Retroviral insertional mutagenesis

Slow transforming retroviruses can efficiently induce oncogenic mutations via insertion of the provirus into the genome of their host cells, and these oncogenic mutations can be identified relatively easily by determining the insertion site of the provirus. Consequently, these viruses have been widely used in genetic screens for mutations involved in the onset of tumorigenesis in various model organisms.

Slow transforming retroviruses induce mono- or oligoclonal tumours with a longer latency of 3-12 months. Tumorigenesis caused by slow transforming retroviruses results from mutations caused by insertion of the proviral retrovirus into the host genome. Elements in the proviral genome that regulate the viral transcript, also act in cis on cellular gene transcripts. Depending on whether the provirus integrates into or in the vicinity of genes, these elements can enhance or disrupt normal transcription and thus induce oncogenic mutations.

Retroviral infection of cells is dependent on the binding of viral envelope proteins to cell surface receptors. After incorporation of the provirus into the genome, cells will start producing viral envelope proteins and cell surface receptors become occupied with these proteins thus inhibiting re-infection of the cell. Recombination of the viral sequences with endogenous viral sequences encoding envelope proteins, gives rise to mutant viruses encoding envelope protein variants that utilize different receptors and thus can re-infect an infected cell. Furthermore, many integrated proviruses appear to harbor defective envelope sequences allowing re-infection by the same virus. In this way, cells can undergo multiple rounds of viral infection and mutations can accumulate in one cell (reviewed in Mikkers and Berns, 2002). The ability to induce multiple mutations in the same cell, combined with the fact that the proviral insertion site can be easily identified by amplification of the genomic sequences flanking the retroviral insertion, makes slow retroviruses very suitable for genetic screens for oncogenic mutations.

Taken from Uren,A.G., Kool,J., Berns,A., and van Lohuizen,M. (2005). Retroviral insertional mutagenesis: past, present and future. Oncogene 24, 7656-7672.