MUTATIONS IN MOUSE ARISTALESS-LIKE4 CAUSE STRONGS LUXOID POLYDACTYLY

Mutations that affect vertebrate limb development provide insight into pattern formation, evolutionary biology and human birth defects. Patt erning of the limb axes depends on several interacting signaling cente rs; one of these, the zone of polarizing activity (ZPA), comprises a g roup of mesenchymal cells along the posterior aspect of the limb bud t hat express sonic hedgehog (Shh) and plays a key role in patterning th e anterior-posterior (AP) axis, The mechanisms by which the ZPA and Sh h expression are confined to the posterior aspect of the limb bud mese nchyme are not well understood. The polydactylous mouse mutant Strong' s luxoid (1st) exhibits an ectopic anterior ZPA and expression of Shh that results in the formation of extra anterior digits. Here we descri be a new chlorambucil-induced deletion allele, lst(Alb), that uncovers the Ist locus. Integration of the ist genetic and physical maps sugge sted the mouse Aristaless-like4 (Alx4) gene, which encodes a paired-ty pe homeodomain protein that plays a role in limb patterning, as a stro ng molecular candidate for the Strong's luxoid gene. In genetic crosse s, the three Ist mutant alleles fail to complement an Alx4 gene-target ed allele, Molecular and biochemical characterization of the three Ist alleles reveal mutations of the Alx4 gene that result in loss of func tion. Alx4 haploinsufficiency and the importance of strain-specific mo difiers leading to polydactyly are indicative of a critical threshold requirement for Alx4 in a genetic program operating to restrict polari zing activity and Shh expression in the anterior mesenchyme of the lim b bud, and suggest that mutations in Alx4 may also underlie human poly dactyly.