Plaque formation by vacuolating virus SV40

SV40: Cell transformation and tumorigenesis

The story of SV40-induced tumorigenesis and cellular transformation is intimately entwined with the development of modern molecular biology. Because SV40 and other viruses have small genomes and are relatively easy to manipulate in the laboratory, they offered tractable systems for molecular analysis. Thus, many of the early efforts to understand how eukaryotes replicate their DNA, regulate expression of their genes, and translate mRNA were focused on viral systems. The discovery that SV40 induces tumors in certain laboratory animals and transforms many types of cultured cells offered the first opportunity to explore the molecular basis for cancer. The goal of this article is to highlight some of the experiments that have led to our current view of SV40-induced transformation and to provide some context as to how they contributed to basic research in molecular biology and to our understanding of cancer.

Influence of SV40 Genome on the Replication of an Adenovirus-SV40 "Hybrid" Population

Feldman, L. A. (Baylor University College of Medicine, Houston, Tex.), J. L. Melnick, and F. Rapp. Influence of SV40 genome on the replication of an adenovirus-SV40 "hybrid" population. J. Bacteriol. 90:778-782. 1965.-Replication of a type 7 adenovirus-SV40 hybrid population in primary African green monkey kidney cells was accompanied by the formation of SV40 tumor antigen, adenovirus antigens, and cytopathic changes characteristic of adenovirus infection. Prior infection of the cultures with SV40 stimulated replication of nonintegrated adenovirus 7 but did not enhance the replication of the hybrid virus. These results suggest that the population of the adenovirus-SV40 hybrid studied contains many particles carrying SV40 information. Replication of SV40 virus was not enhanced by co-infection with nonintegrated adenovirus 7 or with the adenovirus-SV40 hybrid. Cytosine arabinoside strongly inhibited replication of the adenovirus-SV40 hybrid population in African green monkey kidney cells. Enhanced replication of nonintegrated adenovirus 7 by SV40 was blocked by cytosine arabinoside; this block could be reversed by 2-deoxycytidine or deoxycytidine triphosphate.

Phylogenetic analysis of polyomavirus simian virus 40 from monkeys and humans reveals genetic variation

A phylogenetic analysis of 14 complete simian virus 40 (SV40) genomes was conducted in order to determine strain relatedness and the extent of genetic variation. This analysis included infectious isolates recovered between 1960 and 1999 from primary cultures of monkey kidney cells, from contaminated poliovaccines and an adenovirus seed stock, from human malignancies, and from transformed human cells. Maximum-parsimony and distance methods revealed distinct SV40 clades. However, no clear patterns of association between genotype and viral source were apparent. One clade (clade A) is derived from strain 776, the reference strain of SV40. Clade B contains isolates from poliovaccines (strains 777 and Baylor), from monkeys (strains N128, Rh911, and K661), and from human tumors (strains SVCPC and SVMEN). Thus, adaptation is not essential for SV40 survival in humans. The C terminus of the T-antigen (T-ag-C) gene contains the highest proportion of variable sites in the SV40 genome. An analysis based on just the T-ag-C region was highly congruent with the whole-genome analysis; hence, sequencing of just this one region is useful in strain identification. Analysis of an additional 16 strains for which only the T-ag-C gene was sequenced indicated that further SV40 genetic diversity is likely, resulting in a provisional clade (clade C) that currently contains strains associated with human tumors and human strain PML-1. Four other polymorphic regions in the genome were also identified. If these regions were analyzed in conjunction with the T-ag-C region, most of the phylogenetic signal could be captured without complete genome sequencing. This report represents the first whole-genome approach to establishing phylogenetic relatedness among different strains of SV40. It will be important in the future to develop a more complete catalog of SV40 variation in its natural monkey host, to determine if SV40 strains from different clades vary in biological or pathogenic properties, and to identify which SV40 strains are transmissible among humans.

Serological cross-reactivities between antibodies to simian virus 40, BK virus, and JC virus assessed by virus-like-particle-based enzyme immunoassays

Enzyme immunoassays (EIAs) for detection of serum antibodies to simian virus 40 (SV40), BK virus (BKV), and JC virus (JCV) were developed by using virus-like-particles (VLPs) produced in insect cells from recombinant baculoviruses expressing the VP1 protein of the respective virus. Rhesus macaque sera with neutralizing antibodies to SV40 showed a high level of reactivity in the SV40 VLP-based EIA, and these sera also showed lower levels of reactivity in the BKV and JCV VLP-based EIAs. Rhesus macaque sera negative for neutralizing antibodies to SV40 were negative in all three EIAs. Competitive binding assays showed that SV40 VLPs inhibited BKV reactivity. In rhesus macaque sera, high optical density (OD) values for antibodies to SV40 VLPs were correlated with high OD values for antibodies to BKV but not with high OD values for antibodies to JCV VLPs. Human sera with neutralizing antibodies to SV40 were more reactive to SV40 VLPs than human sera without neutralizing antibodies to SV40. The greater SV40 reactivities of human sera were correlated with greater reactivities to BKV VLPs but not JCV VLPs. These data suggest that cross-reactivity with BKV antibodies may account for part of the low-level SV40 reactivity seen in human sera. With their greater versatility and their suitability for large-scale testing, the VLP-based EIAs for SV40, BKV, and JCV are likely to contribute to a better understanding of the biology of these viruses.

High-fidelity PCR amplification of infectious copies of the complete simian virus 40 genome from plasmids and virus-infected cell lysates

We describe here a long-polymerase chain reaction (PCR) method that can be used to amplify complete simian virus 40 (SV40) DNA with high fidelity, and we show that authentic, viable virus can be produced from molecular clones of the PCR-amplified viral DNAs. A commercial long-PCR kit that employed a combination of Taq and GB-D polymerases was used, together with a pair of overlapping primers that recognized a unique EcoRI site in the SV40 genome. Efficient amplification required linearization of the circular SV40 genomic DNAs with EcoRI. Entire SV40 genomes were successfully PCR-amplified from an SV40 plasmid and from two different SV40-infected cell lysates and were cloned into pUC-19. Three separate segments of the cloned viral genomes were DNA sequenced, and no nucleotide changes relative to the parental virus were detected, suggesting that the viral DNAs had been amplified with high fidelity. Each PCR clone was infectious, and no differences were detected in the growth characteristics of viruses derived from these clones as compared to the original viral strain. The procedure we utilized shortens and simplifies the molecular cloning of small double-stranded DNA viruses and will be useful for viral diagnostic tests and for recovery of virus from clinical samples. The results of these experiments have broad implications, as the methodology is applicable to many systems.

Artificial modification of the viral regulatory region improves tissue culture growth of SV40 strain 776

We describe here changes in the regulatory region of SV40 that influence its growth potential in cultured cells. Laboratory strains of papovaviruses BK and JC differ in the sequence of their regulatory regions from archetypes that have not been passaged in cell culture. These archetypes lack sequence repeats in the regulatory region; duplications that occur upon passage in cell culture confer selective growth advantage. Changes within the enhancer-promoter region of the well-characterized 776 strain of papovavirus SV40 that might affect its growth in tissue culture cells have not been documented. We measured the effect upon the growth of SV40 (776 strain) in CV-1 cells either of adding an additional 72-basepair (bp) enhancer element or of duplicating the entire 21-bp repeat region. SV40 growth in tissue culture was improved by reiteration of enhancer elements, whereas no growth advantage was conferred by tandem duplication of the 21-bp repeats. Viral DNA infectivity in CV-1 cells was directly proportional to the number of 72-bp elements but was unaffected by tandemly repeated 21-bp repeat elements. This study suggests that the 776 strain of SV40 is an evolutionary intermediate and that tissue-culture-adapted strains of SV40 do not accurately reflect the replication potential of natural isolates from primate hosts.

The cellular secretory pathway is not utilized for biosynthesis, modification, or intracellular transport of the simian virus 40 large tumor antigen

Unlike most proteins, which are localized within a single subcellular compartment in the eucaryotic cell, the simian virus 40 (SV40) large tumor antigen (T-ag) is associated with both the nucleus and the plasma membrane. Current knowledge of protein processing would predict a role for the secretory pathway in the biosynthesis and transport of at least a subpopulation of T-ag to account for certain of its chemical modifications and for its ability to reach the cell surface. We have examined this prediction by using in vitro translation and translocation experiments. Preliminary experiments established that translation of T-ag was detectable with as little as 0.1 microgram of the total cytoplasmic RNA from SV40-infected cells. Therefore, by using a 100-fold excess of this RNA, the sensitivity of the assays was above the limits necessary to detect the theoretical fraction of RNA equivalent to the subpopulation of plasma-membrane-associated T-ag (2 to 5% of total T-ag). In contrast to a control rotavirus glycoprotein, the electrophoretic mobility of T-ag was not changed by the addition of microsomal vesicles to the in vitro translation mixture. Furthermore, T-ag did not undergo translocation in the presence of microsomal vesicles, as evidenced by its sensitivity to trypsin treatment and its absence in the purified vesicles. Identical results were obtained with either cytoplasmic RNA from SV40-infected cells or SV40 early RNA transcribed in vitro from a recombinant plasmid containing the SP6 promoter. SV40 early mRNA in infected cells was detected in association with free, but not with membrane-bound, polyribosomes. Finally, monensin, an inhibitor of Golgi function, failed to specifically prevent either glycosylation or cell surface expression of T-ag, although it did depress overall protein synthesis in TC-7 cells. We conclude from these observations that the constituent organelles of the secretory pathway are not involved in the biosynthesis, modification, or intracellular transport of T-ag. The initial step in the pathway of T-ag biosynthesis appears to be translation on free cytoplasmic polyribosomes. With the exclusion of the secretory pathway, we suggest that T-ag glycosylation, palmitylation, and transport to the plasma membrane are accomplished by previously unrecognized cellular mechanisms.

Absence of a structural basis for intracellular recognition and differential localization of nuclear and plasma membrane-associated forms of simian virus 40 large tumor antigen

The simian virus 40 large tumor antigen (T-ag) is found in both the nuclei (nT-ag) and plasma membranes (mT-ag) of simian virus 40-infected or -transformed cells. It is not known how newly synthesized T-ag molecules are recognized, sorted, and transported to their ultimate subcellular destinations. One possibility is that these events depend upon structural differences between nT-ag and mT-ag. To test this possibility, we compared the structures of nT-ag and mT-ag from simian virus 40-infected cells. No differences between the two forms of T-ag were detected by migration in polyacrylamide gels, by Staphylococcus aureus V8 partial proteolytic mapping of methionine- or proline-containing peptides, or by two-dimensional tryptic peptide mapping of methionine-containing peptides. The carboxy-terminal, methionine-containing tryptic peptide was identified in the two-dimensional maps and was shown to be identical in nT-ag and mT-ag. Thus, a structural basis for the recognition and differential localization of T-ags could not be demonstrated. The carboxy terminus of the T-ag encoded by mutant dlA2413 is derived from the alternate open reading frame of the simian virus 40 early region, in analogy with the theoretical early gene product, T*-ag. We used this mutant to identify peptides unique to T*-ag. None of these peptides were detected in maps of mT-ag; only wild-type T-ag-specific peptides were found. These findings suggest that T*-ag does not represent the membrane-associated form of T-ag, but that mT-ag is encoded within the same reading frame used for nT-ag.

Absence of a structural basis for intracellular recognition and differential localization of nuclear and plasma membrane-associated forms of simian virus 40 large tumor antigen

The simian virus 40 large tumor antigen (T-ag) is found in both the nuclei (nT-ag) and plasma membranes (mT-ag) of simian virus 40-infected or -transformed cells. It is not known how newly synthesized T-ag molecules are recognized, sorted, and transported to their ultimate subcellular destinations. One possibility is that these events depend upon structural differences between nT-ag and mT-ag. To test this possibility, we compared the structures of nT-ag and mT-ag from simian virus 40-infected cells. No differences between the two forms of T-ag were detected by migration in polyacrylamide gels, by Staphylococcus aureus V8 partial proteolytic mapping of methionine- or proline-containing peptides, or by two-dimensional tryptic peptide mapping of methionine-containing peptides. The carboxy-terminal, methionine-containing tryptic peptide was identified in the two-dimensional maps and was shown to be identical in nT-ag and mT-ag. Thus, a structural basis for the recognition and differential localization of T-ags could not be demonstrated. The carboxy terminus of the T-ag encoded by mutant dlA2413 is derived from the alternate open reading frame of the simian virus 40 early region, in analogy with the theoretical early gene product, T*-ag. We used this mutant to identify peptides unique to T*-ag. None of these peptides were detected in maps of mT-ag; only wild-type T-ag-specific peptides were found. These findings suggest that T*-ag does not represent the membrane-associated form of T-ag, but that mT-ag is encoded within the same reading frame used for nT-ag.

Construction and characterization of an SV40 mutant defective in nuclear transport of T antigen

An SV40-adenovirus 7 hybrid virus, PARA(cT), has been described that is defective for the nuclear transport of SV40 large tumor antigen. An SV40(cT) mutant was constructed using SV40 early and late region DNA fragments derived from PARA(cT) and wild-type SV40 respectively. The SV40(cT)-3 construct is defective for viral replication, but can be propagated in COS-1 cells. T antigen induced by SV40(cT)-3 is localized in the cytoplasm of infected cells. The cT mutation also inhibits the transport of wild-type T antigen; COS-1 cells lose their constitutive expression of nuclear T antigen after infection with SV40(cT)-3. Sequence analysis revealed that the cT mutation results in the replacement of a positively charged lysine in wild-type T antigen with a neutral asparagine at amino acid number 128, demonstrating that the alteration of a single amino acid is sufficient to abolish nuclear transport. Implications of the cT mutation on possible mechanisms for the transport of proteins to the nucleus are discussed.

Optimal conditions for titration of SV40 by the plaque assay method

The parameters of the Simian Virus 40 (SV40) plaque assay on African green monkey kidney cells were optimized for reproducibility and maximum plaquing efficiency. Plaques were visible as early as 8 days postinfection; maximum titers were obtained with a 10- to 11-day incubation period. Titers read 12-16 days postinfection were not significantly higher than those observed after 10-11 days. Adsorption volumes greater than 0.1 ml/60 mm Petri dish decreased plaque forming units (PFUs) detected. Times greater than 60 min for adsorption of virus to the cell monolayer did not significantly increase the titer; adsorption times less than 60 min resulted in decreased titers. Under standard conditions, 3 ml of overlay medium containing 0.8% agar was applied following virus adsorption and again on days 5 and 10. Concentrations of fetal calf serum (FCS) in the overlay medium of 2.5 to 7.5% gave equal plaque formation. FCS concentrations of 1 and 10% resulted in slightly decreased and increased plaquing efficiencies respectively. Of the reagents tested, agar or agarose containing overlay media produced plaques of maximum number and size. An overlay of methyl cellulose resulted in the same number of plaques, but their size was reduced by approximately 70% relative to those observed in agar; thus longer incubation times were required. Gum tragacanth overlay medium was actually inhibitory to plaque development. DEAE-dextran, dextran sulfate, or DMSO added to agar overlay medium did not enhance plaque number or size, nor did they shorten the incubation period required for their detection.

Interaction of a simian papovavirus and adenoviruses. I. Induction of adenovirus tumor antigen during abortive infection of simian cells

Feldman, Lawrence A. (Baylor University College of Medicine, Houston, Tex.), Janet S. Butel, and Fred Rapp. Interaction of a simian papovavirus and adenoviruses. I. Induction of adenovirus tumor antigen during abortive infection of simian cells. J. Bacteriol. 91:813-818. 1966.-Adenovirus types 2, 7, and 12 undergo an abortive growth cycle in green monkey kidney cells; they induce the formation of adenovirus tumor antigen, but synthesis of adeno capsid antigen and infectious adenovirus was observed only when cultures were concomitantly infected with a simian papovavirus (SV40). Several other viruses, including herpes simplex and measles which replicate in monkey cells, and rabbit papilloma and human wart papovaviruses which do not, failed to stimulate adenovirus replication in the monkey cells. Adenovirus tumor antigen was detected 8 to 10 hr postinfection by immunofluorescent techniques. The antigen induced by adenovirus types 2 and 7 appeared as intranuclear masses; adenovirus type 12 tumor antigen also appeared as cytoplasmic and nuclear flecks. Sera from hamsters bearing tumors induced by adenovirus type 12 cross-reacted with tumor antigens induced by types 2 and 7 but not with antigens induced by SV40.

Simian Virus 40: Isolation of Two Plaque Types

Two different morphological plaque types of simian virus 40 were isolated from three of six strains. Increased uptake of neutral red gave one type of plaque a uniform red appearance, whereas the center of the other remained unstained. The viruses showed stability of plaque morphology on passage in tissue culture; cross neutralization studies identified both as simian virus 40.