Spontaneous frequency of 6-thioguanine resistant mutants in CHO-AS52 cells after prolonged culturing in the absence of selective agents.

The Escherichia coli xanthine-guanine phosphoribosyl transferase (XPT) containing Chinese hamster ovary (CHO-AS52) cells were studied in our laboratory to evaluate its general applicability in genotoxicity testing of industrial chemicals. Our initial effort was to evaluate the stability of the 6-thioguanine sensitive phenotype. The CHO-AS52 cells were cloned and cultured continuously for over 600 days in the absence of any selective agents. The spontaneous 6-thioguanine resistant mutant frequency was quantified periodically. The frequency was found to increase continuously with time from approximately 20 x 10(-6) to a plateau of approximately 250 x 10(-6) in 122 days. The mutant frequency fluctuated around the plateau value up to the last day of our study, at 648 days after cloning. A similar observation was made for cells that were recloned or treated with aminopterin (to selectively kill 6-TG resistant mutants) at approximately 450 days after the initial cloning. Our data suggest that the xpt gene was stably incorporated in the CHO cells.