EPEN-11. Understanding and overcoming mechanisms of dormancy in ependymoma

Abstract Despite significant advances in molecular-guided diagnostics and stratification, progression-free and overall survival rates for posterior fossa group A ependymoma (EPN-PFA) have shown little improvement over recent decades. The absence of therapeutic innovation beyond surgical resection and adjuvant radiotherapy has left patients with relapsed, locally recurring disease facing poor outcomes. This underscores an urgent need to elucidate the molecular mechanisms underlying local tumor recurrence. We hypothesize that a dormant cell population persists post radiotherapy and contributes to disease relapse. Data analysis of single-nuclei RNA sequencing from EPN-PFA patient tissue revealed senescence-associated signaling enriched within distinct tumor cell clusters, supporting this hypothesis. Molecularly characterized EPN-PFA cell lines cultured as spheroids re-grew after intensified single dose (up to 60 Gy) or fractionated irradiation (14 x 1.8 Gy) after several months, confirming radio resistance of an EPN subpopulation in vitro. To explore therapeutic strategies targeting dormant cells, we leveraged our comprehensive radiosensitizer drug screen (137 compounds) using EPN-PFA cell lines grown as spheroids. This screen identified synergistic interactions between irradiation and several drug candidates, with ‘senolytic’ compounds including navitoclax and venetoclax as consistent top performers across different radiation schedules and dose intensities. Validation experiments in vitro demonstrated that combining irradiation with ‘senolytic’ drugs significantly reduced spheroid size and cell viability. Ongoing work combines in vitro and in vivo regrowth studies with serial single-cell sequencing to delineate the characteristics of radioresistant cell populations and uncover further actionable vulnerabilities to overcome dormancy. Our findings provide a promising foundation to further explore novel therapeutic paradigms targeting dormant EPN cell populations, with the aim of improving long-term outcomes for patients with EPN-PFA.