The Higher Structure of Chromatin in the LCR of the β-Globin Locus Changes during Development

The β-globin locus control region (LCR) is able to enhance the expression of all globin genes throughout the course of development. However, the chromatin structure of the LCR at the different developmental stages is not well defined. We report DNase I and micrococcal nuclease hypersensitivity, chromatin immunoprecipitation analyses for histones H2A, H2B, H3, and H4, and 3C (chromatin conformation capture) assays of the normal and mutant β-globin loci, which demonstrate that nucleosomes at the DNase I hypersensitive sites of the LCR could be either depleted or retained depending on the stages of development. Furthermore, MNase sensitivity and 3C assays suggest that the LCR chromatin is more open in embryonic erythroblasts than in definitive erythroblasts at the primary- and secondary-structure levels; however, the LCR chromatin is packaged more tightly in embryonic erythroblasts than in definitive erythroblasts at the tertiary chromatin level. Our study provides the first evidence that the occupancy of nucleosomes at a DNase I hypersensitive site is a developmental stage-related event and that embryonic and adult cells possess distinct chromatin structures of the LCR.     

Nuclear factors regulating erythroid progenitor cell differentiation and gene expression

Nuclear factors are effector molecules that directly regulate proliferation and differentiation of progenitor cells. Gene targeting experiments in mice have demonstrated that several nuclear factors are essential at different stages of erythroid development including cell cycling factors (Rb), and both ubiquitous (c-myb) and erythroid-specific (GA TA-1) DNA binding factors. In addition, DNA binding factors are required to establish the DNase I hypersensitive sites in the human β-globin locus control region (LCR). By 'opening' chromatin in the β-globin locus early in development, the LCR permits correct temporal and spatial expression of the globin genes in the maturing cells of the erythroid lineage.