H3F3A K27M mutations drive a repressive transcriptome by modulating chromatin accessibility independent of H3K27me3 in Diffuse Midline Glioma.

BACKGROUND: Heterozygous histone H3.3K27M mutation is a primary oncogenic driver of Diffuse Midline Glioma (DMG). H3.3K27M inhibits the Polycomb Repressive Complex 2 (PRC2) methyltransferase activity, leading to global reduction and redistribution of the repressive H3 lysine 27 tri-methylation (H3K27me3). This epigenomic rewiring is thought to promote gliomagenesis, but the precise role of K27M in gene regulation and tumorigenesis remains incompletely understood. RESULTS: We established isogenic DMG patient-derived cell lines using CRISPR-Cas9 editing to create H3.3 wild-type (WT), H3.3K27M, and combinations with EZH2 and EZH1 co-deletion, thereby eliminating PRC2 function and H3K27me3. RNA-seq and ATAC-seq analysis revealed that K27M exerts a novel epigenetic effect independent of PRC2 inhibition. While PRC2 loss led to widespread gene induction including HOX gene clusters, and activation of biological pathways, K27M induced a balanced gene deregulation with an overall repressive effect on pathway activity. Genes uniquely affected by K27M, independent of PRC2 loss, showed concordant changes in chromatin accessibility, with upregulated genes becoming more accessible. Importantly, xenografts of H3.3K27M/EZH1/2 WT cells formed tumors, whereas /EZH1/2 knockout cells did not, demonstrating a PRC2-independent role of K27M in tumorigenesis. CONCLUSION: Our findings reveal that the H3.3K27M mutation alters chromatin accessibility and uniquely deregulates gene expression independent of H3K27 methylation loss. These PRC2-independent functions of K27M contribute to changes in biological pathway activity and are necessary for tumor development, highlighting novel mechanisms of K27M-driven gliomagenesis.