To explore the response of ? globin locus with established chromatin domains upon their exposure to new transcriptional environments, we transferred the chromatin-packaged ? globin locus of undifferentiated human embryonic stem cells (hESCs) or hESC-derived erythroblasts into an adult transcriptional environment. Distinct globin expression patterns were observed. In hESC-derived erythroblasts where both ? and ? globin were active and marked by similar chromatin modifications, ? globin was immediately silenced upon transfer, whereas ? globin continued to be expressed for months, implying that different transcriptional environments were required for their continuing expression. Whereas ? globin was silent both in hESCs and in hESC-derived erythroblasts, ? globin was only activated upon transfer from hESCs, but not in the presence of dominant ? globin transferred from hESC-derived erythroblasts, confirming the competing nature of ? versus ? globin expression. With time, however, silencing of ? globin occurred in the adult transcriptional environment with concurrent activation of ?-globin, accompanied by a drastic change in the epigenetic landscape of ? and ? globin gene regions without apparent changes in the transcriptional environment. This switching process could be manipulated by overexpression or downregulation of certain transcription factors. Our studies provide important insights into the interplay between the transcription environment and existing chromatin domains, and we offer an experimental system to study the time-dependent human globin switching.
The beta-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 beta-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.
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