Polycomb group (PcG) proteins are essential regulators of stem cells. PcG and trithorax group proteins mark developmental regulator gene promoters with bivalent domains consisting of overlapping repressive and activating histone modifications to keep them poised for activation in embryonic stem cells. Bmi1, a component of PcG complexes, maintains the self-renewal capacity of adult stem cells, but its role in multipotency remains obscure. Here we show that Bmi1 is critical for multipotency of hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs). B cell lineage developmental regulator genes, Ebf1 and Pax5, appeared to be transcriptionally repressed by bivalent domains before lineage commitment. Loss of Bmi1 resulted in a resolution of bivalent domains at the Ebf1 and Pax5 loci, leading to their premature expression in HSC/MPPs accompanied by accelerated lymphoid specification and a marked reduction in HSC/MPPs. Thus, Bmi1 is required to reinforce bivalent domains at key developmental regulator gene loci to maintain lineage specification poised for activation in adult stem cells.
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