The incidence and mortality of brain tumors have not changed over the last 3 decades and pose a significant burden on the healthcare system of the United States. Temozolomide (TMZ) is the preferred chemotherapeutic agent for the treatment of brain tumors and base excision repair (BER) is a major mechanism for the repair of TMZ-induced DNA base lesions. BER inhibition, either by interrupting the delicate balance of the expression of key BER proteins or by chemical inhibitors, enhances cytotoxicity of chemotherapeutic DNA damaging agents such as TMZ. Understanding the mechanisms of enhanced cytotoxicity brought on by BER inhibition has great public health significance. By using DNA polymerase ß (Polß) deficiency as a model of BER inhibition, I report that DNA damage-induced cytotoxicity due to Polß deficiency triggers cell death dependent on PARP activation yet independent of poly(ADP-ribose) (PAR) or PAR-catabolite signaling. Cell death is rescued by the NAD+ metabolite NMN and is synergistic with inhibition of NAD+ biosynthesis demonstrating that DNA damage-induced cytotoxicity mediated via BER inhibition is primarily dependent on cellular metabolite bioavailability. I offer a mechanistic justification for the elevated alkylation-induced cytotoxicity of Polß deficient cells, suggesting a linkage between DNA repair, cell survival and cellular bioenergetics. Resistance to TMZ is partially attributed to efficient repair of TMZ-induced DNA lesions. Using the human glioma cell lines LN428 and T98G, I report here that potentiation of TMZ via BER inhibition (methoxyamine (MX), the PARP inhibitors PJ34 and ABT-888 or depletion (knockdown) of PARG) is greatly enhanced by increasing BER initiation via over-expression of MPG. I also show that MX-induced potentiation of TMZ in MPG expressing glioma cells is abrogated by elevated-expression of the rate-limiting BER enzyme Polß, suggesting that cells proficient for BER readily repair AP sites even in the presence of MX. This study demonstrates that increased initiation of BER via MPG over-expression, together with inhibition of repair following initiation, further sensitizes glioma cells to alkylating agent TMZ, suggesting that the expression level of MPG might be used to predict the effectiveness of BER inhibition-induced potentiation of TMZ in glioma cells
