In this article we will discuss about Oncogenic Virus that produce tumours:- 1. Classification of Viral Oncogenesis 2. Mechanism of Viral Oncogenesis 3. Oncogenes 4. Oncogenic Retroviruses 5. Oncogenic DNA Viruses.
Contents:
- Classification of Viral Oncogenesis
- Mechanism of Viral Oncogenesis
- Oncogenes
- Oncogenic Retroviruses
- Oncogenic DNA Viruses
1. Classification of Viral Oncogenesis:
Oncogenic viruses are those viruses that produce tumours in their natural hosts, experimental animals, or in cell cultures.
They are classified according to their nucleic acid content in their genome. Many classes of RNA viruses are responsible for tumour production in animals but only one family of RNA viruses (Retro viridae) can produce the tumour.
2. Mechanism of Viral Oncogenesis:
In cell transformation by tumour viruses, the initial stage of adsorption, penetration and un-coating are exactly the same as for productive infection, but in subsequent two stages, they differ:
A. In case of oncogenic DNA viruses, the DNA is integrated in the host cell genome as pro-phage. The integrated viral genome is rearranged. As the integrated viral DNA is either defective or incompletely, no infectious virus is produced, instead the host cell undergoes neoplastic transformation under its influence.
B. Retroviruses replicate by a unique manner by an enzyme, reverse transcriptase (RT) carried by the viruses. RT constructs a DNA copy of the RNA genome of the virus and the DNA copy (pro-phage) becomes integrated with DNA of the host cell, where it may remain latent for variable periods. Only when activated, the integrated provirus acts as template for translation of progeny viral RNA and cell transformation.
3. Oncogenes:
Genes that are responsible for the induction of tumours are known as oncogenes which were first discovered in retro virus, Rous sarcoma virus (RSV) that can transform cells. Besides their important role in new plastic transformation, they are also responsible for normal cell growth, division, and differentiation.
The process of transformation and neoplasia is due to in correct or illegimate expression of these genes. Martin and Vogt first identified viral gene (v- sre) in Rous sarcoma virus and later a similar src gene was identified in genome of normal chicken cells which is called cellular src (C-src) gene.
More than 30 oncogenes have been identified. A new class of human cancer genes called “tumour suppressor genes“, growth suppressor genes or and-oncogenes have been identified.
4. Oncogenic Retroviruses:
They are responsible for tumours of reticuloendothelial and hematopoietic system (leukaemia, lymphoma) or of connective tissue (sarcoma). They are also called as “acutely transforming viruses” (ATVS), because they produce tumours within a short period of six months after injection into a suitable host.
i. Avian Sarcoma Leukosis Viral (ALV) Complex:
They are caused by antigenically related viruses which caused lymphoma, leukaemia and sarcoma.
ii. Murine Leukosis Virus:
There are several strains of murine leukaemia and sarcoma viruses which are derived from Mo-Mu Lv during passages in mice and rats.
iii. Mouse Mammary Tumour Viruses (MMTV):
They may be endogenous or exogenous viruses. The endogenous ones have no oncogenic role, whereas the exogenous ones are oncogenic. Mammary cancers develop only in susceptible females after a latent period of 6-12 months.
iv. Sarcoma Leukaemia Viruses of Animals:
Leukaemia, lymphosarcoma in cat is induced by Feline leukaemia virus (Felv); Lymphosarcoma in cattle is due to bone leukaemia virus.
v. Human T-cell Leukaemia Virus (HTLV):
Adult T-cell leukaemia is induced by HTLV- 1 but having cell leukaemia of T-cell type is due to HTLV-II.
5. Oncogenic DNA Viruses:
1. Papova Viruses:
They induce benign warts and papilloma in natural hosts. Human papilloma virus (HPV) type 6 and 11 are incriminate in pre malignant lesions of the female and male genital tract.
SV 40 polyoma viruses produce tumours in mice when injected.
2. Herpes Virus:
Herpes simplex type 1 and 2 and Cytomegalovirus transform cultured cells at a very low frequency. They transformed hamster cells, when injected into another hamster, form tumours. In woman, Herpes simplex type 2 is responsible for cervical carcinoma.
No transforming gene is involved Epstein-Barr (EB) virus is associated with Burkitt’s lymphoma in Africa and nasopharyngeal carcinoma in Chinese male population. It is believed that EB virus transforms normal lymphocytes into lymphoblast in immuno-compromised child.
3. Hepatitis B Virus:
It is implicated in hepato-cellular carcinoma. It does not carry any oncogene and is integrated in tumour cells as Hepatitis B virus DNA.
4. Pox Virus:
Yaba virus produces histiocytoma (benign tumour) in natural host (monkey) while shope fibroma virus induces fibroma in rabbit. Molluscum contagiosum virus produces benign growths in humans.