Neoplasia II PDF

Neoplasia II (Molecular Basis of Cancer: Role of Genetic and Epigenetic Alterations)  Neoplasia Genetic disorder of cell growth that is triggered by acquired or less commonly inherited mutations affecting a single cell and its clonal progeny - Gr...

Neoplasia II (Molecular Basis of Cancer: Role of Genetic and Epigenetic Alterations)  Neoplasia Genetic disorder of cell growth that is triggered by acquired or less commonly inherited mutations affecting a single cell and its clonal progeny - Growth advantage = Excessive proliferation “New growth” - parenchyma + stroma = NEOPLASM - Parenchyma: classifies the type of tumor - Stroma: growth and spread * The term tumor is now equated with neoplasm  Neoplasm Benign Malignant Localized at their site Invade and destroy of origin adjacent structures Generally amenable and spread to distant to surgical removal sites (metastasize) Genetic Predisposition and Interactions Between Environmental and Inherited Factors  Carcinogens Exposure of cells to a sufficient dose of chemical carcinogenic agent may cause permanent DNA damage (mutations) Chemicals that can cause initiation of carcinogenesis fall into two categories: direct acting and indirect acting. Direct-acting agents do not require metabolic conversion to become carcinogenic, while indirect- acting agents are converted to an ultimate carcinogen by endogenous metabolic pathways.  Indirect-acting carcinogens are mostly metabolized by cytochrome P-450–dependent monooxygenases forming DNA adducts DNA adducts are covalent interactions between reactive carcinogen chemical species and DNA Polymorphisms in cytochrome P-450 may influence carcinogenesis. Benzo[a]pyrene (BaP) is a pro-carcinogen found in tobacco smoke. It undergoes metabolic activation by the enzyme cytochrome P4501A1 (CYP1A1) forming DNA adducts ROLE OF GENETIC AND EPIGENETIC ALTERATIONS  Mutations that contribute to the acquisition of cancer hallmarks are referred to as driver mutations. The first driver mutation that starts a cell on the path to malignancy is the initiating mutation, which is typically maintained in all the cells of the subsequent cancer. However, because no single mutation appears to be fully transforming, development of a cancer requires that the “initiated” cell acquire a number of additional driver mutations, each of which also contributes to the development of cancer  The time it takes to accumulate driver mutations is unknown in most cancers, but it appears to be lengthy. The persistence of initiated cells during this long preclinical prodrome is consistent with the idea that cancers arise from cells with stem cell–like properties, so-called cancer stem cells, that have a capacity for self- renewal and long-term persistence  Survival of the Fittest Early on, all the cells in a tumor are genetically identical, being the progeny of a single founding transformed cell. During the expansion process, individual tumor cells acquire additional mutations at random. As a result of this tumor evolution, their constituent cells are often extremely heterogeneous genetically even though cancers are clonal in origin.  Survival of the Fittest The diversity tumor subclones compete for access to nutrients and those that are most fit come to dominate the tumor mass. This pernicious tendency of tumors to become more aggressive over time is referred to as tumor progression. Tumors that recur after therapy are almost always found to be resistant if the same treatment is given again, presumably because therapy selects for subclones that, by chance, have a genotype that allows them to survive.  Epigenetic aberrations may either enhance or dampen gene expression and they also contribute to the malignant properties of cancer cells. The epigenetic state of the cell dictates which genes are expressed, which in turn determines the lineage commitment and differentiation state of both normal and neoplastic cells. Unlike DNA mutations, epigenetic changes are potentially reversible by drugs that inhibit DNA-modifying or histone-modifying factors such as silencing of some tumor suppressor genes Cellular and Molecular Hallmarks of Cancer  Self-sufficiency in growth signals Tumors have the capacity to proliferate without external stimuli, usually as a consequence of oncogene activation. Cells expressing oncoproteins are thus freed from normal checkpoints and proliferate excessively.  Key Concepts 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 participates in signaling pathways that drives cell proliferation, which may result from a variety of aberrations. They are constitutively active and resistant to control by external signals.  Cell Cycle Surveillance Mechanisms Quality control checkpoints embedded within the cell cycle ensure that cells with genetic imperfections do not complete replication. The G1-S checkpoint monitors DNA integrity before irreversibly committing cellular resources to DNA replication. The G2-M restriction point makes sure that there has been accurate genetic replication before the cell actually divides.  Cell Cycle Surveillance Mechanisms When cells do detect DNA irregularities, checkpoint activation delays cell cycle progression and triggers DNA repair mechanisms. If the genetic derangement is too severe to be repaired, cells either undergo apoptosis or enter a nonreplicative state called senescence Proto-oncogene Examples ERBB1 (EGFR) Adenocarcinoma, lungs ERBB2 (HER) Breast Carcinoma MYC Burkitt Lymphoma ABL Chronic myelogenous leukemia HST1 Osteosarcoma PDGFB Astrocytoma ALK Adenocarcinoma, lungs FGF3 Stomach, bladder, and breast cancer, melanoma BRAF Melanoma, leukemias, colon carcinoma, others  Oncoproteins and Cell Growth Many oncogenes encode growth factor receptors, of which receptor tyrosine kinases are arguably the most important in cancer. These oncogenic receptors lead to growth factor–independent activity which deliver mitogenic signals even in the absence of growth factor in the environment. Receptor tyrosine kinase activation stimulates RAS. Point mutations of RAS family genes is the most common type of abnormality involving proto-oncogenes in human cancers.  Oncoproteins and Cell Growth Mutations in the form of chromosomal translocations or rearrangements creates fusion genes encoding constitutively active tyrosine kinases. This oncogenic mechanism involves the ABL tyrosine kinase especially in chronic myeloid leukemia (CML). The ABL gene is translocated from its normal location on chromosome 9 to chromosome 22, where it fuses with the BCR gene leading to a fusion gene  Oncogene Addiction Oncogene addiction: tumor cells are highly dependent on the activity of one oncoproteins. Treatment with BCR-ABL inhibitors in CML offers remarkable therapeutic response. Unfortunately, it does not lead to cure. This is because there are rare CML “stem cells” that do not require BCR-ABL signals for their survival. Treatment of patients with advanced melanomas with BRAF inhibitors has produced striking clinical responses.  Transcription Factors The ultimate consequence of deregulated mitogenic signaling pathways is inappropriate and continuous stimulation of nuclear transcription factors that drive growth-promoting genes Among transcription factors, MYC is most commonly affected in cancer. The MYC proto-oncogene is expressed in virtually all eukaryotic cells. MYC can be considered as master transcriptional regulator of cell growth  Functions of MYC Cell cycle progression Protein synthesis Metabolic reprogramming and Warburg effect Upregulates expression of telomerase Can 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 Gain-of-function mutations in D cyclin genes and CDK4, which promote unregulated G1/S progression and thus function as oncogenes Loss-of-function mutations in genes that inhibit G1/S progression. - The two most important tumor suppressor genes, RB and TP53  Insensitivity to growth-inhibitory signals Whereas oncogenes drive the proliferation of cells, the products of most tumor suppressor genes apply brakes to cell proliferation. Tumors may not respond to molecules that inhibit the proliferation of normal cells, usually because of inactivation of tumor suppressor genes that encode components of growth inhibitory pathways.  TP53: Guardian of the Genome TP53, a tumor suppressor gene that regulates cell cycle progression, DNA repair, cellular senescence, and apoptosis, is the most frequently mutated gene in human cancers. The p53 protein is the central monitor of stress in the cell and can be activated by anoxia, inappropriate signaling by mutated oncoproteins, or DNA damage.  MDM2 and related proteins of the MDM2 family stimulate the degradation of p53 E6 proteins encoded by high-risk HPV subtypes bind to p53 and promote its degradation by the proteasome. In addition, E6 upregulates the expression of telomerase With loss of p53 function, DNA damage goes unrepaired, driver mutations accumulate in oncogenes and other cancer genes, and the cell marches along a dangerous path leading to malignant transformation.  Li-Fraumeni syndrome Li-Fraumeni syndrome: these are individuals who inherit one mutated TP53 allele which predisposes them to malignancy because only one additional “hit” in the remaining normal allele is needed. The spectrum of tumor development is broad wherein the most common types are sarcomas, breast cancers, leukemias, brain tumors, and carcinomas of the adrenal cortex. They are more likely to suffer from multiple primary tumors.  RB: Governor of Proliferation Normal growth factor signaling leads to RB hyperphosphorylation and inactivation, thus promoting cell cycle progression by releasing E2F which then activates transcription of S-phase genes When hypophosphorylated, RB exerts antiproliferative effects by binding and inhibiting E2F transcription factors that regulate genes required for cells to pass through the G1/S phase cell cycle checkpoint.  The antiproliferative effect of RB is abrogated in cancers through a variety of mechanisms, including: Loss-of-function RB mutations Amplifications of the CDK4 and cyclin D genes Loss-of-function mutations affecting cyclin-dependent kinase inhibitors (e.g., p16/INK4a) HPV viral E7 protein binds the hypophosphorylated (active) form of RB and promotes its degradation via the proteasome pathway, and also binds and inhibits p21 and p27, two important cyclin-dependent kinase inhibitors. 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. APC is a component of the WNT signaling pathway Germline loss-of-function mutations are associated with familial adenomatous polyposis (FAP), an autosomal dominant disorder causing the development of thousands of adenomatous polyps in the colon during their teens or 20s.  E-Cadherin β-catenin also binds to the cytoplasmic tail of E-cadherin, a cell surface protein that maintains intercellular adhesiveness. Epithelial injury, disrupts the interaction between E-cadherin and β-catenin allowing it to translocate to the nucleus and promote proliferation. Mutation of the E-cadherin/β-catenin axis is a characteristic of many carcinomas which can contribute to easy disaggregation of cells, which can then invade locally or metastasize. Tumor Suppressor Mutations Associated Malignancies PTEN Cowden syndrome (variety of benign skin, GI, and CNS growths; breast, endometrial, and thyroid carcinoma) PTCH Gorlin syndrome (basal cell carcinoma, medulloblastoma, several benign tumors) VHL Von Hippel-Lindau syndrome (cerebellar hemangioblastoma, retinal angioma, renal cell carcinoma) STK11 Peutz-Jeghers syndrome (GI polyps, GI cancers, pancreatic carcinoma, and other carcinomas) CDH1 Gastric carcinoma, lobular breast cancer WT1 Wilms Tumor BRCA 1 and BRCA 2 Familial breast and ovarian carcinomas  Altered cellular metabolism Tumor cells undergo a metabolic switch to aerobic glycolysis also called as the Warburg effect At the heart of the Warburg effect lies a simple question:  Why is it advantageous for a cancer cell to rely on seemingly inefficient glycolysis (which generates 2 molecules of ATP per molecule of glucose) instead of oxidative phosphorylation (which generates 36 molecules of ATP per molecule of glucose)?  Warburg Effect Aerobic glycolysis provides rapidly dividing tumor cells with metabolic intermediates that are needed for the synthesis of cellular components, whereas mitochondrial oxidative phosphorylation does not. Thus, while “pure” oxidative phosphorylation yields abundant ATP, it fails to produce any carbon moieties that can be used to build cellular components that are needed for growth (proteins, lipids, and nucleic acids). Receptor tyrosine kinase/PI3K/AKT signaling It upregulates the activity of glucose transporters and multiple glycolytic enzymes, thus increasing glycolysis Promotes shunting of mitochondrial intermediates to pathways leading to lipid biosynthesis and stimulates factors that are required for protein synthesis. Inhibit pyruvate kinase leading to the buildup of upstream glycolytic intermediates, which are siphoned off for synthesis of DNA, RNA, and protein. MYC Drives changes in gene expression that support anabolic metabolism and cell growth which includes multiple glycolytic enzymes and glutaminase, which is required for mitochondrial utilization of glutamine, another important source of intermediates needed for biosynthesis of cellular components Autophagy Autophagy is a state of severe nutrient deficiency in which cells not only arrest their growth but also cannibalize their own organelles, proteins, and membranes as carbon sources for energy production. Tumor cells may use autophagy to become “dormant,” a state of metabolic hibernation that allows cells to survive making them resistant to therapies that kill actively dividing cells  Oncometabolism It consists of mutations in enzymes that participate in the Krebs cycle most importantly mutations in isocitrate dehydrogenase (IDH) The proposed steps in the oncogenic pathway involving IDH are as follows: The mutated protein loses it IDH function and instead acquires a new enzymatic activity that catalyzes the production of 2-hydroxyglutarate (2-HG) 2-HG inhibits several members of the TET family, including TET2 Loss of TET2 activity leads to abnormal patterns of DNA methylation The net effect of TET2 loss in lineages in which TET2 is a tumor suppressor is the upregulation of RAS and receptor tyrosine kinase signaling.  Evasion of Cell Death Tumor cells frequently contain mutations in genes that result in resistance to apoptotic cell death. The intrinsic pathway appears to be the primary arbitrator of life and death in cancer cells, as cancer cells are subject to a number of intrinsic stresses that can initiate apoptosis: DNA damage, metabolic disturbances, and increased misfolded proteins These stresses are enhanced manyfold when tumors are treated with chemotherapy or radiation (which kill tumor cells mainly by inducing apoptosis).  Evasion of Cell Death Loss of TP53 function due to overexpression of its inhibitor MDM2 Overexpression of anti-apoptotic members of the BCL2 family leading to the protection of tumor cells from apoptosis.  Limitless Replicative Potential All cancers contain cells that are immortal and have limitless replicative potential. Three interrelated factors appear to be critical to the immortality of cancer cells: (1) evasion of senescence (2) evasion of mitotic crisis (3) the capacity for self-renewal  Evasion of senescence Most human cells have the capacity to divide 60 to 70 times. After this, the cells become senescent, permanently leaving the cell cycle and never dividing again. Senescent state is associated with upregulation of p53 and INK4a/p16 and in part by maintaining RB in a hypophosphorylated state. Cell cycle checkpoint disruption in cancers may be caused by a wide variety of acquired genetic and epigenetic aberrations that may allow cells to bypass senescence.  Evasion of mitotic crisis While cells that are resistant to senescence have increased replicative capacity, they are not immortal; instead, they eventually enter a phase referred to as mitotic crisis and die. Mitotic crisis has been ascribed to progressive shortening of telomeres, special DNA sequences at the ends of chromosomes that bind several protective protein complexes Telomere maintenance is seen in virtually all types of cancers through reactivation of telomerase  Cancer Stem Cells Cancer stem cells may arise through transformation of a normal stem cell or through acquired genetic lesions that impart a stem-like state on a more mature cell. It is hypothesized that like normal stem cells, cancer stem cells have a high intrinsic resistance to conventional therapies due to a low rate of cell division and the expression of factors such as multiple drug resistance-1 (MDR1) that counteract the effects of chemotherapeutic drugs.  Self-renewal Self-renewal means that each time a stem cell divides at least one of the two daughter cells remains a stem cell. In a symmetric division, both daughter cells remain stem cells; such divisions may occur during embryogenesis, when stem cell pools are expanding, or during times of stress. In an asymmetric division, only one daughter cell remains a stem cell  Angiogenesis Even if a solid tumor possesses all the genetic aberrations that are required for malignant transformation, it cannot enlarge beyond 1 to 2 mm in diameter unless it has the capacity to induce angiogenesis. Hypoxia triggers angiogenesis through the actions of HIF1α on the transcription of the pro-angiogenic factor VEGF. - Bevacizumab  Angiogenesis VEGF: promotes migration and proliferation of endothelial cells and vasodilation via nitric oxide FGF-2: stimulate proliferation of endothelial cells Ang 1 & 2: structural maturation of new vessels PDGF: recruits smooth muscle cells TGF-B: suppresses endothelial proliferation and migration and enhances the production of ECM  Invasion and Metastasis Local invasion of tumor cells may damage or destroy vital structures and is a prerequisite for distant spread. Although many of these locally invasive cells enter the bloodstream each day, very few produce metastases. The breakaway cells must overcome the challenges of avoiding immune defenses and adapting to a microenvironment (e.g., lymph node, bone marrow, or brain) that is quite different from that of the site of origin of the tumor.  Four Steps in Tissue Invasion Loosening of cell-cell contacts Roles E-cadherins  of – Epithelial-Mesenchymal the extracellular matrix: Transition silencing (SNAIL and TWIST)  Degradation Regulator of ofcellECM proliferation by binding and displaying growth factors and by signaling via cellular integrin family Cathepsin Metalloproteinases, receptors. D, Theand ECM provides a depot for latent growth factors that Urokinase can be activated within foci of injury or inflammation. Attachment to remodeled ECM components  Prevention of anoikis - because of expression of other integrins that mitigate the loss of adhesion Migration of tumor cells  vascular dissemination, tissue homing, and colonization  Antitumor Effector Mechanisms Cells of the immune system can recognize and eliminate cells displaying abnormal antigens The principal immune mechanism of tumor eradication is killing of tumor cells by CTLs specific for tumor antigens CTL responses against tumors are initiated by recognition of tumor antigens by antigen presenting cells (APCs). Once activated by APCs, tumor-specific CTLs can migrate from lymph nodes to the tumor and kill tumor cells, directly and serially, without any assistance from other cell types.  The ability of CTLs to kill tumor cells underlies the ferocious antitumor activity of CTLs engineered to express chimeric antigen receptors (so-called CAR- T cells) against lineage-specific surface antigens found on certain tumors. CAR T-cell therapy for cancer | Booking Health Mechanisms of Immune Evasion by Cancers  Mechanisms of Immune Evasion by Cancers Selective outgrowth of antigen- negative variants During tumor progression, strongly immunogenic antigen expressing subclones may be eliminated, and tumor cells that survive are those that have lost their antigens.  Mechanisms of Immune Evasion by Cancers Loss or reduced expression of MHC molecules: Tumor cells may fail to express normal levels of HLA class I molecules, thereby losing the ability to display cytosolic antigens and escaping attack by cytotoxic T cells. Such cells, however, may trigger NK cells if the tumor cells express ligands for NK cell activating receptors Mechanisms of Immune Evasion by Cancers Secretion of immunosuppressive factors Tumors may secrete TGF-β in large quantities which can inhibit the movement of T cells from the vasculature into the tumor bed. Induction of regulatory T cells (Tregs) Some studies suggest that tumors produce factors that favor the development of immunosuppressive Tregs, which could also contribute to “immunoevasion.”  Engagement of pathways that inhibit T-cell activation Molecular basis of PD-1/PD-L1 immunotherapies. Upper panels: The... | Download Scientific Diagram (researchgate.net)  Tumor cells actively inhibit tumor immunity by upregulating negative regulatory “checkpoints” that suppress immune responses (CTLA-4 and PD-L1/2) The most common toxicities associated with checkpoint blockade are autoimmunity and/or inflammatory damage to organs. This is predictable because the physiologic function of inhibitory receptors and ligands is to maintain tolerance to self antigens  Genomic Instability DNA Mismatch Repair Factors: Hereditary nonpolyposis colon cancer / Lynch syndrome Nucleotide Excision Repair Factors: Xeroderma pigmentosum Homologous Recombination Repair Factors: Bloom syndrome, Ataxia Telangiectasia, Fanconi anemia, Familial breast cancers  Cancer-Enabling Inflammation Release of factors that promote proliferation Infiltrating leukocytes and activated stromal cells secrete a wide variety of growth factors such as EGF, as well as proteases that can liberate growth factors from the ECM Removal of growth suppressors The growth of epithelial cells is normally suppressed by cell–cell and cell–ECM interactions. Proteases released by inflammatory cells can degrade the adhesion molecules that mediate these interactions, removing a barrier to growth.  Cancer-Enabling Inflammation Enhanced resistance to cell death Detachment of epithelial cells from basement membranes and from cell–cell interactions leads to a form of cell death called anoikis. It is suspected that tumor-associated macrophages prevent anoikis by expressing adhesion molecules such as integrins that promote direct physical interactions with the tumor cells.  Cancer-Enabling Inflammation Inducing angiogenesis Inflammatory cells release numerous factors, including VEGF, which stimulate angiogenesis. Activating invasion and metastasis Proteases released from macrophages foster tissue invasion by remodeling the ECM, while factors such as TNF and EGF may directly stimulate tumor cell motility. TGF-B may promote EMT  Cancer-Enabling Inflammation Evading immune destruction  Soluble factors released by macrophages and other stromal cells includes TGF-β and a number of other factors that either favor the recruitment of immunosuppressive Tregs Emerging evidence in human disease that advanced cancers contain alternatively activated (M2) macrophages which produces cytokines that promote angiogenesis, fibroblast proliferation, and collagen deposition.  Epigenetic Changes Epigenetic changes have been implicated in many aspects of the malignant phenotype including the expression of cancer genes, the control of differentiation and self-renewal, and drug sensitivity and drug resistance. A cancer cell’s lineage or differentiation state, like that of normal cells, is dependent on epigenetic modifications that produce a pattern of gene expression that characterizes that particular cell type.  Epigenetic Changes Histone methylation Methylation of histone lysine residues can lead to transcriptional activation or repression, depending on which histone residue is marked. Histone acetylation Lysine residues are acetylated by histone acetyltransferases (HATs), whose modifications tend to open the chromatin and increase transcription. In turn, these changes can be reversed by histone deacetylases (HDACs), leading to chromatin condensation.  Epigenetic Changes Histone phosphorylation Serine residues can be modified by phosphorylation; depending on the specific residue, the DNA may be opened for transcription or condensed and inactive. High levels of DNA methylation in gene regulatory elements typically result in transcriptional silencing Chromatin organizing factors Much less is known about these proteins, which are believed to bind to noncoding regions and control long-range looping of DNA, thus regulating the spatial relationships between enhancers and promoters that control gene expression Thank You! Lecture Must Know Cytochrome P-450–dependent monooxygenases: Mostly involved in the metabolism of indirect carcinogen Receptor tyrosine kinases: arguably the most important growth receptor involved in cancer RAS family genes: most common type of abnormality involving proto-oncogenes MYC  Most commonly affected transcription factor in cancer  Considered as master transcriptional regulator of cell growth TP53  Guardian of the Genome  The most frequently mutated gene in human cancers  Li-Fraumeni syndrome  Inhibited by HPV viral protein E6  Lecture Must Know RB  Governor of Proliferation  hypophosphorylated, RB exerts antiproliferative effects  hyperphosphorylated, RB exerts proliferative effects  Retinoblastoma  Inhibited by HPV viral protein E7 APC Gatekeeper of Colonic Neoplasia Holds β-catenin activity in check by forming a “destruction complex” that leads to its proteasomal degradation in the absence of WNT signaling Familial adenomatous polyposis (FAP) Lecture Must Know Oncometabolism Krebs cycle Mutation in IDH - production of 2-HG - Loss of TET2 activity - upregulation of RAS and receptor tyrosine kinase signaling Warburg Effect: glycolysis (carbon moieties) > oxidative phosphorylation Lecture Exercise A patient suffering from Li-Fraumeni syndrome is mostly like due to a mutation involving? Answer: A A. Guardian of the Genome B. Governor of Proliferation C. Master transcriptional regulator of cell growth D. Gatekeeper of Colonic Neoplasia Lecture Exercise A patient with Lynch syndrome who presented with colon cancer and endometrial cancer has a genomic instability characterized by? A. DNA Mismatch Repair Factors Answer: A B. Nucleotide Excision Repair Factors C. Homologous Recombination Repair Factors D. DNA deacetylation Lecture Exercise Ways of Immune Evasion by cancer cells? A. Selective outgrowth of antigen-negative variants Answer: D B. Loss or reduced expression of MHC molecules C. Secretion of immunosuppressive factors D. All of the above Lecture Exercise A patient with invasive lobular carcinoma of the bilateral breast underwent biopsy. Under the microscope the tumor cells appear dyscohesive and typically infiltrating as single cells. The dyscohesive nature of this tumor is due to dysregulation of ? A. PTEN B. VHL Answer: D C. WT1 D. CDH1 Lecture Exercise TRUE or FALSE. Metastasis is guaranteed when locally invasive cells enter the bloodstream. Answer: False Reference (Robbins Pathology) Vinay Kumar MBBS MD FRCPath, Abul K. Abbas MBBS, Jon C. Aster MD PhD - Robbins & Cotran Pathologic Basis of Disease (Robbins Pathology)-Elsevier (2020)