To assess the functions of the three human MutT-type enzymes, MTH1, MTH2, and NUDT5, mutation induction by an oxidized form of dGTP, 8-hydroxy-2'-deoxyguanosine 5'-triphosphate (8-OH-dGTP; 7,8-dihydro-8-oxo-2'-deoxyguanosine 5'-triphosphate), was examined using human 293T cells treated with their specific siRNAs. Shuttle plasmid DNA containing the supF gene was first transfected into the cells, and then 8-OH-dGTP was introduced by means of osmotic pressure. Escherichia coli cells were transformed with the DNAs replicated in the treated cells. The knockdown of the MTH1, MTH2, and NUDT5 proteins increased the A:T --> C:G substitution mutations induced by 8-OH-dGTP. In addition, the increase in the induced mutation frequency was more evident in the triple-knockdown cells. These results indicate that all three of the human MTH1, MTH2, and NUDT5 proteins act as a defense against the mutagenesis induced by oxidized dGTP.
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