Biological Agents
Various forms of parasites have been associated with many animal and plant cancers, although the
actual linking mechanisms remain unknown or unproved. For example, the blood flukes that cause schistosomiasis frequently seem also to cause bladder cancer, a cancer particularly prevalent in Egypt. The most clearly established biological agents, however, are the
oncogenic (cancer-causing) viruses that commonly induce the formation of neoplasms in lower animals. A very few of these viruses are now strongly linked with some human cancers, and at least one has definitely been proved to cause a form of leukemia.
Among the viruses strongly linked with human cancers are a number of papilloma viruses and a herpes virus, called the Epstein-Barr virus, that causes the disease known as infectious mononucleosis. This latter virus is also suspected of causing the malignancy called Burkitt's lymphoma, prevalent in Africa, and a cancer of the nose and throat that commonly occurs in China. Another human cancer related to a virus infection is a liver carcinoma that sometimes follows a hepatitis-B infection. Another link established between a human cancer and a virus is that between T-cell leukemia and a form of retrovirus called HTLV-1; the cancer appears to be endemic in certain parts of Japan, the West Indies, and the U.S. Southeast.
Oncogenic viruses are divided into DNA and RNA viruses, depending on genome structure. The DNA viruses mainly insert their genetic information directly into the cells of their hosts, although the Epstein-Barr virus instead appears to exist in multiple copies as nucleic material known as a plasmid in the host cell's nucleus, separate from the host DNA. The RNA viruses such as the HTLV-1 virus require first that their genetic information be transcribed into DNA by the enzyme reverse transcriptase, supplied by the virus.
All forms of oncogenic viruses contain one or more genes that are essential for the transformation of the infected cell into a neoplastic cell. Such genes, termed
oncogenes, are best characterized in the genomes of oncogenic RNA viruses. It is now apparent that many oncogenes have closely related counterparts in the normal cellular genome of the cells they infect. The viral form of the oncogene, however, has a different structure and appears to be activated and expressed abnormally by one mechanism or another, leading to neoplastic transformation of the cell. Some oncogenic viruses may activate the normal cellular counterparts of oncogenes, called proto-oncogenes, by one of several mechanisms, causing the neoplastic transformation to occur. Possibly similar mechanisms may result from the action of chemicals or radiation or both, resulting in the activation of proto- and/or cellular oncogenes.