Transcriptional interferences at the Hoxa4/Hoxa5 locus: importance of correct Hoxa5 expression for the proper specification of the axial skeleton.

We have previously described a Hoxa5 mutant mouse line in which specification of axial identity is perturbed and viability is markedly reduced. In the present study, we assay the Hoxa5 mutation in different genetic backgrounds and carry out a complete analysis of skeletal transformations. Although Hoxa5 is expressed over a large domain during embryogenesis, homeotic transformations of the axial skeleton are confined between cervical vertebra C3 and thoracic vertebra T2, which corresponds to the specific expression domain of the major Hoxa5 transcript. Loss of Hoxa5 function also affects the formation of the acromion in the appendicular skeleton. Disruption of the adjacent Hoxa4 gene leads to similar homeotic transformations of the cervicothoracic vertebrae. To discriminate the respective role of each gene, we generated transheterozygous animals carrying inactivated Hoxa4 and Hoxa5 alleles on different chromosomes. Compound heterozygous mutants exhibit homeotic transformations in the cervicothoracic transition region more reminiscent to those observed in Hoxa5 homozygous mutants. Although the Hoxa5 mutation does not significantly affect Hoxa4 expression, the pattern of Hoxa5 expression is impaired in cis by the Hoxa4 mutation, specifically in the cervicothoracic region of the prevertebral column. The expression of Hoxa5 in this particular domain is also perturbed by the Hoxa5 mutation itself, raising the possibility of regional autoregulation. Altogether, these results demonstrate the crucial role of Hoxa5 in the specification of the cervical and upper thoracic region of the skeleton and establish the importance of its correct expression for the proper patterning of the embryo.