Spectrum of spontaneously occurring mutations in the hprt gene of V79 Chinese hamster cells.

A total of 76 independent spontaneous mutants in the hprt gene of V79 Chinese hamster cells have been analyzed. These mutants were obtained in two different laboratories, 17 and 59 mutants in sets 1 and 2, respectively, under different cell culture conditions. Mutation analysis was performed by amplification of hprt cDNA with the polymerase chain reaction and direct sequencing of the products. The data obtained showed similar spectra of spontaneous mutations in both sets of mutants, suggesting that culture does not play a major role in spontaneous mutagenesis. The majority of the mutations were base substitutions (greater than 60%), with twice as many transversions as transitions. Base changes were evenly distributed throughout the structural gene, including the splice junctions. All types of base substitutions appeared in comparable frequencies, except for A.T to T.A transversions, which were almost absent. The fraction of deletion mutations was low (13%). A striking feature of the observed mutation spectra is that one third of the spontaneous mutations analyzed involved aberrant splicing of the hprt primary transcript, with exon 4 being affected most frequently, indicating that splice mutations are a common mechanism of mutation in the hprt gene.