1. Cancer 19

2. 19 Cancer The Development and Causes of Cancer Tumor Viruses Oncogenes Tumor Suppressor Genes Molecular Approaches to Cancer Treatment

3. Introduction Cancer results from a breakdown of the regulatory mechanisms that govern normal cell behavior. Cancer cells grow and divide in an uncontrolled manner, spreading throughout the body and interfering with the function of normal tissues and organs.

4. Introduction Cancer ultimately has to be understood at the molecular and cellular levels. Studies of cancer cells have also illuminated the mechanisms that regulate normal cell behavior.

5. The Development and Causes of Cancer The loss of growth control exhibited by cancer cells is the net result of accumulated abnormalities in multiple cell regulatory systems. It is reflected in several aspects of cell behavior that distinguish cancer cells from their normal counterparts.

6. The Development and Causes of Cancer There are more than 100 types of cancer. A tumor is any abnormal proliferation of cells. Benign tumors remain confined to the original location, neither invading surrounding normal tissue nor spreading to distant body sites.

7. The Development and Causes of Cancer A malignant tumor can invade surrounding normal tissue and spread throughout the body via the circulatory or lymphatic systems (metastasis). Only malignant tumors are properly referred to as cancers.

8. Figure 19.1 A cancer of the pancreas

9. The Development and Causes of Cancer Most cancers are in three main groups: Carcinomas—malignancies of epithelial cells (about 90% of human cancers). Sarcomas—solid tumors of connective tissue such as muscle, bone, cartilage, and fibrous tissue (rare in humans).

10. The Development and Causes of Cancer Leukemias and lymphomas arise from the blood-forming cells and immune system cells, respectively. Tumors are further classified according to tissue of origin and type of cell involved.

11. The Development and Causes of Cancer Only a few types of cancer occur frequently. The four most common cancers are prostate, breast, lung, and colon/rectum. Lung cancer, by far the most lethal, is responsible for nearly 30% of all cancer deaths.

12. Table 19.1 Most Frequent Cancers in the United States

13. The Development and Causes of Cancer A fundamental feature of cancer is tumor clonality—tumors develop from single cells that begin to proliferate abnormally. The single-cell origin has been demonstrated by analysis of X chromosome inactivation patterns.

14. Figure 19.2 Tumor clonality

15. The Development and Causes of Cancer The development of cancer is a multistep process: cells gradually become malignant through a progressive series of alterations. One indication of this is that most cancers develop late in life. Most cancers develop as a consequence of multiple abnormalities, which accumulate over many years.

16. Figure 19.3 Increased rate of cancer with age

17. The Development and Causes of Cancer At the cellular level, development of cancer is a multistep process: Mutation and selection for cells with progressively increasing capacity for proliferation, survival, invasion, and metastasis.

18. The Development and Causes of Cancer Tumor initiation: mutation leads to abnormal proliferation of a single cell, which grows into a population of clonal tumor cells. Tumor progression: additional mutations occur within cells of the tumor population.

19. Figure 19.4 Stages of tumor development

20. The Development and Causes of Cancer Tumor cells have high frequency of mutations and chromosome abnormalities. Some mutations may confer selective advantage, such as rapid growth.

21. The Development and Causes of Cancer Clonal selection: Descendents of these cells become dominant. Clonal selection continues throughout tumor development, so tumors continuously become more rapid- growing and increasingly malignant.

22. The Development and Causes of Cancer Colon carcinoma is an example of tumor progression. Proliferation of colon epithelial cells gives rise to a small benign neoplasm (an adenoma or polyp). Clonal selection leads to growth of adenomas of increasing size and proliferative potential.

23. The Development and Causes of Cancer Malignant carcinomas arise from the benign adenomas; tumor cells invade the underlying connective tissue. Eventually the cancer cells penetrate the colon walls and invade other abdominal organs, blood, and lymphatic vessels.

24. Figure 19.5 Development of colon carcinomas (Part 1)

25. Figure 19.5 Development of colon carcinomas (Part 2)

26. The Development and Causes of Cancer Carcinogens are substances that cause cancer. Radiation and many chemical carcinogens damage DNA and induce mutations: Solar ultraviolet radiation—the major cause of skin cancer. Aflatoxin—produced by some molds that contaminate peanuts and stored grains.

27. The Development and Causes of Cancer Chemicals in tobacco smoke include benzo( α )pyrene, dimethylnitrosamine, and nickel compounds. It is estimated that smoking is responsible for nearly one-third of all cancer deaths.

28. Figure 19.6 Structure of representative chemical carcinogens

29. The Development and Causes of Cancer Other carcinogens are tumor promoters that stimulate cell proliferation. Hormones, particularly estrogens, are tumor promoters in some human cancers. Exposure to excess estrogen significantly increases the likelihood that a woman will develop uterine cancer.

30. The Development and Causes of Cancer Asbestos The bacterium Heliobacter pylori causes stomach cancer. Some viruses cause cancer, including liver cancer and cervical carcinoma. Studies of tumor viruses have helped elucidate molecular events in the development of all types of cancers.

31. The Development and Causes of Cancer Cancer cells have characteristic properties that distinguish them from normal cells and contribute to malignancy: 1. Uncontrolled proliferation 2. Reduced dependence on growth factors 3. Reduced cell adhesion molecules 4. Secretion of proteases

32. The Development and Causes of Cancer 5. Promotion of angiogenesis 6. Abnormal differentiation 7. Failure to undergo apoptosis 8. Capacity for unlimited replication

33. The Development and Causes of Cancer 1. In culture, normal cells display density-dependent inhibition: They proliferate until reaching a finite cell density, determined partly by availability of growth factors. They then cease proliferating and are arrested in the G 0 stage of the cell cycle.

34. The Development and Causes of Cancer Cancer cells are not restricted by growth factor availability or cell–cell contact. They don’t respond to the signals that cause normal cells to cease proliferation, but grow to high densities in culture.

35. Figure 19.7 Density-dependent inhibition

36. The Development and Causes of Cancer Normal fibroblasts show contact inhibition: They migrate across a culture dish until making contact with a neighboring cell. Normal cells adhere to each other, forming an orderly array. Tumor cells continue moving after contact, migrating over adjacent cells, growing in disordered, multilayered patterns.

37. Figure 19.8 Contact inhibition

38. The Development and Causes of Cancer 2. Many cancer cells can grow in the absence of growth factors required by normal cells. Some cancer cells produce growth factors that stimulate their own proliferation (autocrine growth stimulation).

39. Figure 19.9 Autocrine growth stimulation

40. The Development and Causes of Cancer Reduced growth factor dependence can also result from abnormalities in intracellular signaling systems. Example: unregulated activity of growth factor receptors or other proteins (e.g., Ras proteins or protein kinases).

41. The Development and Causes of Cancer 3. Cancer cells are less regulated by cell– cell and cell–matrix interactions. Most cancer cells are less adhesive than normal cells, due to reduced expression of cell surface adhesion molecules. Loss of E-cadherin is important in development of carcinomas (epithelial cancers).

42. The Development and Causes of Cancer Reduced adhesion molecules make cancer cells less restrained by interactions with other cells and the matrix, contributing to their ability to invade and metastasize. Many tumor cells are rounder than normal; they are less firmly attached to either the matrix or neighboring cells.

43. The Development and Causes of Cancer 4. Cancer cells secrete proteases that digest extracellular matrix components, allowing them to invade adjacent normal tissues. Example: Proteases that digest collagen allow carcinomas to penetrate the basal laminae and invade underlying connective tissue.

44. The Development and Causes of Cancer 5. Cancer cells secrete growth factors that promote formation of new blood vessels (angiogenesis). When a tumor reaches about a million cells, new blood vessels are needed to supply oxygen and nutrients. The new capillaries are easily penetrated by tumor cells, contributing to metastasis.

45. The Development and Causes of Cancer 6. Most cancer cells don’t differentiate normally. This is coupled to abnormal proliferation; normal cells cease cell division once fully differentiated. Cancer cells are blocked at an early stage of differentiation.

46. The Development and Causes of Cancer Example: Leukemias All types of blood cells are derived from stem cells in the bone marrow. Once they are fully differentiated, cell division ceases. Leukemic cells don’t undergo terminal differentiation; they are arrested at early stages and retain the capacity for proliferation.

47. Figure 19.10 Defective differentiation and leukemia

48. The Development and Causes of Cancer Growth of leukemias, and some solid tumors, may be driven by proliferation of a subpopulation of cancer stem cells, rather than proliferation of all cells in the tumor. Chronic myeloid leukemia arises from oncogenic transformation of the hematopoietic stem cells.

49. The Development and Causes of Cancer 7. Many cancer cells fail to undergo programmed cell death or apoptosis and have longer life spans than normal cells. Tumor cells are often able to survive in the absence of growth factors required by normal cells.

50. The Development and Causes of Cancer Normal cells also undergo apoptosis following DNA damage, while many cancer cells do not. This contributes to resistance of cancer cells to irradiation and many chemotherapeutic drugs, which act by damaging DNA.

51. The Development and Causes of Cancer 8. Normal cells have limited amounts of telomerase and gradually lose telomeres, leading to cessation of replication. Cancer cells express high levels of telomerase, allowing them to maintain chromosome ends for an indefinite number of divisions.

52. The Development and Causes of Cancer Study of tumor induction was advanced by development of in vitro assays to detect cell transformation— conversion of normal cells to tumor cells. Assays are designed to detect transformed cells, which display the in vitro growth properties of tumor cells.

53. The Development and Causes of Cancer Focus assay: Based on the ability to recognize a group of transformed cells as a morphologically distinct “focus” against a background of normal cells on the surface of a culture dish.

54. Figure 19.11 The focus assay

55. The Development and Causes of Cancer The focus assay takes advantage of three properties of transformed cells: Altered morphology, loss of contact inhibition, and loss of density- dependent inhibition of growth.

56. The Development and Causes of Cancer Cells transformed in vitro can form tumors following inoculation into susceptible animals. This supports in vitro transformation as a valid indicator of the formation of cancer cells.

57. Tumor Viruses Tumor viruses can directly cause cancer in humans or animals. They have played a critical role in cancer research by serving as models for cell transformation. They have small genomes, allowing identification of viral genes responsible for cancer induction.

58. Table 19.2 Tumor Viruses

59. Tumor Viruses Hepatitis B and C viruses are the main causes of liver cancer. The viruses infect liver cells and can lead to long-term chronic infections, associated with a high risk of cancer. The molecular mechanisms by which they cause cancer are not well understood.

60. Tumor Viruses A viral protein may interact with p53 protein. The chronic tissue damage and inflammation in hepatitis B results in continual proliferation of liver cells, and may contribute to tumor development.

61. Tumor Viruses Small DNA tumor viruses: Papillomaviruses: About 100 different types infect epithelial cells. Some cause benign tumors (e.g., warts); others cause malignant carcinomas, particularly cervical and other anogenital cancers.

62. Tumor Viruses Early detection and treatment of cervical cancer is made possible by the Pap smear test. The cancer cells are easily detected by microscopic examination. Vaccines have also been developed for cervical cancer viruses.

63. Tumor Viruses Polyomavirus Merkel cell polyomavirus was identified in 2008 as the cause of a rare skin cancer, Merkel cell carcinoma. SV40 and the adenoviruses don’t cause human cancers, but have been important as models for understanding cell transformation.

64. Tumor Viruses Replication of these viruses leads to host cell lysis and release of progeny virus particles. But if viral replication is blocked, expression of specific viral genes results in transformation of the infected cell.

65. Figure 19.12 Polyomavirus replication and transformation

66. Tumor Viruses Genes that lead to cell transformation are the same genes that function in early stages of lytic infection. Early region genes (expressed right after infection) stimulate host gene expression and DNA synthesis. This can lead to transformation if the viral DNA becomes stably integrated.

67. Tumor Viruses Cell transformation by human papillomaviruses results from expression of early-region genes E6 and E7. The proteins bind and inactivate host cell tumor suppressor proteins Rb and p53. The transforming proteins of SV40 and adenoviruses similarly target Rb and p53.

68. Figure 19.13 The genome of a human papillomavirus

69. Tumor Viruses Herpesviruses are among the most complex animal viruses, with genomes of 100 to 200 kb. Kaposi’s sarcoma-associated herpesvirus and Epstein-Barr virus cause human cancers.

70. Tumor Viruses Epstein-Barr virus can also transform human B lymphocytes in culture, but the mechanisms are not fully understood. The transforming protein (LMP1) mimics a surface receptor on B lymphocytes and activates signaling pathways that stimulate proliferation and inhibit apoptosis.

71. Tumor Viruses Kaposi’s sarcoma cells secrete cytokines and growth factors that drive tumor development. Interestingly, the transforming proteins of the virus appear to act at least in part by stimulating growth factor secretion.

72. Tumor Viruses Retroviruses cause cancer in many animals, including humans. Human T-cell lymphotropic virus type I (HTLV-I) causes adult T-cell leukemia.

73. Tumor Viruses AIDS is caused by the retrovirus HIV. HIV does not cause cancer directly but AIDS patients have a high incidence of malignancies, particularly lymphomas and Kaposi’s sarcoma. They are associated with infection by other viruses and develop as a consequence of immunosuppression.

74. Tumor Viruses Most retroviruses contain only three genes (gag, pol, and env) that are required for virus replication but play no role in cell transformation. Retroviruses of this type induce tumors only rarely, if at all.

75. Figure 19.14 A typical retrovirus genome

76. Tumor Viruses Other retroviruses contain genes that induce cell transformation and are potent carcinogens. Rous sarcoma virus (RSV) is the prototype of these highly oncogenic retroviruses. Studies of RSV led to identification of the first viral oncogene.

77. Oncogenes Oncogenes are specific genes that can induce cell transformation. Studies of viral oncogenes led to identification of cellular oncogenes involved in development of non-virus- induced cancers.

78. Oncogenes RSV transforms chicken embryo fibroblasts in culture and induces sarcomas. The closely related avian leukosis virus (ALV) replicates in the same cells without inducing transformation. This suggested that RSV contains specific genetic information for transformation.

79. Figure 19.15 Cell transformation by RSV and ALV

80. Oncogenes In the 1970s, studies of RSV mutants revealed a single gene responsible for RSV tumor induction. Because RSV causes sarcomas, the oncogene was called src. It is not present in ALV. It encodes the first tyrosine kinase to be identified.

81. Figure 19.16 The RSV and ALV genomes

82. Oncogenes More than 40 oncogenic retroviruses have been isolated. All have at least one oncogene that is not required for replication but is responsible for cell transformation. Many oncogenes encode components of signaling pathways that stimulate cell proliferation.

83. Table 19.3 Representative Retroviral Oncogenes

84. Oncogenes Most viruses are streamlined to replicate as efficiently as possible, so the existence of viral oncogenes not involved in replication was surprising. Research into the origin of these genes led to identification of cellular oncogenes in human cancers.

85. Oncogenes A clue to the origin of oncogenes came from isolation of Abelson leukemia virus from mice that had been injected with nontransforming virus. One mouse developed a lymphoma from which a highly oncogenic virus was isolated. It contained the oncogene abl.

86. Figure 19.17 Isolation of Abelson leukemia virus

87. Oncogenes This suggested the hypothesis that retroviral oncogenes are derived from genes of the host cell. Thus, normal cells must contain genes that are closely related to the retroviral oncogenes.

88. Oncogenes This was demonstrated in 1976 by Varmus and Bishop, who showed that a cDNA probe for src hybridized to closely- related sequences in DNA of normal chicken cells. src-related sequences were also found in normal DNAs in many vertebrates and appeared to be highly conserved in evolution.

89. Key Experiment, Ch. 19, p. 740 (3)

90. Oncogenes Proto-oncogenes: normal-cell genes from which retroviral oncogenes originate. They often encode proteins in the signaling pathways that control normal cell proliferation (e.g., src, ras, raf ). Oncogenes are abnormally expressed or mutated forms of the proto-oncogenes.

91. Oncogenes Retroviral oncogenes differ from proto- oncogenes: Transcription in viral oncogenes is controlled by viral promoters and enhancers. Viral oncogenes are usually expressed at much higher levels than proto-oncogenes, or in inappropriate cells.

92. Oncogenes Oncogenes often encode proteins that differ in structure and function from normal proteins. Oncogenes such as raf are expressed as fusion proteins with viral sequences at the amino terminus. Loss of regulatory domains generate proteins that function in an unregulated manner.

93. Figure 19.18 The Raf oncogene protein

94. Oncogenes Many oncogenes differ from the proto-oncogenes by point mutations, resulting in single amino acid substitutions, which can also lead to unregulated activity.

95. Oncogenes Evidence for involvement of cellular oncogenes in human tumors was found in gene transfer experiments in 1981. DNA from a human bladder carcinoma was found to induce transformation of mouse cells in culture, indicating that the tumor contained an oncogene.

96. Figure 19.19 Detection of a human tumor oncogene by gene transfer

97. Oncogenes Gene transfer assays and other experimental approaches have detected cellular oncogenes in many types of human tumors.

98. Table 19.4 Representative Oncogenes of Human Tumors

99. Oncogenes The first human oncogene identified was the homolog of the rasH oncogene of Harvey sarcoma virus. Three members of the ras gene family (rasH, rasK, and rasN) are the oncogenes most frequently encountered in human tumors.

100. Oncogenes ras oncogenes are not present in normal cells; they are generated in tumor cells from mutations during tumor development. They differ from their proto-oncogenes by point mutations resulting in single amino acid substitutions. The mutations are caused by chemical carcinogens.

101. Figure 19.20 Point mutations in ras oncogenes

102. Oncogenes ras genes encode guanine-binding proteins that function in transduction of mitogenic signals from many growth factor receptors. Ras proteins alternate between active (GTP-bound) and inactive (GDP- bound) states.

103. Oncogenes Mutations of ras oncogenes maintain the Ras proteins constitutively in the active GTP-bound conformation. Oncogenic Ras proteins don’t respond to GAP (GTPase-activating protein), resulting in decreased GTPase activity.

104. Oncogenes Many cancer cells have abnormal chromosome structure, including translocations, duplications, and deletions. These can lead to generation of oncogenes.

105. Oncogenes In Burkitt’s lymphoma, chromosome translocation inserts c-myc oncogene into an immunoglobulin locus, where it is expressed in an unregulated manner. c-myc encodes a transcription factor normally induced in response to growth factor stimulation.

106. Figure 19.21 Translocation of c-myc

107. Oncogenes Translocations often result in rearrangements of coding sequences and abnormal gene products. Example: Translocation of the abl proto- oncogene from chromosome 9 to chromosome 22 in chronic myeloid leukemia.

108. Figure 19.22 Translocation of abl (Part 1)

109. Oncogenes The translocation leads to fusion of abl with bcr, and production of a Bcr/Abl fusion protein. This results in unregulated activity of Abl tyrosine kinase, leading to cell transformation.

110. Figure 19.22 Translocation of abl (Part 2)

111. Oncogenes Oncogenes can be activated by gene amplification, resulting in elevated expression. Amplification is 1000 times more common in tumor cells, and may play a role in the progression of tumors to more rapid growth and increasing malignancy.

112. Oncogenes Oncogene proteins can play many roles in growth factor-stimulated signal transduction pathways. In the ERK pathway, oncogene proteins include polypeptide growth factors, growth factor receptors, intracellular signaling proteins, and transcription factors.

113. Figure 19.23 Oncogenes and the ERK signaling pathway

114. Oncogenes Many oncogenes encode growth factor receptors, mostly tyrosine kinases. These receptors can be converted to oncogene proteins by alterations of amino-terminal domains, which would normally bind extracellular growth factors.

115. Oncogenes The receptor for platelet-derived growth factor (PDGFR) is converted to an oncogene by a chromosome translocation and replacement of the amino terminus by a transcription factor called Tel.

116. Oncogenes Tel sequences of the Tel/PDGFR fusion protein dimerize in the absence of growth factor binding. This results in constitutive activity of the intracellular kinase domain and unregulated production of a proliferative signal from the oncogene protein.

117. Figure 19.24 Mechanism of Tel/PDGFR oncogene activation

118. Oncogenes Many oncogenes encode transcriptional regulatory proteins that are normally induced in response to growth factors. Transcription of the fos proto-oncogene is induced by phosphorylation of Elk-1 by ERK.

119. Oncogenes Fos and Jun dimerize to form AP-1 transcription factor, which activates transcription of cyclin D1.

120. Figure 19.25 The AP-1 transcription factor

121. Oncogenes The gene encoding cyclin D1 can become an oncogene (CCND1) by chromosome translocation or gene amplification. Constitutive expression of cyclin D1 drives cell proliferation in the absence of normal growth factor stimulation. Cdk4 is also activated as an oncogene by point mutations in melanomas.

122. Oncogenes Components of other signaling pathways can also act as oncogenes. Wnt proteins were identified as oncogenes in mouse breast cancers; mutations convert the downstream target of Wnt signaling, β -catenin, to an oncogene (CTNNB1) in human colon cancers.

123. Figure 19.26 Oncogenic activity of  -catenin (Part 1)

124. Figure 19.26 Oncogenic activity of  -catenin (Part 2)

125. Oncogenes Oncogenic activity of some transcription factors results from inhibition of cell differentiation. Mutated forms of thyroid hormone receptor (ErbA) and retinoic acid receptor (PML/RAR α ) are oncogene proteins in chicken erythroleukemia and human acute promyelocytic leukemia.

126. Oncogenes The mutated oncogene receptors interfere with the action of their normal homologs, blocking cell differentiation and maintaining the leukemic cells in an actively proliferating state.

127. Figure 19.27 Action of the PML/RAR  oncogene protein

128. Oncogenes Acute promyelocytic leukemia can be treated by retinoic acid, which induces differentiation and blocks continued cell proliferation.

129. Oncogenes Several oncogenes encode proteins that promote cell survival. Oncogenes that encode growth factors, growth factor receptors, and signaling proteins such as Ras, stimulate cell proliferation and also prevent cell death.

130. Oncogenes The PI 3-kinase/Akt signaling pathway helps prevent apoptosis of many cells. The genes encoding PI 3-kinase and Akt act as oncogenes in both retroviruses and human tumors.

131. Figure 19.28 Oncogenes and cell survival

132. Oncogenes bcl-2 oncogene increases expression of Bcl-2, which blocks apoptosis. Identification of bcl-2 as an oncogene demonstrated the significance of apoptosis in cancer development. It also led to discovery of the role of bcl-2 and related genes as central regulators of apoptosis.

133. Tumor Suppressor Genes Tumor suppressor genes normally inhibit cell proliferation and tumor development. In many tumors, these genes are lost or inactivated, contributing to abnormal proliferation of tumor cells.

134. Tumor Suppressor Genes Tumor suppression was first noticed during somatic cell hybridization experiments in 1969. Hybrids of normal and tumor cells were not capable of forming tumors, suggesting there were genes in the normal cell that suppressed tumors.

135. Figure 19.29 Suppression of tumorigenicity by cell fusion

136. Tumor Suppressor Genes The first tumor suppressor gene was identified in studies of retinoblastoma, an inherited childhood eye tumor. About 50% of children of an affected parent develop retinoblastoma, indicating a single dominant gene that confers susceptibility to tumor development.

137. Figure 19.30 Inheritance of retinoblastoma

138. Tumor Suppressor Genes One defective copy of the Rb tumor suppressor gene is inherited, but development of retinoblastoma requires a second somatic mutation leading to loss of the normal Rb allele. Noninherited retinoblastoma is thus very rare, since its development requires two independent somatic mutations.

139. Figure 19.31 Mutations of Rb during retinoblastoma development (Part 1)

140. Figure 19.31 Mutations of Rb during retinoblastoma development (Part 2)

141. Figure 19.31 Mutations of Rb during retinoblastoma development (Part 3)

142. Figure 19.31 Mutations of Rb during retinoblastoma development (Part 4)

143. Tumor Suppressor Genes The function of Rb as a negative regulator of tumorigenesis was indicated by chromosome morphology. Visible deletions of chromosome 13q14 were found in some retinoblastomas, suggesting that loss (rather than activation) of the Rb gene led to tumor development.

144. Figure 19.32 Rb deletions in retinoblastoma

145. Tumor Suppressor Genes Rb is involved in other tumors; it is lost or inactivated in many bladder, breast, and lung carcinomas. Also, oncogene proteins of several DNA tumor viruses, including SV40, adenoviruses, and human papillomaviruses, bind to Rb and inhibit its activity.

146. Figure 19.33 Interaction of Rb proteins with oncogene proteins of DNA tumor viruses

147. Tumor Suppressor Genes Additional tumor suppressor genes have now been identified. Mutations of some tumor suppressor genes appear to be the most common molecular alterations leading to human tumor development.

148. Table 19.5 Representative Tumor Suppressor Genes

149. Tumor Suppressor Genes p53 was the second tumor repressor gene identified. It is inactivated in many cancers, including leukemias, lymphomas, sarcomas, brain tumors, and carcinomas. Mutations of p53 play a role in about 50% of all cancers.

150. Tumor Suppressor Genes The proteins encoded by most tumor suppressor genes inhibit cell proliferation or survival. In many cases, tumor suppressor proteins inhibit the same cell regulatory pathways that are stimulated by the products of oncogenes.

151. Tumor Suppressor Genes The PTEN tumor suppressor gene encodes a lipid phosphatase that dephosphorylates PIP 3. This inhibits PI 3-kinase and Akt, which can both act as oncogenes by promoting cell survival, and stimulating cell proliferation.

152. Figure 19.34 Suppression of cell proliferation and survival by PTEN

153. Tumor Suppressor Genes Several tumor suppressor genes encode transcriptional regulatory proteins. Smad2 and Smad4 encode transcription factors that are activated by TGF- β signaling and lead to inhibition of cell proliferation. The TGF- β receptor is also encoded by a tumor suppressor gene (T β RII).

154. Tumor Suppressor Genes Products of Rb and INK4 tumor suppressor genes regulate the cell cycle at the point also affected by cyclin D1 and Cdk4, which can both act as oncogenes. Rb inhibits passage through the restriction point in G 1. INK4 encodes the Cdk inhibitor p16.

155. Tumor Suppressor Genes Mutational inactivation of Rb in tumors thus removes a key negative regulator of cell cycle progression. p16 inhibits Cdk4, 6/cyclin D activity. Inactivation of INK4 leads to elevated activity of Cdk4, 6/cyclin D, resulting in uncontrolled phosphorylation of Rb.

156. Figure 19.35 Inhibition of cell cycle progression by Rb and p16

157. Tumor Suppressor Genes p53 gene product regulates both cell cycle progression and apoptosis. DNA damage leads to induction of p53, which activates transcription of both proapoptotic and cell cycle inhibitory genes.

158. Figure 19.36 Action of p53

159. Tumor Suppressor Genes Cells lacking p53 fail to undergo apoptosis in response to DNA damage, leading to increased mutation frequencies. Loss of p53 also interferes with apoptosis induced by other stimuli, such as growth factor deprivation and oxygen deprivation.

160. Tumor Suppressor Genes BRCA1 and BRCA2 genes (responsible for some inherited breast and ovarian cancers) are stability genes that maintain the integrity of the genome. Mutations and inactivation of stability genes leads to high mutation frequency in oncogenes or tumor suppressor genes.

161. Tumor Suppressor Genes MicroRNAs (miRNAs) are major regulators of gene expression in eukaryotes. They act post-transcriptionally, to inhibit translation and/or induce mRNA degradation, contributing to regulation of about half of protein-coding genes.

162. Tumor Suppressor Genes Expression of miRNAs is low in tumors, suggesting they may act as tumor suppressors. Example: let-7 targets oncogenes including rasK and c-myc.

163. Figure 19.37 Action of let-7 miRNA as a tumor suppressor

164. Tumor Suppressor Genes Other miRNAs can act as oncogenes. Example: The miRNAs designated miR- 17-92 are amplified in some tumors and target mRNAs encoding proteins that inhibit cell cycle progression or promote apoptosis.

165. Tumor Suppressor Genes Development of cancer is a multistep process in which normal cells gradually progress to malignancy. Accumulated damage to multiple genes eventually results in increased proliferation, survival, invasiveness, and metastatic potential.

166. Tumor Suppressor Genes Large-scale genome sequencing has been used to analyze mutations in oncogenes and tumor suppressor genes in thousands of individual cancers. About 150 mutated genes have been identified in tumors, but only subsets are involved in cancers of any particular type.

167. Tumor Suppressor Genes Genes consistently mutated in colon cancers include rasK and PI3K oncogenes, and APC and p53 tumor suppressor genes. Breast cancers have frequent mutations in p53 and PI3K.

168. Figure 19.38 Genetic alterations in colorectal and breast carcinomas (Part 1)

169. Figure 19.38 Genetic alterations in colorectal and breast carcinomas (Part 2)

170. Tumor Suppressor Genes Different mutations can affect the same signaling pathway. Example: in colon cancer, mutations in either rasK or B-raf stimulate ERK signaling in the tumor cells. (Raf is downstream of Ras).

171. Tumor Suppressor Genes The large number of different mutations in tumors affect a much smaller number of complementary pathways that regulate cell proliferation, survival, and genome stability.

172. Figure 19.39 Pathways affected by human oncogenes and tumor suppressor genes

173. Molecular Approaches to Cancer Treatment Much has been learned about the molecular defects responsible for the development of cancers. Advances in molecular biology contribute to development of new approaches to cancer prevention and treatment.

174. Molecular Approaches to Cancer Treatment The most effective way to deal with cancer is to prevent development of the disease. Second-best is reliable early detection of premalignant stages that can be treated easily.

175. Molecular Approaches to Cancer Treatment Many cancers can be cured by localized treatment, before they metastasize. Early stages of colon cancer (adenomas) are usually curable by minor surgical procedures.

176. Figure 19.40 Survival rates of patients with colon carcinoma

177. Molecular Approaches to Cancer Treatment Molecular biology can be used to identify individuals with inherited susceptibilities to cancer development. Mutations in tumor suppressor genes, oncogenes (ret and cdk4), stability genes (BRCA1 and BRCA2), and others, can be detected with genetic testing.

178. Molecular Approaches to Cancer Treatment Some patients at high risk may choose prophylactic surgery to prevent cancer from developing. Monitoring of high-risk individuals can allow early detection and treatment.

179. Molecular Approaches to Cancer Treatment Most cancer drugs damage DNA or inhibit DNA replication and are toxic to normal cells as well, especially cells that are continually replaced by division of stem cells (hematopoietic cells, epithelial cells of the gastrointestinal tract, hair follicle cells)

180. Molecular Approaches to Cancer Treatment One new approach uses drugs that interfere with angiogenesis (blood vessel formation) or disrupt tumor blood vessels. These drugs block proliferation of endothelial cells, and are less toxic to normal cells.

181. Molecular Approaches to Cancer Treatment Some drugs are targeted specifically against oncogenes. This is tricky, since proto-oncogenes play important roles in normal cells and can be targeted also. But several selective oncogene-targeted therapies have been developed.

182. Table 19.6 Representative Oncogene-Targeted Therapies

183. Molecular Approaches to Cancer Treatment The first of these therapies was used to treat acute promyelocytic leukemia. The gene for retinoic acid receptor (RAR α ) is fused with PML to form the PML/RAR α oncogene; the resulting protein blocks cell differentiation. High doses of retinoic acid inactivates the PML/RAR α protein.

184. Molecular Approaches to Cancer Treatment Monoclonal antibody treatments have been developed against extracellular targets, such as growth factors or cell surface receptors.

185. Molecular Approaches to Cancer Treatment Herceptin: a monoclonal antibody against the ErbB-2 oncogene protein, which is over-expressed in many breast cancers as a result of amplification of erbB-2 gene. Erbitux; monoclonal antibody against EGF receptor (the ErbB oncogene protein); used to treat colorectal cancer.

186. Molecular Approaches to Cancer Treatment Small molecule inhibitors of oncogene proteins, such as protein kinases, are also being developed. An inhibitor of the Bcr/Abl tyrosine kinase (called imatinib or Gleevec) effectively blocks proliferation of chronic myeloid leukemia cells.

187. Molecular Medicine, Ch. 19, p. 763

188. Figure 19.41 Effect of imatinib on mortality from chronic myeloid leukemia

189. Molecular Approaches to Cancer Treatment A few patients develop resistance to imatinib, which most often results from mutations of the Bcr/Abl protein kinase domain that prevents imatinib binding. Analysis of resistant mutants has resulted in design of new inhibitors that are currently being tested.

190. Molecular Approaches to Cancer Treatment Imatinib also inhibits the PDGF receptor and Kit tyrosine kinases. Kit is an oncogene in about 90% of gastrointestinal stromal tumors. Imatinib is also active against other types of tumors in which the PDGF receptor is activated as an oncogene.

191. Molecular Approaches to Cancer Treatment Two small molecule inhibitors of the EGF receptor (gefitinib and erlotinib) are effective for some lung cancers. These cancers have mutations resulting in constitutive activation of the EGF receptor tyrosine kinase. Inhibition of the EGF receptor was an effective treatment.

192. Molecular Approaches to Cancer Treatment Vemurafenib is approved for treatment of melanoma. It has high affinity for the mutated B-Raf oncogene proteins in many melanomas. In 2012, an inhibitor of Hedgehog signaling was approved for basal cell carcinomas with oncogenic mutations of the Hedgehog receptor Smoothened.

193. Molecular Approaches to Cancer Treatment Oncogene addiction: sensitivity of tumors to inhibition of activated oncogenes. Proliferation and survival of tumor cells become dependent on the oncogene. In normal cells, alternative signaling pathways can compensate if one pathway is blocked.

194. Molecular Approaches to Cancer Treatment Effectiveness of many oncogene- targeted therapies is limited by development of resistance. Resistance can result from mutations in the targeted kinase, activation of other tyrosine kinases, and activation of downstream oncogenic pathways.

195. Molecular Approaches to Cancer Treatment Preventing drug resistance, possibly by combining multiple targeted therapies, is a major focus of ongoing research.

196. Molecular Approaches to Cancer Treatment The apparent dependence of cancer cells on mutationally-activated oncogenes offers the promise that oncogene-targeted drugs combined with genomic sequencing of tumors may lead to major advances in personal cancer treatment.