The T-box transcription factor Eomesodermin governs haemogenic competence of yolk sac mesodermal progenitors.

Extra-embryonic mesoderm (ExM)-composed of the earliest cells that traverse the primitive streak-gives rise to the endothelium as well as haematopoietic progenitors in the developing yolk sac. How a specific subset of ExM becomes committed to a haematopoietic fate remains unclear. Here we demonstrate using an embryonic stem cell model that transient expression of the T-box transcription factor Eomesodermin (Eomes) governs haemogenic competency of ExM. Eomes regulates the accessibility of enhancers that the transcription factor stem cell leukaemia (SCL) normally utilizes to specify primitive erythrocytes and is essential for the normal development of Runx1+ haemogenic endothelium. Single-cell RNA sequencing suggests that Eomes loss of function profoundly blocks the formation of blood progenitors but not specification of Flk-1+ haematoendothelial progenitors. Our findings place Eomes at the top of the transcriptional hierarchy regulating early blood formation and suggest that haemogenic competence is endowed earlier during embryonic development than was previously appreciated.We would like to acknowledge Michal Maj and Line Ericsen, and Kevin Clark in the flow cytometry facilities at the Dunn School and WIMM respectively for providing cell sorting services. The WIMM facility is supported by the MRC HIU; MRC MHU (MC_UU_12009); NIHR Oxford BRC and John Fell Fund (131/030 and 101/517), the EPA fund (CF182 and CF170) and by the WIMM Strategic Alliance awards G0902418 and MC_UU_12025. We thank Neil Ashley for his help on 10x sample preparation and sequencing. The WIMM Single Cell Core Facility was supported by the MRC MHU (MC_UU_12009), the Oxford Single Cell Biology Consortium (MR/M00919X/1) and the WT ISSF (097813/Z/11/B#) funding. The facility was supported by WIMM Strategic Alliance awards G0902418 and MC_UU_12025. We also thank the High-Throughput Genomics Group (Wellcome Trust (WT) Centre for Human Genetics, funded by WT 090532/Z/09/Z), for generating sequencing data. We thank Valerie Kouskoff for providing the iRunx1 ES cell line, Supat Thongjuea and Guanlin Wang for advice with the scRNA-Seq analysis, Joey Riepsaame for advice with CRISP-R experiments, and Doug Higgs, Hedia Chagraoui, Dominic Owens, Andrew Nelson and Arne Mould for helpful discussions. M.D.B and C.P are supported by programmes in the MRC Molecular Hematology Unit Core award (Grant number: MC_UU_12009/2 M.D.B. and MC_UU_12009/9 C.P.). L.G. was supported by a Clarendon PhD studentship and the MRC Molecular Haematology Unit. The work was supported by grants from the Wellcome Trust (214175/Z/18/Z E.J.R, 10281/Z/13/Z L.T.G.H). L.T.G.H was supported by a Clarendon Fund Scholarship and Trinity College Titley Scholarship. E.J.R. is a Wellcome Trust Principal Fellow