The bHLH transcription factor dHAND controls Sonic hedgehog expression andestablishment of the zone of polarizing activity during limb development

Limb outgrowth and patterning of skeletal elements are dependent on complex tissue interactions involving the zone of polarizing activity (ZPA) in the posterior region of the limb bud and the apical ectodermal ridge. The pept ide morphogen Sonic hedgehog (SHH) is expressed specifically in the ZPA and , when expressed ectopically, is sufficient to mimic its functions, inducin g tissue growth and formation of posterior skeletal elements, We show that the basic helix-loop-helix transcription factor dHAND is expressed posterio rly in the developing limb prior to Shh and subsequently occupies a broad d omain that encompasses the Shh expression domain. In mouse embryos homozygo us for a dHAND null allele, limb buds are severely underdeveloped and Shh i s not expressed, Conversely, misexpression of dHAND in the anterior region of the limb bud of transgenic mice results in formation of an additional ZP A, revealed by ectopic expression of Shh and its target genes, and resultin g limb abnormalities that include preaxial polydactyly with duplication of posterior skeletal elements. Analysis of mouse mutants in which Hedgehog ex pression is altered also revealed a feedback mechanism in which Hedgehog si gnaling is required to maintain the full dHAND expression domain in the dev eloping limb. Together, these findings identify dHAND as an upstream activa tor of Shh expression and important transcriptional regulator of limb devel opment.