HEMATOPOIESIS AND STEM CELLS Stem cell–specific epigenetic priming and B cell–specific transcriptional activation at the mouse Cd19 locus

Low-level expression of multiple lineagespecific genes is a hallmark of hematopoietic stem cells (HSCs). HSCs predominantly express genes specific for the myeloid or megakaryocytic-erythroid lineages, whereas the transcription of lymphoid specific genes appears to begin after lymphoid specification. It has been demonstrated for a number of genes that epigenetic priming occurs before gene expression and lineage specification; however, little is known about how epigenetic priming of lymphoid genes is regulated. To address the question of how B cell–restricted expression is established, we studied activation of the Cd19 gene during hematopoietic development. We identified a B cell–specific upstream enhancer and showed that the developmental regulation of Cd19 expression involves precisely coordinated alterations in transcription factor binding and chromatin remodeling at Cd19 cis-regulatory elements. In multipotent progenitor cells, Cd19 chromatin is first remodeled at the upstream enhancer, and this remodeling is associated with binding of E2A. This is followed by the binding of EBF and PAX5 during B-cell differentiation. The Cd19 promoter is transcriptionally activated only after PAX5 binding. Our experiments give important mechanistic insights into how widely expressed and B lineage– specific transcription factors cooperate to mediate the developmental regulation of lymphoid genes during hematopoiesis. (Blood. 2008;112:1673-1682)