Seven new mutations in the nicotinamide adenine dinucleotide reduced-cytochrome b(5) reductase gene leading to methemoglobinemia type I

CYB5R3 homozygous pathogenic variant as a rare cause of cyanosis in the newborn

Detailed below is a very illustrative case of a rare pathology of recessive congenital methemoglobinemia. The patient, a newborn female, was homozygous for c.535G > A, p.(Ala179Thr) a pathogenic variant in the CYB5R3 gene. The reported population frequency of the allele is 0.853%, demonstrating why it is remarkable to find both parents are heterozygous carriers without consanguinity. A brief review of previously published cases is also presented.

Recommendations for diagnosis and treatment of methemoglobinemia

Methemoglobinemia is a rare disorder associated with oxidization of divalent ferro‐iron of hemoglobin (Hb) to ferri‐iron of methemoglobin (MetHb). Methemoglobinemia can result from either inherited or acquired processes. Acquired forms are the most common, mainly due to the exposure to substances that cause oxidation of the Hb both directly or indirectly. Inherited forms are due either to autosomal recessive variants in the CYB5R3 gene or to autosomal dominant variants in the globin genes, collectively known as HbM disease. Our recommendations are based on a systematic literature search. A series of questions regarding the key signs and symptoms, the methods for diagnosis, the clinical management in neonatal/childhood/adulthood period, and the therapeutic approach of methemoglobinemia were formulated and the relative recommendations were produced. An agreement was obtained using a Delphi‐like approach and the experts panel reached a final consensus >75% of agreement for all the questions.

Clinical, metabolic, and molecular genetic characterization of hereditary methemoglobinemia caused by cytochrome b5 reductase deficiency in 30 dogs

Genotype–phenotype correlations of humans and dogs with hereditary methemoglobinemia are not yet well characterized. We determined total hemoglobin and methemoglobin (MetHb) concentrations, cytochrome b₅ reductase (CYB5R) enzyme activities, genotypes, and clinical signs in 30 dogs with persistent cyanosis without cardiopulmonary disease. Erythrocytic CYB5R enzyme activities were low in all dogs assayed. Owner-reported quality of life ranged from subclinical to occasional exertional syncope. Two previously reported and two novel CYB5R3 missense variants were identified among the methemoglobinemic cohort and were predicted to impair enzyme function. Two variants were recurrent: a homozygous Ile194Leu substitution was found in Pomeranians and other small dogs, and a homozygous Arg219Pro change occurred predominately in pit bull terriers. The other two variants were Thr202Ala and Gly76Ser substitutions in single dogs. Of the two common CYB5R3 genotypes, Arg219Pro was associated with a more severe metabolic phenotype. We conclude that CYB5R3 deficiency is the predominate cause of canine hereditary methemoglobinemia. Although this finding is unlikely to alter the clinical approach to hereditary methemoglobinemia in dogs, it demonstrates the possibility of how genotype–phenotype cohort analysis might facilitate precision medicine in the future in veterinary medicine.

Mutation update: Variants of the CYB5R3 gene in recessive congenital methemoglobinemia

NADH-cytochrome b5 reductase 3 deficiency is an important genetic cause of recessive congenital methemoglobinemia (RCM) and occurs worldwide in autosomal recessive inheritance. In this Mutation Update, we provide a comprehensive review of all the pathogenic mutations and their molecular pathology in RCM along with the molecular basis of RCM in 21 new patients from the Indian population, including four novel variants: c.103A>C (p.Thr35Pro), c.190C>G (p.Leu64Val), c.310G>T (p.Gly104Cys), and c.352C>T (p.His118Tyr). In this update, over 78 different variants have been described for RCM globally. Molecular modeling of all the variants reported in CYB5R3 justifies association with the varying severity of the disease. The majority of the mutations associated with the severe form with a neurological disorder (RCM Type 2) were associated with the FAD-binding domain of the protein while the rest were located in another domain of the protein (RCM Type 1).

Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene

Recessive congenital methemoglobinemia type II is a very rare autosomal recessive hematologic disorder due to NADH-cytochrome b5 reductase deficiency, usually caused by full-stop mutations or deletions. This disease classically presents with mild neonatal cyanosis, early onset severe progressive developmental delay, movement disorders, and progressive microcephaly. We report two siblings with recessive congenital methemoglobinemia type II whose evaluation revealed a novel p.Arg92Trp missense mutation of the CYB5R3 gene and a peculiar imaging finding of basal ganglia hypoplasia. Brain magnetic resonance imaging was performed at age 10 months in the older sibling and at age three months in the younger sibling. It revealed similar findings of bilateral small size of the lentiform and caudate nuclei and reduced frontotemporal brain volume. Our patient cases highlight that basal ganglia hypoplasia is an interesting clue to the very rare and frequently unsuspected diagnosis of recessive congenital methemoglobinemia type II, that may explain the associated movement disorders. The novel missense mutation is one of very few identified missense mutations known to cause severe type II recessive congenital methemoglobinemia.

Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia)

Type I congenital methemoglobinemia is an autosomal recessive disorder. A high frequency of congenital methemoglobinemia has been reported among Native Americans inhabiting the Yukon-Kuskokwim Delta. Other rare cases of congenital methemoglobinemia of types I and II have been reported in Japan and other countries. In Russia-namely, in Yakutia-a high frequency of type I congenital methemoglobinemia has been reported. In 2009, the Consultation Polyclinic of the Pediatric Center in Yakutsk city established a registry of children with congenital methemoglobinemia. In total, 43 patients were registered between 2005 and 2009. The median methemoglobin level was 13.5% (ranging between 4.2% and 33.9%) and physical examination revealed cyanosis of the skin and mucus membranes. There were significant positive relationships between percentage of methemoglobin and erythrocyte count, hemoglobin concentration, and hematocrit among male patients, consistent with an upregulation of the hypoxic response. The prevalence per 100,000 children ranged from 12.7 to 47.0 in 3 geographic regions of Yakutia. Further research is needed to clarify the clinical consequences of congenital methemoglobinemia in the children of Yakutia and the reasons for the high variability in the prevalence of the condition.

Inborn defects in the antioxidant systems of human red blood cells

Red blood cells (RBCs) contain large amounts of iron and operate in highly oxygenated tissues. As a result, these cells encounter a continuous oxidative stress. Protective mechanisms against oxidation include prevention of formation of reactive oxygen species (ROS), scavenging of various forms of ROS, and repair of oxidized cellular contents. In general, a partial defect in any of these systems can harm RBCs and promote senescence, but is without chronic hemolytic complaints. In this review we summarize the often rare inborn defects that interfere with the various protective mechanisms present in RBCs. NADPH is the main source of reduction equivalents in RBCs, used by most of the protective systems. When NADPH becomes limiting, red cells are prone to being damaged. In many of the severe RBC enzyme deficiencies, a lack of protective enzyme activity is frustrating erythropoiesis or is not restricted to RBCs. Common hereditary RBC disorders, such as thalassemia, sickle-cell trait, and unstable hemoglobins, give rise to increased oxidative stress caused by free heme and iron generated from hemoglobin. The beneficial effect of thalassemia minor, sickle-cell trait, and glucose-6-phosphate dehydrogenase deficiency on survival of malaria infection may well be due to the shared feature of enhanced oxidative stress. This may inhibit parasite growth, enhance uptake of infected RBCs by spleen macrophages, and/or cause less cytoadherence of the infected cells to capillary endothelium.

Human cytochrome b5 reductase: structure, function, and potential applications

Cytochrome b5 reductase is a flavoprotein that is produced as two different isoforms that have different localizations. The amphipathic microsomal isoform, found in all cell types with the exception of erythrocytes, consists of one hydrophobic membrane-anchoring domain and a larger hydrophilic flavin catalytic domain. The soluble cytochrome b5 reductase isoform, found in human erythrocytes, is a truncated protein that is encoded by an alternative transcript and consists of the larger domain only. Cytochrome b5 reductase is involved in the transfer of reducing equivalents from the physiological electron donor, NADH, via an FAD domain to the small molecules of cytochrome b5. This protein has received much attention from researchers due to its involvement in many oxidation and reduction reactions, such as the reduction of methemoglobin to hemoglobin. Autosomal cytochrome b5 reductase gene deficiency manifests with the accumulation of oxidized Fe+3 and recessive congenital methemoglobinemia in humans. In this article, we provide a comprehensive overview of the structure and function of cytochrome b5 reductase from different eukaryotic sources and its potential use in the food industry, biosensor, and diagnostic areas.

Methemoglobin reductase deficiency: novel mutation is associated with a disease phenotype of intermediate severity

BACKGROUND

Cytochrome b5 reductase (CB5R) deficiency is a recessively inherited autosomal disorder that is either benign (type I) or associated with severe neurological problems (type II). Specific mutations in the CYB5R gene are not exclusive to each type.

OBSERVATION

Two cyanotic children with developmental delay but with slow progression were investigated for CB5R deficiency. A novel mutation, p.Arg58Pro, was independently detected in both cases.

CONCLUSIONS

The clinical variability and severity of the disease reflect the combined effects of impaired function of the 2 mutant enzymes. As illustrated by these 2 cases, inheritance of p.Arg58Pro with either p.Gly76Ser or pLeu188del causes a clinical condition more severe than type I and less severe than the type II cases reported to date.

Molecular basis of two novel mutations found in type I methemoglobinemia

Congenital methemoglobinemia due to NADH-cytochrome b5 reductase 3 (CYB5R3) deficiency is an autosomal recessive disorder that occurs sporadically worldwide, although endemic clusters of this disorder have been identified in certain ethnic groups. It is present as two distinct phenotypes, type I and type II. Type I methemoglobinemia is characterized by CYB5R3 enzyme deficiency restricted to erythrocytes and is associated with benign cyanosis. The less frequent type II methemoglobinemia is associated with generalized CYB5R3 deficiency affecting all cells and is lethal in early infancy. Here we describe the molecular basis of type I methemoglobinemia due to CYB5R3 deficiency in four patients from three distinct ethnic backgrounds, Asian Indian, Mexican and Greek. The CYB5R3 gene of three probands with type I methemoglobinemia and their relatives were sequenced revealing several putative causative mutations; in one subject multiple mutations were present. Two novel mutations, S54R and F157C, were identified and the previously described A179T, V253M mutations were also identified. All these point mutations mapped to the NADH binding domain and or the FAD binding domain. Each has the potential to sterically hinder cofactor binding causing instability of the CYB5R3 protein. Wild-type CYB5R3, as well as two of these novel mutations, S54R and F157C, was amplified, cloned, and purified recombinant peptide obtained. Kinetic and thermodynamic studies of these proteins show that the above mutations lead to decreased thermal stability.

Recessive hereditary methemoglobinemia: two novel mutations in the NADH-cytochrome b5 reductase gene

We report the clinical and molecular characteristics of 6 new patients with recessive hereditary methemoglobinemia due to cytochrome b5 reductase deficiency. One patient was affected by Type-II disease with cyanosis and severe progressive neurological dysfunction, whereas the others displayed the benign Type-I phenotype. Methemoglobin levels ranged from 12.1% to 26.2% and cytochrome b5 reductase activity from 0 to 10% of normal. Eight different mutations were detected among the twelve mutated alleles identified, one splicing mutation, two stop codon, and five missense. Two mutations c. 82 C>T(Gln27STOP) and c. 136 C>T(Arg45Trp) are new. Prenatal diagnosis was performed in the family with Type-II disease.

Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency

Some 60 years ago, Quentin Gibson reported the first hereditary disorder involving an enzyme when he deduced that familial methaemoglobinaemia was caused by an enzymatic lesion associated with the glycolysis pathway in red blood cells. This disorder, now known as recessive congenital methaemoglobinaemia (RCM), is caused by NADH-cytochrome b5 reductase (cb(5)r) deficiency. Two distinct clinical forms, types I and II, have been recognized, both characterized by cyanosis from birth. In type II, the cyanosis is accompanied by neurological impairment and reduced life expectancy. Cytochrome b(5) reductase is composed of one FAD and one NADH binding domain linked by a hinge region. It is encoded by the CYB5R3 (previously known as DIA1) gene and more than 40 mutations have been described, some of which are common to both types of RCM. Mutations associated with type II tend to cause incorrect splicing, disruption of the active site or truncation of the protein. At present the description of the sequence variants of cb(5)r in the literature is confusing, due to the use of two conventions which differ by one codon position. Herein we propose a new system for nomenclature of cb(5)r based on recommendations of the Human Genome Variation Society. The development of a heterologous expression system has allowed the impact of naturally occurring variants of cb(5)r to be assessed and has provided insight into the function of cb(5)r.

A mutation in Arabidopsis cytochrome b5 reductase identified by high-throughput screening differentially affects hydroxylation and desaturation

As a model for analyzing the production of novel fatty acids in oilseeds, we used the genetic and molecular techniques available for Arabidopsis to characterize modifying mutations affecting the accumulation of hydroxy fatty acids in the seeds of Arabidopsis plants that express a transgene for the castor bean fatty acid hydroxylase, FAH12. We developed a high-throughput analytical system and used it to identify three complementation classes of mutations with reduced hydroxy fatty acid accumulation from among Arabidopsis M3 seed samples derived from chemical mutagenesis. We identified one of the mutations by positional cloning as a single base pair change in a gene encoding NADH:cytochrome b5 reductase (CBR1, At5g17770). When expressed in yeast, the mutant form of the enzyme was less active and less stable than the wild-type enzyme. Characterization of homozygous mutant lines with and without the FAH12 transgene (FAH12 cbr1-1 and cbr1-1, respectively) indicated that the only detectable consequence of the cbr1-1 mutation was on desaturase and hydroxylase reactions in the developing seed. The leaf and root fatty compositions, as well as the growth, development and seed production of mutant plants were indistinguishable from wild type. Interestingly, while the cbr1-1 mutation reduced the accumulation of hydroxy fatty acids in seeds by 85%, the effects on 18:1 and 18:2 desaturation reactions were much less (<25% and <60%, respectively). These results suggest that there is competition in developing seeds among the several reactions that utilize reduced cytochrome b5.

Electrolyzed-reduced water reduced hemodialysis-induced erythrocyte impairment in end-stage renal disease patients

Chronic hemodialysis (HD) patients increase erythrocyte susceptibility to hemolysis and impair cell survival. We explored whether electrolyte-reduced water (ERW) could palliate HD-evoked erythrocyte impairment and anemia. Forty-three patients undergoing chronic HD were enrolled and received ERW administration for 6 month. We evaluated oxidative stress in blood and plasma, erythrocyte methemoglobin (metHb)/ferricyanide reductase activity, plasma metHb, and proinflammatory cytokines in the chronic HD patients without treatment (n=15) or with vitamin C (VC)- (n=15), vitamin E (VE)-coated dialyzer (n=15), or ERW treatment (n=15) during an HD course. The patients showed marked increases (15-fold) in blood reactive oxygen species, mostly H(2)O(2), after HD without any treatment. HD resulted in decreased plasma VC, total antioxidant status, and erythrocyte metHb/ferricyanide reductase activity and increased erythrocyte levels of phosphatidylcholine hydroperoxide (PCOOH) and plasma metHb. Antioxidants treatment significantly palliated single HD course-induced oxidative stress, plasma and RBC PCOOH, and plasma metHb levels, and preserved erythrocyte metHb /ferricyanide reductase activity in an order VC>ERW>VE-coated dialyzer. However, ERW had no side effects of oxalate accumulation easily induced by VC. Six-month ERW treatment increased hematocrit and attenuated proinflammatory cytokines profile in the HD patients. In conclusion, ERW treatment administration is effective in palliating HD-evoked oxidative stress, as indicated by lipid peroxidation, hemolysis, and overexpression of proinflammatory cytokines in HD patients.

Expression of a novel P275L variant of NADH:cytochrome b5 reductase gives functional insight into the conserved motif important for pyridine nucleotide binding

The clinical disorder of recessive congenital methemoglobinemia (RCM, OMIN 250800) is associated with mutations in NADH:cytochrome b5 reductase (cb5r) and manifests as cyanosis from birth. Screening a cyanotic infant indicated elevated methemoglobin levels and decreased cb5r activity suggesting RCM. Sequencing the DIA1 gene encoding cb5r revealed a novel mutation, C27161T (NCBI accession number: NT_011520), resulting in replacement of proline at amino acid 275 with leucine (P275L). To understand how this mutation would affect cb5r's function, the P275L variant was expressed in a heterologous expression system and spectroscopic, thermodynamic, and thermostability studies were performed. The leucine substitution at residue 275 was found to significantly decrease the affinity towards the physiological reducing substrate, NADH, without affecting the activity of the P275L variant. From the rat model, residue 275 is predicted to be part of a conserved "CGPPPM" motif important for the binding and correct positioning of the NADH reducing substrate. Thus P275 influences the interaction with NADH which was confirmed by the change in affinity towards the physiological reducing substrate.

Effects of vitamin C infusion and vitamin E-coated membrane on hemodialysis-induced oxidative stress

Chronic hemodialysis (HD) patients manifest anemia and atherosclerosis with associated oxidative stress. We explored whether intravenous infusion of vitamin C (VC) and/or use of vitamin E (VE)-coated dialysis membrane could palliate HD-evoked oxidative stress. Eighty patients undergoing chronic HD were enrolled and randomly assigned into four groups: HD with intravenous VC (n=20), HD with VE-coated dialyzer (n=20), HD with both (n=20), and HD with neither (n=20). We evaluated oxidative stress in blood and plasma, erythrocyte methemoglobin/ferricyanide reductase (red blood cells (RBC)-MFR) activity, plasma methemoglobin, and pro-inflammatory cytokines in these patients. All patients showed marked increases (14-fold) in blood reactive oxygen species (ROS) after HD. The types of ROS were mostly hydrogen peroxide, and in lesser amounts, O2*- and HOCl. HD resulted in decreased plasma VC, total antioxidant status, and RBC-MFR activity and increased plasma and erythrocyte levels of phosphatidylcholine hydroperoxide (PCOOH) and methemoglobin. Intravenous VC significantly palliated HD-induced oxidative stress, plasma and RBC levels of PCOOH, and plasma methemoglobin levels and preserved RBC-MFR activity. The VE-coated dialyzer effectively prevented RBCs from oxidative stress, although it showed a partial effect on the reduction of total ROS activity in whole blood. In conclusion, intravenous VC plus a VE-coated dialyzer is effective in palliating HD-evoked oxidative stress, as indicated by hemolysis and lipid peroxidation, and by overexpression of proinflammation cytokines in HD patients. Using VE-coated dialyzer per se is, however, effective in reducing lipid peroxidation and oxidative damage to RBCs.

Identification and characterization of the novel FAD-binding lobe G75S mutation in cytochrome b(5) reductase: an aid to determine recessive congenital methemoglobinemia status in an infant

NADH-cytochrome b(5) reductase deficiency results clinically in either type I or type II recessive congenital methemoglobinemia. The more severe type II form is associated with a global deficiency of cytochrome b(5) reductase and is characterized by cyanosis with neurological dysfunction. In contrast, the only symptom for type I is cyanosis. We have identified a novel G to A mutation at position 15,635 in the DIAI gene of a 4-month-old baby that results in a glycine to serine substitution at codon 75 in the cytochrome b(5) reductase protein. The G75S mutation, located in the FAD-binding lobe of cytochrome b(5) reductase, was found in association with the previously described V252M variant. The V252M mutation is present in the NADH-binding domain and associated with both types I and II recessive congenital methemoglobinemia. Since the G75S and V252M mutations represent radical changes in differing regions of cytochrome b(5) reductase, generating and characterizing these variants singly and in combination using a rat heterologous expression system would provide insight into the differences between types I and II disease at the molecular level. Although all three variants were found to retain stoichiometric levels of FAD with spectroscopic and thermodynamic properties comparable to those of native cytochrome b(5) reductase, all exhibited decreased catalytic efficiency and reduced protein stability reflecting the position of the mutations in the primary structure. The G75S variant retained only 11% of the catalytic efficiency of the wild-type enzyme. Thus, cytochrome b(5) reductase deficient patients who are heterozygous for either FAD- or NADH-binding lobe mutations can exhibit the clinically less severe type I phenotype.

Recessive congenital methaemoglobinaemia: functional characterization of the novel D239G mutation in the NADH-binding lobe of cytochrome b5 reductase

Type I recessive congenital methaemoglobinaemia (RCM), caused by the reduced form of nicotinamide adenine dinucleotide (NADH)-cytochrome b(5) reductase (cytb(5)r) deficiency, manifests clinically as cyanosis without neurological dysfunction. Two mutations, E255- and G291D, have been identified in the NADH-binding lobe of cytb(5)r in previously reported patients, and we have detected a further novel mutation, D239G, in this lobe in two unrelated Irish families. Although one family belongs to the genetically isolated Traveller Community, which separated from the general Irish population during the 1845-48 famine, the D239G mutation was present on the same haplotype in both families. Three known cytb(5)r mutations were also identified, including the R159- mutation, which causes loss of the entire NADH-binding lobe and had previously been reported in an individual with type II RCM. Characterization of the three NADH-binding lobe mutants using a heterologous expression system revealed that all three variants retained stoichiometric levels of flavin adenine dinucleotide with spectroscopic and thermodynamic properties comparable with those of native cytb(5)r. In contrast to the E255- and G291D variants, the novel D239G mutation had no adverse impact on protein thermostability. The D239G mutation perturbed substrate binding, causing both decreased specificity for NADH and increased specificity for NADPH. Thus cytb(5)r deficient patients who are heterozygous for an NADH-binding lobe mutation can exhibit the clinically less severe type I phenotype, even in association with heterozygous deletion of the NADH-binding lobe.

Prenatal diagnosis of recessive congenital methaemoglobinaemia type II: novel mutation in the NADH-cytochrome b5 reductase gene leading to stop codon read-through

A case of type II recessive congenital methaemoglobinaemia (RCM) observed in a Lebanese subject with a novel mutation in NADH-cytochrome b5 reductase gene is described. A homozygous mutation CAC to AA identified at Thr 295 with an out-of-frame 1-bp deletion leads to a frameshift with translational read-through of the natural stop codon. The molecular mechanism is demonstrated by an in vitro translation study. The model of mutated cytochrome b5 reductase protein possessing 46 additional amino acids was obtained by homology modelling. The mutation causes an alteration of hydrophobicity in the carboxyl-terminal portion, resulting in the conformation being drastically disturbed by the presence of 46 supplementary amino acids. The identical mutation was found in the heterozygous state in the patient's parents and sister. Identification of this new mutation enabled us to perform the molecular prenatal diagnosis of type II RCM at the DNA level.

Disorders of oxidised haemoglobin

Methaemoglobinaemia arises from the production of non-functional haemoglobin containing oxidised Fe(3+) which results in reduced oxygen supply to the tissues and manifests as cyanosis in the patient. It can develop by three distinct mechanisms: genetic mutation resulting in the presence of abnormal haemoglobin, a deficiency of methaemoglobin reductase enzyme and toxin-induced oxidation of haemoglobin. The normal haemoglobin fold forms a pocket to bind the haem and stabilise its complex with molecular oxygen, simultaneously preventing spontaneous oxidation of the Fe(2+) ion chelated by the haem pyrroles and the globin histidines. In the abnormal, M forms of haemoglobin (Hb Ms) amino acid substitution in or near the haem pocket creates a propensity to form methaemoglobin instead of oxyhaemoglobin in the presence of molecular oxygen. Normally, haemoglobin continually oxidises but significant accumulation of methaemoglobin is prevented by the action of a group of methaemoglobin reductase enzymes. In the autosomal recessive form of methaemoglobinaemia there is a deficiency of one of these reductase enzymes thereby allowing accumulation of oxidised Fe(3+) in methaemoglobin. Oxidising drugs and other toxic chemicals may greatly enhance the normal spontaneous rate of methaemoglobin production and if levels exceed 70% of total haemoglobin, vascular collapse occurs resulting in coma and death. Under these conditions, if the source of toxicity can be eliminated methaemoglobin levels will return to normal. Disorders of oxidised haemoglobin are relatively easily diagnosed and in most cases, except for the presence of congenitally defective haemoglobin M, can be treated successfully.

Cytochrome b5 reductase: the roles of the recessive congenital methemoglobinemia mutants P144L, L148P, and R159*

Recessive congenital methemoglobinemia (RCM, OMIM 250800) arises from defects in either the erythrocytic or microsomal forms of the flavoprotein, cytochrome b5 reductase (cb5r) and was the first disease to be directly associated with a specific enzyme deficiency. Of the 33 verified mutations in cb5r that give rise to either the type I (erythrocytic) or type II (generalized) forms of RCM, three of the mutations, corresponding to P144L, L148P, and R159*, are located in a segment of the primary sequence composed of residues G143 to V171 which serves as a "hinge" or "linker" region between the FAD- and NADH-binding lobes of the protein. With the exception of R159*, which produces a truncated non-functional cb5r resulting in type II RCM, the type I methemoglobinemias resulting from the P144L or L148P mutations have been proposed to be due to decreased enzyme stability. Utilizing a recombinant form of the rat cb5r enzyme, we have generated the P144L, L148P, and P144L/L148P mutants, purified the resulting proteins to homogeneity and characterized their spectroscopic, kinetic, and thermodynamic properties. The three mutant proteins retained full complements of FAD with the P144L and L148P variants being spectroscopically indistinguishable from wild-type cb5r. In contrast, kinetic analyses revealed that the P144L, L148P, and P144L/L148P variants retained only 28, 31, and 8% of wild-type NADH:cytochrome b5 reductase activity, respectively, together with significant alterations in affinity for both NADH and NAD+. In addition, FAD oxidation-reduction potentials were 32, 19, and 65 mV more positive for the mutants than the corresponding FAD/FADH2 couple in native cb5r (E0'=-272 mV). Thermal and proteolytic stability measurements indicated that all three mutants were less stable than the wild-type protein while differential spectroscopy indicated altered pyridine nucleotide binding in all three variants. These results demonstrate that the "hinge" region is important in maintaining the correct orientation of the flavin- and pyridine nucleotide-binding lobes within the protein for efficient electron transfer and that the P144L and L148P mutations disrupt the normal registration of the FAD- and NADH-binding lobes resulting in altered affinities for both the physiological reducing substrate, NADH and its product, NAD+.

Cytochrome b5 oxidoreductase: expression and characterization of the original familial ideopathic methemoglobinemia mutations E255- and G291D

NADH:cytochrome b5 oxidoreductase catalyzes the transfer of reducing equivalents from the physiological electron donor, NADH, to two molecules of cytochrome b5. Utilizing a heterologous expression system for the soluble, catalytic domain of the rat microsomal enzyme, we have produced two mutants, corresponding to E255- and G291D. These mutants correspond to the two specific mutations that were identified over a half century later following diagnosis of the original cases of type I recessive congenital methemoglobinemia (RCM). We have purified both the E255- and G291D variants to homogeneity to determine the molecular basis for type I RCM in these individuals. Both the E255- and G291D variants retained a full complement of FAD and exhibited absorption and CD spectroscopic properties comparable to those of the wild-type protein. Oxidation-reduction potentiometric titrations yielded standard midpoint potentials (E0') for the FAD/FADH2 couple of -271 and -273 mV for the E255- and G291D variants, respectively, which were comparable to the value of -268 mV obtained for the wild-type protein and confirmed that the redox potential of the flavin was unaffected by either mutation. Thermal and proteolytic stability studies revealed that while the G291D variant exhibited stability comparable to that of wild-type, the E255- variant was markedly less stable, indicative of an altered conformation. Initial-rate kinetic studies revealed that both mutants had decreased catalytic activity (kcat), with the E255- and G291D variants retaining approximately 38 and 58% of wild-type activity, respectively. However, the affinity for NADH (KmNADH) was decreased approximately 100-fold for E255- compared to only approximately 1.3-fold for G291D, results supported by the spectroscopic binding constant (Ks) obtained for G291D. These results indicate that the properties of both the E255- and G291D cytochrome b5 oxidoreductase mutants are similar to those of other variants that have been identified as resulting in the type I form of RCM.

Deflavination and reconstitution of flavoproteins

Flavoproteins are ubiquitous redox proteins that are involved in many biological processes. In the majority of flavoproteins, the flavin cofactor is tightly but noncovalently bound. Reversible dissociation of flavoproteins into apoprotein and flavin prosthetic group yields valuable insights in flavoprotein folding, function and mechanism. Replacement of the natural cofactor with artificial flavins has proved to be especially useful for the determination of the solvent accessibility, polarity, reaction stereochemistry and dynamic behaviour of flavoprotein active sites. In this review we summarize the advances made in the field of flavoprotein deflavination and reconstitution. Several sophisticated chromatographic procedures to either deflavinate or reconstitute the flavoprotein on a large scale are discussed. In a subset of flavoproteins, the flavin cofactor is covalently attached to the polypeptide chain. Studies from riboflavin-deficient expression systems and site-directed mutagenesis suggest that the flavinylation reaction is a post-translational, rather than a cotranslational, process. These genetic approaches have also provided insight into the mechanism of covalent flavinylation and the rationale for this atypical protein modification.

Heterologous expression of enzymopenic methemoglobinemia variants using a novel NADH:cytochrome c reductase fusion protein

Hereditary enzymopenic methemoglobinemia is a rare disease that predominantly results from defects in either the erythrocytic (type I) or microsomal (type II) forms of the enzyme NADH:cytochrome b5 reductase (EC 1.6.2.2). All 25 currently identified type I and type II methemoglobinemia mutants have been expressed in Escherichia coli using a novel six histidine-tagged rat cytochrome b5/cytochrome b5 reductase fusion protein designated NADH:cytochrome c reductase (H6NCR). All 25 H6NCR variants were isolated and demonstrated to result in two groups of expression products. The first group of 16 mutants, which included the majority of the type I mutants, included K116Q, P131L, L139P, T183S, M193V, S194P, P211L, L215P, A245T, A245V, C270Y, E279K, V305R, V319M, M340-, and F365-, and yielded full-length fusion proteins that retained variable levels of NADH:cytochrome c reductase (NADH:CR) activity, ranging from approximately 2% (M340-) to 92% (K116Q) of that of the wild-type fusion protein. In contrast, the remaining nine mutants that represented the majority of the type II variants, comprised a second group that included Y109*, R124Q, Q143*, R150*, P162H, V172M, R226*, C270R, and R285*, and resulted in truncated H6NCR variants that retained the amino-terminal cytochrome b5 domain but were devoid of NADH:CR activity due to the absence of the cytochrome b5 reductase flavin domain. Kinetic analyses of the first group of full-length mutant fusion proteins indicated that values for both kcat and Km(NADH) were decreased and increased, respectively, indicating that the various mutations affected both substrate affinity and/or turnover. However, for the second group, the truncated products were the result of incomplete production of the carboxyl-terminal flavin-containing domain or instability of the expression products due to improper folding and/or lack of flavin incorporation.

Compound heterozygosity of two missense mutations in the NADH-cytochrome b5 reductase gene of a Polish patient with type I recessive congenital methaemoglobinaemia

A case of type I methaemoglobinaemia observed in a Polish subject with compound heterozygosity for two mutations in the reduced nicotinamide adenine dinucleotide (NADH) cytochrome b5 reductase (b5R) gene is described. One is a novel mutation 647T-->C which leads to substitution of isoleucine by threonine at position 215 (I215T). This maternal mutation was found in several family members. A previously known mutation, 757G-->A, leads to the replacement of valine by methionine at position 252 (V252M). The latter mutation was found also in the father and one of the two brothers. The effects of these mutations were analysed on a model of the human b5R protein obtained by homology modelling. Although both amino acid substitutions are located in the NADH-binding domain, the whole protein structure, especially the region between the flavin adenine dinucleotide and NADH-binding domains, is disturbed. The structural changes in the I215T mutant are less prominent than those in the V252M mutant. We presume that the 647T-->C mutation is a type I mutation, however, it has not been observed in the homozygous state.

Familial idiopathic methemoglobinemia revisited: original cases reveal 2 novel mutations in NADH-cytochrome b5 reductase

In 1943, the first description of familial idiopathic methemoglobinemia in the United Kingdom was reported in 2 members of one family. Five years later, Quentin Gibson (then of Queen's University, Belfast, Ireland) correctly identified the pathway involved in the reduction of methemoglobin in the family, thereby describing the first hereditary trait involving a specific enzyme deficiency. Recessive congenital methemoglobinemia (RCM) is caused by a deficiency of reduced nicotinamide adenine dinucleotide (NADH)-cytochrome b5 reductase. One of the original propositi with the type 1 disorder has now been traced. He was found to be a compound heterozygote harboring 2 previously undescribed mutations in exon 9, a point mutation Gly873Ala predicting a Gly291Asp substitution, and a 3-bp in-frame deletion of codon 255 (GAG), predicting loss of glutamic acid. A brother and a surviving sister are heterozygous; each bears one of the mutations. Thirty-three different mutations have now been recorded for RCM. The original authors' optimism that RCM would provide material for future genetic studies has been amply justified.

Prenatal diagnosis in two families with autosomal, p47(phox)-deficient chronic granulomatous disease due to a novel point mutation in NCF1

OBJECTIVE

Prenatal diagnosis was required in two unrelated families with chronic granulomatous disease (CGD) patients who lacked expression of p47(phox) protein; thus a search for mutations in NCF1 was undertaken.

METHODS

Gene scanning was applied to establish the relative number of coding and pseudo-NCF1 genes. PCRs specific for coding NCF1 cDNA and coding NCF1 exon-7 genomic DNA were devised.

RESULTS

The normal 1:2 ratio of coding and pseudo-NCF1 genes was found in the patients. Sequencing of the RT-PCR product specific for mRNA from the coding NCF1 genes revealed a novel homozygous G579A mutation in both patients, changing the TGG codon for Trp193 into the TAG stop codon. This mutation was confirmed in genomic DNA. The parents of both patients were found to be heterozygotes for this mutation. In the chorionic villus DNA of the first family a heterozygous G579A mutation was found. Postpartum, functional NADPH oxidase tests were normal. In the second family, this mutation was present in homozygous form in the chorionic villus DNA. Following termination of the pregnancy, the diagnosis of p47(phox)-deficient CGD was confirmed on DNA extracted from fetal blood.

CONCLUSION

This is the first report of prenatal diagnosis in p47(phox)-deficient CGD.