Novel Insights into the Role of O6-Methylguanine-DNA Methyltransferase in Glioblastoma Angiogenesis, Invasion, and Proliferation

Abstract

Glioblastoma multiforme (GBM) is characterized by aberrant angiogenesis, rapid proliferation, and widespread invasion through the brain parenchyma. Thus, the prognosis for these tumors remains extremely poor despite aggressive multimodal treatment including chemotherapy with the alkylating agent temozolomide (TMZ). The expression of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) further contributes to treatment resistance by repairing tumoral alkylation damage induced by TMZ, highlighting the requirement for alternative therapeutic options. Though MGMT promoter methylation is a prognostic indicator of response to TMZ, it has also been correlated with increased rate of mutation in GBM and phenotypic alterations in other tumors. Therefore, we aimed to identify whether MGMT may play a greater role in GBM pathology and therapeutic response beyond mediation of alkylating chemotherapy resistance. We determined through gene expression profiling that MGMT expression in GBM cells induced genetic alterations in several functional pathways. Of particular interest in GBM pathology, MGMT overexpression elicited a switch of the angiogenic balance towards an anti-angiogenic profile, as determined by altered expression of vascular endothelial growth factor and its receptors. Furthermore, we established an inverse relationship between MGMT expression and invasion using MGMT overexpression and knockdown models and panels of established and patient-derived primary GBM cell lines. Importantly, our in vitro data were validated in primary GBM tumor biopsy samples showing that MGMT promoter methylation (i.e. MGMT gene silencing) was significantly associated with increased immunohistochemical expression of SPARC, a well-known marker of GBM invasion. Concurrently, in isogenic models we found that MGMT expression positively correlated with increased proliferation, consistent with the well-established theory that invasion and proliferation are mutually exclusive in GBM. Of great importance, the clinical relevance of our investigations was further demonstrated by our striking finding that decreased MGMT expression was associated with increased invasiveness following treatment with angiogenic inhibitors. Our study therefore provides new insight into the aggressiveness of GBM by highlighting potentially novel roles for the DNA repair protein MGMT. Moreover, our data suggest the potential utilization of MGMT as a biomarker for response to angiogenic inhibitors, which constitutes an important step in developing more effective therapies for patients suffering from this devastating disease.