Comparative cis-regulatory analyses identify new elements of the mouse Hoxc8 early enhancer.

The Hoxc8 early enhancer is a 200 bp region that controls the early phase of Hoxc8 expression during mouse embryonic development. This enhancer defines the domain of Hoxc8 expression in the neural tube and mesoderm of the posterior regions of the developing embryo. Five distinct cis-acting elements, A-E, were previously shown to govern early phase Hoxc8 expression. Significant divergence between mammalian and fish Hoxc8 early enhancer sequences and activities suggested additional cis-acting elements. Here we describe four additional cis-acting elements (F-I) within the 200 bp Hoxc8 early enhancer region identified by comparative regulatory analysis and transgene-mutation studies. These elements affect posterior neural tube and mesoderm expression of the reporter gene, either singly or in combination. Surprisingly, these new elements are missing from the zebrafish and Fugu Hoxc8 early enhancer sequences. Considering that fish enhancers direct robust reporter expression in transgenic mouse embryos, it is tempting to postulate that fish and mammalian Hoxc8 early enhancers utilize different sets of elements to direct Hoxc8 early expression. These observations reveal a remarkable plasticity in the Hoxc8 early enhancer, suggesting different modes of initiation and establishment of Hoxc8 expression in different species. We postulate that extensive restructuring and remodeling of Hox cis-regulatory regions occurring in different taxa lead to relatively different Hox expression patterns, which in turn may act as a driving force in generating diverse axial morphologies.