Genome-wide candidate HAND2 target regions in mouse embryonic tissues

The bHLH transcriptional regulator HAND2 performs essential functions during development of the embryonic limb, heart and branchial arches, however, its genomic target regions remained largely unkown. Here, we uncover the range of candidate HAND2 target cis-regulatory modules in the major Hand2 expressing embryonic tissues during midgestation and focus on the transcriptional networks controlled by HAND2 in the developing limb. Our functional and genetic analysis reveals that in the early limb mesenchyme HAND2 directly regulates the expression of several transcription factors involved in the morphogenesis of limb proximal skeletal elements. In addition, HAND2 operates as a key factor in the establishment of anterior and posterior compartments, partly through direct binding to a Gli3 cis-regulatory module. Furthermore, HAND2 refines the posterior boundary of Gli3 expression domain through a relay mechanism mediated by Tbx3. These HAND2-dependent regulatory interactions are required to set the boundaries defining the localized expression of the Shh organizer region, which allows progression of distal limb outgrowth. Embryonic tissues including limb buds, hearts and branchial arches from 150 Hand23xFLAG or wild-type control embryos (NMRI strain) were pooled and processed for ChIP-Seq.