Carcinogenesis and Carcinogens

Introduction to Carcinogenesis - The Bad Seed

• Carcinogenesis: The complex, multistep process transforming normal cells into cancer cells.
• Driven by accumulated genetic (DNA mutations) & epigenetic alterations.
• Key Stages:
  • Initiation: Rapid, irreversible genetic damage (mutation) by a carcinogen.
  • Promotion: Reversible, prolonged clonal expansion of initiated cells; non-mutagenic.
  • Progression: Irreversible; further mutations yield malignant phenotype (invasion, metastasis).

⭐ Most human cancers are of clonal origin, arising from a single genetically altered precursor cell.

Molecular Basis of Cancer - Blueprint for Disaster

• Core principle: Cancer arises from non-lethal genetic damage, leading to uncontrolled cell proliferation.
• Key targets of this damage - four classes of normal regulatory genes:
  • Proto-oncogenes: Promote cell growth (e.g., RAS, MYC, HER2). Activation (gain-of-function mutations) converts them to oncogenes.
  • Tumor Suppressor Genes (TSGs): Inhibit cell growth or promote DNA repair (e.g., RB, TP53, APC). Inactivation (loss-of-function mutations) contributes to cancer.
  • Apoptosis Regulating Genes: Control programmed cell death (e.g., BCL2 family). Alterations lead to evasion of apoptosis (e.g., overexpression of anti-apoptotic BCL2).
  • DNA Repair Genes: Maintain genomic integrity (e.g., BRCA1/2, MSH/MLH genes). Inactivation leads to genomic instability (mutator phenotype), accelerating mutation accumulation.
• Carcinogenesis: A multistep process involving accumulation of multiple genetic (mutations) & epigenetic (e.g., methylation) alterations.

⭐ TP53 ("guardian of the genome") is the most commonly mutated gene in human cancers (involved in >50% of cases), critical for cell cycle arrest, DNA repair, and apoptosis induction upon DNA damage. Its inactivation is a major step in many cancers.

Chemical Carcinogenesis - Toxic Triggers

• Direct-acting: No metabolic conversion needed (e.g., alkylating agents).
• Indirect-acting (Procarcinogens): Require metabolic activation, primarily by cytochrome P-450 enzymes.
  • Activated forms (ultimate carcinogens) bind DNA, forming DNA adducts → mutations.
• Key Examples & Associated Cancers:
  • Polycyclic Aromatic Hydrocarbons (PAHs) (e.g., benzopyrene in tobacco smoke, soot): Lung, skin.
  • Aromatic Amines (e.g., $\beta$-naphthylamine): Bladder.
  • Aflatoxin B1 (Aspergillus flavus on improperly stored grains): Hepatocellular carcinoma (HCC).
  • Nitrosamines (from nitrites in preserved foods): Gastric.
  • Vinyl chloride (plastics industry): Liver angiosarcoma.
  • Arsenic (contaminated water, pesticides): Skin (squamous cell carcinoma), lung, liver angiosarcoma.
  • Asbestos: Lung carcinoma, mesothelioma.
  • Nickel, Chromium: Lung.

⭐ Aflatoxin B1, a product of Aspergillus, is a potent natural hepatocarcinogen strongly associated with G:C → T:A transversions in codon 249 of the TP53 gene.

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Radiation & Microbial Carcinogenesis - Invisible Invaders

• Radiation:
  • UV (Sunlight): Pyrimidine dimers (NER). Cancers: SCC, BCC, Melanoma. Xeroderma Pigmentosum ↑ risk.
  • Ionizing (X-rays, nuclear): DNA ds-breaks. Cancers: Leukemia, Thyroid (children), Breast, Lung.
• Microbial:
  • Viruses:
    • HPV (16, 18): Cervical Ca (E6→p53↓, E7→Rb↓).
    • EBV: Burkitt's (t(8;14)), Nasopharyngeal Ca, Hodgkin's.
    • HBV/HCV: HCC (chronic inflammation).
    • HTLV-1: ATLL (Tax protein).
    • HHV-8: Kaposi Sarcoma.
  • Bacteria:
    • H. pylori: Gastric AdenoCa, MALT Lymphoma (CagA).

⭐ EBV links to Burkitt lymphoma via t(8;14) c-myc translocation.

High‑Yield Points - ⚡ Biggest Takeaways

• Chemical carcinogenesis: initiation (DNA damage), promotion (clonal expansion), progression (malignancy).
• Aflatoxin B1 (Aspergillus) causes HCC via TP53 mutation (codon 249).
• HPV 16, 18: E6 degrades p53; E7 inhibits Rb, causing cervical cancer.
• EBV is linked to Burkitt's lymphoma and nasopharyngeal carcinoma.
• UVB radiation causes skin cancer (SCC, BCC, melanoma) via pyrimidine dimers.
• H. pylori is a risk for gastric adenocarcinoma and MALT lymphoma.
• Vinyl chloride → hepatic angiosarcoma; Asbestos → mesothelioma, lung cancer.