Modulating native GABA A receptors in medulloblastoma with positive allosteric benzodiazepine-derivatives induces cell death.

PURPOSE: Pediatric brain cancer medulloblastoma (MB) standard-of-care results in numerous comorbidities. MB is comprised of distinct molecular subgroups. Group 3 molecular subgroup patients have the highest relapse rates and after standard-of-care have a 20% survival. Group 3 tumors have high expression of GABRA5, which codes for the α5 subunit of the γ-aminobutyric acid type A receptor (GABAA R). We are advancing a therapeutic approach for group 3 based on GABAA R modulation using benzodiazepine-derivatives.

METHODS: We performed analysis of GABR and MYC expression in MB tumors and used molecular, cell biological, and whole-cell electrophysiology approaches to establish presence of a functional 'druggable' GABAA R in group 3 cells.

RESULTS: Analysis of expression of 763 MB tumors reveals that group 3 tumors share high subgroup-specific and correlative expression of GABR genes, which code for GABAA R subunits α5, β3 and γ2 and 3. There are ~ 1000 functional α5-GABAA Rs per group 3 patient-derived cell that mediate a basal chloride-anion efflux of 2 × 109  ions/s. Benzodiazepines, designed to prefer α5-GABAA R, impair group 3 cell viability by enhancing chloride-anion efflux with subtle changes in their structure having significant impact on potency. A potent, non-toxic benzodiazepine ('KRM-II-08') binds to the α5-GABAA R (0.8 µM EC50 ) enhancing a chloride-anion efflux that induces mitochondrial membrane depolarization and in response, TP53 upregulation and p53, constitutively phosphorylated at S392, cytoplasmic localization. This correlates with pro-apoptotic Bcl-2-associated death promoter protein localization.

CONCLUSION: GABRA5 expression can serve as a diagnostic biomarker for group 3 tumors, while α5-GABAA R is a therapeutic target for benzodiazepine binding, enhancing an ion imbalance that induces apoptosis.