Polyoma virus mutant with normal transforming ability but impaired tumorigenic potential

Evaluating the sensitivity of newborn rats and newborn hamsters to oncogenic DNA

To evaluate the risk of residual cellular DNA in vaccines manufactured in tumorigenic cell lines, we have been establishing in vivo assays to quantify the oncogenic activity of DNA. We had generated three oncogene-expression plasmids: pMSV-T24-H-ras, which expresses activated H-ras; pMSV-c-myc, which expresses c-myc; and pMSV-T24-H-ras/MSV-c-myc, which expresses both oncogenes. Tumors were induced in mice by pMSV-T24-H-ras plus pMSV-c-myc or by pMSV-T24-H-ras/MSV-c-myc. Because newborn hamsters and newborn rats have been recommended for oncogenicity testing of the DNA from tumorigenic mammalian cell-substrates used for vaccine production, we evaluated their sensitivity. Newborn hamsters and rats were inoculated with different doses of pMSV-T24-H-ras/MSV-c-myc to determine their sensitivity to tumor induction and with the single-oncogene-expression plasmids to determine whether single oncogenes could induce tumors. Newborn rats were more sensitive than newborn hamsters, and activated H-ras but not c-myc induced tumors in newborns of both rodent species. DNA from four cell lines established from tumors induced by pMSV-T24-H-ras/MSV-c-myc was inoculated into newborn rats. Because no tumors were induced by this cellular DNA, which should be optimal as it contains both oncogenes linked and present in several copies, we conclude that available in vivo models are not sensitive enough to detect the oncogenicity of cellular DNA.

Mutations in polyomavirus large T affecting immortalization of primary rat embryo fibroblasts

To clarify the relationship between various functions of the polyomavirus large T antigen and the contribution of this oncogene toward neoplastic transformation, we have analyzed the properties of mutants with in-frame deletions in the second large T exon. dl45, dl96, and dl97 have retained the ability to immortalize primary rat embryo fibroblasts and to trans-activate viral promoters. dl8, dl23, and dl300, which are deficient immortalization, are also deficient in transactivation. However, a newly constructed mutant, designated dl141, which is deficient in immortalization, is still able to trans-activate both the polyoma and SV40 late promoters. This indicates that the ability to trans-activate promoters is not sufficient to confer on the large T antigen the ability to immortalize primary cells.

Mutations in polyomavirus middle T antigen affecting tumorigenesis

P155 is a polyomavirus mlt mutant with normal transforming ability but impaired tumorigenic potential. The mutation, a 12-bp deletion (nucleotides 1348-1359), removes amino acids 372 to 375 from middle T and affects its ability to function in tumorigenesis (C. Gelinas, S. Masse, and M. Bastin, 1984, J. Virol. 51, 242-246). We used deletion loop mutagenesis to introduce point mutations within the wild-type sequence spanned by the P155 deletion. A mutant phenotype resembling that of P155 could be produced by as little as one alanine to valine substitution at residue 373. The mutants were impaired in their ability to induce tumors in rats but they could still transform established cell lines or primary fibroblasts in culture. To define the biochemical defect, we examined the mutant middle T antigen both for association with pp60c-src, the cellular src gene product, as well as its pattern of phosphorylation. No obvious differences explaining the phenotype were observed. The mutant middle T associated with, and activated pp60c-src, but exhibited a slightly altered pattern of phosphorylation, presumably because of additional sites on the middle T protein.

A substitution in a nonconserved region of polyomavirus large T antigen which causes a thermosensitive mutation

The temperature-sensitive defect of the tsP155 mutant of polyomavirus (Py) maps in the large T antigen (LT) coding sequence of a viral DNA diverging markedly from that of extensively characterized wild-types (WTs) such as A2 and CSP. We have sequenced about 600 base pairs (bp) "early" DNA encompassing the mutated site in tsP155, as well as the corresponding DNA segment from a revertant virus (RtsP155). As expected, tsP155 was found to be more closely related to CSP than to A2. Out of 3 single bp differences between tsP155 and CSP, 2 were common to tsP155 and RtsP155. The only substitution exclusive to tsP155 was a G----C transversion at bp 2658 which canceled the HaeIII site at bp 2657. Heteroduplexes inclusive of tsP155 DNA and of a 312-bp-long fragment of RtsP155 DNA yielded recombinant viruses growing under restrictive conditions whose DNAs had all regained the HaeIII site at bp 2657. These findings clearly identify the ts mutation with the tranversion at bp 2658, which is expected to change Ala 701 for a Pro in LT. We discuss this substitution in relation to the phenotype of tsP155.

Site-directed mutagenesis of the polyomavirus genome: replication-defective large T mutants with increased immortalization potential

We used the deletion loop mutagenesis procedure to direct point mutations into a small region of the polyomavirus genome, at 0.97 map units, affecting the structure of both the middle and large T antigens. We describe the construction of six middle T mutants which have retained the ability to transform rat cells in culture and four large T mutants, three of which are deficient in viral DNA replication and capable of immortalizing primary rat embryo fibroblasts very efficiently.

Mutants of polyomavirus middle-T antigen

Polyomavirus middle-T antigen induces the transformation of established cell lines in culture and is known to interact with and/or modulate the activity of several enzymes (pp60c.src, protein kinase C and phosphatidylinositol kinase) in vitro. This review is a compilation of the reported mutants of middle-T antigen and their biochemical and biological properties as they relate to the transformation event. The mutants of polyomavirus middle-T antigen have been previously classified phenotypically. Given the now large number of mutants, the classification presented here is based upon the position within the molecule. A model of middle-T is presented in which the protein is considered as consisting of three domains: a hydrophobic domain (the putative membrane-binding domain), the amino-terminal half of the molecule (the putative pp60c.src-binding domain) and the intervening amino acids (the putative modulatory domain). A current model for the induction of transformation by polyomavirus middle-T is presented.

Recombinant retroviruses encoding simian virus 40 large T antigen and polyomavirus large and middle T antigens

We used a murine retrovirus shuttle vector system to construct recombinants capable of constitutively expressing the simian virus 40 (SV40) large T antigen and the polyomavirus large and middle T antigens as well as resistance to G418. Subsequently, these recombinants were used to generate cell lines that produced defective helper-free retroviruses carrying each of the viral oncogenes. These recombinant retroviruses were used to analyze the role of the viral genes in transformation of rat F111 cells. Expression of the polyomavirus middle T antigen alone resulted in cell lines that were highly tumorigenic, whereas expression of the polyomavirus large T resulted in cell lines that were highly tumorigenic, whereas expression of the polyomavirus large T resulted in cell lines that were unaltered by the criteria of morphology, anchorage-independent growth, and tumorigenicity. More surprisingly, SV40 large T-expressing cell lines were not tumorigenic despite the fact that they contained elevated levels of cellular p53 and had a high plating efficiency in soft agar. These results suggest that the SV40 large T antigen is not an acute transforming gene like the polyomavirus middle T antigen but is similar to the establishment genes such as myc and adenovirus EIa.

Recombinant retroviruses encoding simian virus 40 large T antigen and polyomavirus large and middle T antigens

We used a murine retrovirus shuttle vector system to construct recombinants capable of constitutively expressing the simian virus 40 (SV40) large T antigen and the polyomavirus large and middle T antigens as well as resistance to G418. Subsequently, these recombinants were used to generate cell lines that produced defective helper-free retroviruses carrying each of the viral oncogenes. These recombinant retroviruses were used to analyze the role of the viral genes in transformation of rat F111 cells. Expression of the polyomavirus middle T antigen alone resulted in cell lines that were highly tumorigenic, whereas expression of the polyomavirus large T resulted in cell lines that were highly tumorigenic, whereas expression of the polyomavirus large T resulted in cell lines that were unaltered by the criteria of morphology, anchorage-independent growth, and tumorigenicity. More surprisingly, SV40 large T-expressing cell lines were not tumorigenic despite the fact that they contained elevated levels of cellular p53 and had a high plating efficiency in soft agar. These results suggest that the SV40 large T antigen is not an acute transforming gene like the polyomavirus middle T antigen but is similar to the establishment genes such as myc and adenovirus EIa.

mlt Mutation in the polyomavirus genome impairing a function of the middle T protein

The DNA from polyomavirus mlt mutant P155 transforms cells in culture as efficiently as wild-type DNA but has a much lower tumorigenic potential when injected into newborn rodents. The mutant has a 12-base-pair deletion between nucleotides 1347 and 1360, i.e., in a region which encodes parts of the middle and large T antigens (G elinas et al., J. Virol. 43:1072-1081, 1982). To determine which of the two viral gene functions was affected by the mutation, we transferred the latter into a modified polyomavirus genome encoding exclusively the middle T protein. Our results show that the P155 mutation alters a function of the polyomavirus middle T protein required for the induction of the tumorigenic process in vivo. Beside the 12-base-pair deletion at 96.3 map units, there is no other alteration in the coding sequence of P155 middle T with respect to that of P16, the wild-type parental strain. We conclude, therefore, that the deletion is the lesion affecting the tumorigenic potential of mutant P155 .

Tumorigenic activity of polyoma virus and SV40 DNAs in newborn rodents

A procedure has been developed whereby the oncogenicity of the DNA from polyoma (Py) virus and Simian virus 40 (SV40) can be tested directly by injecting recombinant DNA into newborn rodents. Injection of 0.2-2.0 micrograms of linear DNA induced the development of subcutaneous liposarcomas and fibrosarcomas at the site of inoculation. Coinjection of high-molecular-weight rat DNA as carrier had little or no effect on tumor formation but plasmids pBR322, pAT153 , and pML2 behaved as strong inhibitors. Tumor induction by injecting DNA into newborn rodents provides an in vivo equivalent to a transformation assay but appears to be a more stringent and rigorous criterion of oncogenic transformation. The oncogenic potential of Py virus in newborn hamsters could be expressed by a recombinant encoding only the middle T protein, although with average tumor latencies 5-10 times longer than those observed with wild-type Py DNA. Py middle T required the cooperation from small T to induce tumors in newborn rats. SV40 DNA was tumorigenic only in newborn hamsters. delta 2005 DNA which is unable to produce the SV40 small T antigen was much less active and required a latent period about twice that of wild-type SV40 DNA. However, its tumorigenic potential was restored by addition of the Py small T antigen gene. This indicates that Py and SV40 small T antigens are interchangeable and that they probably play an identical role in malignant transformation. Finally, evidence was provided that intermolecular recombination or recombination between DNA fragments can occur in vivo.

Role of the three polyoma virus early proteins in tumorigenesis

A modified polyoma virus genome which can encode the middle T protein but not the large or small T proteins transforms rat cells in culture with an efficiency about 20% that of the wild-type genome. Although middle T-transformed cells grow as tumors when transplanted into nude mice or syngeneic rats, the middle T gene alone is totally inactive when used in a more stringent and rigorous assay for tumorigenicity such as the injection of DNA into newborn rats. Thus, functions other than those expressed by middle T antigen are required for the elaboration of all the properties associated with tumorigenesis. To assess whether a complementary function could be exerted by the large or the small T antigen, we constructed plasmids containing two modified early regions which independently encoded middle T and one of the two other proteins. Both recombinants were tumorigenic in newborn rats. Cell lines derived by transfer of these plasmids under no special selective conditions did not acquire the property of growth in low-serum medium but exhibited the same tumorigenic properties as wild-type polyoma DNA-transformed cells. Furthermore, a recombinant which encoded the middle and small T antigens, but not the large T antigen, was tumorigenic in newborn rats. Although the small T antigen provides a complementary function for tumorigenicity, it cannot complement the middle T antigen for an efficient induction of transformation of cultured cells. This suggests that the complementary function exerted by the small T antigen is different from that of the N-terminal fragment of the large T protein.

Role of the three polyoma virus early proteins in tumorigenesis

A modified polyoma virus genome which can encode the middle T protein but not the large or small T proteins transforms rat cells in culture with an efficiency about 20% that of the wild-type genome. Although middle T-transformed cells grow as tumors when transplanted into nude mice or syngeneic rats, the middle T gene alone is totally inactive when used in a more stringent and rigorous assay for tumorigenicity such as the injection of DNA into newborn rats. Thus, functions other than those expressed by middle T antigen are required for the elaboration of all the properties associated with tumorigenesis. To assess whether a complementary function could be exerted by the large or the small T antigen, we constructed plasmids containing two modified early regions which independently encoded middle T and one of the two other proteins. Both recombinants were tumorigenic in newborn rats. Cell lines derived by transfer of these plasmids under no special selective conditions did not acquire the property of growth in low-serum medium but exhibited the same tumorigenic properties as wild-type polyoma DNA-transformed cells. Furthermore, a recombinant which encoded the middle and small T antigens, but not the large T antigen, was tumorigenic in newborn rats. Although the small T antigen provides a complementary function for tumorigenicity, it cannot complement the middle T antigen for an efficient induction of transformation of cultured cells. This suggests that the complementary function exerted by the small T antigen is different from that of the N-terminal fragment of the large T protein.