Medial HOXA genes demarcate haematopoietic stem cell fate during human development.
Pluripotent stem cells (PSCs) may provide a potential source of blood forming stem cells for transplantation; however, unknown molecular barriers prevent the function of these cells. Using two-step differentiation, human embryonic stem cells (hESCs) differentiated in culture into multipotent blood-forming cells that looked like blood stem cells found during human development, but exhibited poor functionality. Gene expression analysis of culture-derived blood forming cells revealed that, despite their molecular resemblance to real blood stem cells, HOXA genes remained suppressed. Depletion of individual HOXA genes disrupted blood stem cell function and caused changes in gene expression that resembled those derived in culture. Increasing the level of HOXA genes in culture prolonged the maintenance of blood forming cells but was insufficient to confer full function. Stimulation of unique developmental signalling pathway during the development of blood forming cells from blood vessel wall induced the HOXA genes, and maintained blood forming cells longer in culture. Thus, HOXA gene marks the establishment of blood stem cell fate and controls their identity and function.
Pluripotent stem cells (PSCs) may provide a potential source of haematopoietic stem/progenitor cells (HSPCs) for transplantation; however, unknown molecular barriers prevent the self-renewal of PSC-HSPCs. Using two-step differentiation, human embryonic stem cells (hESCs) differentiated in vitro into multipotent haematopoietic cells that had the CD34(+)CD38(-/lo)CD90(+)CD45(+)GPI-80(+) fetal liver (FL) HSPC immunophenotype, but exhibited poor expansion potential and engraftment ability. Transcriptome analysis of immunophenotypic hESC-HSPCs revealed that, despite their molecular resemblance to FL-HSPCs, medial HOXA genes remained suppressed. Knockdown of HOXA7 disrupted FL-HSPC function and caused transcriptome dysregulation that resembled hESC-derived progenitors. Overexpression of medial HOXA genes prolonged FL-HSPC maintenance but was insufficient to confer self-renewal to hESC-HSPCs. Stimulation of retinoic acid signalling during endothelial-to-haematopoietic transition induced the HOXA cluster and other HSC/definitive haemogenic endothelium genes, and prolonged HSPC maintenance in culture. Thus, medial HOXA gene expression induced by retinoic acid signalling marks the establishment of the definitive HSPC fate and controls HSPC identity and function.