Cancer Biology BIO544 Notes PDF

CANCER BIOLOGY BIO544 TABLE OF CONTENTS 1. Genetics and Environment in Tumor Development o Rous sarcoma virus (RSV) and its discovery. o Hypothesis regarding RSV’s effect on progenitor cells. o Viral transformation and the role of viral genes in maintaining the transformed...

CANCER BIOLOGY BIO544 TABLE OF CONTENTS 1. Genetics and Environment in Tumor Development o Rous sarcoma virus (RSV) and its discovery. o Hypothesis regarding RSV’s effect on progenitor cells. o Viral transformation and the role of viral genes in maintaining the transformed phenotype. 2. DNA Viruses and Cancer o Discovery of DNA viruses like the Shope papilloma virus in rabbits. o SV40 virus in poliovirus vaccines, its effects, and classification as a tumor virus. o Human adenovirus and its similarities to SV40. o Herpes viruses and their role in cancer, including Epstein-Barr virus (EBV). 3. Properties of Transformed Cells o Altered morphology, loss of contact inhibition, anchorage-independent growth. o Immortalization and high saturation density. o Tumorigenicity, increased glucose transport (FDG PET-SCAN). 4. Tumor Virus Transmission o Integration of viral DNA into the host genome during replication. o Mechanism of viral genome integration, including retroviruses like HIV and human T-cell leukemia virus 1 (HTLV-1). 5. Oncogenes and Tumor Suppressor Genes o Src gene and MYC oncogene. o P53 gene and its dual role as an oncogene and tumor suppressor. o Insertational mutagenesis, chromosomal rearrangements, and translocations (e.g., MYC, Philadelphia chromosome). o EGFR mutations and their role in non-small cell lung cancer (NSCLC). 6. Growth Factors, Receptors, and Cancer o Signaling pathways (EGFR, PDGF, Src protein). o Dimerization and phosphorylation in receptor signaling. o Autocrine and paracrine signaling in tumor cells. o Overexpression of growth factors and receptors in cancers like NSCLC and Kaposi's sarcoma. 7. Cell Cycle and Cancer o Role of cyclins and CDKs in cell cycle regulation (Cyclin A/CDK2, MPF). o G1/S checkpoint and its importance in cancer cell proliferation. o Role of Rb protein as a cell cycle controller and p53 as a checkpoint regulator. 8. P53 and Apoptosis o P53’s role in inducing apoptosis in response to DNA damage. o Wild-type vs. mutant p53 pathways in cancer development. o Mutant p53’s influence on glucose uptake and survival under stress. 9. Mechanisms of Apoptosis and Necrosis o Intrinsic and extrinsic pathways of apoptosis. o Difference between apoptosis (programmed cell death) and necrosis (passive cell death due to injury). o Role of autophagy and its relationship with apoptosis. 10. Replicative Senescence o Hayflick limit and its role in cell division. o Mechanisms by which cancer cells overcome senescence, including telomerase activation. o Markers for senescence (e.g., beta-galactosidase). 11. Telomerase and Tumorigenesis o Role of telomerase in tumor formation.  o Regulation of telomerase expression by oncogenes like c-myc. 12. Evolution of Tumorigenesis o Multistep process of cancer formation. o Field cancerization and accumulation of mutations over time. o Cancer stem cells and their role in tumor heterogeneity. 13. Genomic Integrity and Cancer o Structural DNA variations leading to tumor formation. o DNA adduct formation due to carcinogen exposure (e.g., benzo[a]pyrene in lung cancer). o Inherited defects in DNA repair mechanisms (e.g., xeroderma pigmentosum, BRCA1/2 mutations). 14. Tumor Suppressor Genes o Loss of heterozygosity (LOH) and mechanisms leading to tumorigenesis. o Promoter methylation and its role in inactivating tumor suppressor genes (e.g., NF1 in Ras signaling). o Consequences of loss of tumor suppressors like APC (colon cancer) and VHL (kidney cancer). 15. Mutant P53 and Cancer Cell Metabolism o Mutant p53’s role in enhancing cancer cell metabolism and survival. o Warburg effect (increased glycolysis in cancer cells) and the role of GLUT-1. o Mutant p53’s interference with apoptosis and autophagy. 16. Apoptosis and Metastasis o How cancer cells evade apoptosis and promote metastasis. o Inactivation of p53 as a potential anti-cancer target. 17. Autophagy in Cancer o Process of autophagy and its relationship with tumor aggression. o Autophagy’s mutual exclusivity with apoptosis. 18. Cancer Hallmarks o Overview of the "10 Hallmarks of Cancer." o Key processes such as evading apoptosis, replicative immortality, and sustained angiogenesis. 19. Cell Cycle Checkpoints and Tumor Suppression o Role of tumor suppressors like p53 and Rb in regulating the cell cycle. o Mechanisms through which cancer cells bypass these checkpoints. 20. Evolution of Tumor Formation o Bert Vogelstein’s research on the genetic progression of colon cancer. o Multistep tumorigenesis and genetic mutations involving APC, KRAS, and P53. 21. Cancer Stem Cells o Identification and role of cancer stem cells in tumor formation and heterogeneity. o Experiments by John Dick’s lab on acute myeloid leukemia (AML) and cancer stem cells. 22. Carcinogens and DNA Damage o Role of environmental carcinogens like aflatoxin and heterocyclic amines in mutagenesis. o Mechanisms of DNA damage and repair, including DNA adduct formation. TOPIC-1 1. Rous Sarcoma Virus (RSV) and its Discovery  Discovery by Peyton Rous (1911): o Context: RSV was discovered when Peyton Rous demonstrated that certain types of cancer in chickens could be transferred by a virus. o Significance: This discovery introduced the concept that viruses could induce cancer, which was groundbreaking because it linked cancer with infectious agents, suggesting that external factors could cause cancer.  Virus Characteristics: o RSV is a retrovirus: It has RNA as its genetic material, which is converted to DNA once inside the host cell, integrating into the host genome. o Transmission: RSV can induce cancer-like characteristics in infected cells, leading to sarcoma (a type of tumor in connective tissues) in chickens. o Oncogenes Discovery: The discovery of RSV paved the way for identifying oncogenes—genes that, when altered or abnormally activated, can cause cells to become cancerous. 2. Hypothesis Regarding RSV’s Effect on Progenitor Cells  Viral Infection in Progenitor Cells: o Hypothesis: RSV infects progenitor cells—cells that have the potential to differentiate into various types of cells—leading to abnormal growth and the development of cancer. o Mechanism: When RSV infects a progenitor cell, its genetic material is integrated into the cell’s DNA, causing disruptions in normal cellular control mechanisms. o Abnormal Proliferation: The virus may activate or introduce oncogenes that push progenitor cells to multiply uncontrollably, a key feature in tumorigenesis.  Transformation vs. Normal Differentiation: o In a typical environment, progenitor cells differentiate into specialized cells following controlled signaling pathways. o RSV disrupts these pathways, leading to transformation rather than normal differentiation, resulting in cells that proliferate without control. 3. Viral Transformation and the Role of Viral Genes in Maintaining the Transformed Phenotype  Transformation Process: o Viral transformation occurs when RSV inserts its genetic material into the host cell’s DNA, leading to permanent changes that make the cell cancerous. o Key Gene - Src Oncogene:  RSV contains the src gene, which is a type of oncogene.  Src is crucial because it encodes a protein kinase that alters cell signaling pathways, particularly those controlling cell division and survival, thus maintaining the cancerous state of the cell.  Role of Viral Genes in Phenotype Maintenance: o Continuous Expression: RSV’s viral genes must continue to be expressed in the infected cell for it to retain its transformed (tumor-like) phenotype. o Oncogene Dependency: The cancerous cells become dependent on the activity of the src oncogene for their survival and proliferation, meaning that if the expression of src stops, the cells often revert to a non-cancerous state. o Mechanisms of Transformation:   RSV genes alter signaling pathways, suppressing mechanisms like apoptosis (programmed cell death), which would typically remove damaged or abnormal cells.  These viral genes also interfere with cell cycle regulation, keeping the cell in a continuous state of division, characteristic of tumor cells.  Implications for Cancer Research: o Understanding RSV and its viral transformation mechanisms has provided insights into the general processes by which cells become cancerous. o It established the concept that specific genes, when mutated or abnormally expressed, can lead to cancer—a fundamental principle in modern cancer research. TOPIC-2 1. Discovery of DNA Viruses like the Shope Papilloma Virus in Rabbits  Shope Papilloma Virus Discovery (1930s): o Context: Richard E. Shope identified a virus causing papillomas (wart-like tumors) in rabbits. o Significance: The Shope papilloma virus was one of the first DNA viruses found to cause tumors, providing early evidence that DNA viruses could lead to cancer.  Mechanism: o The virus causes benign tumors that can sometimes progress to malignant cancer, showing how viral infections can lead to both benign and cancerous growths. o Pathway: It introduces viral DNA into host cells, leading to changes in cell growth and replication control, which increases the risk of cancerous transformations.  Impact on Cancer Research: o This discovery helped establish the viral oncogenesis theory, contributing to later research on DNA viruses and their roles in tumor formation in humans. 2. SV40 Virus in Poliovirus Vaccines, Its Effects, and Classification as a Tumor Virus  SV40 (Simian Virus 40) Discovery and Contamination Issue: o Discovered in the 1950s as a contaminant in polio vaccines produced from monkey kidney cells. o Significance: It became apparent that SV40 could transform cells and cause tumors in lab animals, raising concerns about its effects on humans.  Tumor Virus Classification: o SV40 is classified as a tumor virus due to its ability to induce transformation in various cell types, leading to the formation of tumors. o Oncogene Mechanism: The virus contains large T-antigen, a protein that interacts with tumor suppressor proteins like p53 and Rb in host cells, disabling these cell- cycle regulators. o Result: This leads to uncontrolled cell division and the inhibition of apoptosis, fostering a cancerous state within infected cells.  Human Relevance: o Although SV40 is known to cause tumors in animals, its role in human cancer remains inconclusive. However, the study of SV40 has been instrumental in understanding viral interactions with cell cycle regulators. 3. Human Adenovirus and Its Similarities to SV40  Human Adenovirus Discovery and Characteristics: o Adenoviruses were discovered in human tonsils and adenoid tissue and can cause respiratory and gastrointestinal infections.  o Oncogenic Potential: Certain strains of adenovirus can cause tumors in animals, although they are generally not linked to human cancers.  Similarities to SV40: o Interaction with Tumor Suppressors: Like SV40, adenoviruses produce E1A and E1B proteins that interfere with p53 and Rb tumor suppressor pathways. o Mechanism: E1A and E1B proteins promote viral replication but also result in unregulated cell division, similar to the large T-antigen in SV40.  Research Implications: o Studying adenoviruses has provided valuable insight into viral oncogenesis mechanisms and has led to advances in gene therapy, where modified adenoviruses are used as delivery vehicles for therapeutic genes. 4. Herpes Viruses and Their Role in Cancer, Including Epstein-Barr Virus (EBV)  Overview of Herpesviruses: o Herpesviruses are a large family of DNA viruses, some of which are linked to cancers in humans. They have a unique ability to establish latent infections, persisting in the body for life.  Epstein-Barr Virus (EBV): o Discovery and Association with Cancer: EBV, discovered in the 1960s, is known to cause Burkitt's lymphoma, nasopharyngeal carcinoma, and is associated with other lymphomas and cancers, especially in immunocompromised individuals. o Mechanism:  EBV infects B lymphocytes and can cause these cells to proliferate excessively, leading to cancer.  The virus expresses several proteins and RNA molecules that manipulate the host cell's growth control, evading immune detection and promoting cell survival.  Key Proteins: EBNA (Epstein-Barr Nuclear Antigen) proteins and latent membrane proteins (LMPs) are critical for transformation and maintaining the cancerous state in infected cells.  Other Herpesviruses Linked to Cancer: o Kaposi's Sarcoma-associated Herpesvirus (KSHV): Causes Kaposi’s sarcoma, common in individuals with weakened immune systems, such as those with HIV/AIDS. o Mechanism:  Like EBV, KSHV induces cell transformation and promotes angiogenesis (formation of new blood vessels), supporting tumor growth.  Impact on Cancer Research and Treatment: o Herpesviruses have highlighted the role of persistent viral infection in cancer development. o They demonstrate how viruses can evade immune responses, establish latency, and later reactivate, contributing to cancer in susceptible individuals, which has influenced approaches to immunotherapy and vaccine development. TOPIC-3 1. Altered Morphology, Loss of Contact Inhibition, and Anchorage-Independent Growth  Altered Morphology: o Transformed (cancerous) cells often exhibit distinct morphological changes compared to normal cells. o Characteristics: These cells may become rounder, have larger nuclei, and show irregular shapes and sizes.  o Significance: Changes in morphology are due to cytoskeletal alterations and other cellular adaptations that support uncontrolled growth and survival.  Loss of Contact Inhibition: o Definition: Contact inhibition is a mechanism in normal cells where cell division stops when cells touch each other, maintaining a controlled, monolayer growth. o In Transformed Cells: Cancer cells lose this property, allowing them to continue dividing even when in contact with neighboring cells, resulting in overcrowded, multilayered cell clusters. o Impact: Loss of contact inhibition is a hallmark of transformed cells, contributing to their invasive growth potential, leading to tumor formation in tissues.  Anchorage-Independent Growth: o Definition: Normal cells typically require a solid surface to attach to (like the extracellular matrix) for growth and survival. o In Transformed Cells: Cancer cells can grow without attachment, meaning they can survive and proliferate in suspension or in soft agar (a common laboratory test to assess anchorage independence). o Significance: This property is crucial for metastasis, as it enables cancer cells to survive as they move through the body, potentially leading to tumor spread. 2. Immortalization and High Saturation Density  Immortalization: o Definition: Normal cells undergo a limited number of divisions before they enter senescence (a non-dividing, aging state). Transformed cells, however, can bypass this limit, making them effectively “immortal.” o Mechanism: This is often due to the reactivation of telomerase, an enzyme that extends telomeres (the protective ends of chromosomes), preventing the usual DNA shortening that triggers senescence. o Significance: Immortalization enables transformed cells to continuously proliferate without aging, which is essential for the development and maintenance of tumors over extended periods.  High Saturation Density: o Definition: Saturation density is the maximum cell density a culture can reach under given conditions. Normal cells stop growing at lower densities once resources are limited. o In Transformed Cells: Cancer cells can grow to much higher densities, often beyond what is seen in normal cell cultures. o Cause: This is due to deregulated growth signals and a reduced need for growth factors, allowing transformed cells to proliferate even under nutrient-deprived or crowded conditions. o Impact: High saturation density contributes to the aggressive growth of tumors and the formation of dense cell clusters within tumors. 3. Tumorigenicity and Increased Glucose Transport (FDG PET-SCAN)  Tumorigenicity: o Definition: Tumorigenicity is the ability of transformed cells to form tumors when introduced into an appropriate host, such as in laboratory animal models. o Mechanism: Transformed cells acquire mutations that support unchecked growth, survival, and often, the evasion of immune detection, which enables them to form tumors in vivo. o Significance: Tumorigenicity is a defining property of cancer cells and is commonly assessed in experimental cancer research to confirm the cancerous nature of cell lines.  Increased Glucose Transport (FDG PET-SCAN):  o Metabolic Shift in Cancer Cells: Transformed cells rely more heavily on glycolysis for energy production, even in the presence of oxygen, known as the Warburg effect. This shift requires more glucose uptake to meet the cell’s metabolic demands. o FDG PET-Scan Utility: Fluorodeoxyglucose (FDG) is a glucose analog used in PET scans to detect increased glucose uptake, a hallmark of many cancerous tissues. o Explanation of PET Scanning: When FDG is injected into the body, it accumulates in areas with high glucose uptake, such as tumors, allowing for imaging of metabolic activity and providing a diagnostic tool for locating cancerous growths. o Impact: This increased glucose transport not only supports rapid cell division but also makes transformed cells detectable in PET scans, aiding in cancer diagnosis and monitoring. TOPIC-4 1. Integration of Viral DNA into the Host Genome During Replication  Process of Viral DNA Integration: o Some viruses, particularly certain DNA viruses and retroviruses, can integrate their genetic material into the host cell’s DNA as part of their replication cycle. o Significance: This integration allows the viral DNA to become a permanent part of the host genome, leading to persistent infections and, in some cases, contributing to cellular transformation and cancer development.  Tumorigenic Potential: o When viral DNA is integrated near or within critical host genes, it can disrupt normal cellular processes. o Example: If the viral DNA integrates near a proto-oncogene or tumor suppressor gene, it can either activate oncogenes or disable tumor suppressor functions, which may lead to uncontrolled cell growth and tumor formation.  Latency and Persistent Infection: o Integration allows the virus to remain latent within the host cell, where it may not immediately produce active infection or symptoms, enabling long-term persistence. o Reactivation: In certain conditions, such as immunosuppression, the virus may reactivate, increasing the risk of cancer or disease progression. 2. Mechanism of Viral Genome Integration, Including Retroviruses Like HIV and HTLV-1  Retroviral Integration Process: o Retroviruses like HIV (Human Immunodeficiency Virus) and HTLV-1 (Human T- cell Leukemia Virus-1) have an RNA genome that is reverse-transcribed into DNA once they enter the host cell. o Reverse Transcription:  Enzyme: Retroviruses use the enzyme reverse transcriptase to convert their RNA genome into complementary DNA (cDNA).  Integration Site: The viral cDNA is then transported to the host cell’s nucleus, where it integrates into the host genome.  Integration Enzyme - Integrase: o Retroviruses carry an enzyme called integrase, which is responsible for inserting the viral DNA into the host cell’s DNA. o Process:  Integrase cuts the host DNA at specific sites, allowing the viral DNA to be inserted.  This integration is generally random, meaning the viral DNA could disrupt genes at the integration site, potentially leading to cancer if it affects critical genes.   Mechanisms in Specific Retroviruses: o HIV (Human Immunodeficiency Virus):  Pathogenicity: While HIV is not typically classified as a tumor virus, its chronic infection and immune system suppression can increase the risk of certain cancers, such as Kaposi's sarcoma and non-Hodgkin lymphoma.  Integration and Persistence: HIV’s integration into the host genome enables lifelong infection, with the virus remaining latent in cells like CD4+ T cells. o HTLV-1 (Human T-cell Leukemia Virus-1):  Oncogenesis: HTLV-1 is directly linked to adult T-cell leukemia/lymphoma (ATLL) due to its ability to transform T-cells.  Viral Proteins: HTLV-1 expresses Tax and HBZ proteins, which disrupt normal cell cycle regulation and immune signaling pathways, promoting T- cell proliferation and survival.  Random Integration: HTLV-1 integration is random but can cause transformation when it activates oncogenes or deactivates tumor suppressors within infected T-cells.  Cancer Implications: o Both HIV and HTLV-1 demonstrate how retroviruses can contribute to cancer through integration and immune system impacts. o Understanding these mechanisms has been key for developing targeted therapies, such as integrase inhibitors for HIV, which prevent the virus from integrating its DNA into the host genome, helping to control infection. TOPIC-5. Src Gene and MYC Oncogene  Src Gene: o Discovery: Src was one of the first oncogenes discovered in Rous sarcoma virus, where it was shown to induce tumors in chickens. o Function: Src encodes a tyrosine kinase, an enzyme that phosphorylates proteins involved in signal transduction pathways, controlling cell growth and division. o Mechanism in Cancer:  In its oncogenic form, Src remains constantly active, leading to unchecked cell proliferation and survival.  This uncontrolled signaling drives the growth of transformed cells, making Src a key player in tumorigenesis.  MYC Oncogene: o Role in Cell Cycle Regulation: MYC is a transcription factor that regulates the expression of genes involved in cell growth, metabolism, and apoptosis. o Mechanism in Cancer:  When MYC is overexpressed or mutated, it leads to excessive cell division and growth, a hallmark of cancer cells.  MYC is implicated in various cancers, including Burkitt's lymphoma, where a chromosomal translocation leads to its overexpression, promoting tumor formation. 2. P53 Gene and Its Dual Role as an Oncogene and Tumor Suppressor  P53 as a Tumor Suppressor: o Function: Known as the “guardian of the genome,” p53 is a crucial tumor suppressor that responds to DNA damage by inducing cell cycle arrest, DNA repair, or apoptosis (programmed cell death). o Mechanism in Cancer Prevention:   When DNA damage occurs, p53 prevents the propagation of mutated DNA by stopping the cell cycle, allowing time for repair.  If the damage is irreparable, p53 induces apoptosis, removing damaged cells to prevent cancer.  Loss of p53 Function in Cancer: o Mutations in the TP53 gene (encoding p53) are found in over 50% of human cancers. o Without functional p53, cells accumulate mutations and can progress toward a cancerous state due to the lack of cell cycle control and apoptosis.  p53 as an Oncogene: o In some cases, certain p53 mutations produce proteins that act in a dominant- negative fashion, interfering with normal p53 function and promoting tumorigenesis. o This unique aspect allows p53 mutations to contribute to cancer, both by loss of tumor suppression and through gain-of-function mutations that actively promote cancer progression. 3. Insertational Mutagenesis, Chromosomal Rearrangements, and Translocations  Insertational Mutagenesis: o Definition: This process occurs when a viral or foreign DNA sequence integrates into the host genome, potentially near an oncogene or tumor suppressor gene. o Mechanism: This insertion can activate oncogenes or inactivate tumor suppressor genes, leading to uncontrolled cell division and cancer. o Example: Retroviruses like HTLV-1 can integrate near oncogenes, driving cancerous growth through random insertions that disrupt gene regulation.  Chromosomal Rearrangements and Translocations: o Definition: Translocations occur when parts of chromosomes are rearranged or exchanged, which can lead to gene fusions or dysregulation of important genes. o MYC Translocation in Burkitt’s Lymphoma:  In Burkitt’s lymphoma, the MYC gene is translocated to a region near the immunoglobulin gene, resulting in MYC overexpression and uncontrollable cell division. o Philadelphia Chromosome:  A chromosomal translocation between chromosomes 9 and 22 creates the BCR-ABL fusion gene in chronic myelogenous leukemia (CML).  Mechanism: This fusion gene encodes a constantly active tyrosine kinase that drives leukemic cell growth and proliferation, forming the basis for targeted therapies like imatinib (Gleevec) to inhibit BCR-ABL activity. 4. EGFR Mutations and Their Role in Non-Small Cell Lung Cancer (NSCLC)  EGFR (Epidermal Growth Factor Receptor): o Role in Cell Signaling: EGFR is a receptor tyrosine kinase that plays a critical role in cell proliferation, differentiation, and survival by activating pathways like RAS/MAPK and PI3K/AKT.  Mutations in NSCLC: o In non-small cell lung cancer, specific mutations in the EGFR gene lead to constant activation of the receptor, even without growth signals. o Types of Mutations:  Common mutations in NSCLC include exon 19 deletions and L858R point mutations in exon 21, which alter the receptor’s configuration, enabling constant signaling for cell growth. o Impact of EGFR Mutations:  These mutations result in increased cell proliferation, reduced apoptosis, and enhanced metastatic potential, driving NSCLC progression.   Therapeutic Targeting: o EGFR mutations have become a key target for cancer therapy, leading to the development of tyrosine kinase inhibitors (TKIs) such as erlotinib and gefitinib, which specifically inhibit the abnormal EGFR activity in NSCLC, improving patient outcomes. TOPIC-6 1. Signaling Pathways (EGFR, PDGF, Src Protein)  Epidermal Growth Factor Receptor (EGFR): o Role: EGFR is a receptor tyrosine kinase (RTK) involved in cellular growth, survival, and differentiation. o Signaling Pathway: When activated by its ligand, EGFR triggers pathways such as RAS/MAPK and PI3K/AKT, which promote cell proliferation and inhibit apoptosis. o Implications in Cancer: Mutations or overexpression of EGFR can lead to constant signaling, contributing to cancers like non-small cell lung cancer (NSCLC).  Platelet-Derived Growth Factor (PDGF): o Function: PDGF is a growth factor that binds to PDGF receptors (PDGFRs) on cell surfaces, initiating cell division and growth. o Signaling Pathway: PDGF activates downstream pathways, including MAPK, PI3K, and JAK/STAT, which are important for wound healing and tissue repair. o Cancer Relevance: Overexpression or mutation of PDGFR is linked to cancers, such as glioblastoma, where it drives uncontrolled cell proliferation.  Src Protein: o Function: Src is a non-receptor tyrosine kinase involved in several signaling pathways that control cell growth, adhesion, and survival. o Significance in Cancer: As an oncogene, Src is often overactive in cancers, promoting invasive behavior and metastatic potential in tumor cells through uncontrolled signaling. 2. Dimerization and Phosphorylation in Receptor Signaling  Dimerization: o Definition: Dimerization is the process by which two receptor molecules bind together, often as a response to ligand binding. o Mechanism: Many receptor tyrosine kinases (RTKs), like EGFR and PDGFR, require dimerization to become fully active. Ligand binding brings two receptor molecules together, forming a dimer. o Importance: Dimerization is essential for activating the receptor’s kinase domain, enabling it to initiate intracellular signaling cascades.  Phosphorylation: o Role in Activation: After dimerization, the receptors undergo autophosphorylation on specific tyrosine residues within their intracellular domains. o Mechanism: Phosphorylated tyrosines serve as docking sites for other signaling proteins, propagating the signal through pathways that promote cell survival, division, and migration. o Cancer Relevance: In many cancers, dimerization and phosphorylation of RTKs are dysregulated, either through mutations that mimic ligand binding or overexpression of receptors, leading to persistent activation of growth-promoting signals. 3. Autocrine and Paracrine Signaling in Tumor Cells  Autocrine Signaling:  o Definition: In autocrine signaling, cells produce signaling molecules (like growth factors) that bind to receptors on their own surface. o Mechanism in Tumor Cells: Many cancer cells overproduce growth factors, allowing them to stimulate their own growth by binding to receptors on their surface. o Example in Cancer: Some tumors, such as gliomas, produce PDGF and have elevated levels of PDGFR, creating a self-sustaining growth loop that drives tumor proliferation.  Paracrine Signaling: o Definition: Paracrine signaling involves the release of signaling molecules that affect nearby cells rather than the cell itself. o Role in Tumor Microenvironment:  Tumor cells often release growth factors that affect surrounding stromal cells, creating an environment conducive to tumor growth and metastasis.  For example, cancer cells may release VEGF (vascular endothelial growth factor), which promotes blood vessel formation (angiogenesis), supplying the tumor with oxygen and nutrients.  Significance in Cancer Progression: o Both autocrine and paracrine signaling contribute to tumor survival and growth. Autocrine loops allow tumors to become independent of external growth signals, while paracrine signaling modifies the tumor microenvironment to support expansion and metastasis. 4. Overexpression of Growth Factors and Receptors in Cancers like NSCLC and Kaposi's Sarcoma  Non-Small Cell Lung Cancer (NSCLC): o EGFR Overexpression and Mutation:  NSCLC is often associated with overexpression or mutations in the EGFR gene, leading to constant activation of growth-promoting pathways, even in the absence of external growth factors.  Impact: This overactivation drives excessive cell proliferation, survival, and resistance to apoptosis, key features in NSCLC.  Targeted Therapy: EGFR inhibitors, such as erlotinib and gefitinib, have been developed to block this overactive signaling in NSCLC, offering a targeted approach to treatment.  Kaposi's Sarcoma: o Human Herpesvirus 8 (HHV-8): Kaposi’s sarcoma is associated with infection by HHV-8, which stimulates the overexpression of growth factors such as VEGF. o Mechanism of Tumor Promotion:  HHV-8 activates pathways that promote cell proliferation, survival, and angiogenesis, essential for Kaposi’s sarcoma lesions, which are highly vascularized.  Autocrine and Paracrine Signaling: In Kaposi’s sarcoma, infected cells produce cytokines and growth factors that support tumor growth and angiogenesis, fostering tumor progression and invasiveness. TOPIC-7 1. Role of Cyclins and CDKs in Cell Cycle Regulation (Cyclin A/CDK2, MPF)  Cyclins and Cyclin-Dependent Kinases (CDKs): o Cyclins: These are proteins whose levels fluctuate throughout the cell cycle, helping to regulate its progression.  o CDKs (Cyclin-Dependent Kinases): CDKs are enzymes activated by binding to specific cyclins. This binding triggers phosphorylation events that drive the cell through various phases of the cycle. o Importance in Cell Cycle: Cyclins and CDKs work together to ensure that cells move smoothly from one phase of the cell cycle to the next. Abnormalities in this regulation can lead to uncontrolled cell division, a hallmark of cancer.  Cyclin A/CDK2: o Role in S Phase and G2 Phase: Cyclin A binds to CDK2, driving the cell from the G1 phase into the S phase, where DNA synthesis occurs, and later into the G2 phase. o Cancer Implications: Overexpression or dysregulation of Cyclin A/CDK2 can lead to rapid cell proliferation, as it may bypass essential cell cycle checkpoints, leading to tumor formation.  Maturation Promoting Factor (MPF): o Components and Role: MPF is a complex of Cyclin B and CDK1 and is critical for initiating mitosis (M phase). o Mechanism: MPF activates processes that lead to chromosomal condensation, nuclear envelope breakdown, and spindle formation—preparing the cell for division. o Relevance to Cancer: Overactivation of MPF can drive unchecked cell division, contributing to the rapid cell proliferation seen in cancer. 2. G1/S Checkpoint and Its Importance in Cancer Cell Proliferation  G1/S Checkpoint Function: o Role: This checkpoint ensures that the cell is ready for DNA replication in the S phase, assessing factors like DNA integrity and size. o Mechanism: The G1/S checkpoint relies on cyclin D/CDK4 and CDK6, which phosphorylate the Rb protein (Retinoblastoma protein) to move the cell cycle forward.  Importance in Cancer: o Checkpoint Bypass: Cancer cells often evade this checkpoint, allowing them to progress to the S phase even with DNA damage or insufficient resources, leading to genetic instability and mutation accumulation. o Oncogene Influence: Mutations in oncogenes, like Cyclin D or CDKs, can cause this checkpoint to be disregarded, enabling cancer cells to proliferate without the typical controls.  Therapeutic Targeting: o CDK Inhibitors: Targeting the cyclins/CDKs involved in the G1/S checkpoint, particularly CDK4/6 inhibitors, has become a therapeutic strategy in certain cancers, as it helps restore cell cycle control. 3. Role of Rb Protein as a Cell Cycle Controller and p53 as a Checkpoint Regulator  Rb (Retinoblastoma Protein): o Function: Rb is a tumor suppressor protein that regulates the cell cycle by controlling the G1/S checkpoint. It prevents the cell from entering the S phase by inhibiting the transcription factor E2F. o Mechanism in Cell Cycle Control:  When phosphorylated by Cyclin D/CDK4, Rb releases E2F, allowing the transcription of genes necessary for DNA replication and S phase entry.  In its unphosphorylated form, Rb binds to E2F, blocking cell cycle progression. o Cancer Connection:   Mutations or loss of Rb function can lead to unchecked cell cycle progression and proliferation, a common occurrence in many cancers, such as retinoblastoma and small cell lung cancer.  Loss of Rb: Without functional Rb, cells lack a critical checkpoint, leading to abnormal cell cycle regulation and contributing to oncogenesis.  p53 as a Checkpoint Regulator: o Function and Importance: p53 is a tumor suppressor that plays a vital role in the G1/S and G2/M checkpoints. It responds to DNA damage by halting the cell cycle to allow for repair or inducing apoptosis if the damage is irreparable. o Mechanism:  When DNA damage is detected, p53 activates p21, a CDK inhibitor, which binds to and inactivates cyclin/CDK complexes, effectively stopping the cell cycle.  Inducing Apoptosis: If repair is not possible, p53 activates pro-apoptotic genes to trigger cell death, preventing the propagation of damaged DNA. o Cancer Implications:  Mutations in the TP53 gene (encoding p53) are found in over half of all human cancers.  Loss of p53 Function: Without p53, cells can bypass key checkpoints, allowing DNA damage and mutations to accumulate, leading to genetic instability and tumor progression.  Therapeutic Relevance: Because of its crucial role, p53 function is a focal point in cancer therapy research, aiming to restore or mimic its tumor- suppressing actions in p53-deficient cancers. TOPIC-8 1. P53’s Role in Inducing Apoptosis in Response to DNA Damage  Function as a Tumor Suppressor: o p53 is known as the “guardian of the genome” for its essential role in maintaining genomic stability. o Activation: When DNA damage or cellular stress is detected, p53 is activated through post-translational modifications, such as phosphorylation, which stabilizes and accumulates p53 in the nucleus.  Apoptosis Pathway: o p53 initiates apoptosis (programmed cell death) if DNA damage is severe and irreparable, preventing damaged cells from proliferating and potentially transforming into cancer cells. o Mechanism:  Transcriptional Activation: p53 upregulates pro-apoptotic genes, such as BAX, PUMA, and NOXA, which lead to mitochondrial outer membrane permeabilization.  This triggers the release of cytochrome c from the mitochondria, activating caspases (proteins that execute apoptosis).  Fas Death Receptor: p53 also increases the expression of the Fas receptor, another apoptosis pathway that promotes cell death through caspase activation.  Importance in Cancer Prevention: o p53’s role in apoptosis is crucial for preventing the propagation of cells with DNA mutations. Its ability to induce cell death acts as a failsafe against cancer development by eliminating damaged cells before they can accumulate additional mutations. 2. Wild-Type vs. Mutant p53 Pathways in Cancer Development  Wild-Type (Normal) p53: o Role in Cell Cycle Control: Wild-type p53 regulates the cell cycle and induces cell cycle arrest at the G1/S checkpoint, allowing for DNA repair. o Response to Damage: If DNA damage is minor, wild-type p53 pauses the cell cycle to enable repair. If the damage is severe, it triggers apoptosis to remove the cell. o Tumor Suppression: By enforcing DNA repair or cell death, wild-type p53 helps maintain genome integrity, significantly reducing cancer risk.  Mutant p53: o Loss of Function: In many cancers, the TP53 gene is mutated, resulting in a loss of p53’s ability to control the cell cycle and initiate apoptosis. o Gain of Function Mutations: Some mutant forms of p53 acquire new properties that can actively promote cancer by supporting cell survival, proliferation, and resistance to cell death. o Cancer Implications:  Without functional p53, cells can bypass DNA damage checkpoints, allowing mutations to accumulate and cells to survive despite severe genomic instability, a common feature in cancer progression.  Therapeutic Challenge: Cancers with mutant p53 are often more aggressive and resistant to therapy, as these cells lack the apoptosis pathway and continue proliferating under conditions that would typically trigger cell death. 3. Mutant p53’s Influence on Glucose Uptake and Survival Under Stress  Altered Metabolism in Cancer Cells: o Mutant p53 is known to promote metabolic adaptations that support cancer cell survival, particularly under stress conditions such as nutrient scarcity or hypoxia. o Increased Glucose Uptake:  Mutant p53 can upregulate glucose transporters, such as GLUT1, which enhances glucose uptake.  Warburg Effect: This shift toward increased glucose uptake fuels aerobic glycolysis (the Warburg effect), where cancer cells rely on glycolysis over oxidative phosphorylation, even in the presence of oxygen. This provides a rapid source of energy and biosynthetic precursors for dividing cells.  Survival Under Cellular Stress: o Mutant p53 can interfere with the normal stress responses, enabling cancer cells to survive under unfavorable conditions. o Mechanism:  Mutant p53 promotes antioxidant pathways that protect cancer cells from oxidative damage, allowing them to survive and proliferate in stress-induced environments.  Autophagy Suppression: Mutant p53 may also inhibit autophagy (a cell survival process under starvation), forcing cells to rely on glycolysis and thereby enhancing their dependency on glucose.  Cancer Progression and Resistance: o By promoting glucose uptake and survival under stress, mutant p53 contributes to tumor growth and resistance to therapies. o Therapeutic Implications: Targeting the metabolic shifts induced by mutant p53, such as by inhibiting glycolysis or glucose transport, is an area of interest in cancer treatment, particularly for p53-mutant tumors. TOPIC-9 1. Intrinsic and Extrinsic Pathways of Apoptosis  Apoptosis Overview: o Apoptosis, or programmed cell death, is a tightly regulated process that allows cells to die in an orderly way, removing damaged or unnecessary cells without causing inflammation. o Significance: Apoptosis plays a critical role in development, tissue homeostasis, and prevention of cancer by eliminating cells with DNA damage.  Intrinsic Pathway (Mitochondrial Pathway): o Triggered by Internal Signals: This pathway is activated by signals within the cell, often due to DNA damage, oxidative stress, or lack of survival factors. o Mitochondrial Involvement: Key players in the intrinsic pathway are the Bcl-2 family proteins, which regulate mitochondrial membrane permeability.  Pro-apoptotic Proteins: Proteins like BAX and BAK promote apoptosis by making the mitochondrial membrane permeable.  Release of Cytochrome c: This permeability allows cytochrome c to escape from the mitochondria into the cytoplasm.  Formation of Apoptosome: Cytochrome c, in the cytoplasm, binds to Apaf- 1 and procaspase-9, forming the apoptosome, which activates caspase-9. o Caspase Cascade: Caspase-9 then activates downstream executioner caspases (e.g., caspase-3 and caspase-7), leading to the orderly breakdown of cellular components.  Extrinsic Pathway (Death Receptor Pathway): o Triggered by External Signals: This pathway is activated by death ligands, such as Fas ligand (FasL) or tumor necrosis factor (TNF), binding to death receptors on the cell surface. o Death Receptors: Binding of ligands to receptors (e.g., Fas receptor or TNF receptor) causes the receptors to cluster and recruit adaptor proteins like FADD. o Caspase Activation: This clustering leads to the formation of the death-inducing signaling complex (DISC), which activates caspase-8. o Execution Phase: Caspase-8 then activates executioner caspases, like caspase-3, resulting in the systematic breakdown of the cell. o Cross-Talk with Intrinsic Pathway: In some cases, caspase-8 can cleave and activate BID, a pro-apoptotic protein, linking the extrinsic and intrinsic pathways by promoting mitochondrial cytochrome c release. 2. Difference Between Apoptosis (Programmed Cell Death) and Necrosis (Passive Cell Death Due to Injury)  Apoptosis (Programmed Cell Death): o Process: Apoptosis is an active, energy-dependent process involving caspases and organized dismantling of the cell. o Characteristics:  Cell Shrinkage: The cell shrinks and forms apoptotic bodies, which are phagocytosed by neighboring cells without causing inflammation.  DNA Fragmentation: DNA is systematically fragmented, and cellular components are packed into membrane-bound vesicles. o Purpose: Apoptosis is essential for normal development, immune function, and the removal of damaged cells, preventing potential tumor formation.  Necrosis (Passive Cell Death): o Process: Necrosis is typically triggered by external injury, like trauma, infection, or ischemia, and does not require energy. o Characteristics:  Cell Swelling and Lysis: Necrotic cells swell and rupture, releasing their contents into the extracellular space.   Inflammation: This release triggers an inflammatory response, which can further damage surrounding tissues. o Outcome: Necrosis can lead to tissue damage and scarring, contrasting with the controlled cell removal seen in apoptosis.  Functional Differences: o Apoptosis serves as a controlled, non-inflammatory means of eliminating cells, crucial for maintaining tissue health. o Necrosis, by contrast, is often a harmful, uncontrolled process that results in tissue injury and inflammation. 3. Role of Autophagy and Its Relationship with Apoptosis  Autophagy Overview: o Autophagy is a self-digestion process where cells break down and recycle their own components, particularly under conditions of stress, such as nutrient deprivation. o Mechanism:  During autophagy, damaged organelles and proteins are engulfed by autophagosomes, which then fuse with lysosomes to degrade and recycle cellular components.  Energy Source: This process provides energy and building blocks for survival under adverse conditions.  Relationship with Apoptosis: o Protective Role: Autophagy can act as a survival mechanism to delay apoptosis, helping cells manage stress and avoid premature death. o Dual Role in Cell Death: In cases of extreme or prolonged stress, autophagy may shift from being protective to promoting cell death, though this is typically non- inflammatory and distinct from apoptosis.  Cross-Talk with Apoptosis Pathways: o Some proteins, such as p53 and Bcl-2 family proteins, regulate both autophagy and apoptosis, linking these pathways. o In Cancer: Tumor cells can exploit autophagy to survive in nutrient-poor environments, aiding in their growth and therapy resistance. However, autophagy can also suppress tumor initiation by preventing the accumulation of damaged organelles and DNA.  Therapeutic Implications: o Targeting the balance between autophagy and apoptosis is a focus in cancer therapy, as enhancing autophagy may help degrade damaged cells, while inhibiting it can push cancer cells toward apoptosis. TOPIC-10 1. Hayflick Limit and Its Role in Cell Division  Hayflick Limit: o Definition: The Hayflick limit refers to the observation that normal somatic cells have a finite number of divisions, after which they enter a non-dividing state known as replicative senescence. o Discovery: Leonard Hayflick discovered in the 1960s that human cells in culture could only divide approximately 40-60 times before becoming senescent.  Role in Cellular Aging and Tumor Suppression:  o Mechanism: Each time a cell divides, its telomeres (protective caps on the ends of chromosomes) shorten. Eventually, telomeres become too short to protect chromosomes, triggering the cell to enter senescence. o Senescence as a Tumor Suppressor Mechanism:  By limiting the number of divisions, the Hayflick limit prevents the accumulation of mutations over time, reducing the risk of cancer.  Preventing Uncontrolled Growth: Senescence acts as a natural barrier to unlimited cell proliferation, a characteristic of cancer cells. 2. Mechanisms by Which Cancer Cells Overcome Senescence, Including Telomerase Activation  Overcoming the Hayflick Limit in Cancer: o Cancer cells develop ways to bypass replicative senescence, allowing them to divide indefinitely. o Mechanisms:  Telomerase Activation: Most cancer cells activate telomerase, an enzyme that adds telomeric DNA to chromosome ends, effectively restoring and maintaining telomere length.  Telomerase Mechanism: Telomerase has an RNA template that it uses to extend telomeres, compensating for telomere shortening and enabling indefinite division.  Cancer Relevance: Telomerase is typically inactive in most somatic cells but is reactivated in approximately 85-90% of cancers, making it a target for cancer therapies.  Alternative Lengthening of Telomeres (ALT): Some cancer cells use the ALT pathway, a telomerase-independent mechanism that elongates telomeres through recombination-based processes.  Mechanism in ALT: ALT involves DNA repair and recombination proteins that help maintain telomere length, supporting continuous proliferation in the absence of telomerase.  Implications for Cancer Growth and Therapy: o Immortality of Cancer Cells: By overcoming senescence, cancer cells achieve a key hallmark of cancer: the ability to divide indefinitely, contributing to tumor growth and metastasis. o Therapeutic Targeting: Inhibiting telomerase in cancer cells is a potential therapeutic approach to induce senescence or apoptosis in tumors. 3. Markers for Senescence (e.g., Beta-Galactosidase)  Senescence-Associated Beta-Galactosidase (SA-β-gal): o Marker of Senescence: SA-β-gal is one of the most commonly used markers for detecting senescent cells. o Mechanism: Senescent cells show increased lysosomal beta-galactosidase activity, which can be detected by a staining method that produces a blue color at a specific pH. o Applications in Research: SA-β-gal staining is widely used in laboratory research to identify senescent cells in tissue samples or cell cultures.  Other Senescence Markers: o p16^INK4a and p21: These are cyclin-dependent kinase inhibitors that are upregulated in senescent cells to enforce cell cycle arrest.  Function: p16^INK4a and p21 halt the cell cycle by inhibiting CDKs, preventing the cell from progressing through the G1/S checkpoint, thus maintaining the senescent state.   Diagnostic Use: High levels of p16^INK4a and p21 serve as indicators of cellular senescence, especially in aging tissues and cancer research. o DNA Damage Response Markers:  Senescent cells often show persistent DNA damage, which can be detected by markers like γH2AX (a phosphorylated histone protein indicating DNA double-strand breaks).  Implications in Aging and Cancer: The presence of DNA damage markers is consistent with the stress-induced premature senescence seen in many cancer cells and aged tissues.  Secretory Phenotype in Senescent Cells: o Senescence-Associated Secretory Phenotype (SASP): Senescent cells secrete pro- inflammatory cytokines, growth factors, and proteases that can influence nearby cells.  Impact on Tumor Microenvironment: SASP factors can promote inflammation and even stimulate the growth of nearby cancer cells, linking senescence to cancer progression in certain contexts.  Therapeutic Targeting of SASP: Reducing SASP factors is an emerging area in therapy to mitigate their potentially tumor-promoting effects in the cancer microenvironment. TOPIC-11 1. Role of Telomerase in Tumor Formation  Function of Telomerase: o Enzyme Composition: Telomerase is a ribonucleoprotein complex composed of a catalytic protein subunit, TERT (telomerase reverse transcriptase), and an RNA component that serves as a template for adding telomeric repeats. o Telomere Maintenance: Telomerase adds repetitive DNA sequences to the ends of chromosomes, known as telomeres, which protect chromosomes from degradation and prevent DNA loss during replication. o Normal Cell Limitations: In most normal somatic cells, telomerase is inactive or present at low levels, leading to telomere shortening with each division. This ultimately triggers replicative senescence or apoptosis to prevent excessive proliferation.  Role in Tumorigenesis: o Overcoming Senescence: Cancer cells often reactivate telomerase, allowing them to bypass replicative senescence and evade the Hayflick limit. By maintaining telomere length, telomerase enables cancer cells to divide indefinitely. o Chromosomal Stability: Telomerase helps maintain chromosomal integrity by preventing telomere erosion, which would otherwise result in chromosome instability, fusions, and cell death. o Hallmark of Cancer: The ability to evade replicative senescence and achieve immortality through telomerase reactivation is a hallmark of cancer. Studies indicate that telomerase is active in approximately 85-90% of cancers, underscoring its essential role in tumor growth.  Therapeutic Targeting of Telomerase: o Potential Cancer Therapy: Since telomerase is mostly inactive in normal cells but reactivated in most cancer cells, it serves as a potential target for anti-cancer therapies. o Approaches: Therapies aimed at inhibiting telomerase activity, blocking TERT expression, or targeting telomerase’s function in maintaining telomeres are under investigation. The goal is to induce telomere shortening in cancer cells, pushing them back into senescence or apoptosis. 2. Regulation of Telomerase Expression by Oncogenes like c-Myc  Role of c-Myc in Telomerase Activation: o Oncogene Function: c-Myc is a transcription factor that regulates genes involved in cell growth, metabolism, and proliferation. It is often overexpressed in cancers, contributing to uncontrolled cell division. o Direct Regulation of TERT: c-Myc directly binds to the promoter region of the TERT gene (encoding the catalytic subunit of telomerase) and activates its transcription.  Mechanism: c-Myc upregulates TERT expression, thereby increasing telomerase activity in cells where it is normally low or absent, such as somatic cells.  Cancer Relevance: The overexpression of c-Myc leads to the activation of telomerase, which supports unlimited cell division, allowing cancer cells to bypass senescence and grow unchecked.  c-Myc and Cell Cycle Control: o Enhanced Proliferation: By activating telomerase and other cell cycle-related genes, c-Myc accelerates the cell cycle, contributing to rapid and continuous cell division in tumors. o Metabolic Shifts: c-Myc also promotes metabolic changes that support high rates of growth and energy production in cancer cells, further sustaining tumorigenesis.  Additional Regulatory Mechanisms of Telomerase: o Other Oncogenes and Pathways: Beyond c-Myc, other oncogenic pathways, such as RAS and AKT, can indirectly influence telomerase activity by activating transcription factors or signaling cascades that lead to TERT upregulation. o Epigenetic Regulation: In some cancers, telomerase expression is regulated by epigenetic changes (e.g., DNA methylation or histone modifications) at the TERT promoter, allowing cancer cells to maintain high telomerase levels without gene mutations.  Therapeutic Implications of Telomerase and c-Myc Interaction: o Targeting c-Myc or its downstream effects on TERT expression could limit telomerase activity, slowing cancer cell proliferation and promoting senescence. o Understanding how telomerase is regulated by oncogenes like c-Myc highlights potential avenues for developing anti-cancer therapies aimed at disrupting the telomerase-mediated immortality of cancer cells. TOPIC-12 1. Multistep Process of Cancer Formation  Sequential Genetic and Epigenetic Changes: o Cancer is a multistep process involving the accumulation of multiple genetic and epigenetic changes over time. o Stages of Progression: Tumorigenesis generally progresses through stages such as hyperplasia (increased cell growth), dysplasia (abnormal cell appearance and organization), in situ carcinoma (early-stage cancer contained within the tissue), and eventually invasive carcinoma (cancer that spreads to surrounding tissues). o Mutations in Key Genes: Mutations often affect oncogenes, tumor suppressor genes, and genes involved in DNA repair.  Oncogenes (e.g., RAS, MYC): Mutations can activate these genes, promoting uncontrolled cell growth.  Tumor Suppressors (e.g., p53, Rb): Loss or inactivation of these genes removes growth restraints, allowing cancerous cells to proliferate.  Clonal Evolution:  o Tumors evolve through clonal expansion—each mutation that confers a growth advantage results in a clone of cells with that mutation. As new mutations accumulate, some clones outcompete others, leading to tumor growth. o Selection Pressure: Environmental factors, immune responses, and therapy create selective pressures, favoring mutations that enable survival and resistance.  Hallmarks of Cancer: o The multistep model aligns with the “hallmarks of cancer,” which are essential traits for cancer progression, including sustained proliferation, resistance to cell death, invasion, and metastasis. 2. Field Cancerization and Accumulation of Mutations Over Time  Field Cancerization: o Definition: Field cancerization refers to a phenomenon where a large area of cells within a tissue accumulates genetic or epigenetic changes, creating a “field” that is predisposed to developing cancer. o Origins: It is thought to arise from exposure to carcinogens (e.g., tobacco in lung cancer, UV in skin cancer) or underlying genetic predispositions that affect a specific area of tissue. o Implications for Cancer Development:  Cells within this “field” have mutations that may not yet be sufficient for cancer formation but make them more susceptible to acquiring additional mutations.  Over time, a subset of these altered cells may accumulate further mutations, eventually leading to tumor formation in the pre-damaged area.  Accumulation of Mutations Over Time: o Stepwise Accumulation: Cancer cells typically accumulate mutations gradually, through environmental exposure (e.g., chemicals, radiation), errors in DNA replication, and defects in DNA repair mechanisms. o Age as a Risk Factor: The accumulation of mutations over time makes cancer more common with age, as older individuals have had more time for mutations to build up in their cells. o Early vs. Late Mutations:  Initiating Mutations: These occur early in the field and provide a foundation for further mutations. They often involve tumor suppressor genes or DNA repair genes.  Driver and Passenger Mutations: Driver mutations promote cancer progression, while passenger mutations do not contribute directly but can increase overall genetic instability. 3. Cancer Stem Cells and Their Role in Tumor Heterogeneity  Cancer Stem Cells (CSCs): o Definition: CSCs are a subpopulation of cancer cells with stem cell-like properties, including the ability to self-renew and differentiate into various cell types within the tumor. o Origin: CSCs may arise from normal stem cells or from differentiated cells that acquire mutations enabling stem-like behavior.  Role in Tumor Heterogeneity: o Tumor Diversity: CSCs give rise to a range of differentiated cells within the tumor, creating tumor heterogeneity (the presence of diverse cell types within a single tumor).  o Clonal Evolution and Plasticity: CSCs contribute to clonal evolution, allowing different subclones to adapt to various conditions, such as nutrient availability or therapeutic interventions. o Hierarchical Structure: Many tumors are organized hierarchically, with CSCs at the top, generating other cell types that make up the bulk of the tumor.  Resistance to Therapy: o Survival Mechanisms: CSCs are often resistant to traditional cancer therapies (e.g., chemotherapy and radiation) due to their efficient DNA repair, quiescence (dormant state), and drug-efflux pumps. o Recurrence and Metastasis: After therapy, CSCs can survive and regenerate the tumor, leading to relapse or metastasis. o Therapeutic Targeting: Targeting CSCs specifically, alongside other cancer cells, is a strategy to reduce tumor heterogeneity and improve long-term treatment outcomes by preventing recurrence. TOPIC-13 1. Structural DNA Variations Leading to Tumor Formation  Types of Structural DNA Variations: o Structural variations involve large-scale changes in DNA structure, including deletions, duplications, inversions, translocations, and copy number variations (CNVs). o Chromosomal Translocations: In cancer, translocations often result in the fusion of genes, leading to new gene products with abnormal functions.  Example: The Philadelphia chromosome in chronic myelogenous leukemia (CML) is a result of a translocation between chromosomes 9 and 22, creating the BCR-ABL fusion gene with constant tyrosine kinase activity, driving cancer cell proliferation. o Gene Amplifications: Certain genes, like MYC and HER2, can become amplified in cancer, leading to overexpression and uncontrolled growth.  Contribution to Tumorigenesis: o Loss of Tumor Suppressors: Structural variations can lead to the loss or inactivation of tumor suppressor genes (e.g., p53 or Rb), removing growth control and allowing cells to proliferate unchecked. o Oncogene Activation: Translocations and amplifications can activate oncogenes, giving cells growth advantages and promoting tumor progression.  Genomic Instability as a Hallmark of Cancer: o Cancer cells exhibit high levels of genomic instability, with frequent structural variations that contribute to clonal evolution and tumor heterogeneity. o Therapeutic Implications: Targeting specific structural variations (e.g., BCR-ABL in CML) has become a cornerstone of personalized cancer therapies, demonstrating the importance of understanding these changes. 2. DNA Adduct Formation Due to Carcinogen Exposure (e.g., Benzo[a]pyrene in Lung Cancer)  Definition of DNA Adducts: o DNA adducts form when a chemical compound binds covalently to DNA, leading to alterations that can cause mutations if not repaired properly.  Carcinogen Example: Benzo[a]pyrene: o Source: Benzo[a]pyrene is a polycyclic aromatic hydrocarbon (PAH) found in cigarette smoke and other pollutants, making it a common lung carcinogen. o Mechanism:   Benzo[a]pyrene is metabolized into benzo[a]pyrene diol epoxide (BPDE) in the body, which binds to guanine bases in DNA, forming bulky DNA adducts.  Mutagenic Potential: These adducts disrupt DNA structure, leading to mispairing during replication, resulting in mutations that can activate oncogenes (e.g., KRAS) or inactivate tumor suppressor genes (e.g., p53).  Cancer Implications: o Lung Cancer: DNA adducts from benzo[a]pyrene are strongly associated with mutations in the TP53 gene in lung cancer, where they play a significant role in tumorigenesis. o Biomarker Potential: Detection of DNA adducts in tissues can serve as a biomarker of carcinogen exposure and early mutagenic events, aiding in cancer risk assessment and prevention efforts. 3. Inherited Defects in DNA Repair Mechanisms (e.g., Xeroderma Pigmentosum, BRCA1/2 Mutations)  Xeroderma Pigmentosum (XP): o Disease Overview: XP is a rare genetic disorder characterized by extreme sensitivity to UV radiation due to inherited defects in nucleotide excision repair (NER), a DNA repair mechanism responsible for repairing UV-induced DNA damage. o Mechanism:  Individuals with XP have mutations in one of the genes (e.g., XPA, XPB, XPC) involved in the NER pathway.  Consequences: Inability to repair UV-induced DNA damage leads to the accumulation of mutations, particularly in skin cells, resulting in a high risk of skin cancers at a young age. o Cancer Implications: XP patients have a greatly increased risk of developing skin cancers, illustrating how inherited defects in DNA repair can predispose individuals to cancer.  BRCA1 and BRCA2 Mutations: o Role of BRCA1/2: BRCA1 and BRCA2 are tumor suppressor genes that play essential roles in the homologous recombination repair (HRR) pathway, which repairs double-strand breaks (DSBs) in DNA. o Mechanism of Tumor Suppression:  BRCA1 and BRCA2 help maintain genomic stability by repairing DSBs accurately.  When these genes are mutated, DSBs accumulate, leading to chromosomal instability and increasing the risk of cancer. o Cancer Risk:  Inherited mutations in BRCA1 or BRCA2 significantly increase the risk of breast, ovarian, prostate, and pancreatic cancers.  Therapeutic Relevance: Tumors with BRCA mutations are sensitive to PARP inhibitors, which block an alternative DNA repair pathway, leading to cell death by exploiting the already compromised DNA repair machinery.  Cancer Predisposition and Genomic Instability: o Inherited defects in DNA repair mechanisms increase cancer risk by allowing mutations to accumulate unchecked, leading to genomic instability, which is a hallmark of cancer. o Familial Cancer Syndromes: Conditions like Lynch syndrome, Fanconi anemia, and Li-Fraumeni syndrome are also linked to inherited DNA repair defects, underscoring the connection between DNA repair and cancer susceptibility. TOPIC-14 1. Loss of Heterozygosity (LOH) and Mechanisms Leading to Tumorigenesis  Definition of Loss of Heterozygosity (LOH): o LOH refers to the loss of one allele of a gene in a cell that originally had two different alleles (one mutated, one normal). o Significance in Cancer: In tumor suppressor genes, LOH results in the cell losing its only functional copy of the gene, effectively eliminating its tumor-suppressing activity and increasing the risk of cancer.  Mechanisms of LOH: o Mitotic Recombination: During cell division, homologous chromosomes may undergo recombination, which can result in both copies of the chromosome carrying the mutated gene, leading to LOH. o Gene Deletion: Physical deletion of part or all of the chromosome carrying the normal allele can lead to LOH. o Chromosomal Nondisjunction: Failure of chromosomes to separate properly during cell division can result in a cell inheriting two copies of the mutated gene and losing the normal copy.  Role in Tumorigenesis: o Two-Hit Hypothesis: Many tumor suppressor genes follow the Knudson “two-hit” hypothesis, which states that both alleles of a tumor suppressor gene must be inactivated to drive cancer formation. o Examples: Common inactivation of tumor suppressors by LOH occurs in p53, Rb, and BRCA1/2 genes, contributing to a wide range of cancers due to loss of growth regulation and DNA repair mechanisms. 2. Promoter Methylation and Its Role in Inactivating Tumor Suppressor Genes (e.g., NF1 in Ras Signaling)  Definition of Promoter Methylation: o Epigenetic Modification: Promoter methylation is the addition of methyl groups to the CpG islands (cytosine-phosphate-guanine sequences) in gene promoter regions. o Silencing Mechanism: This modification prevents the transcription of genes by blocking transcription factors and recruiting repressive proteins, effectively silencing the gene.  Tumor Suppressor Gene Inactivation: o Epigenetic Silencing of Tumor Suppressors: Promoter methylation is a common mechanism for inactivating tumor suppressor genes without requiring genetic mutations. o Example - NF1 in Ras Signaling:  NF1 (Neurofibromin 1) is a tumor suppressor gene that negatively regulates Ras signaling, which is crucial for controlling cell proliferation.  Methylation and Inactivation: Promoter methylation of NF1 results in decreased expression, leading to overactive Ras signaling, promoting uncontrolled cell growth and increasing cancer risk.  Cancer Implications: o Prevalence in Various Cancers: Promoter methylation inactivates multiple tumor suppressor genes, including p16, RASSF1A, and BRCA1 across different cancers. o Potential Therapeutic Target: Demethylating agents (e.g., 5-azacytidine) are being studied as cancer therapies to restore the function of epigenetically silenced tumor suppressors. 3. Consequences of Loss of Tumor Suppressors like APC (Colon Cancer) and VHL (Kidney Cancer)  APC (Adenomatous Polyposis Coli) in Colon Cancer:  o Role in Cell Signaling: APC is a critical tumor suppressor involved in the Wnt signaling pathway. It helps regulate β-catenin, a protein that controls cell proliferation and differentiation. o Mechanism in Colon Cancer:  When APC is functional, it promotes the degradation of β-catenin, preventing excessive cell growth.  Mutations in APC: In colorectal cancer, mutations in APC prevent β-catenin degradation, allowing it to accumulate in the nucleus, where it activates genes that drive cell proliferation. o Cancer Development:  Loss of APC function is an early event in the development of colorectal tumors, leading to the formation of polyps and, eventually, malignant tumors.  Familial Adenomatous Polyposis (FAP): Individuals with inherited APC mutations develop numerous polyps and have a high risk of colon cancer.  VHL (Von Hippel-Lindau) in Kidney Cancer: o Function of VHL: The VHL protein is a tumor suppressor that regulates the degradation of hypoxia-inducible factor (HIF), a protein involved in cellular responses to low oxygen levels. o Mechanism in Kidney Cancer:  Under normal oxygen conditions, VHL tags HIF for degradation, maintaining low HIF levels and preventing uncontrolled cell growth and angiogenesis.  Mutations in VHL: In renal cell carcinoma, loss of VHL function leads to the accumulation of HIF, which activates genes involved in angiogenesis (e.g., VEGF) and cell proliferation, promoting tumor growth. o Cancer and Syndrome Implications:  VHL mutations contribute to clear cell renal cell carcinoma, a common type of kidney cancer.  Von Hippel-Lindau Disease: Inherited VHL mutations predispose individuals to multiple tumors, including kidney cancer, hemangioblastomas, and pheochromocytomas.  Implications of Tumor Suppressor Loss: o The loss of APC and VHL exemplifies how inactivation of tumor suppressors can disrupt critical cellular pathways, promoting unregulated cell growth and tumorigenesis. o Therapeutic Targeting: Understanding these pathways has led to targeted therapies, such as VEGF inhibitors for kidney cancer and Wnt pathway inhibitors in colorectal cancer, designed to counteract the effects of tumor suppressor loss. TOPIC-15. Mutant p53’s Role in Enhancing Cancer Cell Metabolism and Survival  Mutant p53 and Altered Metabolic Pathways: o While wild-type p53 typically acts to suppress tumor growth, mutations in p53 not only disable its tumor-suppressing functions but also contribute to metabolic adaptations that support cancer cell survival. o Gain-of-Function Mutations: Some mutations in p53 lead to “gain-of-function” effects, where mutant p53 acquires new capabilities that enhance cancer cell metabolism and resistance to stress. o Support for Glycolysis: Mutant p53 increases the expression of enzymes involved in glycolysis, helping cancer cells generate energy rapidly and sustain growth, even under low oxygen conditions (hypoxia).  Promoting Anabolic Processes:  o Increased Glucose Uptake: Mutant p53 enhances the expression of glucose transporters, particularly GLUT-1, which increases glucose uptake to fuel the high metabolic demands of proliferating cancer cells. o Shift Toward Biosynthesis: Mutant p53 favors metabolic pathways that support biosynthesis, providing necessary precursors (nucleotides, lipids, amino acids) for rapid cell division.  Enhanced Survival Under Stress: o Oxidative Stress: Mutant p53 enhances the antioxidant capacity of cancer cells by regulating enzymes that neutralize reactive oxygen species (ROS), helping cancer cells survive oxidative stress. o Resistance to Therapy: This enhanced survival mechanism allows mutant p53 cancer cells to withstand stress conditions, including those induced by chemotherapy and radiotherapy, leading to therapy resistance. 2. Warburg Effect (Increased Glycolysis in Cancer Cells) and the Role of GLUT-1  Warburg Effect: o Definition: The Warburg effect describes a shift in cancer cell metabolism where cells rely on glycolysis for energy production, even in the presence of adequate oxygen. This contrasts with normal cells, which prefer oxidative phosphorylation in aerobic conditions. o Metabolic Adaptation: Glycolysis is less efficient at generating ATP compared to oxidative phosphorylation, but it produces intermediate molecules that are essential for rapid cell growth and division.  Role of GLUT-1 in Cancer Metabolism: o Increased Glucose Uptake: Cancer cells require more glucose to sustain the high rate of glycolysis. GLUT-1 (Glucose Transporter 1) is frequently upregulated in cancer cells, facilitating increased glucose uptake from the extracellular environment. o Link to p53 Mutations: Mutant p53 contributes to the upregulation of GLUT-1, driving the Warburg effect and supporting cancer cell survival in hypoxic conditions often found within tumors. o Cancer Diagnosis and Imaging: The increased glucose uptake in cancer cells forms the basis for FDG-PET scans (fluorodeoxyglucose positron emission tomography), which detect areas of high glucose metabolism in the body and help in cancer diagnosis.  Implications of the Warburg Effect: o Advantages for Cancer Cells: By prioritizing glycolysis, cancer cells can quickly adapt to fluctuating oxygen levels and meet their biosynthetic needs for rapid proliferation. o Therapeutic Targeting: Targeting glycolytic pathways, GLUT-1 transporters, or other metabolic dependencies of cancer cells offers a potential strategy for cancer therapy, particularly in tumors with p53 mutations that drive the Warburg effect. 3. Mutant p53’s Interference with Apoptosis and Autophagy  Inhibition of Apoptosis: o Loss of Pro-Apoptotic Function: Wild-type p53 promotes apoptosis in response to DNA damage and stress, thereby preventing damaged cells from proliferating. Mutant p53 lacks this function, allowing cancer cells to avoid programmed cell death. o Gain-of-Function in Apoptosis Resistance: Some forms of mutant p53 actively interfere with apoptosis signaling by interacting with other pro-apoptotic proteins (e.g., BAX and PUMA) and preventing their activation.  o Cancer Implications: By evading apoptosis, cancer cells with mutant p53 can survive and proliferate despite DNA damage and cellular stress, which contributes to tumor growth and resistance to therapies that rely on apoptosis induction.  Interference with Autophagy: o Role of Autophagy: Autophagy is a process in which cells degrade and recycle their components, especially under stress or nutrient-deprived conditions, supporting survival by generating energy and clearing damaged organelles. o Mutant p53 and Autophagy Suppression:  Mutant p53 can inhibit autophagy by blocking autophagy-related signaling pathways, like those involving AMPK or mTOR, leading to the accumulation of damaged cellular components.  Impact on Metabolism: By inhibiting autophagy, mutant p53 ensures that cancer cells remain reliant on glycolysis and other energy sources, further supporting the Warburg effect. o Consequences for Cancer Cells:  Autophagy suppression helps cancer cells survive under stress conditions, particularly in nutrient-deprived environments, such as poorly vascularized areas within tumors.  Therapeutic Implications: Targeting mutant p53’s influence on autophagy is a potential strategy to disrupt the metabolic flexibility of cancer cells and induce cell death in tumors with p53 mutations. TOPIC-16 1. How Cancer Cells Evade Apoptosis and Promote Metastasis  Mechanisms of Apoptosis Evasion: o Loss of Pro-Apoptotic Signals:  Cancer cells often reduce the expression of pro-apoptotic proteins, such as BAX, BID, and PUMA, which are crucial for initiating apoptosis.  p53 Inactivation: Many cancers inactivate the p53 tumor suppressor gene, which plays a central role in apoptosis. Without p53, cancer cells avoid cell death even when they accumulate genetic damage. o Overexpression of Anti-Apoptotic Proteins:  Proteins like Bcl-2 and Bcl-xL prevent the release of cytochrome c from mitochondria, blocking the intrinsic pathway of apoptosis.  Cancer Progression: By upregulating anti-apoptotic proteins, cancer cells survive in stressful environments, such as nutrient-poor or low-oxygen (hypoxic) conditions within tumors. o Survival Pathways Activation:  Cancer cells activate survival pathways, such as PI3K/AKT and NF-κB signaling, which promote cell survival, growth, and resistance to apoptotic signals.  Therapy Resistance: These pathways contribute to resistance to chemotherapy and radiotherapy, which often rely on inducing apoptosis to kill cancer cells.  Link Between Apoptosis Evasion and Metastasis: o Enhanced Mobility and Invasion:  Cells that evade apoptosis acquire characteristics that support metastasis, including increased motility, resistance to anoikis (a form of apoptosis that normally occurs when cells detach from the extracellular matrix), and invasion capabilities.  EMT (Epithelial-Mesenchymal Transition): Through EMT, cancer cells lose their cell adhesion properties and gain migratory and invasive abilities. EMT is associated with a reduction in apoptotic sensitivity, aiding metastasis.  o Survival in the Circulatory System:  Metastatic cancer cells can resist apoptosis in the bloodstream or lymphatic system, allowing them to survive during transit to distant sites.  Immune Evasion: By avoiding apoptosis, cancer cells can evade immune surveillance mechanisms that typically identify and eliminate abnormal cells. o Establishment at Secondary Sites:  Once they reach distant organs, metastatic cells must survive in a foreign microenvironment. Evasion of apoptosis helps them adapt to new tissue conditions, establishing metastatic colonies. 2. Inactivation of p53 as a Potential Anti-Cancer Target  Role of p53 in Apoptosis: o Tumor Suppressor Function: p53 is often called the “guardian of the genome” because it protects cells from becoming cancerous by inducing apoptosis, cell cycle arrest, or DNA repair in response to DNA damage or cellular stress. o Apoptotic Induction: In its active form, p53 promotes the expression of pro- apoptotic genes, such as BAX, PUMA, and NOXA, which trigger mitochondrial pathways leading to cell death.  Consequences of p53 Inactivation in Cancer: o Loss of Apoptotic Control: When p53 is inactivated by mutations (common in over 50% of cancers), cells evade apoptosis, allowing them to accumulate mutations and divide uncontrollably. o Increased Cancer Aggressiveness: p53-deficient tumors tend to be more aggressive, therapy-resistant, and prone to metastasis due to the lack of apoptotic control. o Mutant p53 Gain-of-Function: In addition to losing normal function, some p53 mutations acquire new abilities that promote cell survival, proliferation, and metastasis, making mutant p53 an active driver of cancer progression.  Targeting p53 Pathways for Anti-Cancer Therapy: o Restoration of p53 Function:  Researchers are exploring ways to restore p53 function in cancer cells. Small molecules like PRIMA-1 and APR-246 have shown promise in reactivating mutant p53, restoring its pro-apoptotic effects in cancer cells.  Gene Therapy: Techniques to deliver functional p53 genes directly to tumors are under investigation, with the goal of re-establishing normal p53 activity and inducing cancer cell apoptosis. o Targeting Downstream Pathways:  When p53 cannot be reactivated, therapies can target downstream pathways impacted by p53 inactivation. For example, drugs that inhibit MDM2, an oncogenic protein that binds and inactivates p53, have shown efficacy in cancers with functional p53 by preventing its degradation.  Exploiting Synthetic Lethality: p53-deficient cells rely on alternative DNA repair pathways. Targeting these pathways can cause cancer cells to accumulate lethal DNA damage, a strategy used in PARP inhibitors for BRCA-mutant cancers. o Enhancing Apoptosis Sensitivity:  Therapies that increase sensitivity to apoptosis by inhibiting survival pathways (e.g., Bcl-2 inhibitors like venetoclax) or by reactivating caspase signaling are promising approaches for p53-deficient cancers.  Combination Therapies: Combining apoptosis-enhancing drugs with conventional therapies can improve outcomes in cancers with p53 mutations, as it lowers the resistance to treatments that typically rely on apoptosis induction. TOPIC-17 1. Process of Autophagy and Its Relationship with Tumor Aggression  Autophagy Overview: o Definition: Autophagy is a cellular process in which cells degrade and recycle their own components, including damaged organelles and misfolded proteins. This process helps maintain cellular homeostasis, especially under stress or nutrient deprivation. o Mechanism:  Initiation: Autophagy begins with the formation of an isolation membrane or phagophore that encloses cellular components, forming an autophagosome.  Fusion with Lysosome: The autophagosome then fuses with a lysosome, where hydrolytic enzymes break down the contents, providing the cell with energy and molecular building blocks.  Regulation: Key regulators of autophagy include proteins like AMPK (activates autophagy under low energy) and mTOR (inhibits autophagy in nutrient-rich conditions).  Autophagy in Cancer: o Dual Role of Autophagy:  In early-stage or low-grade tumors, autophagy can act as a tumor suppressor by preventing the accumulation of damaged organelles and maintaining cellular integrity.  In advanced cancers, however, autophagy often promotes survival and tumor aggression by helping cancer cells adapt to harsh conditions. o Autophagy and Tumor Aggression:  Survival under Stress: Tumor cells rely on autophagy to survive in nutrient- deprived, hypoxic tumor microenvironments. Autophagy provides a source of nutrients and energy, enabling cells to withstand stress.  Therapy Resistance: Autophagy helps cancer cells resist treatments like chemotherapy and radiation, which create stress within the cells. By recycling cellular components, autophagy can prevent therapy-induced cell death, contributing to treatment resistance.  Metastasis and Invasion: Increased autophagic activity is associated with more aggressive cancer behaviors, including metastasis. For example, cancer cells that undergo epithelial-mesenchymal transition (EMT) often display elevated autophagy, which supports their migration and invasion into other tissues.  Therapeutic Targeting of Autophagy: o Inhibition in Advanced Cancers: In cancers where autophagy promotes tumor growth, autophagy inhibitors (e.g., chloroquine and hydroxychloroquine) are being explored to prevent cancer cells from adapting to stress, potentially making them more susceptible to conventional treatments. o Induction in Early-Stage Cancers: In some contexts, promoting autophagy can help eliminate damaged cells, acting as a tumor-suppressive strategy in the early stages of cancer development. 2. Autophagy’s Mutual Exclusivity with Apoptosis  Relationship between Autophagy and Apoptosis: o Distinct Processes:  Autophagy is a survival mechanism where cells degrade parts of themselves to cope with stress.   Apoptosis, in contrast, is a form of programmed cell death where cells systematically dismantle themselves, usually in response to severe damage or irreversible stress. o Mutual Exclusivity:  Autophagy and apoptosis are generally mutually exclusive; when one