Abstract
The transcription factor Runx1 (AML1) is a central regulator of hematopoiesis and is required for the formation of definitive hematopoietic stem cells (HSCs). Runx1 is alternatively expressed from two promoters: the proximal (P2) prevails during primitive hematopoiesis, while the distal (P1) dominates in definitive HSCs. Although some transcription factor binding sites and cis-regulatory elements have been identified, a mechanistic explanation for the alternative promoter usage remains elusive. We investigated DNA methylation of known Runx1 cis-elements at stages of hematopoietic development in vivo and during differentiation of murine embryonic stem cells (ESCs) in vitro. In vivo, we find loss of methylation correlated with the primitive to definitive transition at the P1 promoter. In vitro, hypomethylation, acquisition of active chromatin modifications, and increased transcriptional activity at P1 are promoted by direct interaction with HOXB4, a transcription factor that confers definitive repopulation status on primitive hematopoietic progenitors. These data demonstrate a novel role for DNA methylation in the alternative promoter usage at the Runx1 locus and identify HOXB4 as a direct activator of the P1 promoter. This epigenetic signature should serve as a novel biomarker of HSC potential in vivo, and during ESC differentiation in vitro.