3040 – RUNX1 CIS-REGULATION AND EFFECTOR FUNCTION DURING HUMAN ENDOTHELIAL-TO-HAEMATOPOIETIC TRANSITION

The first haematopoietic stem and progenitor cells (HSPCs) in the embryo arise through a process known as endothelial-to-haematopoietic transition (EHT). In a subset of endothelial cells referred to as haemogenic endothelium (HE), the endothelial transcriptional programme is gradually replaced by a haematopoietic one, promoting haematopoietic commitment and ultimately EHT. This process is critically dependent on the transcription factor RUNX1. There is currently limited knowledge on the transcriptional regulation and downstream function of RUNX1 during human EHT. Here, using an in vitro human induced pluripotent stem cell (hiPSC) differentiation model, we identified five candidate EHT RUNX1 enhancers, characterised by H3K27ac and open chromatin, one of which is only accessible in HE and four are accessible in haematopoietic cells. Through gene regulatory network (GRN) analysis, performed on joint single-cell chromatin accessibility and gene expression profiling data, we identified a set of candidate upstream RUNX1 activators and repressors. These included known RUNX1 regulators (e.g. GATA2, MEIS1, EPAS1) as well as potentially novel ones. To identify the downstream target genes of RUNX1, we profiled RUNX1-binding sites genome-wide in hiPSC-derived HE, where most of these sites were not acetylated and were associated with endothelial genes, suggesting RUNX1 might directly repress the endothelial programme. Together, our data are expected to improve our understanding of the regulatory mechanisms underlying human EHT.