Viral Carcinogens
One of the first observations that cancer may be caused by transmissiblcell-free extracts from sarcomas in chickens could transmit sarcomas to other animals injected with these extracts e agents was by Peyton Rous in 1911 when he
demonstrated that
This was subsequently discovered to represent viral transmission of cancer by the Rous sarcoma virus RSV
At present, several human viruses are known to have oncogenic properties, and several have been causally linked to human cancers it is estimated that 15% of all human tumors worldwide are caused by viruses&
Viruses may cause or increase the risk of malignancy through several mechanisms
Including direct transformation, expression of oncogenes that interfere with cell-cycle checkpoints or DNA repair, expression of cytokines or other growth factors, and alteration of the immune system
Oncogenic viruses may be RNA or DNA viruses, Oncogenic RNA viruses are retroviruses and contain a reverse transcriptase
After the viral infection, the single stranded RNA viral genome is transcribed into a double-stranded DNA copy
which is then integrated into the chromosomal DNA of the cell Retroviral infection of the cell is permanent, thus integrated DNA sequences remain in the host chromosome
Oncogenic transforming retroviruses carry oncogenes derived from cellular genes
These cellular genes, referred to as proto oncogenes, usually are involved in mitogenic signaling and growth control, and include protein kinase, G proteins, growth factors, and transcription factors
Integration of the provirus upstream of a protooncogene may produce chimeric virus-cell transcripts and recombination during the next round of replication that could lead to incorporation of the cellular gene into the viral genome
Unlike the oncogenes of the RNA viruses, those of the DNA tumor viruses are viral, not cellular in origin
These genes are required for viral replication utilizing the host cell machinery
In permissive hosts, infection with an oncogenic DNA virus may result in a productive lytic infection, leading to cell death and the release of newly formed viruses
In non permissive cells, the viral DNA can be integrated into the cellular chromosomal DNA, and some of the early viral genes can be synthesized persistently,leading to transformation of cells to neoplastic state
Uno-compromised individuals are at elevated risk most patient,s infected with oncogenic viruses do not develop cancer when cancer does develop
It usually occurs several years after the viral infection
It is estimated, for example, that the risk of the hepatocellular carcinoma among hepatitis C virus-infected individuals is 1 to 3 % after 30 years
There may be synergy between various environmental factors and viruses in carcinogenesis
Factors that predispose to hepatocellular carcinoma
Among hepatitis C virus-infected include heavy alcohol intake. hepatitis B co-infection and possibly diabetes
some examples for selected viral carcinogens
Epstein Bar Virus
Related to Burkitt,s lymphoma Hodgkins disease Immunosuppression related lymphoma Nasopharyngeal carcinoma
Related to Burkitt,s lymphoma Hodgkins disease Immunosuppression related lymphoma Nasopharyngeal carcinoma
Human T -cell lymphotropic virus
related to Adult T - cell leukemia and lymphoma
related to Adult T - cell leukemia and lymphoma
tags:cancer,viral,carcinogens,causes
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