SV40 Totally Explained

Everything about Sv40 totally explained

SV40 is an abbreviation for Simian vacuolating virus 40 or Simian virus 40, a polyomavirus that's found in both monkeys and humans. Like other polyomaviruses, SV40 is a DNA virus that has the potential to cause tumors, but most often persists as a latent infection. The virion is a naked icosahedral virion that consists of a closed circular dsDNA which is has 5kb. The virion adheres to cell surface receptors MHC class 1 by the virion glycoprotein VP1. Penetration into the cell is throug a caveolin vesicle. Inside the cell nucleus the cellular RNA polymerase II acts to promote early gene expression. This results in an mRNA that's spliced into two segments. The small and Large T antigen result from this. The Large T antigen has two functions 5% will go to the plasma membrane of the cell and 95% will go back to the nucleus driven by an amino acid sequence lys lys lys arg lys val glu. Once in the nucleus the Large T binds three viral DNA sites, I, II, and III. Binding of sites I, and II autoregulates early RNA synthesis. Binding site II happens for each round of replication. Binding site I initiates DNA replication at the ORI. Early transcription gives two spliced RNA's that are both 19s. Late transcription gives both a longer 16s which synthesises the major viral capsid protein VP1 and the smaller 19s which gives Vp2, and Vp3 through leaky scanning. All of the proteins besides the 5%of Large T go back to the nucleus because assembly of the viral particle happens in the nucleus. Eventual release of the viral partical is cytolitic and results in cell death. The early promoter for SV40 contains three elements. The TATA box is the transcriptional start site. The 21 base pair repeats contain six GC boxes and are the site that determines the direction of transcription. Also the 72 base pair repeats are transcriptional enhanders. When the SP1 protein interacts with the 21bp repeats it binds either the first or the last three GC boxes. Binding of the first three initiates early expression and binding of the last three initiates late expression. The function of the 72 bp repeats is to enhance the amount of stable RNA and increase the rate of synthesis. This is done by binding(dimerization) with the AP1(Activator Protein 1) to give a primary transcript that's 3' polyadenylated and 5' capped. The virus was first identified in 1960 in cultures of rhesus monkey kidney cells that were being used to produce polio vaccine. It was named for the effect it produced on infected green monkey cells, which developed an unusual number of vacuoles. The complete DNA sequence of the virus was sequenced by Walter Fiers and his team at the University of Ghent (Belgium) in 1978. The virus is dormant and shows no visible effects in Rhesus monkeys. The virus has been found in many macaque populations in the wild, where it rarely causes disease. However, in monkeys that are immunodeficient—due to, for example, infection with Simian immunodeficiency virus—SV40 acts much like the human JC and BK polyomaviruses, producing kidney disease and sometimes a demyelinating disease similar to PML. In other species, particularly hamsters, SV40 causes a variety of tumors, generally sarcomas. In rats, the oncogenic SV40 Large T-antigen was used to establish a brain tumor model for PNETs and medulloblastomas.
The molecular mechanisms by which the virus reproduces and alters cell function were previously unknown, and research into SV40 vastly increased biologists' understanding of gene expression and the regulation of cell growth.
The hypothesis that SV40 might cause cancer in humans has been a particularly controversial area of research. Several different methods have been used to detect SV40 in a variety of human cancers, although how reliable these detection methods are, and whether SV40 has any role in causing these tumors, remains unclear. As a result of these uncertainties, academic opinion remains divided, with some arguing that this hypothesis isn't supported by the data, and others arguing that some cancers may involve SV40. However, the United States National Cancer Institute announced in 2004 that although SV40 does cause cancer in some animal models, "substantial epidemiological evidence has accumulated to indicate that SV40 likely doesn't cause cancer in humans". This announcement is based on two recent studies.

p53 Damage and carcinogenicity

SV40 is believed to suppress the transcriptional properties of the tumor-suppressing p53 in humans through the SV40 Large T-antigen and SV40 Small T-antigen. p53 is responsible for initiating cell death ("apoptosis"), or cell cycle arrest when a cell is damaged. A mutated p53 gene may contribute to uncontrolled cellular proliferation, leading to a tumor.
SV40 may act as a cocarcinogen with crocidolite to cause mesothelioma (review )
When SV40 infects nonpermissive cells such as 3T3 mouse cells the dsDNA of SV40 becomes covalently integrated. In nonpermissive cells only the early gene expression occurs and this leads to transformation, or oncogenesis. The nonpremissive host needs the Large T antigen and the small t antigen in order to function. The small t antigen interacts with and integrates with the cellular phosphatase pp2A. This causes the cell to lose the ability to initiate transcription.

Polio vaccine contamination

Soon after its discovery, SV40 was identified in the injected form of the polio vaccine produced between 1955 and 1961. This is believed to be due to kidney cells from infected monkeys being used to amplify the vaccine virus during production. Both the Sabin vaccine (oral, live virus) and the Salk vaccine (injectable, killed virus) were affected; the technique used to inactivate the polio virus in the Salk vaccine, by means of formaldehyde, didn't reliably kill SV40.
It was difficult to detect small quantities of virus until the advent of PCR testing; since then, stored samples of vaccine made after 1962 have tested negative for SV40, but no samples prior to 1962 could be found. Thus, although over 10 million people received the potentially contaminated batches of vaccine, there's no way to know whether they were exposed to the virus, and if so, whether it was in a quantity and by a route that would cause infection. It is also unknown how widespread the virus was among humans before the 1950s, though one study found that 12% of a sample of German medical students in 1952 had SV40 antibodies. Although horizontal transmission between people has been proposed, isn't clear if this actually happens and if it does, how frequently it occurs.
An analysis presented at the Vaccine Cell Substrate Conference in 2004 suggested that vaccines used in the former Soviet bloc countries, China, Japan, and Africa, could have been contaminated up to 1980, meaning that hundreds of millions more could have been exposed to the virus knowingly.

Treatment in the popular press

Claims have been made detailing the controversy surrounding SV40 research. One book by a pair of investigative journalists contains statements indicating that researchers were penalized for reporting the findings of a potential cause and effect relationship between the early polio vaccine, SV40 and cancer. The book further alleges falsification of research due to financial conflicts of interest. An additional book written by Kevin Trudeau, the creator of the "Natural Cures" infomercial alleges that harm caused to the public by the SV40 virus in polio vaccines has been covered up.

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