Role of the Dlx homeobox genes in proximodistal patterning of the branchial arches: mutations of Dlx-1, Dlx-2, and Dlx-1 and -2 alter morphogenesis of proximal skeletal and soft tissue structures derived from the first and second arches.

The Dlx homeobox gene family is expressed in a complex pattern within the embryonic craniofacial ectoderm and ectomesenchyme. A previous study established that Dlx-2 is essential for development of proximal regions of the murine first and second branchial arches. Here we describe the craniofacial phenotype of mice with mutations in Dlx-1 and Dlx-1 and -2. The skeletal and soft tissue analyses of mice with Dlx-1 and Dlx-1 and -2 mutations provide additional evidence that the Dlx genes regulate proximodistal patterning of the branchial arches. This analysis also elucidates distinct and overlapping roles for Dlx-1 and Dlx-2 in craniofacial development. Furthermore, mice lacking both Dlx-1 and -2 have unique abnormalities, including the absence of maxillary molars. Dlx-1 and -2 are expressed in the proximal and distal first and second arches, yet only the proximal regions are abnormal. The nested expression patterns of Dlx-1, -2, -3, -5, and -6 provide evidence for a model that predicts the region-specific requirements for each gene. Finally, the Dlx-2 and Dlx-1 and -2 mutants have ectopic skull components that resemble bones and cartilages found in phylogenetically more primitive vertebrates.