Functional comparison of the Hoxa 4, Hoxa 10, and Hoxa 11 homeoboxes

A number of models attempt to explain the functional relationships of Hox genes. The functional equivalence model states that mammalian Hox-encoded proteins are largely functionally equivalent, and that Hox quantity is more important than Hox quality. In this report, we describe the results of two homeobox swaps. In one case, the homeobox of Hoxa 11 was replaced with that of the very closely related Hoxa 10. Developmental function was assayed by analyzing the phenotypes of all possible allele combinations, including the swapped allele, and null alleles for Hoxa 11 and Hoxd 11. This chimeric gene provided wild-type function in the development of the axial skeleton and male reproductive tract, but served as a hypomorph allele in the development of the appendicular skeleton, kidneys, and female reproductive tract. In the other case, the Hoxa 11 homeobox was replaced with that of the divergent Hoxa 4 gene. This chimeric gene provided near recessive null function in all tissues except the axial skeleton, which developed normally. These results demonstrate that even the most conserved regions of Hox genes, the homeoboxes, are not functionally interchangeable in the development of most tissues. In some cases, developmental function tracked with the homeobox, as previously seen in simpler organisms. Homeoboxes with more 5' cluster positions were generally dominant over more 3' homeoboxes, consistent with phenotypic suppression seen in Drosophila. Surprisingly, however, all Hox homeoboxes tested did appear functionally equivalent in the formation of the axial skeleton. The determination of segment identity is one of the most evolutionarily ancient functions of Hox genes. It is interesting that Hox homeoboxes are interchangeable in this process, but are functionally distinct in other aspects of development.