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.

FIG. 1.

Tumor induction in hamsters by parental and recombinant SV40 viruses. (A) Influence of the viral regulatory region. Black bars indicate viruses with simple regulatory regions (1E); shaded bars indicate viruses with complex regulatory regions (2E). The difference between the two groups (1E versus 2E) was statistically significant (66/152 [43%] versus 18/155 [12%]; P = 0.0001). (B) Influence of the T-ag variable domain. Sequences unique to a given strain are indicated by markings shown in the figure.

FIG. 2.

Transformation in vitro by SV40 parental and recombinant viruses. Primary mouse embryo fibroblasts were infected at 10 PFU per cell. Replicate plates were stained, and transformed foci were counted at 3 and 6 weeks p.i. The relative numbers of transformed foci are plotted and normalized to the value for strain 776(2E) at 3 weeks. Bay(1E), Baylor(1E).

FIG. 3.

SV40 regulatory region rearrangements detected in hamster tumors. (A) Schematic representation of the SV40 reference strain [776(2E)] showing segments within the regulatory region. ori, origin of DNA replication; G/C, G/C-rich region; 72, 72-bp enhancer element; MLP(+1), major late promoter start site. Numbers above or below the figure identify specific nucleotide positions in 776(2E). The relative borders of specific regulatory region segments are indicated by arrows above or below the figure. Tandemly repeated sequences are depicted by white boxes inscribed with the numbers 21 or 72. (B to E) Regulatory region structures of viruses inoculated (top row) and sequences recovered (bottom row) from hamster tumors. In the tumor induced by 776(1E) in hamster 1263, only the rearranged regulatory region was detected; the original structure was not detected. In tumors from hamsters 1721, 1713, and 982, both original and rearranged regulatory region sequences were detected.