Causes of cancer viral carcinogens

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

Hepatitis C
  Related to Hepatocellular carcinoma
  Human immunodeficiency virus orAIDS 

Related to Kaposi,s sarcoma and non hodgkin,s lymphoma
Human papilloma virus type 16 and 18

Related to Cervical and  Anal cancers
 Human T -cell lymphotropic virus

 related to Adult T - cell leukemia and lymphoma

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