Base excision repair of adenine/8-oxoguanine mispairs by an aphidicolin-sensitive DNA polymerase in human cell extracts

Replication of DNA containing 8-oxo-7,8-dihydroguanine (8oxoG) can generate 8oxoG/A base pairs which, if uncorrected, lead to G-->T transversions. It is generally accepted that the repair of these promutagenic base pairs in human cells is initiated by the MutY DNA glycosylase homolog (hMYH). Here we provide biochemical evidence that human cell extracts perform base excision repair (BER) on both DNA strands of an 8oxoG/A mismatch. At early repair times the specificity of nucleotide incorporation indicates a preferential insertion of C opposite 8oxoG leading to the formation of 8oxoG/C pairs. This is followed by repair synthesis on the opposite DNA strand that is consistent with hOGG1-mediated correction of 8oxoG/C to G/C. Repair synthesis on either strand is completely inhibited by aphidicolin suggesting that a replicative DNA polymerase is involved in the gap filling. This is the first demonstration that repair of 8oxoG/A base pairs is by two BER events likely mediated by Poldelta/epsilon. We suggest that the Poldelta/epsilon-mediated BER is the general mode of repair when BER lesions are formed at replication forks.