Abstract 630: Elucidation of PAX3::FOXO1 tumor initiation mechanisms in human induced pluripotent stem cell models.

Abstract Rhabdomyosarcoma (RMS) is the most common pediatric soft tissue sarcoma. Alveolar RMS (ARMS) is driven by either t(2;13)(q35;q14) or t(1;13)(p36;q14) resulting in the PAX3-FOXO1 (P3F) and PAX7-FOXO1 (P7F) fusion oncoproteins, respectively and has a poor prognosis. A deeper understanding of P3F mediated tumorigenesis is needed to discover novel targets. Current model systems fail to recapitulate the human disease in terms of timing, location, and the formation of other tumor types. Previously, our lab generated an ARMS model system derived from human induced pluripotent stem cells (iPSCs), in which forced P3F expression during endothelial directed differentiation blocked endothelial maturation instead reprogramming cells to skeletal muscle-like that form ARMS tumors in mice. Building off this model, we generated a doxycycline inducible iARMS model driven by degradable P3F-FKBP12F36V (ddP3F cells) allowing for fine control over P3F expression. Degradation of P3F did not significantly reduce viability or proliferation but reduced the ability of ddP3F cells to undergo myogenic differentiation. P3F-negative ddP3F cells continued to proliferate for multiple passages and retained the ability to form foci. Transcriptional analyses revealed ARMS cell states remained stable upon P3F loss. Taken together, this data shows that P3F is important for ARMS fate initiation but not maintenance, indicating further examination of the initiation event was needed for insight into ARMS biology. To elucidate the P3F-mediated initiation mechanism, I turned to our previously established iARMS model. I utilized CUT Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 630.