EXTH-09. HSP90 INHIBITION AS A NOVEL THERAPY FOR DIFFUSE INTRINSIC PONTINE GLIOMA (DIPG)

Diffuse intrinsic pontine glioma (DIPG) is an aggressive pediatric brain tumor. The mean age of onset is 7-9 years with a median survival of 9 months following diagnosis. Histone 3 (H3) mutations (H3K27M) have been identified in approximately 80% of patients, representing an intriguing target, but how to do this is unclear. To address this, we performed a synthetic lethality drug screen of over 2400 compounds on isogenic cells expressing H3K27M and empty vector. 37 drugs were synthetically lethal with H3K27M, including HSP90 inhibitors. In cancer, HSP90 has a higher isoelectric point which can be targeted specifically by PU-H71. Using PU-H71, HSP90 complexed with oncogenic clients can be inhibited to reduce tumor cell viability. We tested PU-H71 on primary patient derived DIPG lines demonstrating caspase-3/7 mediated cell death within 24 hours (n=3) and an IC50 of 100-200 nM (n=5) at 72 hours. HSP90 is a molecular chaperone that supports protein activation and localization; thus, its inhibition may allow simultaneous targeting of multiple oncogenic pathways. To determine which pathways are targeted in DIPG we used PU-H71-conjugated beads to pull-down HSP90 complexed with client proteins. LC-MS/MS characterization of the HSP90 interactome from three DIPG cell lines (SU-DIPG XVII, SU-DIPGXXV, SU-DIPG 50) identified 339 overlapping proteins including MAPK3, SMS, SRM, CTSP1, OAT, GMPS, and PYG. Pathway enrichment analysis highlighted roles in cell cycle regulation and metabolic processes. Treating DIPG with PU-H71 successfully reduced tumor cell viability but PU-H71 has poor brain penetration. To overcome this, a molecular isoform of PU-H71 has been developed, PU-HZ151, which will be used to test the in vivo efficacy of HSP90 inhibition for DIPG.

EPCO-16. ONCOHISTONE INTERACTOME PROFILING UNCOVERS MECHANISMS OF CHROMATIN DISRUPTION AND IDENTIFIES POTENTIAL THERAPEUTIC TARGETS IN PEDIATRIC HIGH-GRADE GLIOMA

Mutations in histone H3 at amino acids 27 (H3K27M) and 34 (H3G34R) occur with high-frequency in pediatric high-grade glioma. H3K27M mutations have been shown to lead to global disruption of H3K27me3 through dominant negative PRC2 inhibition with accompanying gains in H3K36me3, while H3G34R mutations lead to local losses of H3K36me3 through inhibition of SETD2. However, the mechanism of action of these mutants on the broader landscape of chromatin-associated proteins remains unknown. Importantly, proteins with differential associations with oncohistones could be targeted therapeutically. Here we profiled the interactomes of the H3.1K27M, H3.3K27M and H3.3G34R oncohistones using BioID to gain an unbiased measure of their interaction landscapes. Among the differential interactors all 3 mutants lost interaction with H3K9 methyltransferases, while H3G34R also had reduced interaction with DNA methyltransferases accompanied by genome-wide DNA hypomethylation. In contrast, H3K27M mutants had increased association with transcription factors, consistent with the activation of transcription induced by the global loss of H3K27me3. H3K9me3 was reduced in H3K27M-containing nucleosomes, and cis-H3K9 methylation was required for H3K27M to exert its effect on global H3K27me3. Depletion of H3K9 methyltransferases with shRNA or treatment with H3K9 methyltransferase inhibitors was lethal to H3.1K27M, H3.3K27M and H3.3G34R mutant pHGG cell lines, underscoring the importance of H3K9 methylation for oncohistone-mutant gliomas and suggesting it could make an attractive therapeutic target.