Targeting gene expression to haemopoietic stem cells: a chromatin-dependent upstream element mediates cell type-specific expression of the stem cell antigen CD34.

The ability to target heterologous gene expression to haemopoietic stem cells will allow biological manipulation of this compartment and facilitate gene therapy of blood disorders. To identify regulatory elements with this potential, we have analysed the transcriptional regulation of the murine stem cell antigen CD34. Within haemopoiesis CD34 is expressed in stem and early progenitor cells, but it is also expressed in certain non-haemopoietic cell types, including fibroblasts. Comparison of CD34 chromatin in haemopoietic progenitor cells (416B and M1) and fibroblasts (Swiss 3T6) revealed several DNase I hypersensitive regions, one of which, a cluster of sites centred 3 kb upstream of exon 1, was specific to haemopoietic progenitors. This element stimulated expression by approximately two orders of magnitude in CD34+ haemopoietic progenitors, but not in CD34+ fibroblasts or in CD34- haemopoietic cells (18.8). Enhancer function was dependent upon chromosomal integration, although position-independent expression was not obtained. However, largely position-independent expression was conferred by addition of a downstream element which was hypersensitive in all the cell types analysed. We conclude that the murine CD34 gene is regulated by a chromatin-dependent, upstream enhancer element which acts in conjunction with a downstream domain-controlling element to confer high level gene expression in haemopoietic progenitor cells.