Mutations associated with base excision repair deficiency and methylation-induced genotoxic stress

The long-term effect of exposure to DNA alkylating agents is entwined with the cell's genetic capacity for DNA repair and appropriate DNA damage responses. A unique combination of environmental exposure and deficiency in these responses can lead to genomic instability; this "gene-environment interaction" paradigm is a theme for research on chronic disease etiology. In the present study, we used mouse embryonic fibroblasts with a gene deletion in the base excision repair (BER) enzymes DNA beta-polymerase (beta-pol) and alkyladenine DNA glycosylase (AAG), along with exposure to methyl methanesulfonate (MMS) to study mutagenesis as a function of a particular gene-environment interaction. The beta-pol null cells, defective in BER, exhibit a modest increase in spontaneous mutagenesis compared with wild-type cells. MMS exposure increases mutant frequency in beta-pol null cells, but not in isogenic wild-type cells; UV light exposure or N-methyl-N'-nitro-N-nitrosoguanidine exposure increases mutant frequency similarly in both cell lines. The MMS-induced increase in mutant frequency in beta-pol null cells appears to be caused by DNA lesions that are AAG substrates, because overexpression of AAG in beta-pol null cells eliminates the effect. In contrast, beta-pol/AAG double null cells are slightly more mutable than the beta-pol null cells after MMS exposure. These results illustrate that BER plays a role in protecting mouse embryonic fibroblast cells against methylation-induced mutations and characterize the effect of a particular combination of BER gene defect and environmental exposure.