Mechanisms of Retrovirus-induced Cancers

Retroviruses reverse-transcribe their RNA into double-stranded DNA called provirus. The viral integrase enzyme catalyzes incorporation of this proviral DNA into the host genome. Once integrated, the provirus becomes part of the host chromosome and is replicated along with host DNA during cell division.

Integrated proviral DNA can activate cancer through three mechanisms: promoter and enhancer sequences in the provirus may activate neighboring host genes triggering abnormal growth; viral proteins may indirectly activate host genes causing aberrant expression; or the provirus may carry oncogenes that drive tumor formation upon expression in the host cell.

Acute retroviruses like Rous sarcoma virus cause cancer within weeks of infection. Non-acute retroviruses induce cancer only after long latent periods and affect only some infected hosts. Additional factors including immunosuppression, somatic mutations, genetic predisposition, and carcinogen exposure influence cancer progression in non-acute infections.

Reverse transcriptase is a viral enzyme that converts the retroviral RNA genome into single-stranded complementary DNA (cDNA). This cDNA then replicates to form double-stranded DNA, which subsequently enters the host nucleus for integration into the host genome. This process is essential for establishing persistent infection.

Retroviruses are oncogenic because their integrated proviral DNA interferes with host genes regulating cell growth and progression. When viral genes or products disrupt normal cellular controls, they transform normal cells into cancerous cells, leading to uncontrolled proliferation and tumor development.

Host factors significantly influence whether retroviral infection leads to cancer. Immunosuppression, somatic mutations, genetic predisposition, and exposure to carcinogens all contribute to cancer progression following viral infection. These factors determine whether infected individuals develop cancer and how quickly progression occurs.

Upon entering the host cytoplasm, the retroviral core disassembles to release its contents, including viral genetic material and enzymes like reverse transcriptase and integrase. These components are then available to perform their respective functions in converting and integrating the viral genome into the host DNA.