Expression of the E. coli Fpg protein in CHO cells lowers endogenous oxypurine clustered damage levels and decreases accumulation of endogenous Hprt mutations

Endogenous DNA damage clusters-two or more oxidized bases, abasic sites, or strand breaks within about 20 base pairs on opposing strands-can accumulate in unirradiated mammalian cells, and may be significant origins of spontaneous detrimental biological effects. Factors determining the levels of such endogenous clusters are largely unknown. To determine if cellular repair genotype can affect endogenous cluster levels in mammalian cells, the authors examined cluster levels, growth rates, and mutant frequencies in Chinese hamster ovary cells expressing the Escherichia coli glycosylase fpg protein, whose principal substrates are oxidized purines. In cells expressing high levels of fpg protein, the levels of oxypurine clustered damages were decreased while those of oxypyrimidine clusters and abasic clusters were unchanged. Furthermore, in these cells, the growth rates were increased and the level of spontaneous background mutants in the hypoxanthine guanine phosphoribosyl transferase gene was decreased. These results suggest that endogenous clusters are potentially detrimental DNA damages, and that their levels-as well as the detrimental consequences of their presence-can be effectively reduced by increased cellular activity of specific DNA repair proteins.