An iron rheostat controls hematopoietic stem cell fate.

Mechanisms governing the maintenance of blood-producing hematopoietic stem and multipotent progenitor cells (HSPCs) are incompletely understood, particularly those regulating fate, ensuring long-term maintenance, and preventing aging-associated stem cell dysfunction. We uncovered a role for transitory free cytoplasmic iron as a rheostat for adult stem cell fate control. We found that HSPCs harbor comparatively small amounts of free iron and show the activation of a conserved molecular response to limited iron-particularly during mitosis. To study the functional and molecular consequences of iron restriction, we developed models allowing for transient iron bioavailability limitation and combined single-molecule RNA quantification, metabolomics, and single-cell transcriptomic analyses with functional studies. Our data reveal that the activation of the limited iron response triggers coordinated metabolic and epigenetic events, establishing stemness-conferring gene regulation. Notably, we find that aging-associated cytoplasmic iron loading reversibly attenuates iron-dependent cell fate control, explicating intervention strategies for dysfunctional aged stem cells.

PU.1-Dependent Enhancer Inhibition Separates Tet2-Deficient Hematopoiesis from Malignant Transformation.

Cytosine hypermethylation in and around DNA-binding sites of master transcription factors, including PU.1, occurs in aging hematopoietic stem cells following acquired loss-of-function mutations of DNA methyl-cytosine dioxygenase ten-eleven translocation-2 (TET2), albeit functional relevance has been unclear. We show that Tet2-deficient mouse hematopoietic stem and progenitor cells undergo malignant transformation upon compromised gene regulation through heterozygous deletion of an upstream regulatory region (UREΔ/WT) of the PU.1 gene. Although compatible with multilineage blood formation at young age, Tet2-deficient PU.1 UREΔ/WT mice develop highly penetrant, transplantable acute myeloid leukemia (AML) during aging. Leukemic stem and progenitor cells show hypermethylation at putative PU.1-binding sites, fail to activate myeloid enhancers, and are hallmarked by a signature of genes with impaired expression shared with human AML. Our study demonstrates that Tet2 and PU.1 jointly suppress leukemogenesis and uncovers a methylation-sensitive PU.1-dependent gene network as a unifying molecular vulnerability associated with AML. SIGNIFICANCE: We identify moderately impaired PU.1 mRNA expression as a biological modality predisposing Tet2-deficient hematopoietic stem and progenitor cells to malignant transformation. Our study furthermore uncovers a methylation-sensitive PU.1 gene network as a common feature of myeloid leukemia potentially allowing for the identification of patients at risk for malignant transformation. See related commentary by Schleicher and Pietras, p. 378. This article is highlighted in the In This Issue feature, p. 369.

Thrombopoietin receptor-independent stimulation of hematopoietic stem cells by eltrombopag.

Eltrombopag (EP), a small-molecule thrombopoietin receptor (TPO-R) agonist and potent intracellular iron chelator, has shown remarkable efficacy in stimulating sustained multilineage hematopoiesis in patients with bone marrow failure syndromes, suggesting an effect at the most immature hematopoietic stem and multipotent progenitor level. Although the functional and molecular effects of EP on megakaryopoiesis have been studied in the past, mechanistic insights into its effects on the earliest stages of hematopoiesis have been limited. We investigated the effects of EP treatment on hematopoietic stem cell (HSC) function using purified primary HSCs in separation-of-function mouse models, including a TPO-R-deficient strain, and stem cells isolated from patients undergoing TPO-R agonist treatment. Our mechanistic studies showed a stimulatory effect on stem cell self-renewal independently of TPO-R. Human and mouse HSCs responded to acute EP treatment with metabolic and gene expression alterations consistent with a reduction of intracellular labile iron pools that are essential for stem cell maintenance. Iron preloading prevented the stem cell stimulatory effects of EP. Moreover, comparative analysis of stem cells in the bone marrow of patients receiving EP showed a marked increase in the number of functional stem cells compared to patients undergoing therapy with romiplostim, another TPO-R agonist lacking an iron-chelating ability. Together, our study demonstrates that EP stimulates hematopoiesis at the stem cell level through iron chelation-mediated molecular reprogramming and indicates that labile iron pool-regulated pathways can modulate HSC function.