A variant X-linked chronic granulomatous disease patient (X91+) with partially functional cytochrome b

X-ray structure and enzymatic study of a bacterial NADPH oxidase highlight the activation mechanism of eukaryotic NOX

NADPH oxidases (NOX) are transmembrane proteins, widely spread in eukaryotes and prokaryotes, that produce reactive oxygen species (ROS). Eukaryotes use the ROS products for innate immune defense and signaling in critical (patho)physiological processes. Despite the recent structures of human NOX isoforms, the activation of electron transfer remains incompletely understood. SpNOX, a homolog from Streptococcus pneumoniae, can serves as a robust model for exploring electron transfers in the NOX family thanks to its constitutive activity. Crystal structures of SpNOX full-length and dehydrogenase (DH) domain constructs are revealed here. The isolated DH domain acts as a flavin reductase, and both constructs use either NADPH or NADH as substrate. Our findings suggest that hydride transfer from NAD(P)H to FAD is the rate-limiting step in electron transfer. We identify significance of F397 in nicotinamide access to flavin isoalloxazine and confirm flavin binding contributions from both DH and Transmembrane (TM) domains. Comparison with related enzymes suggests that distal access to heme may influence the final electron acceptor, while the relative position of DH and TM does not necessarily correlate with activity, contrary to previous suggestions. It rather suggests requirement of an internal rearrangement, within the DH domain, to switch from a resting to an active state. Thus, SpNOX appears to be a good model of active NOX2, which allows us to propose an explanation for NOX2's requirement for activation.

Structure of the core human NADPH oxidase NOX2

NOX2 is the prototypical member of the NADPH oxidase NOX superfamily and produces superoxide (O₂^•-), a key reactive oxygen species (ROS) that is essential in innate and adaptive immunity. Mutations that lead to deficiency in NOX2 activity correlate with increased susceptibility to bacterial and fungal infections, resulting in chronic granulomatous disease. The core of NOX2 is formed by a heterodimeric transmembrane complex composed of NOX2 (formerly gp91) and p22, but a detailed description of its structural architecture is lacking. Here, we present the structure of the human NOX2 core complex bound to a selective anti-NOX2 antibody fragment. The core complex reveals an intricate extracellular topology of NOX2, a four-transmembrane fold of the p22 subunit, and an extensive transmembrane interface which provides insights into NOX2 assembly and activation. Functional assays uncover an inhibitory activity of the 7G5 antibody mediated by internalization-dependent and internalization-independent mechanisms. Overall, our results provide insights into the NOX2 core complex architecture, disease-causing mutations, and potential avenues for selective NOX2 pharmacological modulation.

Variant Type X91+ Chronic Granulomatous Disease: Clinical and Molecular Characterization in a Chinese Cohort

PURPOSE

We aimed to report the clinical and immunological characteristics of variant type X91⁺ chronic granulomatous disease (CGD) in a Chinese cohort.

METHODS

The clinical manifestations and immunological phenotypes of patients with X91⁺ CGD were collected. A dihydrorhodamine (DHR) analysis was performed to evaluate neutrophil function. Gp91^phox protein expression was determined using extracellular staining with the monoclonal antibody (mAb) 7D5 and flow cytometry.

RESULTS

Patients with X91⁺ CGD accounted for 8% (7/85) of all patients with CGD. The median age of onset in the seven patients with X91⁺ CGD was 4 months. Six patients received the BCG vaccine, and 50% (3/6) had probable BCG infections. Mycobacterium tuberculosis infection was prominent. The most common sites of infection were the lung (6/7), lymph nodes (5/7), and soft tissue (3/7). Two patients experienced recurrent oral ulcers. The stimulation index (SI) of the patients with X91⁺ CGD ranged widely from 1.9 to 67.3. The difference in the SI among the three groups of patients (X91⁺ CGD, X91^- CGD, and X91⁰ CGD) was statistically significant (P = 0.0071). The three groups showed no significant differences in onset age, diagnosis age, or severe infection frequency. CYBB mutations associated with X91⁺ CGD were commonly located in the second transmembrane or intracellular regions. Three novel X91⁺ CGD-related mutations (c.1462-2 A > T, c.1243C > T, and c.925G > A) were identified.

CONCLUSIONS

Variant type X91⁺ CGD may result in varied clinical manifestations. Moreover, the laboratory findings might indicate a moderate neutrophil SI. We should deepen our understanding of variant X91⁺ CGD to prevent missed diagnoses.

High Efficiency Gene Correction in Hematopoietic Cells by Donor-Template-Free CRISPR/Cas9 Genome Editing

The CRISPR/Cas9 prokaryotic adaptive immune system and its swift repurposing for genome editing enables modification of any prespecified genomic sequence with unprecedented accuracy and efficiency, including targeted gene repair. We used the CRISPR/Cas9 system for targeted repair of patient-specific point mutations in the Cytochrome b-245 heavy chain gene (CYBB), whose inactivation causes chronic granulomatous disease (XCGD)—a life-threatening immunodeficiency disorder characterized by the inability of neutrophils and macrophages to produce microbicidal reactive oxygen species (ROS). We show that frameshift mutations can be effectively repaired in hematopoietic cells by non-integrating lentiviral vectors carrying RNA-guided Cas9 endonucleases (RGNs). Because about 25% of most inherited blood disorders are caused by frameshift mutations, our results suggest that up to a quarter of all patients suffering from monogenic blood disorders could benefit from gene therapy employing personalized, donor template-free RGNs.

Crystal structures and atomic model of NADPH oxidase

NADPH oxidases (NOXs) are the only enzymes exclusively dedicated to reactive oxygen species (ROS) generation. Dysregulation of these polytopic membrane proteins impacts the redox signaling cascades that control cell proliferation and death. We describe the atomic crystal structures of the catalytic flavin adenine dinucleotide (FAD)- and heme-binding domains of Cylindrospermum stagnale NOX5. The two domains form the core subunit that is common to all seven members of the NOX family. The domain structures were then docked in silico to provide a generic model for the NOX family. A linear arrangement of cofactors (NADPH, FAD, and two membrane-embedded heme moieties) injects electrons from the intracellular side across the membrane to a specific oxygen-binding cavity on the extracytoplasmic side. The overall spatial organization of critical interactions is revealed between the intracellular loops on the transmembrane domain and the NADPH-oxidizing dehydrogenase domain. In particular, the C terminus functions as a toggle switch, which affects access of the NADPH substrate to the enzyme. The essence of this mechanistic model is that the regulatory cues conformationally gate NADPH-binding, implicitly providing a handle for activating/deactivating the very first step in the redox chain. Such insight provides a framework to the discovery of much needed drugs that selectively target the distinct members of the NOX family and interfere with ROS signaling.

A novel missense mutation in the NADPH binding domain of CYBB abolishes the NADPH oxidase activity in a male patient with increased susceptibility to infections

Chronic granulomatous disease (CGD) is a primary immunodeficiency caused by mutations in the five structural genes (CYBB, CYBA, NCF1, NCF2, and NCF4) that typically results in a decrease in function or inability to generate a respiratory burst, leading to defective killing of pathogens, including fungi and intracellular bacteria. Mutations in CYBB, encoding the gp91phox (also known as NOX2) result in X-linked CGD account for approximately 65% of CGD cases. Here, we aimed the characterization of a novel missense mutation c.1226C > A/p.A409E in the CYBB gene in a patient with X-linked CGD. Relevant clinical data of a male patient whose family was positive for XCGD was reviewed. Oxidative burst and NADPH protein expression was evaluated by flow cytometry, while Genetic analysis was performed by Sanger sequencing. Monocyte-derived macrophages (MDMs) were evaluated for their capacity for phagocytosis and growth suppression of the intracellular Mycobacterium tuberculosis (M. tuberculosis). We thus report the absence of an oxidative burst in the phagocytes of the patient. Flow cytometry evaluation revealed a normal expression of NADPH oxidase components in neutrophils and genetic analysis proved the existence of a novel missense c.1226C > A mutation in the CYBB gene resulting in p.A409E. Further, we have showed that the patient's MDMs were unhindered in their ability to take up mycobacteria normally. Instead, the MDMs failed to control the intracellular proliferation of M. tuberculosis, a phenotype that improved in the presence of recombinant human interferon-gamma (rhIFN-γ). This work expands the genetic spectrum of X-linked CGD and demonstrates improvement in macrophage function in X91⁺CGD patient by rhIFN-γ.

Role of putative second transmembrane region of Nox2 protein in the structural stability and electron transfer of the phagocytic NADPH oxidase

Flavocytochrome b(558) (cytb) of phagocytes is a heterodimeric integral membrane protein composed of two subunits, p22(phox) and gp91(phox). The latter subunit, also known as Nox2, has a cytosolic C-terminal "dehydrogenase domain" containing FAD/NADPH-binding sites. The N-terminal half of Nox2 contains six predicted transmembrane α-helices coordinating two hemes. We studied the role of the second transmembrane α-helix, which contains a "hot spot" for mutations found in rare X(+) and X(-) chronic granulomatous disease. By site-directed mutagenesis and transfection in X-CGD PLB-985 cells, we examined the functional and structural impact of seven missense mutations affecting five residues. P56L and C59F mutations drastically influence the level of Nox2 expression indicating that these residues are important for the structural stability of Nox2. A53D, R54G, R54M, and R54S mutations do not affect spectral properties of oxidized/reduced cytb, oxidase complex assembly, FAD binding, nor iodonitrotetrazolium (INT) reductase (diaphorase) activity but inhibit superoxide production. This suggests that Ala-53 and Arg-54 are essential in control of electron transfer from FAD. Surprisingly, the A57E mutation partially inhibits FAD binding, diaphorase activity, and oxidase assembly and affects the affinity of immunopurified A57E cytochrome b(558) for p67(phox). By competition experiments, we demonstrated that the second transmembrane helix impacts on the function of the first intracytosolic B-loop in the control of diaphorase activity of Nox2. Finally, by comparing INT reductase activity of immunopurified mutated and wild type cytb under aerobiosis versus anaerobiosis, we showed that INT reduction reflects the electron transfer from NADPH to FAD only in the absence of superoxide production.

Hematologically important mutations: X-linked chronic granulomatous disease (third update)

Chronic granulomatous disease (CGD) is an immunodeficiency disorder affecting about 1 in 250,000 individuals. The disease is caused by a lack of superoxide production by the leukocyte enzyme NADPH oxidase. Superoxide is used to kill phagocytosed micro-organisms in neutrophils, eosinophils, monocytes and macrophages. The leukocyte NADPH oxidase is composed of five subunits, of which the enzymatic component is gp91-phox, also called Nox2. This protein is encoded by the CYBB gene on the X chromosome. Mutations in this gene are found in about 70% of all CGD patients. This article lists all mutations identified in CYBB in the X-linked form of CGD. Moreover, apparently benign polymorphisms in CYBB are also given, which should facilitate the recognition of future disease-causing mutations.

First report of clinical, functional, and molecular investigation of chronic granulomatous disease in nine Jordanian families

INTRODUCTION

Chronic granulomatous disease is a rare inherited immunodeficiency syndrome caused by mutations in four genes encoding essential nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex components.

MATERIAL AND METHODS

Clinical, functional, and molecular investigations were conducted in 15 Jordanian CGD patients from nine families.

RESULTS AND DISCUSSION

Fourteen patients were children of consanguineous parents and suffered from autosomal recessive (AR) CGD forms with mutations in the CYBA, NCF1, and NCF2 genes encoding p22phox, p47phox, and p67phox proteins, except for one patient in whom the mutation's location was not found. One patient had an extremely rare X(+)CGD subtype resulting from a novel missense mutation (G1234C) in exon 10 of CYBB. We found a genetic heterogeneity in the Jordanian families with a high frequency of rare ARCGD, probably because consanguineous marriages are common in Jordan. No clear correlation between the severity of the clinical symptoms and the CGD types could be established.

[The X+ chronic granulomatous disease as a fabulous model to study the NADPH oxidase complex activation]

Chronic granulomatous disease (CGD) is a rare inherited disorder in which phagocytes lack NADPH oxidase activity. Patients with CGD suffer from recurrent bacterial and fungal infections because of the absence of superoxide anions (O2- degrees ) generatingsystem. The NADPH oxidase complex is composed of a membranous cytochrome b558, cytosolic proteins p67phox, p47phox, p40phox and two small GTPases Rac2 and Rap1A. Cytochrome b558 consists of two sub-units gp91phox and p22phox. The most common form of CGD is due to mutations in CYBB gene encoding gp91phox. In some rare cases, the mutated gp91phox is normally expressed but is devoided of oxidase activity. These variants called X+ CGD, have provided interesting informations about oxidase activation mechanisms. However modelization of such variants is necessary to obtain enough biological material for studies at the molecular level. A cellular model (knock-out PLB-985 cells) has been developed for expressing recombinant mutated gp91phox for functional analysis of the oxidase complex. Recent works demonstrated that this cell line genetically deficient in gp91phox is a powerful tool for functional analysis of the NADPH oxidase complex activation.

Pollen NAD(P)H Oxidases and Their Contribution to Allergic Inflammation

This article provides an overview of NADPH oxidase and its role in allergic inflammation. A background and historical perspectives of NADPH oxidase are first provided, followed by a detailed overview of mammalian NADPH oxidase subunits and their functional organization. Plant NADPH oxidase, the authors' discovery of NADPH oxidase in pollens, and their contribution to allergic inflammation are then discussed, concluding with a discussion of future directions and outstanding questions that require attention.

Molecular approaches in the diagnosis of primary immunodeficiency diseases

Over 120 inherited primary immunodeficiency diseases (PIDs) are known to exist. The genes responsible for many of these diseases have also been identified. Recent advances in diagnostic procedures have enabled these to be identified earlier and appropriately treated. While a number of approaches are available to identify mutations, direct sequencing remains the gold standard. This approach identifies the exact genetic change with substantial precision. We suggest that a sensitive and economical approach to mutation detection could be the direct sequencing of cDNA followed by the confirmatory sequencing of the corresponding exon. While screening techniques such as single-stranded conformation polymorphism (SSCP), heteroduplex analysis (HA), denaturing gradient gel electrophoresis (DGGE), and denaturing high-performance liquid chromatography (dHPLC) have proven useful, each has inherent advantages and disadvantages. We discuss these advantages and disadvantages and also discuss the potential of future sequencing technologies such as pyrosequencing, combinatorial sequencing-by-hybridization, multiplex polymerase colony (polony), and resequencing arrays as tools for future mutation detection. In addition we briefly discuss several high-throughput SNP detection technologies.

Characterization of six novel mutations in the CYBB gene leading to different sub-types of X-linked chronic granulomatous disease

Chronic granulomatous disease is an inherited disorder in which phagocytes lack a functional NADPH oxidase and so cannot generate superoxide anions (O(2) (-)). The most common form is caused by mutations in CYBB encoding gp91 phox, the heavy chain of flavocytochrome b(558) (XCGD). We investigated 11 male patients and their families suspected of suffering from X-linked CGD. These XCGD patients were classified as having different variants (X91(0), X91(-) or X91(+)) according to their cytochrome b(558) expression and NADPH oxidase activity. Nine patients had X91(0) CGD, one had X91(-) CGD and one had X91(+) CGD. Six mutations in CYBB were novel. Of the four new X91(0) CGD cases, three were point mutations: G65A in exon 2, G387T in exon 5 and G970T in exon 9, leading to premature stop codons at positions Try18, Try125 and Glu320, respectively, in gp91 phox. One case of X91(0) CGD originated from a new 1005G deletion detected in exon 9. Surprisingly, four nonsense mutations in exon 5 led to the generation of two mRNAs, one with a normal size containing the mutation and the other in which exon 5 had been spliced. A novel X91(-) CGD case with low gp91 phox expression was diagnosed. It was caused by an 11-bp deletion in the linking region between exon 12 and intron 12, activating a new cryptic site. Finally, a new X91(+) CGD case was detected, characterized by a missense mutation Leu505Arg in the potential NADPH-binding site of gp91 phox. No clear correlation between the severity of the clinical symptoms and the sub-type of XCGD could be established.

Gene therapy for chronic granulomatous disease

Chronic granulomatous disease (CGD) is a congenital immune deficiency that is a promising therapeutic target for gene replacement into haematopoietic stem cells (HSCs). CGD results from mutations in any one of four genes encoding subunits of the superoxide-generating NADPH oxidase of phagocytes. Life-threatening, recurrent bacterial and fungal infections, as well as inflammatory granulomas, are the hallmarks of the disease. NADPH oxidase activity can be reconstituted by retroviral- or lentiviral-mediated gene transfer to human CGD marrow in vitro and in xenograft transplant models. Gene transfer studies in knockout mouse models that resemble the human disease suggest that correction of oxidase activity in a minority of phagocytes will be of clinical benefit. Phase I clinical studies in unconditioned CGD patients showed transient expression of small numbers of gene-corrected neutrophils. Areas of research at present include efforts to enhance gene transfer rates into repopulating HSCs using vectors that transduce quiescent cells, and to increase the engraftment of genetically corrected HSCs using non-myeloablative conditioning and drug resistance genes for selection.

Functional analysis of two-amino acid substitutions in gp91 phox in a patient with X-linked flavocytochrome b558-positive chronic granulomatous disease by means of transgenic PLB-985 cells

Chronic granulomatous disease (CGD) is a rare inherited disorder in which phagocytes lack NADPH oxidase activity. The most common form is caused by mutations in the CYBB gene encoding gp91 phox protein, the heavy chain of cytochrome b(558), which is the redox element of NADPH oxidase. In some rare cases, the mutated gp91 phox is normally expressed but no NADPH oxidase can be detected. This type of CGD is called X91(+) CGD. We have previously reported an X(+) CGD case with a double-missense mutation in gp91 phox. Transgenic PLB-985 cells have now been made to study the impact of each single mutation on oxidase activity and assembly to rule out a possible new polymorphism in the CYBB gene. The His303Asn/Pro304Arg gp91 phox transgenic PLB-985 cells exactly mimic the phenotype of the neutrophils of the X(+) CGD patient. The His303Asn mutation is sufficient to inhibit oxidase activity in intact cells and in a broken cell system, whereas in the Pro304Arg mutant, residual activity suggests that the Pro304Arg substitution is less devastating to oxidase activity than the His303Asn mutation. The study of NADPH oxidase assembly following the in vitro and in vivo translocation of cytosolic factors p47 phox and p67 phox has demonstrated that, in the double mutant and in the His303Asn mutant, NADPH oxidase assembly is abolished, although the translocation is only attenuated in Pro304Arg mutant cells. Thus, even though the His303Asn mutation has a more severe inhibitory effect on NADPH oxidase activity and assembly than the Pro304Arg mutation, neither mutation can be considered as a polymorphism.

The NADPH oxidase of professional phagocytes--prototype of the NOX electron transport chain systems

The NADPH oxidase is an electron transport chain in "professional" phagocytic cells that transfers electrons from NADPH in the cytoplasm, across the wall of the phagocytic vacuole, to form superoxide. The electron transporting flavocytochrome b is activated by the integrated function of four cytoplasmic proteins. The antimicrobial function of this system involves pumping K+ into the vacuole through BKCa channels, the effect of which is to elevate the vacuolar pH and activate neutral proteases. A number of homologous systems have been discovered in plants and lower animals as well as in man. Their function remains to be established.

Regulation of neutrophil function by Rac GTPases

Rac plays a central role in regulating neutrophil responses to inflammatory signals, including actin remodeling, chemotaxis, and superoxide production by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Rac-GTP is a component of the membrane-assembled NADPH oxidase complex, and new evidence suggests that Rac-GTP interacts directly with the oxidase flavocytochrome, in addition to binding to the regulatory p67 subunit, to regulate electron transfer both independently and cooperatively from NADPH to molecular oxygen. Other new studies suggest that Rac-GTP plays a dual role in NADPH oxidase activation, and can initiate signaling pathways leading to translocation of cytosolic oxidase subunits in addition to functioning in the assembled enzyme complex. Rac activation in response to neutrophil chemoattractants may be regulated in large part by a newly identified guanine nucleotide exchange factor, P-Rex1, which is activated by either phosphatidylinositols or Gbetagamma subunits. Multiple Rac GTPase activating proteins are present in neutrophils and may also modulate levels of Rac-GTP. The importance of Rac in a broad range of neutrophil functions is shown by the variety of defects seen in neutrophils from Rac2 knockout mice and from a patient with recurrent infections and a dominant-negative mutation in Rac2.

Current molecular models for NADPH oxidase regulation by Rac GTPase

Reactive oxygen species (ROS) have been increasingly recognized as important components of cell signaling in addition to their well-established roles in host defense. The formation of ROS in phagocytic and nonphagocytic cells involves membrane-localized and Rac guanosine triphosphatase (GTPase)-regulated reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase(s). We discuss here the current molecular models for Rac GTPase action in the control of the phagocytic leukocyte NADPH oxidase. As a mechanistically detailed example of Rac GTPase signaling, the NADPH oxidase provides a potential paradigm for signaling by Rho family GTPases in general.

A comparison of the NADPH oxidase in human sperm and white blood cells

The mechanism of reactive oxygen species (ROS) generation in human sperm has recently been found to depend upon a novel NADPH-oxidase (NOX5) resembling the multicomponent NADPH-oxidase of white blood cells (WBCs). The purpose of our study was to compare the ROS producing activity of NOX5 in sperm and NADPH-oxidase of WBCs, and to investigate the role of protein kinase C (PKC) in NOX5 activation. A combination of electron paramagnetic resonance (EPR), chemiluminescence (CL), and nitroblue tetrazolium (NBT) dye reduction were used to monitor ROS production by sperm. The involvement of PKC in NOX5 activation was investigated using myristate acetate (PMA), and the PKC inhibitor GF-109203X. The presence of b cytochrome in NOX5 was investigated by spectrophotometry. PMA-stimulated WBCs produced superoxide dismutase -- inhibitable EPR signals for both superoxide and hydroxyl radicals. Sperm did not produce these spectra with or without PMA stimulation. WBCs generated significantly increased levels of CL and reduced NBT after PMA stimulation; whereas sperm did not increase the CL response or reduce NBT. Adenosine triphosphate (ATP) levels in WBCs were significantly reduced after PMA stimulation, whereas sperm ATP levels did not change. The characteristic spectra of b cytochrome observed after dithionite reduction of WBCs was not observed with sperm under similar conditions. These results indicate that the ROS producing activity of NOX5 is significantly lower than the WBC NADPH-oxidase, and suggest that the activation mechanism of NOX5 in sperm is independent of PKC.

Molecular and functional characterization of a new X-linked chronic granulomatous disease variant (X91+) case with a double missense mutation in the cytosolic gp91phox C-terminal tail

We report here two atypical cases of X-linked CGD patients (first cousins) in which cytochrome b(558) is present at a normal level but is not functional (X91+). The mutations were localized by single-strand conformational polymorphism of reverse transcriptase-polymerase chain reaction amplified fragments and then identified by sequence analysis. They consisted in two base substitutions (C919 to A and C923 to G), changing His303 to Asn and Pro304 to Arg in the cytosolic gp91phox C-terminal tail. Mismatched polymerase chain reaction and genomic DNA sequencing showed that mothers had both wild-type and mutated alleles, confirming that this case was transmitted in an X-linked fashion. A normal amount of FAD was found in neutrophil membranes, both in the X91+ patients and their parents. Epstein-Barr virus-transformed B lymphocytes from the X91+ patients acidified normally upon stimulation with arachidonic acid, indicating that the mutated gp91phox still functioned as a proton channel. A cell-free translocation assay demonstrated that the association of the cytosolic factors p47phox and p67phox with the membrane fraction was strongly disrupted. We concluded that residues 303 and 304 are crucial for the stable assembly of the NADPH oxidase complex and for electron transfer, but not for its proton channel activity.

Exploration of the diaphorase activity of neutrophil NADPH oxidase

In the O2- generating flavocytochrome b, the membrane-bound component of the neutrophil NADPH oxidase, electrons are transported from NADPH to O2 in the following sequence: NADPH --> FAD --> heme b -->O2. Although p-iodonitrotetrazolium (INT) has frequently been used as a probe of the diaphorase activity of the neutrophil flavocytochrome b, the propensity of its radical to interact reversibly with O2 led us to question its specificity. This study was undertaken to reexamine the interaction of INT with the redox components of the neutrophil flavocytochrome b. Two series of inhibitors were used, namely the flavin analog 5-deaza FAD and the heme inhibitors bipyridyl and benzylimidazole. The following results indicate that INT reacts preferentially with the hemes rather than with the FAD redox center of flavocytochrome b and is not therefore a specific probe of the diaphorase activity of flavocytochrome b. First, in anaerobiosis, reduced heme b in activated membranes was reoxidized by INT as efficiently as by O2 even in the presence of concentrations of 5-deaza FAD which fully inhibited the NADPH oxidase activity. Second, the titration curve of dithionite-reduced heme b in neutrophil membranes obtained by oxidation with increasing amounts of INT was strictly superimposable on that of dithionite-reduced hemin. Third, INT competitively inhibited the O2 uptake by the activated NADPH oxidase in a cell-free system. Finally, the heme inhibitor bipyridyl competitively inhibited the reduction of INT in anaerobiosis, and the oxygen uptake in aerobiosis.

Identification of a spectrally stable proteolytic fragment of human neutrophil flavocytochrome b composed of the NH2-terminal regions of gp91(phox) and p22(phox)

A heme-bearing polypeptide core of human neutrophil flavocytochrome b(558) was isolated by applying high performance, size exclusion, liquid chromatography to partially purified Triton X-100-solubilized flavocytochrome b that had been exposed to endoproteinase Glu-C for 1 h. The fragment was composed of two polypeptides of 60-66 and 17 kDa by SDS-polyacrylamide gel electrophoresis and retained a native heme absorbance spectrum that was stable for several days when stored at 4 degrees C in detergent-containing buffer. These properties suggested that the majority of the flavocytochrome b heme environment remained intact. Continued digestion up to 4.5 h yielded several heme-associated fragments that were variable in composition between experiments. Digestion beyond 4.5 h resulted in a gradual loss of recoverable heme. N-Linked deglycosylation and reduction and alkylation of the 1-h digestion fragment did not affect the electrophoretic mobility of the 17-kDa fragment but reduced the 60-66-kDa fragment to 39 kDa. Sequence and immunoblot analyses identified the fragments as the NH(2)-terminal 320-363 amino acid residues of gp91(phox) and the NH(2)-terminal 169-171 amino acid residues of p22(phox). These findings provide direct evidence that the primarily hydrophobic NH(2)-terminal regions of flavocytochrome b are responsible for heme ligation.

Molecular basis for Rac2 regulation of phagocyte NADPH oxidase

A Rac GTPase-regulated multiprotein NADPH oxidase is critical for the formation of reactive oxygen species (ROS) in phagocytic leukocytes and other nonphagocytic cells. NADPH oxidase reduces molecular oxygen to form superoxide anion in a two-step process. Electrons are initially transferred from NADPH to cytochrome b-associated FAD, then to cytochrome b heme and finally to molecular oxygen. We show here that Rac is required for both electron-transfer reactions. Mutational and biophysical analysis shows that Rac and p67phox independently regulate cytochrome b to catalyze the transfer of electrons from NADPH to FAD. However, they must interact with each other to induce the subsequent transfer of electrons from FAD to cytochrome b heme and molecular oxygen. This two-step model of regulation by Rac GTPase may provide a means of more effectively controlling the inflammatory responses of phagocytic leukocytes.

Autosomal recessive chronic granulomatous disease caused by defects in NCF-1, the gene encoding the phagocyte p47-phox: mutations not arising in the NCF-1 pseudogenes

Chronic granulomatous disease (CGD) is a primary immunodeficiency caused by defects in any one of 4 genes encoding phagocyte NADPH oxidase subunits. Unlike other CGD subtypes, in which there is great heterogeneity among mutations, 97% of affected alleles in patients previously reported with A47(0) CGD carry a single mutation, a GT deletion (DeltaGT) in exon 2 of the p47-phox gene, NCF-1. This unusually high incidence results from recombination events between NCF-1 and its highly homologous pseudogenes, in which DeltaGT originates. In 50 consecutive patients with A47(0) CGD, 4 were identified who were heterozygous for DeltaGT in NCF-1, and for the first time, 2 were identified whose DNA appeared normal at this position. To avoid co-amplification of pseudogene sequence and to enable the identification of mutations in these patients, allele-specific polymerase chain reaction was used to amplify alleles not containing DeltaGT. In each of the 4 patients who were heterozygous for DeltaGT, an additional novel mutation was identified. These were 2 missense mutations, G125 --> A in exon 2 (predicting Arg42 --> Gln) and G784 --> A in exon 8 (Gly262 --> Ser), and 2 splice junction mutations at the 5' end of intron 1, gt --> at and gtg --> gtt. The first of 2 patients who appeared normal at the GT position was a compound heterozygote with the G125 --> A transition on one allele and a deletion of G811 on the other. In the second of these patients, only a single defect was detected, G574 --> A, which predicts Gly192 --> Ser but is likely to result in defective splicing because it represents the final nucleotide of exon 6.

Molecular analysis of 9 new families with chronic granulomatous disease caused by mutations in CYBA, the gene encoding p22phox

Chronic granulomatous disease is a rare inherited disorder caused by nonexistent or severely decreased phagocyte superoxide production that results in a severe defect in host defense and consequent predisposition to microbial infection. The enzyme responsible for generating the superoxide, NADPH oxidase, involves at least 5 protein components. The absence of, or a defect in, any 1 of 4 of these proteins (p22phox, p47phox, p67phox, or gp91phox) gives rise to the known types of chronic granulomatous disease. One of the rarest forms of the disease is due to defects in the CYBA gene encoding p22phox, which together with gp91phox forms flavocytochromeb558, the catalytic core of NADPH oxidase. To date, only 9 kindreds with p22phoxdeficiency have been described in the literature comprising 10 mutant alleles. Four polymorphisms in the CYBA gene have also been reported. Here we describe 9 new, unrelated kindreds containing 12 mutations, 9 of which are novel. In addition, we report 3 new polymorphisms. The novel mutations are (a) deletion of exons 2 and 3, (b) a missense mutation in exon 3 (T155→C), (c) a splice site mutation at the 5′ end of intron 3, (d) a missense mutation in exon 2 (G74→T), (e) a nonsense mutation in exon 1 (G26→A), (f) a missense mutation in exon 4 (C268→T), (g) a frameshift in exon 3 due to the insertion of C at C162, (h) a nonsense mutation in exon 2 (G107→A), and (i) a missense mutation in exon 2 (G70→A).

Molecular analysis of 9 new families with chronic granulomatous disease caused by mutations in CYBA, the gene encoding p22phox

Chronic granulomatous disease is a rare inherited disorder caused by nonexistent or severely decreased phagocyte superoxide production that results in a severe defect in host defense and consequent predisposition to microbial infection. The enzyme responsible for generating the superoxide, NADPH oxidase, involves at least 5 protein components. The absence of, or a defect in, any 1 of 4 of these proteins (p22phox, p47phox, p67phox, or gp91phox) gives rise to the known types of chronic granulomatous disease. One of the rarest forms of the disease is due to defects in the CYBA gene encoding p22phox, which together with gp91phox forms flavocytochromeb558, the catalytic core of NADPH oxidase. To date, only 9 kindreds with p22phoxdeficiency have been described in the literature comprising 10 mutant alleles. Four polymorphisms in the CYBA gene have also been reported. Here we describe 9 new, unrelated kindreds containing 12 mutations, 9 of which are novel. In addition, we report 3 new polymorphisms. The novel mutations are (a) deletion of exons 2 and 3, (b) a missense mutation in exon 3 (T155→C), (c) a splice site mutation at the 5′ end of intron 3, (d) a missense mutation in exon 2 (G74→T), (e) a nonsense mutation in exon 1 (G26→A), (f) a missense mutation in exon 4 (C268→T), (g) a frameshift in exon 3 due to the insertion of C at C162, (h) a nonsense mutation in exon 2 (G107→A), and (i) a missense mutation in exon 2 (G70→A).

Genetic, biochemical, and clinical features of chronic granulomatous disease

The reduced nicotinamide dinucleotide phosphate (NADPH) oxidase complex allows phagocytes to rapidly convert O2 to superoxide anion which then generates other antimicrobial reactive oxygen intermediates, such as H2O2, hydroxyl anion, and peroxynitrite anion. Chronic granulomatous disease (CGD) results from a defect in any of the 4 subunits of the NADPH oxidase and is characterized by recurrent life-threatening bacterial and fungal infections and abnormal tissue granuloma formation. Activation of the NADPH oxidase requires translocation of the cytosolic subunits p47phox (phagocyte oxidase), p67phox, and the low molecular weight GT-Pase Rac, to the membrane-bound flavocytochrome, a heterodimer composed of the heavy chain gp91phox and the light chain p22phox. This complex transfers electrons from NADPH on the cytoplasmic side to O2 on the vacuolar or extracellular side, thereby generating superoxide anion. Activation of the NADPH oxidase requires complex rearrangements between the protein subunits, which are in part mediated by noncovalent binding between src-homology 3 domains (SH3 domains) and proline-rich motifs. Outpatient management of CGD patients relies on the use of prophylactic antibiotics and interferon-gamma. When infection is suspected, aggressive effort to obtain culture material is required. Treatment of infections involves prolonged use of systemic antibiotics, surgical debridement when feasible, and, in severe infections, use of granulocyte transfusions. Mouse knockout models of CGD have been created in which to examine aspects of pathophysiology and therapy. Gene therapy and bone marrow transplantation trials in CGD patients are ongoing and show great promise.

Functional Analysis of NADPH Oxidase in Granulocytic Cells Expressing a ▵488-497 gp91phox Deletion Mutant

Chronic granulomatous disease (CGD) is a group of inherited disorders in which phagocytes are unable to generate superoxide (O2−) due to genetic defects in any 1 of 4 essential NADPH oxidase components. Mutations in the X-linked gene for gp91phox, the large subunit of the flavocytochromeb558 heterodimer, account for the majority of CGD. An X-CGD patient in which a splice junction mutation results in an in-frame deletion of 30 nucleotides encoding amino acids 488 to 497 of gp91phox (▵488-497 gp91phox) has previously been reported. In this study, we generated myeloid PLB-985 cells expressing the mutant ▵488-497 gp91phox to further characterize its functional properties. These cells mimicked the phenotype of the patient’s neutrophils with normal expression of a nonfunctional ▵488-497 gp91phox flavocytochrome. Translocation of p47phox and p67phox to ▵488-497 gp91phox PLB-985 plasma membranes was not affected, as determined both in activated intact cells and in the cell-free system. Furthermore, a synthetic peptide corresponding to residues 488-497 of gp91phox was relatively ineffective in inhibiting O2− production in the cell-free oxidase assay (IC50, ∼500 μmol/L), suggesting that residues 488-497 of gp91phox are not directly involved in oxidase assembly. Mutant ▵488-497 gp91phox flavocytochrome failed to support iodonitrotetrazolium (INT) reduction, showing a disruption of electron transfer from NADPH to the FAD center of gp91phox. However, the FAD binding capacity of the mutant flavocytochrome was normal, as measured by equilibrium dialysis. Taken together, these results suggest that the ▵488-497 deletion in gp91phox disrupts electron transfer to FAD, either due to a defect in NADPH binding or to impaired delivery of electrons from NADPH.

Genetic analysis of 13 families with X-linked chronic granulomatous disease reveals a low proportion of sporadic patients and a high proportion of sporadic carriers

X-linked chronic granulomatous disease (X-CGD) is the most common type of CGD, whose responsible gene has been identified and termed as CYBB, according to the gp91-phox, a subunit of cytochrome b558. Although approximately 200 different mutations of the gp91-phox gene have been reported, no precise study of the proportion of sporadic cases in X-CGD, based on molecular genetic analysis, has been reported. We made a genetic analysis of six newly identified X-CGD patients together with that of eight previously reported X-CGD patients. The mutations newly detected were three missense mutations, two splice mutations, and one insertion of 2 bases. All of the mutations were novel. Twelve mothers (two of them came from the same family) and four maternal grandmothers from 13 different X-CGD families were available for further genetic studies. It was revealed that a proportion of sporadic patients was low and that of sporadic carriers was high. These results suggest that the mutation for the disease originates mainly from male gametes.

Spectrophotometric determination of neutrophil cytochrome b558 of chronic granulomatous disease

BACKGROUND

Chronic granulomatous disease (CGD) is an inherited disease characterized clinically by severe recurrent bacterial infections from infancy. This disease is a disorder of the formation of superoxide (O2-) by the neutrophil NADPH oxidase system, mostly due to defects in cytochrome b558 (cyt b558), which is one of the oxidase components. Diagnosis of CGD has been performed by the assay of the O2- forming activity, immunological determination of defects in the oxidase components, and or spectrophotometry of cyt b558. However, spectrophotometric analysis of the b-type heme is difficult with small amounts of blood from infant CGD patients, as the limited amounts of neutrophils are contaminated with a relatively high ratio of hemoglobin (Hb) that interferes with the heme spectrum of cyt b558. This report presents an accurate method for the spectrophotometric analysis of cyt b558 in a small amount of CGD neutrophils that were treated with CO gas in a safe procedure instead of the previously reported CO-bubbling method.

METHODS AND RESULTS

The difference of the reduced minus oxidized cyt b558 spectrum was measured under no interference from oxy Hb at the alpha and beta bands and differentiated as d[delta A]/d lambda (lambda = wavelength) to obtain further evidence for the defects of the cyt b558 heme spectrum. The interference from CO-insensitive met Hb was eliminated by subtracting the absorption peak at the Soret (gamma) band of the contaminating met Hb, which was estimated from the CO-treated and untreated spectra of the same, hemolyzed sample.

CONCLUSIONS

This spectrophotometric method is feasible for the determination of abnormality and heme content of cyt b558 with a small amount of CGD neutrophils in 10-20 mL of blood even in the presence of contaminating Hb.

Interactions between the components of the human NADPH oxidase: intrigues in the phox family

The human NADPH oxidase is a very intriguing enzyme; although its catalytic unit is retained within cytochrome b558, various additional proteins are required for activity of the NADPH oxidase. In the past few years substantial progress has been made to elucidate the protein-protein interactions and the activation events involved. The following facts have become evident: (1) activation of rac and subsequent interaction with p67-phox is crucial for the interaction of p67-phox with cytochrome b558, and probably with gp91-phox; (2) p47-phox interacts with p22-phox, and phosphorylation of 379Ser of p47-phox is obligatory for this event; (3) p47-phox and p67-phox regulate each other's translocation in a positive sense (see also reference 71). To put it differently: it is vital to gain insight in the intrigues within the phox family and associated characters to fully understand NADPH oxidase activation.

The FRE1 ferric reductase of Saccharomyces cerevisiae is a cytochrome b similar to that of NADPH oxidase

Plasma membrane preparations from strains of the yeast Saccharomyces cerevisiae gave a reduced minus oxidized spectrum characteristic of a b-type cytochrome and very similar to the spectrum of flavocytochrome b558 of human neutrophils. The magnitude of the signal correlated with the level of ferric reductase activity and the copy number of the FRE1 gene, indicating that the FRE1 protein is a cytochrome b. Sequence similarities with the flavin binding site of flavocytochrome b558 and other members of the ferredoxin-NADP reductase family, together with increased levels of noncovalently bound FAD and iodonitrotetrazolium violet reductase activity in membranes from a yeast strain overexpressing ferric reductase, suggested that the FRE1 protein may also carry a flavin group. Potentiometric titrations indicated that FRE1, like neutrophil NADPH oxidase, has an unusually low redox potential, in the region of -250 mV, and binds CO.

Detection of gp91-phox precursor protein in B-cell lines from patients with X-linked chronic granulomatous disease as an indicator for mutations impairing cytochrome b558 biosynthesis

NADPH oxidase cytochrome b558 consists of two subunits, gp91-phox and p22-phox, defects of which result in chronic granulomatous disease (CGD). The nature of the interaction between these subunits has yet to be determined. Absence of p22-phox in autosomal CGD patient-derived B-cell lines results in detectable levels of an incompletely glycosylated gp91-phox precursor. We have detected this same precursor species in four cell lines from patients with the X-linked form of the disease due to mutations in gp91-phox. Such mutations should delineate regions of gp91-phox important for its biosynthesis, including stable association with p22-phox. One mutation mapped to the putative FAD-binding domain, one mapped to a potential haem-binding domain, and two involved the region encoded by exon 3.

The NADPH oxidase and chronic granulomatous disease

Chronic granulomatous disease (CGD) is characterized by severe, protracted and often fatal infection, which results from a failure of the NADPH oxidase enzyme system in the patient's phagocytes to produce superoxide. The NADPH oxidase enzyme system is composed of a number of interacting components, the absence of any one of which causes failure of the system as a whole. Investigation of individuals with CGD has led to the identification of the different protein components and the genes coding for them. CGD is particularly well suited to treatment by gene therapy and is likely to be one of the earliest monogenic conditions to be successfully treated in this way.

Cytochrome b-245 of the neutrophil superoxide-generating system contains two nonidentical hemes. Potentiometric studies of a mutant form of gp91phox

Analysis of potentiometric titrations of the cytochrome b-245 from a X+ chronic granulomatous disease patient with an Arg54 --> Ser mutation in gp91phox indicates that the mutant form of the cytochrome contains two nonidentical hemes with midpoint potentials of Em7 = -220 and Em7 = -300 mV. In the light of this information, reanalysis of redox titrations of wild-type cytochrome b-245 implies that it probably also contains two separate heme centers with midpoint potentials of Em7 = -225 and Em7 = -265 mV. The effect of the Arg54 --> Ser substitution is to reduce the midpoint potential of one of the heme centers by approximately 35 mV and suggests possible interaction between Arg54 and a heme propionate side chain.