Combined epigenetic therapy with FACT and BET inhibitors remodels chromatin and disrupts oncogenic transcription in Diffuse Midline Glioma [ATAC-seq]

Aberrant epigenetic regulation is a hallmark of Diffuse Midline Glioma (DMG), an incurable pediatric brain tumor. The H3K27M driver histone mutation leads to transcriptional dysregulation, indicating that targeting the epigenome and transcription may be a key therapeutic strategy against this highly aggressive cancer. One such target is the Facilitates Chromatin Transcription (FACT) histone chaperone. We found FACT to be enriched at developmental gene promoters, coinciding with regions of open chromatin and binding motifs of core DMG regulatory transcription factors. Furthermore, FACT interacted and co-localized with the Bromodomain and Extra-Terminal Domain (BET) protein BRD4 at promoters and enhancers, suggesting functional cooperation between FACT and BRD4 in DMG. In vitro, a combinatorial therapeutic approach using the FACT inhibitor CBL0137, coupled with BET inhibition revealed potent and synergistic cytotoxicity across a range of DMG cultures, with H3K27M-mutant cells demonstrating heightened sensitivity. These results were recapitulated in vivo, significantly extending survival in three independent orthotopic PDX models of DMG. Mechanistically, we show that CBL0137 treatment decreased chromatin accessibility, synergizing with BET inhibition to disrupt transcription, silencing several key oncogenes including MYC, PDGFRA and MDM4, alongside alterations to the splicing landscape. Combined, these data highlight the therapeutic promise of simultaneously targeting FACT and BRD4 in DMG, proposing a novel strategy for combating this devastating pediatric brain tumor.