Viral carcinogenesis

Between 20 and 30% of human cancers are caused by infectious agents. These can be multicellular parasites, bacteria, or viruses. Whereas bacteria and parasites cause cancer by indirect mechanisms, such as inflammation and immune deregulation, viruses infect human cells directly and initiate molecular changes that lead to uncontrolled proliferation. Both DNA and RNA viruses, such as retroviruses, can cause cancer; however, the ability to promote cancer is a by-product of viral evolution and a low frequency event, often the result of faulty virus replication. Most human oncogenes, such as MYC, SRC, RAS, were first discovered because they had been pirated by retroviruses. The function of key human tumour suppressor proteins, such as RB and TP53, were identified because they bind to and are inactivated by DNA tumour viruses. Because viruses are foreign, vaccines can be devised that target them. More recently it has become possible to prevent two types of cancer caused by viruses—cervical cancer caused by human papillomavirus (HPV) and hepatocarcinoma caused by hepatitis B virus (HBV)—by preventing the initial infection using vaccination. Study of the relationship between infective agents and cancer has therefore achieved two main goals: to unravel basic mechanisms of carcinogenesis and to make some cancers preventable.