Mutation rate at the hprt locus in human cancer cell lines with specific mismatch repair-gene defects

Spontaneous mutation rates at the hypoxanthine-guanine phosphoribosyltransferase (hprt) locus were measured in human cancer cell lines defectivein the mismatch repair (MMR) genes hMLH1, hPMS2, or GTBP, as well as in acell line carrying mutations in both hMLH1 and hPMS2. The mutation rate wasdetermined by quantitating mutant frequency increases within a singleculture as a function of cell division. These MMR- deficient cell linesexhibited a 50- to 750-fold increase in mutation rate relative to aMMR-proficient cancer cell line. From lowest to highest, the spontaneousmutation rates relative to the MMR-gene defects studied here are asfollows: hMLH1- < GTBP- < hPMS2- < hMLH1- / hPMS2-. In addition, acell line in which MMR was restored by chromosome transfer exhibited amutation rate 12-fold below the MMR- deficient parental cell line. Thesedata support the notion that MMR plays an important role in controlling therate of spontaneous mutation and suggest that different MMR-gene defectsmay vary in their ability to repair different types of DNA mismatches, thusleading to measurable quantitative differences in spontaneous mutagenesis.Furthermore, a difference in mutation rates was observed between ahPMS2-defective cell line (3.1 x 10(-5) mutations/cell/generation) and twohMLH1- defective cell lines (4.0 x 10(-6) and 7.3 x 10(-6)mutations/cell/generation). Assuming the hPMS2- and hMLH1-gene productsonly function in the proposed hMutL alpha heterodimer, then defects ineither gene should yield comparable mutation rates. These data suggest thathPMS2 plays a critical role in MMR, while additional hMLH1 homologues orhPMS2 alone may function to partially complement defects in hMLH1.