Ewing's sarcoma is the second most common bone malignancy in children. It is associated with recurrent chromosomal translocations that produce fusions between a relatively unknown gene, EWS, and one of the members of the ets family of transcription factors. Chromosomal translocation t(11;22)(q24;q12) is the most frequently found translocation in Ewing's sarcoma that produces the EWS/FLI1 fusion gene. These chimeric oncoproteins are believed to function as aberrant transcription factors that exert their oncogenic features through deregulation of downstream target genes.

Identification of downstream targets of EWS/FLI1 is a key step towards understanding the biology of Ewing's sarcoma and could potentially lead to the development of novel therapies. One such target is thrombospondin, an antiangiogenic factor that is strongly suppressed by the EWS/FLI1 oncoprotein. Suppression of thrombospondin is at least in part by transcriptional regulation and requires binding of EWS/FLI1 to DNA. Ectopic expression of thrombospondin in Ewing's tumor cell lines delays tumor formation in SCID mice. Additionally, we observed a marked decrease in the number of blood vessels in tumors with thrombospondin expression. Taken together, these observations suggest that EWS/FLI1 regulates angiogenesis via suppression of thrombospondins. Furthermore, angiogenic pathways could potentially serve as therapeutic targets for Ewing's sarcoma.

The identity of the cell of origin of Ewing's sarcoma has remained elusive due to lack of morphological and biochemical markers on the tumors. The problem is exacerbated by the fact that the fusion oncogene is not tolerated by most primary cells. To address this important issue, we first identified cell types based on the similarity of their expression profiles to those of EWS/FLI1 modulated genes in the Ewing's tumor background. Identification of a IMR-90 fibroblasts lead to a series of experiments where a combination of few genetic alterations resulted in a cellular background that could tolerate EWS/FLI1 expression. Moreover, these cells had the capacity for limited anchorage independent growth, indicating partial transformation by the fusion gene. These cells represent the foundation of a novel model system for studying Ewing's sarcoma pathogenesis.