cell growth and prevent cells from have various functions in a cell’s normal activities. Proto‐ becoming malignant (FIGURE 16.9a). The existence of such genes oncogenes can be converted into oncogenes (i.e., activated) by originally came to light from studies in the late 1960s in which several mechanisms (FIGURE 16.10): normal and malignant rodent cells were fused to one another. Some of the cell hybrids formed from this type of fusion lost their 1. The gene can be mutated in a way that alters the properties of malignant characteristics, suggesting that a normal cell possesses the gene product so that it no longer functions normally factors that can suppress the uncontrolled growth of a cancer cell. (Figure 16.10, path a). Further evidence for the existence of tumor‐suppressor genes 2. The gene can become duplicated one or more times, resulting was gathered from observations that specific regions of particu- in gene amplification and excess production of the encoded lar chromosomes are consistently deleted in cells of certain types protein (Figure 16.10, path b). Proto-oncogene Normal cell growth Mutated tumor- suppressor gene Mutated proto- oncogene has become oncogene Normal cell growth Normal Copies of the mutated cell tumor-suppressor gene Normal growth on both homologues cell growth Loss of growth Normal Loss control cell of growth growth control (a) (b) FIGURE 16.9 Contrasting effects of mutations in tumor‐suppressor genes (a) and oncogenes (b). Whereas a mutation in one of the two copies (alleles) of an oncogene may be sufficient to cause a cell to lose growth control, both copies of a tumor‐suppressor gene must be knocked out to induce the same effect. As discussed shortly, oncogenes arise from proto‐oncogenes as the result of gain‐of‐function mutations, that is, mutations that cause the gene product to exhibit new functions that lead to malignancy. Tumor‐suppressor genes, in contrast, suffer loss‐of‐function mutations and/or epigenetic inactivation that render them unable to restrain cell growth. karp_c16.indd 680 10/14/2017 5:25:14 PM  16.5 Tumor‐Suppressor Genes: The RB Gene 681 Encoded protein with altered structure/function Mutation or deletion a Increased synthesis Gene duplication of encoded protein Regulatory Proto- region oncogene b c OR Protein encoded A DNA by proto-oncogene A protein-coding gene regulatory translocated from distant sequence site fuses with portion of translocated gene causing formation from distant Synthesis of a protein of a fusion gene site alters containing portions expression of encoded by different downstream gene genes. The fusion protein is no longer under normal control Increased synthesis of encoded protein FIGURE 16.10 Activation of a proto‐oncogene to an oncogene. Activation can be accomplished in several ways as indicated in this figure. In pathway a, a mutation in the gene alters the structure and function of the encoded protein. In pathway b, gene amplification results in overexpres- sion of the gene. In pathway c, a rearrangement of the DNA brings a new DNA segment into the vicinity or up against the gene, altering either its expression or the structure of the encoded protein. 3. A chromosome rearrangement can occur that brings a DNA the cell can become fully malignant. Even then, the cell may not sequence from a distant site in the genome into close proxim- exhibit all of the properties required to invade surrounding tis- ity of the gene, which can either alter the expression of the sues or to form secondary colonies by metastasis. Mutations in gene or the nature of the gene product (Figure 16.10, path c). additional genes, such as those encoding cell‐adhesion mole- Any of these genetic alterations can cause a cell to become less cules or extracellular proteases (discussed in Human Perspective, responsive to normal growth controls. Oncogenes act domi- Chapter 11), may be required before these cells acquire a meta- nantly, which is to say that a single copy of an oncogene can static phenotype. cause the cell to express the altered phenotype, regardless of We can now turn to the functions of the products encoded whether there is a normal, unactivated copy of the gene on the by both tumor‐suppressor genes and oncogenes and examine homologous chromosome (Figure 16.9b). Researchers have how mutations in these genes can cause a cell to become taken advantage of this property to identify oncogenes by intro- malignant. ducing the DNA suspected of containing the gene into cultured cells and monitoring the cells for evidence of altered growth properties (Section 16.1). We saw earlier that the development of a human malignancy 16.5 Tumor‐Suppressor Genes: requires more than a single genetic alteration. The reason The RB Gene becomes more apparent with the understanding that there are two types of genes responsible for tumor formation. As long as a The transformation of a normal cell to a cancer cell is accom- cell has its full complement of tumor‐suppressor genes, it is panied by the loss of function of one or more tumor‐suppressor thought to be protected against the effects of an oncogene for genes. High‐throughput sequencing studies have identified reasons that will be evident when the functions of these genes are hundreds of genes that are implicated as tumor suppressors in discussed below. Most tumors contain alterations in both tumor‐ humans. Some of the better characterized genes, listed in suppressor genes and oncogenes, suggesting that the loss of a Table 16.1, include genes that encode transcription factors tumor‐suppressor function within a cell must be accompanied (e.g., TP53 and WT1), cell cycle regulators (e.g., RB and INK4a), by the conversion of a proto‐oncogene into an oncogene before components that regulate G proteins (NF1), a phosphoinositide karp_c16.indd 681 10/14/2017 5:25:16 PM  682 CHAPTER 16 Cancer TABLE 16.1 Tumor‐Suppressor Genes Gene Primary tumor Proposed function Inherited syndrome APC Colorectal Binds β‐catenin acting as transcription factor Familial adenomatous polyposis BRCA1 Breast DNA repair Familial breast cancer MSH2, MLH1 Colorectal Mismatch repair HNPCC E‐Cadherin Breast, colon, etc. Cell adhesion molecule Familial gastric cancer INK4a Melanoma, pancreatic p16: Cdk inhibitor Familial melanoma ARF: stabilizes p53 NF1 Neurofibromas Activates GTPase of Ras Neurofibromatosis type 1 NF2 Meningiomas Links membrane to cytoskeleton Neurofibromatosis type 2 TP53 Sarcomas, lymphomas, etc. Transcription factor (cell cycle and apoptosis) Li‐Fraumeni syndrome PTEN Breast, thyroid PIP3 phosphatase Cowden disease RB Retinal Binds E2F (cell cycle transcription regulation) Retinoblastoma VHL Kidney Protein ubiquitination and degradation von Hippel‐Lindau syndrome WT1 Wilms tumor of kidney Transcription factor Wilms tumor phosphatase (PTEN), and a protein that regulates protein deg- chromosome missing the retinoblastoma gene have a strong dis- radation (VHL).2 In one way or another, most of the proteins position toward developing retinoblastoma. In fact, approxi- encoded by tumor‐suppressor genes act as negative regulators mately 10 percent of individuals who inherit a chromosome with of cell proliferation, which is why their elimination promotes an RB deletion never develop the retinal cancer. How is it that a uncontrolled cell growth. The products of tumor‐suppressor small percentage of these predisposed individuals escape the genes also help maintain genetic stability, which may be a pri- disease? mary reason that tumors contain such an aberrant karyotype The genetic basis of retinoblastoma was explained in 1971 (Figure 16.5). Some tumor‐suppressor genes are involved in by Alfred Knudson of the University of Texas. Knudson pro- the development of a wide variety of different cancers, whereas posed that the development of retinoblastoma requires that others play a role in the formation of one or a few cancer both copies of the RB gene of a retinal cell be either eliminated types. or mutated before the cell can give rise to a retinoblastoma. In It is common knowledge that members of some families are other words, the cancer arises as the result of two independent at high risk of developing certain types of cancers. Although “hits” in a single cell. In cases of sporadic retinoblastoma, the these inherited cancer syndromes are rare, they provide an tumor develops from a retinal cell in which both copies of the opportunity to identify tumor‐suppressor genes that, when miss- RB gene have undergone successive spontaneous mutation ing, contribute to the development of both inherited and spo- (FIGURE 16.11a). Because the chance that both alleles of the radic (i.e., noninherited) forms of cancer. The first tumor‐suppressor same gene will be the target of debilitating mutations in the gene to be studied and eventually cloned—and one of the most same cell is extremely unlikely, the incidence of the cancer in important—is associated with a rare childhood cancer of the ret- the general population is extremely low. In contrast, the cells of ina of the eye, called retinoblastoma. The gene responsible for this a person who inherits a chromosome with an RB deletion are disorder is named RB. The incidence of retinoblastoma follows already halfway along the path to becoming malignant. two distinct patterns: (1) It occurs at high frequency and at young Mutation or deletion of the remaining RB allele in any of the age in members of certain families, and (2) it occurs sporadically cells of the retina produces a cell that lacks a normal RB gene at an older age among members of the population at large. The and thus cannot produce a functional RB gene product fact that retinoblastoma runs in certain families suggested that (Figure 16.11b). This explains why individuals who inherit an the cancer can be inherited. Examination of cells from children abnormal RB gene are so highly predisposed to developing the suffering from retinoblastoma revealed that one member of the cancer. The second “hit” fails to occur in approximately 10 per- thirteenth pair of homologous chromosomes was missing a small cent of these individuals, who do not develop the disease. piece from the interior portion of the chromosome. The deletion Knudson’s hypothesis was subsequently confirmed by examin- was present in all of the children’s cells—both the cells of the reti- ing cells from patients with an inherited disposition to retino- nal cancer and cells elsewhere in the body—indicating that the blastoma and finding that, as predicted, both alleles of the gene chromosomal aberration had been inherited from one of the were missing or mutated in the cancer cells. Individuals with parents. sporadic retinoblastomas had normal cells that lacked RB Retinoblastoma is inherited as a dominant genetic trait mutations and tumor cells in which both alleles of the gene because members of high‐risk families that develop the disease were mutated. inherit one normal allele and one abnormal allele. But unlike Although deficiencies in the RB gene are first manifested in most dominantly inherited conditions, such as Huntington’s dis- the development of retinal cancers, this is not the end of the ease, where an individual who inherits a missing or an altered story. People who suffer from the inherited form of retinoblas- gene invariably develops the disorder, children who inherit a toma are also at high risk of developing other types of tumors 2 later in life, particularly soft‐tissue sarcomas (tumors of mesen- For the present chapter, which deals primarily with human biology, we will fol- chymal rather than epithelial origin). The consequences of RB low a convention that is commonly used: human genes are written in capital let- ters (e.g., APC), mouse genes are written with the first letter capitalized (e.g., mutations are not confined to persons who inherit a mutant Brca1), and viral genes are written in lower case (e.g., src). allele. Mutations in RB alleles are a common occurrence in karp_c16.indd 682 10/14/2017 5:25:16 PM  16.5 Tumor-Suppressor Genes: The RB Gene 683 Retinal cell RB gene Retinal cell Mutated RB gene inherited from parent Normal cell growth Spontaneous mutation in one copy of RB gene Spontaneous Normal mutation in second Normal cell copy of RB gene cell growth growth Spontaneous Normal Normal mutation in second cell cell copy of RB gene growth Loss growth of growth control Normal Loss cell of growth growth control (a) (b) FIGURE 16.11 Mutations in the RB gene that can lead to retinoblastoma. (a) In sporadic (i.e., nonfamilial) cases of the disease, an individual begins life with two normal copies of the RB gene in the zygote, and retinoblastoma occurs only in those rare individuals in whom a given retinal cell accumulates independent mutations in both alleles of the gene. (b) In familial (i.e., inherited) cases of the disease, an individual begins life with one abnormal allele of the RB gene, usually present as a deletion. Thus all the cells of the retina have at least one of their two RB genes nonfunctional. If the other RB allele in a retinal cell becomes inactivated, usually as the result of a point mutation, that cell gives rise to a retinal tumor. sporadic breast, prostate, and lung cancers among individuals E2F–pRB complex is associated with DNA but acts as a gene who have inherited two normal RB alleles. When cells from these repressor rather than a gene activator. As the end of G1 tumors are cultured in vitro, the reintroduction of a wild‐type RB approaches, the pRB subunit of the pRB–E2F complex is phos- gene back into the cells is generally sufficient to suppress their phorylated by the cyclin‐dependent kinases that regulate the cancerous phenotype, indicating that the loss of this gene func- G1–S transition. Once phosphorylated, pRB releases its bound tion contributes significantly to tumorigenesis. Let’s look more E2F, allowing the transcription factor to activate gene expres- closely at the role of the RB gene. sion, which marks the cell’s irreversible commitment to enter S The importance of the cell cycle in cell growth and prolif- phase. A cell that loses pRB activity as the result of RB mutation eration was discussed in Chapters 14 and 15, where it was noted would be expected to lose its ability to inactivate E2F, thereby that factors that control the cell cycle can play a pivotal role in removing certain restraints over the entry to S phase. E2F is the development of cancer. In its best studied role, the protein only one of dozens of proteins capable of binding to pRB, sug- encoded by the RB gene, pRB, helps regulate the passage of cells gesting that pRB has numerous other functions. The complexity from the G1 stage of the cell cycle into S phase, during which of pRB interactions is also suggested by the fact that the protein DNA synthesis occurs. As discussed in Section 14.3, the transi- contains at least 16 different serine and threonine residues that tion from G1 to S is a time of commitment for the cell; once a can be phosphorylated by cyclin‐dependent kinases. It is likely cell enters S phase, it invariably proceeds through the remain- that phosphorylation of different combinations of amino acid der of the cell cycle and into mitosis. The transition from G1 to residues allows the protein to interact with different down- S is accompanied by the activation of many different genes that stream targets. encode proteins ranging from DNA polymerases to cyclins and The importance of pRB as a negative regulator of the cell histones. Among the transcription factors involved in activating cycle is demonstrated by the fact that DNA tumor viruses genes required for S‐phase activities are members of the E2F (including adenoviruses, human papilloma virus, and SV40) family of transcription factors, which are key targets of pRB. A encode a protein that binds to pRB, blocking its ability to bind to model depicting the role of pRB in controlling E2F activity is E2F. The ability of these viruses to induce cancer in infected cells illustrated in FIGURE 16.12. During G1, E2F proteins are nor- depends on their ability to block the negative influence that pRB mally bound to pRB, which prevents the E2F molecules from has on progression of a cell through the cell cycle. By using these activating a number of genes encoding proteins required for S‐ pRB‐blocking proteins, these viruses accomplish the same result phase activities (e.g., cyclin E and DNA polymerase ). Studies as when the RB gene is deleted, leading to the development of have shown (as indicated in step 1 of Figure 16.12) that the tumors. karp_c16.indd 683 10/14/2017 5:25:20 PM  684 CHAPTER 16 Cancer E2F pRB The Role of p53: Guardian of the 1 Genome Gene repression The importance of p53 as an antitumor weapon is most evident Cdk activation leads to from the fact that TP53 is the most commonly mutated gene in pRb phosphorylation and dissociation from E2F human cancers; approximately half of all human tumors contain cells with point mutations or deletions in both alleles of the TP53 E2F gene (FIGURE 16.13a). Furthermore, tumors composed of cells 2 bearing TP53 mutations are correlated with a poorer prognosis + P than those containing a wild‐type TP53 gene. Clearly, the elimi- pRB nation of TP53 function is an important step in the progression P P of many cancer cells toward the fully malignant state. Transcription Pancreatic cancer 3 E2F Ovarian cancer Colon cancer Gene activation Lung cancer Liver cancer Stomach cancer mRNA Kidney cancer Prostate cancer Encoded protein Breast cancer Melanoma 4 0 10 20 30 40 50 60 70 80 90 100 % Mutations of TP53 gene (a) 5 G1 S FIGURE 16.12 The role of pRB in controlling transcription of genes required for progression of the cell cycle. During most of G1, G245 the unphosphorylated pRB is bound to the E2F protein. The E2F–pRB R249 complex binds to regulatory sites in the promoter regions of numerous R175 R248 genes involved in cell cycle progression, acting as a transcriptional repressor that blocks gene expression. Repression probably involves R273 the methylation of lysine 9 of histone H3 that modulates chromatin architecture (Section 6.3). Activation of the cyclin‐dependent kinase (Cdk) leads to the phosphorylation of pRB, which can no longer bind the E2F protein (step 2). In the pathway depicted here, loss of the bound pRB converts the DNA‐bound E2F into a transcriptional activator, leading to expression of the genes being regulated (step 3). R282 The mRNA is translated into proteins (step 4) that are required for the progression of cells from G1 into S phase of the cell cycle (step 5). Other roles of pRB have been identified but are not discussed. 16.6 Tumor‐Suppressor Genes: (b) The TP53 Gene FIGURE 16.13 The role of the tumor suppressor gene TP53 in human cancer. (a) The frequency with which both alleles of the TP53 The TP53 gene may have more to do with the development of gene are mutated in different types of cancers. The data refers to the human cancer than any other component of the genome. The most common form of each of these 10 types of cancers. (b) p53 gene gets its name from the product it encodes, p53, which is a function is particularly sensitive to mutations in its DNA‐binding domain. polypeptide having a molecular mass of 53,000 Daltons. In 1990, p53 functions as a tetramer, each subunit of which consists of several domains with different functions. This image shows a ribbon drawing of TP53 was recognized as the tumor‐suppressor gene that, when the DNA‐binding domain. The six amino acid residues most often absent, is responsible for a rare inherited disorder called Li‐ mutated in p53 molecules that have been debilitated in human cancers Fraumeni syndrome. Victims of this disease are afflicted with a are indicated in a single‐letter nomenclature (Figure 2.26). These very high incidence of various cancers, including breast and residues occur at or near the protein–DNA interface and either directly brain cancer and leukemia. Like individuals with the inherited impact the binding of the protein to DNA or alter its conformation. form of retinoblastoma, persons with Li‐Fraumeni syndrome SOURCE: (a) From Janet E. Dancey, et al., reprinted from Cell 148:412, inherit one normal and one abnormal (or deleted) allele of the 2012. with permission from Elsevier. (b) from Y. Cho, S. Gorina, P. D. TP53 tumor‐suppressor gene and are thus highly susceptible to Jeffrey, N. P. Pavletich, Science 265:352, 1994. Reprinted with permis- cancers that result from random mutations in the normal allele. sion from AAAS. karp_c16.indd 684 10/14/2017 5:25:22 PM  16.6 Tumor‐Suppressor Genes: The TP53 Gene 685 Why is the presence of p53 so important in preventing a cell the genetic integrity required for controlled growth (FIGURE 16.14). from becoming malignant? For one thing, p53 seems to bind a very Several studies have shown that established tumors in mice will long list of different proteins, as well as DNA, and is involved in a undergo regression when the activity of their p53 genes is restored. diverse array of cellular activities. In its best studied role, p53 serves This finding suggests that tumor development continues to as a transcription factor that acts as crucial player in a cell’s response depend on the absence of a functional TP53 gene, even after its to stress. When a cell sustains DNA damage, p53 responds by alter- cells become genetically unstable. For these reasons, the develop- ing the expression of a large number of genes involved in cell cycle ment of therapies that restore p53 function to p53‐deficient cells regulation, apoptosis, and/or senescence. The importance of the has become an active area of research. The most advanced ther- transcription‐regulating role of p53 is evident in Figure 16.13b, apy based on this strategy involves injection into the tumor of an which shows the location of the six mutations most commonly adenovirus that carries a wild‐type TP53 gene. This approach has found to disable p53 in human cancers; all of them map in the been used widely in China, but a similar adenoviral vector has not region of the protein that interacts with DNA. One of the best stud- been approved by the FDA at the time of this writing. ied genes activated by p53 encodes a protein called p21 that inhib- The level of p53 in a healthy G1 cell is very low, which keeps its the cyclin‐dependent kinase that normally drives a cell through its potentially lethal action under control. However, if a G1 cell the G1 checkpoint. As the level of p53 rises in the damaged G1 cell, sustains genetic damage, as occurs if the cell is subjected to ultra- expression of the p21 gene is activated, and progression through violet light or chemical carcinogens, the concentration of p53 the cell cycle is arrested (see Figure 14.9). This gives the cell time to rises rapidly. A similar response can be elicited simply by inject- repair the genetic damage before it initiates DNA replication. ing a cell with DNA containing broken strands. The increase in When both copies of the TP53 gene in a cell have been mutated so p53 levels is not due to increased expression of the gene but to an that their product is no longer functional, the cell can no longer increase in the stability of the protein. In unstressed cells, p53 has produce the p21 inhibitor or exercise the feedback control that pre- a half‐life of a few minutes. p53 degradation is facilitated by a vents it from entering S phase when it is not prepared to do so. protein called MDM2, which binds to p53 and escorts it out of Failure to repair DNA damage leads to the production of abnormal the nucleus and into the cytosol. Once in the cytosol, MDM2 cells that have the potential to become malignant. adds ubiquitin molecules to the p53 molecule, leading to its Cell cycle arrest is not the only way that p53 protects an destruction by a proteasome (Section 6.20). How does DNA organism from developing cancer. Alternatively, p53 can direct a damage lead to stabilization of p53? We saw in Section 14.4 that genetically damaged cell along a pathway that leads to death by persons suffering from ataxia telangiectasia lack a protein kinase apoptosis or necrosis, thereby ridding the body of cells with a called ATM and are unable to respond properly to DNA‐ malignant potential. p53 is thought to direct cell death through damaging radiation. ATM is normally activated following DNA several pathways, including the activation of expression of the damage, and p53 is one of the proteins ATM phosphorylates. BAX gene, whose encoded protein initiates apoptosis The phosphorylated version of the p53 molecule is no longer (Figure 15.40). Not all actions of p53 are dependent on its activa- able to interact with MDM2, which stabilizes existing p53 mole- tion of transcription. p53 is also capable of binding directly to cules in the nucleus and allows them to activate the expression of several members of the Bcl‐2 family proteins (page 661) in a man- genes such as p21 and BAX (see Figure 16.16). ner that stimulates apoptosis. For example, p53 can bind to Bax Some tumor cells have been found that contain a wild‐type proteins at the outer mitochondrial membrane, directly trigger- TP53 gene but extra copies of MDM2. Such cells are thought to ing membrane permeabilization and release of apoptotic factors. produce excessive amounts of MDM2, which prevents p53 from If both alleles of TP53 should become inactivated, a cell that is building to required levels to stop the cell cycle or induce apopto- carrying damaged DNA fails to be destroyed, even though it lacks sis following DNA damage (or other oncogenic stimuli). A major Repair Division with damage before division (mutation, aneuploidy) DNA p53 level DNA No p53 Tumor damage rises damage No G1 G1 arrest arrest or apoptosis Mitotic failure and cell death (a) (b) (c) FIGURE 16.14 A model for the function of p53. (a) Cell division does not normally require the involvement of p53. (b) If, however, the DNA of a cell becomes damaged as the result of exposure to mutagens, the level of p53 rises and acts either to arrest the progression of the cell through G1 or to direct the cell toward apoptosis. (c) If both copies of the TP53 gene are inactivated, the cell loses the ability to arrest the cell cycle or commit the cell to apoptosis following DNA damage. As a result, the cell either dies from mitotic failure or continues to proliferate with genetic abnormali- ties that may lead to the formation of a malignant growth. SOURCE: (a–c) D. P. Lane, reprinted with permission from Nature 358:15, 1992; copyright 1992. Nature by Nature Publishing Group. Reproduced with permission of Nature Publishing Group in the format reuse in a book/textbook via Copyright Clearance Center. karp_c16.indd 685 10/14/2017 5:25:23 PM  686 CHAPTER 16 Cancer Untreated 5-Fluorouracil Etoposide Adriamycin (+/+) (+/–) (–/–) FIGURE 16.15 Experimental demonstration of the role of p53 in the survival of cells treated with chemotherapeutic agents. Cells were cultured from mice that had two functional alleles of the gene encoding p53 (top row), one functional allele of the gene (middle row), or were lacking a functional allele of the gene (bottom row). Cultures of each of these cells were grown either in the absence of a chemotherapeutic agent (first column) or in the presence of one of the three compounds indicated at the top of the other three columns. It is evident that the compounds had a dramatic effect on arresting growth and inducing cell death (apoptosis) in normal cells, whereas the cells lacking p53 continued to proliferate in the presence of these compounds. SOURCE: From Scott W. Lowe, H. E. Ruley, T. Jacks, and D. E. Housman, Cell 74:959, 1993, with permission from Elsevier. effort is underway to develop drugs that block the interaction become apoptotic—as long as they possess a functioning TP53 between MDM2 and p53 in an attempt to restore p53 activity in gene. If cancer cells lose p53 function, they often cannot be cancer cells that retain this key tumor suppressor. The relation- directed into apoptosis and they become highly resistant to fur- ship between MDM2 and p53 has also been demonstrated using ther treatment (FIGURE 16.15). This may be the primary reason gene knockouts. Mice that lack a gene encoding MDM2 die at an why tumors that typically lack a functional TP53 gene (e.g., colon early stage of development, presumably because their cells cancer, prostate cancer, and pancreatic cancer) respond much undergo p53‐dependent apoptosis. This interpretation is sup- more poorly to radiation and chemotherapy than tumors that ported by the finding that mice lacking genes that encode both possess a wild‐type copy of the gene (e.g., testicular cancer and MDM2 and p53 (double knockouts) survive to adulthood but are childhood acute lymphoblastic leukemias). highly prone to cancer. Because these embryos cannot produce p53, they don’t require a protein such as MDM2 that facilitates p53 destruction. This observation illustrates an important princi- The Role of p53 in Promoting ple in cancer genetics: even if a “crucial” gene such as RB or TP53 is not mutated or deleted, the function of that gene can be affected Senescence as the result of alterations in other genes whose products are part We have seen how p53 can direct a potential cancer cell into either of the same pathway as the “crucial” gene. In this case, overexpres- growth arrest or apoptosis. Recent studies indicate that p53 also sion of MDM2 can have the same effect as the absence of p53. As controls signaling pathways that lead to cellular senescence, long as the tumor‐suppressor pathway is blocked, the tumor‐ another mechanism that has evolved as a barrier that stops way- suppressor gene itself need not be mutated. Numerous studies indi- ward cells from developing into malignant tumors. Unlike apop- cate that both the p53 and pRB pathways have to be inactivated, totic cells, senescent cells can remain alive and metabolically one way or another, to allow the progression of most tumor cells. active but are permanently arrested in a nondividing state, as Because of its ability to trigger apoptosis, p53 plays a pivotal exemplified by the senescent melanocytes found in moles (dis- role in treatment of cancer by radiation and chemotherapy. It was cussed on page 680). In other cases, senescent cells may be ingested assumed for many years that cancer cells are more susceptible by phagocytic immune cells. Senescence can be triggered in an than normal cells to drugs and radiation because cancer cells otherwise normal cell by the experimental activation of an onco- divide more rapidly. But some cancer cells divide more slowly gene, such as Ras, which might occur with some frequency during than their normal counterparts, yet they are still more sensitive the day‐to‐day activities of dividing cells in a normal tissue. to drugs and radiation. An alternate theory suggests that normal Studies suggest that oncogene activation triggers a period of accel- cells are more resistant to drugs or radiation because, once they erated division after which the senescence program takes effect sustain genetic damage, they either arrest their cell cycle until the and slams on the brakes. This is the apparent route that is taken damage is repaired or they undergo apoptosis. In contrast, can- during the formation of benign moles. One of the pathways lead- cer cells that have sustained DNA damage are more likely to ing to senescence involves expression of a tumor‐suppressor gene karp_c16.indd 686 10/14/2017 5:25:23 PM  16.7 Other Tumor‐Suppressor Genes 687 called INK4a, which is often disabled in human cancers mitotic chromosomes. Loss of APC function could therefore lead (Table 16.1). INK4a encodes two separate tumor‐suppressor pro- directly to abnormal chromosome segregation and aneuploidy teins (proteins that are translated in alternate reading frames of (page 602). The presence of mutated APC DNA has been found the mRNA): p16, which is an inhibitor of cyclin‐dependent in the blood of persons with early‐stage colon cancer, which kinases required for progression through the cell cycle, and ARF, raises the possibility of a diagnostic test for the disease. which stabilizes p53 by inhibiting MDM2. The precise role of p53 It is estimated that breast cancer strikes approximately one in in directing cells toward the senescent state remains unclear, but eight women living in the United States, Canada, and Europe. Of inactivation of the TP53 gene within senescent cells can cause the these cases, 5–10 percent are due to the inheritance of a gene that cells to resume their progress toward full malignancy. predisposes the individual to development of the disease. After an Whether p53 moves a cell toward cell cycle arrest, apoptosis, intensive effort by several laboratories, two genes named BRCA1 or senescence apparently depends on the type of posttransla- and BRCA2 were identified in the mid‐1990s as being responsible tional modifications to which it is subjected. As in the case of the for the majority of the inherited cases of breast cancer. BRCA core histones (page 230), modifications include phosphoryla- mutations also predispose a woman to the development of ovar- tion, acetylation, methylation, and ubiquitination and affect ian cancer, which has an especially high mortality rate. more than three dozen residues within the p53 molecule. It was pointed out in Section 14.4, that cells possess check- Moreover, the fact (1) that the TP53 transcript can be alternately points that halt progression of the cell cycle following DNA dam- spliced into numerous p53 isoforms, (2) that these p53 proteins age. The BRCA proteins are part of one or more large protein can interact with a host of different proteins, and (3) that p53 has complexes that respond to DNA damage and activate DNA repair been found to influence several other major tumor‐related path- by means of homologous recombination. Cells with mutant BRCA ways (e.g., DNA repair, glucose metabolism, and autophagy) add proteins accumulate chromosomal breaks and exhibit a highly additional layers of complexity to the p53 story. Dissecting the aneuploid karyotype. In cells with a functional TP53 gene, failure roles of these various factors in the function of this “multitask- to repair DNA damage leads to the activation of p53, which causes ing” protein will be a daunting challenge. the cell to either arrest cell cycle progress or undergo apoptosis, as illustrated in FIGURE 16.16. We have seen in this chapter that apoptosis is one of the body’s primary mechanisms of ridding itself of potential tumor 16.7 Other Tumor‐Suppressor cells. The mechanism of apoptosis was discussed in the last Genes chapter, as were pathways that promoted cell survival rather than cell destruction. The best studied cell‐survival pathway Although mutations in RB and TP53 are associated with a wide involves the activation of a kinase called PKB (AKT) by the variety of human malignancies, mutations in a number of other phosphoinositide PIP3. PIP3, in turn, is formed by the catalytic tumor‐suppressor genes are detected in only a few types of activity of the lipid kinase PI3K (see Figure 15.25). Activation of cancer. the PI3K/PKB pathway leads to an increased likelihood that a Familial adenomatous polyposis coli (FAP) is an inherited cell will survive a stimulus that normally would lead to its disorder in which individuals develop hundreds or even thou- destruction. Whether a cell lives or dies following a particular sands of premalignant polyps (adenomas) from epithelial cells event depends to a large degree on the balance between proap- that line the colon wall. If not removed, cells within some of these optotic and antiapoptotic signals. Mutations that affect this bal- polyps are very likely to progress to a fully malignant stage. The ance, such as those that contribute to the overexpression of PKB cells of patients with this condition were found to contain a dele- or PI3K, can shift this balance in favor of cell survival, which can tion of a small portion of chromosome 5, which was subsequently provide a potential cancer cell with a tremendous advantage. identified as the site of a tumor‐suppressor gene called adenoma- Another protein that can affect the balance between life and tous polyposis coli, or APC. A person inheriting an APC deletion death of a cell is the lipid phosphatase, PTEN, which removes is in a similar position to one who inherits an RB deletion: If the the phosphate group from the 3‐position of PIP3, converting the second allele of the gene is mutated in a given cell, the protective molecule into PI(4,5)P2, which cannot activate PKB. Cells in value of the gene function is lost. The loss of the second allele of which both copies of the PTEN gene are inactivated tend to have APC causes the cell to lose growth control and proliferate to form an excessively high level of PIP3, which leads to an overactive a polyp rather than differentiating into normal epithelial cells of population of PKB molecules. When a normal PTEN gene is the intestinal wall. The conversion of cells in a polyp to the more introduced into tumor cells that lack a functioning copy of the malignant state, characterized by the ability to metastasize and gene, the cells typically undergo apoptosis, as would be expected. invade other tissues, is presumably gained by the accumulation of Like the other tumor‐suppressor genes listed in Table 16.1, additional mutations, including those in TP53 (see Figure 16.20). mutations in PTEN cause a rare hereditary disease characterized Mutated APC genes are found not only in inherited forms of by an increased risk of cancer, and such mutations are also found colon cancers, but also in the majority of sporadic colon tumors, in a variety of sporadic cancers. suggesting that the gene plays a major role in the development of Mutation or deletion is not the only mechanism by which this disease. APC is known to suppress the Wnt pathway, which tumor‐suppressor genes can be inactivated. Tumor suppressor activates the transcription of genes (e.g., MYC and CCND1) that genes, such as BRCA1 or PTEN, are often rendered nonfunctional promote cell proliferation. APC has also been identified as a as the result of epigenetic mechanisms, such as DNA methylation microtubule plus‐end binding protein and is believed to play a or histone modification, which silences transcription of the gene role in the attachment of microtubules to the kinetochores of (Section 6.16). karp_c16.indd 687 10/14/2017 5:25:23 PM  688 CHAPTER 16 Cancer and metabolism (see Figure 16.18).3 Oncogenic RAS mutants typically encode a protein whose GTPase activity cannot be stim- 1 DNA damage ulated, which leaves the molecule in an active GTP‐bound form, sending continuous proliferation signals along the pathway. Despite extensive efforts to develop anti‐RAS strategies for can- cer therapy, no drugs that block RAS function have yet to be BRCA1 2b 2a approved. The functions of a number of oncogenes are summa- _ + rized in FIGURE 16.17 and discussed below.4 X BRCA1 X BRCA2 BRCA2 Failed Repair Oncogenes That Encode Growth repair Factors or Their Receptors 3a Checkpoint activation The first connection between oncogenes and growth factors was P made in 1983, when it was discovered that the cancer‐causing sim- p53 MDM2 ian sarcoma virus contained an oncogene (sis) derived from the 3b 4a 4b 3 The human genome actually contains three different RAS genes and three dif- p21 ferent RAF genes that are active in different tissues. Of these, KRAS and BRAF are Bax most often implicated in tumor formation. 4 The reader is referred to the Human Perspective of Chapter 11 for a discussion Cell cycle arrest of genes that encode cell‐surface molecules and extracellular proteases that play Apoptosis an important role in tissue invasion and metastasis. FIGURE 16.16 DNA damage initiates activity of a number 1 of proteins encoded by both tumor‐suppressor genes and proto‐ Growth factors oncogenes. In this simplified figure, DNA damage is seen to cause e.g., PDGF, EGF 2 double‐strand breaks in the DNA (step 1) that are repaired by a Growth factor receptors e.g., EGF receptor (HER2) proposed multiprotein complex that includes BRCA1 and BRCA2 (step 2a). Mutations in either of the genes that encode these proteins can block the repair process (step 2b). If DNA damage is not repaired, a checkpoint is activated that leads to a rise in the level of p53 activity SRC (step 3a). The p53 protein is normally inhibited by interaction with the RAS 3 RAF protein MDM2 (step 3b). p53 is a transcription factor that activates Protein kinases or proteins expression of either (1) the p21 gene (step 4a), whose product (p21) that activate protein kinases causes cell cycle arrest, or (2) the BAX gene (step 4b), whose product (Bax) causes apoptosis. p53 activation can also promote cellular 7 Metabolic senescence, but the pathway is unclear. enzymes e.g. IDH1 Cyclin SOURCE: Reprinted with permission after J. Brugarolas and T. Jacks, CDK 4 Nature Med 3:721, 1997, copyright 1997, Nature Medicine by Nature Proteins that IC α KG Publishing Group. Reproduced with permission of Nature Publishing control cell cycle Group in the format reuse in a book/textbook via Copyright Clearance 2–HG e.g., CYCLIN D1, Center. CDK2 8 Proteins that 5 affect apoptosis 6 e.g., BCL-2 Transcription 16.8 Oncogenes BCL-2 factors e.g., MYC, HIF As described above, oncogenes encode proteins that promote the loss of growth control and the conversion of a