Neoplasia "Molecular Basis of Cancer" PDF

# Neoplasia "Molecular Basis of Cancer" ## Hallmarks of Cancer The diagram shows a circle with 10 smaller circles around it. - The 10 smaller circles correspond to ten hallmarks of Cancer: **1. Sustaining proliferative signaling** **2. Evading growth suppressors** **3. Resisting cell death** **...

# Neoplasia "Molecular Basis of Cancer" ## Hallmarks of Cancer The diagram shows a circle with 10 smaller circles around it. - The 10 smaller circles correspond to ten hallmarks of Cancer: **1. Sustaining proliferative signaling** **2. Evading growth suppressors** **3. Resisting cell death** **4. Deregulating cellular energetics** **5. Inducing angiogenesis** **6. Enabling replicative immortality** **7. Tumor-promoting inflammation** **8. Activating invasion and metastasis** **9. Genome Instability (Mutator Phenotype)** **10. Avoiding immune destruction** - The larger circle represents a cancer cell. - The arrows show the connections between each hallmark, how they can affect each other. ## Nonlethal Genetic Damage * Nonlethal genetic damage (mutation) lies at the heart of carcinogenesis. * This damage can be caused by * Environmental exposures * Inherited in the germline * Spontaneous and random * A tumor is formed by the clonal expansion of a single precursor cell that has incurred genetic damage. * This means that tumors are clonal. ## The Principal Targets of Cancer-Causing Mutations * The targets are * Growth-promoting proto-oncogenes, * Growth-inhibiting tumor suppressor genes, * Genes that regulate programmed cell death (apoptosis) * Genes involved in DNA repair ## Development of Cancer Through Stepwise Acquisition of Complementary Mutations This diagram shows a series of steps involved in the development of cancer: - **Normal Cell** - The first step is a normal cell. - **Initiated Precursor with Stem Cell-Like Properties** - The next step is an initiated precursor with stem cell-like properties. This is when the normal cell gets damaged by a carcinogen-induced mutation. The initiated Precursor also undergoes a mutation affecting its genomic Integrity. - **Precursor With Mutator Phenotype** - The next step is the Precursor with a mutator phenotype. This cell undergoes additional driver mutations. - **Founding Cancer Cell** - Another step in cancer development is the Founding cancer cell. This cell undergoes some additional mutations including the emergence of subclones. - **Genetically Heterogeneous Cancer** - Genetically Heterogeneous cancer is the last step in cancer development. This stage involves further genetic evolution and leads to diagnosis. ## Cellular and Molecular Hallmarks of Cancer All cancers display eight fundamental changes in cell physiology. These changes are considered the hallmarks of Cancer. * Self-sufficiency in growth signals * Insensitivity to growth-inhibitory signals * Altered cellular metabolism. * Evasion of apoptosis * Limitless replicative potential (immortality) * Sustained angiogenesis * Ability to invade and metastasize * Ability to evade the host immune response ## Acquired (Environmental) DNA Damaging Agents - **Acquired (environmental) DNA damaging agents:** * Chemicals * Radiation * Viruses - **Normal cell** with successful DNA repair. - **DNA Damage** - The normal cell can experience DNA damage. If DNA damage is not repaired, this can lead to mutations in the genome of the somatic cells. - **Inherited Mutations:** * Genes affecting DNA repair. * Genes affecting cell growth or apoptosis. - **Activation of growth-promoting oncogenes** and the **inactivation of tumor suppressor genes** leads to an unregulated cell proliferation. This can also lead to decreased apoptosis. - **Clonal Expansion** of the cell. The cells can also experience additional mutations. - **Angiogenesis** (the formation of new blood vessels) - **Escape from immunity** - **Tumor Progression** - **Malignant Neoplasm (Cancer)** - **Invasion and Metastasis** ## Normal Cell Cycle * It is important to understand the molecular regulation of the normal cell cycle, since cell cycle abnormalities are fundamental to cancer growth and many of the genes that cause cancer perturb the cell cycle. * The orderly progression of cells through the various phases of the cell cycle is orchestrated by cyclins and cyclin-dependent kinases (CDKs), and by their inhibitors. ## Main Cell-Cycle Components and Their Inhibitors * **Cyclin-Dependent Kinases (CDK)** * **CDK Inhibitors** * **Checkpoint Components** ## Cell-Cycle Inhibitors * The activity of cyclin-CDK complexes is tightly regulated by inhibitors, called CDK inhibitors. * Two main classes of CDK inhibitors: the Cip/Kip and the INK4/ARF families. * These inhibitors function as tumor suppressors and are frequently altered in tumors. * **Cip/Kip-p21 p27 p57** * **INK4/ARF- p16INK4, p14ARF** ## Cell-Cycle Checkpoints * The cell cycle has its own internal controls, called checkpoints. * Two main checkpoints: One at the G1/S transition and another at the G2/M. * To function properly, cell-cycle checkpoints require sensors of DNA damage, signal transducers, and effector molecules. ## G1/S Checkpoint * The S phase is the point of no return in the cell cycle, and before a cell makes the final commitment to replicate, the G1/S checkpoint checks for DNA damage. * If DNA damage is present, the DNA repair machinery and mechanisms that arrest the cell cycle are put in motion. * The delay in cell-cycle progression provides the time needed for DNA repair; if the damage is not repairable, apoptotic pathways are activated to kill the cell. * Thus, the G1/S checkpoint prevents the replication of cells that have defects in DNA, which would be perpetuated as mutations or chromosomal breaks in the progeny of the cell. * DNA damaged after its replication can still be repaired as long as the chromatids have not separated. ## Normal Cell Cycle Diagram The diagram shows a circle with four stages: - G1 - S - G2 - M (Mitosis) - G0 - The circle represents the progression of cells through the cell cycle. - At the G1/S checkpoint, there is a check for DNA damage. - At the G2/M checkpoint, there is a check for damaged or unduplicated DNA. ## Diagram Showing Cell Cycle Components The image shows a diagram of the cell cycle. - It includes the following components: * CDK inhibitors * Cyclin D/CDK4 & CDK6, * RB, * Cyclin E/CDK2 * Cyclin A/CDK1 & CDK2 * Cyclin B/CDK1 * G1, S, G2, M phases ## Checkpoint Components * The sensors and transducers of DNA damage appear to be similar for the G1/S and G2/M checkpoints. * The checkpoint effector molecules differ, depending on the cell-cycle stage at which they act. * In the G1/S checkpoint, cell-cycle arrest is mostly mediated through p53, which induces the cell-cycle inhibitor p21. * Arrest of the cell cycle by the G2/M checkpoint involves both p53-dependent and independent mechanisms. * Defect in cell-cycle checkpoint components is a major cause of genetic instability in cancer cells. ## Self-Sufficiency in Growth Signals * **Oncogenes - Genes that promote autonomous cell growth in cancer cells.** * **Protooncogenes - Their normal cellular counterparts; genes in normal cells which encode proteins that have normal function in the cell.** ## Protooncogenes and Oncogenes * Protooncogenes are physiologic regulators of cell proliferation and differentiation. * Oncogenes are characterized by the ability to promote cell growth in the absence of normal mitogenic stimulus. ## Protooncogenes and Oncogenes * Oncogene products, called oncoproteins, resemble the normal products of protooncogenes with the exception that oncoproteins are devoid of important regulatory elements. * Their production in the transformed cells becomes constitutive, that is, not dependent on growth factors or other external signals. * Because oncoproteins are constitutively expressed, they endow the cell with selfsufficiency in growth. * "Enemies with in' ## Functional Category of Oncogenes * Growth Factors. * Growth Factor Receptors. * Proteins Involved in Signal Transduction. * Nuclear Regulatory Proteins. * Cell-Cycle Regulators. ## Selected Oncogenes, Their Mode of Activation, and Associated Human Tumors ### Growth Factors | Category | Proto-Oncogene | Mode of Activation in Tumor | Associated Human Tumor | |---|---|---|---| | Growth Factors | PDGFB | Overexpression | Astrocytoma | | Growth Factors | HST1 | Overexpression | Osteosarcoma | | Growth Factors | FGF3 | Amplification | Stomach cancer, Bladder cancer, Breast cancer, Melanoma | | Growth Factors | TGFA | Overexpression | Astrocytomas | | Growth Factors | HGF | Overexpression | Hepatocellular carcinomas, Thyroid Cancer | ### Growth Factor Receptors | Category | Proto-Oncogene | Mode of Activation in Tumor | Associated Human Tumor | |---|---|---|---| | Growth Factor Receptors | ERBB1 (EGFR) | Mutation | Adenocarcinoma of lung, Breast carcinoma | | Growth Factor Receptors | ERBB2 (HER) | Amplification | Breast carcinoma | | Growth Factor Receptors | FLT3 | Point mutation | Leukemia | | Growth Factor Receptors | RET | Point mutation | Multiple endocrine neoplasia 2A and B, familial medullary thyroid carcinomas | | Growth Factor Receptors | PDGFRB | Overexpression, translocation | Gliomas, leukemias | | Growth Factor Receptors | KIT | Point mutation | Gastrointestinal stromal tumor, seminomas, leukemias | | Growth Factor Receptors | ALK | Translocation, fusion gene formation | Adenocarcinoma of lung, certain lymphomas | | Growth Factor Receptors | ALK | Point mutation | Neuroblastoma | ### Proteins Involved in Signal Transduction | Category | Proto-Oncogene | Mode of Activation in Tumor | Associated Human Tumor | |---|---|---|---| | Proteins Involved in Signal Transduction | KRAS | Point mutation | Colon, lung, and pancreatic tumors | | Proteins Involved in Signal Transduction | HRAS | Point mutation | Bladder and kidney tumor | | Proteins Involved in Signal Transduction | NRAS | Point mutation | Melanomas, hematologic malignancies | | Proteins Involved in Signal Transduction | GNAQ | Point mutation | Uveal melanoma | | Proteins Involved in Signal Transduction | GNAS | Point mutation | Pituitary adenoma, other endocrine tumors | | Proteins Involved in Signal Transduction | ABL | Translocation | Chronic myelogenous leukemia | | Proteins Involved in Signal Transduction | BRAF | Point mutation, Translocation | Melanomas, leukemias, colon carcinoma, others | | Proteins Involved in Signal Transduction | NOTCH1 | Point mutation, Translocation | Leukemias, lymphomas, breast carcinoma | | Proteins Involved in Signal Transduction | JAK2 | Translocation | Myeloproliferative disorders | ### Nuclear Regulatory Proteins | Category | Proto-Oncogene | Mode of Activation in Tumor | Associated Human Tumor | |---|---|---|---| | Nuclear Regulatory Proteins | MYC | Translocation | Burkitt lymphoma | | Nuclear Regulatory Proteins | NMYC | Amplification | Neuroblastoma | ### Cell Cycle Regulators | Category | Proto-Oncogene | Mode of Activation in Tumor | Associated Human Tumor | |---|---|---|---| | Cell Cycle Regulators | CCND1 (Cyclin D1) | Translocation | Mantle cell lymphoma, multiple myeloma | | Cell Cycle Regulators | CCND1 (Cyclin D1) | Amplification | Breast and esophageal cancers | | Cell Cycle Regulators | CDK4 | Amplification or point mutation | Glioblastoma, melanoma, sarcoma | ## Diagram Showing Growth Factor Signaling Pathways in Cancer There are three main pathways: - The Growth Factor Receptor Pathway - The RAS Pathway - The PI3K Pathway - All three pathways are involved in cell proliferation and cell growth. - Mutations in these pathways can lead to cancer. ## Growth Factors * Normal cells require stimulation by growth factors to proliferate. * Cancer cells, however, acquire the ability to synthesize the same growth factors to which they are responsive, creating an autocrine loop. * Glioblastomas express both platelet-derived growth factor (PDGF) and the PDGF receptor tyrosine kinases. ## Growth Factor Receptors * Receptor tyrosine kinases are the most important in cancer. * The oncogenic versions receptors are associated with mutations that lead to constitutive, growth factor-independent tyrosine kinase activity. * The mutant receptors deliver continuous mitogenic signals to the cell, even in the absence of growth factor in the environment. * **Salient examples of clinical importance:** * ERBB1 encodes the epidermal growth factor receptor (EGFR) * Point mutations result in constitutive activation of the EGFR tyrosine kinase. e.g. lung adenocarcinomas * ERBB2 gene is amplified in certain breast carcinomas, leading to overexpression of the HER2 receptor and constitutive tyrosine kinase activity. * Gene rearrangements activate other receptor tyrosine kinases, such as the tyrosine kinase ALK. ## Downstream Components of the Receptor Tyrosine Kinase Signaling Pathway * RAS Mutations: Point mutations of RAS family genes constitute the most common type of abnormality involving proto-oncogenes in human tumors. * 15-20% of all human cancers have a RAS mutation * Normally, RAS is activated by receptors to exchange GDP for GTP * Activated RAS returns to ground state by its intrinsic GTPase activity * GTPase activating proteins (GAPs) augment this process[1000 fold] * Mutant forms of RAS bind GAP but their GTPase activity is not augmented * K-ras-colon, pancreatic, cholangioca * H-ras-bladder ca * N-ras –haematological tus ## Oncogenic BRAF and PI3K Mutations * BRAF is a serine/threonine protein kinase * Like activating RAS mutations, activating mutations in BRAF stimulate each of these downstream kinases and ultimately activate transcription factors. Mutations in BRAF have been detected in close to 100% of hairy cell leukemias, more than 60% of melanomas, 80% of benign nevi. * PI3K mutations affect the catalytic subunits and generally result in an increase in enzyme activity. For example, about 30% of breast carcinomas have gain-of-function mutations involving the a-isoform of the PI3K catalytic subunit. ## Alterations in Nonreceptor Tyrosine Kinases * Chronic myelogenous leukemia (CML) and some acute lymphoblastic leukemias, the ABL gene is translocated from its normal abode on chromosome 9 to chromosome 22 where it fuses with the BCR gene. * The resultant chimeric gene encodes a constitutively active, oncogenic BCR-ABL tyrosine kinase. ## Diagram Showing the Translocation of ABL from Chromosome 9 to Chromosome 22 - This diagram shows the translocation of the ABL gene from chromosome 9 to chromosome 22, resulting in the formation of a BCR-ABL hybrid gene. ## Transcription Factors * All signal transduction pathways converge on the nucleus where the expression of the target genes takes place. * Transcription factors of this class include the products of the MYC, MYB, JUN, FOS, and REL proto-oncogenes. Of these, MYC is most commonly involved in human tumors. * The MYC proto-oncogene is expressed in virtually all eukaryotic cells and belongs to the immediate early response genes, which are rapidly and transiently induced by RAS/MAPK signaling following growth factor stimulation of quiescent cells. ## Oncogenes, Oncoproteins: Transcription Factors * c-MYC, c-FOS, c-JUN * MYC is most common mutated. * c-MYC (chr 8) translocation to chr 14(Ig gene) increased MYC protein. - Burkitt's Lymphoma example. * c-MYC amplified in colon, breast cancer. * L-MYC amplification small cell CA lung ## Diagram Showing the Translocation of c-MYC from Chromosome 8 to Chromosome 14 - This diagram shows the translocation of the c-MYC gene from chromosome 8 to chromosome 14, creating an Ig-MYC hybrid gene. This hybrid gene is expressed at a higher level than the normal MYC gene. - The increased expression of MYC protein can lead to cell growth and cancer. * MYC is one of a handful of transcription factors that can act together to reprogram somatic cells into pluripotent stem cells. ## Cyclins and Cyclin-Dependent Kinases * Progression of cells through the cell cycle is orchestrated by cyclin-dependent kinases (CDKs), which are activated by binding to cyclins, so-called because of the cyclic nature of their production and degradation. The CDK-cyclin complexes phosphorylate crucial target proteins that drive cells forward through the cell cycle. * There are two main cell cycle checkpoints: one at the G1/S transition and the other at the G2/M transition, each of which is tightly regulated by a balance of growth promoting and growth suppressing factors, as well as by sensors of DNA damage. ## Oncogenes, Oncoproteins, and Unregulated Cell Proliferation * **Proto-oncogenes:** Normal cellular genes whose products promote cell proliferation. * **Oncogenes:** Mutated or overexpressed versions of proto-oncogenes that function autonomously, having lost dependence on normal growth-promoting signals. * **Oncoprotein:** A protein encoded by an oncogene that drives increased cell proliferation through one of several mechanisms. ## Oncogenes, Oncoproteins * Constitutive expression of growth factors and their cognate growth factor receptors, setting up an autocrine cell signaling loop * Mutations in growth factor receptors, non-receptor tyrosine kinases, or downstream signaling molecules that lead to constitutive signaling, such as: * Activation of the EGF receptor tyrosine kinase by point mutations (lung cancer). * Activation of the HER2 receptor tyrosine kinase by gene amplification (breast cancer). * Activation of the JAK2 tyrosine kinase by point mutations (myeloproliferative disorders). * Activation of the ABL nonreceptor tyrosine kinase by chromosomal translocation and creation of a BCR-ABL fusion gene (chronic myelogenous leukemia, acute lymphoblastic leukemia) * Activation of RAS by point mutations (many cancers) * Activation of the PI3K and BRAF serine/threonine kinases by point mutations (many cancers) * Increased expression of MYC, a master transcription factor that regulates genes needed for rapid cell growth by deregulation through chromosomal translocations (Burkitt lymphoma, other hematologic malignancies); gene amplification (neuroblastoma); increased activity of upstream signaling pathways (many cancers) * Mutations that increase the activity of cyclin-dependent kinase 4 (CDK4)/D cyclin complexes, which promote cell cycle progression. ## Diagram Showing Cell Cycle Components - This diagram shows a circle with four stages: * G1 * S * G2 * M (Mitosis) * G0 - The circle represents the progression of cells through the cell cycle. - The diagram also includes: * CDK inhibitors * Cyclin D/CDK4 & CDK6 * Various cyclins: A, B, E * RB * CDK1 and CDK2 ## Insensitivity to Growth Inhibition: Tumor Suppressor Genes * The products of most tumor suppressor genes apply brakes to cell proliferation. * Abnormalities in these genes lead to failure of growth inhibition. * **Tumor suppressor proteins form a network of checkpoints that prevent uncontrolled growth.** - RB and p53 are part of a regulatory network that recognizes genotoxic stress from any source and responds by shutting down proliferation. ## Diagram Showing the Role of RB in Regulating the G1-S Checkpoint of the Cell Cycle - The diagram shows two pathways: * Growth inhibitor pathway * Growth Factor pathway - The growth inhibitor pathway includes the following: * TGF-B * p53 * CDK inhibitors (p16, INK4a) * Cyclins D/CDK4, 6 * Cyclin E/CDK2 * Hypophosphorylated RB * E2F * Histone methyltransferase - The Growth Factor Pathway includes the following: * EGF and PDGF * Cyclins D/CDK 4, 6 * Cyclin E/CDK2 * Hyperphosphorylated RB * E2F * Histone-deacetylase - The diagram shows how RB is regulated by growth inhibitors and growth factors - RB is a tumor suppressor protein that controls the cell cycle. - When RB is hypophosphorylated, it binds to E2F, which prevents the transcription of S-phase genes. - As a result, the cell cycle is arrested in G1. - When RB is hyperphosphorylated, it releases E2F, which allows the transcription of S-phase genes. - This allows the cell cycle to progress to S-phase and proceed with DNA replication. ## Loss of Normal Cell Cycle Control is Central to Malignant Transformation - Loss of normal cell cycle control is central to malignant transformation. - At least one of four key regulators of the cell cycle (p16/INK4a, cyclin D, CDK4, RB) is dysregulated in the vast majority of human cancers. ## TP53: Guardian of the Genome * A tumor suppressor gene that regulates: * Cell cycle progression * DNA repair * Cellular senescence * Apoptosis * Is the most frequently mutated gene in human cancers. * Loss-of-function mutations in TP53, located on chromosome 17p13.1, are found in more than 50% of cancers. * In most cases, mutations are present in both TP53 alleles and are acquired in somatic cells * Individuals inherit one mutated TP53 allele (Li-Fraumeni syndrome). * 25-fold greater chance of developing a malignant tumor by age 50 than the general population. * The most common types of tumors are sarcomas, breast cancer, leukemias, brain tumors, and carcinomas of the adrenal cortex. ## TP53 * TP53 encodes the protein p53, which is tightly regulated at several levels. * MDM2 and related proteins of the MDM2 family stimulate the degradation of p53; these proteins are frequently overexpressed in malignancies with normal TP53 alleles. * The transforming proteins of several DNA viruses bind p53 and promote its degradation. e.g. E6 protein of high-risk human papilloma viruses. * p53 is virtually undetectable in normal cells. * In stressed cells, p53 is released from the inhibitory effects of MDM2 via two major mechanisms: * DNA damage and hypoxia. Key initiators of p53 activation. * Protein kinases, ataxia telangiectasia mutated (ATM) and ataxia-telangiectasia and Rad3 related (ATR). ## TP53 Activation - ATM and ATR stimulate the phosphorylation of a number of proteins, including p53 and MDM2 which leads to disruption in the binding and degradation of p53 by MDM2, allowing p53 to accumulate. ## TP53 Function - Once activated, p53 opposes neoplastic transformation by inducing either: * Transient cell cycle arrest. * Senescence (permanent cell cycle arrest). * Programmed cell death (apoptosis). ## Diagram Showing the P53 Pathway - This diagram shows the p53 pathway, a signaling pathway for DNA damage. - The diagram includes: * Normal cell. * Cell with mutations or a loss of p53. * Oncogenic Stress (hypoxia). * DNA damage. * P53 accumulation. * P53's effects on target genes: * p21 (CDK inhibitor) * GADD45 (DNA repair) * BAX (apoptosis) * G1 cell cycle arrest. * Senescence. * Apoptosis. * Mutant Cells with an expansion of additional mutations. * With loss of p53 function, DNA damages goes unrepaired; driver mutations accumulate in oncogenes and other cancer genes, and the cell marches blindly along a dangerous path leading to malignant transformation. ## APC: Gatekeeper of Colonic Neoplasia * Adenomatous polyposis coli (APC) is a member of the class of tumor suppressors that function by downregulating growth promoting signaling pathways. * Germline loss-of-function mutations involving the APC (5q21) locus are associated with familial adenomatous polyposis, an autosomal disorder that causes thousands of adenomatous polyps in the colon during the teens or 20s. * Almost invariably, one or more of these polyps undergoes malignant transformation, giving rise to colon cancer.