Developmentally regulated extended domains of DNA hypomethylation encompass highly transcribed genes of the human β-globin locus

Objective: DNA methylation has long been implicated in developmental β-globin gene regulation. However, the mechanism underlying this regulation is unclear, especially because these genes do not contain CpG islands. This has led us to propose and test the hypothesis that, just as for histone modifications, developmentally specific changes in human β-like globin gene expression are associated with long-range changes in DNA methylation. Materials and Methods: Bisulfite sequencing was used to determine the methylation state of individual CpG dinucleotides across the β-globin locus in uncultured primary human erythroblasts from fetal liver and bone marrow, and in primitive-like erythroid cells derived from human embryonic stem cells. Results: β-globin locus CpGs are generally highly methylated, but domains of DNA hypomethylation spanning thousands of base pairs are established around the most highly expressed genes during each developmental stage. These large domains of DNA hypomethylation are found within domains of histone modifications associated with gene expression. We also find hypomethylation of a small proportion of γ-globin promoters in adult erythroid cells, suggesting a mechanism by which adult erythroid cells produce fetal hemoglobin. Conclusion: This is one of the first reports to show that changes in DNA methylation patterns across large domains around non-CpG island genes correspond with changes in developmentally regulated histone modifications and gene expression. These data support a new model in which extended domains of DNA hypomethylation and active histone marks are coordinately established to achieve developmentally specific gene expression of non-CpG island genes.