The viruses that contain DNA as their genetic material and induce tumour are called DNA tumour viruses; for example, papova viruses (SV40, polyoma- and papilloma- viruses), adenoviruses herpes viruses, poxviruses and hepatitis B virus. SV40, polyoma and adenoviruses are the most widely studied viruses.
Papilloma viruses cause papillomas or warts (benign tumour) of cutaneous and mucosal epithelia in humans and other animals, therefore, they are known as papilloma viruses. They are isolated from cows (bovine papilloma virus), rabbits (shope papilloma virus), and many other animals. Though papilloma viruses are known to be associated with cervical carcinoma which is the most common among women with several sex partners, yet no direct evidence of cause of cervical carcinoma, penile cancer or benign genital warts by papilloma viruses is available.
It has been found that different papilloma viruses have similar organization of genome. The two regions i.e. E6 and E2 to E5 of genome in some papilloma viruses have been found to encode open reading frame and to transform the infected cells. In addition, two isolates i.e. human papilloma virus type 16 (HPV-16) and human papilloma virus Type 18 (HPV-18) are the most common in cervical carcinomas.
Hepatitis B Virus
Chronic Hepatitis B virus (HBV) infection leads to liver cancer in human throughout the world. The virus is least frequent in Canada and most frequent in China and Africa. Persistent infection of HBV also cause cirrhosis. Thus, HBV infection results in 80-90% of liver cancer and, in turn, 500,000 deaths annually throughout the world.
As far as mechanism of development of liver cancer in human is concerned, it is not fully known. It can be assumed that HBV integrates into cellular DNA and activate a cellular proto-oncogene. In some cultured tumour cells, deletion chromosome at the site of viral integration has been observed. Therefore , HBV may also act as mutagen.
Epstein- Barr Virus
Epstein-Barr virus (EBV) belongs to family herpes virus. EBV virions are enveloped. Capsid is icosahedral that encloses viral genome. It causes a tumour of mature B cells called Burkitt's lymphoma. This disease is named after the name of discoverer, Burkitt. EBV is found throughout the world but most frequently in children in certain parts of East Africa. Burkitt's lymphoma is found in 90% children of 6 years age in some tropical areas like east Africa, whereas children of the same age group in the USA have 30-40% of disease. EBV is also associated with nasopharyngeal carcinoma and mononucleosis. The later is a disease of excessive lymphoid cell proliferation spread through kissing. Nasopharyngeal carcinoma is a tumour of adult in China caused by EBV. It has been found that this disease is contributed by smoke, chemicals and other environmental factors.
Burkitt's lymphoma possesses specific chromosome translocations on which the immunoglobulin gene loci and the c-myc proto-oncogene are present. The myc oncogene is the gene of several defective genes of avian retroviruses that induce myelocytomas , sarcomas and carcinomas which certainly play role in cell division and expressed in normal growing cells and tumour cells. In humans, c-myc gene is located on the long arm of chromosome 8 at band q 24 and spans 5 kb. In t (8:14) translocations immunoglobulin heavy constant chain is fused head to head with c-myc. The fusion alters the c-myc gene expression resulting in the development of tumour.
Tumour cells generally secrete antibodies that possess highly characteristics chromosomes translocations. The translocations involve in chromosome 8, and one of the three chromosomes that carry antibody light or heavy-chain genes (i) chromosome 14 (heavy chain genes), (ii) chromosome 2 (gamma light-chain genes) or (iii) chromosome 22 (kappa light-chain genes). The translocations which involve chromosome 8 and 14 are designated as (8:14). The t (8:14) is the most common in Burkitt's lymphoma.
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