SUMMARY Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children, while undifferentiated pleomorphic sarcoma (UPS) is one of the most common soft tissue sarcomas diagnosed in adults. To investigate the myogenic cell(s) of origin of these sarcomas, we used Pax7-CreER and MyoD-CreER mice to transform Pax7+ and MyoD+ myogenic progenitors by expressing oncogenic KrasG12D and deleting p53 in vivo. Pax7-CreER mice developed RMS and UPS, while MyoD-CreER mice developed UPS. Using gene set enrichment analysis, RMS and UPS each clustered specifically within their human counterparts. These results suggest that RMS and UPS have distinct and overlapping cells of origin within the muscle lineage. Taken together, we have established novel mouse models of soft tissue sarcoma from muscle stem and progenitor cells. SIGNIFICANCE Although muscle stem cells have been presumed to be a cell of origin for RMS, studies with constitutive Cre drivers expressed in Myf6-expressing cells or adipocyte P2-expressing cells suggest that cells of origin for RMS can be differentiated myofibers or adipogenic precursors, respectively. However, recent studies have demonstrated that Myf6 is expressed in muscle stem cell precursors, revealing a potential limitation of utilizing constitutive Cre drivers for cell of origin studies. Here, using inducible CreER mice, we mutate genes relevant to human RMS specifically in Pax7-expressing or MyoD-expressing cells. Our results suggest that RMS can be initiated in muscle stem cells, while UPS can be initiated in activated (Pax7+MyoD+) satellite cells.

Some patients with soft tissue sarcoma (STS) report a history of injury at the site of their tumor. While this phenomenon is widely reported, there are relatively few experimental systems that have directly assessed the role of injury in sarcoma formation. We recently described a mouse model of STS whereby p53 is deleted and oncogenic Kras is activated in muscle satellite cells via a Pax7CreER driver following intraperitoneal injection with tamoxifen. Here, we report that after systemic injection of tamoxifen, the vast majority of Pax7-expressing cells remain quiescent despite mutation of p53 and Kras. The fate of these muscle progenitors is dramatically altered by tissue injury, which leads to faster kinetics of sarcoma formation. In adult muscle, quiescent satellite cells will transition into an active state in response to hepatocyte growth factor (HGF). We show that modulating satellite cell quiescence via intramuscular (IM) injection of HGF increases the penetrance of sarcoma formation at the site of injection, which is dependent on its cognate receptor c-MET. Unexpectedly, the tumor promoting effect of tissue injury also requires c-Met. These results reveal a mechanism by which HGF/c-MET signaling promotes tumor formation after tissue injury in a mouse model of primary STS, and they may explain why some patients develop a STS at the site of injury.