Analyses of mutation in pSV2gpt-transformed CHO cells

We have developed a system to study mutations which affect expression of the E. coli xanthine-guanine phosphoribosyl transferase (XPRT) gene (gpt) in hypoxanthine-guanine phosphoribosyl transferase-deficient (HPRT-) Chinese hamster ovary (CHO) cells that have been transformed by the plasmid pSV2gpt. Several gpt-transformed cell lines have been isolated and characterized with respect to integrated pSV2gpt sequences, expression of the gpt gene, and cytotoxic and mutagenic responses to UV light. While the gpt-transformed CHO and wild-type CHO-K1-BH4 cell lines have similar cytotoxic responses to UV light, the gpt-transformed cell lines respond differently from the parental CHO-K1-BH4 cell line in terms of mutation induction. As with CHO-K1-BH4 HPRT mutants, spontaneous or induced XPRT mutants derived from the gpt+ cell lines can be selected for 6-thioguanine resistance (TGr). Analysis of cell-free extracts from a number of these TGr clones indicates that the mutant phenotype is due to the absence of XPRT activity. One transformant, designated AS52, has previously been described in limited detail. Here we describe additional characteristics of this cell line, as well as several related transformants.