The Ewing's sarcoma fusion protein, EWS-FLI, binds Runx2 and blocks osteoblast differentiation.

Ewing's sarcomas are highly aggressive round cell tumors of bone and soft tissues that afflict children and young adults. The majority of these tumors harbor the t(11;22) translocation and express the fusion protein EWS-FLI. Modern molecular profiling experiments indicate that Ewing's tumors originate from mesenchymal precursors in young individuals. EWS-FLI alters the morphology of mesenchymal cells and prevents lineage specification; however, the molecular mechanisms for differentiation arrest are unclear. We recently showed that EWS-FLI binds Runx2, a master regulator of osteoblast differentiation. In this report, we demonstrate that FLI sequences within EWS-FLI are responsible for interactions with Runx2. EWS-FLI blocks the expression of osteoblastic genes in a multipotent progenitor cell line that requires Runx2 to integrate bone morphogenic protein (Bmp)2 signaling while increasing proliferation and altering cell morphology. These results demonstrate that EWS-FLI blocks the ability of Runx2 to induce osteoblast specification of a mesenchymal progenitor cell. Disrupting interactions between Runx2 and EWS-FLI1 may promote differentiation of the tumor cell.