A MUTATIONAL ANALYSIS OF THE 5'-HOXD GENES - DISSECTION OF GENETIC INTERACTIONS DURING LIMB DEVELOPMENT IN THE MOUSE

Using gene targeting in mice, we have undertaken a systematic mutation al analysis of the homeobox-containing 5' HoxD genes, In particular, w e have characterized the limb defects observed in mice with independen t targeted disruptions of hoxd-12 and hoxd-13. Animals defective for h oxd-12 are viable, fertile, and appear outwardly normal yet have minor autopodal defects in the forelimb which include a reduction in the bo ne length of metacarpals and phalanges, and a malformation of the dist al carpal bone d4, The limb phenotypes observed in hoxd-13 mutant mice are more extensive, including strong reductions in length, complete a bsences, or improper segmentations of many metacarpal and phalangeal b ones, Additionally, the d4 carpal bone is not properly formed and ofte n produces an extra rudimentary digit, To examine the genetic interact ions between the 5' HoxD genes, we bred these mutant strains with each other and with our previously characterized horn-Il mouse to produce a series of trans-heterozygotes, Skeletal analyses of these mice revea l that these genes interact in the formation of the vertebrate limb, s ince the trans-heterozygotes display phenotypes not present in the ind ividual heterozygotes, including more severe carpal, metacarpal and ph alangeal defects, Some of these phenotypes appear to be accounted for by a delay in the ossification events in the autopod, which lead to ei ther the failure of fusion or the elimination of cartilaginous element s, Characteristically, these mutations Lead to the overall truncation of digits II and V on the forelimb, Additionally, some trans-animals s how the growth of an extra postaxial digit VI, which is composed of a bony element resembling a phalange. The results demonstrate that these genes interact in the formation of the limb, In addition to the previ ously characterized paralogous interactions, a multitude of interactio ns between Hox genes is used to finely sculpt the forelimb, The 5' Hox genes could therefore act as a major permissive genetic milieu that h as been exploited by evolutionary adaptation to form the tetrapod limb s.