Influence of the Viral Regulatory Region on Tumor Induction by Simian Virus 40 in Hamsters

Abstract

Most of the simian virus 40 (SV40) genome is conserved among isolates, but the noncoding regulatory region and the genomic region encoding the large T-antigen C terminus (T-ag-C) may exhibit considerable variation. We demonstrate here that SV40 isolates differ in their oncogenic potentials in Syrian golden hamsters. Experimental animals were inoculated intraperitoneally with 10 7 PFU of parental or recombinant SV40 viruses and were observed for 12 months to identify genetic determinants of oncogenicity. The viral regulatory region was found to exert a statistically significant influence on tumor incidence, whereas the T-ag-C played a minor role. Viruses with a single enhancer (1E) were more oncogenic than those with a two-enhancer (2E) structure. Rearrangements in the 1E viral regulatory region were detected in 4 of 60 (6.7%) tumors. Viral loads in tumors varied, with a median of 5.4 SV40 genome copies per cell. Infectious SV40 was rescued from 15 of 37 (40%) cell lines established from tumors. Most hamsters with tumors and many without tumors produced antibodies to T antigen. All viruses displayed similar transforming frequencies in vitro, suggesting that differences in oncogenic potential in vivo were due to host responses to viral infection. This study shows that SV40 strains differ in their biological properties, suggests that SV40 replicates to some level in hamsters, and indicates that the outcome of an SV40 infection may depend on the viral strain present.Simian virus 40 (SV40) is a member of the family Polyomaviridae and is known for its ability to induce malignancies in the Syrian golden hamster (Mesocricetus auratus) model (9-11, 16, 20, 32). SV40 was discovered as an inadvertent contaminant of early forms of poliovirus and adenovirus vaccines (9, 46) that were prepared in primary cultures of kidney cells from rhesus monkeys, which are often naturally infected with the virus (9, 43, 51). Since its discovery, SV40 has been an important model for studies of virus-induced cancers and of viral effects on eukaryotic cell processes (1, 5). SV40 has been found to cause human infections and to be associated with some human malignancies (9, 21, 52).Phylogenetic analysis has recently established that strains of SV40 exist and can be grouped into clades or genogroups (18). Strains are identified by nucleotide differences at the C terminus of the large tumor antigen (T-ag) gene (T-ag-C) that result in amino acid changes in the protein. The SV40 large T-ag protein is essential for viral replication and is the major viral oncoprotein (1, 9, 32). Strains of SV40 can diverge in the structures of their noncoding regulatory regions (9,23,26,45), generating what are termed variants. SV40 variants containing a partial or complete duplication of the 72-bp enhancer element or other sequence rearrangements are designated as having complex regulatory regions, and those with one enhancer are designated as having a simple or archetypal regulatory region structure (23,26). It has been demonstrated that increased numbers of enha...

Polyomavirus SV40: Model Infectious Agent of Cancer

“…A panel of recombinants was constructed that mixed different regulatory regions and T-ag-C variable domains. Interestingly, the parental and recombinant viruses displayed similar transforming frequencies in vitro in mouse cells (Sroller et al 2008 ) . However, when 21-day-old weanling hamsters were inoculated intraperitoneally, the oncogenic potential in vivo of the viruses ranged from 83% to 0% (Fig.…”

“…First, many virus-inoculated but tumor-free animals develop antibodies to SV40 T-ag, with more responders among animals inoculated with 2E viruses as compared to 1E viruses (Sroller et al 2008 ;McNees et al 2009 ) . SV40 T-ag is not a component of the virus particle, but rather is synthesized in infected cells.…”

“…Subcutaneous inoculation usually results in sarcomas at the site of injection; intrapleural and intraperitoneal inoculation leads to a high proportion of mesotheliomas (Cicala et al 1993 ;Sroller et al 2008 ) ; intracerebral inoculation produces ependymomas (Kirschstein and Gerber 1962 ) ; and intravenous inoculation induces a spectrum of tumors, especially lymphomas, mesotheliomas, and osteosarcomas (Diamandopoulos 1973 ;Carbone et al 1989 ;McNees et al 2009 ) . These studies showed that SV40 has a broad tissue tropism with the capacity to transform a variety of target cell types.…”

“…Recent studies have established that the viral regulatory region is a genetic determinant of SV40 oncogenic potential whereas the T-ag variable domain exerts negligible infl uence (Sroller et al 2008 ) . These two genomic regions were described above as being sites of variation among SV40 isolates.…”

“…Although viral regulatory region rearrangements are commonly observed in SV40-infected immunocompromised monkeys, those changes do not appear to be required for induction of progressive multifocal leukoencephalopathy disease in those animals (Dang et al 2005 ) . Numerous examples of complex rearrangements in the viral regulatory region from various sources have been described ; Lednicky and Butel 2001 ;Sroller et al 2008 ) . As the regulatory region is not invariant, it cannot be used for strain identifi cation purposes.…”