EXTH-06. GLIOBLASTOMA PATIENT SPECIFIC TRANSCRIPTOME REMODELING IN RESPONSE TO MDM2 ANTAGONIST TREATMENT OF WILD-TYPE P53 CANCER STEM CELLS IS ASSOCIATED WITH SENSITIVITY AND STRATEGIES FOR COMBINATION THERAPY

Abstract

Abstract Over 70% of glioblastomas are p53 wild-type and these patients could benefit from p53 reactivation treatment. Here we determined the sensitivity of a glioblastoma patient-derived cancer stem cells (CSCs) panel to three MDM2 antagonists (MDM2a) to test to what extent transcriptional reprogramming is affected by genomic background and correlates to response to acute treatment with MDM2 antagonists. IC50 concentrations and area above the curve (AAC) were measured from dose response curves obtained from 4 and 7-day treatment. The inhibitors were specific to wt-p53 CSCs, but these presented a wide range of sensitivity. Seven wt-p53 CSCs were treated RG7112 IC50 concentrations or DMSO control for 24h (n=4). RNA was isolated for Illumina Truseq stranded mRNA libraries sequenced at 30M depth. Quantified raw counts were processed using NOISeq R package to determine differentially expressed genes (DEG) between control and treated samples. In addition to the expected high representation of upregulated p53 targets in all cell lines, we observed significan cell specific transcriptome alterations, including enrichment of survival pathways, such as mTOR, ERK and NFkB in the less sensitive CSC lines, which did not readily correlate with the individual genomic landscapes. E2F targets, G2-M cell cycle check point and DNA-repair pathways were highly enriched in the downregulated genes. Combination of MDM2a with radiation treatment (RT) was synergistic to a subset of CSCs. As validation we found that MDM2a treatment was effective in sensitizing a resistant orthotopic mouse glioblastoma PDX to fractionated RT, with maximum efficacy when treatment started 24h prior to RT vs simultaneously (Log-rank Mantel-Cox test: p=0.0006 vs 0.0277). The integration of the treatment-mediated transcriptional patterns with differential sensibility to MDM2 antagonists and genomic landscape of the CSC panel provides a platform to identify targets for combination therapies, which is the most promising clinical application of the reactivation of wt-p53 in GBMs.