In this article we will discuss about viruses and cancer.
Cancer (L. Cancer = crab) is a cellular phenomenon of uncontrolled growth and, at present, is one of the most serious medical problems. Cancerous cells in the body grow profusely due to neoplasia (abnormal new cell growth and reproduction because of loss of regulation) leading to the formation of large masses of cells called tumors (L. tumere = to swell).
All tumours are not seriously harmful; the body has ability to wall off some tumors so that they do not spread. Such non-invasive tumors are called benign. The tumors which are harmful are called malignant as they invade the body and destroy normal body tissues and organs. In advanced stages however, the malignant tumors spread even to other parts of the body and initiate new tumors, a process called metastasis.
How does a Normal Cell become Cancerous?
The growth and division of normal cells is regulated by at least two types of genes, namely, proto-oncogenes and tumor suppressor genes. The former promote growth but they function under the control of the latter which are growth-restraining. Changes in either or both types of these genes results in an uncontrolled growth and, therefore, to cancer.
Scientists believe that there are many causes of cancer. Possibly as many as 30-60% of cancers may be related to diet. Many chemicals are carcinogenic and may cause cancer by inducing gene mutations or interfering with normal cell differentiation.
Gene mutations may also result in by physical stimuli such as ultraviolet radiation or X-rays. However, some viruses are thought to be directly related to cancers as they bring about the genetic change that results in initiation of tumor formation.
Viral Cancers in Animals and Humans:
Viruses cause malignant tumors in animals, as such, tumors have been isolated from many animals such as fish, mice, rat, squirrels, dogs, deer and horses. The polyoma virus has been isolated from mice and the simian virus 40(SV 40) from monkeys.
Although we have no clear evidence that the viruses cause cancers in animals as no infective virus has been isolated from cell cultures, but some evidences are there to show the association of viruses with human cancers.
At present, viruses have been implicated in the genesis of at least eight human cancers:
1. Electron microscopic and immunological studies show that there is association of Epstein-Barr virus (EB virus), a herpesvirus which is one of the best studied human cancer viruses, with Burkitt’s lymphoma, a malignant tumor of Jaw and abdomen of children, occurring in certain regions of Africa.
2. EB virus has also been found associated with nasopharyngea carcinoma found in certain Chinese populations.
3. Some strains of human papillomaviruses have been isolated from malignant tissues (not from normal tissues) from patients suffering from skin and cervical cancer.
4. Hepatitis B virus has been found associated with hepatocellular carcinoma (a type of liver cancer) and can be integrated into the human genome.
5. Hepatitis C virus results in cirrhosis of the liver that may lead to liver cancer.
6. Human herpesvirus-8 has been found associated with the development of Kaposi’s sarcoma.
7. Human T-cell lymphotropic virus-1 (HTLV-1) seems able to cause T-cell leukemia.
8. Human T-cell lymphotropic virus-2 (HTLV-2) is found associated with hairy-cell leukemia.
How Viruses Cause Human Cancer?
Although viruses are known to cause cancers in animals since many years, it is still uncertain that they cause cancers in humans. However, it is well established that some kinds of human tumors are strongly associated with infection by specific viruses.
The human T-cell lymphotropic viruses (HTLV-1 and HTLV-2) appear to transform T-cells into tumor cells by producing a regulatory protein that sometimes activates genes related to cell-division as well as virus reproduction. Some oncogenic viruses possess one or more very effective promoters or enhancers.
Whenever, these viruses integrate themselves next to an oncogene in cell-genome, the promoter or enhancer is thought to stimulate its transcription, resulting in a cancer. With the possible exception of HTLV-1, it is not yet known clearly how the viruses associated with human cancers actually aid in cancer development.