DDDR-30. IDENTIFICATION OF DRIVERS OF RESISTANCE IN BRAFV600E PEDIATRIC HIGH-GRADE GLIOMA AND NOVEL THERAPEUTIC TARGETS

Abstract

Abstract The BRAFV600E mutation has recently been discovered as a driver mutation in a subset of pediatric high-grade gliomas (pHGG), with tumors driven by this mutation responding to targeted BRAF and MEK inhibitors. Unfortunately, resistance inevitability develops, resulting in disease progression. There is currently limited understanding of the mechanisms underlying drug resistance in BRAFV600E pHGG, and no effective counter-therapies exist. This study aimed to identify driver pathways of resistance in BRAFV600E pHGG and subsequently novel therapeutic targets. BRAF inhibitor-resistant, MEK inhibitor-resistant and BRAF+MEK inhibitor-resistant BRAFV600E pHGG cultures were derived through chronic exposure of a BRAFV600E patient-derived culture to vemurafenib (BRAF inhibitor), trametinib (MEK inhibitor) or a combination of both drugs, respectively. Cytotoxicity assays confirmed resistance. All resistant cell lines spontaneously changed from a spheroid phenotype to adherent growth, indicating changes in tumor characteristics. RNAseq identified > 1500 genes of interest, with subsequent Gene Set Enrichment Analysis identifying key pathways as novel drivers of resistance, the top-ranked gene sets being neural development and plasma membrane/cell adhesion. Approximately 45% of gene sets were enriched across all drug-resistant lines, indicating overlapping resistance mechanisms. Interrogation of the receptor tyrosine kinase (RTK) gene sets identified potent upregulation of several RTKs. Notably, overexpression of the RTK, EGFR, was confirmed through RNAseq and western blot as a potential key mediator of the acquired resistance. Combined treatment with vemurafenib and the EGFR inhibitor, dacomitinib, resulted in synergistic activity against both vemurafenib-resistant BRAFV600E pHGG cells and matched parental cells, providing a viable therapeutic option for drug-resistant BRAFV600E pHGG. Overall, novel drivers of resistance have been identified in BRAFV600E pHGG. EGFR overexpression was pinpointed as a key mediator and dual BRAF/EGFR inhibition displayed promising efficacy as a therapeutic option. Future studies will continue to elucidate the mechanisms of resistance using multi-omic approaches and the in vivo efficacy of dual BRAF/EGFR inhibition.