PDTM-39. CRACKING THE HISTONE CODE: REVEALING THERAPEUTIC EPIGENETIC TARGETS THROUGH HISTONE H3 TAIL ANALYSIS OF DIFFUSE INTRINSIC PONTINE GLIOMA

Abstract

Abstract INTRODUCTION Diffuse intrinsic pontine glioma (DIPG) is a highly morbid pediatric cancer. Up to 80% harbor a Histone H3K27M mutation, which alters wild type Histone H3 protein post-translational modifications (PTMs) and genomic enrichment patterns to impact chromatin structure and transcription regulation. We previously identified tumorigenic patterns of H3K27Ac/bromodomain co-enrichment in DIPG, and demonstrated pre-clinical efficacy of bromodomain inhibition (JQ1). Here, we employ a novel proteomics platform, developed at our institution, to further elucidate the impact of H3K7M mutation on glioma histone codes and response to bromodomain inhibition. METHODS Epiproteomic analysis was performed on pediatric glioma cell lines (H3K27 WT n=2, H3K27M n=2) to characterize 95 distinct Histone H3.3 and H3.1 N-terminal tail modification states. Cells were treated with JQ1 or DMSO, and collected at 0h, 24h, 48h. Histones were extracted from isolated nuclei, immunopurified, and analyzed by LC-MS/MS. Results were integrated with RNA-Seq and ChIP Seq results (H3K27M, H3K27Ac, H3K27me3, H3K4me1, H3K4me3) from the same DIPG cell lines. Pediatric glioma tissues (H3K27M WT n=3, H3K27M n= 9) were similarly analyzed to validate cell line results. RESULTS Cell PTM profiles cluster by H3 mutation status on unsupervised analysis. Relative H3 PTM abundance were compared across cell lines by tumor location, H3 mutation status, and in response to treatment. Significant differential genomic enrichment H3K27M and H3.3 WT proteins, H3K27Me3 and H3K27Ac were observed between mutant and wild type cell lines with epigenetic-targeted therapy, correlating with cell transcriptomes. CONCLUSIONS Histone H3 tail epiproteomic analysis reveals DIPG histone codes in situ, revealing the effects of bromodomain inhibition on the tumor epigenetic landscape and providing new insight to the mechanism of tumor formation and therapy response. Further investigation of the utility of these signatures as biomarkers for diagnosis and longitudinal monitoring of treatment response are therefore currently underway.