SV40 became a highly controversial subject after it was revealed that millions were exposed to the virus after receiving a contaminated polio vaccine.[1]

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 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.

[edit]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.[4] 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.[5]

As a result of these uncertainties, academic opinion remains divided, with some arguing that this hypothesis is not supported by the data,[6] and others arguing that some cancers may involve SV40.[7][8]

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”.[9] This announcement is based on two recent studies.[10][11]

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 asbestos to cause both Peritoneal and Pleural Mesothelioma [12] (review [13]).

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 having been used to grow 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 initially be found. Then, in 1997, Herbert Ratner of Oak Park, Illinois, gave some vials of 1955 Salk vaccine to researcher Michele Carbone.

Dr. Ratner, the Health Commissioner of Oak Park at the time the Salk vaccine was introduced, had kept these vials of vaccine in a refrigerator for over forty years.,[14][15] Upon testing this vaccine, Dr. Carbone discovered that it contained not only the SV40 strain already known to have been in the Salk vaccine (containing two 72-bp enhancers) but also the same slow-growing SV40 strain currently being found in some malignant tumors and lymphomas (containing one 72-bp enhancers).[16]

It is 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,[17] it 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[18] 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.[19][20][21] 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.[22]The book further alleges falsification of research due to financial conflicts of interest.[23]