Tumor Viruses (World of Microbiology and Immunology)
Tumor viruses are those viruses that are able to infect cells and cause changes within the cell's operating machinery such that the cell's ability to regulate its growth and division is destroyed and the cells become cancerous.
Human papillomavirus, hepatitis B, Epstein-Barr virus, human T-cell leukemia virus, SV-40, and Rous sarcoma virus are all tumor viruses.
The ability of the Rous sarcoma virus to cause sarcomas (cancers of connective tissue) has been known since 1911, when Peyton Rous demonstrated that a sarcoma material from chicken could be filtered and the filtered fluid was still capable of inducing the cancer. The virus was both the first oncogenic (cancer-causing) virus to be discovered and (although not known until much later) the first retrovirus to be discovered. Another, well-known example of a retrovirus is HIV.
There are some 90 types of human papillomavirus, based upon the genetic sequence of their genomes. The target of the viral infection is a certain type of epithelial cell known as stratified squamous epithelium. The cells can be located on the surface of the skin, or can be mucosal cells in regions of the body such as the genital tract. For example, two human papillomaviruses are the most common cause of genital warts. While these warts are noncancerous, other types of papillomavirus result in the development of cervical cancer. Furthermore, human papillomavirus types 16 and 18 are the main cause of genital tract malignancies. The virus is transmitted from person to person typically via sexual contact.
How human papillomavirus triggers the uncontrolled growth that is a hallmark of cancerous cells is still unknown. Studies have determined that in cells that have not yet become cancerous, the viral genetic material is not associated with the cell's genetic material, and that production of new virus particles is still occurring. However, in cancerous cells the viral genetic material has been integrated into that of the host and no new virus particles are being made. Whether the integration event is a trigger for cancerous growth is not known.
The hepatitis B virus is associated with liver damage and liver cancer. The virus is transmitted from person to person via contaminated blood (which commonly occurs via sharing of needles), breast milk, and possibly saliva. Over 90% of all hepatitis B infections are cleared as the immune system responds to the infection. However, in some 5% of those infected the infection becomes chronic. Infected individuals can be asymptomatic, but remain carriers of the virus and thus able to pass on the virus to others.
Chronic infection with the hepatitis B virus greatly elevates the chances of developing cancer of the liver. Because the virus can be present for decades before the damage of liver cancer is diagnosed, the best strategy is the preemptive use of hepatitis B vaccine in those offspring born to mothers who are known to be positive for the virus.
As for human papillomavirus, the molecular mechanism by which hepatitis B virus triggers cancerous growth of cells is unknown. The periodic response of the immune system to the virus may over time favor the expression of genes whose products are involved in overriding growth and division control mechanisms.
The Epstein-Barr virus is linked to two specific cancers. One is called Burkitt's lymphoma, a cancer of the B-cell components of the immune system. The lymphoma is a common cancer of children and occurs almost exclusively in the central region of Africa. The region's high rate of malaria may play a role in the prevalence of the lymphoma, as malaria causes an increase in the number of the already-infected B-cells. The rapid increase in the virally infected B-cells might cause a genetic malfunction that leads to tumor development. The second cancer associated with the virus is nasopharyngeal carcinoma. This cancer is restricted to the coastal region of China, for as yet unknown reasons.
Human T-cell leukemia virus causes cancer in the T-cell components of the immune system. Infection is widespread in Japan and areas of Africa, and is spreading to western nations including the United States.
A virus designated SV-40, which is harbored by species of monkeys, is isolated from a sizable number of cancer sufferers.
See also Oncogene; Oncogenetic research