THE ALVEOLAR RHABDOMYOSARCOMA PAX3 FKHR FUSION PROTEIN IS A TRANSCRIPTIONAL ACTIVATOR/

Chimeric transcription factors, created by gene fusions as the result of chromosomal translocations, have been implicated in the pathogenesi s of several pathologically disparate solid tumors. The PAX3/FKHR fusi on gene, formed by a t(2;13)(q35;q14) in alveolar rhabdomyosarcoma, en codes a hybrid protein that contains both PAX3 DNA binding domains, th e paired box and homeodomain, linked to the bisected DNA binding domai n of FKHR, a member of the forkhead family of transcription factors: H ere we report that PAX3 and PAX3/FKHR display similar, but not identic al transactivation activities when tested with model Pax recognition s equences. No functional role could be ascribed solely to the residual FKHR binding domain present in the fusion protein, but FKHR was found to contribute a strong carboxyl terminal activation domain replacing t he one located in the unrearranged PAX3 gene. We show that the native PAX3/FKHR protein present in tumor cells with this translocation has t ranscriptional characteristics similar to the in vitro expressed prote in. The ability of the PAX3/FKHR hybrid protein to bind DNA in a seque nce specific manner and to transactivate the expression of artificial reporter genes suggests that its aberrant expression could subvert the transcriptional programs that normally control the growth, differenti ation, and survival of primitive myogenic precursors in vivo.