HGG-38. DE NOVO PYRIMIDINE SYNTHESIS INHIBITION INDUCES REPLICATION CATASTROPHE MEDIATED CELL DEATH IN DIFFUSE MIDLINE GLIOMA

Diffuse midline gliomas (DMG) are aggressive and lethal pediatric brain tumors that cannot be cured by conventional therapeutic modalities. Using a genome wide CRISPR screen we identified the de novo pyrimidine biosynthesis pathway as a metaboilic vulnerability in DMGs. BAY2402234 is a small molecule inhibitor of DHODH -a rate liminting enzyme in the de novo pyrimidine biosynthesis pathway. BAY2402234 induces cell death in DMG cells at low nanomolar concentrations while sparing adult glioblastoma cells and normal astrocytes. Further investigations revealed drammatic reduction in cellular UMP pools, the precursor for all pyrimidine nucleotides, after DHODH inhibition, specifically in DMG cells. Cytotoxicity of DHODH inhibition in DMG cells is rescued by exogenous uridine, supporting UMP depletion as the mechanism underlying DMG cell death and also showing that cell death is an “on target” response to BAY2402234. Cell death induced by BAY2402234 is a consequence of replication fork stalling as evident by accumulation of chromatin-bound RPA foci and g-H2AX. Stalled replication forks eventually collapse, resulting in replication catastrophy and apoptosis. Cytotoxic effects of DHODH inhibition are further exacerbated by inhibition of the intra-S checkpoint protein, ATR. Combined treatment of DMG cells with DHODH and ATR inhibitors resulted in enhanced accumulation of chromatin-bound RPA, g-H2AX, replication fork collapse and apoptosis. Importantly, in vivo studies verify that both BAY2402234 (DHODHi), and BAY1895344 (ATRi), cross the blood-brain barrier, accumulate in the brain at therapeutically relevant concentrations, and induce DNA damage in intracranial DMG xenografts in mice. Taken together, our studies have identified DHODH inhibition as a DMG-specific vulnerability resulting in cell death; the mechanism of DHODHi-induced cell death led us to identify combined inhibition of DHODH and ATR as a synergistic therapy against DMG tumors.