Carcinogenesis: Infection

1. Human T-Cell Leukemia Virus Type 1 (HTLV-1)

- only known human RNA retrovirus associated with cancer

- Similar to AIDS, the HTLV-1 targets CD4+ T cells for neoplastic transformation

- stimulates the production of T cells by the TAX gene that encodes IL-2 and its receptor, thereby setting up an autocrine system for proliferation

- At the same time a paracrine pathway increases production of GM-CSF that acts to increase the secretion of other T-cell mitogens, like IL-1.

- Along with these growth-promoting activities, there is also the inhibition of growth-suppressive pathways.

- human infection requires transmission of infected T-cells via sexual intercourse, blood products or breast feeding

2. Human Papillomavirus (HPV)

- Definitely cause benign squamous papillomas (warts) and also can cause the genesis of several cancers such as squamous cell carcinoma of the cervix and anal, perianal, vulvar, and penile cancers.

- Oncogenic potential of HPV is related to products of two early viral genes, E6 and E7. Together they act to simulate the loss of tumor suppressor genes, activate cyclins, inhibit apoptosis, and combat cellular senescence.

- Infection of HPV alone is not sufficient for carcinogenesis. Cotransfection with mutated RAS gene leads to full malignant transformation.

3. Epstein-Barr Virus (EBV)

- implicated in pathogenesis of Burkitt lymphoma, post-transplant lymphoproliferative disease, primary central nervous system lymphoma in AIDS patients, a subset of other AIDS related lymphomas, subset of Hodgkin lymphoma, and nasophyrngeal carcinoma

- all are B-cell tumors, except for nasopharyngeal carcinoma

- molecular basis by EBV-encoded gene LMP-1 acting as oncogene to promote B-cell proliferation by activating signaling pathways that mimic B-cell activation by B-cell surface molecule CD40.

4. Hepatitis B Virus (HBV)

- Strong evidence linking HBV infection with hepatocellular carcinoma (liver cancer), but mode of tumor production is not fully known

- oncogenic effect seems to be multifactorial : 1) Causes chronic liver cell injury, followed by regeneration that predisposes the cells to mutations, 2) HBV encoded regulatory element HBx disrupts normal liver growth of infected liver by transcriptional activation of several growth controlling genes, 3) cytosolic signal transduction pathways are turned on (ex/ RAS-MAP kinases), 4) viral integration seems to cause secondary rearrangements of chromosomes, including multiple deletions that may include tumor suppressor genes.

5. Human Herpesvirus 8 (HHV8)

- associated with rare group of tumors present as malignant effusions

- tumor cells infected with HHV-8 that encodes proteins homologous to several oncoproteins, ex/ cyclin D1

- patients with these primary effusion lymphomas are usually immunosuppressed

Category: Pathology Notes