SV40

SV40 is an abbreviation for Simian vacuolating virus 40 or Simian virus 40, a polyomavirus that is 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.

SV40 became a highly controversial subject after it was revealed that millions were exposed to the virus after receiving a contaminated polio vaccine.

History
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 viral genome was sequenced by Walter Fiers and his team at the University of Ghent (Belgium) in 1978. The virus is dormant and is asymptomatic 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&mdash;due to, for example, infection with Simian immunodeficiency virus&mdash;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.

Virology
SV40 consists of an unenveloped icosahedral virion with a closed circular dsDNA genome of 5kb. The virion adheres to cell surface receptors of MHC class 1 by the virion glycoprotein VP1. Penetration into the cell is through a caveolin vesicle. Inside the cell nucleus, the cellular RNA polymerase II acts to promote early gene expression. This results in an mRNA that is spliced into two segments. The small and large T antigens result from this. The large T antigen has two functions: 5% will go to the plasma membrane of the cell and 95% will return to the nucleus. Once in the nucleus the large T antigen binds three viral DNA sites, I, II, and III. Binding of sites I, and II autoregulates early RNA synthesis. Binding to site II takes place in each cell cycle. Binding site I initiates DNA replication at the origin of replication. Early transcription gives two spliced RNAs that are both 19s. Late transcription gives both a longer 16s, which synthesizes 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, return to the nucleus because assembly of the viral particle happens in the nucleus. Eventual release of the viral particles is cytolytic and results in cell death.

Transcription
The early promoter for SV40 contains three elements. The TATA box is located approximately 20 base-pairs upstream from 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 enhancers. When the SP1 protein interacts with the 21 bp 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 is 3' polyadenylated and 5' capped.

Theorized role in human disease
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 is not 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 does not 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 regulated 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 co-carcinogen 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 nonpermissive 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, did not reliably kill SV40.

It was difficult to detect small quantities of virus until the advent of PCR; 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 is 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, is not 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 unknowingly.

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.

CDC FAQ

 * Frequently Asked Questions about Cancer, Simian Virus 40 (SV40), and Polio Vaccine, Science Coordination and Innovation, United States Centers for Disease Control

NIH 1997 Conference on SV40

 * Simian Virus 40 (SV40:) A Possible Human Polyomavirus Workshop Monday January 27, 1997, Morning Session, transcript of 1997 National Institutes of Health conference on SV40 in humans, (part 1 of 3), United States Food and Drug Administration (FDA)
 * Simian Virus 40 (SV40:) A Possible Human Polyomavirus Workshop Monday January 27, 1997 Afternoon Session, transcript of 1997 National Institutes of Health conference on SV40 in humans (part 2 of 3), United States Food and Drug Administration (FDA)
 * Simian Virus 40 (SV40:) A Possible Human Polyomavirus Workshop, Tuesday, January 28, 1997, transcript of 1997 National Institutes of Health conference on SV40 in humans  (part 3 of 3 ), United States Food and Drug Administration (FDA)

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