EPCO-41. THE NUCLEOSOME REMODELING AND DEACETYLASE COMPLEX UNDERLIES SELECTIVE DEPENDENCY IN H3 G34-MUTANT DIFFUSE HEMISPHERIC GLIOMA

Pediatric-type diffuse high-grade gliomas are fatal central nervous system malignancies of childhood. Recurrent mutations in histone H3.3 at amino acid 34 (glycine to arginine/valine) are a defining molecular feature of diffuse hemispheric glioma, H3 G34-mutant (H3 G34-mutant DHG), which arises preferentially in adolescents and young adults. Single-nuclei sequencing of age and anatomically matched H3 wild-type and H3 G34-mutant primary patient tumors identified aberrant neuronal-like transcriptional programs enriched for signatures of cortical interneurons derived from the medial ganglionic eminence. The extent to which these molecular signatures reflect cancer dependencies in H3 G34-mutant DHG is poorly understood. To identify transcriptional vulnerabilities governing cellular state and underlying developmental lineage, we performed CRISPR screening targeting the DNA-binding domains of 1,427 proteins in H3 G34-mutant DHG (n=6) and H3 K27-altered diffuse midline glioma (DMG) (n=3). Through these efforts, we identified developmentally relevant transcription factor vulnerabilities corresponding to neuronal and oligodendrocyte precursor cell-like hierarchies comprising H3 G34-mutant DHG and H3 K27-altered DMG cellular states, respectively. Intriguingly, we also identified multiple members of the nucleosome remodeling and deacetylase (NuRD) complex as selective dependencies in H3 G34-DHG compared to H3 K27-altered DMG and 32 other cancer cell line models screened using the same DNA-binding domain library. Genetic perturbation of the holo-NuRD structural subunit MBD3 conferred an upregulation of genes involved in neuronal differentiation and synapse formation consistent with NuRD-mediated modulation of the aberrant neuronal-like cell hierarchy observed in H3 G34-mutant DHG. To therapeutically target NuRD-associated class I HDAC activity, patient-derived H3 G34-mutant DHG cell lines were treated with quisinostat, a highly potent class I and II HDAC inhibitor. Pharmacologic inhibition of HDAC activity was sufficient to impair cell growth in vitro. Ongoing experiments are evaluating dependencies in vivo. Our results may suggest a differential requirement for chromatin repressive complex dysregulation in the pathogenesis of oncohistone-mutant pediatric-type diffuse high-grade gliomas.