Additional enhancer copies, with intact cdx binding sites, anteriorize Hoxa-7/lacZ expression in mouse embryos: evidence in keeping with an instructional cdx gradient.

Expression of a Hoxa-7/lacZ reporter construct in transgenic mouse embryos is shifted anteriorly when the upstream enhancer is multimerized. The shift occurs in spinal ganglia, neurectoderm and in both paraxial and lateral plate mesoderms. Much of the multimer effect is inhibited by destruction of a single caudal (cdx) binding motif in the additional copies of the enhancer. These observations are in agreement with earlier enhancer multimerization analyses made for Hoxb-8 (Charite et al., 1998). Our findings therefore provide further evidence that the anterior limit of a Hox gene's expression domain is normally dependent upon and is determined by, the dosage of transcription factor(s) which bind to its enhancer element(s) and that these factors may be, or must include, the cdx proteins. We consider these findings in terms of both instructional (morphogen-like) gradient and timing models for the establishment of Hox gene expression domains. Enhancer multimerization results in an earlier onset of Hoxa-7/lacZ activity in the embryo. In neurectoderm at 8.7 days and in mesoderm at 10.5 days, the anterior boundaries of expression are located posterior to those seen at some earlier stages of development. We discuss how these findings are in keeping with a model where Hox expression boundaries become set along instructional cdx gradients, formed by cdx decay in cells moving away from the primitive streak region.