Aberrant subcellular targeting of the G185R neutrophil elastase mutant associated with severe congenital neutropenia induces premature apoptosis of differentiating promyelocytes

Neutrophil Elastase Defects in Congenital Neutropenia

Severe congenital neutropenia (SCN) is a rare hematological condition with heterogenous genetic background. Neutrophil elastase (NE) encoded by ELANE gene is mutated in over half of the SCN cases. The role of NE defects in myelocytes maturation arrest in bone marrow is widely investigated; however, the mechanism underlying this phenomenon has still remained unclear. In this review, we sum up the studies exploring mechanisms of neutrophil deficiency, biological role of NE in neutrophil and the effects of ELANE mutation and neutropenia pathogenesis. We also explain the hypotheses presented so far and summarize options of neutropenia therapy.

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.

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.

Hermansky-Pudlak syndrome: Mutation update

Hermansky-Pudlak syndrome (HPS) is a group of 10 autosomal recessive multisystem disorders, each defined by the deficiency of a specific gene. HPS-associated genes encode components of four ubiquitously expressed protein complexes: Adaptor protein-3 (AP-3) and biogenesis of lysosome-related organelles complex-1 (BLOC-1) through -3. All individuals with HPS exhibit albinism and a bleeding diathesis; additional features occur depending on the defective protein complex. Pulmonary fibrosis is associated with AP-3 and BLOC-3 deficiency, immunodeficiency with AP-3 defects, and gastrointestinal symptoms are more prevalent and severe in BLOC-3 deficiency. Therefore, identification of the HPS subtype is valuable for prognosis, clinical management, and treatment options. The prevalence of HPS is estimated at 1-9 per 1,000,000. Here we summarize 264 reported and novel variants in 10 HPS genes and estimate that ~333 Puerto Rican HPS subjects and ~385 with other ethnicities are reported to date. We provide pathogenicity predictions for missense and splice site variants and list variants with high minor allele frequencies. Current cellular and clinical aspects of HPS are also summarized. This review can serve as a manifest for molecular diagnostics and genetic counseling aspects of HPS.

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.

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.

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.

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.

The risk of hemophagocytic lymphohistiocytosis in Hermansky-Pudlak syndrome type 2

Genetic disorders of lymphocyte cytotoxicity predispose patients to hemophagocytic lymphohistiocytosis (HLH). Reduced lymphocyte cytotoxicity has been demonstrated in Hermansky-Pudlak syndrome type 2 (HPS2), but only a single patient was reported who developed HLH. Because that patient also carried a potentially contributing heterozygous RAB27A mutation, the risk for HLH in HPS2 remains unclear. We analyzed susceptibility to HLH in the pearl mouse model of HPS2. After infection with lymphocytic choriomeningitis virus, pearl mice developed all key features of HLH, linked to impaired virus control caused by a moderate defect in CTL cytotoxicity in vivo. However, in contrast to perforin-deficient mice, the disease was transient, and all mice fully recovered and controlled the infection. An additional heterozygous Rab27a mutation did not aggravate the cytotoxicity defect or disease parameters. In the largest survey of 22 HPS2 patients covering 234 patient years, we identified only 1 additional patient with HLH and 2 with incomplete transient HLH-like episodes, although cytotoxicity or degranulation was impaired in all 16 patients tested. HPS2 confers a risk for HLH that is lower than in Griscelli or Chediak-Higashi syndrome, probably because of a milder defect in cytotoxicity. Preemptive hematopoietic stem cell transplantation does not appear justified in HPS2.

ELANE mutations in cyclic and severe congenital neutropenia: genetics and pathophysiology

The 2 main forms of hereditary neutropenia are cyclic (CN) and severe congenital (SCN) neutropenia. CN is an autosomal dominant disorder in which neutrophil counts fluctuate with 21-day periodicity. SCN consists of static neutropenia, with promyelocytic maturation arrest in the bone marrow. Unlike CN, SCN displays frequent acquisition of somatic mutations in the gene CSF3R. CN is caused by heterozygous mutations in the gene ELANE, encoding neutrophil elastase. SCN is genetically heterogeneous but is most frequently associated with ELANE mutations. We discuss how the mutations provide clues into the pathogenesis of neutropenia and describe current hypotheses for its molecular mechanisms.

Familial hemophagocytic lymphohistiocytosis: a model for understanding the human machinery of cellular cytotoxicity

Cytotoxic T lymphocytes, natural killer cells, and NKT cells are effector cells able to kill infected cells. In some inherited human disorders, a defect in selected proteins involved in the cellular cytotoxicity mechanism results in specific clinical syndromes, grouped under the name of familial hemophagocytic lymphohistiocytosis. Recent advances in genetic studies of these patients has allowed the identification of different genetic subsets. Additional genetic immune deficiencies may also induce a similar clinical picture. International cooperation and prospective trials resulted in refining the diagnostic and therapeutic approach to these rare diseases with improved outcome but also with improved knowledge of the mechanisms underlying granule-mediated cellular cytotoxicity in humans.

Congenital neutropenia: diagnosis, molecular bases and patient management

The term congenital neutropenia encompasses a family of neutropenic disorders, both permanent and intermittent, severe (<0.5 G/l) or mild (between 0.5-1.5 G/l), which may also affect other organ systems such as the pancreas, central nervous system, heart, muscle and skin. Neutropenia can lead to life-threatening pyogenic infections, acute gingivostomatitis and chronic parodontal disease, and each successive infection may leave permanent sequelae. The risk of infection is roughly inversely proportional to the circulating polymorphonuclear neutrophil count and is particularly high at counts below 0.2 G/l.When neutropenia is detected, an attempt should be made to establish the etiology, distinguishing between acquired forms (the most frequent, including post viral neutropenia and auto immune neutropenia) and congenital forms that may either be isolated or part of a complex genetic disease.Except for ethnic neutropenia, which is a frequent but mild congenital form, probably with polygenic inheritance, all other forms of congenital neutropenia are extremely rare and have monogenic inheritance, which may be X-linked or autosomal, recessive or dominant.About half the forms of congenital neutropenia with no extra-hematopoietic manifestations and normal adaptive immunity are due to neutrophil elastase (ELANE) mutations. Some patients have severe permanent neutropenia and frequent infections early in life, while others have mild intermittent neutropenia.Congenital neutropenia may also be associated with a wide range of organ dysfunctions, as for example in Shwachman-Diamond syndrome (associated with pancreatic insufficiency) and glycogen storage disease type Ib (associated with a glycogen storage syndrome). So far, the molecular bases of 12 neutropenic disorders have been identified.Treatment of severe chronic neutropenia should focus on prevention of infections. It includes antimicrobial prophylaxis, generally with trimethoprim-sulfamethoxazole, and also granulocyte-colony-stimulating factor (G-CSF). G-CSF has considerably improved these patients' outlook. It is usually well tolerated, but potential adverse effects include thrombocytopenia, glomerulonephritis, vasculitis and osteoporosis. Long-term treatment with G-CSF, especially at high doses, augments the spontaneous risk of leukemia in patients with congenital neutropenia.

High-mobility group box 1, oxidative stress, and disease

Oxidative stress and associated reactive oxygen species can modify lipids, proteins, carbohydrates, and nucleic acids, and induce the mitochondrial permeability transition, providing a signal leading to the induction of autophagy, apoptosis, and necrosis. High-mobility group box 1 (HMGB1) protein, a chromatin-binding nuclear protein and damage-associated molecular pattern molecule, is integral to oxidative stress and downstream apoptosis or survival. Accumulation of HMGB1 at sites of oxidative DNA damage can lead to repair of the DNA. As a redox-sensitive protein, HMGB1 contains three cysteines (Cys23, 45, and 106). In the setting of oxidative stress, it can form a Cys23-Cys45 disulfide bond; a role for oxidative homo- or heterodimerization through the Cys106 has been suggested for some of its biologic activities. HMGB1 causes activation of nicotinamide adenine dinucleotide phosphate oxidase and increased reactive oxygen species production in neutrophils. Reduced and oxidized HMGB1 have different roles in extracellular signaling and regulation of immune responses, mediated by signaling through the receptor for advanced glycation end products and/or Toll-like receptors. Antioxidants such as ethyl pyruvate, quercetin, green tea, N-acetylcysteine, and curcumin are protective in the setting of experimental infection/sepsis and injury including ischemia-reperfusion, partly through attenuating HMGB1 release and systemic accumulation.

HAX1 mutations causing severe congenital neuropenia and neurological disease lead to cerebral microstructural abnormalities documented by quantitative MRI

Biallelic mutations in the gene encoding HCLS-associated protein X-1 (HAX1) cause autosomal recessive severe congenital neutropenia (SCN). Some of these patients have neurological abnormalities including developmental delay, cognitive impairment, and/or epilepsy. Recent genotype-phenotype studies have shown that mutations in HAX1 affecting transcripts A (NM_006118.3) and B (NM_001018837.1) cause the phenotype of SCN with neurological impairment, while mutations affecting isoform A but not B lead to SCN without neurological aberrations. In this study, we identified a consanguineous family with two patients suffering from SCN and neurological disease caused by a novel, homozygous genomic deletion including exons 4-7 of the HAX1 gene. Quantitative MRI analyses showed generalized alterations in cerebral proton density in both of the patients, as well as in an additional unrelated patient with another HAX1 mutation (Arg86X) known to be associated with neurological manifestations. This study provides first in vivo evidence of aberrant neuroimaging findings associated with HAX1 deficiency in SCN patients.

Inherited defects in lymphocyte cytotoxic activity

The granule-dependent cytotoxic activity of lymphocytes plays a critical role in the defense against virally infected cells and tumor cells. The importance of this cytotoxic pathway in immune regulation is evidenced by the severe and often fatal condition, known as hemophagocytic lymphohistiocytic syndrome (HLH) that occurs in mice and humans with genetically determined impaired lymphocyte cytotoxic function. HLH manifests as the occurrence of uncontrolled activation of T lymphocytes and macrophages infiltrating multiple organs. In this review, we focus on recent advances in the characterization of effectors regulating the release of cytotoxic granules, and on the role of this cytotoxic pathway in lymphocyte homeostasis and immune surveillance. Analysis of the mechanisms leading to the occurrence of hemophagocytic syndrome designates gamma-interferon as an attractive therapeutic target to downregulate uncontrolled macrophage activation, which sustains clinical and biological features of HLH.

Digenic mutations in severe congenital neutropenia

Severe congenital neutropenia a clinically and genetically heterogeneous disorder. Mutations in different genes have been described as causative for severe neutropenia, e.g. ELANE, HAX1 and G6PC3. Although congenital neutropenia is considered to be a group of monogenic disorders, the phenotypic heterogeneity even within the yet defined genetic subtypes points to additional genetic and/or epigenetic influences on the disease phenotype. We describe congenital neutropenia patients with mutations in two candidate genes each, including 6 novel mutations. Two of them had a heterozygous ELANE mutation combined with a homozygous mutation in G6PC3 or HAX1, respectively. The other 2 patients combined homozygous or compound heterozygous mutations in G6PC3 or HAX1 with a heterozygous mutation in the respective other gene. Our results suggest that digenicity may underlie this disorder of myelopoiesis at least in some congenital neutropenia patients.

Novel genetic etiologies of severe congenital neutropenia

Severe congenital neutropenia (SCN) comprises a heterogenous group of primary immunodeficiency disorders collectively characterized by paucity of mature neutrophils. In recent years, progress has been made with respect to the elucidation of genetic causes underlying syndromic and non-syndromic variants of SCN. Most cases of autosomal dominant SCN are associated with mutations in the neutrophil elastase (ELA-2/ELANE) gene, autosomal recessive forms of this disorder can be caused by mutations in the gene encoding the mitochondrial protein HAX-1. Rarely, SCN can be caused by mutations in the gene encoding the transcription factor GFI1 or activating mutations in the Wiskott-Aldrich syndrome (WAS) gene, respectively. More recently, a complex disorder associating SCN and developmental aberrations was identified, caused by mutations in the glucose-6-phosphatase catalytic subunit 3 (G6PC3) gene. Despite our increasing knowledge of the genetic etiologies of SCN, the molecular pathophysiology underlying these disorders remains only partially understood.

Unrelated cord blood transplantation in children with severe congenital neutropenia

SCN is an inherited hematological disorder with severe neutropenia and recurrent infections. Although there are some reports that recombinant rhG-CSF improves clinical outcome, allogeneic HSCT appears to be the only curative treatment for these patients. We report here two children with SCN successfully treated by CBT from unrelated donors. They were refractory to rhG-CSF treatment and have no identical family donor. Bu + CY were given as conditioning. Case 1 and Case 2 received 6/6 and 5/6 HLA-matched unrelated umbilical cord blood, respectively. The number of infused nucleated cells was 6, 18 x 10(7)/kg and CD34(+) cell number was 3, 74 x 10(5)/kg in Case 1. Those cell numbers were 8, 8 x 10(7)/kg and 5, 34 x 10(5)/kg for Case 2, respectively. Neutrophil/platelet engraftments were 45/49 days in Case 1 and 24/36 days in Case 2. Grade II cutaneous acute GVHD was seen in Case 2 that was treated successfully with prednisolone. Both patients are well with normal hematological findings and full donor chimerism for post-transplant 20 and 24 months, respectively. We conclude that UCB can be considered as a safe source of stem cell in patients with SCN who need urgent HSCT.

Congenital cytopenias and bone marrow failure syndromes

Congenital bone marrow failure syndromes (CBMFS) are extremely uncommon diseases that can present in the neonate. The objective of this article is to review the presentation, diagnosis, pathophysiology, and management of CBMFS in relation to neonatology. CBMFS should be considered when a single or multiple blood cell lineages are low secondary to failure of production. Diagnosis in the neonatal period requires a high index of suspicion. In this particular age group, CBMFS should be considered when the neonate has a family history of CBMFS, is small for gestational age, or has other physical abnormalities. History and physical examination can lead to the diagnosis. CBMFS are often associated with a predisposition to cancer later in life.

Genetic heterogeneity in severe congenital neutropenia: how many aberrant pathways can kill a neutrophil?

PURPOSE OF REVIEW

Severe congenital neutropenia is a primary immunodeficiency in which lack of neutrophils causes inadequate innate immune host response to bacterial infections. Severe congenital neutropenia occurs with sporadic, autosomal dominant, autosomal recessive and X-linked recessive inheritance, as well as in a variety of multisystem syndromes. A principal stimulus for this review is the identification of novel genetic defects and pathophysiological insights into the role of neutrophil apoptosis.

RECENT FINDINGS

The recent findings include identification of mutations in HAX1 in autosomal recessive severe congenital neutropenia (Kostmann disease), a large epidemiological study estimating the risk of progression from severe congenital neutropenia to leukemia, a better understanding of how heterozygous mutations in neutrophil elastase (ELA2) cause severe congenital neutropenia, molecular characterization of a novel syndromic form of severe congenital neutropenia called p14 deficiency and new animal models for several syndromic forms of severe congenital neutropenia.

SUMMARY

We consider the numerous genes mutated in severe congenital neutropenia, the many attempts to make animal models of severe congenital neutropenia, and the results from both human and mouse studies investigating the molecular mechanisms of neutrophil apoptosis. Investigations of how severe congenital neutropenia genes and apoptosis pathways are connected should lead to a better understanding of the pathogenesis of neutropenia and apoptosis pathways relevant to many cell types.

A molecular classification of congenital neutropenia syndromes

Current knowledge on the molecular pathogenesis of severe congenital neutropenia indicates that the clinical diagnosis includes a heterogeneous group of disorders following different patterns of inheritance. Similarly, multifaceted syndromes associated with neutropenia can be classified molecularly, which in turn allows for a better understanding of the basis of the neutropenia. Many of the neutropenia disorders can be treated with G-CSF (filgrastim) to increase the neutrophil count, thereby reducing infection morbidity and mortality. In some instances hematopoietic stem cell transplantation remains the only curative treatment currently available. This review describes and classifies, on a molecular basis, both primary congenital neutropenia and multifaceted syndromes associated with neutropenia.

Mutations of the ELA2 gene found in patients with severe congenital neutropenia induce the unfolded protein response and cellular apoptosis

Severe congenital neutropenia (SCN) is an inborn disorder of granulopoiesis. Mutations of the ELA2 gene encoding neutrophil elastase (NE) are responsible for most cases of SCN and cyclic neutropenia (CN), a related but milder disorder of granulopoiesis. However, the mechanisms by which these mutations disrupt granulopoiesis are unclear. We hypothesize that the ELA2 mutations result in the production of misfolded NE protein, activation of the unfolded protein response (UPR), and ultimately apoptosis of granulocytic precursors. Expression of mutant NE but not wild-type NE strongly induced BiP/GRP78 mRNA expression and XBP1 mRNA splicing, 2 classic markers of the UPR. The magnitude of UPR activation by a specific ELA2 mutation correlated with its associated clinical phenotype. Consistent with the UPR model, expression of mutant NE in primary human granulocytic precursors increased expression of CHOP (DDITS) and induced apoptosis in a protease-independent fashion. Most strikingly, UPR activation and decreased NE protein expression were detected in primary granulocytic precursors from SCN patients. Collectively, these data provide strong support for a UPR model of SCN disease pathogenesis and place SCN in a growing list of human diseases caused by misfolded proteins.

G-CSF treatment of severe congenital neutropenia reverses neutropenia but does not correct the underlying functional deficiency of the neutrophil in defending against microorganisms

The treatment of children affected by severe congenital neutropenia (SCN) with G-CSF strongly reduces the risk of sepsis by reversing neutropenia. However, SCN patients who respond to the treatment with the growth factor still have an elevated risk of succumbing to sepsis. Because the disease is usually caused by heterozygous mutations of ELA2, a gene encoding for neutrophil elastase (NE), we have investigated in G-CSF–responder and nonresponder patients affected by SCN the expression of polypeptides that constitute the antimicrobial machinery of these cells. In peripheral blood–derived neutrophils of patients with heterozygous mutations of ELA2 who were treated with G-CSF, NE was nearly absent as detected by immunofluorescence and immunoblotting, suggesting that production of the mutant protein interferes with normal gene expression. This defect was associated with abnormal expression of other granule-associated proteins such as myeloperoxidase, lactoferrin, cathepsin G, and human-neutrophil-peptide. Moreover, in one patient with partial response to G-CSF, we observed an impairment of neutrophil antimicrobial activity against Candida albicans, and, to a lower extent against Escherichia coli. Thereby, we propose that the treatment with G-CSF is not sufficient to correct all of the functional deficiency of neutrophils, and this might account for the consistent risk of infections observed in SCN patients.

Kostmann syndrome or infantile genetic agranulocytosis, part two: Understanding the underlying genetic defects in severe congenital neutropenia

Congenital neutropenia in man was first reported 50 years ago by the Swedish paediatrician Rolf Kostmann. He coined the term 'infantile genetic agranulocytosis' for this condition, which is now known as Kostmann syndrome. Recent studies have revealed mutations in ELA-2, encoding the neutrophil granule protease, neutrophil elastase, in autosomal dominant neutropenia, and mutations in HAX-1, encoding an anti-apoptotic protein, in autosomal recessive neutropenia.

CONCLUSION

Future studies should aim to clarify the mechanisms underlying the evolution of secondary malignancies in these patients.

LEF-1 is a decisive transcription factor in neutrophil granulopoiesis

We found that lymphoid enhancer-binding factor 1 (LEF-1) is a decisive transcription factor in granulopoiesis controlling proliferation, proper lineage commitment, and granulocytic differentiation via regulation of its target genes C/EBP-alpha, cyclin D1, c-myc, and survivin. Myeloid progenitor cells of patients with severe congenital neutropenia (CN) showed a severe downregulation of LEF-1 and its target genes expression. Expression of neutrophil elastase (NE) is also severely reduced in CN myeloid progenitors. Intriguingly, ELA2 gene promoter is positively regulated by direct binding of LEF-1 or LEF-1 target gene C/EBP-alpha. In summary we demonstrated that LEF-1 is not only crucial in lymphopoiesis, but also in myelopoiesis, documenting new functions of LEF-1.

Neutrophil elastase depends on serglycin proteoglycan for localization in granules

Granule proteins play a major role in bacterial killing by neutrophils. Serglycin proteoglycan, the major intracellular proteoglycan of hematopoietic cells, has been proposed to play a role in sorting and packing of granule proteins. We examined the content of major neutrophil granule proteins in serglycin knockout mice and found neutrophil elastase absent from mature neutrophils as shown by activity assay, Western blotting, and immunocytochemistry, whereas neutrophil elastase mRNA was present. The localization of other neutrophil granule proteins did not differ between wild-type and serglycin knockout mice. Differential counts and neutrophil ultrastructure were unaffected by the lack of serglycin, indicating that defective localization of neutrophil elastase does not induce neutropenia itself, albeit mutations in the neutrophil elastase gene can cause severe congenital neutropenia or cyclic neutropenia. The virulence of intraperitoneally injected Gram-negative bacteria (Klebsiella pneumoniae) was increased in serglycin knockout mice compared with wild-type mice, as previously reported for neutrophil elastase knockout mice. Thus, serglycin proteoglycan has an important role in localizing neutrophil elastase in azurophil granules of neutrophils, while localization of other granule proteins must be mediated by other mechanisms.

Double de novo mutations ofELA2 in cyclic and severe congenital neutropenia

Heterozygous mutations of ELA2, encoding the protease neutrophil elastase (NE), cause either autosomal dominant cyclic neutropenia or severe congenital neutropenia (SCN). Three hypotheses have been proposed for how allelic mutations produce these different disorders: 1) disruption of proteolytic activity; 2) mislocalization of the protein; or 3) destabilization of the protein resulting in induction of the unfolded protein response. As with other dominant diseases with reduced reproductive fitness, sporadic cases can result from new mutations not inherited from either parent. Here we report an exceptional genetic phenomenon in which both a cyclic neutropenia patient and an SCN patient each possess two new ELA2 mutations. Because of the rarity of the phenomenon, we investigated the origins of the mutations and found that both arise nonmosaically and in cis from the paternally-inherited allele. Moreover, these cases offer a unique opportunity to investigate molecular pathways distinguishing these two forms of hereditary neutropenia. We have characterized the mutants separately and in combination, with respect to their effects on proteolysis, subcellular trafficking, and induction of the unfolded protein response. Each pair of mutations acts more or less additively to produce equivalent net effects on reducing proteolytic activity and induction of the unfolded protein response, yet each has different and somewhat opposing effects on disturbing subcellular localization, thus offering support for a role for protein mistrafficking as a disease mechanism.

Severe congenital neutropenia: inheritance and pathophysiology

PURPOSE OF REVIEW

Severe congenital neutropenia is a heterogeneous disorder of hematopoiesis characterized by a maturation arrest of granulopoiesis at the level of promyelocytes with peripheral blood absolute neutrophil counts below 0.5 x 10/l. In this review we summarize our current knowledge on inheritance and pathophysiolgy of congenital neutropenia.

RECENT FINDINGS

There are two major subtypes of congenital neutropenia as judged by inheritance: autosomal dominant trait defined by neutrophil elastase mutations consisting of 60% of patients and autosomal recessive trait comprising approximately 30% of patients. This genetic heterogeneity suggests that several pathologic mechanisms may lead to the same phenotype due to downregulation of common myeloid transcription factors. Lymphoid enhancer-binding factor 1 is the most promising candidate, as its abrogation together with downregulation of lymphoid enhancer-binding factor 1 target genes is compatible with this phenotype. Congenital neutropenia is considered as a preleukemic syndrome, since after 10 years of observation the cumulative incidence for leukemia is 21%. Acquired granulocyte colony-stimulating factor receptor mutations are detected in approximately 80% of congenital neutropenia patients who developed acute myeloid leukemia.

SUMMARY

Congenital neutropenia is a congenital disorder of hematopoiesis inherited by autosomal dominant or recessive traits. Downregulation of lymphoid enhancer-binding factor 1 is involved in the pathophysiology of all congenital neutropenia patients. Congenital neutropenia patients with acquired granulocyte colony-stimulating factor receptor mutations define a group with high risk for development of leukemia.

Neutrophil elastase sorting involves plasma membrane trafficking requiring the C-terminal propeptide

The primary granules/secretory lysosomes of neutrophils store mature neutrophil elastase (NE) as a luminal protein after proteolytic removal of N-terminal and C-terminal pro-peptides from a proform of NE. The N-terminal pro-peptide prevents premature activation that might be toxic to the cell, but the C-terminal pro-peptide has no defined function. In this study, we investigated the role of the C-terminal pro-peptide in trafficking of NE by expressing, in rat basophilic leukemia (RBL) cells, both wild-type NE and the mutant NE/Delta248-267, which lacks the C-terminal pro-peptide. Both transfected proteins were found to be targeted to secretory lysosomes. In addition, results from antibody ligation and cell-surface biotinylation indicated that proform of NE was targeted to the plasma membrane, and then subjected to endocytosis. The results were supported by the detection of targeting of the proform to the plasma membrane followed by internalization both in RBL cells and normal granulopoietic precursor cells. Targeting of NE to the plasma membrane required the C-terminal pro-peptide as NE/Delta248-267 expressed in RBL cells bypassed plasma membrane trafficking. Our results indicate targeting of a population of NE to the plasma membrane and internalization dependent on the C-terminal NE pro-peptide.

Neutrophil elastase in cyclic and severe congenital neutropenia

Mutations in ELA2 encoding the neutrophil granule protease, neutrophil elastase (NE), are the major cause of the 2 main forms of hereditary neutropenia, cyclic neutropenia and severe congenital neutropenia (SCN). Genetic evaluation of other forms of neutropenia in humans and model organisms has helped to illuminate the role of NE. A canine form of cyclic neutropenia corresponds to human Hermansky-Pudlak syndrome type 2 (HPS2) and results from mutations in AP3B1 encoding a subunit of a complex involved in the subcellular trafficking of vesicular cargo proteins (among which NE appears to be one). Rare cases of SCN are attributable to mutations in the transcriptional repressor Gfi1 (among whose regulatory targets also include ELA2). The ultimate biochemical consequences of the mutations are not yet known, however. Gene targeting of ELA2 has thus far failed to recapitulate neutropenia in mice. The cycling phenomenon and origins of leukemic transformation in SCN remain puzzling. Nevertheless, mutations in all 3 genes are capable of causing the mislocalization of NE and may also induce the unfolded protein response, suggesting that there might a convergent pathogenic mechanism focusing on NE.

Severe Congenital Neutropenia

Severe congenital neutropenia (CN) includes a variety of hematologic disorders characterized by severe neutropenia, with absolute neutrophil counts (ANC) below 0.5 x 10(9)/L, and associated with severe systemic bacterial infections from early infancy. One subtype of CN, Kostmann syndrome, is an autosomal recessive disorder, characterized histopathologically by early-stage maturation arrest of myeloid differentiation. CN with similar clinical features occurs as an autosomal dominant disorder and many sporadic cases also have been reported. This genetic heterogeneity suggests that several pathophysiological mechanisms may lead to this common clinical phenotype. Recent studies on the genetic bases of CN have detected inherited or spontaneous point mutations in the neutrophil elastase gene (ELA 2) in about 60% to 80% of patients and, less commonly, mutations in other genes. Acquisition of additional genetic defects during the course of the disease, for example, granulocyte colony-stimulating factor (G-CSF) receptor gene mutations and cytogenetic aberrations, indicates an underlying genetic instability as a common feature for all congenital neutropenia subtypes. Data on more than 600 patients with CN collected by the Severe Chronic Neutropenia International Registry (SCNIR) demonstrate that, regardless of the particular CN subtype, more than 95% of these patients respond to recombinant human (rHu)G-CSF with ANCs that can be maintained above 1.0 x 10(9)/L. Adverse events include mild splenomegaly, osteoporosis, and malignant transformation into myelodysplasia (MDS)/leukemia. If and how G-CSF treatment impacts on these adverse events is not fully understood. In recent analyses the influence of the G-CSF dose required to achieve neutrophil response (ANC >1,000/microL) in the risk of developing acute myeloid leukemia (AML) has been reported. Hematopoietic stem cell transplantation (HSCT) is still the only treatment available for patients who are refractory to G-CSF treatment.

Strong evidence for autosomal dominant inheritance of severe congenital neutropenia associated with ELA2 mutations

OBJECTIVE

To investigate cases of severe congenital neutropenia (SCN) to ascertain SCN inheritance after determining that the same sperm donor was used by 4 different families to impregnate mothers.

STUDY DESIGN

Because the donor sperm was not available, alternative methods were used to determine whether the sperm donor transmitted SCN. DNA isolated from leukocytes was used to sequence the ELA2 gene in the affected children and their mothers. ELA2 was amplified by polymerase chain reaction (PCR), and the product was sequenced. PCR was also performed with genomic DNA from the mothers and affected children using a set of 22 microsatellite PCR primers on chromosomes 14 and 19 to establish linkage to the paternal allele.

RESULTS

None of the mothers had a mutation in ELA2, but all 5 affected children had the same mutation affecting the fourth exon at site S97L. Linkage mapping analysis confirmed that all affected children had the same paternal allele on chromosome 19, which contains ELA2.

CONCLUSIONS

Our findings indicate that the father provided consistent haplotypes leading to the expression of SCN in all affected children, supporting an autosomal dominant inheritance in which ELA2 mutations occur.

Lethal hemophagocytic lymphohistiocytosis in Hermansky-Pudlak syndrome type II

Griscelli syndrome (GS) was diagnosed in a 2-year-old patient with oculocutaneous albinism and immunodeficiency, but sequencing of RAB27a revealed only a heterozygous mutation. Due to impaired natural killer (NK) and T-cell cytotoxicity implying a high risk of developing hemophagocytic lymphohistiocytosis (HLH), he was prepared for hematopoietic stem cell transplantation (HSCT). Unexpectedly, a severe bleeding episode occurred that led to the demonstration of disturbed platelet aggregation, reduced plateletdense granules, and impaired platelet degranulation. In combination with neutropenia, this suggested the diagnosis of Hermansky-Pudlak syndrome type II (HPSII) and a novel homozygous mutation in AP3B1 was detected. None of the 3 reported HPSII patients had developed HLH, and our patient seroconverted to Epstein-Barr virus (EBV) without clinical symptoms. HSCT was therefore withheld, and granulocyte-colony-stimulating factor (G-CSF) therapy was initiated and prevented further bacterial infections. At 3 years of age, however, the patient developed, without an obvious trigger, fulminant HLH that was resistant to therapy. This patient shows that careful clinical and molecular diagnosis is essential to differentiate the complex disorders of lysosomal trafficking. HPSII belongs to the group of familial hemophagocytic syndromes and may represent an indication for HSCT.

Mutations in neutrophil elastase causing congenital neutropenia lead to cytoplasmic protein accumulation and induction of the unfolded protein response

Severe congenital neutropenia (SCN) and cyclic neutropenia (CyN) are sporadic or inherited hematologic disorders of myelopoiesis. Heterozygous mutations in the gene encoding neutrophil elastase (ELA2) have been reported in both diseases. We used an inducible system to express a panel of ELA2 mutations and found for almost all mutants disruption of intracellular neutrophil elastase (HNE) protein processing at different levels. This disruption resulted in cytoplasmic accumulation of a nonfunctional protein, thereby preventing its physiologic transport to azurophil granules. Furthermore, the secretory capacity of the mutant proteins was greatly diminished, indicating alteration of the regulated and the constitutive pathways. Through analysis of primary granulocytes from SCN patients carrying ELA2 mutations, we found an identical pattern of intracellular accumulation of mutant HNE protein in the cytoplasm. Moreover, cells expressing mutant HNE protein exhibited a significant increase in apoptosis associated with up-regulation of the master ER chaperone BiP, indicating that disturbance of intracellular trafficking results in activation of the mammalian unfolded protein response.

Increased CCAAT enhancer-binding protein epsilon (C/EBPepsilon) expression and premature apoptosis in myeloid cells expressing Gfi-1 N382S mutant associated with severe congenital neutropenia

Granulocyte-colony-stimulating factor (G-CSF) stimulates the activation of multiple signaling pathways, leading to alterations in the activities of transcription factors. Gfi-1 is a zinc finger transcriptional repressor that is required for granulopoiesis. How Gfi-1 acts in myeloid cells is poorly understood. We show here that the expression of Gfi-1 was up-regulated during G-CSF-induced granulocytic differentiation in myeloid 32D cells. Truncation of the carboxyl terminus of the G-CSF receptor, as seen in patients with acute myeloid leukemia evolving from severe congenital neutropenia, disrupted Gfi-1 up-regulation by G-CSF. Ectopic expression of a dominant negative Gfi-1 mutant, N382S, which was associated with severe congenital neutropenia, resulted in premature apoptosis and reduced proliferation of cells induced to differentiate with G-CSF. The expression of neutrophil elastase (NE) and CCAAT enhancer-binding protein epsilon (C/EBPepsilon) was significantly increased in 32D cells expressing N382S. In contrast, overexpression of wild type Gfi-1 abolished G-CSF-induced up-regulation of C/EBPepsilon but had no apparent effect on NE up-regulation by G-CSF. Notably, G-CSF-dependent proliferation and survival were inhibited upon overexpression of C/EBPepsilon but not NE. These data indicate that Gfi-1 down-regulates C/EBPepsilon expression and suggest that increased expression of C/EBPepsilon as a consequence of loss of Gfi-1 function may be deleterious to the proliferation and survival of early myeloid cells.

Severe congenital neutropenia: genetics and pathogenesis

Severe congenital neutropenia (SCN) is a rare hematological disease characterized by a selective decrease in circulating neutrophils, bone marrow maturation arrest at the promyelocyte stage, and occurrence of infections. Studies of SCN patients revealed impaired proliferative characteristics and accelerated apoptosis of bone marrow progenitor cells. Sequencing analysis indicated a heterozygous deletion or substitution mutations in the neutrophil elastase (NE gene) in many patients. Expression of NE mutants but not normal NE accelerated apoptosis of human HL-60 progenitor cells similar to that seen with actual patient cells. Our data indicate that impaired survival of bone marrow myeloid progenitor cells arises from expression of mutant NE which contributes to neutropenia in SCN. In an unusual clinical set of studies during which SCN developed in five children from four families impregnated by the same sperm donor, we identified the responsible donor by employing genomic DNA studies.

Mechanisms of Disordered Granulopoiesis in Congenital Neutropenia

Neutrophils are critical components of the innate immune response, and persistent neutropenia is associated with a marked susceptibility to infection. There are a number of inherited clinical syndromes in which neutropenia is a prominent feature. A study of these rare disorders has provided insight into the mechanisms regulating normal neutrophil homeostasis. Tremendous progress has been made at defining the genetic basis of these disorders. Herein, progress in understanding the genetic basis and molecular mechanisms of these disorders is discussed. We have focused our discussion on inherited disorders in which neutropenia is the sole or major hematopoietic defect.