Abstract 4738: Overcoming therapy resistance in rhabdomyosarcoma by targeting the PI3K-MAPK pathway

Abstract Rhabdomyosarcoma (RMS) is a devastating pediatric cancer. There has not been any significant change in the treatment options for the last 4 decades, especially for relapsed or advanced disease. There is an urgent need to identify alternative treatment options to improve survival outcomes of RMS patients. RMS is comprised of two major subtypes as defined by the presence or absence of PAX3/PAX7-FOXO1 gene fusion. Fusion-negative RMS (FN RMS) is characterized by alterations in the RAS-PI3K-MAPK pathway in 90% of the cases. However, the role of the PI3K-MAPK pathway in therapy resistance and disease relapse remains poorly characterized. We have generated two FN-RMS lines, RD and SMS-CTR, with resistance to the standard-of-care chemotherapeutic agent, vincristine, through long-term culturing in incrementally increased concentrations of the drug. By RNA sequencing, vincristine-resistant RD and SMS-CTR cells demonstrated increased expression of genes in the RAS-PI3K-MAPK pathway compared to the parental lines. We hypothesize that increased activity of the PI3K-MAPK pathway contributes to therapy resistance in FN RMS. To test the hypothesis, we first showed that targeted disruption of PIK3CA by shRNA inhibited the cell growth of vincristine-resistant FN RMS cells. The same cells also showed increased sensitivity to the MEK inhibitor, trametinib, as well as the PIK3Cα inhibitor, alpelisib, compared to the parental cells. We then showed that zebrafish FN-RMS tumors treated with the combination of vincristine and alpelisib significantly inhibited tumor growth compared to the treatment with each agent alone. Through integrative analysis of RNA sequencing and ATAC-seq data, we identified MYOD1 and NFATC2 as among the top candidate upstream regulators. Targeted disruption of either MYOD1 or NFATC2 by CRISPR/Cas9 reduced cell growth of vincristine-resistant FN RMS cells and altered expression of the genes in the PI3K-MAPK pathway, implicating MYOD1 and NFATC2 in driving abnormal activity of this pathway. To assess the relevance of our findings in the clinical settings, we showed that selective components in the PI3K-MAPK pathways are up-regulated in 3 matched primary-recurrence/metastasis paraffinized tissue samples of human FN RMS by the NanoString GeoMx Digital Spatial Profiler (DSP) platform. Our findings indicate that targeting the PI3K-MAPK pathway is a promising targeted therapy option for overcoming therapy resistance in RMS. Citation Format: Yadong Wang, Eleanor Chen. Overcoming therapy resistance in rhabdomyosarcoma by targeting the PI3K-MAPK pathway 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 (6Suppl): Abstract nr 4738.