Blood

Rarely, recipients of adenoviral vector-based vaccines experience a severe thrombotic thrombocytopenic condition referred to as vaccine-induced immune thrombotic thrombocytopenia (VITT). VITT is a transient prothrombotic process, although recent data suggests that VITT anti-platelet factor 4 (PF4) antibodies are more persistent than antibodies seen in heparin-induced thrombocytopenia. Whether anti-PF4 antibody persistence in VITT is related to the continued persistence of antibody clones from the acute phase or the development of novel antibodies is unclear. To study this, acute and follow-up samples were obtained from six Ad26.COV2.S-associated VITT patients, with a median time to follow-up of 244 days from acute presentation (Range, 114-664 days). Upon affinity-enrichment of antibodies, mono/oligoclonal PF4/heparin-reactive anti-PF4 antibodies were observed despite negative results in serum protein electrophoresis and the more sensitive "Mass-Fix" technique. This finding distinguishes VITT from monoclonal gammopathy of thrombotic significance where monoclonal antibodies are observed in native sera. Anti-PF4 antibody abundance decreased over time, with no evidence of novel anti-PF4 antibody production after acute presentation. Although previous studies indicate a stereotypical pairing of VITT antibodies with lambda light chains, one VITT patient produced anti-PF4 antibodies with a kappa light chain, suggesting immunological heterogeneity. While none of these six antibodies caused long-term thrombocytopenia or thrombosis, platelet-activating anti-PF4 antibodies were seen four years after the acute event in an additional ChAdOx1 nCoV-19-associated VITT patient. These antibodies continued to cause chronic low-grade thrombocytopenia, highlighting the potential for long-term sequelae in what is generally viewed as a transient thrombotic thrombocytopenic syndrome.

Key pointsO_LIWe report findings from a phase 2 study of MY008211A among Chinese men and women aged [&ge;]18 years with paroxysmal nocturnal hemoglobinuria C_LIO_LIIncreases in hemoglobin of [&ge;]20 g/L were maintained for up to 44 weeks of treatment with MY008211A in all 34 patientsiv C_LI Explanation of noveltyParoxysmal nocturnal hemoglobinuria is characterized by red blood cell (RBC) destruction and a prothrombotic state.v Treatments exist such as complement 5 inhibitors but these carry the risk for iatrogenic extravascular hemolysis and anemia.vi As reported here, the novel, oral complement factor B inhibitor MY008211A yielded increases in hemoglobin and RBC levels, while adverse events over 44 weeks were largely mild to moderate in severity, and infections generally consisted of respiratory infections.vii Paroxysmal nocturnal hemoglobinuria (PNH) is a life-threatening disease characterized by red blood cell (RBC) destruction, blood clots, and impaired bone marrow function.viii We evaluated the efficacy and safety of 3 dosages of MY008211A, a novel complement factor B inhibitor,ix for treating PNH.x This was a multicenter, open-label, phase 2, dose-finding study of MY008211A among Chinese men and women with complement inhibitor-naive PNH and signs of active hemolysis.xi Patients with hemoglobin <100 g/L were assigned to oral MY008211A 400 mg twice daily (BID), 600 mg BID, or 800 mg once daily (QD) for 12 weeks and could then continue treatment with 400 mg BID during a 32-week extension.xii The primary endpoint was the proportion of patients achieving an increase in hemoglobin concentration of [&ge;]20 g/L vs baseline on day (D)84, without RBC transfusions after 4 weeks of dosing.xiii Safety assessments included adverse events (AEs).xiv Fifteen, 9, and 10 patients were assigned to MY008211A 400 mg BID, 600 mg BID, and 800 mg QD, respectively.xv All patients completed the study and its 32-week extension.xvi On D84, all 34 patients achieved increases in hemoglobin concentration of [&ge;]20 g/L from baseline;xvii all patients maintained this increase at D308.xviii Through D308, grade [&ge;]3 AEs occurred in 5 (33%), 5 (56%), and 4 (40%) patients in the 400-, 600-, and 800-mg groups, respectively.xix There were no deaths.xx In this multicenter, open-label study of 3 dosages of MY008211A for PNH, all patients achieved and maintained increases in hemoglobin of [&ge;]20 g/L from baseline without RBC transfusions.

BACKGROUNDWiskott-Aldrich Syndrome (WAS) is a rare, X-linked, life-threatening inborn error of immunity and platelet disorder caused by WAS protein (WASP)-encoding gene mutations. Etuvetidigene autotemcel (etu-cel) is an autologous gene therapy (GT) consisting of hematopoietic stem progenitor cell (HSPCs) transduced ex vivo with a lentiviral vector encoding human WAS cDNA. METHODSEtu-cel was intravenously infused after rituximab and reduced-intensity conditioning. Data from WAS patients treated in two prospective open-label clinical trials (phase I/II n=8; phase III n=10) and one expanded access program (EAP) (n=9) were integrated to evaluate efficacy and safety of etu-cel. Primary efficacy endpoints were overall survival, rate of severe infections from 6 to 18 months after GT and rate of moderate/severe bleeding episodes in the first 12 months post-treatment compared with 1 year prior to GT. Secondary efficacy endpoints included engraftment of gene-corrected cells, WASP expression, T-cell function, platelet count, autoimmunity and eczema over time. Safety endpoints included adverse events (AEs), immune response to transgene, development of replication-competent lentivirus (RCL) and abnormal clonal proliferation (ACP). RESULTSMedian follow-up was 5.7 years (range: 0.4-13.3). Median age at treatment was 2.6 years (range: 1.0-35.1). Overall survival was 96%; one EAP subject died early post-GT due to deterioration of a pre-existing neurological condition. The rate of severe infections per person-year of observation (PYO) decreased from 2.00 (95% CI: 1.50-2.61) pre-GT to 0.15 (95% CI: 0.04-0.39) in the 6-18 months period post-GT. The rate of moderate and severe bleeding events per PYO decreased from 2.00 (95% CI: 1.50-2.61) to 0.80 (95% CI: 0.49-1.22) in the 12 months after GT. After GT, multilineage engraftment of gene-corrected cells was sustained over time. WASP expression in platelets and lymphocytes increased. Platelet count, T-cell functionality, eczema and autoimmunity improved. The most common adverse event [&ge;] grade 3 was device related infection. Etu-cel was well-tolerated with no treatment-related adverse events and no evidence of insertional oncogenesis. CONCLUSIONSWith up to 13 years follow-up, etu-cel demonstrates a favorable benefit-risk profile, leading to sustained long-term clinical benefit. (Funded by GlaxoSmithKline [GSK], Orchard Therapeutics, Fondazione Telethon; ClinicalTrials.gov numbers: NCT01515462, NCT03837483)

Germline RUNX1 mutations lead to familial platelet disorder with associated myeloid malignancies (FPDMM), which is characterized by thrombocytopenia and a life-long risk (35-45%) of hematological malignancies. We recently launched a longitudinal natural history study for patients with FPDMM at the NIH Clinical Center. Among 29 families with research genomic data, 28 different germline RUNX1 variants were detected. Besides missense mutations enriched in Runt homology domain and loss-of-function mutations distributed throughout the gene, splice-region mutations and large deletions were detected in 6 and 7 families, respectively. In 24 of 54 (44.4%) non-malignant patients, somatic mutations were detected in at least one of the clonal hematopoiesis of indeterminate potential (CHIP) genes or acute myeloid leukemia (AML) driver genes. BCOR was the most frequently mutated gene (in 9 patients), and multiple BCOR mutations were identified in 4 patients. Mutations in 7 other CHIP or AML driver genes (DNMT3A, TET2, NRAS, SETBP1, SF3B1, KMT2C, and LRP1B) were also found in more than one non-malignant patient. Moreover, three unrelated patients (one with myeloid malignancy) carried somatic mutations in NFE2, which regulates erythroid and megakaryocytic differentiation. Sequential sequencing data from 19 patients demonstrated dynamic changes of somatic mutations over time, and stable clones were more frequently found in elderly patients. In summary, there are diverse types of germline RUNX1 mutations and high frequency of somatic mutations related to clonal hematopoiesis in patients with FPDMM. Monitoring dynamic changes of somatic mutations prospectively will benefit patients clinical management and reveal mechanisms for progression to myeloid malignancies. Key PointsO_LIComprehensive genomic profile of patients with FPDMM with germline RUNX1 mutations. C_LIO_LIRising clonal hematopoiesis related secondary mutations that may lead to myeloid malignancies. C_LI

The JAK2V617F (JAK2VF) driver mutation is found in 95% of patients with polycythemia vera (PV), a progressive myeloproliferative neoplasm. Current treatments suppress excessive hematopoiesis but lack specificity for targeting JAK2VF cells, are unable to deplete mutant stem/progenitor cells and ultimately result in drug resistance. We discovered that the FDA-approved antibiotic, linezolid (LZD), ameliorates the PV phenotype across multiple model systems. LZD suppressed cell proliferation and STAT5 signaling, altered the cell cycle, and increased apoptosis of JAK2VF-harboring human erythroleukemia cells, but not in wild-type acute leukemia cells. Computational modelling indicated that LZD interacts specifically with mutant JAK2VF but not with wild-type JAK2 protein. We further showed that, in JAK2VF mice that faithfully recapitulate human PV, LZD mitigates disease burden by selectively targeting JAK2VF stem cells thereby normalizing spleen size and blood counts. LZD also inhibited hematopoietic colony formation by patient-derived peripheral blood mononuclear cells, with the more primitive progenitors being preferred targets. Importantly, LZD selectively decreased JAK2VF+ colony numbers, without impacting wild-type JAK2 colonies. In all, the data provide a firm foundation for evaluating LZD-like molecules as an effective therapy for PV and other myeloproliferative neoplasms. Key pointsO_LILinezolid acts as a JAK2V617FIZselective inhibitor in PV mouse models and PV patient samples while sparing wildIZtype hematopoiesis. C_LIO_LILinezolid acts directly on JAK2V617F hematopoietic stem cells. C_LI

BackgroundRare oncogenic driver events, particularly affecting the expression or splicing of driver genes, are suspected to substantially contribute to the large heterogeneity of hematologic malignancies. However, their identification remains challenging. MethodsTo address this issue, we generated the largest dataset to date of matched whole genome sequencing and total RNA sequencing of hematologic malignancies from 3,760 patients spanning 24 disease entities. Taking advantage of our dataset size, we focused on discovering rare regulatory aberrations. Therefore, we called expression and splicing outliers using an extension of the workflow DROP (Detection of RNA Outliers Pipeline) and AbSplice, a variant effect predictor that identifies genetic variants causing aberrant splicing. We next trained a machine learning model integrating these results to prioritize new candidate disease-specific driver genes. ResultsWe found a median of seven expression outlier genes, two splicing outlier genes, and two rare splice-affecting variants per sample. Each category showed significant enrichment for already well-characterized driver genes, with odds ratios exceeding three among genes called in more than five samples. On held-out data, our integrative modeling significantly outperformed modeling based solely on genomic data and revealed promising novel candidate driver genes. Remarkably, we found a truncated form of the low density lipoprotein receptor LRP1B transcript to be aberrantly overexpressed in about half of hairy cell leukemia variant (HCL-V) samples and, to a lesser extent, in closely related B-cell neoplasms. This observation, which was confirmed in an independent cohort, suggests LRP1B as a novel marker for a HCL-V subclass and a yet unreported functional role of LRP1B within these rare entities. ConclusionsAltogether, our census of expression and splicing outliers for 24 hematologic malignancy entities and the companion computational workflow constitute unique resources to deepen our understanding of rare oncogenic events in hematologic cancers.

Thrombosis remains the leading cause of morbidity and mortality for patients (pts) with polycythemia vera (PV), yet PV clinical trials are not powered to identify interventions that improve thrombosis-free survival (TFS). Such trials are infeasible in a contemporary PV cohort, even when selecting "high-risk" pts based on Age >60 and thrombosis history, because thousands of patients would be required for a short-term study to meet TFS endpoint. To address this problem, we used artificial intelligence and machine learning (ML) to dynamically predict near-term (1-year) thrombosis risk in PV pts with high sensitivity and positive predictive value (PPV) to enhance pts selection. Our automation-driven data extraction methods yielded more than 16 million data elements across 1,448 unique variables (parameters) from 11,123 clinical visits for 470 pts. Using the AutoGluon framework, the Random Forest ML classification algorithm was selected as the top performer. The full (309-parameter) model performed very well (F1=0.91, AUC=0.84) when compared with the current ELN gold-standard for thrombosis risk stratification in PV (F1=0.1, AUC=0.39). Parameter engineering, guided by Gini feature importance identified the 21 parameters (top-21) most important for accurate prediction. The top-21 parameters included known, suspected and previously unappreciated thrombosis risk factors. To identify the minimum number of parameters required for the accurate ML prediction, we tested the performance of every possible combination of 3-9 parameters from top-21 (>1.6M combinations). High-performing models (F1> 0.8) most frequently included age (continuous), time since dx, time since thrombosis, complete blood count parameters, blood type, body mass index, and JAK2 mutant allele frequency. Having trained at tested over 1.6M practical ML models with a feasible number of parameters (3-9 parameters in top-21 most predictive), it is clear that study cohorts of patients with PV at high near-term thrombosis risk can be identified with high enough sensitivity and PPV to power a clinical trial for TFS. Further validation with external, multicenter cohorts is ongoing to establish a universal ML model for PV thrombosis that would facilitate clinical trials aimed at improving TFS.

We show that clonal hematopoiesis is associated with an increased incidence of autoimmune hemolytic anemia. The hazard ratios of autoimmune hemolytic anemia and immune thrombocytopenia associated with clonal hematopoiesis were similar.

Alpha-thalassemia is an autosomal recessive disease with increasing worldwide prevalence. The molecular basis is due to mutation or deletion of one or more duplicated -globin genes, and disease severity is directly related to the number of allelic copies compromised. The most severe form, -thalassemia major (TM), results from loss of all four copies of -globin and has historically resulted in fatality in utero. However, in utero transfusions now enable survival to birth. Postnatally, patients face challenges similar to {beta}-thalassemia, including severe anemia and erythrotoxicity due to imbalance of {beta}-globin and -globin chains. While curative, hematopoietic stem cell transplantation (HSCT) is limited by donor availability and potential transplant-related complications. Despite progress in genome editing treatments for {beta}-thalassemia, there is no analogous curative option for patients suffering from -thalassemia. To address this, we designed a novel Cas9/AAV6-mediated genome editing strategy that integrates a functional -globin gene into the {beta}-globin locus in TM patient-derived hematopoietic stem and progenitor cells (HSPCs). Incorporation of a truncated erythropoietin receptor transgene into the -globin integration cassette dramatically increased erythropoietic output from edited HSPCs and led to the most robust production of -globin, and consequently normal hemoglobin. By directing edited HSPCs toward increased production of clinically relevant RBCs instead of other divergent cell types, this approach has the potential to mitigate the limitations of traditional HSCT for the hemoglobinopathies, including low genome editing and low engraftment rates. These findings support development of a definitive ex vivo autologous genome editing strategy that may be curative for -thalassemia. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=177 SRC="FIGDIR/small/555926v2_ufig1.gif" ALT="Figure 1"> View larger version (41K): org.highwire.dtl.DTLVardef@1b63dacorg.highwire.dtl.DTLVardef@18b25a1org.highwire.dtl.DTLVardef@53835corg.highwire.dtl.DTLVardef@d52bf5_HPS_FORMAT_FIGEXP M_FIG C_FIG

Germline variants in DDX41 are the most frequent genetic predisposition to adult hematologic malignancies. The most common variants are truncating, implicating loss of function in the pathogenesis. However, non-truncating variants account for 30-40% of cases, and their impact on essential DDX41 functions remains unknown. We utilized a genetic complementation assay to assess the functionality of 10 recurrent germline non-truncating variants of DDX41. All variants restored viability to Ddx41-deficient hematopoietic progenitor cells at exogenous expression levels. In contrast, the hotspot mutant p.R525H, which is somatically acquired at disease onset in >50% of patients, failed to restore viability. CRISPR-based modeling in cell lines and mice revealed heterogeneity: some variants were non-functional at endogenous expression levels whereas others maintained complete functionality, supporting normal cell proliferation and even lifelong hematopoiesis in a homozygous setting. Notably, co-expression of p.R525H with some variants caused impaired hematopoietic progenitor cell viability, indicating a dominant-negative effect of p.R525H. In contrast, other variants, all classified as variants of unknown significance, were unaffected by the presence of p.R525H. A screen of 100 disease-associated variants confirmed that many non-truncating germline variants are susceptible to p.R525H-mediated dominant-negative effects, whereas wild-type DDX41 is not. These findings indicate that DDX41 variant curation is complicated by variable effects on functionality and variant-specific interactions with somatically-acquired DDX41 mutations. The dominant-negative effect of p.R525H provides a mechanistic basis for the conclusion of recent patient cohort analyses that co-occurrence with a somatic hotspot mutation is a reliable indicator of DDX41-driven disease in carriers of non-truncating variants.

Background Thalassemia is one of the most common monogenic disorders worldwide, current screening strategies combining hematological testing with molecular assays still carry a risk of missed diagnoses and undesirable efficiency, particularly for complex structural variants and rare mutations. Methods In this prospective double-blind, multicenter cohort study of 3,842 participants (3,362 pregnant women and 480 male partners), we conducted a head-to-head comparison to systematically evaluate the incremental clinical value and detection performance of single-molecule nanopore sequencing in thalassemia (SMITH) against conventional hematological testing and next-generation sequencing (NGS). Findings The overall concordance rate between NGS and SMITH was 98.6% (3789/3842). The discrepant cases (n=53) were directly attributed to the superior detection capabilities of SMITH, which successfully identified complex structural rearrangements-including 45 -globin gene triplications and four HK alleles-that were missed by NGS. Furthermore, SMITH accurately detected four rare variants (c.134_135insT/, c.-22(C>T)/, {beta}N/{beta}c.316-290delinsAGGGCAATAATTT and {beta}3.5 kb deletion/{beta}N ) and resolved ten trans and three cis configurations within the globin gene allele. Clinically, these technical advantages translated to a 9.3% (5/54) increase in the detection rate of high-risk prenatal couples, effectively preventing one birth affected by moderate-to-severe thalassemia. Additionally, SMITH corrected a diagnostic discrepancy in one case (HK vs. -3.7), sparing the couple from an unnecessary invasive procedure. Interpretation Our findings demonstrate that SMITH provides a powerful platform for resolving globin gene rearrangements, detecting rare variants, and enabling direct haplotype phasing. By effectively eliminating diagnostic blind spots, SMITH is expected to become an optimal method for thalassemia prevention programs. Funding This study was supported by Chinese National Natural Science Foundation Projects 81760037 and 82271894.

JAK2 is a key regulator of cytokine-mediated proliferative signaling in hematopoietic stem and progenitor cells. Activating mutations, most commonly JAK2 V617F, trigger aberrant cytokine signaling driving the pathogenesis of myeloproliferative neoplasms (MPNs). Phosphatidylinositol transfer proteins (PITPs) facilitate phosphoinositide synthesis by delivering phosphatidylinositol to lipid kinases, though their roles in oncogenic signaling have remained poorly defined. Here we show that PITP{beta} is critical for the development of JAK2V617F-driven MPN in mice. Deleting Pitp{beta} across the hematopoietic system, but not Pitp, prolonged 25-week survival of Jak2V617F mice from 10% to 85%. Loss of Pitp{beta} attenuated disease-associated splenomegaly and curtailed erythroid progenitors expansion both in vivo and in vitro. Mechanistically, PITP{beta} is necessary for AKT hyperactivation in hematopoietic progenitors, while STAT5 and ERK signaling remain unaffected. In alignment with this role, PITP{beta} promotes the production of PtdIns(3,4)P2, a phosphoinositide that sustains aberrant AKT signaling in Jak2V617F progenitors. Pharmacologic inhibition of AKT with the FDA-approved inhibitor capivasertib in Jak2V617F-transplanted mice similarly reduced splenomegaly and erythroid proliferation, mimicking the effects of Pitp{beta} loss. Collectively, these results identify a novel PITP{beta}-PtdIns(3,4)P2 signaling axis that selectively maintains pathological AKT activation in JAK2V617F-driven MPN, revealing a promising therapeutic vulnerability.

Shwachman-Diamond syndrome (SDS) is an inherited bone marrow failure syndrome with predisposition to developing leukemia. We found that multiple independent somatic hematopoietic clones arise early in life, most commonly harboring heterozygous mutations in EIF6 or TP53. EIF6 mutations cause functional compensation for the germline deficiency by alleviating the SDS ribosome joining defect, improving translation, and reducing p53 activation. TP53 mutations decrease checkpoint activation without affecting ribosome assembly. We link development of leukemia with acquisition of biallelic TP53 alterations. Our results define distinct pathways of clonal selection driven by germline fitness constraint and provide a mechanistic framework for clinical surveillance.

BACKGROUNDPlatelet factor 4-polyanion enzyme-linked immunosorbent assays (ELISAs) are considered highly sensitive for diagnosing heparin-induced thrombocytopenia (HIT), such that current practice guidelines recommend use of ELISA-negative results to exclude HIT. Once HIT is ruled out, alternative, non-heparin-based anticoagulant treatments are ceased, and heparin reintroduction frequently occurs. METHODSAntigen-based and PF4-dependent functional testing were used to study PF4/polyvinyl sulfonate ELISA-negative platelet-activating antibodies in HIT-suspected patients and mice immunized with PF4/heparin. RESULTSThree patients with clinical presentations consistent with HIT tested negative in an ELISA using PF4-polyvinylsulfonate (PF4/PVS), an antigenic target very commonly used for HIT antibody detection. All three patients demonstrated PF4-dependent platelet activation in functional testing that was sensitive to blockade of platelet Fc{gamma}RIIa receptors and inhibited by high concentrations of heparin, consistent with pathogenic HIT antibodies. Functional testing-based screening of 500 ELISA-negative patients identified three patients whose sera activated platelets in a PF4- and Fc{gamma}RIIa-dependent manner, and had clinical histories consistent with HIT. Five of the six ELISA-negative HIT patients were re-exposed to heparin, which precipitated a decrease in platelet counts in all re-exposed patients, and one patient developed a new thrombus. To advance the study of ELISA-negative HIT antibodies, mice were immunized with PF4/heparin, and functional and antigenic assays were simultaneously used to successfully identify an ELISA-negative, PF4-dependent platelet-activating murine monoclonal antibody that recapitulated the serological characteristics of ELISA-negative HIT patients. CONCLUSIONSRecognition of ELISA-negative HIT is critical to avoid harm due to the cessation of alternative anticoagulation therapy and re-exposure of these patients to heparin.

Heterozygous defects in runt-related transcription factor-1 (RUNX1) are causative of a familial platelet disorder with associated myeloid malignancy (FPDMM). Since RUNX1-deficient animal models do not mimic FPDMMs bleeding disorder or leukemic risk, establishment of a proper model system is critical to understand the underlying mechanisms of the observed phenotype and to identify therapeutic interventions. We previously reported an in vitro-megakaryopoiesis system using human CD34+-hematopoietic stem and progenitor cells that recapitulated the FPDMM quantitative megakaryocyte defect by decreasing RUNX1 expression using a lentiviral short-hairpin RNA (shRNA for RUNX1 or shRX) strategy. We now show that shRX-megakaryocytes have a marked reduction in agonist responsiveness. We then infused shRX-megakaryocytes into immunocompromised NOD-SCID gamma (NSG) mice and demonstrated that these megakaryocytes released fewer platelets than megakaryocytes transfected with a non-targeting shRNA, and these platelets had a diminished half-life. The platelets were also poorly responsive to agonists, unable to correct thrombus formation in NSG mice homozygous for a R1326H mutation in von Willebrand Factor (VWFR1326H), which switches species-binding specificity of the VWF from mouse to human glycoprotein Ib. A small-molecule inhibitor RepSox, which blocks the transforming-growth factor beta pathway, and which rescued defective megakaryopoiesis in vitro, corrected the thrombopoietic defect, platelet half-life and agonist response, and thrombus formation in NSG/VWFR1326H mice. Thus, this model recapitulates the defect in FPDMM megakaryocytes and platelets, identifies previously unrecognized defects in thrombopoiesis and platelet half-life, and demonstrates, for the first time, reversal of RUNX1 deficiencys hemostatic defects by a drug. Key PointsO_LIRUNX1-deficient megakaryocytes exhibit thrombopoietic and platelet defects in NSG/VWFR1326H mice. C_LIO_LIPre-exposure of RUNX1-deficient megakaryocytes to a TGF{beta}1-pathway inhibitor ameliorated both defects, correcting hemostasis. C_LI

Hematopoietic stem cells are regulated by endothelial and mesenchymal stromal cells in the marrow niche1-3. Leukemogenesis was long believed to be solely driven by genetic perturbations in hematopoietic cells but introduction of genetic mutations in the microenvironment demonstrated the ability of niche cells to drive disease progression4-8. The mechanisms by which the stem cell niche induces leukemia remain poorly understood. Here, using cellular barcoding in zebrafish, we found that clones of niche endothelial and stromal cells are significantly expanded in leukemic marrows. The pro-angiogenic peptide apelin secreted by leukemic cells induced sinusoidal endothelial cell clonal selection and transcriptional reprogramming towards an angiogenic state to promote leukemogenesis in vivo. Overexpression of apelin in normal hematopoietic stem cells led to clonal amplification of the niche endothelial cells and promotes clonal dominance of blood cells. Knock-out of apelin in leukemic zebrafish resulted in a significant reduction in disease progression. Our results demonstrate that leukemic cells remodel the clonal and transcriptional landscape of the marrow niche to promote leukemogenesis and provide a potential therapeutic opportunity for anti-apelin treatment.

Despite advances in understanding the genetic abnormalities in myeloproliferative neoplasms (MPNs) and the development of JAK2 inhibitors, there is an urgent need to devise new treatment strategies, particularly for triple negative myelofibrosis (MF) patients whose MPNs lack mutations in the JAK2 kinase pathway and have very poor clinical outcomes. Here we report that MYC copy number gain and increased MYC expression frequently occur in triple negative MF, and that MYC-directed activation of S100A9, an alarmin protein that plays pivotal roles in inflammation and innate immunity, is necessary and sufficient to drive development and progression of MF. Notably, the MYC-S100A9 circuit provokes a complex network of inflammatory signaling that involves various hematopoietic cell types in the bone marrow microenvironment. Accordingly, genetic ablation of S100A9 or treatment with small molecules targeting the MYC-S100A9 pathway effectively ameliorates MF phenotypes, highlighting the MYC-alarmin axis as a novel therapeutic vulnerability for this subgroup of MPNs. SIGNIFICANCEThis study establishes that MYC expression is increased in triple negative MPNs via trisomy 8, that a MYC-S100A9 circuit manifest in these cases is sufficient to provoke myelofibrosis and inflammation in diverse hematopoietic cell types in the BM niche, and that the MYC-S100A9 circuit is targetable in triple negative MPN.

Proinflammatory signaling is a hallmark of myeloproliferative neoplasms (MPNs). Several studies have shown that monocytes are a major source of proinflammatory cytokines and monocyte-derived fibrocytes play a pivotal role in the pathogenesis of myelofibrosis (MF). To further explore the role of monocytes in MF, we generated inducible NrasG12D/+Jak2V617F/+(NJ) mice. Recipients transplanted with NJ bone marrow cells developed MF with an early onset of anemia and monocytosis. In vitro, NJ recipients bone marrow nucleated cells exhibited increased quantity of CD45+CollagenI+ fibrocytes, which were mainly derived from the Ly6chigh monocytes. RNA sequencing identified a significant elevated expression of CD38 (a nicotinamide adenine dinucleotide (NAD)+ hydrolase) in Ly6chigh monocytes from NJ mice, which results in pronounced lower level of NAD+. In humans, CD14+ monocytes from MF patients showed significantly higher expression of CD38 than controls and monocytes from polycythemia vera (PV) patients with grade 1 fibrosis had higher CD38 expression than those without fibrosis. Finally, we tested that boosting NAD+ via pharmacological CD38 targeting or NAD+ precursor supplementation inhibited the differentiation of fibrocytes in vitro and observed that targeting CD38 can effectively prevent the onset of fibrosis in vivo. Collectively, our findings shed light on the role of CD38 in monocytes and suggest potential clinical applications such as use of CD38 as a biomarker of fibrotic progression and potential clinical utility of CD38 inhibition in patients with MF. Key PointsO_LICD38-overexpressing monocytes are increased in MF murine models and MPN patients progressing to fibrotic-phase disease. C_LIO_LIRestoring intracellular NAD+ levels using the CD38 inhibitor 78c prevented the development of fibrotic-phase disease in MPN murine models. C_LI

Clonal hematopoiesis of indeterminate potential (CHIP) is characterized by expansion of mutant hematopoietic stem and progenitor cells (HSPCs) and an increased risk of chronic diseases and cancers. While mutations in DNMT3A, TET2, and ASXL1 are common in CHIP, the contribution of less frequent gene mutations is not well understood. Here, we report MYD88 mutations, including lymphoma-associated and novel variants in blood cells of the general population and newly diagnosed solid cancer patients. MYD88 CHIP mutations in HSPCs activate NF-{kappa}B, indicating a gain-of-function activity. Modeling MYD88 CHIP in mice, Myd88L252P (equivalent of human L265P) expression resulted in a competitive fitness advantage of HSPCs. Myd88L252P HSPCs exhibit a myeloid cell bias and inflammation, leading to hematologic disease. Single-cell RNA sequencing indicated that Myd88L252P expands distinct hematopoietic and immune cell clusters and activates immune-related pathways in HSPCs. An IRAK1/4 inhibitor suppressed MYD88-dependent NF-{kappa}B activation and reversed Myd88L252P cell expansion. Overall, MYD88 mutations contribute to CHIP by inducing innate immune pathways in HSPCs and inflammatory disease.

Myeloproliferative neoplasms (MPN) are clonal disorders of hematopoietic stem cells (HSC) that are most frequently caused by acquired somatic mutations in JAK2. A number of conditional mouse models of JAK2-V617F-driven MPN have been generated that rely on Cre-LoxP mediated activation, resulting in polyclonal disease. To more closely mimic the monoclonal origin of human MPN, transplantations of single purified JAK2-mutant HSCs or bone marrow (BM) at limiting dilutions into lethally irradiated recipient mice have been previously performed. However, irradiation is known to alter the BM microenvironment and also to induce transient aplasia accompanied by elevated cytokine levels that promotes the expansion of the mutant clone. To overcome these limitations, we examined whether JAK2-V617F-mutant HSCs are able to engraft and initiate MPN in non-conditioned recipients. We found that BM from two different MPN models, one expressing the human JAK2-V617F, and another expressing the mouse Jak2-V617F, efficiently engrafted and initiated MPN in non-irradiated immunocompromised Rag2-/- recipients. MPN evolved even in transplantations at limiting dilutions, showing high competitiveness of single JAK2-mutant HSCs. Thus, JAK2-V617F mutant HSCs can outcompete resident non-mutated HSCs in the absence of elevated cytokine levels and without the need of emptying stem cell niches by irradiation. However, only BM from mice expressing the mouse Jak2-V617F engrafted and initiated disease in non-conditioned C57BL/6 mice, while BM from mice expressing the human JAK2-V617F was rejected, indicating that mouse Jak2-V617F is ignored by the immune surveillance. These results provide a possible explanation why JAK2-V617F is so frequently found in healthy individuals with clonal hematopoiesis.