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 developing rational combination treatments for glioblastoma, we analyzed BETi-induced differential gene expression in glioblastoma derived-spheres, and identified 6 distinct response patterns. To uncover emerging actionable vulnerabilities that can be targeted with a second drug, we extracted the 169 significantly disturbed DNA Damage Response genes and inspected their response pattern. The most prominent candidate with consistent downregulation, 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 cells, 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 γ-H2AX, a proxy for DNA double-strand breaks. Importantly, expression of MSH6 and MSH2, which are required for sensitivity to unrepaired O6-methylguanine-lesions, was only briefly affected by BETi. Taken together, the addition of BET-inhibitors to the current standard of care, comprising temozolomide treatment, may sensitize the 50% of patients whose glioblastoma exert an unmethylated MGMT promoter.

Bromodomain and Extraterminal Domain (BET) Protein Inhibition Hinders Glioblastoma Progression by Inducing Autophagy-Dependent Differentiation

“…Subsequently, they have attracted increasing interest as valuable candidates for the clinical treatment of Myc-driven cancer, including GBM [ 17 , 18 , 19 ]. Recent studies have demonstrated that pharmacological BET inhibition is effective in counteracting GBM growth in both in vitro and in vivo models, similar to what has been observed in other tumor models [ 20 , 21 , 22 , 23 ]. However, the molecular mechanisms of BET proteins in GBM tumorigenesis are scarcely understood, and the potential of BET inhibitors in treating GBM is largely unexplored.…”

The pharmacoepigenomic landscape of cancer cell lines reveals the epigenetic component of drug sensitivity

“…1f ). The epigenetic silencing of MGMT is frequently debated as a clinical biomarker 30 and previous work revealed that JQ1 disturbs DNA damage responses by attenuating MGMT expression in glioblastoma cells 31 . While the different treatment responses are often attributed to somatic mutations in cancer genes, this suggests that DNA methylation can function as a complementary mechanism.…”

Temozolomide, Procarbazine and Nitrosoureas in the Therapy of Malignant Gliomas: Update of Mechanisms, Drug Resistance and Therapeutic Implications

“…The finding is of particular interest since corticosteroids like dexamethasone are frequently used during O6AA therapy, and clinical studies show a negative correlation between use and therapy success [78]. In GBM spheroids, some induction of MGMT was evident, albeit at higher dose levels than used in the studies cited above [79]. The epigenetic regulation of MGMT has been extensively reviewed before [18].…”

“…An alternative strategy of tumor sensitization through MGMT inhibition is based on targeting bromodomain and extra-terminal tail (BET) proteins, which have been identified as potential epigenetic targets in cancer, including glioblastoma. The inhibition of these epigenetic modifiers was shown to reduce MGMT without affecting MMR protein expression [79]. Actually, an inhibitor of BET (Trotabresib) was shown to have antitumor activity in patients with high-grade gliomas and was well tolerated in the TMZ+RT concomitant and TMZ adjuvant settings [86].…”