Targeted mutagenesis of the Hox D Locus: a genetic analysis of limb development in the mouse

The Hox Complex is a matrix of 38 transcription factor genes that play a role in the proper development of the vertebrate body plan by specifying regional information along the embryonic axes. The 5' Hox D genes are a subset of this complex that are strongly expressed in the limb buds during embryogenesis, suggesting that they influence the appendicular skeleton. A specific molecular combinatorial Hox code has been proposed as a model for the 5' Hox D genes in patterning the vertebrate limb. To test this model and resolve the role of these genes in development, a genetic analysis is performed by a mutational approach in the mouse. Specifically, the technique of gene targeting is employed to generate mice individually mutant for hoxd-11, hoxd-12, and hoxd-13. These strains have urogenital tract defects (often resulting in sterility), axial homeosis at the lumbar-sacral region, and malformations in the limb skeleton. The limb phenotypes, however, do not resemble any apparent homeotic transformations as predicted by the combinatorial codes but rather are the result of regional malformations in the shapes, length, and segmentation of bones. To extend these findings, the strains are crossed to each other and to other Hox-deficient mice to produce animals with various combinations of mutant alleles. The phenotypes in these animals are more dramatic demonstrating functional redundancy and genetic interaction, especially in the limb. The data allow for a new interpretation where the Hox D genes regulate cell proliferation in a proximodistal direction during growth of the limb bud. As such, these genes may not code for distinct positional information, per se, but rather contribute to the number of cells required for proper limb construction. The results are discussed in the context of three models for generating a limb pattern in vertebrates.