Hematopoietic Stem Cell Dysfunction Research Articles

SF3B1 K700E Alters the Splicing of Metabolism and Stress Response Genes, Attenuating the Effect of DNMT3A Mutations in MDS

Myelodysplastic syndromes (MDS) are clonal stem cell disorders associated with low blood counts and a high risk of leukemic transformation. Genomic sequencing studies have demonstrated that MDS develops due to the acquisition of somatic mutations in hematopoietic stem cells (HSCs). Splicing factor 3b subunit 1 ( SF3B1) mutations, one of the most frequent mutations in MDS, are associated with lower-risk disease characterized by ineffective erythropoiesis. Our group and others have shown that mutations in SF3B1 cause aberrant mRNA splicing in human and murine HSCs, an expansion of long-term HSCs (LT-HSCs) in primary mice, but a competitive disadvantage in transplantation assays. DNMT3A, de novo methyltransferase 3A, is an epigenetic regulator mutated in ~10-15% of MDS. DNMT3A mutations are associated with unfavorable outcomes in MDS including a higher risk of leukemia transformation and shorter overall survival. Murine models of Dnmt3a mutations revealed that Dnmt3a-mutant HSCs have increased self-renewal over lineage-specific differentiation. SF3B1 and DNMT3A mutations co-occur more frequently than expected by chance and arise early in MDS pathogenesis, making them attractive therapeutic targets. MDS patients with SF3B1 and DNMT3A mutations have shorter progression-free survival than those with solely SF3B1 mutations. However, the mechanisms by which these mutations cooperate to cause HSC dysfunction and their possible combined role in leukemogenesis are not well understood. To investigate how Dnmt3a and Sf3b1 mutations disrupt HSC function in vivo we crossed two conditional knock-in mouse models of common point mutations in SF3B1 (K700E) and DNMT3A (R787H, murine equivalent to human DNMT3A R882H). We followed monthly blood counts and evaluated stem and progenitor cell composition in the bone marrow at 6 months. We found that Sf3b1/Dnmt3a double mutant mice display a similar phenotype to Sf3b1K700E mice, with a progressive anemia and an expansion in LT-HSCs. However, when assayed for HSC self-renewal potential, whole bone marrow and LT-HSCs isolated from Sf3b1/Dnmt3a mutant mice display increased serial colony formation capacities similar to Dnmt3a mutant hematopoietic stem and progenitor cells (HSPCs, 5 to 6 passages), but have a competitive disadvantage in secondary and tertiary transplant assays that is less severe than Sf3b1K700E HSPCs but distinct from the competitive advantage of Dnmt3a mutant HSPCs. Transcriptionally, double mutant HSCs clustered closer to Sf3b1 HSCs with shared expression of 74% of upregulated genes and 64% of downregulated genes and have decreased expression of stress response and metabolism genes. These results indicate Sf3b1 mutations may mitigate the preleukemic phenotype due to Dnmt3a mutations in MDS. To identify factors regulated by SF3B1 mutations that may mitigate the effect of DNMT3A loss on leukemic progression, we generated isogenic DNMT3A KO and SF3B1 K700E single and double mutant K562 cell lines using CRISPR/Cas9. Transcriptome analysis demonstrated that SF3B1K700E/DNMT3AKO double mutant clones clustered with and shared similar upregulated (69%) and downregulated (62%) genes as SF3B1 single mutant clones. Similar to our findings in murine HSPCs, pathway analysis of SF3B1 and SF3B1/DNMT3A mutant clones displayed enrichment of stress response and metabolic pathways. Alternative mRNA splicing analysis identified 13 events (3 alternative 3' splice site, 9 intron retention, and 1 skipped exon) in 8 genes that were upregulated in DNMT3A KO clones and downregulated in SF3B1 and SF3B1/DNMT3A mutant clones. We confirmed decreased protein expression of these mis-spliced genes in SF3B1 single and double mutant clones, but not DNMT3A KO clones. These genes include components of the integrated stress response pathway, which are also mis-spliced in SF3B1 and SF3B1/DNMT3A mutant MDS patient samples. Taken together our data suggest that aberrant splicing by mutant SF3B1 prevents metabolic reprogramming and decreases leukemic progression caused by DNMT3A mutation in MDS.

Reduction in Lymphoid-Supporting Factors By Mesenchymal Stromal Cells Drives Myeloid-Biased Hematopoiesis at Middle Age

The bone marrow “niche” for hematopoietic stem cells (HSCs) encompasses various cell types, extracellular matrices and secreted factors that maintain the specialized functions of HSCs such as quiescence, self-renewal, and production of multipotent progenitor cells. In old age, changes in this niche contribute to dysfunction of HSCs and the hematopoietic system including increased inflammation and myeloid-biased molecular programs. We have observed that HSC and hematopoietic aging phenotypes become apparent by mid-age (Young et al. Cell Stem Cell 2021, Young et al. J Exp Med 2016), leading to the hypothesis that evaluating alterations in the niche at this stage would reveal the earliest changes initiating HSC aging. To test this hypothesis, we examined both the hematopoietic and non-hematopoietic compartments in individual middle-aged (12-14 months of age, n = 9) and compared them to young C57BL/6 mice (2-4 months of age, n = 5). Immunostaining of femurs showed an increase in sinusoid volume, vessel dysmorphia, and an increase in megakaryocyte abundance in the bone marrow of middle-aged mice. No alterations were observed in arterioles nor adipocyte volume. This data provides supporting evidence that a subset of HSC niche remodeling phenotypes observed in old age are present by mid-age. We then performed single-cell RNA-seq to comprehensively assess changes in abundance and transcriptomes of hematopoietic and non-hematopoietic cells at mid-age. Analysis of this data revealed significant expansion of HSCs in middle-aged mice. In the expanded HSC population, a reduction in B lymphoid differentiation signatures was observed without alterations in inflammation or myeloid differentiation signatures. Computational predictions suggested that the reduced B lymphopoiesis program in middle-aged HSCs resulted from decreased interaction with soluble factors produced by subsets of mesenchymal stromal cells (MSCs), rather than endothelial cells or other hematopoietic cell compartments. Specifically, reduced MSC-to-HSC interactions were linked to a decline in signaling mediated by adiponectin (ADIPOQ; Adipoq), stem cell factor (SCF; Kitl), midkine (MDK; Mdk), and insulin-like growth factor 1 (IGF1; Igf1). Among these, reductions in Kitl and Igf1 in MSCs were correlated with the loss of B lymphopoiesis programs in HSCs comparing individual middle-aged mice. To test the functional impact of the decline in Igf1 in MSCs on lymphopoiesis, we created three MSC-selective Igf1 conditional knockout mouse models using distinct Cre recombinases (Lepr-Cre, nestin-CreER and Prx1-CreER). In all three of these models, MSC-selective knockout of Igf1 reduced B lymphopoiesis resulting in myeloid-biased hematopoiesis, replicating hematopoietic aging phenotypes observed at middle age. These findings indicate that the decline in B lymphopoiesis from HSCs during aging is initiated by a reduction in lymphoid-supporting factors produced by MSCs in the bone marrow microenvironment, and this occurs independently of inflammation or activation of myeloid-promoting transcriptional programs in HSCs.

Hematopoietic Stem Cells from Mice and Individuals with Sickle Cell Disease Display Premature Senescence and Loss of Function That Is Targetable By Senolytic Therapy

Sickle cell disease (SCD) is an inherited anemia caused by mutations in the β-globin gene, resulting in aberrant hemoglobin polymerization driving hemolysis, vasooclusion, and systemic inflammation. Allogeneic hematopoietic stem cell (HSC) transplantation is potentially curative, and novel autologous therapies involving gene-editing of mobilized HSCs are being developed. However, many SCD patients are unable to undergo autologous gene-editing due to failure to mobilize sufficient numbers of HSCs. Further, the damaging effects of SCD on the HSC pool remain uncharacterized. Therefore, we interrogated young and aged cohorts of humanized SCD mice and non-SCD controls for HSC frequency and function. Immunophenotypic HSC frequency was mildly elevated (1.3-fold, p=0.00316) in 2-month-old SCD mice, relative to controls (n=5-6/group). In contrast, HSC numbers were halved in 6 and 12-month-old SCD mice (n=8-10/group, p=0.00358 and p=0.0148, respectively). Competitive transplantation of whole bone marrow (WBM) from 2 and 6-month-old SCD mice displayed a profound loss of peripheral blood repopulation, relative to age-matched control mice. Quantitative limiting dilution transplantation of WBM from 2 and 6-month-old SCD mice revealed a 4 and 6-fold loss of functional HSCs, relative to control mice. Thus, despite elevated numbers of phenotypic HSCs in young SCD mice, the HSC pool is depleted of functional repopulating cells during SCD. To understand the loss of functional HSCs, we performed bulk RNA-sequencing of purified HSCs from 6-month-old SCD mice. We observed downregulation of 37 genes encoding ribosomal and histone proteins, suggesting impaired histone/ribosome biogenesis, a phenotype associated with senescence. Consistently, pathway enrichment analysis also revealed perturbed p53 activity and a high degree of association with published molecular signatures of senescence. To explore this further, HSCs isolated from 6-month-old SCD mice or age-matched controls were interrogated for senescence associated β-galactosidase (SA-β-gal) activity. On average 19.5% of SCD HSCs were SA-β-gal+ versus only 7.06% of HSCs isolated from controls (n=10-11/group, p=0.004). HSCs isolated from SCD mice also displayed elevated ROS, DNA damage, and expression of molecular markers of senescence (e.g. p21, BCLXL). Importantly, 23.14 ± 5.61% of Lin-CD34+CD38- bone marrow (BM) hematopoietic stem and progenitor cells (HSPCs) isolated from pediatric individuals with SCD were SA-β-gal+. In contrast, only 4.97 ± 2.97% of HSPCs were SA-β-gal+ in age-matched, non-SCD control individuals (n=8 control and n=5 SCD individuals, p=<0.0001). Further, 47.46 ± 17.49% of Lin-CD34+CD38-CD90+CD45RA- BM cells, which are highly enriched for human HSCs, were SA-β-gal+ in young individuals with SCD, relative to only 4.41 ± 2.37% in the BM of age-matched control individuals (n=4 control and n=5 SCD individuals, p=0.0019). We also detected elevated expression of p16, p21, and BCL2 in BM HSPCs isolated from pediatric individuals with SCD, relative to controls (n=6 control and n=9 SCD individuals). In total, these data reveal that SCD pathophysiology aberrantly invokes senescence programs and damages HSC function in mice and humans. Clearance of senescent HSCs from mouse BM has previously been shown to restore aging-related HSC dysfunction. We therefore tested if clearance of senescent HSCs from the BM could restore function to the SCD HSC pool. 2-month-old SCD and control mice were treated with two weekly cycles of 75 mg/kg/day of the senolytic, ABT-263, via oral gavage. 14 days after treatment, WBM was isolated and tested for HSC function and frequency. Senolytic therapy significantly increased HSC numbers in SCD mice BM, relative to vehicle-treated controls (n=5/group). Importantly, the in vivo hematopoietic repopulating activity of WBM from ABT-263-treated SCD mice was comparable to non-SCD controls, revealing a correction of our previously observed severe transplantation defect (n=12/group). In sum, these data reveal premature onset of senescence as a previously unappreciated phenotype of the HSC pool during SCD. This work provides the first proof-of-principle that treatment of individuals with SCD using senolytics may restore function to the HSC pool, and ongoing preclinical work aims to test if senolytic therapy enhances the yield and function of SCD HSCs mobilized for autologous therapy.

Reduced Polycomb Repressor Complex 2 (PRC2) Activity and Increased TP53 Activity Mediate Hematopoietic Stem Cell Dysfunction in RPS19-Mutated Diamond-Blackfan Anemia

Diamond-Blackfan anemia (DBA) is a congenital hypoplastic anemia caused by heterozygous loss-of-function mutations in one of 23 ribosomal protein (RP) genes, with RPS19 being mutated in approximately 25% of patients. While DBA typically manifests in infancy as isolated anemia with reticulocytopenia, older patients can develop multilineage cytopenias and bone marrow (BM) hypocellularity suggesting impaired hematopoietic stem cell (HSC) function. Defects in HSCs have been observed in Rps19-depleted mice but studies on human DBA HSCs have been inconclusive, largely because patient BM cells are difficult to obtain for research. To address this gap, we developed a new, robust, and scalable cellular model using CRISPR/Cas9 to create heterozygous RPS19 loss-of-function mutations in healthy donor CD34 + hematopoietic stem and progenitor cells (HSPCs). Transplantation of RPS19+/- HSPCs into NBSGW immunodeficient mice revealed a defect in BM repopulation that was partially rescued by co-disruption of the TP53 gene (Bhoopalan et al, JCI Insight, 2023). Here we provide further mechanistic insight into this HSC defect by showing that RP imbalance caused by RPS19 deficiency leads to the activation of TP53 and inhibition of Polycomb Repressor Complex 2 (PRC2), which has been previously shown to be required for HSC maintenance. Healthy donor CD34 + HSPC were electroporated with ribonucleoprotein (RNP) complex consisting of Cas9 and guide RNAs (gRNAs) targeting RPS19 or the AAVS1 safe harbor locus as a negative control. RPS19 disruption was verified by next-generation sequencing of the targeted region and by Western blot analysis showing reduction of RPS19 protein. RNA-Seq at 3 days after electroporation showed enrichment of TP53 pathway genes in RPS19-edited HSPCs compared to control cells, confirming our previous observations. Additionally, we noted an enrichment of HSPC genes that were previously shown to harbor the repressive histone mark H3K27me3 in normal HSPCs. Methylation of H3K27 is catalyzed by EZH2, the catalytic component of PRC2, which maintains HSCs by inhibiting transcription of genes that promote differentiation. Gene set enrichment analysis (GSEA) revealed significant induction of PRC2 target genes in RPS19-disrupted HSPCs and in induced pluripotent stem cells (iPSCs) generated from two different DBA patients with heterozygous RPS19 mutations. The “Cleavage under targets and release using nuclease (CUT&RUN)” assay indicated globally reduced EZH2 binding to chromatin in RPS19-disrupted HSPCs. These findings suggest that PRC2 activity is inhibited by RPS19 haploinsufficiency. Previous studies have shown that RPS19 haploinsufficiency can cause accumulation of large RP subunits, RPL5 and RPL11, which bind 5S ribosomal RNA to form 5S RNP. This complex can sequester the ubiquitin ligase MDM2 to inhibit its function. MDM2 ubiquitinates TP53, targeting it for proteasomal degradation. Additionally, MDM2 binds PRC2 to augment its activity. Thus, we theorized that RPS19 haploinsufficiency leads to the accumulation of 5S RNP complex which sequesters MDM2, causing increased TP53 activity and suppression of PRC2 activity, both of which can inhibit HSC maintenance. To test this, we inhibited formation of 5S RNP complex by co-disrupting RPL5 or RPL11 along with RPS19 in CD34 + HSPCs, followed by RNA-Seq analysis or transplantation into NBSGW mice. Remarkably, haploinsufficiency of either large RP gene rescued the BM repopulation defect of RPS19 +/- HSPC (p<0.05) ( Fig. 1A), reduced TP53 activity and improved PRC2 activity (p<0.001 and FDR q<0.01) ( Fig. 1B). In summary, our data support the model that RPS19 haploinsufficiency leads to increased TP53 activity and reduced PRC2 activity through 5S RNP-mediated sequestration of MDM2, causing impairment of HSC function. These studies provide insights into the molecular mechanisms of DBA and could potentially inform new therapies.

Scurvy: A treatable forgotten fatal differential diagnosis and potential etiology of leukemia and aplastic anemia in pediatric population.

Scurvy is a rare nutritional deficiency disease which is less likely to be suspected and it mostly lead to delayed diagnosis. It can present with features which can mislead clinicians to misdiagnose the condition as leukemia or aplastic anemia. This can subject patients to the wrong management which leads to poor outcome and increased preventable morbidity and mortality. Vitamin C deficiency is still prevalent among pediatric population even in the modern days and should no longer be considered as historical condition. Chromosomal fragility has been greatly accounted for the development of leukemia and aplastic anemia secondary to various triggers. The role of vitamin C toward DNA stability, prevention, and control of mutations have been documented. Vitamin C plays a vital role in hematopoiesis by controlling regulation and prevent dysfunction of hematopoietic stem cells. Scurvy deficiency has been a silent growing clinical problem which needs a high index of suspicion for a clinician to pick it. It should be considered as one among potential differential diagnosis of leukemia and aplastic anemia especially in the pediatric population. History of any dietary restriction should be obtained and addressed properly. Serum vitamin C should be among the essential laboratory workout in diagnosis of both leukemia and aplastic anemia. All patients suspected to have such conditions should be screened and supplemented for vitamin C deficiency irrespective of positive confirmatory test results of leukemia or aplastic anemia since the probability of co-occurrence is likely also. Moreover, studies should be conducted to explore the clinical link, if any, between vitamin C deficiency or insufficiency and development of leukemia and aplastic anemia among the pediatric population given its physiological and genomic role in hematopoiesis. Furthermore, the potential pharmacological therapeutic use of vitamin C in treatment of leukemia and aplastic anemia should be determined clinically.

The essential roles of small non-coding RNAs and RNA modifications in normal and malignant hematopoiesis.

Hematopoietic stem cells (HSCs) developing from mesoderm during embryogenesis are important for the blood circulatory system and immune system. Many factors such as genetic factors, chemical exposure, physical radiation, and viral infection, can lead to the dysfunction of HSCs. Hematological malignancies (involving leukemia, lymphoma, and myeloma) were diagnosed in more than 1.3 million people globally in 2021, taking up 7% of total newly-diagnosed cancer patients. Although many treatments like chemotherapy, bone marrow transplantation, and stem cell transplantation have been applied in clinical therapeutics, the average 5-year survival rate for leukemia, lymphoma, and myeloma is about 65%, 72%, and 54% respectively. Small non-coding RNAs play key roles in a variety of biological processes, including cell division and proliferation, immunological response and cell death. With the development of technologies in high-throughput sequencing and bioinformatic analysis, there is emerging research about modifications on small non-coding RNAs, as well as their functions in hematopoiesis and related diseases. In this study, we summarize the updated information of small non-coding RNAs and RNA modifications in normal and malignant hematopoiesis, which sheds lights into the future application of HSCs into the treatment of blood diseases.

Abstract B010: Determining the role of BRD9 in human hematopoietic stem cell aging

Abstract Aging is the number one non-modifiable risk factor for cancer, including myeloid malignancies, such as Acute Myeloid leukemia (AML). Intrinsic changes within hematopoietic stem cells (HSC) with aging contribute to HSC dysfunction and may predispose to leukemia. We have previously shown that aged human HSC present a reduction of H3K27ac at active promoters and enhancers of genes associated with immune and cancer pathways. Concurrent with this epigenetic deregulation there is deregulation of immune-associated genes and downregulation of the bromodomain containing protein-9 (BRD9). This protein is a component of the non-canonical BAF (nc-BAF) chromatin remodeling complex, which binds to both active enhancers and promoters. Notably alteration of BRD9 has been shown to perturb immune and cancer associated pathways, with BRD9 regulating interferon-stimulated genes. However, little is known about the role of this protein in human hematopoiesis. Therefore, we hypothesize that chromatin remodeling due to loss of BRD9 with HSC aging leads to reduction of H3K27ac at active enhancers and promoters, contributing to HSC dysfunction. To investigate this, we treated human CD34+ hematopoietic stem and progenitor cells (HSPC) with dBRD9-A (100 nM) a chemical degrader of BRD9. Loss of BRD9 resulted in enhanced myeloid differentiation in vitro with a concurrent block in erythroid differentiation. Interestingly, amongst the myeloid populations upregulated with loss of BRD9 were monocytic myeloid derived suppressor cells (M-MDSC), immunosuppressive cells that are involved both in cancer pathogenesis and aging. Treatment with the BRD9 bromodomain specific inhibitor, I-BRD9 (1 µM), suggests that while the role of BRD9 in myeloid differentiation is largely bromodomain independent, the bromodomain may regulate erythroid differentiation. Similarly, loss of BRD9 leads to more myeloid biased colonies, suggesting that loss of BRD9 may recapitulate certain features of aged HSC. At the transcriptional level, RNA-seq revealed that loss of BRD9 resulted in few, but important differentially expressed genes, including downregulation of MPO, LIF, EPX, and GZMB (FDR < 0.05). However, gene set enrichment analysis showed that both bromodomain inhibition and loss of BRD9 resulted in downregulation of interferon and STAT3 associated genes, which are known BRD9 targets. Chromatin profiling of H3K27ac with CUT&RUN in young HSPC showed that with loss of BRD9, there is a reduction of H3K27ac at active enhancers that we had previously observed have loss of H3K27ac with human HSC aging (n=2). These active enhancers are associated with immune signaling and regulate hematopoietic transcription factors such as KLF6, RUNX1/2/3, and HIF1A. In conclusion, we have found that the chromatin remodeling protein BRD9 contributes to the reprogramming of age-associated enhancers and regulates human myeloid and erythroid differentiation in vitro, and that its loss may result in an expansion of M-MDSC, which has implications for age-related disorders such as immune dysfunction and cancer. Citation Format: Emmalee R. Adelman, Maria E. Figueroa. Determining the role of BRD9 in human hematopoietic stem cell aging. [abstract]. In: Proceedings of the AACR Special Conference: Cancer Epigenomics; 2022 Oct 6-8; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_2):Abstract nr B010.

Intracellular Iron Overload Induces Metabolic Switching in Active Hematopoietic Stem Cells in Beta-Thalassemia

Beta-thalassemia (BThal) is a complex monogenic disorder, causing severe anemia, ineffective erythropoiesis and multi-organ secondary defects. Iron overload (IO), associated with ineffective erythropoiesis and therapeutic blood transfusions, is one of the main complications of the disease, leading to morbidities in various organs. Despite the improvement in chelation therapies, IO toxicity is still a relevant issue. Correction of BThal is achieved by transplantation of hematopoietic stem cells (HSC) from healthy donors or autologous HSC from patients upon gene therapy. The quality and the engraftment of HSC depend on the bone marrow (BM) microenvironment, thus niche-HSC crosstalk plays a crucial role for transplantation outcome. We provided the first demonstration of impaired function of HSC caused by an altered BM niche in BThal (Aprile et al., Blood 2020). HSC from Hbbth3/+ (th3) BThal mice showed more active cycling profile and response to stress. We reported that IO reduces the hematopoietic supportive capacity of BM mesenchymal stromal cells from patients and recent evidence highlighted that both IO and iron deficiency are sources of cellular stress detrimental to HSC maintenance. Thus, we hypothesized that accumulation of iron might directly affect HSC function in BThal, as a model of chronic IO. Transcriptome analysis of sorted HSC from th3 mice showed a positive enrichment of genes involved in iron homeostasis, such as TfR1, Steap3, Fth1 and Ftl1, suggesting iron uptake and storage. Consistently, BThal HSC displayed higher levels of intracellular free reactive iron, as compared to wild-type (wt) controls (1.7±0.2 calcein MFI fold to wt, p<0.0001). We found that iron specifically accumulates in the mitochondria causing mitochondrial defects, as reduced size and lower membrane potential (0.4±0.05 TMRE/MTG fold to wt, p<0.001). These data, along with the downregulation of mitochondrial biogenesis and mitophagy genes, indicate an accumulation of damaged mitochondria in BThal HSC. In vivo administration of iron dextran to wt mice generated intracellular IO in HSC and was sufficient to decrease the mitochondrial membrane potential at levels comparable to those of th3 HSC, thus suggesting a direct effect of iron on mitochondrial activity. Since mitochondria are the powerhouse of the cell, we wondered if the mitochondrial dysfunction of BThal HSC could affect their production of metabolic energy. Metabolism indeed control HSC function: active HSC are more cycling and, because of their higher energy request, change the metabolism from glycolysis to mitochondrial oxidative phosphorylation (OXPHOS). Interestingly, we found that ATP levels in th3 HSC are lower compared to controls (wt 9.8±2.6 vs. th3 3.7±0.3 ATP nM, p<0.01). In line with mitochondrial dysfunction, in vitro inhibition of OXPHOS did not affect ATP content of BThal HSC since they switch to glycolysis for their energy demand. Consistently, th3 HSC showed positive enrichment of glycolytic genes and higher glucose uptake (wt 1639±163 vs. th3 2401±85 MFI 2NBDG, p<0.05). To unravel the basis of mitochondrial damage caused by IO, we investigated the role of reactive oxygen species (ROS). Iron is one of the sources of ROS and consistently, th3 HSC showed increased activation of antioxidant genes and higher levels of mitochondrial ROS (wt 2507±756 vs. th3 5205±459 MFI MitoSOX, p<0.01). In vivo reduction of mitochondrial ROS by MitoQ rescued mitochondrial activity (th3 0.3±0.07 vs. th3+MitoQ 0.9±0.2 TMRE/MTG fold to wt, p<0.05), thus demonstrating that mitochondrial dysfunction is reversible in BThal. Ongoing experiments will assess the metabolite profile, the restoration of HSC metabolism and in vivo function upon MitoQ and will also test HSC from BThal patients for mitochondrial defects. Overall, our study revealed that the multisystemic complications of IO in BThal also affect HSC. Iron has a direct impact on HSC metabolism by inducing oxidative stress and mitochondrial dysfunction. Despite the active cycling state, BThal HSC switch their metabolism to glycolysis because of the mitochondrial defect. Targeting mitochondrial iron or ROS might provide new tools to preserve HSC function in the setting of autologous transplantation upon gene therapy/editing for BThal. Moreover, this research will add novel insight about the regulation of HSC biology linking iron to mitochondrial metabolism.

Endothelial mTOR Preserves Hematopoietic Stem Cell Fitness By Suppressing Thrombospondin1

Hematopoietic stem cells (HSC) reside in close proximity to blood vessels within the bone marrow (BM) termed 'perivascular niches' which provide instructive cues for maintaining HSC self-renewal potential. The perivascular niche is comprised of endothelial cells (ECs) lining the BM blood vessels (BMECs), and an array of perivascular stromal cells that collectively serve to regulate balanced HSC self-renewal and differentiation. While young HSCs are endowed with robust self-renewal potential, aging is associated with a significant decline in HSC fitness including a loss of their self-renewal ability and a biased differentiation towards the myeloid lineages at the expense of lymphopoiesis. These aging related HSC defects result in anemias, impaired response to immunizations, and an increased risk for developing clonal disorders including acute myeloid leukemias (AML) and myelodysplastic syndromes (MDS). However, mechanisms that drive HSC defects during aging remain poorly understood, and represents a critical barrier towards developing strategies for preservation of hematopoietic fitness during aging. While HSC aging was generally assumed as a predominantly cell-intrinsic process, recent studies have identified that aging of the BM niche plays an instructive role in promoting age-related HSC defects. Supporting this, we recently discovered that aging is associated with a significant decline in mTOR activity within BMECs that impairs their capacity to serve as a HSC-supportive niche. Our findings showed that deletion of mTOR specifically within ECs of young (4 month old) mice (Endothelial Cell specific mTOR Knock Out; mTORECKO) is sufficient to recapitulate HSC aging phenotypes including loss of HSC self-renewal, and a myeloid-biased differentiation. Here, we sought to identify the mechanisms by which endothelial mTOR preserves HSC fitness. To identify candidate factors that promote HSC aging in mTORECKO mice, we performed a plasma proteomic analysis that revealed a putative role for Thrombospondin1 (Thbs1) wherein mTORECKO mice displayed elevated levels of Thbs1. Analysis of cultured BMECs revealed that inhibition of mTOR signaling within BMECs was sufficient to increase Thbs1 expression. Thbs1 is a well-defined antiangiogenic factor that has recently been identified to play a critical role in promoting a multitude of age-related cardiovascular and inflammatory disorders. However, whether Thbs1 regulates HSC activity is not known. To this end, we performed ex vivo HSC expansion transplantation assays which revealed that Thbs1 causes a significant decline in long term HSC engraftment. RNA Seq analysis of HSC expansions revealed that Thbs1 activates chronic inflammatory responses within HSCs that have been suggested to promote aging related HSC defects. To determine whether Thbs1 promotes HSC aging in mTORECKO mice, we crossed them with Thbs1 global knockout mice (Thbs1KO) to generate mTORECKOThbs1KO mice. Analysis of mTORECKOThbs1KO mice revealed that deletion of Thbs1 was sufficient to restore hematopoietic defects in mTORECKO mice including restoration of peripheral blood myeloid bias, BM HSC frequency and hematopoietic progenitor activity. Furthermore, serial HSC transplantation analysis confirmed that deletion of Thbs1 was sufficient to restore HSC self-renewal and lymphoid output in mTORECKO mice, identifying a putative role for Thbs1 as a pro-geronic factor for HSCs. To determine whether Thbs1 promotes HSC dysfunction during physiological aging, we aged Thbs1KO mice for 18 months and analyzed their hematopoietic parameters. Our analysis revealed that deletion of Thbs1 was able to prevent the age-associated increase in peripheral blood myeloid bias and BM HSC frequency. Notably, HSCs derived from aged (18 month old) Thbs1KO mice demonstrated a suppression of inflammatory responses by RNA Seq analysis, and a restoration of self-renewal ability and balanced lineage output in serial HSC transplantation assays, that are indistinguishable from HSCs derived from young (3 month old) mice. Our findings identify Thbs1 as a novel pro-geronic factor for HSC aging that can be targeted to preserve hematopoietic healthspan.

S245: ALTERATION OF BONE MARROW NICHE BY ALLOGENEIC IMMUNE REACTION AFTER HSCT

Background: Allogeneic stem cell transplantation (allo-SCT) has been considered as a curable therapy for hematologic diseases. Post-transplant myelosuppression is one of the life-threatening complications. Hematopoietic stem cell (HSC) functions are tightly regulated by a specialized microenvironment called “niche” in the bone marrow (BM) and we have identified differential contributions of perivascular stromal cells as HSC niche (Asada et al. Nat Cell Biol 2017). Recent studies indicate that BM niche can be targeted by allo-immune reaction but it remains unclear how the perivascular niche cells are involved in the mechanism of HSC dysfunction after allo-SCT. Aims: In this study, we aim to investigate the mechanisms of the impairment of the niche cells by allo-immune reactions, and to explore a targetable pathway for myelosuppression after allo-SCT. Methods: To evaluate the alterations in niche cells after allo-SCT, we utilized allo-SCT mice models (C3H/HeJ into C57BL/6, C57BL/6 into B6D2F1). Hematopoietic cells, including HSCs and mature cells, and perivascular stromal cells in BM were analyzed by flow cytometry (FACS) after transplantation. In imaging studies, we visualized perivascular stromal cells using Nestin-GFP (Nes-GFP), Myh11-CreER/tdTomato, LeptinR-Cre/tdTomato transgenic mice and observed them with confocal laser scanning microscopy. The gene expression analysis of niche factors was performed by real-time PCR. To assess the landscape of transcriptional remodeling in BM after allo-SCT, we performed single-cell RNA sequencing (scRNAseq) of BM cells. Results: FACS analyses showed that Nes-GFP+ stromal cells were dramatically reduced, and HSC recovery was severely impaired in allo-SCT mice compared to control mice at day 21 after SCT. Imaging studies also revealed that perivascular stromal cells are morphologically damaged in allo-SCT mice. Nes-GFP+ stromal cells in allo-SCT mice presented a decreased expression of niche factors, including Cxcl12 and Scf, which are essential for HSC maintenance, indicating the dysfunction of niche cells. We confirmed the impaired niche function in allo-SCT mice by tandem transplantation analysis in vivo, in which the recovery of syngeneic HSCs was also impaired in damaged microenvironment in allo-SCT mice. Prevention of allo-reactive T cell expansion by post-transplant cyclophosphamide ameliorated the niche cell reduction along with improved recovery of HSCs. These results indicate that niche impairment caused by allo-immune reaction leads to HSC dysfunction. The scRNAseq of BM highlighted the elevated expression levels of inflammation response genes, including Cxcl9 and Cxcl10, and MHC classⅡ, together with the deceased expression of niche factors in niche cells in allo-SCT mice. These alterations seemed to be induced by proinflammatory cytokines such as interferon-gamma (INF-γ) derived from allo-T cells, as the increased number of T cells and high levels of INF-γ in BM were observed in allo-SCT mice. Additionally, evaluation of T cell localization by using imaging techniques of BM revealed that allo-T cells reside significant closer to LepR+ niche cells in allo-SCT mice. Since activation of CXCR3, which is a receptor for chemokines Cxcl9 and Cxcl10, has been shown to induce chemotaxis of activated T cells, we treated allo-SCT mice with AMG487, an antagonist of CXCR3, resulted in an improved recovery of both perivascular niche cells and hematopoiesis in allo-SCT mice. Summary/Conclusion: Collectively, allo-immune reaction severely damages perivascular HSC niche cells, leading to impaired recovery of hematopoiesis after allo-SCT.

Molecular regulation of hematopoietic stem cell quiescence.

Hematopoietic stem cells (HSCs) are primarily dormant in a cell-cycle quiescence state to preserve their self-renewal capacity and long-term maintenance, which is essential for the homeostasis of hematopoietic system. Dysregulation of quiescence causes HSC dysfunction and may result in aberrant hematopoiesis (e.g., myelodysplastic syndrome and bone marrow failure syndromes) and leukemia transformation. Accumulating evidence indicates that both intrinsic molecular networks and extrinsic signals regulate HSC quiescence, including cell-cycle regulators, transcription factors, epigenetic factors, and niche factors. Further, the transition between quiescence and activation of HSCs is a continuous developmental path driven by cell metabolism (e.g., protein synthesis, glycolysis, oxidative phosphorylation, and autophagy). Elucidating the complex regulatory networks of HSC quiescence will expand the knowledge of HSC hemostasis and benefit for clinical HSC use. Here, we review the current understanding and progression on the molecular and metabolic regulation of HSC quiescence, providing a more complete picture regarding the mechanisms of HSC quiescence maintenance.

2001 – THE ENDOSOMAL ADAPTOR PROTEIN MYCT1 CONTROLS ENVIRONMENTAL SENSING IN HUMAN HEMATOPOIETIC STEM CELLS

Hematopoietic stem cells (HSC) integrate diverse environmental cues to balance between quiescence, self-renewal and differentiation. However, the molecular programs governing HSC stemness become dysregulated during culture, compromising HSC self-renewal and engraftment ability. Despite recent advances, our ability to expand functional human HSCs in culture for therapeutic use is still limited. We uncovered MYCT1 as a novel regulator of human HSCs that becomes drastically downregulated during culture, concomitantly with loss of engraftment potential. We discovered that MYCT1 is essential for human HSPC ex vivo expansion and engraftment upon transplantation. Immunoprecipitation-mass spectrometry revealed that MYCT1 is a membrane-associated endosomal protein that interacts with signaling receptors with critical functions in HSC biology. MYCT1 knockdown (KD) in human ECs and HSPCs led to hyperactivation of endocytosis, a crucial regulatory step determining the responsiveness to extracellular cues and governing cell fate decisions. Phospho-proteomics, single-cell RNAseq, and functional assays revealed that MYCT1 KD causes widespread dysregulation of signaling pathways, including hypersensitivity to the cytokines in the media, loss of stemness programs, and defective proliferation. Importantly, overexpression of MYCT1 in human HSPCs rescued endocytosis, signaling, and transcriptomic programs that were dysregulated in cultured human HSPCs and further disrupted by MYCT1 KD. These data suggest that MYCT1 governs human HSC stemness by moderating environmental sensing through the control of endocytosis. As MYCT1 expression is downregulated in cultured HSPCs, the inability to properly sense microenvironmental signals may be a key mechanism contributing to culture-associated HSC dysfunction that will need to be overcome to restore transplantability of ex vivo expanded human HSCs. Hematopoietic stem cells (HSC) integrate diverse environmental cues to balance between quiescence, self-renewal and differentiation. However, the molecular programs governing HSC stemness become dysregulated during culture, compromising HSC self-renewal and engraftment ability. Despite recent advances, our ability to expand functional human HSCs in culture for therapeutic use is still limited. We uncovered MYCT1 as a novel regulator of human HSCs that becomes drastically downregulated during culture, concomitantly with loss of engraftment potential. We discovered that MYCT1 is essential for human HSPC ex vivo expansion and engraftment upon transplantation. Immunoprecipitation-mass spectrometry revealed that MYCT1 is a membrane-associated endosomal protein that interacts with signaling receptors with critical functions in HSC biology. MYCT1 knockdown (KD) in human ECs and HSPCs led to hyperactivation of endocytosis, a crucial regulatory step determining the responsiveness to extracellular cues and governing cell fate decisions. Phospho-proteomics, single-cell RNAseq, and functional assays revealed that MYCT1 KD causes widespread dysregulation of signaling pathways, including hypersensitivity to the cytokines in the media, loss of stemness programs, and defective proliferation. Importantly, overexpression of MYCT1 in human HSPCs rescued endocytosis, signaling, and transcriptomic programs that were dysregulated in cultured human HSPCs and further disrupted by MYCT1 KD. These data suggest that MYCT1 governs human HSC stemness by moderating environmental sensing through the control of endocytosis. As MYCT1 expression is downregulated in cultured HSPCs, the inability to properly sense microenvironmental signals may be a key mechanism contributing to culture-associated HSC dysfunction that will need to be overcome to restore transplantability of ex vivo expanded human HSCs.

A FLI1-KLF6 Axis Regulates Aging in Human Hematopoietic Stem and Progenitor Cells and Normalization of KLF6 Levels in Aged Cells Leads to Their Rejuvenation

Aging causes a gradual decline in hematopoietic stem cell (HSC) function, which increases the risk for hematological malignancies. While much has been done in murine models, human HSC aging impairment is less understood. We recently showed that Krüppel-like transcription factor 6 (KLF6) is among the top downregulated genes during human HSC aging, which correlates with H3K27ac loss at several upstream putative enhancers. Moreover, loss of KLF6 in human CD34 + cells resulted in impaired in vitro differentiation, increased colony-forming potential and a transcriptional profile similar to that of aged CD34 + CD38 - cells. We hypothesized that age-acquired deregulation of KLF6 may be a key player in age-related HSC dysfunction and sought to fully characterize this. Thus, we isolated CD34 + cells from young (<32 y.o) and aged (>65 y.o.) healthy donors and performed CRISPR-Cas9 genome editing and transcriptional activation of KLF6, respectively, followed by epigenetic and transcriptional reprogramming, in vivo hematopoietic reconstitution, and analysis of DNA damage, apoptosis, and reactive oxygen species (ROS) levels.KLF6 knock-out (KO) and non-targeting control (NTC) cells from young healthy donors were engrafted into immunodeficient NSGS mice. Hematopoietic reconstitution analysis showed that KLF6 KO cells led to increased myeloid and reduced lymphoid reconstitution in peripheral blood (PB; p<1.62 -7) and an increase in immunophenotypically defined HSC and CD34 + CD38 - progenitor fractions in the bone marrow (BM; p=0.02, and p=0.04, respectively). H3K27ac analysis of KLF6 KO cells revealed a loss of 3,390 ChIP-seq peaks (FDR < 0.05) and 285 peaks gained. Functional annotation using ChIP-Enrich showed that H3K27ac loss associates with myeloid homeostasis, erythroid differentiation and oxidative stress (FDR < 0.05). Three putative enhancer (E) regions upstream of the KLF6 locus showed loss of H3K27ac with aging. Depletion of the E1 but not E2 or E3 regions phenocopied in vitro and in vivo findings of KLF6 KO. Transcription factor (TF) ChIP-seq data analysis revealed FLI1, ERG, and RUNX1 binding overlapping the E1 region. Knockdown of FLI1 but not ERG or RUNX1 led to an increase in KLF6. Notably, FLI1 mRNA levels, but not ERG or RUNX1, are increased during normal aging.We next performed in vitro KLF6 activation in aged CD34 + (KLF6a) cells using a dCas9-VP64 system to test if we could rejuvenate these cells. KLF6a cells exhibited a decrease in their in vitro myeloid differentiation potential, compared to aged NTC CD34 + cells (p<0.0041), and behaved instead similar to young controls. ChIP-seq analysis of KLF6a showed marked decrease of H3K4me1 (n=3,273 peaks) with relatively few regions with increased H3K4me1 (n=602) (FDR < 0.05). In contrast, we observed an increase in H3K27ac (n=3,361 peaks) with only 71 peaks lost compared to aged NTC (FDR < 0.05). Regions that gained H3K27ac in KLF6a were associated with platelet activation, cell junction and adhesion. In vivo analysis of KLF6a cells injected into NSGS mice revealed a significant reduction in the PB myeloid fraction compared to NTC (p<1.2-8), with a concomitant expansion in the lymphoid compartment (p<4.4 -11). BM composition analysis at week 16 showed a decrease in the HSC fraction in KLF6a cells (p=0.0029) as well as a reduction in CD34 +CD38 -, CD34 +CD38 + and MEPs (p=0.036, p<0.0001 and p=0.041, respectively). We next examined the impact of KLF6 modulation on DNA damage and observed that young human KLF6 KO cells had a significant increase in gH2AX and 53BP1 (p<0.0001, for both) whereas KLF6a in aged CD34 + cells exhibited reduced gH2AX and 53BP1 foci in comparison to aged NTC (p<0.0001, for both). In addition, apoptotic levels in KLF6 KO cells were higher than in NTC cells (p=0.006) whereas aged KLF6a cells showed a reduction in the incidence of apoptotic cells compared to NTC (p=0.019). Finally, ROS analysis in young KLF6 KO showed increased levels of total and mitochondrial ROS compared to NTC (p=0.0008 and p<0.0001, respectively) whereas both ROS fractions were reduced in KLF6a cells (p=0.0002 and p<0.0001, respectively).In summary, these results show that the FLI1-KLF6 axis plays a key role in regulating HSPC aging and that KLF6 is required for normal HSPC function and differentiation. In addition, normalization of KLF6 levels in aged HSPCs resulted in reprogramming and rejuvenation HSPCs, confirming the central role of this TF in aging HSPC biology. DisclosuresNo relevant conflicts of interest to declare.

Cytoplasmic Labile Iron Accumulates in Aging Stem Cells Perturbing a Key Rheostat for Identity Control

Bone marrow resident and rarely dividing hematopoietic stem cells (HSC) harbor an extensive self-renewal capacity to sustain life-long blood formation, albeit their function declines during ageing. Various molecular mechanisms confer stem cell identity, ensure long-term maintenance and are known to be deregulated in aged stem cells. How these programs are coordinated, particularly during cell division, and what triggers their ageing-associated dysfunction has been unknown.We have previously uncovered that iron chelator exposure increases the number of functional HSC ex vivo and in vivo (Kao et al., Science Transl Med 2018). While ensuring a sufficient amount of redox active, readily available iron which is required in numerous electron transfer reactions governing fundamental cellular processes, cells tightly regulate the size of the intracellular labile iron pool (LIP) to limit adverse ROS generation. Perturbations in the ability to limit intracellular iron is detrimental for cells and known to compromise HSC maintenance and function via altered redox signaling and increased macromolecule oxidation and damage. The HSC stimulatory effects of iron chelator (IC) treatment and the well characterized central roles of redox active intracellular iron in sustaining basic cell function prompted us to examine a potential regulatory role of the LIP in controlling somatic stem cell function.In this study, we quantified LIP in young and aged HSC and monitored iron homoeostasis pathway activation, hallmarked by the stabilization of transferrin receptor (Tfrc) mRNA, in stem cells for which we developed a single molecule RNA fluorescence in situ hybridization (smRNA FISH) assay enabling the quantification of Tfrc dynamics with unparalleled resolution and sensitivity. We have further used experimental LIP modulation in primary hematopoietic stem cell models to characterize the consequences of iron homeostasis pathway activation in young and aged stem cells; and employed integrated comparative quantitative transcriptomics (single cell RNA-seq) and proteomics along with genetic and pharmacological rescue models to identify the consequences and mechanisms of LIP size alterations. Our findings demonstrate that HSC, containing the lowest amount of cytoplasmic chelatable iron hematopoietic cells, activate a limited iron response during mitosis. Engagement of this iron homeostasis pathway elicits mobilization and β-oxidation of arachidonic acid and enhances stem cell-defining transcriptional programs governed by histone acetyl transferase Tip60/KAT5. We further find an age-associated expansion of the labile iron pool, along with loss of Tip60/KAT5-dependent gene regulation to contribute to the functional decline of ageing HSC, which can be mitigated by iron chelation.Together, our work reveals cytoplasmic redox active iron as a novel rheostat in adult stem cells; it demonstrates a role for the intracellular labile iron pool in coordinating a cascade of molecular events which reinforces HSC identity during cell division and to drive stem cell ageing when perturbed. As loss of iron homeostasis is commonly observed in the elderly, we anticipate these findings to trigger further studies into understanding and therapeutic mitigation of labile iron pool-dependent hematopoietic stem cell dysfunction in a wide range of degenerative and malignant hematologic pathologies. DisclosuresD'Alessandro: Omix Thecnologies: Other: Co-founder; Rubius Therapeutics: Consultancy; Forma Therapeutics: Membership on an entity's Board of Directors or advisory committees.

Arhgef2 regulates mitotic spindle orientation in hematopoietic stem cells and is essential for productive hematopoiesis

AbstractHow hematopoietic stem cells (HSCs) coordinate their divisional axis and whether this orientation is important for stem cell–driven hematopoiesis is poorly understood. Single-cell RNA sequencing data from patients with Shwachman-Diamond syndrome (SDS), an inherited bone marrow failure syndrome, show that ARHGEF2, a RhoA-specific guanine nucleotide exchange factor and determinant of mitotic spindle orientation, is specifically downregulated in SDS hematopoietic stem and progenitor cells (HSPCs). We demonstrate that transplanted Arhgef2−/− fetal liver and bone marrow cells yield impaired hematopoietic recovery and a production deficit from long-term HSCs, phenotypes that are not the result of differences in numbers of transplanted HSCs, their cell cycle status, level of apoptosis, progenitor output, or homing ability. Notably, these defects are functionally restored in vivo by overexpression of ARHGEF2 or its downstream activated RHOA GTPase. By using live imaging of dividing HSPCs, we show an increased frequency of misoriented divisions in the absence of Arhgef2. ARHGEF2 knockdown in human HSCs also impairs their ability to regenerate hematopoiesis, culminating in significantly smaller xenografts. Together, these data demonstrate a conserved role for Arhgef2 in orienting HSPC division and suggest that HSCs may divide in certain orientations to establish hematopoiesis, the loss of which could contribute to HSC dysfunction in bone marrow failure.

The Spliceosomal Component Sf3b1is Essential for Hematopoietic Stem Cell Formation through the Regulation of the Jak/Stat Signaling Pathway

Hematopoietic stem cells (HSCs) maintain multi-lineage hematopoiesis for the lifetime of an organism. During embryogenesis, HSCs first emerge from specialized hemogenic endothelial cells within the dorsal aorta through a process termed the endothelial-to-hematopoietic transition. While much is known about the role of transcription factors and transcriptional control in HSC specification, it is ill defined how post-transcriptional RNA processing influences this first HSC fate decision. Using zebrafish loss-of-function mutants for the pre-mRNA spliceosomal component sf3b1 (splicing factor 3b, subunit 1), we demonstrated that impairing splicing hindered HSC production, specifically at the endothelial-to-hematopoietic transition. Loss-of-function sf3b1hi3394 mutants have diminished expression of thehemogenic endothelial and HSC markers runx1 and gata2b, while the pan-endothelial marker kdrl (kinase insert domain receptor like) and aorta-specific markers notch1b and notch3 are normally expressed at 24 hpf (hours post fertilization). These data indicate that although sf3b1 is ubiquitously expressed, hemogenic endothelium and the downstream HSCs are more sensitive to its loss than other endothelial cells. To uncover the mechanism underlying the defect in hemogenic endothelium and HSCs in sf3b1 mutants, we performed RNA-sequencing on purified kdrl:gfp- positiveendothelial cells from sf3b1 mutants and wild-type siblings at 24 hpf. We observed nearly 900 genes were mis-spliced and of those only 144 were differentially expressed. This group was significantly enriched for genes involved in mRNA processing and Jak (Janus kinase)/Stat (Signaling transducer and activator of transcription) signaling. In particular, target genes of the Jak/Stat pathway were significantly enriched in the downregulated gene list and many components of the signaling pathway were mis-spliced including il6st (interleukin 6 signal transducer), the gene encoding the IL6-family common receptor Gp130 (Glycoprotein 130).To determine if mis-splicing of il6st was sufficient to decrease HSC production, we injected antisense morpholinos into wild-type zebrafish to mimic the sf3b1 mutant - specific exon-skipping event in il6st and found significantly diminished HSC levels at 28 hpf. Stat3 is the major transcription factor downstream of Gp130, thus we ubiquitously overexpressed a constitutively-active form of Stat3 and found it could significantly suppress the HSC defects in sf3b1 mutants. Together, these data indicate that Sf3b1-mediated splicing regulation of the Jak/Stat pathway is critical for HSC emergence. Mutations in SF3B1 are prevalent in myelodysplastic syndrome (MDS), a disorder originating in HSC dysfunction. Altered STAT3 signaling is also common in MDS, thus our data suggest that SF3B1 disruptions in HSCs could be contributing to MDS via mis-regulation of JAK/STAT signaling. DisclosuresNo relevant conflicts of interest to declare.

Mutant ASXL1 induces age-related expansion of phenotypic hematopoietic stem cells through activation of Akt/mTOR pathway.

Somatic mutations of ASXL1 are frequently detected in age-related clonal hematopoiesis (CH). However, how ASXL1 mutations drive CH remains elusive. Using knockin (KI) mice expressing a C-terminally truncated form of ASXL1-mutant (ASXL1-MT), we examined the influence of ASXL1-MT on physiological aging in hematopoietic stem cells (HSCs). HSCs expressing ASXL1-MT display competitive disadvantage after transplantation. Nevertheless, in genetic mosaic mouse model, they acquire clonal advantage during aging, recapitulating CH in humans. Mechanistically, ASXL1-MT cooperates with BAP1 to deubiquitinate and activate AKT. Overactive Akt/mTOR signaling induced by ASXL1-MT results in aberrant proliferation and dysfunction of HSCs associated with age-related accumulation of DNA damage. Treatment with an mTOR inhibitor rapamycin ameliorates aberrant expansion of the HSC compartment as well as dysregulated hematopoiesis in aged ASXL1-MT KI mice. Our findings suggest that ASXL1-MT provokes dysfunction of HSCs, whereas it confers clonal advantage on HSCs over time, leading to the development of CH.

Upfront Alternative Donor Transplant Versus Immunosuppressive Therapy in Patients with Severe Aplastic Anemia Who Lack Fully HLA Matched Related Donor: Systematic Review and Meta-Analysis of Retrospective Studies. on Behalf of the Severe Aplastic Anemia Working Party of European Group for Blood and Marrow Transplantation (SAAWP of EBMT)

Upfront Alternative Donor Transplant Versus Immunosuppressive Therapy in Patients with Severe Aplastic Anemia Who Lack Fully HLA Matched Related Donor: Systematic Review and Meta-Analysis of Retrospective Studies. on Behalf of the Severe Aplastic Anemia Working Party of European Group for Blood and Marrow Transplantation (SAAWP of EBMT)

Can molecular targeting the TNFα-ERK-ETS1-IL27Rα pathway keep us young and healthy by protecting HSCs from aging?

The world's older population grows at an unprecedented rate. Today, there are about 8.5% of people worldwide (617 million) aged 65 and older today, which is projected to continue growing to nearly 17% of the world's population by 2050 (1.6 billion). World population aging has profound implications because aging represents a progressive deterioration of physiological function accompanied by an increase in vulnerability for disease and mortality. Although aging itself is not a disease, it is the greatest risk factor for most chronic human diseases, including cardiovascular and neurodegenerative disease, cancer, diabetes, and osteoarthritis; and a major contributing factor for the increased susceptibility to various infectious diseases such as COVID-19. Age-related changes in the hematopoietic and immune system likely play an important role in the pathogenesis of these diseases in part via induction of immunosenescence and inflammaging.1 These changes have been largely attributed to hematopoietic stem cell (HSC) aging, which leads to a significant decline in HSC self-renewal and long-term hematopoietic reconstitution, a myeloid skewing (ie, increase in myeloid cell but decrease in lymphoid cell production), and clonal hematopoiesis and increased hematological malignancies.2 Both cell-intrinsic and cell-extrinsic alterations have been implicated in driving HSC aging. The cell-intrinsic mechanisms of HSC aging include increased DNA damage,3 loss of polarity and proteostasis,4 impaired autophagy and mitochondrial activities,5 epigenetic reprogramming and senescence.6,7 The cell-extrinsic mechanisms are less studied but may include alterations in bone marrow stromal cell populations, changes in the production of hematopoietic stimulators (such as osteopontin) and inhibitors (such as CCL5) by the stromal cells, and increased production of inflammatory cytokines.8 However, how these cell-extrinsic factors communicate with HSCs to promote HSC aging has not been well established. In a recent Blood publication,9 Dr. Jianwei Wang's group investigated the mechanistic link between age-related inflammation and HSC aging. They found that aging is associated with an upregulation of interleukin 27 receptor α (IL27Rα) expression in HSCs from mouse bone marrow, resulting in a significant expansion of IL27Rα+ HSCs. IL27Rα+ HSCs from both young and old mice exhibited all the phenotypes of aged or senescent HSCs, including inability to self-renew, produce long-term hematopoietic reconstitution, and differentiate into both myeloid and lymphoid cells in a balanced manner after transplantation. In contrast, IL27Rα- HSCs from both old mice and HSCs from old IL27Rα knockout (IL27Rα−/−) mice exhibited significant improvement in HSC functions compared to IL27Rα+ HSCs from the same aged mice. These findings indicate that IL27Rα can be used as a biomarker for aged and senescent HSCs. Next, they investigated the underlying mechanisms contributing to the age-related upregulation of IL27Rα in HSCs. They found that NF-κB and EST1 are important transcription factors that regulate the expression of IL27Rα. However, aging is associated with an increased expression of phosphorylated ERK and ETS1 in HSCs, but with no changes in the NF-κB activation. The activation of the ERK-ETS1 pathway and increased expression of IL27Rα in old HSCs may be attributable to the stimulation by the HSC-extrinsic proinflammatory cytokine TNFα. This is because mouse bone marrow plasma from old mice contained a significantly higher level of TNFα than that from young mice. TNFα stimulated the expression of IL27Rα in c-Kit+ Sca1+ Lineage- (KLS) cells and HSCs in vitro and in vivo, respectively, in an ETS1-dependent manner. The upregulation of IL27Rα in HSCs is likely responsible for mediating TNFα-induced HSC dysfunction because knockout IL27Rα could prevent TNFα-induced HSC dysfunction. Therefore, these findings are highly significant by providing the first link between cell-extrinsic and cell-intrinsic communications that lead to HSC aging via the activation of the TNFα-ERK-ETS1-IL27Rα pathway. It will be of great interest to determine whether molecular targeting this pathway with a small molecule inhibitor or an antibody has the potential to be developed as a novel therapeutic strategy to slowdown HSC aging, which may lead to reduced inflammaging and improvement of hematopoietic and immune functions, particularly under various age-related pathological conditions in which TNFα production is upregulated systemically or locally in the bone marrow microenvironment. However, cautions have to be take into consideration before translating this approach into the clinic because IL27Rα+ HSCs may be primarily responsible for emergency hematopoiesis in the fight against bacterial infection as shown by the authors in this publication.

1016 – REGULATION OF RIBOSOME BIOGENESIS IN HEMATOPOIETIC STEM CELLS

Dysregulation of ribosome biogenesis is the underlying etiology in a group of bone marrow failure syndromes called ribosomopathies. As prominently illustrated by one type of ribosomopathies, Shwachman-Diamond Syndrome (SDS), germline biallelic mutations in three assembly factors essential for the maturation of the 60S ribosome subunit result in progressive hematopoietic stem and progenitor cell (HSPC) failure, suggesting that HSPCs are sensitive to perturbations in ribosome assembly. However, how ribosome assembly is regulated in HSPCs remains poorly understood, as are approaches to restore in hematopoietic stem cell (HSC) function in the context of defective ribosome biogenesis. Here we uncover a novel role for the E3 ubiquitin ligase, HectD1, in regulating HSC function via ribosome biogenesis. HSCs from hematopoietic-specific <i>Hectd1</i> knockout mice exhibit a striking defect in transplantation ability and self-renewal. <i>Hectd1</i>-deficient HSCs fail to be maintained ex vivo, concomitant with a reduction in global protein synthesis and growth rate. The mechanism underlying HSC dysfunction upon <i>Hectd1</i> deficiency is directly linked to aberrant ribosome assembly. We demonstrate that HectD1 ubiquitinates and degrades an assembly factor for the polypeptide exit tunnel of the 60S ribosome in the cytoplasm. Depletion of <i>HectD1</i> led to an accumulation of the anti-association factor eIF6 on the 60S and decreased 80S to 60S ratio, consistent with a subunit joining defect associated with SDS-like diseases. Importantly, we found that restoration of ribosome assembly in <i>HectD1</i>-deficient cells promoted protein synthesis and enhanced HSC reconstitution capacity. These findings highlight the importance of ribosome assembly in HSC regeneration and represent a rare <i>in vivo</i> example of genetic suppression of HSC defects associated with dysfunctional ribosome biogenesis.