Cancer Causing Agents

Cancer-causing agents can be categorized into three groups: oncogenic viruses, chemicals, and radiation. All three lead to the molecular mechanisms of cancer.

A large number of DNA and RNA viruses cause tumours in animals, but in humans it is the DNA viruses that are implicated in most forms of cancer. Only one RNA virus is known to cause cancer in humans. The precise role that viruses play in tumour genesis is not clear, but it seems that they are responsible for causing only one in the series of steps necessary for cancer to develop.

Three DNA viruses — human papillomaviruses, the Epstein-Barr virus, and the hepatitis B virus — are linked to tumours in humans.

More than 70 types of human papillomavirus (HPV) have been described. Some cause benign papillomas of the skin (warts). Other strains, particularly HPV-16 and HPV-18, are linked to genital and anal cancers. These viruses are sexually transmitted. HPV-16 and HPV-18 are found in the majority of squamous-cell carcinomas of the uterine cervix. Genital warts with low malignant potential are associated with HPV-6 and HPV-11.

When transforming DNA viruses infect a cell, they integrate their DNA into the genome of the host. At this point the virus does not reproduce but only produces the proteins necessary to commandeer the DNA synthesis machinery of the host cell. Two of these viral genes, E6 and E7, can act as oncogenes. The proteins they encode bind to the protein products of two important tumour suppressor genes, p53 and RB, respectively, knocking these proteins out of action and allowing the cell to grow and divide.

The E6 and E7 proteins of HPV-16 and HPV-18 bind to the RB and p53 proteins very tightly; in contrast, the E6 and E7 proteins of HPV-6 and HPV-11 (the low-risk types) bind RB and p53 with low affinity. The differences in binding ability of these proteins correlate with their ability to activate cell growth, and they are consistent with the differences in malignant potential of these virus strains.

Epstein-Barr virus (EBV) is a type of herpesvirus that is well known for causing mononucleosis. It also contributes to the pathogenesis of four human tumours: the African form of Burkitt lymphoma, B-cell lymphomas in individuals whose immune systems are impaired from human immunodeficiency virus (HIV, the causative virus of AIDS) infection or the use of immunosuppressant drugs in organ transplantation, nasopharyngeal carcinoma, and some kinds of Hodgkin disease. EBV infects B lymphocytes, one of the principal infection-fighting white blood cells of the immune system. It does not replicate within the B cells; instead, it transforms them into lymphoblasts, which have an indefinite life span. In other words, the virus renders these cells immortal.

Burkitt lymphoma is endemic in certain areas of equatorial Africa and occurs sporadically in other parts of the world. As is the case with other cancer-inducing viruses, it is likely that EBV serves as only the first step toward malignant transformation and that additional mutations are required for bringing about this process.

Hepatitis B virus (HBV) is endemic in Southeast Asia and sub-Saharan Africa, areas that have the world's highest incidence of hepatocellular carcinoma (liver cancer). This and other epidemiological observations, as well as experimental evidence in animal models, have established a clear association between HBV and liver cancer. The precise role of hepatitis B virus in causing liver cancer is not yet understood, but evidence suggests that viral proteins disrupt signal transduction and thereby deregulate cell growth.

Retroviruses have provided some of the most important insights into the molecular cell biology of cancer (see the section Retroviruses and the discovery of oncogenes), and yet only one human retrovirus, the human T-cell leukemia virus type I (HTLV-I), is linked to a human tumour. This virus is associated with a T-cell leukemia/lymphoma that is endemic in the southern islands of Japan and the Caribbean basin but that also is occasionally found elsewhere. HTLV-I infects helper T lymphocytes (the same type of cell that is infected by HIV). Infection occurs when infected T cells are transmitted via sexual intercourse, blood transfusion, or breast feeding. Only about 1 percent of infected individuals will develop leukemia, and then only after a period of 20 to 30 years.

HTLV-I differs from other oncogenic retroviruses in that it does not contain a viral oncogene and does not integrate into specific sites of the human genome to disrupt proto-oncogenes. Although the mechanism of transformation is not clear, a viral protein named tax, which promotes DNA transcription, may be involved in setting up an autocrine (self-stimulating) loop that causes continuous proliferation of infected T cells. When cells are constantly dividing, they are at greater risk from secondary transforming events (mutations) that will ultimately lead to the development of cancer.