Studies on the role of topoisomerases in general, gene- and strand-specific DNA repair.

Using specific inhibitors we have assessed the role of topoisomerases I and II in DNA repair of the overall genome and in both strands of an essential gene, the dihydrofolate reductase (DHFR) gene in chinese hamster ovary (CHO) cells. In these studies we have: (1) used inhibitors of topoisomerases during the repair incubation and (2) studied the DNA repair in cells with altered levels of topoisomerase activity. When cells were allowed to repair after UV irradiation, the gene-specific DNA repair was not affected by either topoisomerase I or topoisomerase II inhibitors alone. However, when topoisomerase I and topoisomerase II inhibitors were added simultaneously the gene- and strand-specific DNA repair were markedly inhibited. In contrast, the overall genome DNA repair was only marginally affected. This suggests that topoisomerases are involved in gene-specific DNA repair and that one type may substitute for the other in the repair process. That concept is further supported by our findings using a mutant cell line with a decreased level of topoisomerase I: gene-specific DNA repair can be inhibited by a topoisomerase II inhibitor alone. By analyzing the steady-state expression of the DHFR gene we find that inhibition of repair in the DHFR gene is not ascribed to an obvious change in the messenger level. Furthermore, using agents other than UV, we observe that the inhibitors have no effect on gene-specific repair of DNA damage introduced by the chemotherapeutic agents cisplatin and nitrogen mustard.