Simian virus 40 and cancer

Since its discovery in 1960 as a contaminant of poliovaccines, Simian Virus 40 (SV40) has been the object of extensive studies to assess whether this oncogenic virus plays a role in human carcinogenesis. Over the last two decades, this question has met with broad scepticism. However, there is increasing evidence linking SV40 to specific types of human cancer, especially malignant mesothelioma. Recently, two laboratories using different experimental approaches independently confirmed that SV40 acts synergistically with environmental fibers to promote mesothelial cell transformation and mesothelioma. Most of the scepticism concerning SV40 and cancer was due to the lack of clear epidemiologic data. However, it is still not clear how SV40 circulates in the human population, making the identification of SV40-exposed versus non-exposed cohorts problematic. Consequently, the most helpful insights into SV40-mediated carcinogenesis have come from molecular pathology, cell and molecular biology, and from animal studies.     

Simian virus 40 sequences in malignant lymphomas in Japan

Recent studies showed that SV40 is detected in >40% of non-Hodgkin's lymphoma (NHL) in United States, suggesting SV40-contaminated poliovaccines widely used during the period 1955-1963 to be a major source of SV40 in NHL. We examined the presence of SV40 sequences in 122 cases with NHL and 3 with Hodgkin's lymphoma from Japan. The detection rate of SV40 sequences in diffuse large B-cell lymphoma (19%) was higher than that in peripheral blood cells of normal healthy volunteers in Japan (4.7%; P < 0.05) reported previously as controls for comparison with the study results from cancer patients, suggesting a role for SV40 in the development of diffuse large B-cell lymphoma. In contrast, the frequency of SV40 sequences in NHL cases born between 1951 and 1963 (12%), during which SV40-contaminated poliovaccines might have been inoculated, is not significantly different from that in cases born before 1950 (11%) or after 1964 (15%). SV40 is a new candidate etiologic factor for malignant lymphoma not only in the United States but also in Japan.     

Simian virus 40 and malignant mesothelioma (Review)

Simian virus 40 (SV40) was recognized as a contaminant of early poliovirus vaccines that were provided to millions of individuals in Europe and in the USA between 1955 and 1963. SV40, a DNA virus of the family of papovaviridae, was proven to be oncogenic in rodents and able to transform human and animal cells in vitro. In 1993 SV40 was accidentally discovered to produce mesotheliomas in hamsters when it was injected in visceral cavities. Afterwards, SV40 DNA sequences were detected with significative frequency in human pleural mesotheliomas by using polymerase chain reaction (PCR) and then SV40 DNA oncogenicity was associated with its large T antigen (Tag). This finding was confirmed by many laboratories, while a few research groups failed to replicate these data and argued that the SV40 DNA detection might be a PCR contamination artefact. In this review the dispute is examined in the light of recent experiments performed to identify molecular and cellular aspects of carcinogenicity and/or co-carcinogenicity of SV40 in human mesothelioma.     

Simian virus 40 and its association with human lymphomas

Simian virus 40 (SV40) is a potent DNA tumor virus that is known to induce cancer in laboratory animals. The neoplasias induced by SV40 in animal models are brain cancers, mesothelioma, bone cancers, and systemic lymphomas. SV40 oncogenesis is mediated primarily by the viral large tumor antigen, which inactivates the tumor suppressor proteins p53 and pRb family members. Evidence indicates that SV40 is an emergent human pathogen and that a significant excess risk of SV40 is associated with primary human brain cancers, malignant mesothelioma, bone cancers, and non-Hodgkin’s lymphoma. Therefore, the major types of tumors induced by SV40 in laboratory animals are the same as those human malignancies found to contain SV40 markers. Experimental and clinical data indicate that SV40 may be functionally important in the development of some of those malignancies. Recently, the Institute of Medicine of the National Academies concluded that SV40 infections could lead to cancer in humans under natural conditions (based on moderate strength biologic evidence). This review examines the data implicating SV40 in the pathogenesis of human lymphomas and discusses future directions to define the causative role for SV40 in these malignancies.     

Simian virus 40 is present in human lymphomas and normal blood

Many independent studies have demonstrated Simian virus 40 (SV40) in normal and neoplastic human tissues. Clonal integration of virus in the DNA of several thyroid and bone tumors suggests a direct role for SV40 in some cancers. However, in most cases the role of SV40 remains unclear. This study determined the presence of SV40, by amplification followed by hybridization, in 266 normal and neoplastic blood and lymphoid samples. Amplification detected SV40 in 14% of non-autoimmune deficiency syndrome (AIDS) lymphomas, 28% of AIDS related lymphoma and 16% of peripheral blood lymphocytes from non-cancerous patients. No SV40 was detected in leukemia samples. Direct Southern blotting of SV40+ samples detected no virus, consistent with less than one viral genome in ten cells. Sequence analysis of SV40 in blood and lymphoid samples found sequences distinct from laboratory strains of SV40. The presence of limited quantities of SV40 in a small proportion of both normal and neoplastic tissues is suggestive of an adventitious presence with no apparent direct role in blood and lymphoid cancers.     

Simian virus 40 (SV40) production from SV40-transformed human amnion cells of established lines.

Sixteen established cell lines of simian virus 40 (SV40)-transformed human amnion cells were examined for SV40 production. Many of these lines produced SV40 for extensive periods. Virus production had not ceased for 2 lines after 18 months, for 3 lines after 12 months, and for 3 lines at 3 months after recovery from "crisis". Three lines became virus-free in the first month, 1 line in the second month, 1 in the third month, and 1 in the fourth month, and 2 lines stopped virus production between 6 and 11 months after recovery. The virus titers were relatively low. Inclusion body-containing cells were infrequent. In contrast, in most cultures of SV40-transformed human fibroblasts rescued from crisis, no infectious virus was demonstrated, although exceptions have been reported. Virus was produced after heterokaryon formation of cells of the virus-free amnion lines with CV-1 cells in the presence of inactivated Sendai virus, as observed for SV40-transformed human fibroblasts. During the crisis period, some of the SV40-transformed amnion cells produced substantial amounts of virus. Titers decreased during the later periods of crisis. The most pronounced decrease in titers was in cultures from which established lines were recovered.     

Early chromosome changes in diploid Chinese hamster cells after infection with Simian virus 40

Primary and secondary Chinese hamster (ChH) embryo cells infected with SV40 exhibited chromosomal changes within one cell generation (24 h). The initial change seen is an increase in the number of polyploid metaphases (>10%). The majority of the polyploid cells are tetraploid (8×), however, higher ploidy values were also observed (16 ×, 32 × and 64 ×). In approximately 20–30% of the polyploid metaphases there were chromosome changes such as breaks, abnormal chromosomes and loss and addition of chromosomes. The remaining polyploids were normal in regard to chromosome number and chromosome morphology. A primary mouse embryo cell system also responded to SV40 infection with an increase in the number of polyploid cells within 48 h.     

Simian virus 40 infection down-regulates the expression of nitric oxide synthase in human mesothelial cells

The cytotoxic effects of asbestos are partly mediated by the production of free radicals, including nitric oxide (NO). SV40 has been suggested to synergize with asbestos in the pathogenesis of malignant mesothelioma. Crocidolite asbestos fibers induced in human mesothelial and malignant mesothelioma cells a significant increase of NO synthase activity and expression, which was absent in SV40-infected cells. Furthermore, SV40 infection prevented the NF kappa B activation elicited by crocidolite in both mesothelial and mesothelioma cells. These data suggest that SV40, by inhibiting the synthesis of NO, could favor the survival of transformed, potentially neoplastic cells.     

Simian virus 40 (SV40)-like DNA sequences not detectable in finnish mesothelioma patients not exposed to SV40-contaminated polio vaccines.

Occupational asbestos exposure can be demonstrated in 80% of mesothelioma cases. A possible role of simian virus 40 (SV40) in the etiology of mesothelioma was raised because several studies reported the presence and expression of SV40-like DNA sequences in human mesotheliomas. It is also known that expression of SV40 large T antigen inhibits cellular Rb and p53. This suggests that SV40 might render infected cells more susceptible to asbestos carcinogenicity. The SV40-like sequences are suggested to have arisen from contaminated polio vaccines. Millions of people in the United States and most European countries were inoculated with SV40-contaminated polio vaccine in 1955-1963. However, in Finland, where polio vaccination started in 1957, no SV40-contaminated vaccine was used. We used a polymerase chain reaction-based method to test for the presence of SV40-like sequences in DNA extracted from the frozen tumor tissues of 49 Finnish mesothelioma patients, most of whom had been occupationally exposed to asbestos. All of the Finnish tumor tissues tested negative for SV40-like sequences. The results suggest that the SV40-like sequences detected in mesothelioma tissue in some previous studies may indeed originate from SV40-contaminated polio vaccines. It is a matter of speculation whether the absence of SV40 infection has contributed to the relatively low incidence of mesothelioma in Finland (1/10(5) in 1990-1995).     

Changes in lactate dehydrogenase enzyme pattern in Chinese hamster cells infected and transformed with Simian virus 40.

The lactate dehydrogenase (LDH) isozyme patterns of consecutive passages of Chinese hamster embryo cultures were monitored. At early passages the population displayed two LDH bands, M4 and M3H; however, at higher passages the cultures exhibited M2H2, M2H, and M4. When primary cultures of Chinese hamster embryo cells were infected with simian virus 40 (SV40), no change in the LDH pattern was observed; however, the total activity of LDH increased. Twenty-three of 25 transformed colonies isolated from SV40-infected primary cells by their ability to grow in methyl cellulose produced only M4 or M4-M3H isozymes bands. Four of the SV40-transformed clones that produced only the M4 isozyme were tested for LDH activity and found to have activities 2.5 to 3 times greater than the control cells. Chinese hamster kidney epithelial cells transformed with SV40 virus had a decrease in the H subunit production, from 57 to 31%, compared with normal kidney epithelial cells. This decrease in H subunit production led to an increase in the cathode-migrating isozymes. Therefore, a shift to the cathode-migrating isozyme was observed in SV40-transformed cells. This change in LDH pattern might represent a reversion to the enzyme pattern present in fetal cells.     

Enhancement of N-methyl-N'-nitro-N-nitrosoguanidine-induced DNA amplification in a Simian virus 40-transformed Chinese hamster cell line by 3-aminobenzamide

A Simian virus 40-transformed Chinese hamster cell line (CO 60) amplifies integrated viral DNA sequences as a response to treatment with a variety of carcinogens. To study a possible involvement of poly(ADP-ribose) synthesis, DNA amplification was induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), an alkylating carcinogen that strongly stimulates poly(ADP-ribose) synthesis. In the presence of 3-aminobenzamide (3AB) (2 mM), a competitive inhibitor of poly(ADP-ribose) polymerase, MNNG-induced amplification was increased two to six times the level induced by MNNG alone. Concomitantly, 3AB reduced cellular poly(ADP-ribose) levels and increased MNNG-induced cytotoxicity, as expected. The effect of 3AB on MNNG-induced amplification depended both on the concentration of 3AB and the duration of its presence after MNNG treatment. By contrast, 3-aminobenzoic acid, a noninhibitory structural analogue of 3AB, had no influence on amplification induced by MNNG. These data strongly suggest an involvement of poly(ADP-ribose) in the process of DNA amplification, as it is shown that inhibition of carcinogen-stimulated poly(ADP-ribose) synthesis by 3AB is correlated with an enhancement of inducible DNA amplification in this cell line.