Bypass of a 5',8-cyclopurine-2'-deoxynucleoside by DNA polymerase b during DNA replication and base excision repair leads to nucleotidemisinsertions and DNA strand breaks

t5,8-Cyclopurine-2-deoxynucleosides including 5,8-cyclo-dA (cdA) and 5,8-cyclo-dG (cdG) are inducedby hydroxyl radicals resulting from oxidative stress such as ionizing radiation. 5,8-cyclopurine-2-deoxynucleoside lesions are repaired by nucleotide excision repair with low efficiency, thereby leading totheir accumulation in the human genome and lesion bypass by DNA polymerases during DNA replicationand base excision repair (BER). In this study, for the first time, we discovered that DNA polymerase (pol) efficiently bypassed a 5R-cdA, but inefficiently bypassed a 5S-cdA during DNA replication and BER.We found that cell extracts from pol wild-type mouse embryonic fibroblasts exhibited significant DNAsynthesis activity in bypassing a cdA lesion located in replication and BER intermediates. However, pol knock-out cell extracts exhibited little DNA synthesis to bypass the lesion. This indicates that pol playsan important role in bypassing a cdA lesion during DNA replication and BER. Furthermore, we demon-strated that pol beta inserted both a correct and incorrect nucleotide to bypass a cdA at a low concentration. Nucleotide misinsertion was significantly stimulated by a high concentration of pol beta, indicating a muta-genic effect induced by pol beta lesion bypass synthesis of a 5',8-cyclopurine-2'-deoxynucleoside. Moreover,we found that bypass of a 5'S-cdA by pol beta generated an intermediate that failed to be extended by polbeta, resulting in accumulation of single-strand DNA breaks. Our study provides the first evidence that pol beta plays an important role in bypassing a 5',8-cyclo-dA during DNA replication and repair, as well as newinsight into mutagenic effects and genome instability resulting from pol beta bypassing of a cdA lesion.