Single nucleotide patch base excision repair is the major pathway for removal of thymine glycol from DNA in human cell extracts

The repair pathways involved in the removal of thymine glycol (TG) from DNA by human cell extracts have been examined. Closed circular DNA constructs containing a single TG at a defined site were used as substrates to determi ne the patch size generated after in vitro repair by cell extracts. Restric tion analysis of the repair incorporation in the vicinity of the lesion ind icated that the majority of TG was repaired through the base excision repai r (BER) pathways. Repair incorporation 5' to the lesion, characteristic for the nucleotide excision repair pathway, was not found. More than 80% of th e TG repair was accomplished by the single-nucleotide repair mechanism, and the remaining TGs were removed by the long patch BER pathway. We also anal yzed the role of the xeroderma pigmentosum, complementation group G (XPG) p rotein in the excision step of BER. Cell extracts deficient in XPG protein had an average 25% reduction in TG incision. These data show that BER is th e primary pathway for repair of TG in DNA and that XPG protein may be invol ved in repair of TG as an accessory factor.