Defects in interstrand cross-link uncoupling do not account for the extreme sensitivity of ERCC1 and XPF cells to cisplatin

The anticancer drug cisplatin reacts with DNA leading to the formation of interstrand and intrastrand cross-links that are the critical cytotoxic lesions. In contrast to cells bearing mutations in other components of the nucleotide excision repair apparatus (XPB, XPD, XPG and CSB), cells defective for the ERCC1-XPF structure-specific nuclease are highly sensitive to cisplatin. To determine if the extreme sensitivity of XPF and ERCC1 cells to cisplatin results from specific defects in the repair of either intrastrand or interstrand cross-links we measured the elimination of both lesions in a range of nucleotide excision repair Chinese hamster mutant cell lines, including XPF- and ERCC1-defective cells. Compared to the parental, repair-proficient cell line all the mutants tested were defective in the elimination of both classes of adduct despite their very different levels of increased sensitivity. Consequently, there is no clear relationship between initial incisions at interstrand cross-links or removal of intrastrand adducts and cellular sensitivity. These results demonstrate that the high cisplatin sensitivity of ERCC1 and XPF cells likely results from a defect other than in excision repair. In contrast to other conventional DNA cross-linking agents, we found that the repair of cisplatin adducts does not involve the formation of DNA double-strand breaks. Surprisingly, XRCC2 and XRCC3 cells are defective in the uncoupling step of cisplatin interstrand cross-link repair, suggesting that homologous recombination might be initiated prior to excision of this type of cross-link.