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Somatic mutations in UBA1 cause vacuoles, E1 ubiquitin-activating enzyme, X-linked, autoinflammatory somatic (VEXAS) syndrome, an adult-onset inflammatory disease with an overlap of hematologic manifestations. VEXAS syndrome is characterized by a high mortality rate and significant clinical heterogeneity. We sought to determine independent predictors of survival in VEXAS and to understand the mechanistic basis for these factors. We analyzed 83 patients with somatic pathogenic variants in UBA1 at p.Met41 (p.Met41Leu/Thr/Val), the start codon for translation of the cytoplasmic isoform of UBA1 (UBA1b). Patients with the p.Met41Val genotype were most likely to have an undifferentiated inflammatory syndrome. Multivariate analysis showed ear chondritis was associated with increased survival, whereas transfusion dependence and the p.Met41Val variant were independently associated with decreased survival. Using in vitro models and patient-derived cells, we demonstrate that p.Met41Val variant supports less UBA1b translation than either p.Met41Leu or p.Met41Thr, providing a molecular rationale for decreased survival. In addition, we show that these 3 canonical VEXAS variants produce more UBA1b than any of the 6 other possible single-nucleotide variants within this codon. Finally, we report a patient, clinically diagnosed with VEXAS syndrome, with 2 novel mutations in UBA1 occurring in cis on the same allele. One mutation (c.121 A>T; p.Met41Leu) caused severely reduced translation of UBA1b in a reporter assay, but coexpression with the second mutation (c.119 G>C; p.Gly40Ala) rescued UBA1b levels to those of canonical mutations. We conclude that regulation of residual UBA1b translation is fundamental to the pathogenesis of VEXAS syndrome and contributes to disease prognosis.

Reduction of adenosine deaminase 2 (ADA2) activity due to autosomal-recessive loss-of-function mutations in the ADA2 gene (previously known as CECR1) results in a systemic vasculitis known as deficiency of ADA2 (DADA2). Neutrophils and a subset of neutrophils known as low-density granulocytes (LDGs) have been implicated in the pathogenesis of vasculitis, at least in part, through the formation of neutrophil extracellular traps (NETs). The study objective was to determine whether neutrophils and NETs play a pathogenic role in DADA2. In vivo evidence demonstrated NETs and macrophages in affected gastrointestinal tissue from patients with DADA2. An abundance of circulating LDGs prone to spontaneous NET formation was observed during active disease in DADA2 and were significantly reduced after remission induction by anti–tumor necrosis factor (TNF) therapy. Increased circulating LDGs were identified in unaffected family members with monoallelic ADA2 mutations. Adenosine triggered NET formation, particularly in neutrophils from female patients, by engaging A1 and A3 adenosine receptors (ARs) and through reactive oxygen species– and peptidylarginine deiminase–dependent pathways. Adenosine-induced NET formation was inhibited by recombinant ADA2, A1/A3 AR antagonists, or by an A2A agonist. M1 macrophages incubated with NETs derived from patients with DADA2 released significantly greater amounts of TNF-a. Treatment with an A2AAR agonist decreased nuclear translocation of NF-?B and subsequent production of inflammatory cytokines in DADA2 monocyte-derived macrophages. These results suggest that neutrophils may play a pathogenic role in DADA2. Modulation of adenosine-mediated NET formation may contribute a novel and directed therapeutic approach in the treatment of DADA2 and potentially other inflammatory diseases.

•VTE is common in VEXAS syndrome occurring in over 40% of patients with frequent recurrences; however, it is not associated with increased mortality.•Given the high risk of VTE, VEXAS patients should receive thromboprophylaxis in high-risk settings unless contraindicated.

•UBA1mutations are primarily responsible for myeloid clonal expansion and both the inflammatory and hematologic phenotype in VEXAS.•Patients with VEXAS have an enrichment of typical CH mutations, particularly in DNMT3Aand TET2.

Wnt signaling is important for T-cell differentiation at the early CD4−CD8− stage and is subsequently downregulated with maturation. To assess the importance of this downregulation, we generated a mouse line (R26-βcat) in which high levels of active β-catenin are maintained throughout T-cell development. Young R26-βcat mice show a differentiation block at the CD4+CD8+ double-positive (DP) stage. These DP cells exhibit impaired apoptosis upon irradiation or dexamethasone treatment. All R26-βcat mice develop T-cell leukemias at 5 to 6 months of age. R26-βcat leukemias remain dependent on β-catenin function but lack Notch pathway activation. They exhibit recurrent secondary genomic rearrangements that lead to Myc overexpression and loss of Pten activity. Because β-catenin activation and Myc translocations were previously found in murine T-cell acute lymphoblastic leukemias (T-ALLs) deficient for Pten, our results suggest that activation of the canonical Wnt pathway is associated with a subtype of Notch-independent T-ALLs that bear Myc gene rearrangements and Pten mutations.

The physiologic role played by plasmacytoid dendritic cells (pDCs) in the induction of innate responses and inflammation in response to pathogen signaling is not well understood. Here, we describe a new mouse model lacking pDCs and establish that pDCs are essential for the in vivo induction of NK-cell activity in response to Toll-like receptor 9 (TLR9) triggering. Furthermore, we provide the first evidence that pDCs are critical for the systemic production of a wide variety of chemokines in response to TLR9 activation. Consequently, we observed a profound alteration in monocyte, macrophage, neutrophil, and NK-cell recruitment at the site of inflammation in the absence of pDCs in response to CpG-Dotap and stimulation by microbial pathogens, such as Leishmania major, Escherichia coli, and Mycobacterium bovis. This study, which is based on the development of a constitutively pDC-deficient mouse model, highlights the pivotal role played by pDCs in the induction of innate immune responses and inflammation after TLR9 triggering.

Point mutations that trigger ligand-independent proteolysis of the Notch1 ectodomain occur frequently in human T-cell acute lymphoblastic leukemia (T-ALL) but are rare in murine T-ALL, suggesting that other mechanisms account for Notch1 activation in murine tumors. Here we show that most murine T-ALLs harbor Notch1deletions that fall into 2 types, both leading to ligand-independent Notch1 activation. Type 1 deletions remove exon 1 and the proximal promoter, appear to be RAG-mediated, and are associated with mRNA transcripts that initiate from 3′ regions of Notch1. In line with the RAG dependency of these rearrangements, RAG2 binds to the 5′ end of Notch1in normal thymocytes near the deletion breakpoints. Type 2 deletions remove sequences between exon 1 and exons 26 to 28 of Notch1, appear to be RAG-independent, and are associated with transcripts in which exon 1 is spliced out of frame to 3′ Notch1exons. Translation of both types of transcripts initiates at a conserved methionine residue, M1727, which lies within the Notch1 transmembrane domain. Polypeptides initiating at M1727 insert into membranes and are subject to constitutive cleavage by γ-secretase. Thus, like human T-ALL, murine T-ALL is often associated with acquired mutations that cause ligand-independent Notch1 activation.

The Notch pathway is frequently activated in T-cell acute lymphoblastic leukemias (T-ALLs). Of the Notch receptors, Notch1 is a recurrent target of gain-of-function mutations and Notch3 is expressed in all T-ALLs, but it is currently unclear how these receptors contribute to T-cell transformation in vivo. We investigated the role of Notch1 and Notch3 in T-ALL progression by a genetic approach, in mice bearing a knockdown mutation in the Ikaros gene that spontaneously develop Notch-dependent T-ALL. While deletion of Notch3has little effect, T cell–specific deletion of floxed Notch1 promoter/exon 1 sequences significantly accelerates leukemogenesis. Notch1-deleted tumors lack surface Notch1 but express γ-secretase–cleaved intracellular Notch1 proteins. In addition, these tumors accumulate high levels of truncated Notch1 transcripts that are caused by aberrant transcription from cryptic initiation sites in the 3′ part of the gene. Deletion of the floxed sequences directly reprograms the Notch1locus to begin transcription from these 3′ promoters and is accompanied by an epigenetic reorganization of the Notch1locus that is consistent with transcriptional activation. Further, spontaneous deletion of 5′ Notch1 sequences occurs in approximately 75% of Ikaros-deficient T-ALLs. These results reveal a novel mechanism for the oncogenic activation of the Notch1gene after deletion of its main promoter.

The Notch pathway is frequently activated in T-cell acute lymphoblastic leukemias (T-ALLs). Of the Notch receptors, Notch1 is a recurrent target of gain-of-function mutations and Notch3 is expressed in all T-ALLs, but it is currently unclear how these receptors contribute to T-cell transformation in vivo. We investigated the role of Notch1 and Notch3 in T-ALL progression by a genetic approach, in mice bearing a knockdown mutation in the Ikaros gene that spontaneously develop Notch-dependent T-ALL. While deletion of Notch3 has little effect, T cell–specific deletion of floxed Notch1 promoter/exon 1 sequences significantly accelerates leukemogenesis. Notch1-deleted tumors lack surface Notch1 but express γ-secretase–cleaved intracellular Notch1 proteins. In addition, these tumors accumulate high levels of truncated Notch1 transcripts that are caused by aberrant transcription from cryptic initiation sites in the 3′ part of the gene. Deletion of the floxed sequences directly reprograms the Notch1 locus to begin transcription from these 3′ promoters and is accompanied by an epigenetic reorganization of the Notch1 locus that is consistent with transcriptional activation. Further, spontaneous deletion of 5′ Notch1 sequences occurs in approximately 75% of Ikaros-deficient T-ALLs. These results reveal a novel mechanism for the oncogenic activation of the Notch1 gene after deletion of its main promoter.

Familial Mediterranean fever (FMF) is an autoinflammatory disease caused by mutations in MEFV, which encodes a 781–amino acid protein denoted pyrin. We have previously shown that pyrin regulates caspase-1 activation and IL-1β production through interaction of its N-terminal PYD motif with the ASC adapter protein, and also modulates IL-1β production by interaction of its C-terminal B30.2 domain with the catalytic domains of caspase-1. We now asked whether pyrin might itself be a caspase-1 substrate, and found that pyrin is cleaved by caspase-1 at Asp330, a site remote from the B30.2 domain. Pyrin variants harboring FMF-associated B30.2 mutations were cleaved more efficiently than wild-type pyrin. The N-terminal cleaved fragment interacted with the p65 subunit of NF-κB and with IκB-α through its 15-aa bZIP basic domain and adjacent sequences, respectively, and translocated to the nucleus. The interaction of the N-terminal fragment with p65 enhanced entrance of p65 into the nucleus. The interaction of N-terminal pyrin with IκB-α induced calpain-mediated degradation of IκB-α, thus potentiating NF-κB activation. Absolute and relative quantities of cleaved pyrin and IκB-α degradation products were substantially increased in leukocytes from FMF patients compared with healthy controls. Our data support a new pyrin/caspase-1 pathway for NF-κB activation.

Somatic mutations in UBA1in hematopoietic stem cells and myeloid cells have recently been described and are associated with adult-onset severe autoinflammatory diseases including relapsing polychondritis, Sweet syndrome, polyarteritis nodosa, and giant cell arteritis. This newly defined syndrome is named VEXAS (vacuoles, E1, X-linked, autoinflammatory, somatic). We performed clinicopathologic, cytogenetic, flow cytometric and molecular bone marrow assessment of 15 patients diagnosed with VEXAS and peripheral cytopenias. All patients were male, with a median age of 64y (IQR 45-80) and all had somatic missense mutations affecting the p.Met41 residue in UBA1with variant allele frequencies (VAF) over 20% from peripheral blood or bone marrow samples and with lineage restriction to mature myeloid cells. Germline mutations at this residue are not reported in any public databases. Peripheral blood findings included macrocytic anemia in all patients (100%), thrombocytopenia in 9/15, and neutropenia in 1/15. All bone marrow aspirates (15/15; 100%) showed prominent vacuolization of both myeloid and erythroid precursors with few vacuoles noted in mature cells. Of the 9 patients tested for serum copper levels all were normal or borderline low. Marrow was hypercellular with granulocytic and megakaryocytic hyperplasia in 12/15 (80%). Definitive WHO criteria for MDS was met in 5/15 (40%) with 3 cases of MDS-MLD, and 2 of MDS-SLD. The remaining cases were suspicious for MDS with low or borderline levels of dyspoiesis that did not exceed greater than 10% of cells in respective lineages meeting criteria for clonal cytopenia of undetermined significance (CCUS). Of the MDS cases, dysplasia was seen in megakaryocytes (5/5), myeloid (2/5) and erythroid (1/5) precursors. Cytogenetic abnormalities were detected in 2/5 MDS patients involving t(3;12)(q21;q13) in one case, and del (5q)/del (13q) in another case. Next generation sequencing analyses were performed in 9/15 patients and showed mutations in DNMT3Ain 2/5 MDS cases (VAF 43% and 44%), CSF1Rin 1/5 MDS (VAF 3.1%), GNA11in 1/5 MDS (VAF3.3%) and EZH2mutation (VAF 22%) in one of the patients without overt MDS. Clonal plasma cell or B-cell populations were diagnosed in 6/15 patients with 3 plasma cell dyscrasias, and 2 cases of monoclonal B-cell lymphocytosis. The most common abnormalities detected by bone marrow flow cytometry analysis were severely reduced or absent B-cell precursors, inverted CD4:CD8 ratios and abnormal expression of CD56 on monocytes.

Young: Novartis: Research Funding.

Tumor necrosis factor (TNF) receptor–associated periodic syndrome (TRAPS) is an autosomal dominant systemic autoinflammatory disease associated with heterozygous mutations in TNF receptor 1 (TNFR1). Here we examined the structural and functional alterations caused by 9 distinct TRAPS-associated TNFR1 mutations in transfected cells and a mouse “knock-in” model of TRAPS. We found that these TNFR1 mutants did not generate soluble versions of the receptor, either through membrane cleavage or in exosomes. Mutant receptors did not bind TNF and failed to function as dominant-negative inhibitors of TNFR1-induced apoptosis. Instead, TRAPS mutant TNFR1 formed abnormal disulfide-linked oligomers that failed to interact with wild-type TNFR1 molecules through the preligand assembly domain (PLAD) that normally governs receptor self-association. TRAPS mutant TNFR1 molecules were retained intracellularly and colocalized with endoplasmic reticulum (ER) markers. The capacity of mutant receptors to spontaneously induce both apoptosis and nuclear factor κB (NF-κB) activity was reduced. In contrast, the R92Q variant of TNFR1 behaved like the wild-type receptor in all of these assays. The inflammatory phenotype of TRAPS may be due to consequences of mutant TNFR1 protein misfolding and ER retention.

Plasmacytoid dendritic cells (PDCs), which produce IFN-α in response to autoimmune complexes containing nuclear antigens, are thought to be critically involved in the pathogenesis of systemic lupus erythematosus (SLE). One of the immunostimulatory components of SLE immune complexes (SLE-ICs) is self DNA, which is recognized through Tlr9 in PDCs and B cells. Small nuclear ribonucleoproteins (snRNPs) are another major component of SLE-ICs in 30% to 40% of patients. In this study, we show that murine PDCs are activated by purified U1snRNP/anti-Sm ICs to produce IFN-α and proinflammatory cytokines and to up-regulate costimulatory molecules. The induction of IFN-α and IL-6 by U1snRNPs in murine bone marrow–derived PDCs required the presence of intact U1RNA and was largely dependent on Tlr7 but independent of Tlr3. Intracellularly delivered isolated U1snRNA and oligoribonucleotides derived from the stem loop regions and the Sm-binding site of U1snRNA efficiently induced IFN-α and IL-6 in Flt3L-cultured DCs in a Tlr7-dependent manner. The U1snRNA component of U1snRNP immune complexes, found in patients with SLE, acts as an endogenous “self” ligand for Tlr7 and triggers IFN-α and IL-6 production in PDCs.

The PU.1 transcription factor is a key regulator of hematopoietic development, but its role at each hematopoietic stage remains unclear. In particular, the expression of PU.1 in hematopoietic stem cells (HSCs) could simply represent “priming” of genes related to downstream myelolymphoid lineages. By using a conditional PU.1 knock-out model, we here show that HSCs express PU.1, and its constitutive expression is necessary for maintenance of the HSC pool in the bone marrow. Bone marrow HSCs disrupted with PU.1 in situ could not maintain hematopoiesis and were outcompeted by normal HSCs. PU.1-deficient HSCs also failed to generate the earliest myeloid and lymphoid progenitors. PU.1 disruption in granulocyte/monocyte-committed progenitors blocked their maturation but not proliferation, resulting in myeloblast colony formation. PU.1 disruption in common lymphoid progenitors, however, did not prevent their B-cell maturation. In vivo disruption of PU.1 in mature B cells by the CD19-Cre locus did not affect B-cell maturation, and PU.1-deficient mature B cells displayed normal proliferation in response to mitogenic signals including the cross-linking of surface immunoglobulin M (IgM). Thus, PU.1 plays indispensable and distinct roles in hematopoietic development through supporting HSC self-renewal as well as commitment and maturation of myeloid and lymphoid lineages.

PU.1 is a hematopoietic-specific transcriptional activator that is absolutely required for the differentiation of B lymphocytes and myeloid-lineage cells. Although PU.1 is also expressed by early erythroid progenitor cells, its role in erythropoiesis, if any, is unknown. To investigate the relevance of PU.1 in erythropoiesis, we produced a line of PU.1-deficient mice carrying a green fluorescent protein reporter at this locus. We report here that PU.1 is tightly regulated during differentiation—it is expressed at low levels in erythroid progenitor cells and down-regulated upon terminal differentiation. Strikingly, PU.1-deficient fetal erythroid progenitors lose their self-renewal capacity and undergo proliferation arrest, premature differentiation, and apoptosis. In adult mice lacking one PU.1 allele, similar defects are detected following stress-induced erythropoiesis. These studies identify PU.1 as a novel and critical regulator of erythropoiesis and highlight the versatility of this transcription factor in promoting or preventing differentiation depending on the hematopoietic lineage.

The Ikaros gene encodes a zinc finger transcription factor that is selectively expressed by all hematopoietic cells. Although Ikaros is required for lymphocyte differentiation, its role in the myeloid lineage is unclear. We show here that Ikaros expression is temporally regulated during neutrophil differentiation: Ikaros is primarily expressed at immature stages and significantly less so in mature neutrophils. Furthermore IkL/L mice, harboring a hypomorphic mutation at the Ikaros locus, exhibit several defects during neutrophil differentiation. (1) IkL/L fetal livers contain high numbers of neutrophil lineage cells, and this increase is reflected in the number of GM-CSF-dependent progenitor cells. (2) The migratory potential and survival of neutrophil progenitors is altered in vitro. (3) Expression of the Gr-1 marker is delayed and repressed. In contrast, neutrophil function appears normal. These data demonstrate that Ikaros regulates early neutrophil differentiation but is dispensable in mature neutrophils.

Familial Mediterranean fever (FMF) is a recessive disorder characterized by episodes of fever and intense inflammation. FMF attacks are unique in their sensitivity to the microtubule inhibitor colchicine, contrasted with their refractoriness to the anti-inflammatory effects of glucocorticoids. The FMF gene,MEFV, was recently identified by positional cloning; it is expressed at high levels in granulocytes and monocytes. The present study investigated the subcellular localization of the normal gene product, pyrin. These experiments did not support previously proposed nuclear or Golgi localizations. Instead fluorescence microscopy demonstrated colocalization of full-length GFP- and epitope-tagged pyrin with microtubules; this was markedly accentuated in paclitaxel-treated cells. Moreover, immunoblot analysis of precipitates of stabilized microtubules with recombinant pyrin demonstrated a direct interaction in vitro. Pyrin expression did not affect the stability of microtubules. Deletion constructs showed that the unique N-terminal domain of pyrin is necessary and sufficient for colocalization, whereas disease-associated mutations in the C-terminal B30.2 (rfp) domain did not disrupt this interaction. By phalloidin staining, a colocalization of pyrin with actin was also observed in perinuclear filaments and in peripheral lamellar ruffles. The proposal is made that pyrin regulates inflammatory responses at the level of leukocyte cytoskeletal organization and that the unique therapeutic effect of colchicine in FMF may be dependent on this interaction.

Acute promyelocytic leukemia (APL) is always associated with chromosomal translocations that disrupt the retinoic acid receptor α (RARα) gene. Whether these translocations relate to a role for endogenous RARα in normal granulopoiesis remains uncertain because most studies addressing this question have used non-physiological overexpression systems. Granulocyte differentiation in cells derived from RARα-deficient (RARα−/−) mice was studied and evaluated in the context of agonist-bound and ligand-free RARα. Our results demonstrate that RARα is dispensable for granulopoiesis, as RARα−/− mice have a normal granulocyte population despite an impaired ability to respond to retinoids. However, although it is not absolutely required, RARα can bidirectionally modulate granulopoiesis. RARα stimulates differentiation in response to exogenous retinoic acid. Furthermore, endogenous retinoids control granulopoiesis in vivo, as either vitamin A–deficient mice or animals treated with an RAR antagonist accumulate more immature granulocytes in their bone marrow. Conversely, RARα acts to limit differentiation in the absence of ligand because granulocyte precursors from RARα−/− mice differentiate earlier in culture. Thus, the block in granulopoiesis exerted by RARα fusion proteins expressed in APL cells may correspond to an amplification of a normal function of unliganded RARα.

Familial Mediterranean fever (FMF) is a recessive disorder characterized by episodes of fever and neutrophil-mediated serosal inflammation. We recently identified the gene causing FMF, designatedMEFV, and found it to be expressed in mature neutrophils, suggesting that it functions as an inflammatory regulator. To facilitate our understanding of the normal function of MEFV, we extended our previous studies. MEFV messenger RNA was detected by reverse transcriptase–polymerase chain reaction in bone marrow leukocytes, with differential expression observed among cells by in situ hybridization. CD34 hematopoietic stem-cell cultures induced toward the granulocytic lineage expressed MEFV at the myelocyte stage, concurrently with lineage commitment. The prepromyelocytic cell line HL60 expressed MEFV only at granulocytic and monocytic differentiation. MEFV was also expressed in the monocytic cell lines U937 and THP-1. Among peripheral blood leukocytes, MEFV expression was detected in neutrophils, eosinophils, and to varying degrees, monocytes. Consistent with the tissue specificity of expression, complete sequencing and analysis of upstream regulatory regions of MEFV revealed homology to myeloid-specific promoters and to more broadly expressed inflammatory promoter elements. In vitro stimulation of monocytes with the proinflammatory agents interferon (IFN) ?, tumor necrosis factor, and lipopolysaccharide induced MEFV expression, whereas the antiinflammatory cytokines interleukin (IL) 4, IL-10, and transforming growth factor ß inhibited such expression. Induction by IFN-? occurred rapidly and was resistant to cycloheximide. IFN-? also induced MEFV expression. In granulocytes, MEFV was up-regulated by IFN-? and the combination of IFN-? and colchicine. These results refine understanding of MEFV by placing the gene in the myelomonocytic-specific proinflammatory pathway and identifying it as an IFN-? immediate early gene.

Erythropoietin (Epo) is the central regulator of red blood cell production and acts primarily by inducing proliferation and differentiation of erythroid progenitor cells. Because a sufficient supply of iron is a prerequisite for erythroid proliferation and hemoglobin synthesis, we have investigated whether Epo can regulate cellular iron metabolism. We present here a novel biologic function of Epo, namely as a potential modulator of cellular iron homeostasis. We show that, in human (K562) and murine erythroleukemic cells (MEL), Epo enhances the binding affinity of iron-regulatory protein (IRP)-1, the central regulator of cellular iron metabolism, to specific RNA stem-loop structures, known as iron-responsive elements (IREs). Activation of IRP-1 by Epo is associated with a marked increase in transferrin receptor (trf-rec) mRNA levels in K562 and MEL, enhanced cell surface expression of trf-recs, and increased uptake of iron into cells. These findings are in agreement with the well-established mechanism whereby high-affinity binding of IRPs to IREs stabilizes trf-rec mRNA by protecting it from degradation by a specific RNase. The effects of Epo on IRE-binding of IRPs were not observed in human myelomonocytic cells (THP-1), which indicates that this response to Epo is not a general mechanism observed in all cells but is likely to be erythroid-specific. Our results provide evidence for a direct functional connection between Epo biology and iron metabolism by which Epo increases iron uptake into erythroid progenitor cells via posttranscriptional induction of trf-rec expression. Our data suggest that sequential administration of Epo and iron might improve the response to Epo therapy in some anemias.

Valent: Novartis: Consultancy, Honoraria, Research Funding.

We have identified a novel fusion gene in an 18-month old child with juvenile myelomonocytic leukemia (JMML) displaying a reciprocal chromosomal translocation t(5;7)(q33;p11.2). Molecular investigation at diagnosis revealed absence of mutations in KRAS, NRAS, PTPN11, or cCBL, but FISH analysis identified a rearrangement involving the PDGFRB gene located on chromosome 5q33. After temporary responses to imatinib (IM) and subsequently nilotinib (NIL) treatment, resistance associated with disease relapses was observed. Employment of the 5’-RACE technique facilitated identification of the PDGFRB fusion partner on chromosome 7p11.2, the NDEL1 gene encoding the nudE neurodevelopmental protein 1-like 1. The NDEL1 gene has not been implicated in any other reciprocal translocation to date, and it is conceivable that its ability to form dimers could drive permanent kinase activation of PDGFRβ. The chimeric mRNA contains the 5´exons 1-5 of NDEL1 fused in frame to the PDGFRB exons 10-22 containing the transmembrane and tyrosine kinase domains.

In chronic myeloid leukemia (CML) and Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL) patients resistant to tyrosine kinase inhibitors (TKIs), BCR-ABL1 mutation status is an essential component of the therapeutic decision algorithm. Capillary Sanger sequencing (SS) is currently the gold standard for mutation screening of the BCR-ABL1 kinase domain (KD), despite key technical limitations including limited sensitivity and no discrimination between compound and polyclonal mutations. Benchtop next-generation sequencers have recently been introduced as potential diagnostic platforms and there is growing interest in their clinical application. In the framework of the IRON-II (Interlaboratory RObustness of Next-generation sequencing) international consortium, 10 laboratories from 7 countries (Italy, Germany, United Kingdom, Spain, Austria, Turkey, Czech Republic) have engaged in the set-up, standardization and validation of a laboratory-developed screening assay for BCR-ABL1KD mutations based on the Roche 454 amplicon deep-sequencing technology.Fusion primers were designed to generate four partially overlapping amplicons by nested reverse transcription (RT)-polymerase chain reaction (PCR), the first amplification step needed to select for the translocated ABL1 allele. Fusion primers were barcoded with multiplex identifiers (MIDs) consisting of 10-base pair tags allowing multiplexing of twelve clinical samples (forty-eight amplicons) in a single NGS run. The assay was designed in a ready-to-use 96-well plate format containing lyophilized oligonucleotide primers.Different primer designs and primer-MID combinations were evaluated for their performances. Sequencing runs generated an average of 97,432 reads (range, 59,459-151,335). For the primer design selected for further evaluation, the coverage per amplicon ranged between 1,449 and 5,997 sequencing reads. To explore the sensitivity and accuracy of the assay, serial dilutions of BaF3 cell lines harboring four different known mutations (Y253F, E255K, T315I, M351T) into an unmutated BaF3 cell line (50%:50%; 25%:75%; 10%:90%; 5%:95%; 2%:98%; 1%:99%) were sequenced in parallel in two distinct laboratories (Bologna and Jena). In both centers, results showed a high linearity of mutation calling and accuracy of mutation detection and quantitation over the entire range of dilutions, down to 1% mutation abundance. Intra-run reproducibility and inter-run reproducibility were confirmed by a series of experiments in which a set of samples was resequenced in the same and in independent runs, respectively, with and without repetition of the RT and PCR steps. Importantly, we demonstrated that reproducibility could be maintained over a wide dynamic range of amplicon coverage (from 100 to 5,000 independent sequencing reads). A total of 554 clinical samples (2,216 amplicons) were analyzed by the 10 laboratories - including 517 clinical samples analyzed in parallel by NGS and SS and 30 clinical samples analyzed in parallel by NGS, SS and conventional pyrosequencing. Three hundred and ninety-four of 398 (99%) variants detected by SS were also detected by NGS. In addition, comparison between NGS, SS and conventional pyrosequencing results showed very good concordance with respect to the estimation of variant abundance. NGS allowed to detect additional, low level mutations (>1% but<10-15%, i.e. undetectable by SS) in 294/554 (53%) samples. In a subset of twenty randomly selected samples, low level mutations were confirmed by independent methods (restriction fragment length polymorphism or allele-specific oligonucleotide-PCR). Compound mutations as against polyclonality could be resolved in all the clinical samples harboring multiple mutations mapping 450 bp apart or closer. Longitudinal retrospective analysis of CML and Ph+ ALL clinical samples showed that NGS could have identified TKI-resistant mutations earlier than SS, thus allowing more timely therapeutic intervention.Our results indicate the technical feasibility, accuracy and robustness of NGS for BCR-ABL1 KD mutation screening and represent an important step forward towards its routine application in a clinical setting. An international ring trial to test inter-laboratory reproducibility of BCR-ABL1 mutation detection by NGS is now about to start.Soverini: Novartis: Consultancy; Bristol-Myers Squibb: Consultancy; ARIAD: Consultancy. Kohlmann:MLL Munich Leukemia Laboratory: Employment. Machova Polakova:Novartis: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding. Lion:Pfizer: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; Novartis: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Hochhaus:ROCHE: Research Funding. Martinelli:NOVARTIS: Consultancy, Speakers Bureau; BMS: Consultancy, Speakers Bureau; PFIZER: Consultancy; ARIAD: Consultancy.

Valent: Novartis: Honoraria, Research Funding. Lion:Novartis, Bristol-Myers- Squibb, Pfizer: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding.

In chronic myeloid leukemia (CML) and Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL) patients resistant to tyrosine kinase inhibitors (TKIs), BCR-ABL1mutation status is an essential component of the therapeutic decision algorithm. Capillary Sanger sequencing (SS) is currently the gold standard for mutation screening of the BCR-ABL1kinase domain (KD), despite key technical limitations including limited sensitivity and no discrimination between compound and polyclonal mutations. Benchtop next-generation sequencers have recently been introduced as potential diagnostic platforms and there is growing interest in their clinical application. In the framework of the IRON-II (Interlaboratory RObustness of Next-generation sequencing) international consortium, 10 laboratories from 7 countries (Italy, Germany, United Kingdom, Spain, Austria, Turkey, Czech Republic) have engaged in the set-up, standardization and validation of a laboratory-developed screening assay for BCR-ABL1KD mutations based on the Roche 454 amplicon deep-sequencing technology.

No relevant conflicts of interest to declare.

No relevant conflicts of interest to declare.

Understanding how multipotent cells commit to each of their terminal fate potentials is an important aspect of stem cell biology. In adult murine hematopoiesis, HSCs with long-term self-renewal potential reside within the Lin −Sca-1+c-Kit+ (LSK) fraction having CD34−, Thy1lo, and Flt3/Flk2−phenotypes. The LSK cells having CD34+, Thy1−, and/or Flt3+ phenotypes are capable of multi-lineage reconstitution only for a short-term, and therefore should contain multipotent progenitors (MPPs). In terms of developmental steps downstream of MPPs, there has been a controversy. The existence of prospectively isolatable progenitors capable of generating only myeloerythroid cells (common myeloid progenitor: CMP) or only lymphocytes (common lymphoid progenitors: CLP) outside the LSK fraction suggests that the first commitment step after the MPP stage is the strict bifurcation of lymphoid vs. myeloid pathway. Recently, however, several studies have suggested that the lineage commitment could occur within the LSK fraction, preceding the proposed bifurcation of myeloid and lymphoid pathways. For example, MPPs expressing Flt3 at a high level retained granulocytes/monocytes (GM) but not megakaryocyte/erythrocyte (MegE) potential together with lymphoid potential, suggesting the existence of common progenitor for GM and lymphoid lineages within the LSK fraction. Based on this data, the coupled loss of self-renewal activity and MegE potential in the early HSC commitment has been proposed. How can we reconcile the controversy in the early hematopoietic lineage map? By utilizing mice having GATA-1 or PU.1 transcriptional reporters, we here present a high-resolution map containing new lineage-restricted progenitor populations within the MPP population of CD34+ LSK phenotype. Initial upregulation of GATA-1 and PU.1 occurred independently at the MPP stage. The GATA-1+ MPP displayed potent myeloerythroid potential without giving rise to lymphocytes, whereas the PU.1+ MPP showed granulocyte/monocyte/lymphoid-restricted progenitor activity without megakaryocyte/erythroid differentiation. Furthermore, GATA-1+ and PU.1+ MPPs possessed huge expansion potential, and differentiated into the original CMPs and CLPs, respectively. Thus, the reciprocal activation of GATA-1 and PU.1 primarily organizes hematopoietic lineage fate decision to form the earliest hematopoietic branchpoint that comprises new, isolatable myeloerythroid and myelolymphoid progenitor populations.

Understanding how multipotent cells commit to each of their terminal fate potentials is an important aspect of stem cell biology. Hematopoietic stem cells (HSCs) of Lin -Sca-1+c-Kit+ (LSK) phenotype have been purified, which were further divided into CD34-long-term and CD34+ short-term (ST)-HSCs. The existence of phenotypically isolatable common lymphoid progenitors (CLPs) and common myeloid progenitors (CMPs) downstream of ST-HSCs suggests that the first commitment step after the HSC stage is the bifurcation of lymphoid vs. myeloid pathway. Recent studies, however, suggest that the loss of MegE potential could be an early event in HSC stage. For example, LSK cells activating RAG-1 or Flt-3 expression retained granulocyte/monocyte (GM) but not megakaryocyte/erythrocyte (MegE) potential together with lymphoid potential, suggesting the existence of common progenitor for GM and lymphoid lineages. Here we report that a fraction of ST-LSK cells expressing high levels of PU.1, a transcription factor necessary for GM and lymphoid development, represents GM/lymphoid bipotent progenitors. In mice harboring knock-in GFP reporter for PU.1, LSK cells were divided into GFP high and GFP low subpopulations. Although PU.1low LSK cells were multipotent, PU.1high LSK cells differentiated only into GM and lymphoid cells in vitro and in vivo. We also found that single PU.1high LSK cells differentiated into GM, T and B cells in vivo. These data formally prove the existence of the third major early progenitor population activating PU.1 at a high level, the granulocyte/monocyte/lymphoid progenitor (GMLP). The existence of prospectively isolatable GMLPs strongly suggests that HSCs sequentially lose MegE then GM potential during their lymphoid commitment.