Oncoviruses come in two different forms: viruses with a DNA genome, such as adenovirus, and viruses with an RNA genome, like the Human T-cell Leukemia viruses and several viruses known to be common in cats, mice and chickens.
The oncogenic mechanism is either to insert additional oncogenic genes in the host DNA, or to enhance already existing oncogenic genes in the genome.
Other oncogenic retroviruses transform cells by integrating into the host gene near a proto-oncogene. If the viral LTRs are close enough to that oncogene, they will upregulate transcription not only of the viral DNA but of the proto-oncogene nearby, causing growth, cell proliferation and by consequence tumour formation.
|Virus||Associated cancer types|
|Hepatitis viruses, including hepatitis B and hepatitis C||Hepatocellular carcinoma (liver cancer).|
|Human T-lymphotropic virus||Tropical spastic paraparesis and adult T-cell leukemia|
|Human papillomaviruses||Cancers of cervix, skin, anus, and penis.|
|Kaposi’s sarcoma-associated herpesvirus||Kaposi’s sarcoma and Body cavity lymphoma|
|Epstein–Barr virus||Burkitt’s lymphoma, Hodgkin’s lymphoma, B lymphoproliferative disease and Nasopharyngeal carcinoma.|
The theory that cancer could be caused by a virus began to be developed in 1911 by Peyton Rous. Rous transmitted solid tumors of chickens by transplanting tissue between them.
By the early 1950s it was known that viruses could remove and incorporate genes and genetic material in cells. It was suggested that these new genes inserted into cells could make the cell cancerous. Many of these viral oncogenes have been discovered and identified to cause cancer.
The main viruses associated with human cancers are human papillomavirus, hepatitis B and hepatitis C virus, Epstein-Barr virus, and human T-lymphotropic virus. Experimental and epidemiological data imply a causative role for viruses and they appear to be the second most important risk factor for cancer development in humans, exceeded only by tobacco usage. The mode of virally-induced tumors can be divided into two, acutely-transforming or slowly-transforming. In acutely transforming viruses, the viral particles carry a gene that encodes for an overactive oncogene called viral-oncogene (v-onc), and the infected cell is transformed as soon as v-onc is expressed. In contrast, in slowly-transforming viruses, the virus genome is inserted, especially as viral genome insertion is an obligatory part of retroviruses, near a proto-oncogene in the host genome. The viral promoter or other transcription regulation elements in turn cause overexpression of that proto-oncogene, which in turn induces uncontrolled cellular proliferation. Because viral genome insertion is not specific to proto-oncogenes and the chance of insertion near that proto-oncogene is low, slowly-transforming viruses have very long tumor latency compared to acutely-transforming viruses, which already carry the viral oncogene.
Hepatitis viruses, including hepatitis B and hepatitis C, can induce a chronic viral infection that leads to liver cancer in 0.47% of hepatitis B patients per year (especially in Asia, less so in North America), and in 1.4% of hepatitis C carriers per year. Liver cirrhosis, whether from chronic viral hepatitis infection or alcoholism, is associated with the development of liver cancer, and the combination of cirrhosis and viral hepatitis presents the highest risk of liver cancer development. Worldwide, liver cancer is one of the most common, and most deadly, cancers due to a huge burden of viral hepatitis transmission and disease.
Advances in cancer research have made a vaccine designed to prevent cancer available. In 2006, the US FDA approved a human papilloma virus vaccine, called Gardasil. The vaccine protects against four HPV types, which together cause 70% of cervical cancers and 90% of genital warts. In March 2007, the US CDC Advisory Committee on Immunization Practices (ACIP) officially recommended that females aged 11-12 receive the vaccine, and indicated that females as young as age 9 and as old as age 26 are also candidates for immunization.