Activation of the unfolded protein response is associated with impaired granulopoiesis in transgenic mice expressing mutant Elane

The metabolic basis of inherited neutropenias

Neutrophils are the shortest-lived blood cells, which requires a prodigious degree of proliferation and differentiation to sustain physiologically sufficient numbers and be poised to respond quickly to infectious emergencies. More than 107 neutrophils are produced every minute in an adult bone marrow-a process that is tightly regulated by a small group of cytokines and chemical mediators and dependent on nutrients and energy. Like granulocyte colony-stimulating factor, the primary growth factor for granulopoiesis, they stimulate signalling pathways, some affecting metabolism. Nutrient or energy deficiency stresses the survival, proliferation, and differentiation of neutrophils and their precursors. Thus, it is not surprising that monogenic disorders related to metabolism exist that result in neutropenia. Among these are pathogenic mutations in HAX1, G6PC3, SLC37A4, TAFAZZIN, SBDS, EFL1 and the mitochondrial disorders. These mutations perturb carbohydrate, lipid and/or protein metabolism. We hypothesize that metabolic disturbances may drive the pathogenesis of a subset of inherited neutropenias just as defects in DNA damage response do in Fanconi anaemia, telomere maintenance in dyskeratosis congenita and ribosome formation in Diamond-Blackfan anaemia. Greater understanding of metabolic pathways in granulopoiesis will identify points of vulnerability in production and may point to new strategies for the treatment of neutropenias.

Human genetic defects in SRP19 and SRPRA cause severe congenital neutropenia with distinctive proteome changes

The mechanisms of coordinated changes in proteome composition and their relevance for the differentiation of neutrophil granulocytes are not well studied. Here, we discover 2 novel human genetic defects in signal recognition particle receptor alpha (SRPRA) and SRP19, constituents of the mammalian cotranslational targeting machinery, and characterize their roles in neutrophil granulocyte differentiation. We systematically study the proteome of neutrophil granulocytes from patients with variants in the SRP genes, HAX1, and ELANE, and identify global as well as specific proteome aberrations. Using in vitro differentiation of human induced pluripotent stem cells and in vivo zebrafish models, we study the effects of SRP deficiency on neutrophil granulocyte development. In a heterologous cell-based inducible protein expression system, we validate the effects conferred by SRP dysfunction for selected proteins that we identified in our proteome screen. Thus, SRP-dependent protein processing, intracellular trafficking, and homeostasis are critically important for the differentiation of neutrophil granulocytes.

Truncated CSF3 receptors induce pro-inflammatory responses in severe congenital neutropenia

Severe congenital neutropenia (SCN) patients are prone to develop myelodysplastic syndrome (MDS) or acute myeloid leukaemia (AML). Leukaemic progression of SCN is associated with the early acquisition of CSF3R mutations in haematopoietic progenitor cells (HPCs), which truncate the colony-stimulating factor 3 receptor (CSF3R). These mutant clones may arise years before MDS/AML becomes overt. Introduction and activation of CSF3R truncation mutants in normal HPCs causes a clonally dominant myeloproliferative state in mice treated with CSF3. Paradoxically, in SCN patients receiving CSF3 therapy, clonal dominance of CSF3R mutant clones usually occurs only after the acquisition of additional mutations shortly before frank MDS or AML is diagnosed. To seek an explanation for this discrepancy, we introduced a patient-derived CSF3R-truncating mutation in ELANE-SCN and HAX1-SCN derived and control induced pluripotent stem cells and compared the CSF3 responses of HPCs generated from these lines. In contrast to CSF3R-mutant control HPCs, CSF3R-mutant HPCs from SCN patients do not show increased proliferation but display elevated levels of inflammatory signalling. Thus, activation of the truncated CSF3R in SCN-HPCs does not evoke clonal outgrowth but causes a sustained pro-inflammatory state, which has ramifications for how these CSF3R mutants contribute to the leukaemic transformation of SCN.

Case report: Five-year periodontal management of a patient with two novel mutation sites in ELANE-induced cyclic neutropenia

Cyclic neutropenia (CyN) is a rare, ELANE-related neutropenia. Oral manifestations are among the initial signs of CyN and an important reason that leads patients to seek professional help. This case report describes a 12-year-old girl with recurrent oral ulcers, severe chronic periodontitis, and pathological tooth migration as the initial and main clinical symptoms of CyN. Two novel mutations in ELANE, c.180T>G (p.I60M) and c.182C>G (p.A61G) associated with CyN were observed. Bioinformatics research indicated lower stability and impaired molecular linkages of the mutant neutrophil elastase (NE) encoded by ELANE. However, the enzyme affinity to the classic substrate Suc–Ala–Ala–Ala–pNA was not substantially changed, suggesting that the impaired integrity and stability of the mutant NE, rather than catalytic deficiency, might be the pathogenic mechanism of ELANE mutation-induced neutropenia. The patient was prescribed scaling and root planing (SRP) and monthly periodontal maintenance without systemic management. Although the routine periodontal treatment was occasionally interrupted by the 2019 coronavirus pandemic, her periodontal devastation remained well-remitted in the 5-year follow-up assessment. The results of this study confirmed the importance of plaque control and proper diagnosis in the periodontal management of such patients and provide better clinical references. In addition, the novel mutations identified in this study expand the spectrum of known ELANE mutations in CyN and further contribute to knowledge regarding its pathogenic mechanism.

Heterozygous variants of CLPB are a cause of severe congenital neutropenia

Severe congenital neutropenia is an inborn disorder of granulopoiesis. Approximately one third of cases do not have a known genetic cause. Exome sequencing of 104 persons with congenital neutropenia identified heterozygous missense variants of CLPB (caseinolytic peptidase B) in 5 severe congenital neutropenia cases, with 5 more cases identified through additional sequencing efforts or clinical sequencing. CLPB encodes an adenosine triphosphatase that is implicated in protein folding and mitochondrial function. Prior studies showed that biallelic mutations of CLPB are associated with a syndrome of 3-methylglutaconic aciduria, cataracts, neurologic disease, and variable neutropenia. However, 3-methylglutaconic aciduria was not observed and, other than neutropenia, these clinical features were uncommon in our series. Moreover, the CLPB variants are distinct, consisting of heterozygous variants that cluster near the adenosine triphosphate-binding pocket. Both genetic loss of CLPB and expression of CLPB variants result in impaired granulocytic differentiation of human hematopoietic progenitor cells and increased apoptosis. These CLPB variants associate with wild-type CLPB and inhibit its adenosine triphosphatase and disaggregase activity in a dominant-negative fashion. Finally, expression of CLPB variants is associated with impaired mitochondrial function but does not render cells more sensitive to endoplasmic reticulum stress. Together, these data show that heterozygous CLPB variants are a new and relatively common cause of congenital neutropenia and should be considered in the evaluation of patients with congenital neutropenia.

Congenital neutropenia: disease models guiding new treatment strategies

PURPOSE OF REVIEW

Myeloid diseases are often characterized by a disturbed regulation of myeloid cell proliferation, survival, and maturation. This may either result in a severe paucity of functional neutrophils (neutropenia), an excess production of mature cells (myeloproliferative disorders) or in clonal expansions of dysplastic or immature myeloid cells (myelodysplasia and acute myeloid leukemia). Although these conditions can be regarded as separate entities, caused by the accumulation of distinct sets of somatic gene mutations, it becomes increasingly clear that they may also evolve as the prime consequence of a congenital defect resulting in severe neutropenia. Prominent examples of such conditions include the genetically heterogeneous forms of severe congenital neutropenia (SCN) and Shwachman-Diamond Syndrome. CSF3 treatment is a successful therapy to alleviate neutropenia in the majority of these patients but does not cure the disease nor does it prevent malignant transformation. Allogeneic stem cell transplantation is currently the only therapeutic option to cure SCN, but is relatively cumbersome, e.g., hampered by treatment-related mortality and donor availability. Hence, there is a need for new therapeutic approaches.

RECENT FINDINGS

Developments in disease modeling, amongst others based on induced pluripotent stem cell and CRISPR/Cas9 based gene-editing technologies, have created new insights in disease biology and possibilities for treatment. In addition, they are fueling expectations for advanced disease monitoring to prevent malignant transformation.

SUMMARY

This review highlights the recent progress made in SCN disease modeling and discusses the challenges that are still ahead of us to gain a better understanding of the biological heterogeneity of the disease and its consequences for patient care.

Diagnosis and therapeutic decision-making for the neutropenic patient

Determining the cause of a low neutrophil count in a pediatric or adult patient is essential for the hematologist's clinical decision-making. Fundamental to this diagnostic process is establishing the presence or lack of a mature neutrophil storage pool, as absence places the patient at higher risk for infection and the need for supportive care measures. Many diagnostic tests, eg, a peripheral blood smear and bone marrow biopsy, remain important tools, but greater understanding of the diversity of neutropenic disorders has added new emphasis on evaluating for immune disorders and genetic testing. In this article, a structure is provided to assess patients based on the mechanism of neutropenia and to prioritize testing based on patient age and hypothesized pathophysiology. Common medical quandaries including fever management, need for growth factor support, risk of malignant transformation, and curative options in congenital neutropenia are reviewed to guide medical decision-making in neutropenic patients.

Neutrophil elastase: Nonsense lost in translation

In this issue of Cell Stem Cell, Daniel Bauer and colleagues investigate the pathogenesis of ELANE-associated severe congenital neutropenia (SCN) and describe two potentially universal gene correction strategies for autosomal dominant disorders (Rao et al., 2021). One exploits nonsense-mediated decay to prevent translation of the mutant allele. The other unexpectedly blocks translation by shortening the 3'-UTR.

Dissecting ELANE neutropenia pathogenicity by human HSC gene editing

Severe congenital neutropenia (SCN) is a life-threatening disorder most often caused by dominant mutations of ELANE that interfere with neutrophil maturation. We conducted a pooled CRISPR screen in human hematopoietic stem and progenitor cells (HSPCs) that correlated ELANE mutations with neutrophil maturation potential. Highly efficient gene editing of early exons elicited nonsense-mediated decay (NMD), overcame neutrophil maturation arrest in HSPCs from ELANE-mutant SCN patients, and produced normal hematopoietic engraftment function. Conversely, terminal exon frameshift alleles that mimic SCN-associated mutations escaped NMD, recapitulated neutrophil maturation arrest, and established an animal model of ELANE-mutant SCN. Surprisingly, only -1 frame insertions or deletions (indels) impeded neutrophil maturation, whereas -2 frame late exon indels repressed translation and supported neutrophil maturation. Gene editing of primary HSPCs allowed faithful identification of variant pathogenicity to clarify molecular mechanisms of disease and encourage a universal therapeutic approach to ELANE-mutant neutropenia, returning normal neutrophil production and preserving HSPC function.

iPSC modeling of stage-specific leukemogenesis reveals BAALC as a key oncogene in severe congenital neutropenia

Severe congenital neutropenia (CN) is a pre-leukemic bone marrow failure syndrome that can evolve to acute myeloid leukemia (AML). Mutations in CSF3R and RUNX1 are frequently observed in CN patients, although how they drive the transition from CN to AML (CN/AML) is unclear. Here we establish a model of stepwise leukemogenesis in CN/AML using CRISPR-Cas9 gene editing of CN patient-derived iPSCs. We identified BAALC upregulation and resultant phosphorylation of MK2a as a key leukemogenic event. BAALC deletion or treatment with CMPD1, a selective inhibitor of MK2a phosphorylation, blocked proliferation and induced differentiation of primary CN/AML blasts and CN/AML iPSC-derived hematopoietic stem and progenitor cells (HSPCs) without affecting healthy donor or CN iPSC-derived HSPCs. Beyond detailing a useful method for future investigation of stepwise leukemogenesis, this study suggests that targeting BAALC and/or MK2a phosphorylation may prevent leukemogenic transformation or eliminate AML blasts in CN/AML and RUNX1 mutant BAALC(hi) de novo AML.

CRISPR-Cas9-Mediated ELANE Mutation Correction in Hematopoietic Stem and Progenitor Cells to Treat Severe Congenital Neutropenia

Severe congenital neutropenia (SCN) is a monogenic disorder. SCN patients are prone to recurrent life-threatening infections. The main causes of SCN are autosomal dominant mutations in the ELANE gene that lead to a block in neutrophil differentiation. In this study, we use CRISPR-Cas9 ribonucleoproteins and adeno-associated virus (AAV)6 as a donor template delivery system to repair the ELANEL172P mutation in SCN patient-derived hematopoietic stem and progenitor cells (HSPCs). We used a single guide RNA (sgRNA) specifically targeting the mutant allele, and an sgRNA targeting exon 4 of ELANE. Using the latter sgRNA, ∼34% of the known ELANE mutations can in principle be repaired. We achieved gene correction efficiencies of up to 40% (with sgELANE-ex4) and 56% (with sgELANE-L172P) in the SCN patient-derived HSPCs. Gene repair restored neutrophil differentiation in vitro and in vivo upon HSPC transplantation into humanized mice. Mature edited neutrophils expressed normal elastase levels and behaved normally in functional assays. Thus, we provide a proof of principle for using CRISPR-Cas9 to correct ELANE mutations in patient-derived HSPCs, which may translate into gene therapy for SCN.

Effect of the unfolded protein response and oxidative stress on mutagenesis in CSF3R: a model for evolution of severe congenital neutropenia to myelodysplastic syndrome/acute myeloid leukemia

Severe congenital neutropenia (SCN) is a rare blood disorder characterised by abnormally low levels of circulating neutrophils. The most common recurrent mutations that cause SCN involve neutrophil elastase (ELANE). The treatment of choice for SCN is the administration of granulocyte-colony stimulating factor (G-CSF), which increases the neutrophil number and improves the survival and quality of life. Long-term survival is however linked to the development of myelodysplastic syndrome/acute myeloid leukemia (MDS/AML). About 70% of MDS/AML patients acquire nonsense mutations affecting the cytoplasmic domain of CSF3R (the G-CSF receptor). About 70% of SCN patients with AML harbour additional mutations in RUNX1. We hypothesised that this coding region of CSF3R constitutes a hotspot vulnerable to mutations resulting from excessive oxidative stress or endoplasmic reticulum (ER) stress. We used the murine Ba/F3 cell line to measure the effect of induced oxidative or ER stress on the mutation rate in our hypothesised hotspot of the exogenous human CSF3R, the corresponding region in the endogenous Csf3r, and Runx1. Ba/F3 cells transduced with the cDNA for partial C-terminal of CSF3R fused in-frame with a green fluorescent protein (GFP) tag were subjected to stress-inducing treatment for 30 days (~51 doubling times). The amplicon-based targeted deep sequencing data for days 15 and 30 samples show that although there was increased mutagenesis observed in all the three genes of interest (partial CSF3R, Csf3r and Runx1), there were more mutations in the GFP region compared with the partial CSF3R region. Our findings also indicate that there is no correlation between the stress-inducing chemical treatments and mutagenesis in Ba/F3 cells. Our data suggest that oxidative or ER stress induction does not promote genomic instability, affecting partial C-terminal of the transduced CSF3R, the endogenous Csf3R and the endogenous Runx1 in Ba/F3 cells that could account for these targets to being mutational hotspots. We conclude that other mechanisms to acquire mutations of CSF3R that help drive the evolution of SCN to MDS/AML.

Severe congenital neutropenia-associated JAGN1 mutations unleash a calpain-dependent cell death programme in myeloid cells

Severe congenital neutropenia (SCN) of autosomal recessive inheritance, also known as Kostmann disease, is characterised by a lack of neutrophils and a propensity for life-threatening infections. Using whole-exome sequencing, we identified homozygous JAGN1 mutations (p.Gly14Ser and p.Glu21Asp) in three patients with Kostmann-like SCN, thus confirming the recent attribution of JAGN1 mutations to SCN. Using the human promyelocytic cell line HL-60 as a model, we found that overexpression of patient-derived JAGN1 mutants, but not silencing of JAGN1, augmented cell death in response to the pro-apoptotic stimuli, etoposide, staurosporine, and thapsigargin. Furthermore, cells expressing mutant JAGN1 were remarkably susceptible to agonists that normally trigger degranulation and succumbed to a calcium-dependent cell death programme. This mode of cell death was completely prevented by pharmacological inhibition of calpain but unaffected by caspase inhibition. In conclusion, our results confirmed the association between JAGN1 mutations and SCN and showed that SCN-associated JAGN1 mutations unleash a calcium- and calpain-dependent cell death in myeloid cells.

Normal peripheral blood neutrophil numbers accompanying ELANE whole gene deletion mutation

The patient reported here, along with collective observations in the literature, suggest that ELANE deletion does not cause neutropenia. Potential therapeutic genome editing involving knockout of the mutant ELANE allele is therefore not expected to produce neutropenia.

Inducible expression of a disease-associated ELANE mutation impairs granulocytic differentiation, without eliciting an unfolded protein response

Severe congenital neutropenia (SCN) is characterized by a near absence of neutrophils, rendering individuals with this disorder vulnerable to recurrent life-threatening infections. The majority of SCN cases arise because of germline mutations in the gene elastase, neutrophil-expressed (ELANE) encoding the neutrophil granule serine protease neutrophil elastase. Treatment with a high dose of granulocyte colony-stimulating factor increases neutrophil production and reduces infection risk. How ELANE mutations produce SCN remains unknown. The currently proposed mechanism is that ELANE mutations promote protein misfolding, resulting in endoplasmic reticulum stress and activation of the unfolded protein response (UPR), triggering death of neutrophil precursors and resulting in neutropenia. Here we studied the ELANE mutation p.G185R, often associated with greater clinical severity (e.g. decreased responsiveness to granulocyte colony-stimulating factor and increased leukemogenesis). Using an inducible expression system, we observed that this ELANE mutation diminishes enzymatic activity and granulocytic differentiation without significantly affecting cell proliferation, cell death, or UPR induction in murine myeloblast 32D and human promyelocytic NB4 cells. Impaired differentiation was associated with decreased expression of genes encoding critical hematopoietic transcription factors (Gfi1, Cebpd, Cebpe, and Spi1), cell surface proteins (Csf3r and Gr1), and neutrophil granule proteins (Mpo and Elane). Together, these findings challenge the currently prevailing model that SCN results from mutant ELANE, which triggers endoplasmic reticulum stress, UPR, and apoptosis.

New insights into the pathomechanism of cyclic neutropenia

Cyclic neutropenia (CyN) is a hematologic disorder in which peripheral blood absolute neutrophil counts (ANCs) show cycles of approximately 21-day intervals. The majority of CyN patients harbor ELANE mutations, but the mechanism of ANC cycling is unclear. We performed analysis of bone marrow (BM) subpopulations in CyN patients at the peak and the nadir of the ANC cycle and detected high proportions of BM hematopoietic stem cells (HSCs) and hematopoietic stem and progenitor cells (HSPCs) at the nadir of the ANC cycle, as compared with the peak. BM HSPCs produced fewer granulocyte colony-forming unit colonies at the ANC peak. To investigate the mechanism of cycling, we found that mRNA expression levels of ELANE and unfolded protein response (UPR)-related genes (ATF6, BiP (HSPA5), CHOP (DDIT3), and PERK (EIF2AK3)) were elevated, but antiapoptotic genes (Bcl-2 (BCL2) and bcl-xL (BCL2L1)) were reduced in CD34⁺ cells tested at the ANC nadir. Moreover, HSPCs revealed increased levels of reactive oxygen species and gH2AX at the ANC nadir. We suggest that in CyN patients, some HSPCs escape the UPR-induced endoplasmic reticulum (ER) stress and proliferate in response to granulocyte colony-stimulating factor (G-CSF) to a certain threshold at which UPR again affects the majority of HSPCs. There is a cyclic balance between ER stress-induced apoptosis of HSPCs and compensatory G-CSF-stimulated HSPC proliferation followed by granulocytic differentiation.

Lentiviral gene therapy and vitamin B3 treatment enable granulocytic differentiation of G6PC3-deficient induced pluripotent stem cells

Induced pluripotent stem cells (iPSCs) from patients with genetic disorders are a valuable source for in vitro disease models, which enable drug testing and validation of gene and cell therapies. We generated iPSCs from a severe congenital neutropenia (SCN) patient, who presented with a nonsense mutation in the glucose-6-phosphatase catalytic subunit 3 (G6PC3) gene causing profound defects in granulopoiesis, associated with increased susceptibility of neutrophils to apoptosis. Generated SCN iPSC clones exhibited the capacity to differentiate into hematopoietic cells of the myeloid lineage and we identified two cytokine conditions, i.e., using granulocyte-colony stimulating factor or granulocyte-macrophage colony stimulating factor in combination with interleukin-3, to model the SCN phenotype in vitro. Reduced numbers of granulocytes were produced by SCN iPSCs compared with control iPSCs in both settings, which reflected the phenotype in patients. Interestingly, our model showed increased monocyte/macrophage production from the SCN iPSCs. Most importantly, lentiviral genetic correction of SCN iPSCs with a codon-optimized G6PC3 transgene restored granulopoiesis and reduced apoptosis of in vitro differentiated myeloid cells. Moreover, addition of vitamin B3 clearly induced granulocytic differentiation of SCN iPSCs and increased the number of neutrophils to levels comparable with those obtained from healthy control iPSCs. In summary, we established an iPSC-derived in vitro disease model, which will serve as a tool to test the potency of alternative treatment options for SCN patients, such as small molecules and gene therapeutic vectors.

Management of tooth extraction in a patient with ELANE gene mutation-induced cyclic neutropenia: A case report

INTRODUCTION

Cyclic neutropenia (CyN) is a rare hematological disease, and patients with CyN often experience an early onset of severe periodontitis and are forced to undergo tooth extraction. Here, we report a case of a patient with CyN who showed different periodicity and oscillations of neutrophil count compared with her mother, despite sharing the same novel genetic mutation.

PATIENT CONCERNS

A 17-year-old Japanese girl who had been diagnosed with CyN shortly after birth presented to our hospital with a complaint of mobility of her teeth and gingivitis. Upon presentation, an intraoral examination was performed and revealed redness and swelling of the marginal and attached gingiva. Radiographs revealed extreme resorption of the alveolar bone and apical lesions in her mandibular lateral incisors. The patient's hematologic data demonstrated a lack of blood neutrophils (0/μL). The patient had no history of dental extraction, and her mother also had a history of CyN.

DIAGNOSES

The patient was diagnosed with severe periodontitis that was associated with CyN. Gene testing showed a novel heterozygous mutation in exon 4 of the ELANE gene (c.538delC, p.Leu180Ser fsX11).

INTERVENTIONS

Based on the clinical findings, we planned to extract the patient's mandibular lateral incisors. Although the tooth extraction was scheduled considering the cyclic variation in neutrophil count, the patient's neutrophil count was 0/μL on the day before the planned extraction. Therefore, granulocyte-colony stimulating factor (G-CSF) was administered to increase the patient's neutrophil count. On the day of the patient's admission for the tooth extraction, she presented with fever (body temperature, 38.5°C), tonsillitis, and stomatitis. The extraction was subsequently delayed, and the patient was administered antibiotics and G-CSF for 4 days. At this time, the neutrophil count increased to 750/μL, and the tooth extraction was carried out safely.

OUTCOMES

The postoperative course was uneventful, and the healing process at the extraction site was excellent.

CONCLUSION

There is a possibility that the periodicity and oscillations of neutrophil count may change with growth in patients with CyN. Therefore, it is important to frequently examine and treat patients with fluctuating neutrophil levels for the management of invasive dental treatment in patients with CyN.

Chaperoning Endoplasmic Reticulum-Associated Degradation (ERAD) and Protein Conformational Diseases

Misfolded proteins compromise cellular homeostasis. This is especially problematic in the endoplasmic reticulum (ER), which is a high-capacity protein-folding compartment and whose function requires stringent protein quality-control systems. Multiprotein complexes in the ER are able to identify, remove, ubiquitinate, and deliver misfolded proteins to the 26S proteasome for degradation in the cytosol, and these events are collectively termed ER-associated degradation, or ERAD. Several steps in the ERAD pathway are facilitated by molecular chaperone networks, and the importance of ERAD is highlighted by the fact that this pathway is linked to numerous protein conformational diseases. In this review, we discuss the factors that constitute the ERAD machinery and detail how each step in the pathway occurs. We then highlight the underlying pathophysiology of protein conformational diseases associated with ERAD.

CRISPR/Cas9-mediated ELANE knockout enables neutrophilic maturation of primary hematopoietic stem and progenitor cells and induced pluripotent stem cells of severe congenital neutropenia patients

A Autosomal-dominant ELANE mutations are the most common cause of severe congenital neutropenia. Although the majority of congenital neutropenia patients respond to daily granulocyte colony stimulating factor, approximately 15 % do not respond to this cytokine at doses up to 50 μg/kg/day and approximately 15 % of patients will develop myelodysplasia or acute myeloid leukemia. “Maturation arrest,” the failure of the marrow myeloid progenitors to form mature neutrophils, is a consistent feature of ELANE associated congenital neutropenia. As mutant neutrophil elastase is the cause of this abnormality, we hypothesized that ELANE associated neutropenia could be treated and “maturation arrest” corrected by a CRISPR/Cas9-sgRNA ribonucleoprotein mediated ELANE knockout. To examine this hypothesis, we used induced pluripotent stem cells from two congenital neutropenia patients and primary hematopoietic stem and progenitor cells from four congenital neutropenia patients harboring ELANE mutations as well as HL60 cells expressing mutant ELANE. We observed that granulocytic differentiation of ELANE knockout induced pluripotent stem cells and primary hematopoietic stem and progenitor cells were comparable to healthy individuals. Phagocytic functions, ROS production, and chemotaxis of the ELANE KO (knockout) neutrophils were also normal. Knockdown of ELANE in the mutant ELANE expressing HL60 cells also allowed full maturation and formation of abundant neutrophils. These observations suggest that ex vivo CRISPR/Cas9 RNP based ELANE knockout of patients’ primary hematopoietic stem and progenitor cells followed by autologous transplantation may be an alternative therapy for congenital neutropenia.

Serpin B1 defect and increased apoptosis of neutrophils in Cohen syndrome neutropenia

Cohen syndrome (CS) is a rare genetic disorder due to mutations in VPS13B gene. Among various clinical and biological features, CS patients suffer from inconsistent neutropenia, which is associated with recurrent but minor infections. We demonstrate here that this neutropenia results from an exaggerate rate of neutrophil apoptosis. Besides this increased cell death, which occurs in the absence of any endoplasmic reticulum stress or defect in neutrophil elastase (ELANE) expression or localization, all neutrophil functions appeared to be normal. We showed a disorganization of the Golgi apparatus in CS neutrophils precursors, that correlates with an altered glycosylation of ICAM-1 in these cells, as evidenced by a migration shift of the protein. Furthermore, a striking decrease in the expression of SERPINB1 gene, which encodes a critical component of neutrophil survival, was detected in CS neutrophils. These abnormalities may account for the excessive apoptosis of neutrophils leading to neutropenia in CS. KEY MESSAGES: Cohen syndrome patients' neutrophils display normal morphology and functions. Cohen syndrome patients' neutrophils have an increased rate of spontaneous apoptosis compared to healthy donors' neutrophils. No ER stress or defective ELA2 expression or glycosylation was observed in Cohen syndrome patients' neutrophils. SerpinB1 expression is significantly decreased in Cohen syndrome neutrophils as well as in VPS13B-deficient cells.

Extended Cleavage Specificity of Human Neutrophil Elastase, Human Proteinase 3, and Their Distant Ortholog Clawed Frog PR3-Three Elastases With Similar Primary but Different Extended Specificities and Stability

Serine proteases are major granule constituents of several of the human hematopoietic cell lineages. Four proteolytically active such proteases have been identified in human neutrophils: cathepsin G (hCG), N-elastase (hNE), proteinase 3 (hPR-3), and neutrophil serine protease 4 (hNSP-4). Here we present the extended cleavage specificity of two of the most potent and most abundant of these enzymes, hNE and hPR-3. Their extended specificities were determined by phage display and by the analysis of a panel of chromogenic and recombinant substrates. hNE is an elastase with a relatively broad specificity showing a preference for regions containing several aliphatic amino acids. The protease shows self-cleaving activity, which results in the loss of activity during storage even at +4°C. Here we also present the extended cleavage specificity of hPR-3. Compared with hNE, it shows considerably lower proteolytic activity. However, it is very stable, shows no self-cleaving activity and is actually more active in the presence of SDS, possibly by enhancing the accessibility of the target substrate. This enables specific analysis of hPR-3 activity even in the presence of all the other neutrophil enzymes with addition of 1% SDS. Neutrophils are the most abundant white blood cell in humans and one of the key players in our innate immune defense. The neutrophil serine proteases are very important for the function of the neutrophils and therefore also interesting from an evolutionary perspective. In order to study the origin and functional conservation of these neutrophil proteases we have identified and cloned an amphibian ortholog, Xenopus PR-3 (xPR-3). This enzyme was found to have a specificity very similar to hPR-3 but did not show the high stability in the presence of SDS. The presence of an elastase in Xenopus closely related to hPR-3 indicates a relatively early appearance of these enzymes during vertebrate evolution.

New monogenic disorders identify more pathways to neutropenia: from the clinic to next-generation sequencing

Neutrophils are the most common type of leukocyte in human circulating blood and constitute one of the chief mediators for innate immunity. Defined as a reduction from a normal distribution of values, neutropenia results from a number of congenital and acquired conditions. Neutropenia may be insignificant, temporary, or associated with a chronic condition with or without a vulnerability to life-threatening infections. As an inherited bone marrow failure syndrome, neutropenia may be associated with transformation to myeloid malignancy. Recognition of an inherited bone marrow failure syndrome may be delayed into adulthood. The list of monogenic neutropenia disorders is growing, heterogeneous, and bewildering. Furthermore, greater knowledge of immune-mediated and drug-related causes makes the diagnosis and management of neutropenia challenging. Recognition of syndromic presentations and especially the introduction of next-generation sequencing are improving the accuracy and expediency of diagnosis as well as their clinical management. Furthermore, identification of monogenic neutropenia disorders is shedding light on the molecular mechanisms of granulopoiesis and myeloid malignancies.

Brief Report: A Novel ELANE Mutation Associated With Inflammatory Arthritis, Defective NETosis, and Recurrent Parvovirus Infection

OBJECTIVE

We describe a 38-year-old woman who presented with a history of inflammatory arthritis, rash, and daily fevers. She was noted to have chronic parvovirus infection with persistently detectable viral titers and a novel mutation in the ELANE gene. ELANE encodes neutrophil elastase, a neutrophil serine protease with important antimicrobial effects, and is found as part of neutrophil extracellular trap (NET) complexes. Pathogenic ELANE mutations have been identified in forms of hereditary neutropenia. However, our patient never had neutropenia. Because of the striking clinical scenario, we investigated this mutation functionally.

METHODS

NET formation by neutrophils was assessed by scanning electron microscopy. Neutrophil activation and neutrophil elastase production were evaluated by flow cytometry and fluorescent substrate-based functional assay, respectively. A multiplex assay was used to quantitate neutrophil inflammatory cytokine production. PyMOL software was used to generate 3-dimensional models of mutant elastase.

RESULTS

Activated neutrophils from the patient demonstrated a significantly decreased ability to form NETs on scanning electron microscopy, as well as quantitative defects in neutrophil activation and neutrophil elastase activity. The patient's neutrophils showed altered levels of interleukin-12 (IL-12) and IL-8, which are key cytokines for antiviral immunity and neutrophil chemotaxis. Three-dimensional mapping revealed that the mutation could alter protein folding and surface charge distribution, potentially perturbing protein trafficking. Thus, the mutation could affect neutrophil function by decreasing NETosis and altering key antiviral activities of neutrophils.

CONCLUSION

This is the first report of a human pathogenic ELANE mutation associated with a defect in NETosis and a distinct syndrome of recurrent viral infection and chronic inflammation.

Severe congenital neutropenias

Severe congenital neutropenias are a heterogeneous group of rare haematological diseases characterized by impaired maturation of neutrophil granulocytes. Patients with severe congenital neutropenia are prone to recurrent, often life-threatening infections beginning in their first months of life. The most frequent pathogenic defects are autosomal dominant mutations in ELANE, which encodes neutrophil elastase, and autosomal recessive mutations in HAX1, whose product contributes to the activation of the granulocyte colony-stimulating factor (G-CSF) signalling pathway. The pathophysiological mechanisms of these conditions are the object of extensive research and are not fully understood. Furthermore, severe congenital neutropenias may predispose to myelodysplastic syndromes or acute myeloid leukaemia. Molecular events in the malignant progression include acquired mutations in CSF3R (encoding G-CSF receptor) and subsequently in other leukaemia-associated genes (such as RUNX1) in a majority of patients. Diagnosis is based on clinical manifestations, blood neutrophil count, bone marrow examination and genetic and immunological analyses. Daily subcutaneous G-CSF administration is the treatment of choice and leads to a substantial increase in blood neutrophil count, reduction of infections and drastic improvement of quality of life. Haematopoietic stem cell transplantation is the alternative treatment. Regular clinical assessments (including yearly bone marrow examinations) to monitor treatment course and detect chromosomal abnormalities (for example, monosomy 7 and trisomy 21) as well as somatic pre-leukaemic mutations are recommended.

How I manage children with neutropenia

Neutropenia, usually defined as a blood neutrophil count <1·5 × 10⁹ /l, is a common medical problem for children and adults. There are many causes for neutropenia, and at each stage in life the clinical pattern of causes and consequences differs significantly. I recommend utilizing the age of the child and clinical observations for the preliminary diagnosis and primary management. In premature infants, neutropenia is quite common and contributes to the risk of sepsis with necrotizing enterocolitis. At birth and for the first few months of life, neutropenia is often attributable to isoimmune or alloimmune mechanisms and predisposes to the risk of severe bacterial infections. Thereafter when a child is discovered to have neutropenia, often associated with relatively minor symptoms, it is usually attributed to autoimmune disorder or viral infection. The congenital neutropenia syndromes are usually recognized when there are recurrent infections, the neutropenia is severe and there are congenital anomalies suggesting a genetic disorder. This review focuses on the key clinical finding and laboratory tests for diagnosis with commentaries on treatment, particularly the use of granulocyte colony-stimulating factor to treat childhood neutropenia.

A Truncated Granulocyte Colony-stimulating Factor Receptor (G-CSFR) Inhibits Apoptosis Induced by Neutrophil Elastase G185R Mutant: IMPLICATION FOR UNDERSTANDING CSF3R GENE MUTATIONS IN SEVERE CONGENITAL NEUTROPENIA

Mutations in ELANE encoding neutrophil elastase (NE) have been identified in the majority of patients with severe congenital neutropenia (SCN). The NE mutants have been shown to activate unfolded protein response and induce premature apoptosis in myeloid cells. Patients with SCN are predisposed to acute myeloid leukemia (AML), and progression from SCN to AML is accompanied by mutations in CSF3R encoding the granulocyte colony-stimulating factor receptor (G-CSFR) in ∼80% of patients. The mutations result in the expression of C-terminally truncated G-CSFRs that promote strong cell proliferation and survival. It is unknown why the CSF3R mutations, which are rare in de novo AML, are so prevalent in SCN/AML. We show here that a G-CSFR mutant, d715, derived from an SCN patient inhibited G-CSF-induced expression of NE in a dominant negative manner. Furthermore, G-CSFR d715 suppressed unfolded protein response and apoptosis induced by an SCN-derived NE mutant, which was associated with sustained activation of AKT and STAT5, and augmented expression of BCL-XL. Thus, the truncated G-CSFRs associated with SCN/AML may protect myeloid precursor cells from apoptosis induced by the NE mutants. We propose that acquisition of CSF3R mutations may represent a mechanism by which myeloid precursor cells carrying the ELANE mutations evade the proapoptotic activity of the NE mutants in SCN patients.

Characterisation of Neutropenia-Associated Neutrophil Elastase Mutations in a Murine Differentiation Model In Vitro and In Vivo

Severe congenital neutropenia (SCN) is characterised by a differentiation block in the bone marrow and low neutrophil numbers in the peripheral blood, which correlates with increased risk of bacterial infections. Several underlying gene defects have been identified in SCN patients. Mutations in the neutrophil elastase (ELANE) gene are frequently found in SCN and cyclic neutropenia. Both mislocalization and misfolding of mutant neutrophil elastase protein resulting in ER stress and subsequent induction of the unfolded protein response (UPR) have been proposed to be responsible for neutrophil survival and maturation defects. However, the detailed molecular mechanisms still remain unclear, in part due to the lack of appropriate in vitro and in vivo models. Here we used a system of neutrophil differentiation from immortalised progenitor lines by conditional expression of Hoxb8, permitting the generation of mature near-primary neutrophils in vitro and in vivo. NE-deficient Hoxb8 progenitors were reconstituted with murine and human forms of typical NE mutants representative of SCN and cyclic neutropenia, and differentiation of the cells was analysed in vitro and in vivo. ER stress induction by NE mutations could be recapitulated during neutrophil differentiation in all NE mutant-reconstituted Hoxb8 cells. Despite ER stress induction, no change in survival, maturation or function of differentiating cells expressing either murine or human NE mutants was observed. Further analysis of in vivo differentiation of Hoxb8 cells in a murine model of adoptive transfer did not reveal any defects in survival or differentiation in the mouse. Although the Hoxb8 system has been found to be useful for dissection of defects in neutrophil development, our findings indicate that the use of murine systems for analysis of NE-mutation-associated pathogenesis is complicated by differences between humans and mice in the physiology of granulopoiesis, which may go beyond possible differences in expression and activity of neutrophil elastase itself.

Neutrophil function in children following allogeneic hematopoietic stem cell transplant

HSCT is a lifesaving procedure for children with malignant and non-malignant conditions. The conditioning regimen renders the patient severely immunocompromised and recovery starts with neutrophil (PMN) engraftment. We hypothesize that children demonstrate minimal PMN dysfunction at engraftment and beyond, which is influenced by the stem cell source and the conditioning regimen. Peripheral blood was serially collected from children at 1 to 12 months following allogeneic HSCT. PMN superoxide (O2-) production, degranulation (elastase), CD11b surface expression, and phagocytosis were assessed. Twenty-five patients, mean age of 10.5 yr with 65% males, comprised the study and transplant types included: 14 unrelated cord blood stem cells (cords), seven matched related bone marrow donors, three matched unrelated bone marrow donors, and one peripheral blood progenitor cells. Engraftment occurred at 24 days. There were no significant differences between controls and patients in PMN O2- production, phagocytosis, CD11b surface expression, and total PMN elastase. Elastase release was significantly decreased <6 months vs. controls (p < 0.05) and showed normalization by six months for cords only. The conditioning regimen did not affect PMN function. PMN function returns with engraftment, save elastase release, which occurs later related to the graft source utilized, and its clinical significance is unknown.

The diversity of mutations and clinical outcomes for ELANE-associated neutropenia

PURPOSE OF REVIEW

Mutations in the gene for neutrophil elastase, ELANE, cause cyclic neutropenia (CyN) and severe congenital neutropenia (SCN). This study summarized data from the Severe Chronic Neutropenia International Registry (SCNIR) on genotype-phenotype relationships of ELANE mutations to important clinical outcomes. We also summarize findings for ELANE mutations not observed in SCNIR patients.

RECENT FINDINGS

There were 307 SCNIR patients with 104 distinctive ELANE mutations who were followed longitudinally for up to 27 years. The ELANE mutations were diverse; there were 65 single amino acid substitutions; 61 of these mutations (94%) were 'probably' or 'possibly damaging' by PolyPhen-2 analysis, and one of the 'benign' mutations was associated with two cases of acute myeloid leukemia (AML). All frame-shift mutations (19/19) were associated with the SCN. The pattern of mutations in the SCN versus CyN was significantly different (P < 10), but some mutations were observed in both groups (overlapping mutations). The cumulative incidence of severe adverse events, that is, myelodysplasia, AML, stem cell transplantation, or deaths was significantly greater for patients with SCN versus those with CyN or overlapping mutations. Specific mutations (i.e. G214R or C151Y) had a high risk for evolution to AML.

SUMMARY

Sequencing is useful for predicting outcomes of ELANE-associated neutropenia.

Pathogenesis of ELANE-mutant severe neutropenia revealed by induced pluripotent stem cells

Severe congenital neutropenia (SCN) is often associated with inherited heterozygous point mutations in ELANE, which encodes neutrophil elastase (NE). However, a lack of appropriate models to recapitulate SCN has substantially hampered the understanding of the genetic etiology and pathobiology of this disease. To this end, we generated both normal and SCN patient-derived induced pluripotent stem cells (iPSCs), and performed genome editing and differentiation protocols that recapitulate the major features of granulopoiesis. Pathogenesis of ELANE point mutations was the result of promyelocyte death and differentiation arrest, and was associated with NE mislocalization and activation of the unfolded protein response/ER stress (UPR/ER stress). Similarly, high-dose G-CSF (or downstream signaling through AKT/BCL2) rescues the dysgranulopoietic defect in SCN patient-derived iPSCs through C/EBPβ-dependent emergency granulopoiesis. In contrast, sivelestat, an NE-specific small-molecule inhibitor, corrected dysgranulopoiesis by restoring normal intracellular NE localization in primary granules; ameliorating UPR/ER stress; increasing expression of CEBPA, but not CEBPB; and promoting promyelocyte survival and differentiation. Together, these data suggest that SCN disease pathogenesis includes NE mislocalization, which in turn triggers dysfunctional survival signaling and UPR/ER stress. This paradigm has the potential to be clinically exploited to achieve therapeutic responses using lower doses of G-CSF combined with targeting to correct NE mislocalization.

Regulation of Neutrophil Serine Proteases by Intracellular Serpins

Neutrophil granules contain serine proteases that are central components of the antimicrobial weapons of the innate immune system. Neutrophil proteases also contribute to the amplification and resolution of inflammatory responses through defined proteolytic cleavage of mediators, cell surface receptors, and extracellular matrix proteins. In the blood and at mucosal surfaces, neutrophil serine proteases are regulated by serpins found in plasma and by non-serpin secreted inhibitors. Distinct mechanisms leading to neutrophil cell death have been described for the granule serine proteases, neutrophil elastase, cathepsin G, and proteinase-3. Granule leakage in neutrophils triggers death pathways mediated by cathepsin G and proteinase-3, and both proteases are tightly regulated by their inhibitor SERPINB1 in a cell intrinsic manner. Although stored in the same types of granules, neutrophil elastase does not significantly contribute to cell death following intracellular release from granules into the cytoplasm. However, heterozygous mutations in ELANE, the gene encoding elastase, are the cause of severe congenital neutropenia, a life-threatening condition characterized by the death of neutrophils at an early precursor stage in the bone marrow. This chapter focuses on recent work exploring the biology of clade B intracellular serpins that inhibit neutrophil serine proteases and their functions in neutrophil homeostasis and serine protease control at sites of inflammation.

Game of clones: the genomic evolution of severe congenital neutropenia

Severe congenital neutropenia (SCN) is a genetically heterogeneous condition of bone marrow failure usually diagnosed in early childhood and characterized by a chronic and severe shortage of neutrophils. It is now well-established that mutations in HAX1 and ELANE (and more rarely in other genes) are the genetic cause of SCN. In contrast, it has remained unclear how these mutations affect neutrophil development. Innovative models based on induced pluripotent stem cell technology are being explored to address this issue. These days, most SCN patients receive life-long treatment with granulocyte colony-stimulating factor (G-CSF, CSF3). CSF3 therapy has greatly improved the life expectancy of SCN patients, but also unveiled a high frequency of progression toward myelodysplastic syndrome (MDS) and therapy refractory acute myeloid leukemia (AML). Expansion of hematopoietic clones with acquired mutations in the gene encoding the G-CSF receptor (CSF3R) is regularly seen in SCN patients and AML usually descends from one of these CSF3R mutant clones. These findings raised the questions how CSF3R mutations affect CSF3 responses of myeloid progenitors, how they contribute to the pre-leukemic state of SCN, and which additional events are responsible for progression to leukemia. The vast (sub)clonal heterogeneity of AML and the presence of AML-associated mutations in normally aged hematopoietic clones make it often difficult to determine which mutations are responsible for the leukemic process. Leukemia predisposition syndromes such as SCN are unique disease models to identify the sequential acquisition of these mutations and to interrogate how they contribute to clonal selection and leukemic evolution.

The amazing ubiquitin-proteasome system: structural components and implication in aging

Proteome quality control (PQC) is critical for the maintenance of cellular functionality and it is assured by the curating activity of the proteostasis network (PN). PN is constituted of several complex protein machines that under conditions of proteome instability aim to, firstly identify, and then, either rescue or degrade nonnative polypeptides. Central to the PN functionality is the ubiquitin-proteasome system (UPS) which is composed from the ubiquitin-conjugating enzymes and the proteasome; the latter is a sophisticated multi-subunit molecular machine that functions in a bimodal way as it degrades both short-lived ubiquitinated normal proteins and nonfunctional polypeptides. UPS is also involved in PQC of the nucleus, the endoplasmic reticulum and the mitochondria and it also interacts with the other main cellular degradation axis, namely the autophagy-lysosome system. UPS functionality is optimum in the young organism but it is gradually compromised during aging resulting in increasing proteotoxic stress; these effects correlate not only with aging but also with most age-related diseases. Herein, we present a synopsis of the UPS components and of their functional alterations during cellular senescence or in vivo aging. We propose that mild UPS activation in the young organism will, likely, promote antiaging effects and/or suppress age-related diseases.

Understanding, treating and avoiding hematological disease: better medicine through mathematics?

This paper traces the experimental, clinical and mathematical modeling efforts to understand a periodic hematological disease-cyclical neutropenia. It is primarily a highly personal account by two scientists from quite different backgrounds of their interactions over almost 40 years and their attempts to understand this intriguing disease. It's also a story of their efforts to offer effective treatments for the patients who suffer from cyclic neutropenia and other conditions causing neutropenia and infections.

Inherited bone marrow failure syndromes in adolescents and young adults

The inherited bone marrow failure syndromes are a diverse group of genetic diseases associated with inadequate production of one or more blood cell lineages. Examples include Fanconi anemia, dyskeratosis congenita, Diamond-Blackfan anemia, thrombocytopenia absent radii syndrome, severe congenital neutropenia, and Shwachman-Diamond syndrome. The management of these disorders was once the exclusive domain of pediatric subspecialists, but increasingly physicians who care for adults are being called upon to diagnose or treat these conditions. Through a series of patient vignettes, we highlight the clinical manifestations of inherited bone marrow failure syndromes in adolescents and young adults. The diagnostic and therapeutic challenges posed by these diseases are discussed.

ER-stress and apoptosis: molecular mechanisms and potential relevance in infection

During ER-stress, one of the responses a cell can choose is apoptosis. Apoptosis generally is a cell's preferred response when other control mechanisms are overwhelmed. We now have a reasonably clear molecular picture what is happening once the apoptotic apparatus has been started. Unclear however are the majority of the upstream pathways that connect other signalling to apoptosis. During ER-stress, confirmed apoptosis-regulating targets are pro- and anti-apoptotic proteins of the Bcl-2-family, whose concerted action induces apoptosis. I will here discuss how mitochondrial apoptosis is triggered, how this is linked to the ER-stress response and in what way this may be relevant during microbial infections.

Induced pluripotent stem cell-derived myeloid phagocytes: disease modeling and therapeutic applications

Myeloid phagocytes (neutrophils, monocytes, macrophages and dendritic cells) have key roles in immune defense, as well as in tissue repair and remodeling. Defective or dysregulated myeloid phagocyte production or function can cause immune dysfunction, blood cell malignancies and inflammatory diseases. The tumor microenvironment can also condition myeloid phagocytes to promote tumor growth. Studies of their physiological and pathophysiological roles and the mechanisms regulating their production and function are crucial for the identification of novel therapeutic targets. In this review, we examine the use of induced pluripotent stem cells to study myeloid phagocytes in human diseases and develop future therapeutic strategies.

Cellular stress pathways in pediatric bone marrow failure syndromes: many roads lead to neutropenia

The inherited bone marrow failure syndromes, like severe congenital neutropenia (SCN) and Shwachman-Diamond syndrome (SDS), provide unique insights into normal and impaired myelopoiesis. The inherited neutropenias are heterogeneous in both clinical presentation and genetic associations, and their causative mechanisms are not well established. SCN, for example, is a genetically heterogeneous syndrome associated with mutations of ELANE, HAX1, GFI1, WAS, G6PC3, or CSF3R. The genetic diversity in SCN, along with congenital neutropenias associated with other genetically defined bone marrow failure syndromes (e.g., SDS), suggests that various pathways may be involved in their pathogenesis. Alternatively, all may lead to a final common pathway of enhanced apoptosis. The pursuit for a more complete understanding of the molecular mechanisms that drive inherited neutropenias remains at the forefront of pediatric translational and basic science investigation. Advances in our understanding of these disorders have greatly increased over the last 10 years concomitant with identification of their genetic lesions. Emerging themes include induction of the unfolded protein response (UPR), defective ribosome assembly, and p53-dependent apoptosis. Additionally, defects in metabolism, disruption of mitochondrial membrane potential, and mislocalization have been found. When perturbed, each of these lead to an intracellular stress that triggers apoptosis in the vulnerable granulocytic precursor.

A lack of secretory leukocyte protease inhibitor (SLPI) causes defects in granulocytic differentiation

We identified diminished levels of the natural inhibitor of neutrophil elastase (NE), secretory leukocyte protease inhibitor (SLPI), in myeloid cells and plasma of patients with severe congenital neutropenia (CN). We further found that downregulation of SLPI in CD34(+) bone marrow (BM) hematopoietic progenitors from healthy individuals resulted in markedly reduced in vitro myeloid differentiation accompanied by cell-cycle arrest and elevated apoptosis. Reciprocal regulation of SLPI by NE is well documented, and we previously demonstrated diminished NE levels in CN patients. Here, we found that transduction of myeloid cells with wild-type NE or treatment with exogenous NE increased SLPI messenger RNA and protein levels, whereas transduction of mutant forms of NE or inhibition of NE resulted in downregulation of SLPI. An analysis of the mechanisms underlying the diminished myeloid differentiation caused by reduced SLPI levels revealed that downregulation of SLPI with short hairpin RNA (shRNA) upregulated nuclear factor κB levels and reduced phospho-extracellular signal-regulated kinase (ERK1/2)-mediated phosphorylation and activation of the transcription factor lymphoid enhancer-binding factor-1 (LEF-1). Notably, microarray analyses revealed severe defects in signaling cascades regulating the cell cycle, including c-Myc-downstream signaling, in myeloid cells transduced with SLPI shRNA. Taken together, these results indicate that SLPI controls the proliferation, differentiation, and cell cycle of myeloid cells.

Neutropenia-associated ELANE mutations disrupting translation initiation produce novel neutrophil elastase isoforms

Hereditary neutropenia is usually caused by heterozygous germline mutations in the ELANE gene encoding neutrophil elastase (NE). How mutations cause disease remains uncertain, but two hypotheses have been proposed. In one, ELANE mutations lead to mislocalization of NE. In the other, ELANE mutations disturb protein folding, inducing an unfolded protein response in the endoplasmic reticulum (ER). In this study, we describe new types of mutations that disrupt the translational start site. At first glance, they should block translation and are incompatible with either the mislocalization or misfolding hypotheses, which require mutant protein for pathogenicity. We find that start-site mutations, instead, force translation from downstream in-frame initiation codons, yielding amino-terminally truncated isoforms lacking ER-localizing (pre) and zymogen-maintaining (pro) sequences, yet retain essential catalytic residues. Patient-derived induced pluripotent stem cells recapitulate hematopoietic and molecular phenotypes. Expression of the amino-terminally deleted isoforms in vitro reduces myeloid cell clonogenic capacity. We define an internal ribosome entry site (IRES) within ELANE and demonstrate that adjacent mutations modulate IRES activity, independently of protein-coding sequence alterations. Some ELANE mutations, therefore, appear to cause neutropenia via the production of amino-terminally deleted NE isoforms rather than by altering the coding sequence of the full-length protein.

A congenital neutrophil defect syndrome associated with mutations in VPS45

BACKGROUND

Neutrophils are the predominant phagocytes that provide protection against bacterial and fungal infections. Genetically determined neutrophil disorders confer a predisposition to severe infections and reveal novel mechanisms that control vesicular trafficking, hematopoiesis, and innate immunity.

METHODS

We clinically evaluated seven children from five families who had neutropenia, neutrophil dysfunction, bone marrow fibrosis, and nephromegaly. To identify the causative gene, we performed homozygosity mapping using single-nucleotide polymorphism arrays, whole-exome sequencing, immunoblotting, immunofluorescence, electron microscopy, a real-time quantitative polymerase-chain-reaction assay, immunohistochemistry, flow cytometry, fibroblast motility assays, measurements of apoptosis, and zebrafish models. Correction experiments were performed by transfecting mutant fibroblasts with the nonmutated gene.

RESULTS

All seven affected children had homozygous mutations (Thr224Asn or Glu238Lys, depending on the child's ethnic origin) in VPS45, which encodes a protein that regulates membrane trafficking through the endosomal system. The level of VPS45 protein was reduced, as were the VPS45 binding partners rabenosyn-5 and syntaxin-16. The level of β1 integrin was reduced on the surface of VPS45-deficient neutrophils and fibroblasts. VPS45-deficient fibroblasts were characterized by impaired motility and increased apoptosis. A zebrafish model of vps45 deficiency showed a marked paucity of myeloperoxidase-positive cells (i.e., neutrophils). Transfection of patient cells with nonmutated VPS45 corrected the migration defect and decreased apoptosis.

CONCLUSIONS

Defective endosomal intracellular protein trafficking due to biallelic mutations in VPS45 underlies a new immunodeficiency syndrome involving impaired neutrophil function. (Funded by the National Human Genome Research Institute and others.).

Expression of the secondary granule proteins major basic protein 1 (MBP-1) and eosinophil peroxidase (EPX) is required for eosinophilopoiesis in mice

Eosinophil activities are often linked with allergic diseases such as asthma and the pathologies accompanying helminth infection. These activities have been hypothesized to be mediated, in part, by the release of cationic proteins stored in the secondary granules of these granulocytes. The majority of the proteins stored in these secondary granules (by mass) are major basic protein 1 (MBP-1) and eosinophil peroxidase (EPX). Unpredictably, a knockout approach targeting the genes encoding these proteins demonstrated that, unlike in mice containing a single deficiency of only MBP-1 or EPX, the absence of both granule proteins resulted in the near complete loss of peripheral blood eosinophils with no apparent impact on any other hematopoietic lineage. Moreover, the absence of MBP-1 and EPX promoted a concomitant loss of eosinophil lineage-committed progenitors in the marrow, identifying a specific blockade in eosinophilopoiesis as the causative event. Significantly, this blockade of eosinophilopoiesis is also observed in ex vivo cultures of marrow progenitors and is not rescued in vivo by adoptive bone marrow engraftment, suggesting a cell-autonomous defect in marrow progenitors. These observations implicate a role for granule protein gene expression as a regulator of eosinophilopoiesis and provide another strain of mice congenitally deficient of eosinophils.