Abstract 2460: Characterizing PAX3-FOXO1 dependent cell death in alveolar rhabdomyosarcoma reveals novel strategies for combination therapy

Abstract

Abstract Rhabdomyosarcoma is the most common soft tissue sarcoma in children. The aggressive alveolar subtype (aRMS) is characterized by chromosomal translocations, most often by t(2;13) resulting in the expression of the oncogenic fusion protein PAX3-FOXO1. Expression of this chimaeric transcription factor is critical for tumorigenesis and cell survival. However, the exact mechanism of cell death after loss of PAX3-FOXO1 activity has not been determined so far. Our aim was thus to characterize the cell death pathways activated upon silencing of PAX3-FOXO1. In addition, we aimed at finding drugs further sensitizing aRMS cells to this mode of cell death. We used combined shRNA and CRISPR approaches, as well as a small molecule screen to demonstrate that after shRNA-mediated silencing of PAX3-FOXO1 expression, aRMS cells undergo intrinsic apoptosis in a NOXA-dependent manner. In accordance, we can show that the BH3-mimetic ABT-263 sensitizes aRMS cells to PAX3-FOXO1 silencing via this cell death pathway. Interestingly, ABT-199 was less effective, suggesting that Bcl-xl plays a major anti-apoptotic role in aRMS. Furthermore, induction of cell death is dependent on PI3K activity and antagonized by GSK3, suggesting that inhibition of PI3K in this sarcoma might have an anti-apoptotic impact. In contrast, combination of ABT-263 or GSK-3 inhibitors with small molecule drugs affecting PAX3-FOXO1 activity such as PLK1 and aurora kinase inhibitors can cooperate to enhance cell death in aRMS cells. These studies demonstrate the importance of elucidating biological mechanisms to guide development of rational drug combinations. Citation Format: Marco Wachtel, Johannes Ommer, Beat Schäfer. Characterizing PAX3-FOXO1 dependent cell death in alveolar rhabdomyosarcoma reveals novel strategies for combination therapy. [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 2460.