Oncogenes function by altering transcriptional events either directly or through control of the cell-signalling pathways.

Both DNA and RNA viruses can harbour oncogenes, which transform cells in a dominant fashion:

• DNA viruses: Of the three classes of DNA tumour viruses -adenovirus, papilloma virus and simian SV40 virus, only the papilloma virus is tumourigenic in man. When the genes of DNA tumour viruses are incorporated into the genome of normal cells, viral-specific proteins are encoded and these push the cell into a continuous synthetic phase which is of benefit to the virus. The proteins expressed during the DNA synthetic phase also stimulate viral replication. The oncogenes incorporated in the genome of DNA viruses are not derived from genes within the human genome and are foreign to the host.

• RNA viruses: Retroviruses are small viruses which contain only about 10 genes in a single strand of RNA. Through the action of reverse (RNA-DNA) transcriptase they copy their genetic material into the DNA of the host cell. Unlike the transforming genes of DNA viruses, those of RNA tumour viruses are not of viral origin but have been 'hijacked' from normal host cells in which they existed as 'proto-oncogenes' which normally regulate cell growth. The proto-oncogene may be incorporated in the virus genome in an incomplete or mutated form which, when re-inserted into the host genome, may lead to the expression of abnormal protein products. Alternatively, re-insertion of the oncogene into a region of the genome different from its normal proto-onco-genic site may bring it under the control of genes which stimulate overexpression of its normal protein product so that it is permanently 'turned on'.

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