Transcriptional regulation of Hhex in hematopoiesis and hematopoietic stem cell ontogeny.

Hematopoietic stem cells (HSCs) emerge during development via an endothelial-to-hematopoietic transition from hemogenic endothelium of the dorsal aorta (DA). Using in situ hybridization and analysis of a knock-in RedStar reporter, we show that the transcriptional regulator Hhex is expressed in endothelium of the dorsal aorta (DA) and in clusters of putative HSCs as they are specified during murine development. We exploited this observation, using the Hhex locus to define cis regulatory elements, enhancers and interacting transcription factors that are both necessary and sufficient to support gene expression in the emerging HSC. We identify an evolutionarily conserved non-coding region (ECR) in the Hhex locus with the capacity to bind the hematopoietic-affiliated transcriptional regulators Gata2, SCL, Fli1, Pu.1 and Ets1/2. This region is sufficient to drive the expression of a transgenic GFP reporter in the DA endothelium and intra-aortic hematopoietic clusters. GFP-positive AGM cells co-expressed HSC-associated markers c-Kit, CD34, VE-Cadherin, and CD45, and were capable of multipotential differentiation and long term engraftment when transplanted into myelo-ablated recipients. The Hhex ECR was also sufficient to drive expression at additional blood sites including the yolk sac blood islands, fetal liver, vitelline and umbilical arteries and the adult bone marrow, suggesting a common mechanism for Hhex regulation throughout ontogenesis of the blood system. To explore the physiological requirement for the Hhex ECR region during hematoendothelial development, we deleted the ECR element from the endogenous locus in the context of a targeted Hhex-RedStar reporter allele. Results indicate a specific requirement for the ECR in blood-associated Hhex expression during development and further demonstrate a requirement for this region in the adult HSC compartment. Taken together, our results identified the ECR region as an enhancer both necessary and sufficient for gene expression in HSC development and homeostasis. The Hhex ECR thus appears to be a core node for the convergence of the transcription factor network that governs the emergence of HSCs.