A mathematical model of homologous recombination in cultured cells

Expression of SV40 large T antigen stimulates reversion of a chromosomal gene duplication in human cells

Transformation of human cells is characterized by altered cell morphology, frequent karyotypic abnormalities, reduced dependence on growth factors and substrate, and rare "immortalization"-clonal acquisition of unlimited proliferative potential. We previously reported a marked increase in DNA rearrangements, arising between two duplicated segments in a transfected plasmid substrate, for five immortal human cell lines relative to three normal fibroblast strains [Finn et al. (1989) Mol. Cell. Biol. 9, 4009-4017]. We have now assessed reversion of a 14-kilobase-pair duplication within the hypoxanthine phosphoribosyl transferase (HPRT) gene locus, in a fibroblast strain during its normal replicative lifespan and after stable transformation with SV40 large-T antigen. Revertants, selected under HPRT-dependent growth conditions immediately after purging preexisting HPRT+ cells, were confirmed as HPRT+ by hypoxanthine incorporation and 6-thioguanine sensitivity. Southern blot analyses indicate loss from most revertant clones of a restriction fragment representing the duplicated HPRT region, as predicted for homologous recombination between the 14-kilobase-pair repeats. Amplification of a subregion of HPRT mRNA implicated deletion of duplicated exons in 93% of revertant colonies. Reversion to HPRT+ was unaltered during the normal in vitro lifespan of these cells, but increased in 9 clones stably transformed with large-T antigen (mean = 3.8-fold; each P < 10(-5)). Stimulation of HPRT-reversion is abrogated in a variety of T-antigen mutants, and depends on continued induction of T antigen by glucocorticoid in two clones tested 10-30 doublings before replicative senescence. Since no immortal subclones arose from these clones, elevated reversion must precede immortalization. Increased DNA rearrangements, in cells expressing T-antigen, could facilitate the rare concurrence of multiple mutations necessary for immortalization.

Intrachromosomal recombination mediated by the polyomavirus large T antigen

We used a spleen necrosis virus-based retroviral vector to introduce the polyomavirus replication origin into rat cells and developed a system to analyze homologous recombination events that do not reconstitute a selectable marker. Introduction of the gene coding for the polyomavirus large T antigen into the cell lines by DNA transfection promoted high-frequency recombination between the two retroviral LTRs, leading to amplification and excision of DNA sequences. To analyze homology requirements, we constructed cell lines carrying only the replication origin without exogenous repeats. Most of the cell lines sustained high-frequency recombination, presumably by undergoing homologous recombination between repetitive DNA lying in the vicinity of the integrated origin. Our results indicate that homologous recombination promoted by large T antigen does not require recombination hot spots in the viral genome other than the replication origin and they explain the cytotoxicity observed in some cell types when large T antigen is expressed in the presence of a functional origin.

High-frequency recombination mediated by polyomavirus large T antigen defective in replication

We investigated the mechanism by which the large T antigen (T-Ag) of both polyomavirus and simian virus 40 (SV40) promotes homologous recombination in mammalian cells. To this end, we constructed a rat cell line, designated Hy5, that carries two mutated copies of the polyomavirus middle-T-Ag (pmt) oncogene lying as direct repeats on the same chromosome. The structure of the viral insert was devised so that intrachromosomal recombination between the pmt repeats reconstitutes wild-type pmt and yields cell populations amenable to selection for the transformed phenotype. Correction of pmt by gene conversion occurred spontaneously at a rate of ca. 1.7 x 10(-7) per cell generation and was masked by another recombination event that also led to the transformation of the Hy5 cell line. This event was identified as chromosomal inversion and overexpression of the upstream pmt copy as a result of homologous recombination between adjacent pBR322 sequences. Both events were promoted by the polyomavirus large T-Ag by several orders of magnitude, as well as by mutants defective in the initiation of viral DNA synthesis. Large T-Ag also promoted reconstitution of wild-type pmt by unequal exchange between sister chromatids, yielding structures compatible with some of the chromosomal aberrations commonly observed in transformed cells. Our data indicate that large T-Ag has a recombination-promoting activity that can be dissociated from its replicative function.