Repair efficiency of (5′S)-8,5′-cyclo-2′-deoxyguanosine and (5′S)-8,5′-cyclo-2′-deoxyadenosine depends on the complementary base

5′-R and 5′-S diastereoisomers of 8,5′-cyclo-2′-deoxyadenosine (cdA) and 8,5′-cyclo-2′-deoxyguanosine (cdG) containing a base-sugar covalent bond are formed by hydroxyl radicals. R-cdA and S-cdA are repaired by nucleotide excision repair (NER) in mammalian cellular extracts. Here, we have examined seven purified base excision repair enzymes for their ability to repair S-cdG or S-cdA. We could not detect either excision or binding of these enzymes on duplex oligonucleotide substrates containing these lesions. However, both lesions were repaired by HeLa cell extracts. Dual incisions by human NER on a 136-mer duplex generated 24–32 bp fragments. The time course of dual incisions were measured in comparison to cis-anti-B[a]P-N2-dG, an excellent substrate for human NER, which showed that cis-anti-B[a]P-N2-dG was repaired more efficiently than S-cdG, which, in turn, was repaired more efficiently than S-cdA. When NER efficiency of S-cdG with different complementary bases was investigated, the wobble pair S-cdG·dT was excised more efficiently than the S-cdG·dC pair that maintains nearly normal Watson-Crick base pairing. But S-cdG·dA mispair with no hydrogen bonds was excised less efficiently than the S-cdG·dC pair. Similar pattern was noted for S-cdA. The S-cdA·dC mispair was excised much more efficiently than the S-cdA·dT pair, whereas the S-cdA·dA pair was excised less efficiently. This result adds to complexity of human NER, which discriminates the damaged base pairs on the basis of multiple criteria.

► NEIL1, NEIL2, Fpg, OGG1, Endo III, Endo V, and Endo VIII do not recognize S-cdG and S-cdA.
► Both S-cdG and S-cdA are repaired by HeLa cell extracts. NER efficiencies followed the order: cis-anti-B[a]P-N2-dG ≫ S-cdG > S-cdA.
► Human NER efficiency of S-cdG and S-cdA depends on the complementary base. Repair efficiency followed the order: S-cdG·dT > S-cdG·dC > S-cdG·dA, although duplex stability in melting temperature (Tm) followed the order: S-cdG·dC > S-cdG·dT > S-cdG·dA.
► NER efficiency of S-cdA followed analogous pattern: S-cdA·dC > S-cdA·dT > S-cdA·dA.