Abstract 4472: Targeting the oncogenic transcription factor EWS-Fli1 by BET bromodomain inhibition in Ewing sarcoma
Authors: Camille JACQUES, François LAMOUREUX, Marc BAUD’HUIN, Lidia RODRIGUEZ-CALLEJA, Thibaut QUILLARD, Pierre PERROT, Franck TIRODE, Françoise REDINI, James E. BRADNER, Dominique HEYMANN, Benjamin ORY
Journal: Cancer Research (2016)

Abstract Objective Ewing Sarcoma is the second most common primitive malignant bone tumor after osteosarcoma. This childhood cancer is defined by a chromosomal translocation leading to the production of a chimerical transcription factor, EWS-Fli1, which is implicated in the progression of this malignancy. As current treatments improve its outcome, therapeutic resistances remain and reduce the survival rates. Despite protocols’ optimization efforts, patients still relapse and it is important to develop new therapeutic approaches. The recently synthesized molecule JQ1, an inhibitor of the BET bromodomain proteins could be a new original weapon against this cancer. BET proteins interact with acetylated histones to regulate the chromatin accessibility to transcription factors and RNA polymerases. As JQ1 was recently shown to reduce the transcription of “super-enhancers”-dependent-genes such as oncogenes, which are highly sensitive to the BET proteins presence, it sounds relevant to hypothesize that EWS-Fli1's expression depends on the activity of these proteins. Method The cell viability, the cell proliferation and the cell's clonogenic potential consequent to a JQ1 treatment were studied in vitro. Using a Ewing Sarcoma nude mice model, the JQ1 effects on both the tumor growth and the animals overall survival were assessed. The EWS-Fli1 expression and ones of its transcriptional targets were evaluated by RT-qPCR and Western blotting and the direct regulation of EWS-Fli1 expression by BRD4 was checked by ChIP-qPCR. Results We demonstrated that JQ1 reduces the viability and the clonogenic capabilities of the cells as well as it leads to a G1-phase blockade, delays the burden of the tumor growth and improves the mice overall survival. Histological analysis shows that JQ1 reduces the vascularization and the cell proliferation within the tumors. Those effects are correlated to the associated silencing of EWS-Fli1 and the consequent modulation of some of its target-genes resulting from the depletion of BRD4 from the EWS-Fli1 promoter. Conclusion Our results shed light on the BET bromodomains’ role as essential regulators of Ewing Sarcoma carcinogenesis. These proteins are indeed required to control EWS-Fli1's expression, consequently impacting its downstream signaling, which is functionally decisive for the maintenance of the tumorigenic features of these cancer-cells. Thus, the BET bromodomain proteins could be promising therapeutic targets in the Ewing tumors context. Citation Format: Camille JACQUES, François LAMOUREUX, Marc BAUD’HUIN, Lidia RODRIGUEZ-CALLEJA, Thibaut QUILLARD, Pierre PERROT, Franck TIRODE, Françoise REDINI, James E. BRADNER, Dominique HEYMANN, Benjamin ORY. Targeting the oncogenic transcription factor EWS-Fli1 by BET bromodomain inhibition in Ewing sarcoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4472.