PARP-mediated PARylation of MGMT is critical to promote repair of temozolomide-induced O6-methylguanine DNA damage in glioblastoma

Background. Temozolomide (TMZ) resistance in glioblastoma multiforme (GBM) is mediated by the DNA repair protein O-6-methylguanine DNA methyltransferase (MGMT). MGMT promoter methylation (occurs in about 40% of patients) is associated with loss of MGMT expression (MGMT-) that compromises DNA repair, leading to a favorable response to TMZ therapy. The 60% of patients with unmethylated MGMT (MGMT+) GBM experience resistance to TMZ; in these patients, understanding the mechanism of MGMT-mediated repair and modulating MGMT activity may lead to enhanced TMZ activity. Here, we report a novel mode of regulation of MGMT protein activity by poly(ADP-ribose) polymerase (PARP). Methods. MGMT-PARP interaction was detected by co-immunoprecipitation. PARylation of MGMT and PARP was detected by co-immunoprecipitation with anti-PAR antibody. O-6-methylguanine (O-6-MetG) adducts were quantified by immunofluorescence assay. In vivo studies were conducted in mice to determine the effectiveness of PARP inhibition in sensitizing GBM to TMZ. Results. We demonstrated that PARP physically binds with MGMT and PARylates MGMT in response to TMZ treatment. In addition, PARylation of MGMT by PARP is required for MGMT binding to chromatin to enhance the removal of O-6-MetG adducts from DNA after TMZ treatment. PARP inhibitors reduced PARP-MGMT binding and MGMT PARylation, silencing MGMT activity to repair O-6-MetG. PARP inhibition restored TMZ sensitivity in vivo in MGMT-expressing GBM. Conclusion. This study demonstrated that PARylation of MGMT by PARP is critical for repairing TMZ-induced O-6-MetG, and inhibition of PARylation by PARP inhibitor reduces MGMT function rendering sensitization to TMZ, providing a rationale for combining PARP inhibitors to sensitize TMZ in MGMT-unmethylated GBM.