Repair of 3-methyladenine and abasic sites by base excision repair mediates glioblastoma resistance to temozolomide

Abstract

Alkylating agents have long played a central role in the adjuvant therapy of glioblastoma (GBM). More recently, inclusion of temozolomide (TMZ), an orally administered methylating agent with low systemic toxicity, during and after radiotherapy has markedly improved survival. Extensive in vitro and in vivo evidence has shown that TMZ-induced O6-methylguanine (O6-meG) mediates GBM cell killing. Moreover, low or absent expression of O6-methylguanine-DNA methyltransferase (MGMT), the sole human repair protein that removes O6-meG from DNA, is frequently associated with longer survival in GBMs treated with TMZ, promoting interest in developing inhibitors of MGMT to counter resistance. However, the clinical efficacy of TMZ is unlikely to be due solely to O6-meG, as the agent produces approximately a dozen additional DNA adducts, including cytotoxic N3-methyladenine (3-meA) and abasic sites. Repair of 3-meA and abasic sites, both of which are produced in greater abundance than O6-meG, is mediated by the base excision repair (BER) pathway, and occurs independently of removal of O6-meG. These observations indicate that BER activities are also potential targets for strategies to potentiate TMZ cytotoxicity. Here we review the evidence that 3-meA and abasic sites mediate killing of GBM cells. We also present in vitro and in vivo evidence that alkyladenine-DNA glycosylase, the sole repair activity that excises 3-meA from DNA, and Ape1, the major human abasic site endonuclease, mediate TMZ resistance in GBMs and represent potential anti-resistance targets.

Reduction of MLH1 and PMS2 confers temozolomide resistance and is associated with recurrence of glioblastoma

“…Previous studies have suggested that MGMT and base excision repair (BER) are involved in TMZ resistance [15, 16]. MGMT also repairs the O 6 -chloroethylguanine residues induced by SN2-type methylating agents such as ACNU [17, 18].…”

Interaction of Functionalized Naphthalenophanes with Abasic Sites in DNA: DNA Cleavage, DNA Cleavage Inhibition, and Formation of Ligand–DNA Adducts

“…In the context of chemotherapy employing DNA‐alkylating drugs (e.g., methyl methanesulfonate (MMS) or temozolomide), the major DNA alkylation product, N 7 ‐methylguanine, undergoes spontaneous depurination, leading to formation of AP sites, as well as a reaction with histone proteins, leading to the generation of DNA–protein cross‐links . Whereas the latter are thought to be cytotoxic, AP sites are instantly processed by the base‐excision DNA repair (BER) pathway, which eliminates drug‐induced DNA damage and leads to chemoresistance . It has been demonstrated that inhibition of BER increases the sensitivity of cancer cells towards DNA‐alkylating drugs in certain cancers, leading to a therapeutically useful synergic effect .…”

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

“…Major DNA adducts generated by TMZ are N 7 ‐methylguanine (N 7 ‐MeG; 60‐80%), N 3 ‐methyladenine (N 3 ‐MeA; 10–20%), and O 6 ‐methylguanine ( O 6 ‐MeG; 5–10%) (Bobola et al ., 2012). Excision of a modified base generates an apurinic/apyrimidinic (AP) DNA site, and consecutive AP endonuclease 1 (APE1) activity creates a single‐stranded DNA site which has the propensity to develop into a double‐strand break if not processed expediently by BER (Helena et al ., 2018).…”

“…We therefore focused on N 7 ‐MeG and N 3 ‐MeA adducts which constitute over 90% of TMZ‐induced base alterations. These are recognized by the BER glycosylase MPG which performs the initial cleavage of the glycosylic bond between the damaged base and deoxyribose to generate an AP site (Bobola et al ., 2012; Kim and Wilson, 2012). Treatment of human GBM cells with RXFP1 agonist CTRP8 increased MPG protein levels and MPG activity as determined by MPG molecular beacon assay, while the cellular levels of other BER proteins, including APE1, DNA polymerase β (polβ), and XRCC1, remained unaffected by CTRP8‐mediated RXFP1 activation.…”