Key Points• IRP1 controls HIF2a mRNA translation in vivo and thereby acts as an upstream regulator of Epo expression. • IRP1 deficiency leads to age-dependent erythropoietic abnormalities and misregulation of body iron metabolism via the HIF2a/Epo pathway.Hypoxia inducible factor 2a (HIF2a) transcriptionally activates several genes in response to hypoxia. Under normoxic conditions, it undergoes oxygen-dependent degradation by the prolyl hydroxylase (PHD)/von Hippel-Lindau (VHL) system. The presence of an iron-responsive element (IRE) within the 59 untranslated region of HIF2a mRNA suggests a further iron-and oxygen-dependent mechanism for translational regulation of its expression via iron regulatory proteins 1 and 2 (IRP1 and IRP2, respectively). We show here that the disruption of mouse IRP1, but not IRP2, leads to profound HIF2a-dependent abnormalities in erythropoiesis and systemic iron metabolism. Thus, 4-to 6-week-old IRP1 2/2 mice exhibit splenomegaly and extramedullary hematopoiesis, which is corrected in older animals. These erythropoietic abnormalities are caused by translational de-repression of HIF2a mRNA and subsequent accumulation of HIF2a, which induces expression of erythropoietin (Epo). Increased levels of circulating Epo lead to reticulocytosis, polycythemia, and suppression of hepatic hepcidin mRNA. This in turn promotes hyperferremia and iron depletion in splenic macrophages due to unrestricted expression of ferroportin. Our data demonstrate that IRP1 is the principal regulator of HIF2a mRNA translation in vivo and provide evidence that translational control of HIF2a expression dominates over PHD/VHL-mediated regulation of HIF2a stability in juvenile IRP1 2/2 mice. (Blood. 2013;122(9):1658-1668