Targeted deletion of 59HS1 and 59HS4 of the b-globin locus control region reveals additive activity of the DNaseI hypersensitive sites

The mammalian b-globin locus is a multigenic, developmentally regulated, tissuespecific locus from which gene expression is regulated by a distal regulatory region, the locus control region (LCR). The functional mechanism by which the b-globin LCR stimulates transcription of the linked b-like globin genes remains unknown. The LCR is composed of a series of 5 DNaseI hypersensitive sites (5*HSs) that form in the nucleus of erythroid precursors. These HSs are conserved among mammals, bind transcription factors that also bind to other parts of the locus, and compose the functional components of the LCR. To test the hypothesis that individual HSs have unique properties, homologous recombination was used to construct 5 lines of mice with individual deletions of each of the 5*HSs of the endogenous murine b-globin LCR. Here it is reported that deletion of 5*HS1 reduces expression of the linked genes by up to 24%, while deletion of 5*HS4 leads to reductions of up to 27%. These deletions do not perturb the normal stagespecific expression of genes from this multigenic locus. In conjunction with previous studies of deletions of the other HSs and studies of deletion of the entire LCR, it is concluded that (1) none of the 5*HSs is essential for nearly normal expression; (2) none of the HSs is required for proper developmental expression; and (3) the HSs do not appear to synergize either structurally or functionally, but rather form independently and appear to contribute additively to the overall expression from the locus. (Blood. 2001;98: 2022-2027)