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Bmi1 is essential for normal and leukemic hematopoiesis, but its target genes in hematopoietic stem cells (HSCs) are incompletely understood. In this issue of Genes & Development, Burgess et al.(pp. 887–900) demonstrate a novel role of Bmi1 in regulating ribosome biogenesis and protein synthesis. Bmi1-deficient HSCs exhibited reduced transplantability, with the up-regulation of ARX and genes involved in ribosome biogenesis. However, depletion of ARX or its known targets, p16 Ink4a/p19 Arf, only partially rescues Bmi1 loss-induced hematopoietic defects. They further demonstrate an increased protein synthesis rate and resultant proteostatic stress in Bmi1−/− HSCs, indicating a novel mechanism by which Bmi1 controls HSC maintenance.