Overcoming Temozolomide Resistance in Glioblastoma via Enhanced NAD(+) Bioavailability and Inhibition of Poly-ADP-Ribose Glycohydrolase

SIMPLE SUMMARY: Glioblastoma is the most prevalent and lethal brain tumor type, often treated with the DNA alkylating agent temozolomide (TMZ). The cytotoxic DNA lesion O(6)-methylguanine only accounts for about 9% of the DNA lesions induced by TMZ. The other DNA lesions (>80%) are quickly repaired by the base excision repair (BER) pathway. However, resistance to cytotoxicity of the O(6)-methylguanine lesion is common in cancer due to defects in the mismatch repair pathway or overexpression of the MGMT repair protein. Therefore, the aim of this study was to find approaches to inhibit the BER pathway to overcome TMZ resistance. We found that combining TMZ with an NAD(+) precursor (dihydronicotinamide riboside) and a PARG inhibitor strongly inhibited BER and overcame TMZ resistance. This combination treatment regimen provides a novel approach to consider for glioblastoma. ABSTRACT: Glioblastoma multiforme (GBM) is an incurable brain cancer with an average survival of approximately 15 months. Temozolomide (TMZ) is a DNA alkylating agent for the treatment of GBM. However, at least 50% of the patients treated with TMZ show poor response, primarily due to elevated expression of the repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) or due to defects in the mismatch repair (MMR) pathway. These resistance mechanisms are either somatic or arise in response to treatment, highlighting the need to uncover treatments to overcome resistance. We found that administration of the NAD(+) precursor dihydronicotinamide riboside (NRH) to raise cellular NAD(+) levels combined with PARG inhibition (PARGi) triggers hyperaccumulation of poly(ADP-ribose) (PAR), resulting from both DNA damage-induced and replication-stress-induced PARP1 activation. Here, we show that the NRH/PARGi combination enhances the cytotoxicity of TMZ. Specifically, NRH rapidly increases NAD(+) levels in both TMZ-sensitive and TMZ-resistant GBM-derived cells and enhances the accumulation of PAR following TMZ treatment. Furthermore, NRH promotes hyperaccumulation of PAR in the presence of TMZ and PARGi. This combination strongly suppresses the cell growth of GBM cells depleted of MSH6 or cells expressing MGMT, suggesting that this regimen may improve the efficacy of TMZ to overcome treatment resistance in GBM.