Abstract
In vitro hematopoiesis systems can be used to define mechanisms for blood cell formation and function, produce cell therapeutics, and model blood cell contributions to systemic disease. Hematopoietic progenitor cell (HPC) production remains inefficient, precluded by knowledge gaps related to specification and morphogenesis of specialized hemogenic endothelial cells, which undergo an endothelial-to-hematopoietic transition (EHT) to form HPCs. We elected to define changes in gene expression and chromatin organization during HPC formation to reveal regulatory mechanisms. Using paired single cell RNA/ATAC sequencing together with Hi-C, we profiled cells before and after EHT. Pathway analysis and pseudotime inferences confirmed a continuum of stromal and endothelial cells undergoing development into HE cells and lineage-based HPCs in vitro. In these cell types, we characterize cis-regulatory elements and transcriptional regulatory activities that facilitate EHT and HPC homeostasis, including for SNAI1, SOX17, TGFβ, STAT4, as well as for GFI1b and KLF1 in megakaryocyte- and erythroid-biased progenitors, respectively. We then leveraged our insights into chromatin organization among in vitro-derived cells to assess enrichments corresponding to human trait variation reported in human genome wide association studies. HPCs revealed locus enrichment for quantitative blood traits and autoimmune disease predisposition, which were particularly enriched in myeloid- and lymphoid-biased populations. Stromal and endothelial cells from our in vitro cultures were specifically enriched for accessible chromatin at blood pressure loci. Our findings reveal genes and mechanisms governing in vitro hematopoietic development and blood cell-related disease pathology.