DIPG-26. H3.3K27M ENHANCES SENSITIVITY TO ATR INHIBITION IN PEDIATRIC GLIOMA

Abstract

Abstract BACKGROUND Diffuse midline glioma (DMG) is an aggressive, uniformly fatal childhood brain tumor. Radiation remains the standard of care but provides limited survival benefit. The most prevalent driver mutation in DMG is lysine to methionine mutation at position 27 (K27M) of histone H3.3, resulting in global loss of H3K27 trimethylation and remodeled chromatin, thus promoting DMG tumorigenesis. We have previously shown that inhibition of de novo pyrimidine synthesis induces replication stress that enhances DMG dependency on ATR. In this study, we sought to explore the contribution of H3K27M to basal replication stress in DMGs and vulnerability to ATR inhibition. METHODS To investigate ATR sensitivity of DMG, we employed ART0380, a potent and selective ATR inhibitor currently in Phase 2 clinical trials in adults. Using isogenic models and a panel of patient tumor-derived DMG cell lines we perform viability assays, immunoblotting, microscopy and flow cytometry-based assays to determine various endpoints/measures of ATR inhibition in DMGs. RESULTS Our data show that H3K27M histone oncoproteins enhance dependency on ATR and ATR inhibition synergizes with radiation to induce DMG cell death. After ART0380 treatment, H3K27M oncoproteins result in greater induction in apoptosis and accumulation of DNA damage (as measured by g-H2AX) compared with their histone H3 wild-type isogenic counterparts. Inhibition of ATR by ART0380 is confirmed by effects on phosphorylation of bona fide ATR targets-CHK1 and RPA. To gain mechanistic insights into how H3K27M modulates dependency on ATR, we have evaluated accumulation of chromatin-bound RPA, R-loops, and frequency of transcription and replication collisions (TRC) in isogenic cells. CONCLUSIONS Our data indicate a heightened sensitivity of H3K27M mutated DMG to ATR inhibition thus identifying a therapeutic window for DMG treatment with ATR inhibitor as monotherapy or in combination with radiotherapy.