The 5′HS4 core element of the human β-globin locus control region is required for high-level globin gene expression in definitive but not in primitive erythropoiesis

To assess the contribution of DNase I-hypersensitive site 4 (HS4) of the β-globin locus control region (LCR) to overall LCR function we deleted a 280 bp fragment encompassing the core element of 5′HS4 from a 248 kb β-globin locus yeast artificial chromosome (β-YAC) and analyzed globin gene expression during development in β-YAC transgenic mice. Four transgenic lines were established; each contained at least one intact copy of the β-globin locus. The deletion of the 5′HS4 core element had no effect on globin gene expression during embryonic erythropoiesis. In contrast, deletion of the 5′HS4 core resulted in a significant decrease of γ and β-globin gene expression during definitive erythropoiesis in the fetal liver and a decrease of β-globin gene expression in adult blood. We conclude that the core element of 5′HS4 is required for globin gene expression only in definitive erythropoiesis. Absence of the core element of HS4 may limit the ability of the LCR to provide an open chromatin domain and/or enhance γ and β-globin gene expression in the adult erythroid cells.