Oncogenic DNA viruses

Cancer

1. Hyperplasia: Uncontrolled proliferation of cells.
2. Anaplasia: Structure abnormality of cells; cells so affected have lose of or reduction in their functions.
3. Metastasis: Ability of a malignant cell to detach itself form a tumor and establish a new tumor at another site within the host.

For a high period, microbiologists have concluded the idea that at some types of cancer are being caused by viruses. The first proof came in 1908, when it was shown that certain chicken leukemias could be transferred to healthy chickens by blood serum. Three years lafter it was shown in that chicken sarcoma can be transmitted by bacteria- free filtrates of tumor extracts. Virus- induce carcinomas in mice were discover in 1936 and it was clearly demonstrated that and agent causing mouse mammary tumors can be transmitted from other to off spring through the mother milk. Many viruses have seen been proved capable of inducing cancer in animal. But for many years these discoveries are not considers relevant to the problem of human cancer. Furthermore, human cancer did not seem to be infectious, there were no conformed isolation of a virus from human cancer cell. Therefore the logic that viruses are the cause of human cancer became less to attention.

Recent year, however, even stronger proof has connected viruses to animal cancers. This more recent finding received the idea that human cancers might be also be caused by viruses. Both RNA and DNA viruses have been found capable of inducing cancer in animals. For example, in 1980 a retrovirus called human T-cell lymphotrophic virus typeI (HTLV-I) was identified as the probable cause of a rare, malignant cancer of humans called adult T-cell leukemia. This leukemia occurs in Japan, Africa, and the Caribbean. In 1982, a related virus, HTVL-II, was identified as the likely cause of some cases of human leukemia called hairy-cell leukemia. (Interestingly, HTVL-I and HTVL-II, are similar in many ways to HIV, the virus that causes AIDS, although HIV is not a cancer virus). Until the discoveries HTLV-II, several other virus have been found in human cancer; for example, hepatitis? Virus was associated with the form of liver carcinoma, and Epstein-barr virus was associated with a form of lymphoma. Even there is strong circumstance evidence that such a viruses can cause human cancer, it is quite unique matter to show that this unequivocally. Such evidence would require either (1) Experiment involving inoculation of human subject with viruses which is out of the question for ethical reasons, or (2) Demonstration that the cancer can be prevented by immunization of human subjects with the appropriate viral vaccine. Although the later stretegy is possible, the viral vaccine have not been introduced and, even if they were, it could require years to find a definitive answer about their effectiveness.

The appearence of cancer is termed oncogenesis (from the Greek words onkos, meanings “mass”); viruses that is responsible for cancer are said to be oncogenic. In animals with cancer caused by oncogenic viruses, the affected cells are transformed, resulting in the formation of tumor. Transformed cells show morphological, biochemical, and other properties distinctly different from uninfected cells or infected cells in which tumors are not produced. Unique characteristics of transformed cells is the hode of contact inhibition, a property of normal, cells. Normal cells recognized cellular boundaries and limit their cells population through the process of contact inhibition, which means they are inhibited from growing when they contact other cells. So, when normal cells are propagated in tissue culture, they form a mono-layer. Transforms cells in cells culture reproduce indefinitely and pile on top of each other. Other properties of transform cells include invasiveness (they produce tumor when injected into animals), chromosomar abnormalities (unusual member of and/ or breaks in chromosome), and an increase uses of sugar transport across cytoplasmic membrane.

Among the RNA viruses, only parts of a family retroviridae (more commonly known as the RNA tumor viruses) cause cancer in animals. They replicate via a DNA intermediates. None of the other RNA viruses, all of which replicate through an RNA intermediates are known to cause cancer. In contrast, among the FDNA viruses, grop of up least three families (herpesviridae, adenoviridae, and papovaviridae) can cause cancer in animal.

Common characteristic of all oncogenic viruses is that the viral genomic in a chance become integrated into the host cell’s DNA replicate along with the host cells chromosome. The host cells does not lyse-a situation same as to the phenomenal of lysogene in bacteria infected with the temperate phages. If the viral genome are RNA, it sarves as a template for the synthesis process of a DNA molecule complementary to it; the enzyme reverse transcriptase is guiding for this synthesis, giving rise to a DNA- RNA hybride. Then a second strand of DNA is synthesized complementary to the first strand. This gives double-stranded DNA molecule synthesized from the viral RNA, which can now be part into the host DNA as a provirus in this way transformation an oncogenesis are induced in the hosat cell.

Oncogenic DNA viruses

Oncogenic viruses are found within several types of DNA-containing viruses. These group include the papovaviruses, the harpes-viruses, and the adenoviruses.

Papovaviruses include the papillomaviruses thsat cause benigm warts in human and other animal have been found in cervical cancdder; polyoma viruses that response several kinds of tumors when injected into new born mice; and simian virus 40 (SV 40), origenerally isolated from cells culture used for the growth of polio viruses for polio vaccines. SV 40 cannot be induce tumors in the monkey, its natural host, but can do so in rodents in the laboratory; So this was not detected when monkey cells (which showed no cytopathic effect even though SV 40 was present) were is initially for the cultivation of viruses for polio vaccine production.

Among the herpes viruses that affect human are the herpes simplex viruses and the Epstein-Barr virus (EVV). The different is the probable cause a Burkitt’s lymphoma, a cancer of the lymphoid system. EVV has an affinity for blood cell called lymphocytes and has the ability for transforming them into rapidly proliferating cells. EBV has also been implicated in Hodgkin’s disease (another form of lymphoma) and in nasopharyngeal carcinoma (a cancer of the nose and throat). These viruses is for the cause of infectious mononucleosis (regarded by some authorities as a self-limiting leukemia). Herpes simplex virus types 1 (HSV 1) is the cause of cold sores, but it has been found in lip cancer. Herpes simplex virus type 2 (HSV 2), which cause more than 80% single-stranded RNA of retrovirus RNA-DNA RNA degraded double-stranded viral DNA host cell DNA with integrated with viral DNA synthesis of complementary stranded of DNA by reverse transcriptase ir.-il RNA slr,nd Complementary DNA strand A second DNA strand is synthesized complementary to the first DNADNA Viral DNA is inserted into host cell DNA Transcription from integrated viral DNA single-stranded RNA of genital herpes infection, has been associated cervical cancer.

Oncogenic RNA viruses

Usually the RNA-containing viruses, only the retroviruses are oncogenic. They harbor the lymphoma, leukemia, and sarcoma viruses of cats, chickens, and mice, as well as mammary tumor viruses of mice. Thus many different types of cells can be transformed these viruses.

The tendency of this viruses to be oncogenic is related to their capacity t become proviruses, which are the double-stranded DNA molecules synthesized from the viral RNA that become integrated into the host cell’s DNA. The major reasons why retroviruses can show cancer is that they introduce new genetic material into the host’s DNA and is responsible for transformation.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.