A Syndrome with Congenital Neutropenia and Mutations in G6PC3

Severe congenital neutropenia was described more than 50 years ago by Kostmann,1,2 and subsequently the disorder was found to consist of a heterogeneous group of diseases.3,4 In these syndromes, the neutropenia is associated with life-threatening bacterial infections early in life. Most patients respond to treatment with recombinant human granulocyte colony-stimulating factor (rhG-CSF), which increases neutrophil counts and decreases the frequency and severity of infections.5 Nonetheless, patients may remain at risk for both infectious complications and the development of clonal disorders of hematopoiesis, such as myelodysplastic syndrome and acute myeloid leukemia.6

Considerable progress has been made in identifying the molecular defects that cause congenital neutropenia.7,8 Many patients with severe congenital or cyclic neutropenia have a heterozygous mutation in ELA2, the gene encoding neutrophil elastase.9-11 We recently identified homozygous mutations in HAX1, the gene encoding HCLS1-associated protein X1, in a subgroup of patients with autosomal recessive severe congenital neutropenia.12 In addition, mutations in WAS, the gene encoding the Wiskott–Aldrich syndrome protein,13,14 and in GFI1, the gene encoding the growth factor independent 1 transcription-repressor protein,15 have been associated with a phenotype resembling Kostmann's syndrome. In many patients with congenital neutropenia, however, the underlying molecular cause remains unknown. Despite insights into the role of apoptosis in congenital neutropenia,12,16,17 the mechanisms of neutropenia and the risk of leukemia in patients with severe congenital neutropenia are incompletely understood. We report a syndrome consisting of severe congenital neutropenia, other congenital abnormalities, and biallelic mutations in G6PC3, the gene encoding glucose-6-phosphatase, catalytic subunit 3.
Methods
Patients and Controls

We took blood and bone marrow samples from patients and healthy volunteers with their written informed consent. The study was approved by the institutional review board of Hannover Medical School.
Analytical Methods

We genotyped microsatellite markers in a whole-genome scan for a family with severe congenital neutropenia, with a pedigree identified as SCN-I. The equipment and protocols for genotyping have been described previously.12 The genetic-linkage analysis was performed with the use of a combination of quantitative and qualitative syllogisms. The quantitative decisions were made with the use of lod scores and optimal recombination fractions computed with Superlink software.18,19 For calculation of lod scores, we modeled neutropenia as a fully penetrant autosomal recessive disease with no phenocopies and a disease-allele frequency of 0.001. The Marshfield map20 was used to select usefully positioned markers for fine mapping. For details, see the Supplementary Appendix, available with the full text of this article at NEJM.org.

Exons and flanking intron–exon boundaries from candidate genes were amplified by polymerase chain reaction (PCR) and analyzed with the use of an ABI PRISM 3130 DNA Sequencer and DNA Sequencing Analysis software, version 3.4 (Applied Biosystems), and Sequencer, version 3.4.1 (Gene Codes Corporation). See the Supplementary Appendix for primer sequences and details of the restriction-length-polymorphism analysis of the frequency of the R253H mutation in healthy controls.

Promyelocytes were sorted by fluorescence-activated cell sorting, as described previously,17 with minor modifications. Analysis of the expression of the G6PC3 and HSPA5/BiP/Grp78 genes was performed with the use of a Roche LightCycler 2.0 (see the Supplementary Appendix).

The complete open reading frames of wild-type and mutant G6PC3 were amplified by PCR and cloned in pYES-cup1 (modified from pYES-NT [Invitrogen]), as described by Ashikov et al.,21 and expressed in Saccharomyces cerevisiae. Centrifugation at 100,000×g produced a microsomal fraction that was assayed for hydrolysis of glucose-6-phosphate to glucose by the addition of 14C-glucose-6-phosphate (MP Biomedicals). Released 14C-glucose was separated from glucose-6-phosphate by anion exchange and measured in the eluate by liquid scintillation.

Whole-cell lysates from primary granulocytes were separated by sodium dodecyl sulfate–polyacrylamide-gel electrophoresis (SDS-PAGE), blotted, and stained with antibodies against phospho-Mcl-1 (Ser159/Thr163), total glycogen synthase kinase 3β (GSK-3β), phospho-GSK-3β (Ser9) (all from Cell Signaling Technology/New England Biolabs), Bip/Grp78 (BD Biosciences), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (Santa Cruz Biotechnology) (see the Supplementary Appendix).

Bone marrow samples from patients and healthy control subjects were subjected to hypotonic lysis. Fixation and electron microscopy were performed as described previously.22

Human G6PC3 complementary DNA (cDNA) was cloned into a bicistronic retroviral vector (MMP)23 containing murine cd24 as a marker gene. RD114-pseudotyped retroviral particles were generated by tripartite transfection of MMP-based vectors together with the envelope plasmid and the packaging plasmid mPD.old.gag/pol into a human embryonic kidney-cell line (HEK293T). Transduction of CD34+ cells and myeloid differentiation were performed as described previously.12

Apoptosis in peripheral-blood neutrophils or in cells that were differentiated into myeloid cells in vitro was induced with the use of tumor necrosis factor α (TNF-α) (50 ng per milliliter), thapsigargin (10 μM), or tunicamycin (5 μg per milliliter) (all from Sigma) and assessed by staining with annexin V (Invitrogen) and propidium iodide (Sigma). In fibroblasts, apoptosis was induced with the use of 5 mM dithiothreitol (Roche). Caspase 3/7 activation was assessed as described previously12 (see the Supplementary Appendix).
Results
Clinical Findings

Table 1Table 1Clinical and Molecular Findings in Patients with G6PC3 Deficiency. lists the main features of the five patients we studied. Patients 1 and 2, who were siblings born to consanguineous parents of Aramean descent, presented with neonatal sepsis. Their extended pedigree is denoted SCN-I (see Fig. 1 in the Supplementary Appendix). Their workup in the first year of life found severe neutropenia with a paucity of mature neutrophils in the peripheral blood and bone marrow. Bone marrow smears contained few granulocytes beyond the stage of promyelocytes or myelocytes (Figure 1A and 1BFigure 1Clinical and Hematologic Phenotype., and Table 1 in the Supplementary Appendix). Erythrocyte counts were normal. Platelet counts in Patient 1 ranged from 73,000 to 425,000 per cubic millimeter, whereas Patient 2 had normal platelet counts. Both patients had unusually prominent subcutaneous veins, venous angiectasia, or both (Figure 1C); Patient 1 had a type 2 atrial septal defect (Figure 1D), and Patient 2 had cor triatriatum and hepatosplenomegaly. Genealogic investigations revealed that the SCN-I pedigree could be extended to include two additional sibships, each with one child affected by severe congenital neutropenia and type 2 atrial septal defect (Patients 3 and 4 in Fig. 1 in the Supplementary Appendix). We also identified a child with severe congenital neutropenia in a second consanguineous pedigree (SCN-II) from the same ethnic background (Patient 5 in Fig. 1 in the Supplementary Appendix). All patients received rhG-CSF, which resulted in an increase in neutrophil counts.
Genetic Studies

Mutations in both ELA2 10 and HAX1 12 were excluded in all five index patients. Linkage analysis provided statistical evidence that the gene of interest in SCN-I is located on chromosome 17q21 between D17S1299 (36.2 Mb, 62.0 cM) and D17S1290 (53.7 Mb, 82.0 cM) (Figure 2AFigure 2Haplotypes on Chromosome 17q, Mutation Analysis of G6PC3, and Assessment of Enzymatic Activity of G6PC3 with the R253H Mutation., and the Supplementary Appendix). We carried out a series of fine mapping steps in SCN-I and SCN-II and were able to genotype an additional 13 microsatellite markers between D17S1299 and D17S1290 in SCN-I and 11 of these 13 microsatellite markers in SCN-II. Table 2 in the Supplementary Appendix shows the single-marker lod scores. On the assumption that the same gene is mutated in all five affected children, the maximal linkage interval spanned from D17S1789 (39.1 Mb, 63.1 cM) to D17S791 (42.2 Mb, 64.2 cM). With the use of D17S932, D17S950, and D17S806, the peak multipoint lod score in SCN-I alone was 4.98, and the peak two-pedigree multipoint lod score was 5.74.

Several candidate genes were identified in the SCN-I linkage interval (see Table 3 in the Supplementary Appendix). Of these, G6PC3, which encodes glucose-6-phosphatase, catalytic subunit 3, and is located in the narrowest possible linkage interval, was a plausible candidate, because abnormal glucose metabolism has been implicated in the neutropenia of type Ib glycogen storage disease.24 DNA sequencing revealed a homozygous missense mutation in exon 6 of the G6PC3 gene (c.G758A, p.R253H) (Figure 2B). This mutation was found in all four affected children in SCN-I and in the one affected child in SCN-II. All parents were heterozygous for the mutation, a finding consistent with autosomal recessive inheritance of a germ-line missense mutation. According to restriction-site analysis, the R253H allele of the G6PC3 gene was not found in 192 healthy central European persons. An in silico sequence analysis using the program Sorting Intolerant from Tolerant (SIFT)25 calculated that the probability that this mutation is benign is 0.01. Analysis with the program PolyPhen26 predicted that the R253H change probably interferes with the function of glucose-6-phosphatase. The wild-type protein is conserved in mammals, amphibians, bony fish, and insects.
Functional Studies
Enzymatic Activity of G6PC3 with the R253H Mutation

Wild-type G6PC3 and G6PC3 with the R253H mutation were expressed in S. cerevisiae. Microsomes were isolated from yeast transfected with wild-type G6PC3 or G6PC3 with the R253H mutation and assayed for phosphatase activity. Wild-type G6PC3 hydrolyzed glucose-6-phosphate and the universal substrate p-nitrophenylphosphate (pNPP), as demonstrated by radioactive (Figure 2C) and spectrometric (Fig. 3 in the Supplementary Appendix) assays, respectively. In contrast, the level of enzymatic activity of G6PC3 with the R253H mutation did not exceed the level of phosphatase activity in yeast transfected with an empty vector.
Apoptosis

Like neutrophils from patients with mutations in ELA2 10 or HAX1, 12 peripheral-blood neutrophils from our patients had an increased rate of spontaneous apoptosis. Apoptosis of neutrophils was markedly accelerated after induction with TNF-α (Figure 3AFigure 3Increased Susceptibility to Apoptosis in Myeloid Cells and Fibroblast Cell Lines.) or tunicamycin (data not shown), as assessed by annexin V staining and a test for activation of caspase 3/7, respectively (see Fig. 4 in the Supplementary Appendix). Since G6PC3 is ubiquitously expressed and since the phenotype of our patients was not restricted to the hematopoietic system, we tested nonhematopoietic cells for susceptibility to apoptosis. Skin fibroblasts from patients with a deficiency of G6PC3 had an increased susceptibility to apoptosis after dithiothreitol-induced stress to the endoplasmic reticulum (Figure 3B).

To provide further evidence that this form of severe congenital neutropenia is caused by mutations in G6PC3, CD34+ hematopoietic stem cells from two patients were isolated and transduced with retroviral constructs containing either the wild-type G6PC3 cDNA sequence and murine cd24 as a reporter gene (MMP-G6PC3-mcd24) or the reporter gene only (MMP-mcd24). After in vitro differentiation in the presence of rhG-CSF and recombinant human granulocyte–macrophage colony-stimulating factor (rhGM-CSF), cells were exposed to tunicamycin to induce apoptosis and analyzed by flow cytometry by gating on mcd24-positive cells. In control-transduced cells from a patient, exposure to tunicamycin induced a high degree of apoptosis (30.0% of annexin V–positive cells and 4.6% of annexin V and propidium iodide–double-positive cells underwent apoptosis). By contrast, in G6PC3-transduced cells, the percentage of cells undergoing induced apoptosis was lower (17.9% of annexin V–positive cells and 1.9% of double-positive cells) (Figure 3C, and Fig. 5 in the Supplementary Appendix). We tested the function of neutrophils in G6PC3-deficient neutrophils. Both phagosomal lysis of Escherichia coli and the oxidative burst were similar to those in neutrophils from healthy control subjects (Fig. 6 in the Supplementary Appendix).
Endoplasmic Reticulum Stress

Endoplasmic reticulum stress and the unfolded-protein response have been linked to the pathophysiology of aberrant organogenesis,27 including structural heart defects28 and congenital neutropenia caused by mutations in ELA2.17,29 Transmission electron microscopy of bone marrow cells from four patients with G6PC3 deficiency showed an enlarged rough endoplasmic reticulum in myeloid progenitor cells as compared with the rough endoplasmic reticulum in corresponding cells from a healthy subject (Figure 4A and 4BFigure 4Pathophysiologic Consequences of G6PC3 Deficiency., and Fig. 7 in the Supplementary Appendix). This finding is consistent with increased endoplasmic reticulum stress. BiP messenger RNA (mRNA), a member of the chaperone family and another marker of endoplasmic reticulum stress, was measured by quantitative real-time PCR in bone marrow promyelocytes that had been isolated by flow cytometry. The level of BiP mRNA was increased in promyelocytes from the two patients we tested as compared with that in promyelocytes from healthy subjects (Figure 4C).
GSK-3β

A signaling circuit linking glucose, GSK-3β, and Mcl-1 has been previously established30,31 in which GSK-3β controls glycogen metabolism, Wnt signaling, and apoptosis.32 Mcl-1, an antiapoptotic member of the Bcl-2 family, is involved in the maintenance of neutrophil viability.33 GSK-3β phosphorylates Mcl-1, thus facilitating its degradation in the proteasome.30 We performed Western blot studies to estimate the levels of GSK-3β, BiP, and Mcl-1 proteins in neutrophils exposed to tunicamycin, an agent that induces endoplasmic reticulum stress. Neutrophils from the two patients we examined had increased levels of BiP (Figure 4E), an increase in the enzymatically active dephosphorylated form of GSK-3β (Figure 4D, and Fig. 8 in the Supplementary Appendix), and increased phosphorylation of Mcl-1 (Figure 4E). To investigate whether intracellular glucose deprivation causes dephosphorylation of GSK-3β, we inhibited glucose metabolism in neutrophils from two healthy subjects with the use of 2-deoxyglucose. Treatment with 2-deoxyglucose induced dephosphorylation of GSK-3β (Figure 4F) and increased apoptosis of neutrophils (Figure 4G), whereas CD3-positive T lymphocytes were resistant to the effects of 2-deoxyglucose (Figure 4G).
G6PC3 Mutations in Other Patients

We assessed the frequency and variety of G6PC3 mutations in a cohort of patients with genetically unclassified severe congenital neutropenia. Of 104 such patients, 7 had distinct biallelic mutations in G6PC3 (Table 1). The mutations include nonsense mutations (Y47X and Y48X) that would abolish the function of the protein if the truncated mRNA was translated. The three other missense mutations were predicted to be deleterious to the protein by SIFT22 analysis, with probabilities of being benign of 0.03 for L185P, 0.00 for G262R, and 0.00 for G260R. None of these additional patients with G6PC3 mutations had mutations in ELA2 or HAX1, a finding suggesting that these three genetic defects are distinct variants of severe congenital neutropenia. None of the patients with G6PC3 deficiency had hypoglycemia or lactic acidosis (see Table 4 in the Supplementary Appendix), as is seen in glycogen storage disorders.

A clinical review of all 12 patients with G6PC3 deficiency found variation in clinical features. Of the 12 patients, 8 had various cardiac malformations and 10 had a phenotype of unusually prominent subcutaneous veins, venous angiectasia, or both (Figure 1, and Fig. 2 in the Supplementary Appendix). Five patients had urogenital malformations, including cryptorchidism and urachal fistula (the urachus is a channel between the bladder of the fetus and the allantois). Two patients had inner-ear hearing loss, and two had delayed growth but no dysmorphic features (Table 1).
Discussion

We have described a congenital neutropenia syndrome with biallelic mutations in G6PC3. Like patients with mutations in HAX1 12 or ELA2, 16 patients with G6PC3 deficiency lacked mature neutrophils in the bone marrow and had increased susceptibility to apoptosis in peripheral neutrophils. Of 12 patients, 8 had structural heart defects (e.g., type 2 atrial septal defect, cor triatriatum, or pulmonary stenosis) and 5 had urogenital defects (e.g., cryptorchidism or urachal fistula). In most patients, an atypical, increased visibility of the superficial veins, angiectasia, or both was prominent. The spectrum of developmental aberrations may depend on factors other than the mutant G6PC3. Perhaps increased susceptibility to apoptosis also affects cardiac or urogenital development in patients with G6PC3 deficiency. There is clinical variation in other neutropenia syndromes, such as Cohen syndrome and cartilage-hair hypoplasia, even though the two syndromes are genetically homogeneous.8 All patients in our cohort had a response to treatment with rhG-CSF, and to date no patient has had a clonal hematopoietic disorder.

Three human genes mediating glucose-6-phosphatase activity have been discovered: G6PC1, G6PC2, and G6PC3. G6PC1, the classic glucose-6-phosphatase in the liver, kidney, and small intestine, catalyzes the hydrolysis of glucose-6-phosphate, an essential step in the gluconeogenic and glycogenolytic pathways. Patients without G6PC1 activity have type Ia glycogen storage disease.34 G6PC2 is expressed only in pancreatic islet cells35,36 and may be involved in glucose-dependent insulin secretion by controlling free glucose levels.37 Statistically significant associations between noncoding polymorphisms in or near G6PC2 and the level of glucose after an overnight fast have been shown.38,39 In contrast to G6PC1 and G6PC2, G6PC3 is ubiquitously expressed.40,41 Glucose-6-phosphate is transported to the endoplasmic reticulum by a glucose-6-phosphate transporter (G6PT).42 The stoichiometry and topologic relationships between the catalytic subunits of glucose-6-phosphatase and G6PT are unclear, but they do have a functional link.42-44 The complex formed between G6PT and G6PC1 (and perhaps also the complex between G6PT and G6PC2) appears to maintain normoglycemia. Our data show that G6PC3 is needed to maintain neutrophil viability and suggest an important role for glucose in the homeostasis of human neutrophils.

Cheung et al. recently described the phenotype of g6pc3-deficient mice that was generated by gene targeting.45 These mice had neutropenia and neutrophil dysfunction; another group had previously shown that murine g6pc3 deficiency results in lowered plasma cholesterol and elevated glucagon levels.46 We could not identify any consistent aberration in neutrophil function or any metabolic aberrations in patients with G6PC3 deficiency. The underlying mechanism of increased apoptosis of neutrophils in the absence of G6PC3 involves increased endoplasmic reticulum stress, which is usually seen in cases of deficient protein folding in the endoplasmic reticulum. In an attempt to counteract potentially toxic effects that can ensue from abnormally folded proteins, cells initiate a rescue program that, if ineffective, leads to apoptosis.47 We have provided evidence that GSK-3β, a key enzyme that regulates cellular differentiation and apoptosis,32 is implicated in this pathway. In the absence of intracellular glucose, GSK-3β is activated and thus can phosphorylate the antiapoptotic molecule Mcl-1, thereby mediating its degradation.30,31 Neutrophils from patients with G6PC3 deficiency have higher levels of nonphosphorylated GSK-3β and phosphorylated Mcl-1 than do neutrophils from unaffected people, a finding suggesting that a decrease in antiapoptotic Mcl-1 accounts for increased apoptosis in G6PC3-deficient neutrophils. These alterations may be in part responsible for the phenotype of G6PC3 deficiency.

Although we cannot rule out the possibility that additional mechanisms could contribute to the increase in apoptosis in G6PC3-deficient neutrophils, our data suggest that G6PC3 acts by a pathway involving GSK-3β to maintain the viability of neutrophils. Evidence of increased endoplasmic reticulum stress has previously been reported in patients with mutations in ELA2, 17,29 and premature apoptosis of neutrophils is known to cause the phenotype of congenital neutropenia.12,16,17 Thus, G6PC3 deficiency is another example of how increased apoptosis of neutrophil granulocytes can cause congenital neutropenia.

Five arguments support our claim that G6PC3 deficiency causes neutropenia. Distinct biallelic G6PC3 mutations were found in two pedigrees and seven singleton patients with congenital neutropenia; sequence analysis predicted that all four missense mutations are likely to affect the function of G6PC3; expression of wild-type G6PC3 and G6PC3 with the R253H mutation in yeast showed that the R253H mutation abrogates enzymatic activity; g6pc3 −/− knockout mice have neutropenia and increased myeloid-cell apoptosis; and the susceptibility to apoptosis in G6PC3-deficient myeloid cells was reduced by retroviral transfer of the wild-type G6PC3 gene.

Drs. Appaswamy and Ashikov contributed equally to this article.

Supported by grants from the Deutsche Forschungsgemeinschaft (DFG KliFo 110-2, to Dr. Klein), the Junior Research Group “Glycomics” (to Dr. Gerardy-Schahn), the Bundesministerium für Bildung und Forschung Bone Marrow Failure Syndromes (to Drs. Welte and Klein), and the European Union (MEXT-CT-2006-042316, to Dr. Grimbacher); and by the Intramural Research Program of the National Institutes of Health, National Library of Medicine (for Dr. Schäffer). Dr. Boztug is a recipient of an Else Kröner Memorial Fellowship. Drs. Klein and Gerardy-Schahn are members of REBIRTH, a Deutsche Forschungsgemeinschaft Cluster of Excellence.

Dr. Welte reports owning equity in Amgen and receiving royalties on a patent for rhG-CSF. No other potential conflict of interest relevant to this article was reported.
Source Information

From Hannover Medical School, Hannover, Germany (K.B., G.A., A.A., J.D., M.G., G.B., J.L.-G., F.N., A.-K.G., H.B., R.G.-S., C.Z., K.W., C. Klein); the National Center for Biotechnology Information, National Institutes of Health, Bethesda, MD (A.A.S.); the University Medical Center Freiburg, Freiburg, Germany (U.S.); St. Anna Children's Hospital, Vienna (M.M.); Children's Hospital, University of Heidelberg, Heidelberg, Germany (J.G.); the University of Freiburg, Freiburg, Germany (C. Kratz); Aghia Sophia Children's Hospital, University of Athens, Athens (T.P.); Centre Hospitalier Universitaire Angers, Angers, France (I.P.); Assistance Publique–Hôpitaux de Paris, Hôpital de la Pitié–Salpétrière, Paris (C.B.-C.); the Immunology, Asthma, and Allergy Research Institute, Tehran University of Medical Sciences, Tehran, Iran (N.R.); Children's Hospital Amsterdamer Straße, Cologne, Germany (K.M.); Saint Georges University Hospital, Beirut, Lebanon (N.I.-H.); and Royal Free Hospital and University College London, London (B.G.).

Address reprint requests to Dr. Klein at the Department of Pediatric Hematology and Oncology, Medical School Hannover, Carl-Neuberg-Straße 1, Hannover D-30625, Germany, or at klein.christoph@mh-hannover.de.

We thank the patients and their families for supporting our study; all colleagues referring and registering patients at the Severe Chronic Neutropenia International Registry; François-Loïc Cosset for the kind gift of the RD114 plasmid; Edelgard Odenwald for analysis of bone marrow smears; Thomas Jack for his help with echocardiography; Jessica Pfannstiel, Maren Sievers, Marie Böhm, Marly Dalton, Martina Wackerhahn, Tanja Reinke, and Gwendoline Leroy for technical assistance; and Dr. Jean Donadieu at the Pediatric Hematology Oncology Service, Hôpital Trousseau, Paris, and Dr. Roula Farah at the Department of Pediatrics, Saint Georges Hospital, Balamand University, Beirut, Lebanon, for excellent collaboration.

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