REPAIR OF (2-CHLOROETHYL)-3-CYCLOHEXYL-1-NITROSOUREA-INDUCED DAMAGE BY MAMMALIAN-CELL EXTRACTS

The repair of damage induced by the alkylating antitumor drug 1-(2-chl oroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) was investigated using an in vitro excision repair system. Hamster cell extracts prepared from t he parental CHO-9 cell line and the ERCC1 mutant 43-3B were both profi cient in the repair of CCNU-induced damage. The in vitro repair of CCN U damage was faster than the repair of UV damage and plasmid substrate s were rapidly and efficiently incised after incubation with either CH O-9 or 43-3B extracts. 7-Methylguanine (7-meG) and 3-methyladenine (3- meA) glycosylases were active to a similar extent in the CHO-9 and 43- 3B extracts. The data indicate that most damage induced by CCNU is rep aired via the ERCC1-independent base excision repair pathway, initiati ng with removal of chloroethylated and hydroxyethylated bases by N-gly cosylases. Yet, the sensitive phenotype of 43-3B cells suggests that t he ERCC1 gene product is required for the removal of a small subset of CCNU-induced lesions that are important for drug cytotoxicity.