Severe nasal clefting and abnormal embryonic apoptosis in Alx3/Alx4 doublemutant mice

A group of mouse aristaless-related genes has been implicated in functions in the development of the craniofacial skeleton. We have generated an AW mu tant allele in which the lacZ coding sequence is inserted in-frame in the A W gene and the sequences encoding the conserved protein domains are deleted . Mice homozygous for this null allele are indistinguishable from wild-type mice. Compound mutants of Alx3 and Alx4, however, show severe craniofacial abnormalities that are absent in Alx4 single mutants. Alx3/Alx4 double mut ant newborn mice have cleft nasal regions. Most facial bones and many other neural crest derived skull elements are malformed, truncated or even absen t. The craniofacial defects in Alx3/Alx4 double mutant embryos become anato mically manifest around embryonic day 10.5, when the nasal processes appear to be abnormally positioned. This most probably leads to a failure of the medial nasal processes to fuse in the facial midline and subsequently to th e split face phenotype. We detected a significant increase in apoptosis loc alised in the outgrowing frontonasal process in embryonic day 10.0 double m utant embryos, which we propose to be the underlying cause of the subsequen t malformations.