Understanding the determinants of cell fate decisions in pediatric rhabdomyosarcomas

Alveolar rhabdomyosarcomas (aRMS) are highly aggressive pediatric sarcomas, associated with the skeletal muscle lineage. These tumors are believed to originate from differentiation defects during myogenesis, the process of muscle development: despite expression of key myogenic regulatory factors, aRMS cells are “blocked” in a proliferative state and are unable to differentiate. Based on our recent findings, aRMS tumors contain different cell populations mirroring the entire muscle developmental process, from progenitor stem-like cells to a small subpopulation of differentiated cells that lost their proliferative potential. Progenitor cells follow a branched cell fate: they can either become cycling myoblasts, and drive worst patient prognosis, or undergo muscle differentiation, and lose the malignant potential. However, at this point it is still unclear how progenitor cell fate is regulated and how it can be therapeutically hijacked towards differentiation. We hypothesize that developmental cues are intrinsic. The objective of our study is to identify the mechanisms that govern cancer cell differentiation in aRMS and to exploit their therapeutic potential. In a first step, we will introduce Cas9 protein into primary patient-derived xenograft (PDX) aRMS cells that have a reporter system for myogenin, a gene upregulated upon differentiation. Second, we will perform a fluorescence-activated cell sorting (FACS)-based genome-wide CRISPR screen by infecting these cells with loss-of-function library and sorting for differentiated cells. Finally, gene targets will be validated by functional knock-out and potential targeted drugs against these genes will be tested both in vitro and in vivo. Our research intends to improve the clinical outcome of RMS patients by promoting cellular differentiation.