A search for allelic recombination in Chinese hamster cell hybrids

Mutants resistant to 6-thioguanine were selected from CHO cells which were either temperature sensitive or proline requiring. These mutants were stable and had low levels of hypoxanthine guanine phosphoribosyl transferase (HGPRT). Hybrids were selected which were heteroallelic at the hgprt locus and complementation between the mutants used was not observed. Interallelic recombination at this locus would generate hgprt+ cells which could be selected in Littlefield's HAT medium. Selection experiments with hybrids containing three different pairs of mutants yielded no recombinants among populations of 4 x 106 - 2 x 107 cells. After treatment with the recombinagen mitomycin C, 3 putative recombinants were detected amongst 1.4 x 107 surviving cells from one hybrid. One of these strains was examined and shown to have a normal level of HGPRT and its heterozygosity at this locus was demonstrated by the segregation of colonies resistant to 6-thioguanine. It cannot be excluded that the rare hgprt+ colonies seen arose by mutation rather than by recombination. Mitotic allelic recombination therefore appears to be a much less frequent event in CHO cells than it is in lower eukaryotes. It is possible that mitotic recombination is effectively suppressed in mammalian cells to prevent the expression of deleterious recessive mutants.