Viruses and Cancer

Overview of Virus-Induced Cancers

Several types of cancer are caused by viruses, often developing long after an initial viral infection. Unlike other forms of cancer, those caused by viruses are not contagious.

• Sarcoma: Cancer of connective tissue.

• Adenocarcinoma: Cancer of glandular epithelial tissue.

The Transformation of Normal Cells Into Tumor Cells

Genetic and Viral Mechanisms

Normal cells can be transformed into tumor cells through genetic mutations and viral integration. This process involves several key genetic elements and steps:

• Proto-oncogenes: Genes that encode proteins involved in stimulating normal cell growth.

• Oncogenes: Mutated proto-oncogenes that transform normal cells into cancerous cells.

• Transformation: The process by which cells acquire distinct properties leading to cancer.

• Oncogenic viruses: Viruses whose genetic material becomes integrated into the host cell's DNA, inducing tumor formation.

• A transformed cell may harbor a tumor-specific transplantation antigen (TSTA) on its surface, which is often irregularly shaped.

DNA Oncogenic Viruses

Major Families and Examples

Several DNA viruses are known to cause cancer by integrating their genetic material into host cells.

• Adenoviridae

• Herpesviridae: Epstein-Barr virus (causes Burkitt's lymphoma)

• Poxviridae

• Papovaviridae: Human papillomavirus (HPV) (causes cervical and anal cancer)

• Hepadnaviridae: Hepatitis B virus

RNA Oncogenic Viruses

Retroviruses and Cancer

RNA viruses, particularly retroviruses, can cause cancer by transcribing their RNA into DNA and integrating it into the host genome.

• Retroviridae: Viral RNA is transcribed to DNA via reverse transcriptase, which can integrate into host DNA.

• HTLV-1 and HTLV-2: Cause adult T cell leukemia and lymphoma.

• FeLV: Feline leukemia virus.

Viruses to Treat Cancer

Oncolytic Viruses

Oncolytic viruses are engineered or naturally occurring viruses that infect and kill tumor cells or stimulate an immune response against tumor cells.

• Oncolytic viruses selectively target tumor cells.

• They may also enhance the body's immune response to cancer.

Latent Viral Infections and Persistent Viral Infections

Definitions and Examples

Some viruses can remain in the host for extended periods, either in a latent or persistent state.

• Latent virus: Remains in asymptomatic host cells for long periods; may reactivate due to changes in immunity.

• All herpesviruses are capable of latency.

• Examples:

  • HSV-1: Cold sores

  • Varicellovirus: Shingles

• Persistent viral infection: Occurs gradually over a long period and is generally fatal.

• Example: Subacute sclerosing panencephalitis (measles virus)

Prions

Proteinaceous Infectious Particles

Prions are infectious proteins that cause neurodegenerative diseases. They are inherited and transmissible by ingestion, transplant, and surgical instruments.

• Spongiform encephalopathies:

  • "Mad cow disease"

  • Creutzfeldt-Jakob disease (CJD)

  • Gerstmann-Sträussler-Scheinker syndrome

  • Fatal familial insomnia

  • Sheep scrapie

Prion Proteins and Disease Mechanism

Prion diseases are caused by the conversion of normal host glycoprotein into an infectious misfolded form.

• PrPC: Normal cellular prion protein, found on the cell surface.

• PrPSc: Scrapie protein; accumulates in brain cells, forming plaques.

• Disease mechanism: Conversion of PrPC into PrPSc, the infectious misfolded form.

Additional info: Prion diseases are unique in that they do not involve nucleic acids, and their transmission can occur via contaminated instruments or ingestion of infected tissue.