A SIGNALING CASCADE INVOLVING ENDOTHELIN-1, DHAND AND MSX1 REGULATES DEVELOPMENT OF NEURAL-CREST-DERIVED BRANCHIAL ARCH MESENCHYME

Numerous human syndromes are the result of abnormal cranial neural cre st development. One group of such defects, referred to as CATCH-22 (ca rdiac defects, abnormal facies, thymic hypoplasia, cleft palate, hypoc alcemia, associated with chromosome 22 microdeletion) syndrome, exhibi t craniofacial and cardiac defects resulting from abnormal development of the third and fourth neural crest-derived branchial arches and bra nchial arch arteries. Mice harboring a null mutation of the endothelin -1 gene (Edn1), which is expressed in the epithelial layer of the bran chial arches and encodes for the endothelin-1 (ET-I) signaling peptide , have a phenotype similar to CATCH-22 syndrome with aortic arch defec ts and craniofacial abnormalities, Here we show that the basic helix-l oop-helix transcription factor, dHAND, is expressed in the mesenchyme underlying the branchial arch epithelium. Further, dHAND and the relat ed gene, eHAND, are downregulated in the branchial and aortic arches o f Edn1-null embryos, In mice homozygous null for the dHAND) gene, the first and second arches are hypoplastic secondary to programmed cell d eath and the third and fourth arches fail to form. Molecular analysis revealed that most markers of the neural-crest-derived components of t he branchial arch are expressed in dHAND-null embryos, suggesting norm al migration of neural crest cells. However, expression of the homeobo x gene, Msx1, was undetectable in the mesenchyme of dHAND-null branchi al arches but unaffected in the limb bud, consistent with the separabl e regulatory elements of Msx1 previously described. Together, these da ta suggest a model in which epithelial secretion of ET-1 stimulates me senchymal expression of dHAND, which regulates Msx1 expression in the growing, distal branchial arch. Complete disruption of this molecular pathway results in growth failure of the branchial arches from apoptos is, while partial disruption leads to defects of branchial arch deriva tives, similar to those seen in CATCH-22 syndrome.