Epigenetic and transcriptional analysis of mesoderm progenitor cells identifies HOPX as a novel regulator of hemogenic endothelium

We analyzed chromatin dynamics and transcriptional activity of human embryonic stem cell (hESC)-derived cardiac progenitor cells (CPCs) and KDR+/CD34+ endothelial cells generated from cardiogenic or hemogenic mesoderm. Using an unbiased algorithm to hierarchically rank genes modulated at the level of chromatin and transcription, we identified novel candidate regulators of mesodermal lineage determination. HOPX, a non-DNA binding homeodomain protein, was identified as a candidate regulator of blood-forming endothelial cells. We used HOPX reporter and knockout hESCs, as well as hopx loss of function studies in zebrafish, to show the requirement of HOPX in vivo and in vitro in hemato-endothelial lineage specification. Loss of HOPX does not impact endothelial fate specification but markedly reduces primitive hematopoiesis acting at least in part through suppression of Wnt/β-catenin signaling. Single cell RNA-seq data during mouse hematopoietic development in vivo confirm a role for HOPX in hematopoietic fate. Taken together, we show that HOPX is a novel regulator of hemato-endothelial fate specification in vitro and in vivo that functionally regulates Wnt signaling to modulate primitive hematopoiesis.