2010 - IDENTIFYING NECESSARY CHROMATIN STATES AND TARGETS FOR HEMOGENIC SPECIFICATION AND REPROGRAMMING

Hematopoietic stem cells (HSCs) originate from a subset of endothelium in the embryo known as hemogenic endothelium. Hemogenic endothelium must undergo an endothelial to hematopoietic transition (EHT) to form HSCs. EHT requires the activity of the transcription factor (TF) RUNX1. Ectopic pan-endothelial expression of RUNX1 in mouse embryos is able to specify embryonic endothelium as hemogenic. However, as compared to embryonic endothelium, RUNX1 is not as effective at specifying fetal endothelium as hemogenic. To understand the difference in the competency of embryonic versus fetal endothelium to be specified as hemogenic, we examined differences in chromatin at these two developmental stages. ATAC-seq revealed that RUNX1 appears to be specifying embryonic endothelium through a canonical EHT program. To orchestrate this EHT transcriptional program,  RUNX1 participates with other hematopoietic TFs, such as FLI1, ETS1, SPI1, and GFI1. Following RUNX1 induction in embryonic endothelium, immune genes and motility related genes are upregulated. In fetal endothelium, however, RUNX1 induction results in a more complex picture. To determine if these differences were due to underlying chromatin differences between fetal and embryonic endothelium, we examined control embryonic and fetal endothelium using stochastic optical reconstruction microscopy (STORM). STORM imaging suggests that there are global differences in chromatin structure between fetal and embryonic endothelium. Fetal endothelium appears to have more densely packed chromatin that is located more peripherally than embryonic endothelium. Taken together, these data show that the reduced specification efficiency by RUNX1 observed between embryonic and fetal endothelium is due, at least in part, to chromatin structure.