Latent Viruses

• Definition of Latency: Viruses that are not actively causing illness but remain present in the body.

• Examples:

  • Chickenpox (Varicella Zoster Virus): Lies dormant in nerve cells and can reactivate to cause shingles.

  • Herpes Viruses: Remain dormant in host cells after initial infection.

  • HIV (Human Immunodeficiency Virus): An RNA virus that uses reverse transcriptase to integrate into the host genome as a provirus, remaining latent until activated.

Latency vs. Active Infection

• Latency in Viruses:

  • Dormancy in tissues without active replication.

  • Reactivation can occur due to various factors such as stress or immune status.

• Comparison with Bacteriophages: Similar to lysogeny in bacteriophage, where the bacteriophage integrates its genome and remains dormant.

Viruses and Cancer

• Viral Induction of Cancer: Approximately 10% of all cancers are virally induced.

• Key Genetic Regulators:

  • Proto-oncogenes: Promote cell division (gas pedal analogy).

  • Tumor Suppressor Genes: Inhibit cell division, stopping it in case of DNA damage (brake pedal analogy).

• Mutation Effects:

  • Mutated proto-oncogenes become oncogenes: lead to uncontrolled cell division.

  • Mutated tumor suppressor genes fail to stop cell division, allowing for further mutations and cancer development.

Mechanisms by Which Viruses Cause Cancer

• Insertion of Oncogenes: Some viruses carry oncogenes that can integrate into host genomes.

• Stimulation of Existing Oncogenes: Some viruses activate proto-oncogenes present in host cells.

• Interference with Tumor Suppression: Viruses can disrupt tumor suppressor genes by integrating their genomes into these regions.

Types of Viruses Inducing Cancer

• Double-Stranded DNA Viruses:

  • Examples: Epstein Barr Virus, Hepatitis B Virus, Human Papillomavirus (HPV).

• Retroviruses:

  • Examples: Human T-cell Leukemia Virus (HTLV), similar to HIV.

• Specific Cancers:

  • Epstein Barr Virus: Associated with Burkitt lymphoma and nasopharyngeal cancers.

  • Hepatitis B Virus: Causes 80% of liver cancer cases.

  • HPV: 99.7% of cervical cancers are linked to HPV.

    • Vaccine: Gardasil 9 covers strains 16, 18, 6, and 11 linked to cancer and genital warts.

Emerging Viruses

• Definition: Newly recognized or changing viruses that have increased in incidence, often with global importance.

• Zoonotic Origin: Many emerging viruses originate from animal sources (e.g., SARS-CoV-2 from bats).

• Factors Contributing to Emergence:

  • Global Travel: Connectivity allows for rapid viral spread.

  • Climate Change: Alters habitats for disease vectors (e.g., mosquitoes carrying dengue).

  • Social and Behavioral Changes: Altered human interaction patterns can promote virus spread.

  • Viral Evolution: Changes in viral genetic information can increase virulence and host adaptation.

Viroids and Prions

• Viroids: Small, uncoated pieces of RNA affecting plants; discovered in 1971.

• Prions: Infectious proteins causing transmissible spongiform encephalopathies (TSEs) by misfolding normal brain proteins.

  • Transmission: Typically through the gastrointestinal tract, resistant to many disinfectants.

  • Examples of Prion Diseases:

    • Bovine Spongiform Encephalopathy (Mad Cow Disease)

    • Creutzfeldt-Jakob Disease: Human form of prion disease.

    • Kuru: Spread through ritualistic cannibalism.

    • Chronic Wasting Disease: Primarily in deer and elk.

Exam Preparation Tips

• Make flashcards of different viruses and their associations with diseases.

• Focus on understanding the mechanisms by which viruses induce cancer and the specific viruses linked to various cancers.

• Watch video resources for more insights, especially regarding emerging viruses and viral chatter.