Transformation of rodent cells by RFV, the human papovavirus with dual genome

Human papovavirus BK early gene regulation in nonpermissive cells

The human papovavirus BK latently infects a majority of the population worldwide, and its DNA has been found in human tumor tissue. BKV is known to be highly oncogenic in rodents, and is capable of transforming cells in vitro. Rearrangements in the transcriptional regulatory sequences controlling expression of the transforming early gene, T antigen, are known to affect both the tumorigenic and transforming properties of this virus. Little is known about the mechanism by which this occurs. We have examined several aspects of BKV early promoter/enhancer regulation in cell types which the virus transforms, baby hamster kidney (BHK) and newborn rat kidney (NRK) cells, and compare them to the same processes in monkey kidney CV1 cells. We find that BKV early transcriptional efficiency requires the same enhancer repeat elements in all three cell types, but that requirements for sequences to the early and late side of these repeats vary between these cells. While the BKV T antigen was found to repress early gene expression from the BKV early promoter in CV1 cells, this effect was lower in BHK cells and essentially absent in NRK cells. The impaired autoregulation observed in rodent cells may be the result of inefficient T antigen production in these cells. DNA replication from the BKV origin was not detected in either BHK or NRK cells. Finally, we find no correlation between the efficiency of the BKV early regulatory region in directing gene expression and the ability to transform NRK cells.

BK virus-transformed inbred hamster brain cells: status of viral DNA in subclones

We have recently reported that viral DNA sequences in inbred LSH hamster brain cells transformed by the GS variant of BK virus (LSH-BR-BK) are present predominantly in a free form (Beth et al., J. Virol. 40:276-284, 1981). In this report, we confirm that the presence of viral DNA sequences in these cells is not due to virus production, since viral capsid proteins were not detected by immunoprecipitation. Furthermore, we examined the status of viral DNA in 15 subclones of this cell line and detected free and integrated viral DNA sequences in only 5 of the subclones. The other 10 subclones contained exclusively integrated viral DNA sequences, as shown by the blot hybridization of high-molecular-weight cell DNA which was uncleaved or digested with HincII, for which there are no sites in viral DNA. The arrangement of viral DNA in these clones was further analyzed by cleavage of cellular DNA with HpaII and HindIII. Mitomycin (0.03 microgram/ml) treatment of subclones containing only integrated sequences resulted in the appearance of free viral DNA sequences in some of these cells. This result supports the postulation that free viral DNA in LSH-BR-BK cells is made up of excision products of observed tandemly repeated integrated sequences. In addition to the large T- and small t-antigens, LSH-BR-BK and all of its 15 subclones contained two antigen species which were larger than large T and one species which was smaller than small t. The number of tumor antigens in the LSH- BR-BK cell line and its subclones with a large copy number in a free form was not more than in the subclones with low copy number and integrated DNA. This suggests that free viral DNA is not a template for tumor antigen production in transformed cells.

Cloning and characterization of BK virus-related DNA sequences from normal and neoplastic human tissues

DNA homologous to that of the known papovarius BK was cloned from the high molecular weight DNA of two human tissues: a normal liver and a kidney carcinoma. The clone isolated from human liver consisted of DNA indistinguishable from prototype BK by restriction enzyme analysis that used ten different enzymes. The DNA cloned from the human carcinoma of the kidney was subject to rearrangement in recombination-deficient bacteria, and exhibited a deletion of a small segment of DNA localized to the BK late region. Restriction fragments representing the BK origin and promoter regions are overrepresented in the tumor-derived clone. The possible significance of retrieval of defective viral genomes from tumor tissues is discussed.

Differential requirement for SV40 early genes in immortalization and transformation of primary rat and human embryonic cells

A series of recombinant plasmids carrying various DNA fragments of SV40 early region were used to test for their ability to immortalize primary cultures of rat embryo (RE) and human embryonic kidney (HEK) cells. When primary RE cells were transfected with plasmids containing an entire early region of wild-type SV40- or a deletion mutant in the small tumor (t) antigen, dl 1410-DNA, they were all immortalized. The immortalized cells could grow in soft-agar medium and produced large tumor (T)-antigen. Cultured RE cells transfected with pW2-t, which contains a deletion in the large-T-specific coding region, also gave rise to continuous cell lines. Interestingly, two of nine RE lines immortalized by pW2-t could also grow in soft-agar medium. The plasmid pW-t8 carrying a similar fragment of SV40 DNA as pW2-t, but lacking the processing and polyadenylation signal sequences, also immortalized RE cells. Surprisingly, the plasmid pD-t1 which contains neither the intact large-T nor the small-t function also immortalized RE cells. However, the RE lines immortalized by pW-t8 or pD-t1 were unable to grow in soft-agar medium and displayed a wide range of growth phenotypes. On the contrary, when primary HEK cells were used for immortalization experiments, only those SV40 plasmids carrying the intact large-T function were able to generate immortalized lines. The growth properties of these immortalized HEK lines can be categorized into two groups. Those HEK lines immortalized by the large T alone grew slightly denser and rounder than their parental normal HEK cells, while those immortalized by both the large-T and small-t antigens grew extremely fast, reached higher density, piled up on each other, and were anchorage independent. In addition, when these SV40 plasmids were used to directly transform primary HEK cells by the focus assay, the large-T clone, pD3-05, only transformed HEK cells to form light foci. Transfection by the large-T plus the small-t sequences either in cis or in trans, did increase the frequency of focus formation, and gave rise to dense foci which could grow in soft-agar medium.

Transformation by purified early genes of simian virus 40

A rapid soft-agar assay using baby hamster kidney (BHK21 cl.13) cells has been developed to establish the functional roles for the large T and small t antigens of SV40 in transformation. Plasmids expressing either large T or small t antigens of SV40 have also been constructed and these plasmids have been used separately or in combination for transformation. A large T clone, pD3-05, containing a deletion in the small t-specific coding region [0.584-0.54 map units (mu)], transformed a low-background subclone of baby hamster kidney (BHK21 cl.13) cell line and F111 rat fibroblasts to anchorage independence at a low level (10-20 and 1%, respectively, of an early region clone from wild type [WT], pW2). A WT-derived small t clone, pW2-t, containing a deletion in the large T-specific coding region (0.373-0.169 mu), did not transform F111 cells, but transformed BHK21 cells at a very low level (about 2% of pW2). Another WT-derived small t clone, pW2-t/B1, containing a larger deletion in the large T-specific coding region (0.512-0.169 mu), did not transform either BHK21 or F111 cells. However, cotransformation with pD3-05 clone and pW2-t or pW2-t/B1 clone increased the frequency of transformation to about the same level as that of pW2. The ability of the small t clones to enhance the transformation efficiency of the large T clone was not due to recombination between the two plasmids, since cotransformation with pD3-05 and a small t clone without the polyadenylation [poly(A)] signal sequence from WT, pW-t8, did not increase the frequency of transformation. When the frequency of transformation was determined by the focus assay using F111 cells, pD3-05 transformed as well as pW2. Also, cotransformation with pD3-05 and pW2-t/B1 did not increase the frequency of focus formation. Therefore, the small t antigen was not required for this morphological transformation.

Multiple origins of the complementary defective genomes of RF and origin proximal sequences of GS, two human papovavirus isolates

It has previously been shown that the genome of RF virus, a variant of the human papovavirus, BK, consists of two DNA species, one (R1a) with a deletion corresponding to the early and the other (R2) with a deletion corresponding to the late region of BKV (A. Pater, M.M. Pater, and G. di Mayorca (1980). J. Virol. 36, 480-487; A. Pater, M. M. Pater, R. M. Dougherty, and G. di Mayorca (1981a). Virology 113, 86-94). In this report transfection experiments are used to show that these DNA species are individually defective for infection and that both DNA molecules are required simultaneously for the infection of human embryonic kidney (HEK) cells. DNA fragments containing the origin of replication in each of the DNA species are analyzed to show that R1a contains three and R2 contains two origins of replication. In addition, several changes in the repeat region proximal to the origin of replication are observed. The changes involve deletions and insertions. Examination of the deleted junctions most often reveals involvement of short stretches of repeated sequences (hot spots) in recombination. Another observed change is the insertion into R2 of a 63-bp sequence which contains no homology to either BK or SV40 DNA. This insertion is into the late-promoter region of this late-region coding DNA and appears to replace a poor "TATA" box in BK wild type with a better TATA box with the correct spacing from the "CAAT" box. A 304-bp fragment containing the origin of replication, early and late promoters, and the repeat units proximal to the origin of replication of GS, another variant of human BKV papovaviruses has also been sequenced. Several rearrangements, including deletions and insertions in the repeat region, are observed. Moreover, when homologous regions of this virus are compared to that of BKV, five base changes are detected, one of which is in the 23-bp origin region. This base change gives this 23-bp palindrome of GS a perfect two-fold rotational symmetry.

Malignant transformation of guinea pig cells after exposure to ultraviolet-irradiated guinea pig cytomegalovirus

Guinea pig cells were malignantly transformed in vitro by ultraviolet (uv)-irradiated guinea pig cytomegalovirus (GPCMV). When guinea pig hepatocyte monolayers were infected with uv-irradiated GPCMV, three continuous epithelioid cell lines which grew in soft agarose were established. Two independently derived GPCMV-transformed liver cells and a cell line derived from a soft agarose clone of one of these lines induced invasive tumors when inoculated subcutaneously or intraperitoneally into nude mice. The tumors were sarcomas possibly derived from hepatic stroma or sinusoid. Transformed cell lines were also established after infection of guinea pig hepatocyte monolayers with human cytomegalovirus (HCMV) or simian virus 40 (SV40). These cell lines also formed colonies in soft agarose and induced sarcomas in nude mice. It is concluded that (i) GPCMV can malignantly transform guinea pig cells; (ii) cloning of GPCMV-transformed cells in soft agarose produced cells that induced tumors with a shorter latency period but with no alteration in growth rate or final tumor size; and (iii) the tumors produced by GPCMV-and HCMV-transformed guinea pig cells were more similar to each other in growth rate than to those induced by SV40-transformed guinea pig cells.

BK virus-transformed inbred hamster brain cells: status of viral DNA in subclones

We have recently reported that viral DNA sequences in inbred LSH hamster brain cells transformed by the GS variant of BK virus (LSH-BR-BK) are present predominantly in a free form (Beth et al., J. Virol. 40:276-284, 1981). In this report, we confirm that the presence of viral DNA sequences in these cells is not due to virus production, since viral capsid proteins were not detected by immunoprecipitation. Furthermore, we examined the status of viral DNA in 15 subclones of this cell line and detected free and integrated viral DNA sequences in only 5 of the subclones. The other 10 subclones contained exclusively integrated viral DNA sequences, as shown by the blot hybridization of high-molecular-weight cell DNA which was uncleaved or digested with HincII, for which there are no sites in viral DNA. The arrangement of viral DNA in these clones was further analyzed by cleavage of cellular DNA with HpaII and HindIII. Mitomycin (0.03 microgram/ml) treatment of subclones containing only integrated sequences resulted in the appearance of free viral DNA sequences in some of these cells. This result supports the postulation that free viral DNA in LSH-BR-BK cells is made up of excision products of observed tandemly repeated integrated sequences. In addition to the large T- and small t-antigens, LSH-BR-BK and all of its 15 subclones contained two antigen species which were larger than large T and one species which was smaller than small t. The number of tumor antigens in the LSH- BR-BK cell line and its subclones with a large copy number in a free form was not more than in the subclones with low copy number and integrated DNA. This suggests that free viral DNA is not a template for tumor antigen production in transformed cells.