EPEN-26. DRUG SCREENING IN PATIENT-DERIVED MODELS OF POSTERIOR FOSSA A EPENDYMOMA REVEALS NOVEL PHARMACOLOGICAL VULNERABILITIES

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Abstract Ependymoma (EPN) is the third most common malignant intracranial pediatric brain tumor. The most common and aggressive EPN subgroup, posterior fossa ependymoma group A (PFA-EPN), occurs predominantly in younger children and has a 5-year progression free survival of only 33%. Despite progress in the molecular characterization and subtyping of EPN, the standard treatment remains surgery with adjuvant radiation therapy. There is currently no approved role for chemotherapy in PFA-EPN. As such, identifying novel therapies for PFA-EPN is an important unmet medical need. To define therapeutic sensitivities for PFA-EPN, we performed a drug screen on three patient-derived PFA-EPN cell lines. We found that PFA-EPN cell lines were sensitive to several clinically relevant drugs, most notably those targeting epigenetic regulators. This is of a particular interest as a common molecular feature of PFA-EPN is the global reduction of the repressive post-translational histone modification H3K27me3 due to transcriptional EZHIP overexpression or H3 K27M mutations. To further characterize our PFA-EPN cell lines, we performed optical genome mapping analysis, revealing previously undescribed structural variants affecting genes encoding proteins responsible for signaling and transcription. One PFA-EPN sample that showed profound sensitivity to EGFR inhibitors, also had a duplication of a genomic region containing the EGFR ligand encoding genes, AREG, BTC, EREG, and EPGN. We are currently working to validate these results in larger cohorts of PFA-EPN models. Collectively, these findings provide new clues on drugs, signaling and putative therapeutic targets in these rare yet aggressive tumors.