Dynamic chromatin organization and regulatory interactions in human endothelial cell differentiation

Abstract

Background: Vascular endothelial cells are a mesoderm-derived lineage with many essential functions, including angiogenesis and coagulation. However, the gene regulatory mechanisms that underpin endothelial specialization are largely unknown, as are the roles of 3D chromatin organization in regulating endothelial cell transcription. Methods: To investigate the relationships between 3D chromatin organization and gene expression in endothelial cell differentiation, we induced endothelial cell differentiation from human pluripotent stem cells and performed Hi-C and RNA-seq assays at specific timepoints in differentiation. Results: Our analyses reveal that long-range intrachromosomal contacts increase over the course of endothelial cell differentiation, as do genomic compartment transitions between active and inactive states. These compartmental states are tightly associated with endothelial transcription. Dynamic topologically associating domain (TAD) boundaries strengthen and converge on an endothelial cell state, and nascent TAD boundaries are linked to the expression of genes that support endothelial cell specification. Relatedly, chromatin pairwise point interactions (DNA loops) increase in frequency during differentiation and are linked to the expression of genes with essential roles in vascular biology, including MECOM, TFPI, and KDR. To identify forms of regulation specific to endothelial cell differentiation, we compared the functional chromatin dynamics of endothelial cells with those of developing cardiomyocytes. Cardiomyocytes exhibit greater long-range cis interactions than endothelial cells, whereas endothelial cells have increased local intra-TAD interactions and much more abundant pairwise point interactions. Conclusions: Genome topology changes dynamically during endothelial differentiation, including acquisition of long-range cis interactions and new TAD boundaries, interconversion of hetero- and euchromatin, and formation of DNA loops. These chromatin dynamics guide transcription in the development of endothelial cells and promote the divergence of endothelial cells from related cell types such as cardiomyocytes.