The Pax3-FKHR oncoprotein is unresponsive to the Pax3-associated repressor hDaxx

The Pax3–FKHR fusion protein is present in alveolar rhabdomyosarcoma and results from the t(2;13) (q35;q14) chromosomal translocation. Its oncogenic activity is dependent on a combination of protein–DNA and protein–protein interactions mediated by the Pax3 homeodomain recognition helix. In this report we demonstrate that human Daxx (hDaxx) interacts with Pax3 in vivo and with DNA‐bound Pax3 in vitro . This interaction is mediated primarily through the homeodomain recognition helix with the additional involvement of the octapeptide domain and its N‐terminal flanking amino acids. Through this interaction hDaxx represses the transcriptional activity of Pax3 by ∼80%. The Pax3–FKHR fusion is unresponsive to this repressive effect despite an observed endogenous interaction with hDaxx in a rhabdomyosarcoma tumor cell line. Therefore, these data support the model that fusion of FKHR to Pax3 not only adds a strong transactivation domain, but also deregulates transcriptional control of Pax3 by overriding the natural repressive effect of hDaxx.