As mentioned earlier in this chapter, viruses are responsible for a number of cancers in animals, and there is evidence that viruses contribute to at least a few cancers in humans (Table 16.6). For example, about 95% of all women with cervical cancer are infected with human papilloma viruses (HPVs). Similarly, infection with the virus that causes hepatitis B increases the risk of liver cancer in some people. The Epstein–Barr virus, which is responsible for mononucleosis, has been linked to several types of cancer that are prevalent in parts of Africa, including Burkitt lymphoma.

Many of the viruses that cause cancer in animals are retroviruses (see Chapter 7). Earlier in this chapter, we learned how studies of the Rous sarcoma retrovirus in chickens led to the identification of oncogenes in humans. Retroviruses sometimes cause cancer by mutating and rearranging host genes, converting proto-oncogenes into oncogenes (Figure 16.12a). Another way in which viruses can contribute to cancer is by altering the expression of host genes (Figure 16.12b). Retroviruses often contain strong promoters to ensure that their own genetic material is transcribed by the host cell. If the provirus inserts near a proto-oncogene, viral promoters can stimulate high levels of expression of the proto-oncogene, leading to cell proliferation.

441