H3K27M co-opts ASCL1 to maintain progenitor states and drive gliomagenesis

Phenotypic plasticity, a hallmark of cancer facilitated by epigenomic reprogramming, represents a significant challenge but also therapeutic opportunity for cancer treatment. How plasticity is regulated in diffuse midline gliomas (DMGs), incurable pediatric brain tumors characterized by oncogenic H3K27M histone mutations, is not understood. Here we demonstrate that H3K27M-driven plasticity requires the lineage-specific, pioneer transcription factor (TF) ASCL1. Prolonged differentiation reveals clinically relevant plasticity in DMGs, which segregate into ASCL1-high, neural progenitor cell (NPC)-like gliomaspheres and ASCL1-low, mesenchymal (MES)-like adherent subpopulations. Direct transcriptional activation by ASCL1 enables super enhancers to sustain NPC programs de-repressed by H3K27M. H3K27M loss induces an irreversible MES-like state, suppresses plasticity by abolishing the ASCL1-high state, and is largely phenocopied by loss of ASCL1. Knocking out ASCL1 in DMG lines robustly inhibited tumor growth and significantly extended survival of recipient mice. Our results suggest targeting epigenetic plasticity to suppress master TF-driven progenitor states as a general therapeutic opportunity in cancer.