Abstract 3900: The BRG1 ATPase helps to maintain the de-differentiation phenotype in alveolar rhabdomyosarcoma
Authors: Laubscher Dominik , Stanton Benjamin Z. , Gryder Berkley , Das Sudipto , Andresson Thorkell , Roschitzki Bernd , Wolski Witold , Ngo QuyAi , Marques Joana G. , Wachtel Marco , Vakoc Christopher , Khan Javed , Schäfer Beat 
Published: September 2020

Abstract
The alveolar subtype of rhabdomyosarcoma (aRMS) is a highly malignant soft tissue tumor, which commonly occurs in childhood and is associated with the skeletal muscle lineage. aRMS is characterized by the expression of the fusion transcription factor PAX3-FOXO1, acting as the main oncogenic driver. Despite the expression of master regulatory factors (MRFs) of skeletal muscle differentiation, aRMS cells are blocked from executing the terminal differentiation program. Since PAX3-FOXO1 plays a role in repressing differentiation, in part by induction of epigenetic changes, we were interested to gain additional knowledge about epigenetic cofactors that mediate this activity of the fusion protein. Here, we identified BRG1, the major catalytic subunit of SWI/SNF complexes, to be essential for tumor cell proliferation in aRMS and the most important regulatory ATPase. We further report that the physiological effects of interference with SWI/SNF complex function involve transcriptional activation of the myogenic differentiation program, accompanied by altered morphology and cell cycle profiles. Proteomic studies suggest that PAX3-FOXO1 is not an integral part of SWI/SNF complexes, but still has close proximity indicating potential functional interaction through chromatin interfaces. Genome-wide ChIP binding profiles in aRMS cells revealed associations of BRG1 with motifs of myogenic factors including MYOG, MYF5, and MYOD, suggesting functional interactions with MRFs. Our data is consistent with a model where BRG1 containing complexes have a repressive function at regulatory elements of downstream differentiation target genes. Ongoing work aims to further characterize BRG1-mediated changes on genome architecture to understand its contribution to differentiation blockade in the background of fusion positive aRMS. We conclude that BRG1 is an essential target to overcome differentiation blockade of aRMS cells. While other SWI/SNF complex members have been proposed previously to be potentially important for RMS oncogenesis, these proteins generally lack druggability. In contrast, efforts to develop BRG1 specific compounds are already underway, including PROTAC compounds. Preclinical studies using these tools will provide novel insights into therapeutic applicability to target SWI/SNF complexes in RMS.
Citation Format: Dominik Laubscher, Benjamin Z. Stanton, Berkley Gryder, Sudipto Das, Thorkell Andresson, Bernd Roschitzki, Witold Wolski, QuyAi Ngo, Joana G. Marques, Marco Wachtel, Christopher Vakoc, Javed Khan, Beat Schäfer. The BRG1 ATPase helps to maintain the de-differentiation phenotype in alveolar rhabdomyosarcoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3900.