The mutagenicity of an oxidized form of dATP, 2-hydroxydeoxyadenosine 
5'-triphosphate (2-OH-dATP), was examined using an SV40 origin-dependent in 
vitro replication system with a HeLa extract. 2-OH-dATP induced mutations in a 
dose-dependent manner and elicited substitution and deletion mutations. Of the 
substitutions, a G.C-->A.T transition including a tandem (CC-->TT) mutation was 
mainly observed. This result agrees with our previous observation that mammalian 
DNA polymerase alpha misincorporates the oxidized nucleotide opposite C, but is 
in contrast to the finding that 2-OH-dATP elicits G.C-->T.A transversions in 
Escherichia coli. This type of mutation was also elicited, but to a lesser 
extent. Interestingly, the mutagenicity of 2-OH-dATP was enhanced in the 
presence of 2-hydroxydeoxyadenosine 5'-diphosphate, an inhibitor of the MTH1 
protein, suggesting that this protein functions in the hydrolysis of 2-OH-dATP 
in the replication reaction mixture, and probably in living cells. These results 
indicate that 2-OH-dATP is mutagenic and that its mutagenicity is suppressed by 
the MTH1 protein in mammalian cells.