Chromatin Structure and Control of β-Like Globin Gene Switching

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Abstract
The human β-globin locus is a complex genetic system widely used for analysis of eukaryotic gene expression. The locus consists of five functional β-like globin genes, ε, Gγ, Aγ, δ, and β, arrayed on the chromosome in the order that they are expressed during ontogeny. Globin gene expression is regulated, in part, by the locus control region, which physically consists of five DNasel-hypersensitive sites located 6-22 Kb upstream of the ε-globin gene. During ontogeny two switches occur in β-globin gene expression that reflect the changing oxygen requirements of the fetus. The first switch from embryonic ε- to fetal γ-globin occurs at six weeks of gestation. The second switch from γ- to adult δ- and β-globin occurs shortly after birth. Throughout the locus, cis-acting elements exist that are dynamically bound by trans-acting proteins, including transcription factors, co-activators, repressors, and chromatin modifiers. Discovery of novel erythroid-specific transcription factors and a role for chromatin structure in gene expression have enhanced our understanding of the mechanism of globin gene switching. However, the hierarchy of events regulating gene expression during development, from extracellular signaling to transcriptional activation or repression, is complex. In this review we attempt to unify the current knowledge regarding the interplay of cis-acting elements, transcription factors, and chromatin modifiers into a comprehensive overview of globin gene switching.