Identifying tumor propagating cells, tumor heterogeneity, and modifiers of Embryonal Rhabdomyosarcoma.

mbryonal rhabdomyosarcoma (ERMS) is a devastating pediatric cancer of muscle. Relapse is the major clinical challenge facing ERMS-patients, with 50% succumbing to their disease.  Thus, there is a clinical imperative to identify pathways that kill relapse-associated cells and/or induce them to differentiate into non-proliferative cell types. Our group has recently identified the tumor-propagating cell (TPC) in zebrafish ERMS.  The TPC is molecularly similar to a muscle satellite cell that expresses myf5, c-met, and m-cadherin. In vivo imaging experiments defined the development of tumors from the earliest stages and revealed that at late stages tumors have distinct regions of cells with TPCs separated from more differentiated cells. Similar heterogeneity is also seen in primary human ERMS and in xenograft transplants in mice. Multiphoton-photon time-lapse and serial imaging of late stage zebrafish ERMS revealed that slow moving myf5-GFP+ TPCs populate newly formed tumor only after seeding by highly migratory mid-differentiated myogenin+ ERMS cells. Moreover, myogenin-H2B-RFP+ ERMS-cells can enter the vasculature and circulate. Counter to what is currently believed in cancer biology, our data suggests that tumor-propagating potential and invasive metastatic activity can be confined to different tumor cell sub-populations. Finally, we have identified Notch signaling as a modulator of ERMS-TPC function.