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DIPG-58. HISTONE H3 WILD-TYPE DIPG/DMG OVEREXPRESSING EZHIP EXTEND THE SPECTRUM OF DIFFUSE MIDLINE GLIOMAS WITH PRC2 INHIBITION BEYOND H3-K27M MUTATION

ABSTRACT: Abstract Diffuse midline gliomas (DMG) H3 K27M-mutant were introduced in the 2016 WHO Classification unifying diffuse intrinsic pontine gliomas (DIPG) and gliomas from the thalamus and spinal cord harboring a histone H3-K27M mutation leading to Polycomb Repressor Complex 2 (PRC2) inhibition. However, few cases of DMG tumors presenting a H3K27 trimethylation loss, but lacking an H3-K27M mutation were reported. To address this question, we combined a retrospective cohort of 10 patients biopsied for a DIPG at the Necker Hospital or included in the BIOMEDE trial (NCT02233049) and extended our analysis to H3-wildtype (WT) diffuse gliomas from other midline locations presenting either H3K27 trimethylation loss or ACVR1 mutation from Necker, ICR, the HERBY trial, the INFORM registry study and the St. Jude PCGP representing 9 additional cases. Genomic profiling identified alterations frequently found in DMG, but none could explain the observed loss of H3K27 trimethylation. Similar observations were previously made in the PF-A subgroup of ependymoma, where the H3K27me3 loss resulted from EZHIP/CXorf67 overexpression rather than H3-K27M mutations. We thus analyzed EZHIP expression and observed its overexpression in all but one H3-WT DMGs compared to H3-K27M mutated tumors (EZHIP negative). Strikingly, based on their DNA methylation profiles, all H3-WT DMG samples analyzed clustered close to H3-K27M DIPG, rather than EZHIP overexpressing PF-A ependymomas. To conclude, we described a new subgroup of DMG lacking H3-K27M mutation, defined by H3K27 trimethylation loss and EZHIP overexpression that can be detected by IHC. We propose that these EZHIP/H3-WT DMGs extend the spectrum of DMG with PRC2 inhibition beyond H3-K27M mutation.

SUBMITTER: Castel D

PROVIDER: S-EPMC7715950 | biostudies-literature | 2020 Dec

REPOSITORIES: biostudies-literature

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Project description:Abstract Effective treatments are urgently needed for the incurable paediatric brainstem tumour Diffuse Midline Glioma (DMG). Most DMGs contain Histone H3 mutations (H3K27M), which produce extensive epigenetic dysregulation by inhibiting EZH2-mediated trimethylation of H3K27 (H3K27me3). Global depletion of the repressive H3K27me3 modification results in aberrantly open chromatin and is central to DMG tumorigenesis. Thus, targeting the epigenome is a promising avenue of treatment for DMG. We found that targeting the histone chaperone complex Facilitates Chromatin Transcription (FACT) with the curaxin drug CBL0137 to have potent pre-clinical efficacy against DMG, leading to a paediatric Phase I/II clinical trial for CBL0137 which includes a DMG/DIPG cohort (NCT04870944). In this project we aim to elucidate CBL0137’s molecular mechanism in DMG. FACT is critical for maintaining chromatin homeostasis during DNA replication, transcription, and repair. We therefore hypothesised that FACT maintains the aberrant epigenetic landscape resulting from H3K27M. Consistently, we found CBL0137 to be more cytotoxic against H3K27M-mutant, compared to H3-WT or isogenic DMG cells lacking the mutation. Furthermore, FACT directly interacts with H3K27M, and FACT inhibition increases both EZH2 catalytic activity and global H3K27me3 levels. We are now using ChIP-seq to discover the genome-wide distribution of FACT and H3K27M, and will interrogate the consequence of CBL0137 treatment on the epigenome and transcriptome. Preliminary results suggest that FACT is located at certain genes co-occupied by H3K27M, the active histone mark H3K27ac, and the BET protein BRD4. This implies that FACT is involved in maintaining open chromatin and perhaps transcription at these regions. Combining CBL0137 with other epigenetic drugs, such as BET inhibitors, could therefore represent a rational therapeutic opportunity for DMG. This work will ultimately inform mechanism-based targeted therapies to combine with CBL0137 to improve its efficacy and uncover valuable new insights into DMG epigenetics and pathobiology.

Dataset Information

DIPG-58. HISTONE H3 WILD-TYPE DIPG/DMG OVEREXPRESSING EZHIP EXTEND THE SPECTRUM OF DIFFUSE MIDLINE GLIOMAS WITH PRC2 INHIBITION BEYOND H3-K27M MUTATION

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