Targeting of human eNOS promoter to the Hprt locus of mice leads to tissue-restricted transgene expression

Phenotypic heterogeneity of the endothelium arises from cell type-specific differences in gene expression. An understanding of the mechanisms that und erlie differential gene expression would provide important insight into the molecular basis of vascular diversity. In standard transgenic assays, mult iple copies of heterologous DNA cassettes are randomly integrated into the mouse genome, resulting in significant line-to-line variation in expression . To overcome these limitations, we have targeted a single copy of a transg ene that contains 1,600 bp of the human endothelial nitric oxide synthase ( eNOS) promoter coupled to the LacZ reporter gene to the X-linked hypoxanthi ne phosphoribosyltransferase (Hprt) locus of mice by homologous recombinati on. The transgene was inserted in either of the orientations relative to th at of the Hprt gene. In mice derived from multiple embryonic stem (ES) cell clones, the expression pattern was limited to a subset of endothelial cell s, cardiomyocytes, and vascular smooth muscle cells. These findings suggest that Hprt locus targeting is a feasible tool for studying endothelial cell -restricted gene regulation.