Comparative analysis of paired- and homeodomain-specific roles in PAX3-FKHR oncogenesis.

The alveolar rhabdomyosarcoma-associated t(2;13) chromosomal translocation produces an oncogenic fusion transcription factor PAX3-FKHR that combines the N-terminal DNA binding domains (paired domain and homeodomain) of PAX3 with the C-terminal activation domain of FKHR. In the context of PAX3-FKHR, the two DNA binding domains can work either cooperatively or autonomously in regulating gene transcription. The latter is a gain-of-function unique to the fusion protein. The biological activities driven by the individual DNA binding domain remains poorly defined. In this study, we express PAX3-FKHR mutants that contain only a single functional DNA binding domain into C2C12 myoblasts, and measured the in vitro and in vivo behaviors of these cells. We show that only the homeodomain-specific PAX3-FKHR mutant recapitulates the in vitro transformation properties of the wild type fusion protein. However, despite the differential responses in vitro, both the paired domain- and the homeodomain-specific PAX3-FKHR mutants promote tumor development from myoblasts in vivo. Our results suggest an important role for the gain of the paired domain- and the homeodomain-transcription activities in the PAX3-FKHR malignant transformation process.