PAX3 and PAX7 exhibit conserved cis-acting transcription repression domains and utilize a common gain of function mechanism in alveolar rhabdomyosarcoma

The t(2;13) and t(1;13) translocations of alveolar rhabdomyosarcoma (ARMS) result in chimeric PAX3-FKHR or PAX7-FKHR transcription factors, respective ly, In each chimera, a PAX DNA-binding domain is fused to the C-terminal FK HR transactivation domain. Previously we demonstrated that PAX3-FKHR is mor e potent than PAX3 because the FKHR transactivation domain is resistant to repression mediated by the PAX3 N-terminus. Here me test the hypothesis tha t the cis-acting repression domain is a conserved feature of PAX3 and PAX7 and that PAX7-FKHR gains function similarly, Using PAX-specific DIVA-bindin g sites, we found that PAX7 was virtually inactive, while PAX7-FKHR exhibit ed activity 600-fold above background and was comparable to PAX3-FKHR, Dele tion analysis showed that the transactivation domains of PAX7 and PAX7-FKHR are each more potent than either full-length protein, and resistance to ci s-repression is responsible for the PAX7-FKHR gain of function. Further del etion mapping and domain swapping experiments with PAX3 and PAX7 showed tha t their transactivation domains exhibit subtle dose-dependent differences i n potency, likely due to regions of structural divergence; while their repr ession domains are structurally and functionally conserved. Thus, the data support the hypothesis and demonstrate that PAX3 and PAX7 utilize a common gain of function mechanism in ARMS.