EPEN-03. USP7 is an interaction partner of EZHIP and potential druggable target in PFA ependymomas

Ependymomas (EPN) arise in the supratentorial brain (ST-EPN), posterior fossa (PF-EPN), or the spinal cord (SP-EPN), in children and adults. Molecular profiling has identified distinct subgroups in each location. Among the three molecular subgroups of PF-EPN, PFAs are characterized by young median age at diagnosis, an overall balanced genome and bad clinical outcome (56 % 10-year OS). Therapy involves tumor resection and radiotherapy, but the role of chemo- or targeted therapies remains to be defined. Recently, we and others identified enhancer of zeste inhibiting protein (EZHIP) as potential driver of PFAs. By inhibiting EZH2, catalytic subunit of the polycomb repressive complex 2 (PRC2), EZHIP prevents the distribution of the epigenetic repressor mark H3K27me3. However, since EZHIP does not possess any known enzymatic functions, it does not seem to be the druggable target urgently searched for in PFA. We therefore focused on essential and potentially druggable interactions. Here, we present ubiquitin-specific protease 7 (USP7), a known cancer regulator with multiple inhibitors available, which has been shown to interact with EZHIP in non-PFA cells. We confirmed this interaction in PFA cells by co-immunoprecipitation and mass spectrometry, and showed that the EZHIP-USP7 interaction is independent of EZH2, a separate interactor of both. Functionally, we show that USP7 de-ubiquitinates EZHIP, preventing its degradation and thus stabilizing it. As EZHIP is essential for PFA cell survival, we aim to target EZHIP indirectly by affecting its stability regulation via USP7. Knockdown experiments showed a susceptibility of PFA cells to a USP7 loss, focusing on proliferation, apoptosis, and expressional changes induced by altered USP7 levels. Moreover, USP7 inhibitors highly affected the survival of different PFA cell lines at a low micromolar IC50 in vitro and in vivo treatments of PFA patient-derived xenografts with USP7 inhibitors are ongoing, hopefully helping to improve targeted therapies for PFA ependymoma patients.