Cells deficient in DNA polymerase beta are hypersensitive to alkylating agent-induced apoptosis and chromosomal breakage

DNA polymerase beta (beta-pol), which is involved in base excision repair, was investigated for its role in protection of cells against various genoto xic agents and cytostatic drugs using beta-pol knockout mouse fibroblasts. We show that cells lacking beta-pol are highly sensitive to induction of ap optosis and chromosomal breakage by methylating agents, such as N-methyl-N- nitro-N-nitrosoguanidine and methyl methanesulfonate and the crosslinking a ntineoplastic drugs mitomycin C and mafosfamide, The cross-sensitivity betw een the agents observed suggests that beta-pol is involved in repair not on ly of DNA methylation lesions but also of other kinds of DNA damage induced by various cytostatic drugs. Cells deficient in beta-pol were not hypersen sitive to cisplatin, melphalan, benzo(n)pyrene diol epoxide, chloroethylnit rosourea, or UV light. Because both established and primary beta-pol knocko ut fibroblasts displayed the hypersensitive phenotype, which, moreover, was complemented by transfection with a beta-pol expression vector, the alkyla ting agent hypersensitivity can clearly be attributed to the beta-pol defic iency. The results demonstrate that beta-pol-driven base excision repair is highly important for protection of cells against cell killing due to apopt osis and induced chromosomal breakage and suggest that incompletely repaire d DNA damage causes chromosomal changes and may act as a trigger of DNA dam age-induced apoptosis.