Divergent transcriptional activities determine limb identity

Limbs develop using a common genetic programme despite widely differing morphologies. This programme is modulated by limb-restricted regulators such as hindlimb (HL) transcription factors Pitx1 and Tbx4 and the forelimb (FL) Tbx5. Both Tbx factors have been implicated in limb patterning and growth, but their relative activities and underlying mechanisms remain unclear. In this paper, we show that Tbx4 and Tbx5 harbour conserved and divergent transcriptional regulatory domains that account for their roles in limb development. In particular, both factors share an activator domain and the ability to stimulate limb growth. However, we find that Tbx4 is the primary effector of HL identity for both skeletal and muscle development; this activity relies on a repressor domain that is inactivated by a human TBX4 small-patella syndrome mutation. We propose that limb identity is largely achieved by default in FL, whereas a specific repressor activity unique to Tbx4 determines HL identity. The embryonic development of forelimbs and hindlimbs is regulated by transcription factors, including Pitx1, Tbx4 and Tbx5. In this study, the contributions of Tbx4 and Tbx5 to limb identity are further delineated, resulting in the demonstration of a role for Tbx4 in hindlimb skeletal and muscle patterning.