Abstract 3838: Targeting epigenetic regulator BMI-1 in alveolar rhabdomyosarcoma

Background: Rhabdomyosarcoma (RMS) is an extremely aggressive soft tissue sarcoma which affects mainly children. There are two subtypes: alveolar rhabdomyosarcoma (ARMS) and embryonal rhabdomyosarcoma (ERMS). ARMS is characterized by PAX-FOXO1 fusion proteins, whereas subsets of ERMS harbor alterations within RAS and TP53 pathways. Currently, the outcomes for ARMS (especially when metastatic) remain dismal, thus underscoring the urgent need to identify novel targets for this cancer. The genomic landscape of many pediatric cancers, including ARMS, is relatively sparse. This led us to ask whether key epigenetic factors are driving tumor aggressiveness and could constitute novel approaches for treating ARMS. Notably, the epigenetic complexes PRC1 and PRC2 are overexpressed in a variety of sarcomas and are associated with worse overall survival. We took a hypothesis-based approach and focused on PRC1. We discovered that B lymphoma Mo-MLV insertion region 1 (BMI-1), a protein member of PRC1, is overexpressed in ARMS cells. BMI-1 is a known oncogene in other cancers, but its potential oncogenic role in ARMS and other pediatric malignancies has not yet been interrogated; thus we aim to study it within this context. Methods: To analyze the function of BMI-1 in ARMS, we depleted the protein in ARMS cell line models by both shRNA/siRNA knockdown and measured expression, cell proliferation and apoptosis. We utilized two small molecule inhibitors, PTC-209 and PTC-028, to obtain IC50s in these cell lines, then determined effects on cell proliferation and apoptosis. Results: We examined RNA-Seq tumor datasets and determined that BMI1 is robustly expressed in ARMS tumors. Additionally, we confirmed that BMI-1 is also overexpressed in ARMS cell lines at the levels of RNA and protein. Next, we depleted BMI-1 using multiple shRNAs and siRNAS and found that this led to striking (~70%) decreases in cell growth. We also observed increased levels of apoptosis within knockdown cells. Given these results, we asked whether small molecule inhibitors of BMI-1 mediated similar phenotypes, and so we used the inhibitors PTC-209 and PTC-028. PTC-209 is a first-generation BMI-1 inhibitor, while PTC-028 is a second-generation orally available inhibitor with higher potency. Both compounds inhibited BMI-1 function and greatly reduced cell proliferation in ARMS cell lines within the nanomolar range; however, as expected, PTC-028 showed a more pronounced effect compared to PTC-209. Conclusions: BMI1 supports proliferation and survival in cell line models of ARMS. Both chemical and pharmacologic inhibition of BMI1 led to striking decreases in cell proliferation. Currently, we are further investigating the molecular impact of BMI1 inhibition, with plans to investigate its effectiveness within an in vivo ARMS model. Targeting BMI-1 pharmacologically could provide a novel therapeutic option for patients with ARMS and may apply more broadly to other sarcomas. Citation Format: Cara E. Shields, Selma M. Cuya, Sarah K. Chappell, Komal Rathi, Shiv Patel, Sindhu Potlapalli, Robert W. Schnepp. Targeting epigenetic regulator BMI-1 in alveolar rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3838.