An SCL 3 ' enhancer targets developing endothelium together with embryonicand adult haematopoietic progenitors

The SCL gene encodes a basic helix-loop-helix transcription factor which is expressed in early haematopoietic progenitors throughout ontogeny and is e ssential for the normal development of blood and blood vessels. Transgenic studies have characterised spatially distinct 5' enhancers which direct lac Z expression to subdomains of the normal SCL expression pattern, but the sa me elements failed to produce appropriate haematopoietic expression, We now describe an SCL 3' enhancer with unique properties. It directed lacZ expre ssion in transgenic mice to extra-embryonic mesoderm and subsequently to bo th endothelial cells and to a subset of blood cells at multiple sites of em bryonic haematopoiesis including the yolk sac, para-aortic splanchnopleura and AGM region. The 3' enhancer also targeted expression to haematopoietic progenitors in both foetal liver and adult bone marrow Purified lacZ(+) cel ls were highly enriched for clonogenic myeloid and erythroid progenitors as well as day-12 spleen colony forming units (CFU-S), Within the total gated population from bone marrow 95% of the myeloid and 90% of the erythroid co lony-forming cells were contained in the lacZ(+) fraction, as were 98% of t he CFU-S, Activation of the enhancer did not require SCL protein. On the co ntrary, transgene expression in yolk sacs was markedly increased in an SCL- /- background, suggesting that SCL is subject to negative autoregulation. A lternatively the SCL-/- environment may alter differentiation of extra-embr yonic mesoderm and result in an increased number of cells capable of expres sing high levels of the transgene. Our data represents the first descriptio n of an enhancer that integrates information necessary for expression in de veloping endothelium and early haematopoietic progenitors at distinct times and sites throughout ontogeny, This enhancer provides a potent tool for th e manipulation of haematopoiesis and vasculogenesis in vivo.