de Bruijn Group: Developmental Haematopoiesis

MRC Molecular Haematology Unit MRC Weatherall Institute of Molecular Medicine Nuffield Division of Clinical Laboratory Sciences

We study the embryonic origins of blood stem cells with the aim to inform the generation of these cells in culture, and ultimate produce clinically relevant blood stem cells for therapeutic purposes.

Haematopoietic stem cells (HSCs) have been used in the clinic for several decades. Yet, our capacity to grow, expand and manipulate HSCs is still limited, which is holding back a more wider application of HSC-based therapies, for example in therapies aimed at correcting single gene haematopoietic disorders by genome editing. The Developmental Haematopoiesis Group aims to learn from the embryo how HSCs are specified and how their self-renewal and differentiation into functional end cells is controlled, and apply this knowledge for the purpose of regenerative medicine.

In the mouse embryo, definitive HSCs are generated in the dorsal aorta and vitelline and umbilical arteries from a specialised subset of endothelium, the haemogenic endothelium (HE). This endothelial-to-haematopoietic transition (EHT) involves the bending out and rounding up of endothelial cells and transition into haematopoietic cell clusters that protrude into the lumen of the arteries.

The transcription factor Runx1 is arguably the most critical regulator of EHT. In mice, homozygous loss of Runx1 results in embryonic lethality and a complete absence of definitive haematopoietic cells. Runx1 is critically required along the transition of HE to definitive haematopoietic stem and progenitor cells, and several of its known transcriptional targets act in this transition. However, the extent of the gene interaction network underlying EHT remains to be established.

To facilitate the identification and isolation of HE and cells undergoing EHT for mechanistic studies, we generated a Runx1 +23 enhancer-reporter model (23GFP). The 23GFP expression marks all HE and emerging haematopoietic stem and progenitor cells, similar to endogenous Runx1. Making use of this reporter in in vivo HSC development and ESC/iPSC differentiation cultures, our goal is to further the understanding of how HSCs are specified and born.

Our research focuses around three main lines: Elucidating the Runx1 cis-regulatory logic in EHT; Characterising the gene interaction network, cellular pathways and niche factors that underlie EHT; and Tracing the origin of HE back to gastrulation to begin to map the factors acting on the precursors of these cells. The insights obtained from these studies will inform protocols to generate definitive HE and blood cells from ESC/iPSC. In addition, RUNX1 mutations are involved in human leukaemia, and a deeper understanding of the role of Runx1 in the regulatory network underlying HSC emergence could also shed light the molecular mechanisms of leukaemia onset and progression.