Ascorbate deficiency increases quiescence and self-renewal in hematopoietic stem cells and multipotent progenitors.

Ascorbate (vitamin C) limits hematopoietic stem cell (HSC) function and suppresses leukemia development, partly by promoting the function of the Tet2 tumor suppressor. In humans, ascorbate is obtained from the diet while in mice it is synthesized in the liver. In this study, we show that deletion of the Slc23a2 ascorbate transporter from hematopoietic cells depleted ascorbate to undetectable levels in HSCs and MPPs without altering plasma ascorbate levels. Slc23a2 deficiency increased HSC reconstituting potential and self-renewal potential upon transplantation into irradiated mice. Slc23a2 deficiency also increased the reconstituting and self-renewal potentials of multipotent hematopoietic progenitors (MPPs), conferring the ability to long-term reconstitute irradiated mice. Slc23a2-deficient HSCs and MPPs divided much less frequently than control HSCs and MPPs. Increased self-renewal and reconstituting potential were observed particularly in quiescent Slc23a2-deficient HSCs and MPPs. The effect of Slc23a2 deficiency on MPP self-renewal was not mediated by reduced Tet2 function. Ascorbate thus regulates quiescence and restricts self-renewal potential in HSCs and MPPs such that ascorbate deficiency confers MPPs with long-term self-renewal potential.

Selective advantage of mutant stem cells in human clonal hematopoiesis is associated with attenuated response to inflammation and aging.

Clonal hematopoiesis (CH) arises when hematopoietic stem cells (HSCs) acquire mutations, most frequently in the DNMT3A and TET2 genes, conferring a competitive advantage through mechanisms that remain unclear. To gain insight into how CH mutations enable gradual clonal expansion, we used single-cell multi-omics with high-fidelity genotyping on human CH bone marrow (BM) samples. Most of the selective advantage of mutant cells occurs within HSCs. DNMT3A- and TET2-mutant clones expand further in early progenitors, while TET2 mutations accelerate myeloid maturation in a dose-dependent manner. Unexpectedly, both mutant and non-mutant HSCs from CH samples are enriched for inflammatory and aging transcriptomic signatures, compared with HSCs from non-CH samples, revealing a non-cell-autonomous effect. However, DNMT3A- and TET2-mutant HSCs have an attenuated inflammatory response relative to wild-type HSCs within the same sample. Our data support a model whereby CH clones are gradually selected because they are resistant to the deleterious impact of inflammation and aging.

Metformin reduces the clonal fitness of Dnmt3aR878H hematopoietic stem and progenitor cells by reversing their aberrant metabolic and epigenetic state.

Clonal hematopoiesis (CH) arises when a hematopoietic stem cell (HSC) acquires a mutation that confers a competitive advantage over wild-type (WT) HSCs, resulting in its clonal expansion. Individuals with CH are at an increased risk of developing hematologic neoplasms and a range of age-related inflammatory illnesses1-3. Therapeutic interventions that suppress the expansion of mutant HSCs have the potential to prevent these CH-related illnesses; however, such interventions have not yet been identified. The most common CH driver mutations are in the DNA methyltransferase 3 alpha (DNMT3A) gene with arginine 882 (R882) being a mutation hotspot. Here we show that murine hematopoietic stem and progenitor cells (HSPCs) carrying the Dnmt3aR878H/+ mutation, which is equivalent to human DNMT3AR882H/+, have increased mitochondrial respiration compared with WT cells and are dependent on this metabolic reprogramming for their competitive advantage. Importantly, treatment with metformin, an oral anti-diabetic drug with inhibitory activity against complex I in the electron transport chain (ETC), reduced the fitness of Dnmt3aR878H/+ HSCs. Through a multi-omics approach, we discovered that metformin acts by enhancing the methylation potential in Dnmt3aR878H/+ HSPCs and reversing their aberrant DNA CpG methylation and histone H3K27 trimethylation (H3K27me3) profiles. Metformin also reduced the fitness of human DNMT3AR882H HSPCs generated by prime editing. Our findings provide preclinical rationale for investigating metformin as a preventive intervention against illnesses associated with DNMT3AR882 mutation-driven CH in humans.

Transcriptional Activation of Regenerative Hematopoiesis via Vascular Niche Sensing.

Transition between activation and quiescence programs in hematopoietic stem and progenitor cells (HSC/HSPCs) is perceived to be governed intrinsically and by microenvironmental co-adaptation. However, HSC programs dictating both transition and adaptability, remain poorly defined. Single cell multiome analysis divulging differential transcriptional activity between distinct HSPC states, indicated for the exclusive absence of Fli-1 motif from quiescent HSCs. We reveal that Fli-1 activity is essential for HSCs during regenerative hematopoiesis. Fli-1 directs activation programs while manipulating cellular sensory and output machineries, enabling HSPCs co-adoptability with a stimulated vascular niche. During regenerative conditions, Fli-1 presets and enables propagation of niche-derived Notch1 signaling. Constitutively induced Notch1 signaling is sufficient to recuperate functional HSC impairments in the absence of Fli-1. Applying FLI-1 modified-mRNA transduction into lethargic adult human mobilized HSPCs, enables their vigorous niche-mediated expansion along with superior engraftment capacities. Thus, decryption of stem cell activation programs offers valuable insights for immune regenerative medicine.

Lipids and the cancer stemness regulatory system in acute myeloid leukemia.

Acute myeloid leukemia (AML) is a heterogeneous disease of impaired myeloid differentiation and a caricature of normal hematopoiesis. Leukemic stem cells (LSCs) are responsible for long-term clonal propagation in AML just as hematopoietic stem cells (HSCs) sustain lifelong hematopoiesis. LSCs are often resistant to standard chemotherapy and are responsible for clinical relapse. Although AML is highly heterogeneous, determinants of stemness are prognostic for AML patient survival and can predict AML drug sensitivity. Therefore, one way to overcome challenges preventing efficacious treatment outcomes is to target LSC stemness. Metabolomic and lipidomic studies of serum and cells from AML patients are emerging to complement genomic, transcriptomic, epigenetic, and proteomic data sets to characterize and stratify AML. Recent studies have shown the value of fractionating LSCs versus blasts when characterizing metabolic pathways and implicate the importance of lipid balance to LSCs function. As more extensive metabolic studies coupled to functional in vivo assays are conducted on highly purified HSCs, bulk AML, and LSCs, the similarities and differences in lipid homeostasis in stem-like versus more mature AML subtypes as well as from normal HSCs are emerging. Here, we discuss the latest findings from studies of lipid function in LSCs, with a focus on sphingolipids (SLs) as stemness/lineage fate mediators in AML, and the balance of fatty acid anabolism and catabolism fueling metabolic flexibility and drug resistance in AML. We also discuss how designing successful strategies to target lipid vulnerabilities and improve AML patient survival should take into consideration the hierarchical nature of AML.

Multiomic Profiling of Central Nervous System Leukemia Identifies mRNA Translation as a Therapeutic Target.

Central nervous system (CNS) dissemination of B-precursor acute lymphoblastic leukemia (B-ALL) has poor prognosis and remains a therapeutic challenge. Here we performed targeted DNA sequencing as well as transcriptional and proteomic profiling of paired leukemia-infiltrating cells in the bone marrow (BM) and CNS of xenografts. Genes governing mRNA translation were upregulated in CNS leukemia, and subclonal genetic profiling confirmed this in both BM-concordant and BM-discordant CNS mutational populations. CNS leukemia cells were exquisitely sensitive to the translation inhibitor omacetaxine mepesuccinate, which reduced xenograft leptomeningeal disease burden. Proteomics demonstrated greater abundance of secreted proteins in CNS-infiltrating cells, including complement component 3 (C3), and drug targeting of C3 influenced CNS disease in xenografts. CNS-infiltrating cells also exhibited selection for stemness traits and metabolic reprogramming. Overall, our study identifies targeting of mRNA translation as a potential therapeutic approach for B-ALL leptomeningeal disease. SIGNIFICANCE: Cancer metastases are often driven by distinct subclones with unique biological properties. Here we show that in B-ALL CNS disease, the leptomeningeal environment selects for cells with unique functional dependencies. Pharmacologic inhibition of mRNA translation signaling treats CNS disease and offers a new therapeutic approach for this condition.This article is highlighted in the In This Issue feature, p. 1.

SUMO wrestling cancer stem cells.

Sumoylation is a reversible post-translational modification implicated in cancer. In this issue of Cell Chemical Biology, Benoit et al. describe an inhibitor of sumoylation that results in anti-proliferative effects in cancer stem cell models via the sumoylation enzyme SAE2.

TFEB-mediated endolysosomal activity controls human hematopoietic stem cell fate.

It is critical to understand how human quiescent long-term hematopoietic stem cells (LT-HSCs) sense demand from daily and stress-mediated cues and then transition into bioenergetically active progeny to differentiate and meet these cellular needs. However, the demand-adapted regulatory circuits of these early steps of hematopoiesis are largely unknown. Here we show that lysosomes, sophisticated nutrient-sensing and signaling centers, are regulated dichotomously by transcription factor EB (TFEB) and MYC to balance catabolic and anabolic processes required for activating LT-HSCs and guiding their lineage fate. TFEB-mediated induction of the endolysosomal pathway causes membrane receptor degradation, limiting LT-HSC metabolic and mitogenic activation, promoting quiescence and self-renewal, and governing erythroid-myeloid commitment. In contrast, MYC engages biosynthetic processes while repressing lysosomal catabolism, driving LT-HSC activation. Our study identifies TFEB-mediated control of lysosomal activity as a central regulatory hub for proper and coordinated stem cell fate determination.

A latent subset of human hematopoietic stem cells resists regenerative stress to preserve stemness.

Continuous supply of immune cells throughout life relies on the delicate balance in the hematopoietic stem cell (HSC) pool between long-term maintenance and meeting the demands of both normal blood production and unexpected stress conditions. Here we identified distinct subsets of human long-term (LT)-HSCs that responded differently to regeneration-mediated stress: an immune checkpoint ligand CD112lo subset that exhibited a transient engraftment restraint (termed latency) before contributing to hematopoietic reconstitution and a primed CD112hi subset that responded rapidly. This functional heterogeneity and CD112 expression are regulated by INKA1 through direct interaction with PAK4 and SIRT1, inducing epigenetic changes and defining an alternative state of LT-HSC quiescence that serves to preserve self-renewal and regenerative capacity upon regeneration-mediated stress. Collectively, our data uncovered the molecular intricacies underlying HSC heterogeneity and self-renewal regulation and point to latency as an orchestrated physiological response that balances blood cell demands with preserving a stem cell reservoir.

Sphingosine-1-phosphate receptor 3 potentiates inflammatory programs in normal and leukemia stem cells to promote differentiation.

Acute myeloid leukemia (AML) is a caricature of normal hematopoiesis, driven from leukemia stem cells (LSC) that share some hematopoietic stem cell (HSC) programs including responsiveness to inflammatory signaling. Although inflammation dysregulates mature myeloid cells and influences stemness programs and lineage determination in HSC by activating stress myelopoiesis, such roles in LSC are poorly understood. Here, we show that S1PR3, a receptor for the bioactive lipid sphingosine-1-phosphate, is a central regulator which drives myeloid differentiation and activates inflammatory programs in both HSC and LSC. S1PR3-mediated inflammatory signatures varied in a continuum from primitive to mature myeloid states across AML patient cohorts, each with distinct phenotypic and clinical properties. S1PR3 was high in LSC and blasts of mature myeloid samples with linkages to chemosensitivity, while S1PR3 activation in primitive samples promoted LSC differentiation leading to eradication. Our studies open new avenues for therapeutic target identification specific for each AML subset.

The Transition from Quiescent to Activated States in Human Hematopoietic Stem Cells Is Governed by Dynamic 3D Genome Reorganization.

Lifelong blood production requires long-term hematopoietic stem cells (LT-HSCs), marked by stemness states involving quiescence and self-renewal, to transition into activated short-term HSCs (ST-HSCs) with reduced stemness. As few transcriptional changes underlie this transition, we used single-cell and bulk assay for transposase-accessible chromatin sequencing (ATAC-seq) on human HSCs and hematopoietic stem and progenitor cell (HSPC) subsets to uncover chromatin accessibility signatures, one including LT-HSCs (LT/HSPC signature) and another excluding LT-HSCs (activated HSPC [Act/HSPC] signature). These signatures inversely correlated during early hematopoietic commitment and differentiation. The Act/HSPC signature contains CCCTC-binding factor (CTCF) binding sites mediating 351 chromatin interactions engaged in ST-HSCs, but not LT-HSCs, enclosing multiple stemness pathway genes active in LT-HSCs and repressed in ST-HSCs. CTCF silencing derepressed stemness genes, restraining quiescent LT-HSCs from transitioning to activated ST-HSCs. Hence, 3D chromatin interactions centrally mediated by CTCF endow a gatekeeper function that governs the earliest fate transitions HSCs make by coordinating disparate stemness pathways linked to quiescence and self-renewal.

Human Aging Alters the Spatial Organization between CD34+ Hematopoietic Cells and Adipocytes in Bone Marrow.

Age-related clonal hematopoiesis is a major risk factor for myeloid malignancy and myeloid skewing is a hallmark of aging. However, while it is known that non-cell-autonomous components of the microenvironment can also influence this risk, there have been few studies of how the spatial architecture of human bone marrow (BM) changes with aging. Here, we show that BM adiposity increases with age, which correlates with increased density of maturing myeloid cells and CD34+ hematopoietic stem/progenitor cells (HSPCs) and an increased proportion of HSPCs adjacent to adipocytes. However, NGFR+ bone marrow stromal cell (NGFR+ BMSC) density and distance to HSPCs and vessels remained stable. Interestingly, we found that, upon aging, maturing myeloid cell density increases in hematopoietic areas surrounding adipocytes. We propose that increased adjacency to adipocytes in the BM microenvironment may influence myeloid skewing of aging HSPCs, contributing to age-related risk of myeloid malignancies.

Relapse-Fated Latent Diagnosis Subclones in Acute B Lineage Leukemia Are Drug Tolerant and Possess Distinct Metabolic Programs.

Disease recurrence causes significant mortality in B-progenitor acute lymphoblastic leukemia (B-ALL). Genomic analysis of matched diagnosis and relapse samples shows relapse often arising from minor diagnosis subclones. However, why therapy eradicates some subclones while others survive and progress to relapse remains obscure. Elucidation of mechanisms underlying these differing fates requires functional analysis of isolated subclones. Here, large-scale limiting dilution xenografting of diagnosis and relapse samples, combined with targeted sequencing, identified and isolated minor diagnosis subclones that initiate an evolutionary trajectory toward relapse [termed diagnosis Relapse Initiating clones (dRI)]. Compared with other diagnosis subclones, dRIs were drug-tolerant with distinct engraftment and metabolic properties. Transcriptionally, dRIs displayed enrichment for chromatin remodeling, mitochondrial metabolism, proteostasis programs, and an increase in stemness pathways. The isolation and characterization of dRI subclones reveals new avenues for eradicating dRI cells by targeting their distinct metabolic and transcriptional pathways before further evolution renders them fully therapy-resistant. SIGNIFICANCE: Isolation and characterization of subclones from diagnosis samples of patients with B-ALL who relapsed showed that relapse-fated subclones had increased drug tolerance and distinct metabolic and survival transcriptional programs compared with other diagnosis subclones. This study provides strategies to identify and target clinically relevant subclones before further evolution toward relapse.

Sphingolipid Modulation Activates Proteostasis Programs to Govern Human Hematopoietic Stem Cell Self-Renewal.

Cellular stress responses serve as crucial decision points balancing persistence or culling of hematopoietic stem cells (HSCs) for lifelong blood production. Although strong stressors cull HSCs, the linkage between stress programs and self-renewal properties that underlie human HSC maintenance remains unknown, particularly at quiescence exit when HSCs must also dynamically shift metabolic state. Here, we demonstrate distinct wiring of the sphingolipidome across the human hematopoietic hierarchy and find that genetic or pharmacologic modulation of the sphingolipid enzyme DEGS1 regulates lineage differentiation. Inhibition of DEGS1 in hematopoietic stem and progenitor cells during the transition from quiescence to cellular activation with N-(4-hydroxyphenyl) retinamide activates coordinated stress pathways that coalesce on endoplasmic reticulum stress and autophagy programs to maintain immunophenotypic and functional HSCs. Thus, our work identifies a linkage between sphingolipid metabolism, proteostatic quality control systems, and HSC self-renewal and provides therapeutic targets for improving HSC-based cellular therapeutics.

A stemness screen reveals C3orf54/INKA1 as a promoter of human leukemia stem cell latency.

There is a growing body of evidence that the molecular properties of leukemia stem cells (LSCs) are associated with clinical outcomes in acute myeloid leukemia (AML), and LSCs have been linked to therapy failure and relapse. Thus, a better understanding of the molecular mechanisms that contribute to the persistence and regenerative potential of LSCs is expected to result in the development of more effective therapies. We therefore interrogated functionally validated data sets of LSC-specific genes together with their known protein interactors and selected 64 candidates for a competitive in vivo gain-of-function screen to identify genes that enhanced stemness in human cord blood hematopoietic stem and progenitor cells. A consistent effect observed for the top hits was the ability to restrain early repopulation kinetics while preserving regenerative potential. Overexpression (OE) of the most promising candidate, the orphan gene C3orf54/INKA1, in a patient-derived AML model (8227) promoted the retention of LSCs in a primitive state manifested by relative expansion of CD34+ cells, accumulation of cells in G0, and reduced output of differentiated progeny. Despite delayed early repopulation, at later times, INKA1-OE resulted in the expansion of self-renewing LSCs. In contrast, INKA1 silencing in primary AML reduced regenerative potential. Mechanistically, our multidimensional confocal analysis found that INKA1 regulates G0 exit by interfering with nuclear localization of its target PAK4, with concomitant reduction of global H4K16ac levels. These data identify INKA1 as a novel regulator of LSC latency and reveal a link between the regulation of stem cell kinetics and pool size during regeneration.

Deletion of the MAD2L1 spindle assembly checkpoint gene is tolerated in mouse models of acute T-cell lymphoma and hepatocellular carcinoma.

Chromosome instability (CIN) is deleterious to normal cells because of the burden of aneuploidy. However, most human solid tumors have an abnormal karyotype implying that gain and loss of chromosomes by cancer cells confers a selective advantage. CIN can be induced in the mouse by inactivating the spindle assembly checkpoint. This is lethal in the germline but we show here that adult T cells and hepatocytes can survive conditional inactivation of the Mad2l1 SAC gene and resulting CIN. This causes rapid onset of acute lymphoblastic leukemia (T-ALL) and progressive development of hepatocellular carcinoma (HCC), both lethal diseases. The resulting DNA copy number variation and patterns of chromosome loss and gain are tumor-type specific, suggesting differential selective pressures on the two tumor cell types.

Chromosome instability induced by Mps1 and p53 mutation generates aggressive lymphomas exhibiting aneuploidy-induced stress.

Aneuploidy is a hallmark of human solid cancers that arises from errors in mitosis and results in gain and loss of oncogenes and tumor suppressors. Aneuploidy poses a growth disadvantage for cells grown in vitro, suggesting that cancer cells adapt to this burden. To understand better the consequences of aneuploidy in a rapidly proliferating adult tissue, we engineered a mouse in which chromosome instability was selectively induced in T cells. A flanked by Lox mutation was introduced into the monopolar spindle 1 (Mps1) spindle-assembly checkpoint gene so that Cre-mediated recombination would create a truncated protein (Mps1(DK)) that retained the kinase domain but lacked the kinetochore-binding domain and thereby weakened the checkpoint. In a sensitized p53(+/-) background we observed that Mps1(DK/DK) mice suffered from rapid-onset acute lymphoblastic lymphoma. The tumors were highly aneuploid and exhibited a metabolic burden similar to that previously characterized in aneuploid yeast and cultured cells. The tumors nonetheless grew rapidly and were lethal within 3-4 mo after birth.

Spindle checkpoint deficiency is tolerated by murine epidermal cells but not hair follicle stem cells.

The spindle assembly checkpoint (SAC) ensures correct chromosome segregation during mitosis by preventing aneuploidy, an event that is detrimental to the fitness and survival of normal cells but oncogenic in tumor cells. Deletion of SAC genes is incompatible with early mouse development, and RNAi-mediated depletion of SAC components in cultured cells results in rapid death. Here we describe the use of a conditional KO of mouse Mad2, an essential component of the SAC signaling cascade, as a means to selectively induce chromosome instability and aneuploidy in the epidermis of the skin. We observe that SAC inactivation is tolerated by interfollicular epidermal cells but results in depletion of hair follicle bulge stem cells. Eventually, a histologically normal epidermis develops within ∼1 mo after birth, albeit without any hair. Mad2-deficient cells in this epidermis exhibited abnormal transcription of metabolic genes, consistent with aneuploid cell state. Hair follicle bulge stem cells were completely absent, despite the continued presence of rudimentary hair follicles. These data demonstrate that different cell lineages within a single tissue respond differently to chromosome instability: some proliferating cell lineages can survive, but stem cells are highly sensitive.

The Lkb1 metabolic sensor maintains haematopoietic stem cell survival.

Haematopoietic stem cells (HSCs) can convert between growth states that have marked differences in bioenergetic needs. Although often quiescent in adults, these cells become proliferative upon physiological demand. Balancing HSC energetics in response to nutrient availability and growth state is poorly understood, yet essential for the dynamism of the haematopoietic system. Here we show that the Lkb1 tumour suppressor is critical for the maintenance of energy homeostasis in haematopoietic cells. Lkb1 inactivation in adult mice causes loss of HSC quiescence followed by rapid depletion of all haematopoietic subpopulations. Lkb1-deficient bone marrow cells exhibit mitochondrial defects, alterations in lipid and nucleotide metabolism, and depletion of cellular ATP. The haematopoietic effects are largely independent of Lkb1 regulation of AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) signalling. Instead, these data define a central role for Lkb1 in restricting HSC entry into cell cycle and in broadly maintaining energy homeostasis in haematopoietic cells through a novel metabolic checkpoint.