Novel role of base excision repair (BER) in mediating cisplatin cytotoxicity.

Using isogenic mouse embryonic fibroblasts and human cancer cell lines, we show that cells defective in BER display a cisplatin specific resistant phenotype. This was accompanied by enhanced repair of cisplatin interstrand crosslinks (ICLs) and ICL-induced DNA double strand breaks (DSBs), but not intrastrand adducts. Cisplatin induces abasic sites with a reduced accumulation in uracil DNA glycosylase (UNG) null cells. We show that cytosines that flank the cisplatin ICLs undergo preferential oxidative deamination in vitro, and AP endonuclease 1 (APE1) can cleave the resulting ICL DNA substrate following removal of the flanking uracil. We also show that DNA polymerase beta has low fidelity at the cisplatin ICL site after APE1 incision. Down-regulating ERCC1-XPF in BER deficient cells restored cisplatin sensitivity. Based on our results, we propose a novel model in which BER plays a positive role in maintaining cisplatin cytotoxicity by competing with the productive cisplatin ICL DNA repair pathways.