P10.14.a Targeting Bromodomains to Impair Dna Damage Response and Epigenetic Control in High-Risk Ependymoma

Abstract BACKGROUND Ependymomas (EPN) account for 10% of all pediatric brain tumors. Based on tumor location and DNA methylation ten molecular groups have been characterized, identifying supratentorial EPN harboring ZFTA gene fusions (including ZFTA-RELA) and posterior fossa group A (PFA) as the most aggressive ones in pediatric patients. Currently, treatment options are limited to local interventions, emphasizing the urge to identify new therapeutic targets. As epigenetic deregulation frequently acts as major driver in EPN aggressiveness, epigenetic drugs emerge as a promising treatment option in these high-risk EPN groups. Bromodomain and extraterminal domain (BET) proteins are crucial epigenetic readers, involved in oncogene expression in various cancer types. Especially in ZFTA fusion-driven tumors, BRD4 is known as co-mediator, suggesting BET inhibition as a possible therapeutic vulnerability in this tumor subgroup. MATERIAL AND METHODS Initial assessments encompassed primary, patient derived (n=7) and high passaged (n=2) cell models of the two most aggressive EPN subgroups. Sensitivity towards BET inhibition was assessed in cell viability assays in short- and long-term treatments. Impact of the treatment with respect to gene- and protein expression was assessed by transcriptomics analyses and Western blotting, respectively. Cerebral organoids and HEK293 cells transfected with ZFTA-RELA plasmids were used to prove dependency of the BETi sensitivity on the oncofusion. RESULTS Our data showed elevated expression of BET proteins in ZFTA-RELA compared to PFA models also resulting in increased sensitivity of these models towards BET inhibitors (BETi). Flow cytometry experiments as well as RNA sequencing proved cell cycle arrest and DNA damage in ZFTA-RELA models in response to BETi. Consequently, BET inhibition exerted synergistic effects in combination with PARP inhibitors. Interestingly, transfection of HEK293 cells as well as cerebral organoids with the ZFTA-RELA oncofusion showed similar effects upon BET inhibition proving dependency of BETi sensitivity on the driving oncofusion in this tumor subtype. CONCLUSION Summarizing, our data indicate BETi treatment as promising therapeutic strategy specifically in ZFTA fusion-driven EPN. Further studies will assess the impact of BETi treatment as well as the combination with PARP inhibitors in vivo and in cerebral organoids driven by the ZFTA-RELA oncofusion.