MLL-AF9– and HOXA9-mediated acute myeloid leukemia stem cell self-renewal requires JMJD1C

Self-renewal is a hallmark of both hematopoietic stem cells (HSCs) and leukemia stem cells (LSCs); therefore, the identification of mechanisms that are required for LSC, but not HSC, function could provide therapeutic opportunities that are more effective and less toxic than current treatments. Here, we employed an in vivo shRNA screen and identified jumonji domain–containing protein JMJD1C as an important driver of MLL-AF9 leukemia. Using a conditional mouse model, we showed that loss of JMJD1C substantially decreased LSC frequency and caused differentiation of MLL-AF9– and homeobox A9–driven (HOXA9-driven) leukemias. We determined that JMJD1C directly interacts with HOXA9 and modulates a HOXA9-controlled gene-expression program. In contrast, loss of JMJD1C led to only minor defects in blood homeostasis and modest effects on HSC self-renewal. Together, these data establish JMJD1C as an important mediator of MLL-AF9– and HOXA9-driven LSC function that is largely dispensable for HSC function.     

ID2 and HIF-1α collaborate to protect quiescent hematopoietic stem cells from activation,differentiation, and exhaustion

Defining mechanism(s) that maintain tissue stem quiescence is important for improving tissue regeneration, cell therapies, aging, and cancer. We report here that genetic ablation of Id2 in adult hematopoietic stem cells (HSCs) promotes increased HSC activation and differentiation, which results in HSC exhaustion and bone marrow failure over time. Id2^Δ/Δ HSCs showed increased cycling, ROS production, mitochondrial activation, ATP production, and DNA damage compared with Id2^+/+ HSCs, supporting the conclusion that Id2^Δ/Δ HSCs are less quiescent. Mechanistically, HIF-1α expression was decreased in Id2^Δ/Δ HSCs, and stabilization of HIF-1α in Id2^Δ/Δ HSCs restored HSC quiescence and rescued HSC exhaustion. Inhibitor of DNA binding 2 (ID2) promoted HIF-1α expression by binding to the von Hippel-Lindau (VHL) protein and interfering with proteasomal degradation of HIF-1α. HIF-1α promoted Id2 expression and enforced a positive feedback loop between ID2 and HIF-1α to maintain HSC quiescence. Thus, sustained ID2 expression could protect HSCs during stress and improve HSC expansion for gene editing and cell therapies.     

Pivotal role for glycogen synthase kinase–3 in hematopoietic stem cell homeostasis in mice

Hematopoietic stem cell (HSC) homeostasis depends on the balance between self renewal and lineage commitment, but what regulates this decision is not well understood. Using loss-of-function approaches in mice, we found that glycogen synthase kinase–3 (Gsk3) plays a pivotal role in controlling the decision between self renewal and differentiation of HSCs. Disruption of Gsk3 in BM transiently expanded phenotypic HSCs in a β-catenin–dependent manner, consistent with a role for Wnt signaling in HSC homeostasis. However, in assays of long-term HSC function, disruption of Gsk3 progressively depleted HSCs through activation of mammalian target of rapamycin (mTOR). This long-term HSC depletion was prevented by mTOR inhibition and exacerbated by β-catenin knockout. Thus, GSK-3 regulated both Wnt and mTOR signaling in mouse HSCs, with these pathways promoting HSC self renewal and lineage commitment, respectively, such that inhibition of Gsk3 in the presence of rapamycin expanded the HSC pool in vivo. These findings identify unexpected functions for GSK-3 in mouse HSC homeostasis, suggest a therapeutic approach to expand HSCs in vivo using currently available medications that target GSK-3 and mTOR, and provide a compelling explanation for the clinically prevalent hematopoietic effects observed in individuals prescribed the GSK-3 inhibitor lithium.     

急性髓系白血病干细胞的研究进展

急性髓系白血病(AML)细胞群是由不同分化阶段的白血病细胞组成，其中最原始的细胞为白血病干细胞(leukemia stem cell,LSC)。虽然LSC所占比例极少，但仅其具有维持白血病细胞克隆的作用。AML的恶性转化发生在干细胞水平。这种发生恶性转化的干细胞的细胞和分子遗传学改变决定了白血病细胞克隆的分化特点，从而形成不同亚型的AML。LSC与正常遣血干细胞(hematopoietic stem cell，HSC)有许多相似之处，它具有自我更新能力和有限的分化潜能，同时又具有其自身独特的特征。LSC具有某些特殊的细胞表面标志，如CD90^-，CD117^-，CD123^ 。与正常的HSC相比，肿瘤抑制性蛋白-死亡相关蛋白激酶和干扰素调节因子1在LSC中高表达。与相对分化的白血病细胞相比，LSC主要处于G0期，其对常规化疗药物无效，因此LSC是白血病复发的根源。虽然LSC具有耐药的特点，但经合适的刺激如蛋白酶体抑制剂MG132的处理，LSC比正常HSC更易于凋亡。本将近年来对LSC的细胞和分子生物学方面研究的进展进行综述。为白血病发病机制及治疗策略的探讨提供新的思路。     

Protein tyrosine phosphatase–σ regulates hematopoietic stem cell–repopulating capacity

Hematopoietic stem cell (HSC) function is regulated by activation of receptor tyrosine kinases (RTKs). Receptor protein tyrosine phosphatases (PTPs) counterbalance RTK signaling; however, the functions of receptor PTPs in HSCs remain incompletely understood. We found that a receptor PTP, PTPσ, was substantially overexpressed in mouse and human HSCs compared with more mature hematopoietic cells. Competitive transplantation of bone marrow cells from PTPσ-deficient mice revealed that the loss of PTPσ substantially increased long-term HSC-repopulating capacity compared with BM cells from control mice. While HSCs from PTPσ-deficient mice had no apparent alterations in cell-cycle status, apoptosis, or homing capacity, these HSCs exhibited increased levels of activated RAC1, a RhoGTPase that regulates HSC engraftment capacity. shRNA-mediated silencing of PTPσ also increased activated RAC1 levels in wild-type HSCs. Functionally, PTPσ-deficient BM cells displayed increased cobblestone area–forming cell (CAFC) capacity and augmented transendothelial migration capacity, which was abrogated by RAC inhibition. Specific selection of human cord blood CD34⁺CD38^–CD45RA^–lin^– PTPσ^– cells substantially increased the repopulating capacity of human HSCs compared with CD34⁺CD38^–CD45RA^–lin^– cells and CD34⁺CD38^–CD45RA^–lin^–PTPσ⁺ cells. Our results demonstrate that PTPσ regulates HSC functional capacity via RAC1 inhibition and suggest that selecting for PTPσ-negative human HSCs may be an effective strategy for enriching human HSCs for transplantation.     

A role for GPx3 in activity of normal and leukemia stem cells

The determinants of normal and leukemic stem cell self-renewal remain poorly characterized. We report that expression of the reactive oxygen species (ROS) scavenger glutathione peroxidase 3 (GPx3) positively correlates with the frequency of leukemia stem cells (LSCs) in Hoxa9+Meis1-induced leukemias. Compared with a leukemia with a low frequency of LSCs, a leukemia with a high frequency of LSCs showed hypomethylation of the Gpx3 promoter region, and expressed high levels of Gpx3 and low levels of ROS. LSCs and normal hematopoietic stem cells (HSCs) engineered to express Gpx3 short hairpin RNA (shRNA) were much less competitive in vivo than control cells. However, progenitor cell proliferation and differentiation was not affected by Gpx3 shRNA. Consistent with this, HSCs overexpressing Gpx3 were significantly more competitive than control cells in long-term repopulation experiments, and overexpression of the self-renewal genes Prdm16 or Hoxb4 boosted Gpx3 expression. In human primary acute myeloid leukemia samples, GPX3 expression level directly correlated with adverse prognostic outcome, revealing a potential novel target for the eradication of LSCs.     

HOXB4基因在造血干细胞自我更新中的调控作用

造血干细胞（hematopoietic stem cell,HSC）具有高度自我更新能力和多向分化潜能,HSC的自我更新是由促进生长的正调控信号和导致凋亡的负调控信号之间的平衡来调控的。在正调控信号中,HOXB4通过激活不同的信号通路增强HSC的自我更新,同时又不影响维持正常稳态造血的调控机制。提高HOXB4的表达水平,能够极大地增强HSC的自我更新功能,但基本不影响细胞的分化、系特异性及终末细胞的形态和功能。不仅如此,HOXB4还可增强胚胎干细胞（embryonic stem cell,ESC）的造血潜能,促进ESC向造血细胞分化。因此,HOXB4和（或）HOXB4的靶基因的表达上调可能在干细胞移植和基因治疗等方面具有广阔的应用前景。本文就HOXB4基因参与调控造血干细胞的自我更新,HOXB4对HSC的分化特异性及终末分化的“零”效应及HOXB4调控HSC自我更新的分子机制等进行综述。     

Heterogeneous disease-propagating stem cells in juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML) is a poor-prognosis childhood leukemia usually caused by RAS-pathway mutations. The cellular hierarchy in JMML is poorly characterized, including the identity of leukemia stem cells (LSCs). FACS and single-cell RNA sequencing reveal marked heterogeneity of JMML hematopoietic stem/progenitor cells (HSPCs), including an aberrant Lin⁻CD34⁺CD38⁻CD90⁺CD45RA⁺ population. Single-cell HSPC index-sorting and clonogenic assays show that (1) all somatic mutations can be backtracked to the phenotypic HSC compartment, with RAS-pathway mutations as a “first hit,” (2) mutations are acquired with both linear and branching patterns of clonal evolution, and (3) mutant HSPCs are present after allogeneic HSC transplant before molecular/clinical evidence of relapse. Stem cell assays reveal interpatient heterogeneity of JMML LSCs, which are present in, but not confined to, the phenotypic HSC compartment. RNA sequencing of JMML LSC reveals up-regulation of stem cell and fetal genes (HLF, MEIS1, CNN3, VNN2, and HMGA2) and candidate therapeutic targets/biomarkers (MTOR, SLC2A1, and CD96), paving the way for LSC-directed disease monitoring and therapy in this disease.     

Sulfatase modifying factor 1–mediated fibroblast growth factor signaling primes hematopoietic multilineage development

Self-renewal and differentiation of hematopoietic stem cells (HSCs) are balanced by the concerted activities of the fibroblast growth factor (FGF), Wnt, and Notch pathways, which are tuned by enzyme-mediated remodeling of heparan sulfate proteoglycans (HSPGs). Sulfatase modifying factor 1 (SUMF1) activates the Sulf1 and Sulf2 sulfatases that remodel the HSPGs, and is mutated in patients with multiple sulfatase deficiency. Here, we show that the FGF signaling pathway is constitutively activated in Sumf1^−/− HSCs and hematopoietic stem progenitor cells (HSPCs). These cells show increased p-extracellular signal-regulated kinase levels, which in turn promote β-catenin accumulation. Constitutive activation of FGF signaling results in a block in erythroid differentiation at the chromatophilic erythroblast stage, and of B lymphocyte differentiation at the pro–B cell stage. A reduction in mature myeloid cells and an aberrant development of T lymphocytes are also seen. These defects are rescued in vivo by blocking the FGF pathway in Sumf1^−/− mice. Transplantation of Sumf1^−/− HSPCs into wild-type mice reconstituted the phenotype of the donors, suggesting a cell autonomous defect. These data indicate that Sumf1 controls HSPC differentiation and hematopoietic lineage development through FGF and Wnt signaling.To catalyze the hydrolysis of their natural substrates, sulfatases must be posttranslationally activated. A consensus sequence in their catalytic domain contains a cysteine that is modified into formylglycine by the formylglycine-generating enzyme encoded by the sulfatase modifying factor 1 (SUMF1) gene (Schmidt et al., 1995; Cosma et al., 2003; Dierks et al., 2003). SUMF1 exerts its activity within the ER; however, it can also be secreted and taken up by distant cells and tissues, where it relocalizes in the ER as an active enzyme (Zito et al., 2007). Multiple sulfatase deficiency (MSD) is a human monogenic disorder in which all of the sulfatase activities are simultaneously defective (Hopwood and Ballabio, 2001). Patients with MSD have mutations in SUMF1 (Cosma et al., 2004). A Sumf1^−/− strain has been generated as a mouse model of MSD, and it shows a complete loss of sulfatase activities, early mortality, congenital growth retardation, skeletal abnormalities, neurological defects, and a generalized inflammatory process in many organs (Settembre et al., 2007).These Sumf1^−/− mice represent an important resource to study developmental defects associated with SUMF1 lack of function. Indeed, SUMF1 also has putative activity during development specification. To date, 17 different sulfatases have been described in humans, and all are activated by SUMF1 (Sardiello et al., 2005). Among this large sulfatase family, Sulf1 and Sulf2 are localized on the cell surface and catalyze hydrolysis of the 6-O-sulfate of the N-acetyl glucosamines of heparan during degradation of heparan sulfate proteoglycans (HSPGs; Morimoto-Tomita et al., 2002). Wingless (Wg)/Wnt belongs to a family of secreted morphogenic proteins that control tissue-specific cell fate decisions during embryogenesis, and that bind to the heparan sulfate moieties on the cell surface of HSPGs (Logan and Nusse, 2004; Bejsovec, 2005). QSulf, the avian orthologue of human Sulf (hSulf), removes the sulfate from heparan sulfate, and releases Wnt from HSPGs. This released Wnt associates with Frizzled (Fz) and LRP5/6 receptors, resulting in inactivation of a multiprotein destruction complex, which is composed of glycogen synthase kinase-3 (GSK-3), Axin2, and adenomatous polyposis coli (APC). This inactivation leads to translocation of β-catenin into the nucleus, where it activates several target genes (Hoppler and Kavanagh, 2007). In addition to its role in embryogenesis, Wnt is involved in controlling proliferation of stem cells. Wnt3a belongs to the Wnt family, and has been shown to enhance self-renewal and maintain totipotency/multipotency of embryonic stem cells and hematopoietic stem cells (HSCs), respectively, through accumulation of β-catenin in the cell nucleus (Reya et al., 2003; Anton et al., 2007). Furthermore, transplantation of Wnt3a-treated BM increases the survival of lethally irradiated mice (Willert et al., 2003).HSCs reside in the BM and are a rare population of adult pluripotent stem cells that have the dual capability of self-renewal and differentiation into all blood cell lineages. Based on their ability to self-renew, HSCs can be defined as long-term (LT-HSCs) and short-term (ST-HSCs) HSCs. LT-HSCs have extensive self-renewal abilities and sustain life-long hematopoiesis, whereas ST-HSCs represent a more committed population with short self-renewal potential (Laiosa et al., 2006a). That Wnt signaling stimulates proliferation and self-renewal of HSCs was suggested >10 yr ago (Austin et al., 1997). More recently, it has been shown that exposure of HSCs to Wnt antagonists in vitro reduces their proliferation ability and that Bcl-2 transgenic HSCs transduced with retroviral vectors encoding β-catenin can fully reconstitute the hematopoietic compartments of recipients (Reya et al., 2003). On the other hand, stable expression of β-catenin from the Rosa26 locus in transgenic mice led to loss of HSC repopulation and multilineage differentiation potential (Kirstetter et al., 2006). Mice with β-catenin deletion in their HSCs show normal hematopoiesis (Cobas et al., 2004). These apparently contradictory data might reveal some novel aspects of the function of the Wnt signaling pathway. Indeed, it appears that different and specific amounts of β-catenin confer divergent phenotypes and differentiation capabilities to HSCs.In addition, a fine-tuned balance between different signaling pathways might be important to control self-renewal and differentiation of HSCs. The Notch and Wnt pathways have been shown to act in synergy to maintain the HSC pool, with Wnt being important to induce proliferation and support viability of HSCs, and Notch being important in the maintenance of HSCs in an undifferentiated state (Duncan et al., 2005). Furthermore, in vivo inhibition of GSK-3β augments the repopulation abilities of transplanted HSCs via modulation of gene targets of both the Wnt and Notch pathways (Trowbridge et al., 2006).Interestingly, the activities of Sulf1 and Sulf2 modulate Wnt signaling by modifying the sulfation state of the heparan sulfates contained in HSPGs, thereby impairing fibroblast growth factor (FGF) signaling. Crystal structure studies have demonstrated that binding of FGF1 and FGF2 to the FGF receptor is stabilized by 6-O-sulfation of the heparan sulfates of the HSPGs (Pellegrini et al., 2000). Thus, through desulfation of these heparan sulfates, hSulf1 can down-regulate FGF-dependent extracellular signal-regulated kinase (ERK) kinase activity (Lai et al., 2003). FGF1 and FGF2 signaling preserves ex vivo expansion of primitive HSCs (Yeoh et al., 2006).Thus, an intriguing question is raised: does Sumf1 act as a master regulator of the signaling of the Wnt and FGF pathways through activation of Sulf1 and/or Sulf2, resulting, in turn, in modulation of developmental signals and of HSC self-renewal and cell lineage commitment?Here, we show that SUMF1 has a role in promoting cell lineage commitment. Sumf1^−/− HSCs and hematopoietic stem progenitor cells (HSPCs) show constitutive activation of the FGF signaling pathway, and the consequent increase in p-ERK leads to GSK3-β phosphorylation and β-catenin accumulation. In turn, Notch is also accumulated. These altered signaling pathways lead to a block of erythroid, myeloid, and lymphoid differentiation in Sumf1^−/− mice. We also provide evidence that Sulf2^−/− mice recapitulates the Sumf1^−/− BM phenotype up to a certain point. In contrast, Ids^−/− and Sgsh^−/− mice, which are two mouse loss-of-function sulfatase models, did not show any relevant hematopoietic differentiation defects. Furthermore, upon transplantation of Sumf1^−/− HSPCs into lethally irradiated WT mice, there was impaired differentiation of the donor cells, which recapitulates the hematopoietic defects seen in the Sumf1^−/− mice; furthermore, in the recipient mice, a decrease in the frequency of LT-HSCs was observed. These findings confirm that the differentiation impairment of the mutant HSCs and their progeny is caused by SUMF1 loss of function, and not by different environmental features.     

Hematopoiesis and RAS-driven myeloid leukemia differentially require PI3K isoform p110α

The genes encoding RAS family members are frequently mutated in juvenile myelomonocytic leukemia (JMML) and acute myeloid leukemia (AML). RAS proteins are difficult to target pharmacologically; therefore, targeting the downstream PI3K and RAF/MEK/ERK pathways represents a promising approach to treat RAS-addicted tumors. The p110α isoform of PI3K (encoded by Pik3ca) is an essential effector of oncogenic KRAS in murine lung tumors, but it is unknown whether p110α contributes to leukemia. To specifically examine the role of p110α in murine hematopoiesis and in leukemia, we conditionally deleted p110α in HSCs using the Cre-loxP system. Postnatal deletion of p110α resulted in mild anemia without affecting HSC self-renewal; however, deletion of p110α in mice with KRAS^G12D-associated JMML markedly delayed their death. Furthermore, the p110α-selective inhibitor BYL719 inhibited growth factor–independent KRAS^G12D BM colony formation and sensitized cells to a low dose of the MEK inhibitor MEK162. Furthermore, combined inhibition of p110α and MEK effectively reduced proliferation of RAS-mutated AML cell lines and disease in an AML murine xenograft model. Together, our data indicate that RAS-mutated myeloid leukemias are dependent on the PI3K isoform p110α, and combined pharmacologic inhibition of p110α and MEK could be an effective therapeutic strategy for JMML and AML.     

Endothelium and NOTCH specify and amplify aorta-gonad-mesonephros–derived hematopoietic stem cells

Hematopoietic stem cells (HSCs) first emerge during embryonic development within vessels such as the dorsal aorta of the aorta-gonad-mesonephros (AGM) region, suggesting that signals from the vascular microenvironment are critical for HSC development. Here, we demonstrated that AGM-derived endothelial cells (ECs) engineered to constitutively express AKT (AGM AKT-ECs) can provide an in vitro niche that recapitulates embryonic HSC specification and amplification. Specifically, nonengrafting embryonic precursors, including the VE-cadherin–expressing population that lacks hematopoietic surface markers, cocultured with AGM AKT-ECs specified into long-term, adult-engrafting HSCs, establishing that a vascular niche is sufficient to induce the endothelial-to-HSC transition in vitro. Subsequent to hematopoietic induction, coculture with AGM AKT-ECs also substantially increased the numbers of HSCs derived from VE-cadherin⁺CD45⁺ AGM hematopoietic cells, consistent with a role in supporting further HSC maturation and self-renewal. We also identified conditions that included NOTCH activation with an immobilized NOTCH ligand that were sufficient to amplify AGM-derived HSCs following their specification in the absence of AGM AKT-ECs. Together, these studies begin to define the critical niche components and resident signals required for HSC induction and self-renewal ex vivo, and thus provide insight for development of defined in vitro systems targeted toward HSC generation for therapeutic applications.     

Self-renewal of multipotent long-term repopulating hematopoietic stem cells is negatively regulated by Fas and tumor necrosis factor receptor activation

Multipotent self-renewing hematopoietic stem cells (HSCs) are responsible for reconstitution of all blood cell lineages. Whereas growth stimulatory cytokines have been demonstrated to promote HSC self-renewal, the potential role of negative regulators remains elusive. Receptors for tumor necrosis factor (TNF) and Fas ligand have been implicated as regulators of steady-state hematopoiesis, and if overexpressed mediate bone marrow failure. However, it has been proposed that hematopoietic progenitors rather than stem cells might be targeted by Fas activation. Here, murine Lin(-)Sca1(+)c-kit(+) stem cells revealed little or no constitutive expression of Fas and failed to respond to an agonistic anti-Fas antibody. However, if induced to undergo self-renewal in the presence of TNF-alpha, the entire short and long-term repopulating HSC pool acquired Fas expression at high levels and concomitant activation of Fas suppressed in vitro growth of Lin(-)Sca1(+)c-kit(+) cells cultured at the single cell level. Moreover, Lin(-)Sca1(+)c-kit(+) stem cells undergoing self-renewal divisions in vitro were severely and irreversibly compromised in their short- and long-term multilineage reconstituting ability if activated by TNF-alpha or through Fas, providing the first evidence for negative regulators of HSC self-renewal.     

S6K1 regulates hematopoietic stem cell self-renewal and leukemia maintenance

Hyperactivation of the mTOR pathway impairs hematopoietic stem cell (HSC) functions and promotes leukemogenesis. mTORC1 and mTORC2 differentially control normal and leukemic stem cell functions. mTORC1 regulates p70 ribosomal protein S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E–binding (eIF4E-binding) protein 1 (4E-BP1), and mTORC2 modulates AKT activation. Given the extensive crosstalk that occurs between mTORC1 and mTORC2 signaling pathways, we assessed the role of the mTORC1 substrate S6K1 in the regulation of both normal HSC functions and in leukemogenesis driven by the mixed lineage leukemia (MLL) fusion oncogene MLL-AF9. We demonstrated that S6K1 deficiency impairs self-renewal of murine HSCs by reducing p21 expression. Loss of S6K1 also improved survival in mice transplanted with MLL-AF9–positive leukemic stem cells by modulating AKT and 4E-BP1 phosphorylation. Taken together, these results suggest that S6K1 acts through multiple targets of the mTOR pathway to promote self-renewal and leukemia progression. Given the recent interest in S6K1 as a potential therapeutic target in cancer, our results further support targeting this molecule as a potential strategy for treatment of myeloid malignancies.     

Niche displacement of human leukemic stem cells uniquely allows their competitive replacement with healthy HSPCs

Allogeneic hematopoietic stem cell (HSC) transplantation (HSCT) is currently the leading strategy to manage acute myeloid leukemia (AML). However, treatment-related morbidity limits the patient generalizability of HSCT use, and the survival of leukemic stem cells (LSCs) within protective areas of the bone marrow (BM) continues to lead to high relapse rates. Despite growing appreciation for the significance of the LSC microenvironment, it has remained unresolved whether LSCs preferentially situate within normal HSC niches or whether their niche requirements are more promiscuous. Here, we provide functional evidence that the spatial localization of phenotypically primitive human AML cells is restricted to niche elements shared with their normal counterparts, and that their intrinsic ability to initiate and retain occupancy of these niches can be rivaled by healthy hematopoietic stem and progenitor cells (HSPCs). When challenged in competitive BM repopulation assays, primary human leukemia-initiating cells (L-ICs) can be consistently outperformed by HSPCs for BM niche occupancy in a cell dose-dependent manner that ultimately compromises long-term L-IC renewal and subsequent leukemia-initiating capacity. The effectiveness of this approach could be demonstrated using cytokine-induced mobilization of established leukemia from the BM that facilitated the replacement of BM niches with transplanted HSPCs. These findings identify a functional vulnerability of primitive leukemia cells, and suggest that clinical development of these novel transplantation techniques should focus on the dissociation of L-IC–niche interactions to improve competitive replacement with healthy HSPCs during HSCT toward increased survival of patients.Acute myeloid leukemia (AML) is a hematological neoplasm with a hierarchical cellular structure that is reminiscent of the normal hematopoietic system (Lapidot et al., 1994; Bonnet and Dick, 1997; Hope et al., 2004). Leukemic stem cells (LSCs), which sit at the top of this hierarchy, are particularly resistant to conventional therapeutic measures, contributing to minimum residual disease and ultimately causing patient relapse (Guzman et al., 2002). More recent insights suggest that the BM microenvironment plays a fundamental role in sheltering LSCs (Konopleva et al., 2002) and specifying their self-renewal properties (Raaijmakers et al., 2010; Schepers et al., 2013; Kode et al., 2014). Therefore, niche-targeted consolidation treatment strategies represent a promising mechanism to effectively compromise LSC self-renewal and eliminate minimum residual disease in AML. To inform novel therapeutic efforts toward this goal, it is necessary to develop a thorough understanding of LSC niche characteristics, in relation to those of hematopoietic stem cells (HSCs).We have previously characterized geographical and molecular features that functionally define the HSC niche in vivo (Guezguez et al., 2013), and in this study we extend these observations by reporting that LSC-enriched populations share an equivalent spatial and functional distribution in BM. Critically, we show that hematopoietic stem and progenitor cells (HSPCs) can rival leukemia-initiating cells (L-ICs) to populate vacant sites within the BM, which has been described to contain a limited number of saturable niches (Colvin et al., 2004; Czechowicz et al., 2007). We further demonstrate that in the context of established leukemic disease, it is necessary to dissociate leukemia-niche interactions before HSC transplantation (HSCT), to achieve competitive healthy reconstitution at the expense of LSC self-renewal.