Tumor Viruses

Both RNA viruses and DNA viruses can induce tumors in humans. Of the RNA viruses only human T-lymphotropic virus (a retrovirus) is known to induce tumors (T cell leukemia in this case). The RNA tumor viruses were the first viruses recognized to be involved in tumor formation.

Several DNA viruses are known to induce tumors including the following:

• Hepadnaviridae such as the hepattis B virus which incudes hepatocellular carcinoma. This cancer affects 200,000 million people.

• Herpesviridae such as the Epstein Barr virus which induces Burkitt's lymphoma ((African) and nasopharyngeal carcinomas

• Papillomaviruses such as HPV which induce papillomas and cervical carcinomas.

The types of RNA and DNA viruses which can induce tumors in rodents is more extensive.

Immortalization of human cells by DNA tumor virus genes has been modeled as a two-step process in which introduction of an immortalizing agent initially extends the lifetime of the primary cell, usually by abrogating the function of the cell cycle regulatory proteins pRb and p53. At some point the cells inter a crisis phase in which the telomeres have shortened beyond a critial point necessary for cell growth and division. The emergence of immortal clones from cells in crisi is usually a relatively rare event and is thought to involve some genomic modificaiton. This second stage is associated with reactivation of telomerase activity.