Mechanisms of viral oncogenesis

Viral Blueprint - The Cancer Playbook

• Direct-acting: Viruses either carry a viral oncogene (v-onc) or integrate their genome near a cellular proto-oncogene (c-onc), causing overexpression (insertional mutagenesis).
• Indirect-acting: Viral proteins can bind to and inactivate cell cycle regulators and tumor suppressors.
• Chronic Inflammation: Persistent viral infection (e.g., HCV, HBV) causes chronic inflammation, leading to ↑ cell turnover and accumulation of mutations.

⭐ High-yield example: Human Papillomavirus (HPV) produces the E6 protein, which leads to the degradation of p53, and the E7 protein, which inhibits the Retinoblastoma (Rb) protein.

DNA Viruses - Cellular Hijackers

DNA viruses typically integrate their genome into host DNA. Their oncoproteins target and neutralize the cell's primary "brakes" on proliferation: the p53 and Rb tumor suppressor proteins. This forces the cell into a state of continuous replication, increasing the risk of malignant transformation.

• Human Papillomavirus (HPV)

  • High-risk serotypes: 16, 18, 31, 33.
  • Viral Oncoproteins:
    • $E6$: Promotes ubiquitin-mediated degradation of the p53 tumor suppressor.
    • $E7$: Binds and inactivates the retinoblastoma (Rb) protein.
  • Mechanism: Inactivating Rb releases E2F transcription factor, pushing the cell past the G1/S checkpoint. Loss of p53 prevents apoptosis in response to this abnormal growth signal.

• Epstein-Barr Virus (EBV)

  • Proteins: Latent Membrane Protein 1 (LMP-1) acts as a constitutively active CD40 receptor, promoting B-cell proliferation. EBNA-2 transactivates host proto-oncogenes like c-myc.
  • Cancers: Burkitt Lymphoma, nasopharyngeal carcinoma.

• Other Key DNA Viruses

  • Hepatitis B (HBV): Primarily drives cancer via chronic inflammation and hepatocyte regeneration. The HBx protein can also transactivate growth-promoting genes.
  • HHV-8 (KSHV): Encodes viral homologs of key cellular proteins (v-cyclin, v-BCL-2) that directly promote cell cycling and inhibit apoptosis.

⭐ High-risk HPV strains (16, 18) cause >70% of cervical cancers. The combined inactivation of both p53 (by $E6$) and Rb (by $E7$) is the critical driver of malignant transformation.

RNA Retroviruses - The Tax Man Cometh

• Human T-lymphotropic Virus (HTLV-1): A deltaretrovirus.
  • Encodes the oncoprotein Tax, a key transcriptional trans-activator.
  • Tax stimulates host cell proliferation by activating genes like $IL-2$ and its receptor, $IL-2R$.
  • This drives polyclonal T-cell expansion, increasing the risk of secondary mutations.
• Mechanism: Indirect action; does not rely on insertional mutagenesis near a proto-oncogene.
• Associated Malignancy: Adult T-cell Leukemia/Lymphoma (ATLL).

⭐ HTLV-1 exhibits a long latency period (decades), with ATLL developing in only <5% of those chronically infected.

High‑Yield Points - ⚡ Biggest Takeaways

• Viral oncogenesis hinges on inactivating tumor suppressors (e.g., p53, Rb) and activating proto-oncogenes.
• HPV uses E6 to degrade p53 and E7 to inhibit Rb, forcing cell cycle progression.
• HBV and HCV induce hepatocellular carcinoma primarily via chronic inflammation and hepatocyte regeneration.
• EBV (Burkitt lymphoma) and HHV-8 (Kaposi's sarcoma) express oncoproteins that mimic host growth factors.
• HTLV-1's Tax protein transactivates host cell genes, driving proliferation in adult T-cell leukemia/lymphoma.