Carcinogenic agents

Chemical Carcinogens - The Poison Pantry

• Mechanism: Multi-step process involving initiation (irreversible DNA damage/mutation) and promotion (clonal expansion of initiated cells).
• Activation: Most are procarcinogens, metabolically activated by cytochrome P-450 monooxygenases (liver) into electrophilic ultimate carcinogens that form DNA adducts.

Key Agent → Associated Cancer:

• Aflatoxins (Aspergillus): → Hepatocellular Carcinoma (HCC)
• Alkylating Agents (chemo): → Leukemia/Lymphoma
• Aromatic amines (textile dyes): → Bladder Cancer
• Arsenic (wood preservatives): → Skin (Squamous Cell), Lung, Liver (Angiosarcoma)
• Asbestos: → Lung Cancer > Mesothelioma
• Cigarette Smoke: → Lung, Bladder, Pancreas, Esophagus
• Ethanol: → Oropharynx, Esophagus (Squamous Cell), Liver (HCC)
• Nitrosamines (smoked foods): → Gastric Cancer
• Vinyl chloride (PVC): → Liver (Angiosarcoma)

⭐ Aflatoxin B₁ induces a signature G→T mutation in codon 249 of the TP53 gene, a molecular fingerprint for diagnosing aflatoxin-induced Hepatocellular Carcinoma.

Radiation Carcinogenesis - Sun's Sinister Side

• UV Radiation (UVR): Primarily from sun exposure; non-ionizing.
  • Mechanism: Forms pyrimidine dimers (T-T, C-T) in DNA, distorting the helix.
  • Repair: Nucleotide Excision Repair (NER) pathway. Defects in NER lead to conditions like Xeroderma Pigmentosum (XP), with ↑ risk of skin cancer.
  • Cancers: Basal Cell Carcinoma (BCC), Squamous Cell Carcinoma (SCC), and Melanoma.

• Ionizing Radiation: From medical sources (X-rays, CT scans) or occupational/environmental exposure (radon, nuclear).
  • Mechanism: Causes double-strand DNA breaks, primarily through generation of hydroxyl free radicals from water.
  • Cancers: Acute Myeloid Leukemia (AML), Chronic Myeloid Leukemia (CML), and Papillary Thyroid Cancer.

⭐ High-Yield Fact: Papillary carcinoma is the most common type of radiation-induced thyroid cancer, often seen years after head and neck irradiation.

Oncogenic Microbes - Viral & Bacterial Villains

• DNA Viruses

  • HPV (Types 16, 18): E6 protein inhibits p53; E7 inhibits Rb. Causes cervical, anogenital, & oropharyngeal SCC.
  • EBV (Epstein-Barr Virus): LMP-1 protein acts as a constitutively active CD40 receptor, promoting B-cell proliferation. Linked to Burkitt & Hodgkin lymphoma, nasopharyngeal carcinoma.
  • HBV & HCV: Chronic inflammation and hepatocyte regeneration drive mutations. HBx protein (HBV) is directly oncogenic. Leads to hepatocellular carcinoma.
  • HHV-8 (KSHV): Causes Kaposi sarcoma by promoting angiogenesis and cell proliferation, especially in immunosuppressed states (e.g., AIDS).

• RNA Virus

  • HTLV-1: Tax protein activates NF-κB, stimulating T-cell proliferation. Causes Adult T-cell Leukemia/Lymphoma (ATLL).

• Bacteria

  • H. pylori: Chronic inflammation. CagA protein disrupts cell signaling, promoting proliferation. Causes gastric adenocarcinoma & MALT lymphoma.

⭐ EBV is classically associated with Burkitt Lymphoma, especially the endemic African type, which often presents with a jaw lesion. The translocation t(8;14) involving the c-MYC gene is a hallmark.

High‑Yield Points - ⚡ Biggest Takeaways

• Aflatoxins (Aspergillus) are strongly linked to hepatocellular carcinoma.
• Alkylating agents from chemotherapy create a risk for secondary leukemia/lymphoma.
• Aromatic amines (e.g., aniline dyes, tobacco smoke) are a major cause of bladder cancer.
• Arsenic exposure is associated with squamous cell carcinoma of the skin, lung cancer, and liver angiosarcoma.
• Asbestos is famously linked to bronchogenic carcinoma and mesothelioma.
• Ionizing radiation is a significant risk factor for papillary thyroid carcinoma and leukemias.