DNA polymerase beta mediates protection of mammalian cells against ganciclovir-induced cytotoxicity and DNA breakage

The efficacy of suicide herpes simplex virus-1 thymidine kinase (HSVtk)/gan ciclovir (GCV) gene therapy is often limited by intrinsic resistance of tum or cells. Here we show that repair of GCV incorporated in DNA is a factor i nvolved in GCV resistance. A protective role of DNA repair in GCV-induced c ell killing is supported by the following findings: (a) GCV-exposed Chinese hamster ovary-HSVtk cells exhibited both reduced repair of GCV and cloning efficiency in the presence of a specific polymerase beta (beta -pol) inhib itor, prunasin; (b) DNA beta -pol-deficient mouse fibroblasts were more sen sitive to the cytotoxic, apoptosis-inducing, and genotoxic (DNA breakage an d chromosomal aberration-inducing) effects of GCV as compared with wild-typ e and beta -pol-complemented cell lines; (c) methoxyamine, an inhibitor of beta -pol-dependent short-patch base excision repair, sensitized wild-type and complemented P-pol cells to GCV, whereas it had no effect on the sensit ivity of beta -pol-null cells to GCV. Because methoxyamine-mediated sensiti zation of beta -pol wild-type and beta -pol-complemented cells to GCV did n ot reach the level of null cells, we suggest that both beta -pol-dependent short- and long-patch base excision repair are involved in protection of ce lls to GCV. Some implications for HSVtk/GCV gene therapy are being discusse d.