DNA double-strand break repair pathways and cellular tolerance to inhibitors of topoisomerase II.

The Chinese hamster ovary cell line xrs-1 is hypersensitive to gamma-radiation. This sensitivity has been attributed to an inability of this cell line to efficiently repair gamma-ray induced double-strand breaks (DSBs). We have recently reported that xrs-1 is also sensitive to topoisomerase II inhibitors that stabilize the cleavable complex. In this study, we have investigated the basis of this sensitivity by monitoring cleavable complex formation and loss in xrs-1 and its parent CHO-KI following treatment with the topoisomerase II inhibitors etoposide and 4'-(9-acridinylamino)methanesulfon-m-anisidide. Our studies indicate that xrs and CHO-K1 cells accumulate drug-induced cleavable complexes at equal rates and to an equal extent. However, studies on the loss of cleavable complexes after drug removal suggest that protein-free DSBs arise in cells treated with topoisomerase II inhibitors. Furthermore, a larger number of these DSBs persist in repair-deficient xrs cells than in repair-proficient Chinese hamster ovary-KI cells. The persistence of DSBs appears to account for the cytotoxic effects of topoisomerase II inhibitors that stabilize the cleavable complex. These results suggest that the xrs repair pathway is required for efficient removal of potentially cytotoxic DSBs that arise in cells treated with topoisomerase II inhibitors.