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Oncogenic Viruses: RNA and DNA Types
Approximately 12-20% of all human cancers worldwide have a viral etiology. Oncogenic viruses are classified based on their genome type - DNA or RNA. Viruses are necessary but not sufficient for tumor induction; long latent periods typically elapse between infection and tumor formation.
- Jawetz Melnick & Adelberg's Medical Microbiology 28E; Sabiston Textbook of Surgery 11E; Harper's Biochemistry 32E
General Principles of Viral Carcinogenesis
| Tenet | Explanation |
|---|---|
| Viruses are seldom complete carcinogens | They act as initiators; co-factors needed |
| Persistent infection required | Long latency between infection and tumor |
| Host factors matter | Immune status, genetics influence outcome |
| Virus strains differ in oncogenic potential | e.g., HPV 16/18 vs. low-risk HPV types |
| Viruses modulate growth control pathways | Target cell cycle checkpoints, apoptosis |
I. DNA ONCOGENIC VIRUSES
DNA tumor viruses encode viral oncoproteins that are needed for viral replication but also disrupt host cellular growth-control pathways. Their oncogenes are viral in origin (unlike RNA viruses). In permissive hosts, they cause lytic infection; in non-permissive hosts, viral DNA integrates into the host genome, and early viral genes are persistently expressed, driving transformation. The key mechanism is binding of viral oncoproteins to p53 and Rb (tumor suppressor proteins).
1. Human Papillomavirus (HPV) - Papillomaviridae
- Genome: dsDNA
- Oncogenic types: HPV 16 and 18 (high risk)
- Cancers: Cervical cancer (no. 1 cause), vulvar/vaginal cancer, penile cancer, oropharyngeal cancer (base of tongue, tonsils), anal cancer
- Mechanism:
- E6 protein binds p53 and targets it for ubiquitin-mediated degradation → loss of apoptosis/cell cycle arrest
- E7 protein binds and inactivates Rb → uncontrolled cell cycle progression (S-phase entry)
- ~5% of all cancer worldwide is attributed to HPV
2. Epstein-Barr Virus (EBV) - Herpesviridae
- Genome: dsDNA (linear, becomes episome in latency)
- Cancers: Burkitt lymphoma, Hodgkin's disease, nasopharyngeal carcinoma, B-cell lymphoma (immunosuppression-related), sinonasal angiogenic T-cell lymphoma
- Mechanism:
- Establishes latency in B lymphocytes
- Three discrete latency programs:
- Latency I → Burkitt lymphoma
- Latency II → Hodgkin disease & nasopharyngeal carcinoma
- Latency III → Immunosuppression-related lymphoma
- In latency, only EBV RNA expressed; drives B cells to become resting memory B cells
3. Hepatitis B Virus (HBV) - Hepadnaviridae
- Genome: Partially dsDNA (unique - has a reverse transcriptase step)
- Cancer: Hepatocellular carcinoma (HCC)
- Mechanism:
- HBV protein HBx acts as a transcriptional transactivator
- Chronic inflammation → liver cirrhosis → HCC
- Integration of HBV DNA into host genome causes chromosomal instability
- Inactivation of p53 by HBx protein
4. Human Herpesvirus 8 (HHV-8 / Kaposi Sarcoma-Associated Herpesvirus, KSHV)
- Genome: dsDNA
- Cancers: Kaposi sarcoma, primary effusion lymphoma (PEL)
- Mechanism:
- Encodes viral homologs of cellular signaling proteins (v-cyclin, v-FLIP)
- v-cyclin activates CDKs → bypasses cell cycle checkpoints
- Strongly associated with HIV/AIDS-related malignancies
5. Merkel Cell Polyomavirus (MCPyV) - Polyomaviridae
- Genome: dsDNA (circular)
- Cancer: Merkel cell carcinoma (rare but aggressive skin tumor)
- Mechanism: Viral T antigen binds Rb → inactivates it → uncontrolled proliferation
II. RNA ONCOGENIC VIRUSES
Most RNA tumor viruses are retroviruses - they carry reverse transcriptase and convert their RNA genome into a DNA provirus, which integrates permanently into the host chromosome. The oncogenes of RNA viruses are of cellular origin (captured proto-oncogenes). Exception: Hepatitis C virus (HCV) is an RNA oncovirus that does NOT have reverse transcriptase.
Types of Oncogenic Retroviruses
| Type | Mechanism | Example |
|---|---|---|
| Highly oncogenic (direct-transforming) | Carry a captured cellular oncogene (v-onc) | Rous Sarcoma Virus (RSV) - carries v-src |
| Weakly oncogenic (slowly transforming) | Provirus integrates near cellular proto-oncogene; strong promoter/enhancer activates it | HTLV-1 |
1. Human T-Cell Lymphotropic Virus Type 1 (HTLV-1) - Retroviridae
- Genome: ssRNA (+) → reverse transcribed to dsDNA provirus
- Cancer: Adult T-cell leukemia/lymphoma (ATL)
- Mechanism:
- Viral protein Tax acts as a transcriptional activator
- Tax activates NF-κB pathway → promotes cell survival and proliferation
- Tax also inactivates p53 and inhibits DNA repair
- Provirus integrates near cellular genes, driving their expression
- Long latency: 20-30 years from infection to ATL
2. Hepatitis C Virus (HCV) - Flaviviridae (Exception)
- Genome: ssRNA (+) - no reverse transcriptase, does NOT form a DNA provirus
- Cancer: Hepatocellular carcinoma (HCC)
- Mechanism:
- HCV core protein and NS5A protein interfere with tumor suppressor pathways
- Chronic infection → persistent inflammation → liver fibrosis/cirrhosis → HCC
- Indirect carcinogenesis via oxidative stress and immune-mediated liver damage
- HCV NS3/4A blocks innate immune signaling (inhibits interferon response)
3. Human Immunodeficiency Virus (HIV) - Retroviridae (Indirect oncogen)
- Genome: ssRNA (+)
- Cancers: Kaposi sarcoma, non-Hodgkin lymphoma, cervical cancer (AIDS-defining)
- Mechanism: Primarily indirect - destroys CD4+ T cells → profound immunosuppression → removes immune surveillance allowing other oncogenic viruses (HHV-8, HPV, EBV) to drive malignancy
Summary Table: Oncogenic Viruses
| Virus | Type | Family | Cancer |
|---|---|---|---|
| HPV 16, 18 | DNA | Papillomaviridae | Cervical, oropharyngeal, vulvar, anal ca |
| EBV | DNA | Herpesviridae | Burkitt lymphoma, Hodgkin's, NPC |
| HBV | DNA | Hepadnaviridae | Hepatocellular carcinoma |
| HHV-8 (KSHV) | DNA | Herpesviridae | Kaposi sarcoma, PEL |
| Merkel cell polyomavirus | DNA | Polyomaviridae | Merkel cell carcinoma |
| HTLV-1 | RNA (Retrovirus) | Retroviridae | Adult T-cell leukemia/lymphoma |
| HCV | RNA (non-retrovirus) | Flaviviridae | Hepatocellular carcinoma |
| HIV | RNA (Retrovirus) | Retroviridae | AIDS-related malignancies (indirect) |
Key Oncogenic Mechanisms - Comparison
| Feature | DNA Oncogenic Viruses | RNA Oncogenic Viruses |
|---|---|---|
| Oncogene origin | Viral (v-onc) | Cellular (proto-oncogene captured) |
| Integration | Viral DNA integrates directly | RNA → reverse transcriptase → DNA provirus → integrates |
| Key targets | p53, Rb tumor suppressor proteins | Oncogene activation (Ras, Src, etc.); Tax activates NF-κB |
| Replication in permissive host | Lytic infection + cell death | Retroviral integration is permanent |
| Example key proteins | HPV E6/E7; HBx; EBV LMP-1 | HTLV-1 Tax; reverse transcriptase |
Important note on HCV: HCV is unique - it is an RNA virus that causes cancer WITHOUT reverse transcription or chromosomal integration, acting primarily through chronic inflammation and indirect disruption of tumor suppressor pathways. This makes it distinct from all other known oncogenic RNA viruses.
Sources: Jawetz Melnick & Adelberg's Medical Microbiology 28E, p. 640-641; Harper's Illustrated Biochemistry 32E, p. 704; Schwartz's Principles of Surgery 11E, p. 1585-1591; Sabiston Textbook of Surgery, p. 3160-3210
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