The lyase activity of the DNA repair protein β-polymerase protects from DNA-damage-induced cytotoxicity

Small DNA lesions such as oxidized or alkylated bases are repaired by the base excision repair (BER) pathway(1). BER includes removal of the damaged base by a lesion-specific DNA glycosylase, strand scission by apurinic/apyrimidinic endonuclease, DNA resynthesis and ligation(2). BER may be further subdivided into DNA beta-polymerase (beta-pol)-dependent single-nucleotide repair and beta-pol-dependent or -independent long patch repair subpathways(3-6). Two important enzymatic steps in mammalian single-nucleotide BER are contributed by beta-pol: DNA resynthesis of the repair patch and lyase removal of 5'-deoxyribose phosphate (dRP)(2). Fibroblasts from beta-pol null mice are hypersensitive to monofunctional DNA-methylating agents, resulting in increases in chromosomal damage, apoptosis and necrotic cell death(3,7). Here we show that only the dRP lyase activity of beta-pol is required to reverse methylating agent hypersensitivity in beta-pol null cells. These results indicate that removal of the dRP group is a pivotal step in BER in vivo. Persistence of the dRP moiety in DNA results in the hypersensitivity phenotype of beta-pol null cells and may signal downstream events such as apoptosis and necrotic cell death.