Ewing sarcoma/primitive neuroectodermal tumors (EWS/PNET) are characterized by 
specific chromosomal translocations most often generating a chimeric EWS/FLI-1 
gene. Depending on the number of juxtaposed exons assembled, several fusion 
types have been described with different incidences and prognoses. To assess the 
impact of each fusion type on the specific phenotypic, tumorigenic, and 
metastatic features of EWS/PNET, we developed an amenable system using a murine 
mesenchymal multipotent C3H10T1/2 cell line. Upon transduction of EWS/FLI-1, 
cells acquired dramatic morphological changes in vitro, including a smaller size 
and "neurite-like" membrane elongations. Chimeric fusion proteins conferred 
oncogenic properties in vitro, including anchorage-independent growth and an 
increased rate of proliferation. Furthermore, EWS/FLI-1 expression blocked 
mineralization, with concomitant repression of osteoblastic genes, and induced a 
dramatic repression of the adipocytic differentiation program. Moreover, 
EWS/FLI-1 promoted an aberrant neural phenotype by the de novo expression of 
specific neural genes. The intramuscular injection of transduced cells led to 
tumor development and the induction of overt osteolytic lesions. Analogously, to 
what was observed in human tumors, type 2 EWS/FLI-1 cells formed primary tumors 
in immunodeficient mice with a higher incidence and a lower latency than cells 
bearing types 1 and 3 fusions. By contrast, cells expressing types 2 and 3 
fusions showed specific metastatic activity with a higher number of macroscopic 
metastases in soft tissues and osteolytic lesions in the limbs as compared to 
type-1-expressing cells. Therefore, the structure of each oncoprotein strongly 
influenced its tumorigenicity and metastagenicity. Thus, this model provides a 
basis for understanding the genetic determinants involved in Ewing tumor 
development and metastatic activity and represents a cellular system to analyze 
other oncoproteins involved in human sarcomagenesis.