N-methylpurine DNA glycosylase and DNA polymerase β modulate BER inhibitor potentiation of glioma cells to temozolomide

Abstract

Temozolomide (TMZ) is the preferred chemotherapeutic agent in the treatment of glioma following surgical resection and/or radiation. Resistance to TMZ is attributed to efficient repair and/or tolerance of TMZ-induced DNA lesions. The majority of the TMZ-induced DNA base adducts are repaired by the base excision repair (BER) pathway and therefore modulation of this pathway can enhance drug sensitivity. N-methylpurine DNA glycosylase (MPG) initiates BER by removing TMZ-induced N3-methyladenine and N7-methylguanine base lesions, leaving abasic sites (AP sites) in DNA for further processing by BER. Using the human glioma cell lines LN428 and T98G, we 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 over-expression of the BER initiating enzyme MPG. We also show that methoxyamine-induced potentiation of TMZ in MPG expressing glioma cells is abrogated by elevated-expression of the rate-limiting BER enzyme DNA polymerase β (Polβ), suggesting that cells proficient for BER readily repair AP sites in the presence of MX. Further, depletion of Polβ increases PARP inhibitor-induced potentiation in the MPG over-expressing glioma cells, suggesting that expression of Polβ modulates the cytotoxic effect of combining increased repair initiation and BER inhibition. This study demonstrates that MPG overexpression, together with inhibition of BER, sensitizes glioma cells to the alkylating agent TMZ in a Polβ-dependent manner, suggesting that the expression level of both MPG and Polβ might be used to predict the effectiveness of MX and PARP-mediated potentiation of TMZ in cancer treatment.

Novel MSH6 Mutations in Treatment-Naïve Glioblastoma and Anaplastic Oligodendroglioma Contribute to Temozolomide Resistance Independently of MGMT Promoter Methylation

“…Unfortunately, regardless of MGMT methylation status, most patients eventually recur and these recurrences are often resistant to temozolomide. Recent evidence indicates that there are several potential mechanisms that contribute to temozolomide resistance in adult and pediatric GBM, including activation of the base excision DNA repair pathway (19,20) and phosphoinostide-3-kinase-mediated upregulation of the HOXA9/ HOXA10 genes (21). Findings from the TCGA and other groups implicate defects in the MMR pathway, frequently through mutation of MSH6, as a mediator of temozolomide resistance in alkylator-exposed, MGMT-methylated GBMs (5-7).…”

Alkylpurine–DNA–N-glycosylase confers resistance to temozolomide in xenograft models of glioblastoma multiforme and is associated with poor survival in patients

“…The production of abasic sites from overexpression of APNG was then exploited using methoxyamine, which binds AP sites caused by APNG activity and makes them recalcitrant to subsequent repair with downstream BER enzymes (39). This stalling of the BER pathway by APNG overexpression and targeting downstream BER genes leads to accumulating single-strand DNA breaks, DNA damage, and apoptosis of these cancer cell lines when subjected to chemotherapeutics including TMZ (40)(41)(42). It would be of great interest to determine whether some GBM patients with extremely high levels of APNG have impaired BER and might respond better to TMZ with methoxyamine.…”

C1q/TNF‐related peptide 8 (CTRP8) promotes temozolomide resistance in human glioblastoma

“…Glioma patients undergoing TMZ treatment with low MPG levels, possibly due to MPG promoter methylation, have a better outcome compared to those with high MPG expression (Agnihotri et al ., 2012). This points to the importance of a proper balance of BER factors and links increased MPG activity and AP site formation to enhanced TMZ resistance in GBM (Tang et al ., 2011). Recently, the drug salinomycin was shown to downregulate the expression of DNA repair factors MPG, MGMT, and Rad51 recombinase and induce endoplasmic reticulum (ER) stress.…”

“…DNA polymerase β adds the complementary base and the X‐ray repair cross‐complementing group 1 (XRCC1)/DNA ligase III complex performs the phosphodiester bond formation to complete BER (Krokan and Bjoras, 2013). Inhibition of BER in MPG overexpressing human glioma sensitizes these cells to TMZ in vitro and in vivo , but this cytotoxic effect is diminished at higher cellular levels of the rate‐limiting BER enzyme DNA polymerase β (Kim and Wilson, 2012; Tang et al ., 2011). …”