Human methyl purine DNA glycosylase and DNA polymerase ß expression collectively predict sensitivity to temozolomide

Over-expression of N-methylpurine DNA glycosylase (MPG) has been suggested as a possible gene therapy approach to sensitize tumor cells to the cell killing effects of temozolomide, an imidazotetrazine-class chemotherapeutic alkylating agent. In the present study, we show that both elevated MPG expression and shRNA-mediated loss of Pol ß expression in human breast cancer cells increases cellular sensitivity to temozolomide. Resistance to temozolomide is restored by complementation of either wild-type human Pol ß or human Pol ß with an inactivating mutation specific to the polymerase active site yet functional for 5′dRP lyase activity. These genetic and cellular studies uniquely demonstrate that over-expression of MPG causes an imbalance in BER leading to an accumulation of cytotoxic 5′dRP lesions and that the 5′dRP lyase activity of Pol ß is required to restore resistance to temozolomide. These results imply that Pol ß dependent 5′dRP lyase activity is the rate-limiting step in BER in these cells and suggests that BER is a tightly balanced pathway for the repair of alkylated bases such as N7-MeG and N3-MeA. Further, we find that 5′dRP-mediated cell death is independent of caspase-3 activation and does not induce the formation of autophagosomes, as measured by GFP-LC3 localization. The experiments presented herein suggest that it will be important to investigate whether an active BER pathway could be partially responsible for the temozolomide-mediated resistance seen in some tumors and that balanced BER protein expression and overall BER capacity may help predict sensitivity to temozolomide.