Replicated anterior zeugopod (raz): a polydactylous mouse mutant with lowered Shh signaling in the limb bud.
2003
A unique limb phenotype is described in a radiation-induced mutant mouse
resulting from an inversion of a proximal segment of chromosome 5. The limb
phenotype in the homozygous mutant presents with two anterior skeletal
elements in the zeugopod but no posterior bone, hence the name replicated
anterior zeugopod, raz . The zeugopod phenotype is accompanied by
symmetrical central polydactyly of hand and foot. The chromosomal inversion
includes the Shh gene and the regulatory locus, located ∼1 Mb
away, within the Lmbr1 gene. In homozygous mutants, the expression of
Shh mRNA and Shh protein is severely downregulated to about 20% of
wild-type limb buds, but Shh expression appears normal throughout the
remainder of the embryo. Correspondingly, Gli3 expression is
upregulated and posteriorly expanded in the raz/raz limb bud. We
propose that the double anterior zeugopod and symmetrical central polydactyly
are due to an increased and uniform concentration of the Gli3 repressor form
because of lowered Shh signaling.
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