DIPG-53. LSD1 DEGRADATION SYNERGIZES WITH HDAC INHIBITION IN PEDIATRIC H3K27M DIFFUSE MIDLINE GLIOMA

H3K27M-mutated diffuse midline glioma (H3K27M DMG) is an aggressive, lethal pediatric brain tumor which resists conventional cancer treatments. To identify new therapeutic targets, our lab executed a focused CRISPR negative selection screen in patient-derived H3K27M cell lines after treatment with the histone deacetylase (HDAC) inhibitor panobinostat, and discovered a strong co-dependence with the histone demethylase LSD1. We further explored the therapeutic potential of this synergistic interaction with a screen of HDAC- and LSD1- targeting drugs to identify synergistic combinations, which show penetration of the blood brain barrier suitable for clinical translation. Interestingly, conventional catalytic LSD1 inhibitors did not seem to synergize with HDAC inhibitors tested on cell viability, while a recently described CoREST/LSD1 degrader phenocopied the effects seen in our CRISPR screen. We hypothesized that the CoREST/LSD1 degrader induces synergy with HDAC inhibitors through elimination of a scaffolding function of LSD1. To prove this, we knocked out LSD1 using CRISPR/Cas9 in H3K27M cells and subsequently treated with a panel of HDAC inhibitors. These showed a significant sensitization compared to controls cells. Further, we confirm the high specificity of the CoREST/LSD1 degrader by performing streptavidin bead pull down with a newly synthesized biotin-conjugated probe. This revealed that it interacts with the entire CoREST complex (members include LSD1, RCOR1, HDAC1/2). In conclusion, we show that degrading CoREST/LSD1 in combination with HDAC inhibition is a synergistic strategy in H3K27M DMG. For future translational studies, we will assess biomarkers upon treatment and analyse compound potency in vivo.