EPCO-13. EZHIP SELECTIVELY REPRESSES GENOME STABILITY PROGRAMS IN NEURAL STEM CELLS TO PROMOTE EPIGENETIC VULNERABILITY AND TRANSFORMATION

Abstract BACKGROUND Posterior fossa group A ependymomas (PFAs) and diffuse midline gliomas (DMGs) are lethal pediatric brain tumors characterized by global loss of H3K27me3. This epigenetic disruption is caused by either EZHIP overexpression or H3.1/H3.3K27M mutations, which are mutually exclusive. EZHIP is aberrantly expressed in nearly all PFAs and approximately 10% of DMGs, while K27M mutations are found in ~80% of DMGs. In normal development, EZHIP expression is restricted to primordial germ cells during meiosis. We used human neural stem cells (NSCs), which give rise to ependymal and oligodendrocyte precursor lineages, to model EZHIP’s developmental effects. These cell types reflect the likely cells of origin for PFAs and DMGs, making them a biologically relevant platform to interrogate early tumorigenic mechanisms. METHODS AND RESULTS EZHIP was expressed in NSCs, astrocytes, and DMG lines. In NSCs, EZHIP induced strong repression of genome stability and mitotic fidelity genes, confirmed by gene set enrichment analysis (NES –2.91, p = 1e–10), and activated stem-like developmental programs. This repression was not observed in differentiated cells or tumor lines, indicating an NSC-specific vulnerability. The repressed targets were highly enriched for E2F-regulated genes (NES –3.61, p = 1e–10). EZHIP-expressing NSCs showed reduced DNA repair capacity, mitotic abnormalities, and continued proliferation. EZHIP is known to impair double-strand break repair by binding PALB2 and blocking BRCA2 recruitment, and to disrupt TopoIIα localization via H3K27me3 depletion, compounding mitotic stress in NSCs. CONCLUSIONS EZHIP disrupts genome maintenance and promotes transformation selectively in neural progenitors during a defined developmental window. Published data show that H3.3K27M induces profound transcriptional reprogramming in week 13 human NSCs, but these effects are diminished or absent by week 19, suggesting a shared, time-limited window of epigenetic vulnerability in both H3K27M- and EZHIP-driven tumors. Understanding this context-dependent repression has important implications for identifying stage-specific therapeutic vulnerabilities in pediatric gliomas.