“Maturational” Globin Switching in Primary Primitive Erythroid Cells.

Mammals have two distinct erythroid lineages. The “definitive” erythroid lineage generates small, enucleated erythrocytes that constitute the predominant cell type in the fetal and postnatal circulation. It is preceded by the “primitive” erythroid lineage, which originates in the yolk sac and generates a semi-synchronous wave of large erythroblasts that terminally differentiate in the bloodstream. This feature provides a unique opportunity to investigate changes in gene expression during erythroid maturation. Here, we have examined expression of the various α- and β-globin genes in purified populations of primary primitive erythroid cells isolated from progressive developmental time-points of mouse embryogenesis. Our studies, using both in situ hybridization and quantitative PCR, indicate that βH1 is the predominant β-globin transcript in the early yolk sac. Thus, unlike the human, the murine β-globin genes are not up-regulated in the order of their chromosomal arrangement. As primitive erythroblasts mature from proerythroblasts to reticulocytes, they undergo a βH1- to εy-globin switch, up-regulate low levels of the adult β1- and β2-globins, and down-regulate ζ-globin. These changes in transcript levels correlate with changes in RNA polymerase II density at their promoters and transcribed regions as assayed by ChIP assays. Furthermore, we found that the εy- and βH1-globin genes in primitive erythroblasts reside within a single large hyperacetylated domain. Taken together, these results are consistent with the notion that this βH1- to εy-globin “maturational” switch is dynamically regulated at the transcriptional level. We conclude that the embryonic to adult globin switch that occurs during murine ontogeny is due not only to the sequential appearance of primitive and definitive erythroid lineages but also to striking changes in globin gene expression that occur as primitive erythroblasts mature in the bloodstream.