Targeted deletion of 5 ' HS1 and 5 ' HS4 of the beta-globin locus control region reveals additive activity of the DNaseI hypersensitive sites

The mammalian beta -globin locus Is a multigenic, developmentally regulated , tissue-specific locus from which gene expression is regulated by a distal regulatory region, the locus control region (LCR). The functional mechanis m by which the beta -globin LCR stimulates transcription of the linked P-li ke globin genes remains unknown. The LCR Is composed of a series of 5 DNase l hypersensitive sites (5'HSs) that form in the nucleus of erythroid precur sors. These HSs are conserved among mammals, bind transcription factors tha t also bind to other parts of the locus, and compose the functional compone nts of the LCR. To test the hypothesis that individual HSs have unique prop erties, homologous recombination was used to construct 5 lines of mice with individual deletions of each of the 5 ' HSs of the endogenous murine beta -globin LCR. Here It Is reported that deletion of 5 ' HS1 reduces expressio n of the linked genes by up to 24%, while deletion of 5 ' HS4 leads to redu ctions of up to 27%. These deletions do not perturb the normal stage-specif ic expression of genes from this multigenic locus. In conjunction with prev ious studies of deletions of the other HSs and studies of deletion of the e ntire LCR, it is concluded that (1) none of the 5 ' HSs is essential for ne arly normal expression; (2) none of the HSs Is required for proper developm ental expression; and (3) the HSs do not appear to synergize either structu rally or functionally, but rather form independently and appear to contribu te additively to the overall expression from the locus.