Requirement for Pbx1 in skeletal patterning and programming chondrocyte proliferation and differentiation

Pbx1 and a subset of homeodomain proteins collaboratively bind DNA as highe r-order molecular complexes with unknown consequences for mammalian develop ment. Pbx1 contributions were investigated through characterization of Pbx1 -deficient mice. Pbx1 mutants died at embryonic day 15/16 with severe hypop lasia or aplasia of multiple organs and widespread patterning defects of th e axial and appendicular skeleton. An obligatory role for Pbx1 in limb axis patterning was apparent from malformations of proximal skeletal elements, but distal structures were unaffected. In addition to multiple rib and vert ebral malformations, neural crest cell-derived skeletal structures of the s econd branchial arch were morphologically transformed into elements reminis cent of first arch-derived cartilages. Although the skeletal malformations did not phenocopy single or compound Hox gene defects, they were restricted to domains specified by Hox proteins bearing Pbx dimerization motifs and u naccompanied by alterations in Hox gene expression. In affected domains of limbs and ribs, chondrocyte proliferation was markedly diminished and there was a notable increase of hypertrophic chondrocytes, accompanied by premat ure ossification of bone. The pattern of expression of genes known to regul ate chondrocyte differentiation was not perturbed in Pbx1-deficient cartila ge at early days of embryonic skeletogenesis, however precocious expression of Cof1a1, a marker of bone formation, was found. These studies demonstrat e a role for Pbx1 in multiple developmental programs and reveal a novel fun ction in co-ordinating the extent and/or timing of proliferation with termi nal differentiation. This impacts on the rate of endochondral ossification and bone formation and suggests a mechanistic basis for most of the observe d skeletal malformations.