Dnar-07. Bet Protein Inhibition Sensitizes Glioblastoma Cells to Temozolomide Treatment by Attenuating MGMT Expression

Abstract

Bromodomain and extra-terminal tail (BET) proteins have been identified as potential epigenetic targets in cancer, including glioblastoma. These epigenetic modifiers link the histone code to gene transcription that can be disrupted with small molecule BET inhibitors (BETi). With the aim of identifying rational combination treatments for glioblastoma, we analyzed BETi-induced differential gene expression in glioblastoma derived-spheres. This identified 6 distinct expression patterns. To uncover emerging actionable vulnerabilities that can be targeted with a second drug, we extracted the significantly disturbed DNA Damage Response genes. Among the constantly downregulated candidates was the O-6-methylguanine-DNA methyltransferase (MGMT) gene, a known resistance factor for alkylating agent therapy in glioblastoma. BETi not only reduced MGMT expression in GBM cell lines, but also inhibited its induction, typically observed upon temozolomide treatment. To determine the potential clinical relevance, we evaluated the specificity of the effect on MGMT expression and MGMT mediated treatment resistance to temozolomide. BETi-mediated attenuation of MGMT expression was associated with reduction of BRD4- and Pol II-binding at the MGMT promoter. On the functional level, we demonstrated that ectopic expression of MGMT under an unrelated promoter was not affected by BETi, while under the same conditions, pharmacologic inhibition of MGMT restored the sensitivity to temozolomide, reflected in an increased level of gammaH2AX. Importantly, members of the mismatch repair pathway, required for sensitivity to unrepaired O6G-lesions, namely MSH6 and MSH2, were only transiently affected by BETi. Taken together, the addition of BET-inhibitors to the current standard of care, comprising temozolomide treatment, may improve the outcome of the 50% of patients whose GBM exert an unmethylated MGMT promoter.