HOXA9 promotes hematopoietic commitment of human embryonic stem cells

Abstract

The molecular determinants regulating the specification of human embryonic stem cells (hESCs) into hematopoietic cells remain elusive. HOXA9 plays a relevant role in leukemogenesis and hematopoiesis. It is highly expressed in hematopoietic stem and progenitor cells (HSPCs) and isdownregulated upon differentiation. Hoxa9-deficient mice display impaired hematopoietic development, and deregulation of HOXA9 expression is frequently associated with acute leukemia. Analysis of the genes differentially expressed in cord blood HSPCs vs hESC-derived HSPCs identified HOXA9 as the most down-regulated gene in hESC-derived HSPCs, suggesting that expression levels of HOXA9 may be crucial for hematopoietic differentiation of hESC. Here we show that during hematopoietic differentiation of hESCs, HOXA9 expression parallels hematopoietic development, but is restricted to the hemogenic precursors (HEP) (CD31+CD34+CD452), and diminishes as HEPs differentiate into blood cells (CD45+). Different gain-of-function andloss-of-functionstudiesrevealthatHOXA9enhanceshematopoieticdifferentiationof hESCs by specifically promoting the commitment of HEPs into primitive and total CD45+ blood cells. Geneexpression analysis suggests that nuclearfactor-kB signaling could be collaborating with HOXA9toincrease hematopoietic commitment. However, HOXA9onits ownisnot sufficienttoconferinvivo longterm engraftment potential to hESC-hematopoietic derivatives, reinforcing the idea that additional molecular regulators are needed for the generation of definitive in vivo functional HSPCs from hESC.