Abstract 1723: Targeting SMARCA1 in rhabdomyosarcoma: Potential therapeutic implications

Abstract Background: Rhabdomyosarcoma (RMS) is the most common soft tissue in pediatric sarcoma, and studies demonstrate that RMS arises from skeletal muscle precursor cells. RMS, genetically divided into two subtypes: PAX-FOXO1 fusion positive RMS (FP-RMS), which is driven by chromosomal translocation involving PAX3 or PAX7 genes with FOXO1 and PAX-FOXO1 fusion negative RMS (FN-RMS), which is marked by mutations in RAS isoforms and some other genes. Chromatin was one of the earliest identified targets for cancer therapy. Several chromatin remodeling proteins are associated with cancer progression processes such as proliferation, differentiation, apoptosis, and tumorigenesis. Components of chromatin remodeling complexes have been classified as both oncogenes and tumor suppressors. The ISWI family protein, SMARCA1 has been implicated in tumorigenesis for several cancer types. ISWI complexes regulate cell differentiation and proliferation in other cell systems, but their impact on myogenesis is not well understood. In this study, we will characterize the function of SMARCA1 in RMS cells and skeletal muscle. We hypothesize that SMARCA1 acts to modulate chromatin accessibility and drive RMS tumorigenic growth. Methods: We employed RNA-seq, ATAC-seq, and CUT&RUN to study the impact of SMARCA1 in RMS cells. Furthermore, phenotypic experiments were performed to determine the influence of SMARCA1 on differentiation and proliferation. Results: SMARCA1 is expressed at a high level in RMS tissues but not in muscle tissues. Our findings show that SMARCA1 is modestly upregulated during differentiation in normal myogenesis. Upon treatment with trametinib, FP-RMS cell lines with SMARCA1 knockout exhibited a dramatic decrease in cell confluency and viability. Intriguingly, the control FP-RMS cell lines demonstrated increased SMARCA1 protein levels in response to trametinib, which could suggest a resistance mechanism to the treatment. Trametinib also affected SMARCA1 mRNA expression in FN-RMS cells. Additionally, it was discovered that the elimination of SMARCA1 not only resulted in a reduction in cell migration but also led to a failure of these cells to develop spheres, unlike the control cells, where the migration gap healed in the usual manner and the spheres were observed. These findings may align with our hypothesis that SMARCA1 regulates chromatin accessibility, potentially impacting adhesion properties and signaling pathways. Analysis of gene clusters in RMS transcriptomic data indicated that SMARCA1 co-expresses with a RAS effector, RALA. SMARCA1 is also clustered with HDAC2. In addition, SMARCA1 interacted with HDAC2 in RMS cells, as determined by co-immunoprecipitation experiments. Conclusions: Our findings suggest that SMARCA1 may influence RMS tumor growth. This study might contribute to a deeper understanding of SMARCA1's role in RMS and support its credentialing as a novel therapeutic avenue. Citation Format: Ashwaq Aljabri, Katie Hebron, Yuliya Kriga, Juan Manuel Caravaca, Aiysha Althobaiti, Stacey Stauffer, Angela Kim, Lauren Stoak, Morgan Porter, Jyoti Shetty, Bao Tran, Matthew Geisler, Judith Davie, Marielle Yohe. Targeting SMARCA1 in rhabdomyosarcoma: Potential therapeutic implications [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1723.