GENETIC-ANALYSIS OF A HOXD-12 REGULATORY ELEMENT REVEALS GLOBAL VERSUS LOCAL MODES OF CONTROLS IN THE HOXD COMPLEX

Vertebrate Herd genes are essential determinants of limb morphogenesis . In order to understand the genetic control of their complex expressi on patterns, we have used a combined approach involving interspecies s equence alignments in parallel with transgenic analyses, followed by i n vivo mutagenesis, Here, we report on the identification of a regulat ory element that is located in the vicinity of the Hoxd-12 gene, While this element is well conserved in tetrapods, little sequence similari ty was scored when compared to the cognate fish DNA, The regulatory po tential of this region XI (RXI) was first assayed in the context of a Hoxd-12/lacZ reporter transgene and shown to direct reporter gene expr ession in posterior limb buds. A deletion of this region was generated by targeted mutagenesis in ES cells and introduced into mice. Analyse s of animals homozygous for the HoxD(RXI) mutant allele revealed the f unction of this region in controlling Hoxd-12 expression in the presum ptive posterior zeugopod where it genetically interacts with Hoxa-11., Downregulation of Hoxd-12 expression was also detected in the trunk s uggesting that RXT may mediate a rather general function in the activa tion of Hoxd-12, These results support a model whereby global as well as local regulatory influences are necessary to build up the complex e xpression patterns of Herd genes during limb development.