Further delineation of the phenotype of severe congenital neutropenia type 4 due to mutations in G6PC3

Drosophila neuronal Glucose-6-Phosphatase is a modulator of neuropeptide release that regulates muscle glycogen stores via FMRFamide signaling

Neuropeptides (NPs) and their cognate receptors are critical effectors of diverse physiological processes and behaviors. We recently reported of a noncanonical function of the Drosophila Glucose-6-Phosphatase (G6P) gene in a subset of neurosecretory cells in the central nervous system that governs systemic glucose homeostasis in food-deprived flies. Here, we show that G6P-expressing neurons define six groups of NP-secreting cells, four in the brain and two in the thoracic ganglion. Using the glucose homeostasis phenotype as a screening tool, we find that neurons located in the thoracic ganglion expressing FMRFamide NPs (FMRFaG6P neurons) are necessary and sufficient to maintain systemic glucose homeostasis in starved flies. We further show that G6P is essential in FMRFaG6P neurons for attaining a prominent Golgi apparatus and secreting NPs efficiently. Finally, we establish that G6P-dependent FMRFa signaling is essential for the build-up of glycogen stores in the jump muscle which expresses the receptor for FMRFamides. We propose a general model in which the main role of G6P is to counteract glycolysis in peptidergic neurons for the purpose of optimizing the intracellular environment best suited for the expansion of the Golgi apparatus, boosting release of NPs and enhancing signaling to respective target tissues expressing cognate receptors.

Drosophila Neuronal Glucose 6 Phosphatase is a Modulator of Neuropeptide Release that Regulates Muscle Glycogen Stores via FMRFamide Signaling

Neuropeptides (NPs) and their cognate receptors are critical effectors of diverse physiological processes and behaviors. We recently reported of a non-canonical function of the Drosophila Glucose-6-Phosphatase ( G6P ) gene in a subset of neurosecretory cells in the CNS that governs systemic glucose homeostasis in food deprived flies. Here, we show that G6P expressing neurons define 6 groups of neuropeptide secreting cells, 4 in the brain and 2 in the thoracic ganglion. Using the glucose homeostasis phenotype as a screening tool, we find that neurons located in the thoracic ganglion expressing FMRFamide neuropeptides ( FMRFa G6P neurons) are necessary and sufficient to maintain systemic glucose homeostasis in starved flies. We further show that G6P is essential in FMRFa G6P neurons for attaining a prominent Golgi apparatus and secreting neuropeptides efficiently. Finally, we establish that G6P dependent FMRFa signaling is essential for the build-up of glycogen stores in the jump muscle which expresses the receptor for FMRFamides. We propose a general model in which the main role of G6P is to counteract glycolysis in peptidergic neurons for the purpose of optimizing the intracellular environment best suited for the expansion of the Golgi apparatus, boosting release of neuropeptides and enhancing signaling to respective target tissues expressing cognate receptors.

SIGNIFICANCE STATEMENT

Glucose-6-phosphtase (G6P) is a critical enzyme in sugar synthesis and catalyzes the final step in glucose production. In Drosophila - and insects in general - where trehalose is the circulating sugar and Trehalose phosphate synthase, and not G6P, is used for sugar production, G6P has adopted a novel and unique role in peptidergic neurons in the CNS. Interestingly, flies lacking G6P show diminished Neuropeptide secretions and have a smaller Golgi apparatus in peptidergic neurons. It is hypothesized that the role of G6P is to counteract glycolysis, thereby creating a cellular environment that is more amenable to efficient neuropeptide secretion.

Severe congenital neutropenia due to G6PC3 deficiency: early and delayed phenotype of a patient

BACKGROUND

Severe Congenital Neutropenia type 4 (SCN4), is a rare autosomal recessive condition, due to mutations in the G6PC3 gene. The phenotype comprises neutropenia of variable severity and accompanying anomalies.

CASE PRESENTATION

We report a male patient with confirmed G6PC3 deficiency presented with recurrent bacterial infections and multi-systemic complications. Our case was the first with a novel homozygous frameshift mutation in G6PC3. The patient demonstrated large platelets on his peripheral blood smear which is a rare presentation of this disease.

CONCLUSION

As SCN4 patients could be easily missed, it is recommended to consider G6PC3 mutation for any case of congenital, unexplained neutropenia.

Novel AGL variants in a patient with glycogen storage disease type IIIb and pulmonary hypertension caused by pulmonary veno-occlusive disease: A case report

Glycogen storage disease type III (GSD-III) is an autosomal recessive metabolic disorder caused by mutations in the AGL gene, and may develop various types of pulmonary hypertension (PH). Here, we report a case of 24-year-old man with GSD-IIIb with two novel null variants in AGL (c.2308 + 2T>C and c.3045_3048dupTACC). He developed multi-drug-resistant pulmonary veno-occlusive disease (PVOD) and was registered as a candidate for lung transplantation. No pathogenic variants were detected in previously known causative genes for pulmonary hypertension and the underlying mechanism of coincidence of two rare diseases was unknown. We discuss the association of the loss of glycogen-debranching enzyme with incident PVOD.

Severe congenital neutropenia due to G6PC3 deficiency: Case series of five patients and literature review

BACKGROUND AND OBJECTIVES

Glucose-6-phosphate catalytic subunit 3 (G6PC3) deficiency is characterized by severe congenital neutropenia with recurrent pyogenic infections, a prominent superficial venous pattern, and cardiovascular and urogenital malformations, caused by an alteration of glucose homeostasis, with increased endoplasmic reticulum stress and cell apoptosis.

METHODS

We reviewed our patients with G6PC3 deficiency diagnosed along the last decade in Mexico; we also searched the PubMed/Medline database for the terms ("G6PC3 deficiency" OR "Dursun syndrome" OR "Severe congenital neutropenia type 4"), and selected articles published in English from 2009 to 2020. Results We found 89 patients reported from at least 14 countries in 4 continents. We describe five new cases from Mexico. Of the 94 patients 56% are male, 48% from Middle East countries, none of them had adverse reactions to live vaccines; all presented with at least one severe infection prior to age 2. 75% had syndromic features, mainly atrial septal defect in 55%, and prominent superficial veins in 62%.

CONCLUSIONS

With a total of 94 patients reported in the past decade, we delineate the most frequent laboratory and genetic features, their treatment, and outcomes, and to expand the knowledge of syndromic and non-syndromic phenotypes in these patients.

Mechanisms of Lower Extremity Vein Dysfunction in Chronic Venous Disease and Implications in Management of Varicose Veins

Chronic venous disease (CVD) is a common venous disorder of the lower extremities. CVD can be manifested as varicose veins (VVs), with dilated and tortuous veins, dysfunctional valves and venous reflux. If not adequately treated, VVs could progress to chronic venous insufficiency (CVI) and lead to venous leg ulcer (VLU). Predisposing familial and genetic factors have been implicated in CVD. Additional environmental, behavioral and dietary factors including sedentary lifestyle and obesity may also contribute to CVD. Alterations in the mRNA expression, protein levels and proteolytic activity of matrix metalloproteinases (MMPs) have been detected in VVs and VLU. MMP expression/activity can be modulated by venous hydrostatic pressure, hypoxia, tissue metabolites, and inflammation. MMPs in turn increase proteolysis of different protein substrates in the extracellular matrix particularly collagen and elastin, leading to weakening of the vein wall. MMPs could also promote venous dilation by increasing the release of endothelium-derived vasodilators and activating potassium channels, leading to smooth muscle hyperpolarization and relaxation. Depending on VVs severity, management usually includes compression stockings, sclerotherapy and surgical removal. Venotonics have also been promoted to decrease the progression of VVs. Sulodexide has also shown benefits in VLU and CVI, and recent data suggest that it could improve venous smooth muscle contraction. Other lines of treatment including induction of endogenous tissue inhibitors of metalloproteinases (TIMPs) and administration of exogenous synthetic inhibitors of MMPs are being explored, and could provide alternative strategies in the treatment of CVD.

Hypothesis: A Novel Neuroprotective Role for Glucose-6-phosphatase (G6PC3) in Brain-To Maintain Energy-Dependent Functions Including Cognitive Processes

The isoform of glucose-6-phosphatase in liver, G6PC1, has a major role in whole-body glucose homeostasis, whereas G6PC3 is widely distributed among organs but has poorly-understood functions. A recent, elegant analysis of neutrophil dysfunction in G6PC3-deficient patients revealed G6PC3 is a neutrophil metabolite repair enzyme that hydrolyzes 1,5-anhydroglucitol-6-phosphate, a toxic metabolite derived from a glucose analog present in food. These patients exhibit a spectrum of phenotypic characteristics and some have learning disabilities, revealing a potential linkage between cognitive processes and G6PC3 activity. Previously-debated and discounted functions for brain G6PC3 include causing an ATP-consuming futile cycle that interferes with metabolic brain imaging assays and a nutritional role involving astrocyte-neuron glucose-lactate trafficking. Detailed analysis of the anhydroglucitol literature reveals that it competes with glucose for transport into brain, is present in human cerebrospinal fluid, and is phosphorylated by hexokinase. Anhydroglucitol-6-phosphate is present in rodent brain and other organs where its accumulation can inhibit hexokinase by competition with ATP. Calculated hexokinase inhibition indicates that energetics of brain and erythrocytes would be more adversely affected by anhydroglucitol-6-phosphate accumulation than heart. These findings strongly support the paradigm-shifting hypothesis that brain G6PC3 removes a toxic metabolite, thereby maintaining brain glucose metabolism- and ATP-dependent functions, including cognitive processes.

Three patients with glucose-6 phosphatase catalytic subunit 3 deficiency

Objectives Severe congenital neutropenia (SCN) is a primary immunodeficiency (PID) characterized by persistent severe neutropenia, recurrent infections, and oral aphthous lesions. Severe congenital neutropenia is caused by various genetic defects such as ELANE, GFI, HAX-1, JAGN1, SRP54, and glucose-6 phosphatase catalytic subunit 3 (G6PC3) deficiency. Clinical features of the patients with G6PC3 deficiency vary from neutropenia to several systemic features in addition to developmental delay. Case presentation In this report, we presented three unrelated patients diagnosed with G6PC3 deficiency. All these patients had short stature, prominent and superficial vascular tissue, cardiac abnormalities (Atrial septal defect (secondary), mitral valve prolapse with mitral insufficiency, pulmonary hypertension) and lymphopenia. Patient 1 (P1) and 2 (P2) had urogenital abnormalities, P2 and P3 had thrombocytopenia. Conclusions We have shown that lymphopenia and CD4 lymphopenia do not rarely accompany to G6PC3 deficiency. Characteristic facial appearance, systemic manifestions, neutropenia could be the clues for the diagnosis of G6PC3 deficiency.

The role of neutrophils in host defense and disease

Neutrophils, the most abundant circulating leukocyte, are critical for host defense. Granulopoiesis is under the control of transcriptional factors and culminates in mature neutrophils with a broad armamentarium of antimicrobial pathways. These pathways include nicotinamide adenine dinucleotide phosphate oxidase, which generates microbicidal reactive oxidants, and nonoxidant pathways that target microbes through several mechanisms. Activated neutrophils can cause or worsen tissue injury, underscoring the need for calibration of activation and resolution of inflammation when infection has been cleared. Acquired neutrophil disorders are typically caused by cytotoxic chemotherapy or immunosuppressive agents. Primary neutrophil disorders typically result from disabling mutations of individual genes that result in impaired neutrophil number or function, and provide insight into basic mechanisms of neutrophil biology. Neutrophils can also be activated by noninfectious causes, including trauma and cellular injury, and can have off-target effects in which pathways that typically defend against infection exacerbate injury and disease. These off-target effects include acute organ injury, autoimmunity, and variable effects on the tumor microenvironment that can limit or worsen tumor progression. A greater understanding of neutrophil plasticity in these conditions is likely to pave the way to new therapeutic approaches.

CDG and immune response: From bedside to bench and back

Glycosylation is an essential biological process that adds structural and functional diversity to cells and molecules, participating in physiological processes such as immunity. The immune response is driven and modulated by protein-attached glycans that mediate cell-cell interactions, pathogen recognition and cell activation. Therefore, abnormal glycosylation can be associated with deranged immune responses. Within human diseases presenting immunological defects are congenital disorders of glycosylation (CDG), a family of around 130 rare and complex genetic diseases. In this review, we have identified 23 CDG with immunological involvement, characterized by an increased propensity to-often life-threatening-infection. Inflammatory and autoimmune complications were found in 7 CDG types. CDG natural history(ies) and the mechanisms behind the immunological anomalies are still poorly understood. However, in some cases, alterations in pathogen recognition and intracellular signaling (eg, TGF-β1, NFAT, and NF-κB) have been suggested. Targeted therapies to restore immune defects are only available for PGM3-CDG and SLC35C1-CDG. Fostering research on glycoimmunology may elucidate the involved pathophysiological mechanisms and open new therapeutic avenues, thus improving CDG patients' quality of life.

Family studies of warts, hypogammaglobulinemia, immunodeficiency, myelokathexis syndrome

PURPOSE OF REVIEW

WHIM syndrome (warts, hypogammaglobulinemia, immunodeficiency, myelokathexis, or WHIMs) is a very rare autosomal dominant immunodeficiency disorder attributable to mutations in CXCR4. We reviewed clinical manifestations in 24 patients in 9 families to expand understanding of this syndrome.

RECENT FINDINGS

Warts, cellulitis and respiratory infections are common in patients with WHIMs. Less commonly these patients have congenital heart disease, human papilloma virus-associated malignancies (cervical and vulvular) and lymphomas. Hearing loss because of recurrent otitis media is another important complication. Treatment with granulocyte colony-stimulating factor is controversial; this review indicates that it is effective to prevent and treat infections based upon long-term observations of patients enrolled in the Severe Chronic Neutropenia International Registry. Understanding the natural history and diversity of this syndrome are important for ongoing clinical trials of novel agents to treat WHIMs.

SUMMARY

WHIM syndrome has diverse manifestations; some features occur consistently in almost all patients, for example, neutropenia, lymphocytopenia and mild hypogammaglobulinemia. However, the clinical consequences are quite variable across patient cohorts and within families. Each complication is important as a cause for morbidity and a source for patient and family concerns.

G6PC3 Deficiency: Primary Immune Deficiency Beyond Just Neutropenia

Glucose-6-phosphatase catalytic subunit 3 (G6PC3) deficiency was recently defined as a new severe congenital neutropenia subgroup remarkable with congenital heart defects, urogenital malformations, endocrine abnormalities, and prominent superficial veins. Here, we report 3 patients with G6PC3 deficiency presenting with recurrent diarrhea, failure to thrive, and sinopulmonary infections leading to bronchiectasis. In patient I and II, a combined immune deficiency was suspected due to early-onset disease with lymphopenia, neutropenia, and thrombocytopenia, along with variable reductions in lymphocyte subpopulations and favorable response to intravenous γ-globulin therapy. Apart from neutropenia, all 3 patients had intermittent thrombocytopenia, anemia, and lymphopenia. All patients had failure to thrive and some of the classic syndromic features of G6PC3 deficiency, including cardiac abnormalities and visibility of superficial veins in all, endocrinologic problems in PI and PIII, and urogenital abnormalities in PII. Our experience suggests that a diagnosis of congenital neutropenia due to G6PC3 may not be as straightforward in such patients with combined lymphopenia and thrombocytopenia. A high index of suspicion and the other syndromic features of G6PC3 were clues to diagnosis. Screening of all combined immune deficiencies with neutropenia may help to uncover the whole spectra of G6PC3 deficiency.

Cardiovascular abnormalities in primary immunodeficiency diseases

In recent years, increasing numbers of patients with primary immune deficiency (PID) are being recognized as also suffering from cardiovascular system (CVS) abnormalities. These CVS defects might be explained by infectious or autoimmune etiologies, as well as by the role of specific genes and the immune system in the development and function of CVS tissues. Here, we provide the first comprehensive review of the clinical, potentially pathogenic mechanisms, and the management of PID, as well as the associated immune and CVS defects. In addition to some well-known associations of PID with CVS abnormalities, such as DiGeorge syndrome and CHARGE anomaly, we describe the cardiac defects associated with Omenn syndrome, calcium channel deficiencies, DNA repair defects, common variable immunodeficiency, Roifman syndrome, various neutrophil/macrophage defects, FADD deficiency, and HOIL1 deficiency. Moreover, we detail the vascular abnormalities recognized in chronic mucocutaneous candidiasis, chronic granulomatous disease, Wiskott–Aldrich syndrome, Schimke immuno-osseus dysplasia, hyper-IgE syndrome, MonoMAC syndrome, and X-linked lymphoproliferative disease. In conclusion, the expanding spectrum of PID requires increased alertness to the possibility of CVS involvement as an important contributor to the diagnosis and management of these patients.

Glycans Instructing Immunity: The Emerging Role of Altered Glycosylation in Clinical Immunology

Protein glycosylation is an important epigenetic modifying process affecting expression, localization, and function of numerous proteins required for normal immune function. Recessive germline mutations in genes responsible for protein glycosylation processes result in congenital disorders of glycosylation and can have profound immunologic consequences. Genetic mutations in immune signaling pathways that affect glycosylation sites have also been shown to cause disease. Sugar supplementation and in vivo alteration of glycans by medication holds therapeutic promise for some of these disorders. Further understanding of how changes in glycosylation alter immunity may provide novel treatment approaches for allergic disease, immune dysregulation, and immunodeficiency in the future.

Genetic analysis and clinical picture of severe congenital neutropenia in Israel

BACKGROUND

The relative frequency of mutated genes among patients with severe congenital neutropenia (SCN) may differ between various ethnic groups. To date, few population-based genetic studies have been reported. This study describes the genetic analysis of 32 Israeli patients with SCN.

PROCEDURES

Clinical data were retrieved from the prospective Israeli Inherited Bone Marrow Failure Registry. Recruitment included living and deceased patients who were diagnosed between 1982 and 2012, for whom molecular diagnosis was performed. ELANE, HAX1 and G6PC3 genes were sequenced in all patients, and GFI-1 and WAS genes were sequenced if other genes were wildtype.

RESULTS

Eleven patients (34%) had heterozygous mutations in ELANE (10 kindreds), eight (25%) had homozygous mutations in G6PC3 (5 kindreds) and 13 (41%) had no detected mutations. No patients had mutations in HAX1 or WAS. Four of the eight patients with G6PC3 mutations had congenital anomalies. The probability of survival for all patients was 50% at age of 18. Deaths were mainly due to sepsis (5 patients, 4/5 not responding to G-CSF, none with G6PC3 mutation). Two patients developed acute myelogenous leukemia (AML) and one myelodysplastic syndrome (MDS), none with G6PC3 mutation.

CONCLUSIONS

We found a unique pattern of SCN mutations in Israel with homozygous G6PC3 mutations in eight (25%) patients, the highest frequency described so far. HAX1 mutations, reported mainly in Sweden and Iran, were absent. Patients with G6PC3 mutations had congenital anomalies, appeared to have a better response to G-CSF, and so far have not developed AML or MDS.

Clinical spectrum and long-term follow-up of 14 cases with G6PC3 mutations from the French Severe Congenital Neutropenia Registry

Background

The purpose of this study was to describe the natural history of severe congenital neutropenia (SCN) in 14 patients with G6PC3 mutations and enrolled in the French SCN registry.

Methods

Among 605 patients included in the French SCN registry, we identified 8 pedigrees that included 14 patients with autosomal recessive G6PC3 mutations.

Results

Median age at the last visit was 22.4 years. All patients had developed various comordibities, including prominent veins (n = 12), cardiac malformations (n = 12), intellectual disability (n = 7), and myopathic syndrome with recurrent painful cramps (n = 1). Three patients developed Crohn’s disease, and five had chronic diarrhea with steatorrhea. Neutropenia was profound (<0.5 × 10⁹/l) in almost all cases at diagnosis and could marginally fluctuate. The bone marrow smears exhibited mild late-stage granulopoeitic defects. One patient developed myelodysplasia followed by acute myelogenous leukemia with translocation (18, 21) at age 14 years, cured by chemotherapy and hematopoietic stem cell transplantation. Four deaths occurred, including one from sepsis at age 5, one from pulmonary late-stage insufficiency at age 19, and two from sudden death, both at age 30 years. A new homozygous mutation (c.249G > A /p.Trp83*) was detected in one pedigree.

Conclusions

Severe congenital neutropenia with autosomal recessive G6PC3 mutations is associated with considerable clinical heterogeneity. This series includes the first described case of malignancy in this neutropenia.

Severe congenital neutropenia due to G6PC3 deficiency: early and delayed phenotype in two patients with two novel mutations

Severe Congenital Neutropenia type 4 (SCN4, OMIM 612541) is a rare autosomal recessive disease due to mutations in the G6PC3 gene. The phenotype comprises neutropenia of variable severity and other anomalies including congenital heart defects, prominent superficial veins, uro-genital anomalies, facial dysmorphism, growth and developmental delay and intermittent thrombocytopenia. In some patients, SCN represents the only manifestation of the disease. Variable findings have been reported at bone marrow examination ranging from a maturation arrest at the myelocyte/promyelocyte stage (either in a hypocellular or hypercellular context) to myelokathexis. Here we report two patients harbouring two novel mutations in the G6PC3 gene, including the first Italian patient even described. Both the patients share profound neutropenia with severe infections early in life; in one case non-hematopoietic stigmata of the syndrome, including evident facial dysmorphism and vascular anomalies, appeared gradually over time, prominently in the second decade. Therefore, G6PC3 defects should be considered in any case of congenital, unexplained neutropenia regardless of the clinical phenotype. Both patients are on G-CSF treatment with no evidence of malignant evolution. Even if G6PC3 deficiency seems not to have a propensity towards malignancy, a careful evaluation is warranted.

Testicular failure in a patient with G6PC3 deficiency

BACKGROUND

Glucose-6-phosphatase-β (G6PC3) deficiency is characterized by congenital neutropenia and variable developmental disorders, including those of the cardiovascular system and the urogenital system (e.g., cryptorchidism) and a peculiar visibility of subcutaneous veins.

METHODS

A patient with clinical findings suggestive of G6PC3 deficiency was investigated. Genetic, hematopathologic, immunologic, and endocrine work-up were performed.

RESULTS

The reported patient had binucleotide deletion mutation in G6PC3 and displayed the full spectrum of clinical manifestations associated with G6PC3 deficiency including neutropenia. The reported patient had normal bone marrow cellularity without increased apoptosis, and his neutrophils displayed normal respiratory burst activity. Endocrine work-up revealed low testosterone levels, which did not respond to human chorionic gonadotropin stimulation, extremely elevated luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels, and undetectable anti-Müllerian hormone, all of which are suggestive of testicular failure or anorchia.

CONCLUSION

Our report extends the knowledge about this syndrome and suggests a role for G6PC3 in testicular differentiation and formation. Urogenital dysmorphism could indeed be unrelated to G6PC3 and secondary to consanguinity. However, given the similar description of urogenital anomalies in previous reports of this syndrome, the dysmorphism in our patient is likely related.

A clinical and molecular review of ubiquitous glucose-6-phosphatase deficiency caused by G6PC3 mutations

The G6PC3 gene encodes the ubiquitously expressed glucose-6-phosphatase enzyme (G-6-Pase β or G-6-Pase 3 or G6PC3). Bi-allelic G6PC3 mutations cause a multi-system autosomal recessive disorder of G6PC3 deficiency (also called severe congenital neutropenia type 4, MIM 612541). To date, at least 57 patients with G6PC3 deficiency have been described in the literature.

G6PC3 deficiency is characterized by severe congenital neutropenia, recurrent bacterial infections, intermittent thrombocytopenia in many patients, a prominent superficial venous pattern and a high incidence of congenital cardiac defects and uro-genital anomalies. The phenotypic spectrum of the condition is wide and includes rare manifestations such as maturation arrest of the myeloid lineage, a normocellular bone marrow, myelokathexis, lymphopaenia, thymic hypoplasia, inflammatory bowel disease, primary pulmonary hypertension, endocrine abnormalities, growth retardation, minor facial dysmorphism, skeletal and integument anomalies amongst others. Dursun syndrome is part of this extended spectrum. G6PC3 deficiency can also result in isolated non-syndromic severe neutropenia. G6PC3 mutations in result in reduced enzyme activity, endoplasmic reticulum stress response, increased rates of apoptosis of affected cells and dysfunction of neutrophil activity.

In this review we demonstrate that loss of function in missense G6PC3 mutations likely results from decreased enzyme stability. The condition can be diagnosed by sequencing the G6PC3 gene. A number of G6PC3 founder mutations are known in various populations and a possible genotype-phenotype relationship also exists. G6PC3 deficiency should be considered as part of the differential diagnoses in any patient with unexplained congenital neutropenia.

Treatment with G-CSF leads to improvement in neutrophil numbers, prevents infections and improves quality of life. Mildly affected patients can be managed with prophylactic antibiotics. Untreated G6PC3 deficiency can be fatal. Echocardiogram, renal and pelvic ultrasound scans should be performed in all cases of suspected or confirmed G6PC3 deficiency. Routine assessment should include biochemical profile, growth profile and monitoring for development of varicose veins or venous ulcers.

Survival and differentiation defects contribute to neutropenia in glucose-6-phosphatase-β (G6PC3) deficiency in a model of mouse neutrophil granulocyte differentiation

Differentiation of neutrophil granulocytes (neutrophils) occurs through several steps in the bone marrow and requires a coordinate regulation of factors determining survival and lineage-specific development. A number of genes are known whose deficiency disrupts neutrophil generation in humans and in mice. One of the proteins encoded by these genes, glucose-6-phosphatase-β (G6PC3), is involved in glucose metabolism. G6PC3 deficiency causes neutropenia in humans and in mice, linked to enhanced apoptosis and ER stress. We used a model of conditional Hoxb8 expression to test molecular and functional differentiation as well as survival defects in neutrophils from G6PC3(-/-) mice. Progenitor lines were established and differentiated into neutrophils when Hoxb8 was turned off. G6PC3(-/-) progenitor cells underwent substantial apoptosis when differentiation was started. Transgenic expression of Bcl-XL rescued survival; however, Bcl-XL-protected differentiated cells showed reduced proliferation, immaturity and functional deficiency such as altered MAP kinase signaling and reduced cytokine secretion. Impaired glucose utilization was found and was associated with ER stress and apoptosis, associated with the upregulation of Bim and Bax; downregulation of Bim protected against apoptosis during differentiation. ER-stress further caused a profound loss of expression and secretion of the main neutrophil product neutrophil elastase during differentiation. Transplantation of wild-type Hoxb8-progenitor cells into irradiated mice allowed differentiation into neutrophils in the bone marrow in vivo. Transplantation of G6PC3(-/-) cells yielded few mature neutrophils in bone marrow and peripheral blood. Transgenic Bcl-XL permitted differentiation of G6PC3(-/-) cells in vivo. However, functional deficiencies and differentiation abnormalities remained. Differentiation of macrophages from Hoxb8-dependent progenitors was only slightly disturbed. A combination of defects in differentiation and survival thus underlies neutropenia in G6PC3(-/-) deficiency, both originating from a reduced ability to utilize glucose. Hoxb8-dependent cells are a model to study differentiation and survival of the neutrophil lineage.

A novel G6PC3 gene mutation in a patient with severe congenital neutropenia

Glucose-6-phosphatase catalytic subunit 3 (G6PC3) deficiency is a newly described syndromic type of severe congenital neutropenia, associated with multiple organ abnormalities including facial, cardiac, and urogenital abnormalities, and increased visibility of superficial veins. The molecular pathophysiology of G6PC3 deficiency is associated with the disturbed glucose homeostasis, increased endoplasmic reticulum stress, and apoptosis in neutrophils. We report a new case of G6PC3 deficiency caused by a novel homozygous G6PC3 gene mutation (p.Leu154Pro). Most remarkable is that the chronic neutropenia that originated from this novel G6PC3 genetic defect is also accompanied by some other unusual manifestations in this patient: myelokathexis and hypercholesterolemia.

G6PC3 mutations cause non-syndromic severe congenital neutropenia

The deficiency of ubiquitously expressed glucose-6-phosphatase (G6PC3) enzyme is known to result in a syndrome characterized by severe congenital neutropenia, prominent superficial venous pattern, congenital heart defects and genito-urinary malformations. Here, we describe four patients from three families with non-syndromic severe congenital neutropenia and identify four G6PC3 mutations as causative in these cases. Thus we demonstrate that G6PC3 mutations also result in a non-syndromic form of severe congenital neutropenia. We propose that G6PC3 deficiency should be considered as part of the differential diagnoses in any patient with unexplained congenital neutropenia. Additionally, we show a relationship between the genotype and non-hematological phenotype of G6PC3 deficiency. These findings may provide an insight into the role of the G6PC3 enzyme and glucose metabolism in developmental pathways.

Ovarian failure in HAX1-deficient patients: is there a gender-specific difference in pubertal development in severe congenital neutropenia or Kostmann disease?

AIM

Severe congenital neutropenia (SCN) is a rare disorder of myelopoiesis characterized by neutropenia, recurrent bacterial infections and a maturation arrest of the myelopoiesis in the bone marrow. Homozygous mutations in the HAX1 gene were described in patients with autosomal recessive SCN or Kostmann disease. Some of these patients display neurological disease. We noted, during the course of clinical management of patients with Kostmann disease, insufficient pubertal development in female patients, but not in our male patients. The study objective was to provide a detailed account of this phenotype and its possible relation to HAX1 mutations.

METHODS

Detailed clinical histories and laboratory investigations of three patients with Kostmann disease belonging to the original kindred in northern Sweden described by Rolf Kostmann are reported.

RESULTS

We report one male patient with normal puberty and two female patients with insufficient pubertal development. Elevated levels of LH and FSH were recorded in both patients. All three patients harbour the same p.Glu190X mutation in the HAX1 gene.

CONCLUSIONS

We show for the first time that female patients with Kostmann disease display primary gonadal insufficiency. This suggests a possible role for HAX1 in the development and/or function of the human ovary.

Neutropenia in primary immunodeficiency

PURPOSE OF REVIEW

Neutropenia is a feature of several primary immunodeficiency diseases (PIDDs). Because of the diverse pathophysiologies of the PIDDs and the rarity of each disorder, data are often lacking, leading to the necessity of empiric treatment. Recent developments in the understanding of neutropenia in several of the PIDDs make a review of the data timely.

RECENT FINDINGS

The category of severe congenital neutropenia continues to expand. Mutations in G6PC3 have been identified as the cause of neutropenia in a minority of previously molecularly undefined cases. Recent advances have broadened our understanding of the pathophysiology and the clinical expression of this disorder. A possible function of the C16orf57 gene has been hypothesized that may explain the clinical overlap between Clerucuzio-type poikiloderma with neutropenia and other marrow diseases. Plerixafor has been shown to be a potentially useful treatment in the warts, hypogammaglobulinemia, infection, and myelokathexis syndrome. Investigations of patients with adenosine deaminase deficient severe combined immunodeficiency have identified neutropenia, and particularly susceptibility to myelotoxins, as a feature of this disorder. Granulocyte-colony stimulating factor is the treatment of choice for neutropenia in PIDD, whereas hematopoietic cell transplantation is the only curative option.

SUMMARY

The number of PIDDs associated with neutropenia has increased, as has our understanding of the range of phenotypes. Additional data and hypotheses have been generated helping to explain the diversity of presentations of neutropenia in PIDDs.

Adult siblings with homozygous G6PC3 mutations expand our understanding of the severe congenital neutropenia type 4 (SCN4) phenotype

Background

Severe congenital neutropenia type 4 (SCN4) is an autosomal recessive disorder caused by mutations in the third subunit of the enzyme glucose-6-phosphatase (G6PC3). Its core features are congenital neutropenia and a prominent venous skin pattern, and affected individuals have variable birth defects. Oculocutaneous albinism type 4 (OCA4) is caused by autosomal recessive mutations in SLC45A2.

Methods

We report a sister and brother from Newfoundland, Canada with complex phenotypes. The sister was previously reported by Cullinane et al., 2011. We performed homozygosity mapping, next generation sequencing and conventional Sanger sequencing to identify mutations that cause the phenotype in this family. We have also summarized clinical data from 49 previously reported SCN4 cases with overlapping phenotypes and interpret the medical histories of these siblings in the context of the literature.

Results

The siblings’ phenotype is due in part to a homozygous mutation in G6PC3, [c.829C > T, p.Gln277X]. Their ages are 38 and 37 years respectively and they are the oldest SCN4 patients published to date. Both presented with congenital neutropenia and later developed Crohn disease. We suggest that the latter is a previously unrecognized SCN4 manifestation and that not all affected individuals have an intellectual disability. The sister also has a homozygous mutation in SLC45A2, which explains her severe oculocutaneous hypopigmentation. Her brother carried one SLC45A2 mutation and was diagnosed with “partial OCA” in childhood.

Conclusions

This family highlights that apparently novel syndromes can in fact be caused by two known autosomal recessive disorders.

Inflammatory bowel disease and T cell lymphopenia in G6PC3 deficiency

PURPOSE

G6PC3 deficiency presents as a complex and heterogeneous syndrome that classically associates severe congenital neutropenia with cardiac and urogenital developmental defects. Here we investigate the findings of T cell lymphopenia and inflammatory bowel disease in a child with G6PC3 deficiency due to compound heterozygous mutations in intron 3 (c.IVS3-1 G>A) and exon 6 (c.G778G/C; p.Gly260/Arg).

METHODS

Histological examination was conducted on all biopsy specimens. Immunophenotyping and lymphocyte proliferation assays were performed. Immunoglobulin levels and vaccine responses were measured.

RESULTS

The patient showed persistent global T cell lymphopenia, with only 8 to 13 % of thymic naive CD31(+)CD45RA(+) cells among CD4 T cells (normal range 27-60 %). Proliferation assays and vaccine responses were within normal limits. The gastrointestinal inflammatory lesions were very closely related to those of glycogen storage disease type 1b, with a Crohn's-like appearance but without granuloma or increased cryptic abscesses. The gastrointestinal disease responded to infliximab therapy. These findings were associated with a polyclonal hypergammaglobuliemia G.

CONCLUSION

G6PC3 deficiency may present with inflammatory bowel disease and T cell lymphopenia. The diagnosis should thus be considered in a patient with chronic congenital neutropenia and gastrointestinal symptoms. Patients with confirmed disease should also undergo T cell phenotyping to rule out cellular immunodeficiency.

Extended spectrum of human glucose-6-phosphatase catalytic subunit 3 deficiency: novel genotypes and phenotypic variability in severe congenital neutropenia

OBJECTIVE

To delineate the phenotypic and molecular spectrum of patients with a syndromic variant of severe congenital neutropenia (SCN) due to mutations in the gene encoding glucose-6-phosphatase catalytic subunit 3 (G6PC3).

STUDY DESIGN

Patients with syndromic SCN were characterized for associated malformations and referred to us for G6PC3 mutational analysis.

RESULTS

In a cohort of 31 patients with syndromic SCN, we identified 16 patients with G6PC3 deficiency including 11 patients with novel biallelic mutations. We show that nonhematologic features of G6PC3 deficiency are good predictive indicators for mutations in G6PC3. Additionally, we demonstrate genetic variability in this disease and define novel features such as growth hormone deficiency, genital malformations, disrupted bone remodeling, and abnormalities of the integument. G6PC3 mutations may be associated with hydronephrosis or facial dysmorphism. The risk of transition to myelodysplastic syndrome/acute myeloid leukemia may be lower than in other genetically defined SCN subgroups.

CONCLUSIONS

The phenotypic and molecular spectrum in G6PC3 deficiency is wider than previously appreciated. The risk of transition to myelodysplastic syndrome or acute myeloid leukemia may be lower in G6PC3 deficiency compared with other subgroups of SCN.

Glucose-6-phosphatase-β, implicated in a congenital neutropenia syndrome, is essential for macrophage energy homeostasis and functionality

Glucose-6-phosphatase-β (G6Pase-β or G6PC3) deficiency, also known as severe congenital neutropenia syndrome 4, is characterized not only by neutropenia but also by impaired neutrophil energy homeostasis and functionality. We now show the syndrome is also associated with macrophage dysfunction, with murine G6pc3(-/-) macrophages having impairments in their respiratory burst, chemotaxis, calcium flux, and phagocytic activities. Consistent with a glucose-6-phosphate (G6P) metabolism deficiency, G6pc3(-/-) macrophages also have a lower glucose uptake and lower levels of G6P, lactate, and ATP than wild-type macrophages. Furthermore, the expression of NADPH oxidase subunits and membrane translocation of p47(phox) are down-regulated, and G6pc3(-/-) macrophages exhibit repressed trafficking in vivo both during an inflammatory response and in pregnancy. During pregnancy, the absence of G6Pase-β activity also leads to impaired energy homeostasis in the uterus and reduced fertility of G6pc3(-/-) mothers. Together these results show that immune deficiencies in this congenital neutropenia syndrome extend beyond neutrophil dysfunction.

Genetic defects in severe congenital neutropenia: emerging insights into life and death of human neutrophil granulocytes

The discovery of genetic defects causing congenital neutropenia has illuminated mechanisms controlling differentiation, circulation, and decay of neutrophil granulocytes. Deficiency of the mitochondrial proteins HAX1 and AK2 cause premature apoptosis of myeloid progenitor cells associated with dissipation of the mitochondrial membrane potential, whereas mutations in ELA2/ELANE and G6PC3 are associated with signs of increased endoplasmic reticulum stress. Mutations in the transcriptional repressor GFI1 and the cytoskeletal regulator WASP also lead to defective neutrophil production. This unexpected diversity of factors suggests that multiple pathways are involved in the pathogenesis of congenital neutropenia.