dHAND-Cre transgenic mice reveal specific potential functions of dHAND during craniofacial development.

Most of the bone, cartilage, and connective tissue of the craniofacial region arise from cephalic neural crest cells. Presumably, patterning differences in crest cells are a result of regional action of transcription factors within the developing pharyngeal arches. The basic helix-loop-helix transcription factor dHAND/HAND2 is expressed throughout much of the neural crest-derived mesenchyme of the pharyngeal arches, suggesting that it plays a crucial role in craniofacial development. However, targeted inactivation of the dHAND gene results in embryonic lethality by E10.5 due to vascular defects, preventing further analysis of the role of dHAND in cephalic neural crest cell development. In order to examine putative roles of dHAND during later stages of embryogenesis, we have used a transgenic lineage marker approach, in which a portion of the dHAND upstream region containing an enhancer that directs dHAND expression to the pharyngeal arches is used to drive Cre recombinase expression. By crossing these dHAND-Cre transgenic mice with R26R mice, we can follow the fate of cells that expressed dHAND at any time during development by examining beta-galactosidase activity. We show that dHAND is first expressed in postmigratory cephalic neural crest cells within the pharyngeal arches. In older embryos, beta-galactosidase-labeled cells are observed in most of the neural crest-derived lower jaw skeleton and surrounding connective tissues. However, labeled cells only contribute to substructures within the middle ear, indicating that our transgene is not globally expressed in cephalic neural crest cells within the pharyngeal arches. Moreover, dHAND-Cre mice will provide a valuable tool for tissue-specific inactivation of gene expression in multiple tissue types of neural crest origin.