Abstract 3885: USP7 is an interaction partner of EZHIP and potential druggable target in PFA ependymomas

Abstract

Abstract Ependymomas (EPN) are tumors of the central nervous system (CNS) that can arise in the supratentorial brain (ST-EPN), hindbrain or posterior fossa (PF-EPN), or anywhere in the spinal cord (SP-EPN), both in children and adults. Molecular profiling has identified distinct subgroups and subtypes in each of the anatomic compartments. In the posterior fossa, three subgroups have been identified: PFA, PFB and PF-SE. Of them, PFA ependymomas are characterized by a young median age at diagnosis, an overall balanced genome and a bad clinical outcome (56% 10-year overall survival). Standard therapy consists of tumor resection and radiotherapy, but working chemo- or targeted therapies will be essential to improve patient outcomes. Recently, we and others identified enhancer of zeste inhibiting protein (EZHIP) as potential main driver of PFA tumorigenicity. By inhibiting EZH2, the 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 itself, it does not serve as the druggable target urgently needed for PFA tumors. Thus, interaction partners are one main focus of ongoing PFA research. In this project, we focused on the ubiquitin-specific protease 7 (USP7), a well-known regulator of cancer pathways with a variety of inhibitors available, which was observed to interact with EZHIP in non-PFA cell lines before. We were able to confirm this interaction of EZHIP and USP7 in PFA cells in vitro by co-immunoprecipitation and mass spectrometry, and could show that the EZHIP-USP7 interaction is independent of EZH2, a separate interactor of both. Functionally, we are testing the hypothesis that USP7 de-ubiquitinates EZHIP, thereby preventing its proteasomal degradation and thus stabilizing the protein. As EZHIP expression is essential to PFA cell survival, we aim to target EZHIP indirectly by interfering with its stability regulation via USP7. RNAi-based knockdown (KD) experiments have been used to test a susceptibility of PFA cells to a loss of USP7, focusing on effects on cell proliferation and apoptosis, but also investigating gene expression changes provoked by a change in USP7 levels. Moreover, multiple USP7 inhibitors already highly affect the survival of different PFA cell lines in vitro. Ongoing experiments with PFA patient-derived xenograft (PDX) models will hopefully confirm the strong effect of USP7 inhibitors in PFA, and help to improve targeted therapy for PFA patients. Citation Format: Anne Jenseit, Aylin Camgöz, Monika Mauermann, Stefan M. Pfister, Marcel Kool. USP7 is an interaction partner of EZHIP and potential druggable target in PFA ependymomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3885.