Abstract 3866: Investigating the role of class I HDACs in Ewing sarcoma pathology

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Abstract Introduction: Recent evidence demonstrated that a low mutation rate seems a general feature of pediatric cancers. Ewing sarcoma (ES) is defined by balanced chromosomal EWS/ETS translocations, which give rise to oncogenic chimeric proteins (EWS-ETS). Other contributing somatic mutations involved in disease development have only been observed at low frequency. Thus, cancer in children is not solely a genetic disease and can neither be understood nor cured presumably without epigenetics. Previously, we identified the histone methyl-transferase enhancer of zeste 2 polycomb repressive complex 2 subunit (EZH2), the enzymatic subunit of the polycomb PRC2 complex, to be over-expressed in ES. RNA interference of EZH2 suppressed tumor development and metastasis in vivo and microarray analysis of EZH2 knock down revealed an EZH2-maintained, undifferentiated, reversible phenotype in ES. Experimental procedures: The role of class I histone deacetylases (HDACs) was determined using different potent inhibitors like Trichostatin A (TSA), Romidepsin (FK22), Entinostat (MS-275) and PCI-34051 as well as a CRISPR/Cas9 approach to knock out specific HDACs. To analyze resulting changes qRT-PCR, Western Blotting, proliferation, apoptosis and invasion assays as well as immunofluorescence experiments were deployed. Results: Interestingly, the effects of gene silencing after RNA interference of EZH2 could be mimicked by treatment of ES with TSA or MS-275. Microarray analysis revealed that treatment with MS-275 or TSA resulted in the induction of a similar pattern of differentiation genes as was previously observed after EZH2 blockade and seems to be dependent on histone deacetylase (HDAC) activity. Furthermore, in proliferation assays ES were more susceptible to treatment with HDAC inhibitors than other small round blue cell tumors such as neuroblastoma or pediatric cALL cells. Treatment of enzymatic inhibitors blocking EZH2 activity could not mimic the results observed after EZH2 RNA interference indicating that EZH2-containing PRC2 complexes may serve as a building block of class I HDAC activity in ES. Conclusion: Class I HDACs seem to be important mediators of the pathognomonic EWS-ETS-mediated transcription program in ES and thus interesting new treatment opportunities for this malignant disease. Citation Format: Kristina von Heyking, Tim Hensel, Julia Singer, Oxana Schmidt, Stefan Burdach, Günther H. Richter. Investigating the role of class I HDACs in Ewing sarcoma pathology [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3866. doi:10.1158/1538-7445.AM2017-3866

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