GENERATION OF A STRONG MUTATOR PHENOTYPE IN YEAST BY IMBALANCED BASE EXCISION-REPAIR

Increased spontaneous mutation is associated with increased cancer ris k Here, by using a model system, we show that spontaneous mutation can be increased several hundred-fold by a simple imbalance between the f irst two enzymes involved in DNA base excision repair. The Saccharomyc es cerevisiae MAG1 3-methyladenine (3MeA) DNA glycosylase, when expres sed at high levels relative to the apurinic/apyrimidinic endonuclease, increases spontaneous mutation by up to approximate to 600-fold in S. cerevisiae and approximate to 200-fold in Escherichia coil. Genetic e vidence suggests that, in yeast, the increased spontaneous mutation re quires the generation of abasic sites and the processing of these site s by the REV1/REV3/REV7 lesion bypass pathway, Comparison of the mutat or activity produced by Mag1, which has a broad substrate range, with that produced by the E. coli Tag 3MeA DNA glycosylase, which has a nar row substrate range, indicates that the removal of endogenously produc ed 3MeA is unlikely to be responsible for the mutator effect of Mag1, Finally, the human AAG 3-MeA DNA glycosylase also can produce a small (approximate to 2-fold) but statistically significant increase in spon taneous mutation, a result which could have important implications for carcinogenesis.