Molecular basis of hereditary methaemoglobinaemia, types I and II: two novel mutations in the NADH-cytochrome b5 reductase gene

Familial Psychomotor Delay of an Uncommon Cause: Type II Congenital Methemoglobinemia

Methemoglobinemia is due to oxidization of divalent ferro-iron of hemoglobin to ferri-iron of methemoglobin (MetHb) which is incapable of transferring oxygen to tissues. This disease may be acquired by intoxication with oxidizing agents or inherited with a mutation of CYB5R3, the gene coding for the methemoglobin reductase or cytochrome B5 reductase 3 responsible for the reduction of MetHb to hemoglobin. We report the case of 2 sisters aged respectively of 15 and 8 months. They were born to a second-degree consanguineous marriage with a history of precocious and unexplained deaths in 3 relatives. Both sisters presented neurological features including psychomotor retardation, microcephaly, and axial hypotonia. Cerebral magnetic resonance imaging revealed cerebral atrophy in both cases associated with hypoplasia of the corpus callosum in the younger child. The association of neurological disability, cyanosis, and hypoxemia prompted a search for methemoglobinemia, with MetHB levels respectively of 26% and 15.8%in the 2 sisters. Initial treatment was based on methylene blue, then ascorbic acid. The genetic study revealed a c.463+8G>C mutation of CYB5R3 confirming the diagnosis of methemoglobinemia type II. The diagnosis of methemoglobinemia, although rare, should be considered in the presence of psychomotor retardation with cyanosis and subacute onset hypoxemia, especially in the presence of a family history.

Molecular Dynamic Simulation Analysis of a Novel Missense Variant in CYB5R3 Gene in Patients with Methemoglobinemia

Background and Objective: Mutations in the CYB5R3 gene cause reduced NADH-dependent cytochrome b5 reductase enzyme function and consequently lead to recessive congenital methemoglobinemia (RCM). RCM exists as RCM type I (RCM1) and RCM type II (RCM2). RCM1 leads to higher methemoglobin levels causing only cyanosis, while in RCM2, neurological complications are also present along with cyanosis. Materials and Methods: In the current study, a consanguineous Pakistani family with three individuals showing clinical manifestations of cyanosis, chest pain radiating to the left arm, dyspnea, orthopnea, and hemoptysis was studied. Following clinical assessment, a search for the causative gene was performed using whole exome sequencing (WES) and Sanger sequencing. Various variant effect prediction tools and ACMG criteria were applied to interpret the pathogenicity of the prioritized variants. Molecular dynamic simulation studies of wild and mutant systems were performed to determine the stability of the mutant CYB5R3 protein. Results: Data analysis of WES revealed a novel homozygous missense variant NM_001171660.2: c.670A > T: NP_001165131.1: p.(Ile224Phe) in exon 8 of the CYB5R3 gene located on chromosome 22q13.2. Sanger sequencing validated the segregation of the identified variant with the disease phenotype within the family. Bioinformatics prediction tools and ACMG guidelines predicted the identified variant p.(Ile224Phe) as disease-causing and likely pathogenic, respectively. Molecular dynamics study revealed that the variant p.(Ile224Phe) in the CYB5R3 resides in the NADH domain of the protein, the aberrant function of which is detrimental. Conclusions: The present study expanded the variant spectrum of the CYB5R3 gene. This will facilitate genetic counselling of the same and other similar families carrying mutations in the CYB5R3 gene.

Neurological and Neuroimaging Features of CYB5R3-Related Recessive Hereditary Methemoglobinemia Type II

Recessive hereditary methemoglobinemia (RHM) due to NADH-cytochrome b5 reductase deficiency is a rare disease caused by pathogenic variants in CYB5R3. Unlike type I, in RHM type II (RHM2), the enzymatic defect affects erythrocytes and all body tissues, thus resulting in cyanosis and neurological impairment. Although the first description of RHM2 dates back to the mid-1950s, detailed clinical and neuroimaging information are available for only a few patients. Here, we describe a new patient with RHM2 that harbors an unreported homozygous 31 Kb deletion involving part of CYB5R3, and showing a peculiar neuroimaging pattern resembling a ponto-cerebellar hypoplasia-like condition. A careful review of the available literature was performed with the aim of better delineating neurological and neuroimaging as well as the genotypic spectra of this extremely rare disease.

Characterization of a novel nicotinamide adenine dinucleotide-cytochrome b5 reductase mutation associated with canine hereditary methemoglobinemia

Hereditary methemoglobinemia associated with nicotinamide adenine dinucleotide-cytochrome b5 reductase (b5R) deficiency is a rare autosomal recessive disorder in animals. Recently, nonsynonymous b5R gene (CYB5R3) variants have been reported to be associated with canine and feline hereditary methemoglobinemia. However, the underlying molecular mechanisms of canine and feline methemoglobinemia caused by these nonsynonymous variants have not yet been reported. Previously, we reported a Pomeranian dog family with hereditary methemoglobinemia, carrying CYB5R3 mutation of an A>C transition at codon 194 in exon 7, replacing an isoleucine residue with leucine (p.Ile194Leu). In this study, we investigated the enzymatic and structural properties of the soluble form of wild-type and Ile194Leu canine b5Rs to characterize the effects of this missense mutation. Our results showed that the kinetic properties of the mutant enzyme were not affected by this amino acid substitution. The secondary structure of the wild-type and Ile194Leu b5Rs detected by circular dichroism showed a similar pattern. However, the mutant enzyme exhibited decreased heat stability and increased susceptibility to trypsin hydrolysis. Moreover, the thermostability and unfolding measurements indicated that the mutant enzyme was more sensitive to temperature-dependent denaturation than the wild-type b5R. We concluded from these results that unstable mutant enzyme properties with normal enzymatic activity would be associated with hereditary methemoglobinemia in the Pomeranian dog family.

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).

Familial Congenital Methemoglobinemia in Pomeranian Dogs Caused by a Missense Variant in the NADH-Cytochrome B5 Reductase Gene

Background

In veterinary medicine, congenital methemoglobinemia associated with nicotinamide adenine dinucleotide (NADH)‐cytochrome b5 reductase (b5R) deficiency is rare. It has been reported in several breeds of dogs, but little information is available about its etiology.

Objectives

To analyze the NADH‐cytochrome b5 reductase gene, CYB5R3, in a Pomeranian dog family with methemoglobinemia suspected to be caused by congenital b5R deficiency.

Animals

Three Pomeranian dogs from a family with methemoglobinemia were analyzed. Five healthy beagles and 5 nonrelated Pomeranian dogs without methemoglobinemia were used as controls.

Methods

Methemoglobin concentration, b5R activity, and reduced glutathione (GSH) concentration were measured, and a turbidity index was used to evaluate Heinz body formation. The CYB5R3 genes of the affected dog and healthy dogs were analyzed by direct sequencing.

Results

Methemoglobin concentrations in erythrocytes of the affected dogs were remarkably higher than those of the control dogs. The b5R activity of the affected dogs was notably lower than that of the control dogs. DNA sequencing indicated that this Pomeranian family carried a CYB5R3 gene missense variant (ATC→CTC at codon 194) that resulted in the replacement of isoleucine (Ile) by leucine (Leu).

Conclusions and Clinical Importance

This dog family had familial congenital methemoglobinemia caused by b5R deficiency, which resulted from a nonsynonymous variant in the CYB5R3 gene. This variation (c.580A>C) led to an amino acid substitution (p.Ile194Leu), and Ile194 was located in the proximal region of the NADH‐binding motif. Our data suggested that this variant in the canine CYB5R3 gene would affect function of the b5R in erythrocytes.

Congenital methemoglobinemia type II in a 5-year-old boy

Congenital Methemoglobinemia is a rare neurologic condition which can mimic other diseases such as epilepsy syndromes and leukodystrophies. The responsible gene, CYB5R3, is not typically included on commonly order neurologic and epilepsy panels. We recommend that laboratories include this gene on these tests which often precede larger-scale genetic studies.

A microplate reader-based method to quantify NADH-cytochrome b5 reductase activity for diagnosis of recessive congenital methaemoglobinemia

OBJECTIVES

Congenital methemoglobinemia due to NADH-cytochrome b5 reductase 3 (CYB5R3) deficiencies is an autosomal recessive disorder that occurs sporadically worldwide, A sensitive, accurate, and rapid analysis of NADH-CYB5R enzyme concentrations is necessary for the diagnosis of RCM. Here we present an alternative microplate method that is based on a standard 96-well microplate format and microplate reader that simplify the quantification of NADH-CYB5R activity.

METHODS

TECAN (Infinite 200 PRO series) microplate reader with Tecan's proven Magellan™ software measured the NADH-CYB5R enzyme activity in 250 normal controls and previously diagnosed 25 cases of RCM due to NADH-CYB5R deficiency in the Indian population using 96-well microplates using 200 μl of total reaction mixture and also compared with standard spectrophotometric assay. We have also studied stability of the hemolysate stored at 4 and -20°C temperature.

RESULTS AND DISCUSSION

Enzyme activity in all 25 samples ranged from 6.09 to 10.07 IU/g Hb (mean ± SD: 8.08 ± 1.99 IU/g Hb) where as normal control ranged (n = 250) between 13.42 and 21.58 IU/g Hb) (mean ± SD: 17.5 ± 4.08 IU/g of Hb). Data obtained from the microplate reader were compared with standard spectrophotometer method and found 100% concordance using both methods. Microplate method allows differentiating between normal, deficient and intermediate enzyme activity. It was observed that samples had significant loss of activity when stored at 4°C and retained stable activity at -20°C for 1 week time.

CONCLUSION

Our new method, incorporating a whole process of enzyme assay into a microplate format is readily applicable and allows rapid monitoring of enzyme assay. It is readily applicable to quantitative assay on pediatric sample as well as large number of samples for population screening.

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.

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.

Simulation study of methemoglobin reduction in erythrocytes. Differential contributions of two pathways to tolerance to oxidative stress

Methemoglobin (metHb), an oxidized form of hemoglobin, is unable to bind and carry oxygen. Erythrocytes are continuously subjected to oxidative stress and nitrite exposure, which results in the spontaneous formation of metHb. To avoid the accumulation of metHb, reductive pathways mediated by cytochrome b5 or flavin, coupled with NADH-dependent or NADPH-dependent metHb reductases, respectively, keep the level of metHb in erythrocytes at less than 1% of the total hemoglobin under normal conditions. In this work, a mathematical model has been developed to quantitatively assess the relative contributions of the two major metHb-reducing pathways, taking into consideration the supply of NADH and NADPH from central energy metabolism. The results of the simulation experiments suggest that these pathways have different roles in the reduction of metHb; one has a high response rate to hemoglobin oxidation with a limited reducing flux, and the other has a low response rate with a high capacity flux. On the basis of the results of our model, under normal oxidative conditions, the NADPH-dependent system, the physiological role of which to date has been unclear, is predicted to be responsible for most of the reduction of metHb. In contrast, the cytochrome b5-NADH pathway becomes dominant under conditions of excess metHb accumulation, only after the capacity of the flavin-NADPH pathway has reached its limit. We discuss the potential implications of a system designed with two metHb-reducing pathways in human erythrocytes.

Expression and characterization of a functional canine variant of cytochrome b5 reductase

Cytochrome b5 reductase (cb5r), a member of the flavoprotein transhydrogenase family of oxidoreductase enzymes, catalyzes the transfer of reducing equivalents from the physiological electron donor, NADH, to two molecules of cytochrome b5. We have determined the correct nucleotide sequence for the putative full-length, membrane-associated enzyme from Canis familiaris, and have generated a heterologous expression system for production of a histidine-tagged variant of the soluble, catalytic diaphorase domain, comprising residues I33 to F300. Using a simple two-step chromatographic procedure, the recombinant diaphorase domain has been purified to homogeneity and demonstrated to be a simple flavoprotein with a molecular mass of 31,364 (m/z) that retained both NADH:ferricyanide reductase and NADH:cytochrome b5 reductase activities. The recombinant protein contained a full complement of FAD and exhibited absorption and CD spectra comparable to those of a recombinant form of the rat cytochrome b5 reductase diaphorase domain generated using an identical expression system, suggesting similar protein folding. Oxidation-reduction potentiometric titrations yielded a standard midpoint potential (Eo') for the FAD/FADH2 couple of -273+/-5 mV which was identical to the value obtained for the corresponding rat domain. Thermal denaturation studies revealed that the canine domain exhibited stability comparable to that of the rat protein, confirming similar protein conformations. Initial-rate kinetic studies revealed the canine diaphorase domain retained a marked preference for NADH versus NADPH as reducing substrate and exhibited kcat's of 767 and 600 s(-1) for NADH:ferricyanide reductase and NADH:cytochrome b5 reductase activities, respectively, with Km's of 7, 8, and 12 microM for NADH, K3Fe(CN)6, and cytochrome b5, respectively. Spectral-binding constants (Ks) determined for a variety of NAD+ analogs indicated the highest and lowest affinities were observed for APAD+ (Ks=71 microM) and PCA+ (Ks=>31 mM), respectively, and indicated the binding contributions of the various portions of the pyridine nucleotide. These results provide the first correct sequence for the full-length, membrane-associated form of C. familiaris cb5r and provide a direct comparison of the enzymes from two phylogenetic sources using identical expression systems that indicate that both enzymes have comparable spectroscopic, kinetic, thermodynamic, and structural properties.

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.

Methemoglobinemia and eccentrocytosis in equine erythrocyte flavin adenine dinucleotide deficiency

This report describes erythrocyte biochemical findings in an adult Spanish mustang mare that exhibited persistent methemoglobinemia, eccentrocytosis, and pyknocytosis that were not related to the consumption or administration of an exogenous oxidant. The methemoglobinemia was attributed to a deficiency in cytochrome-b5 reductase (Cb5R) activity, and the eccentrocytes and pyknocytes were attributed to a marked deficiency in reduced nicotinamide adenine dinucleotide phosphate-dependent glutathione reductase (GR) activity that resulted in decreased reduced glutathione concentration within erythrocytes. The GR activity increased to a near-normal value after addition of flavin adenine dinucleotide (FAD) to the enzyme assay, indicating a deficiency of FAD in erythrocytes. The methemoglobinemia, eccentrocytosis, and pyknocytosis were attributed to deficiency of FAD in erythrocytes because the GR and Cb5R enzymes use FAD as a cofactor. This deficiency in FAD results from a defect in erythrocyte riboflavin metabolism, which has not been documented previously in animals.

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.

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

Cytochrome b(5) reductase (b5R) deficiency manifests itself in 2 distinct ways. In methemoglobinemia type I, the patients only suffer from cyanosis, whereas in type II, the patients suffer in addition from severe mental retardation and neurologic impairment. Biochemical data indicate that this may be due to a difference in mutations, causing enzyme instability in type I and complete enzyme deficiency or enzyme inactivation in type II. We have investigated 7 families with methemoglobulinemia type I and found 7 novel mutations in the b5R gene. Six of these mutations predicted amino acid substitutions at sites not involved in reduced nicotinamide adenine dinucleotide (NADH) or flavin adenine dinucleotide (FAD) binding, as deduced from a 3-dimensional model of human b5R. This model was constructed from comparison with the known 3-dimensional structure of pig b5R. The seventh mutation was a splice site mutation leading to skipping of exon 5 in messenger RNA, present in heterozygous form in a patient together with a missense mutation on the other allele. Eight other amino acid substitutions, previously described to cause methemoglobinemia type I, were also situated in nonessential regions of the enzyme. In contrast, 2 other substitutions, known to cause the type II form of the disease, were found to directly affect the consensus FAD-binding site or indirectly influence NADH binding. Thus, these data support the idea that enzyme inactivation is a cause of the type II disease, whereas enzyme instability may lead to the type I form.

A case of methemoglobinemia type II due to NADH-cytochrome b5 reductase deficiency: determination of the molecular basis

Clinical, biochemical and molecular findings in a patient with methemoglobinemia type II are described. Furthermore, a comparison between methemoglobinemia type I and type II, both caused by a deficiency of NADH-cytochrome b5 reductase (b5R), is made. Although the clinical pictures of type I and II are strikingly different, mutations in the diaphorase (DIA1) gene located on chromosome 22 have been described in both types. In the present patient, two newly identified mutations, both leading to a stop codon in exon 4 (Gln77Ter) and in exon 6 (Arg160Ter), were found. Identification of different mutations at different positions in the DIA1 gene might shed light on the clinical and biochemical differences between methemoglobinemia type I and type II.

A novel mutation in the NADH-cytochrome b5 reductase gene of a Chinese patient with recessive congenital methemoglobinemia

Recessive congenital methemoglobinemia due to nicotinamide adenine dinucleotide (NADH)-cytochrome b5 reductase (b5R) deficiency is classified into 2 clinical types: type 1 (erythrocyte type) and type 2 (generalized type). We found a Chinese family with type 1 recessive congenital methemoglobinemia, the patients from which were diagnosed according to clinical symptoms and b5R enzyme activity in the blood cells. To learn the molecular basis of type 1 recessive congenital methemoglobinemia in this Chinese family, we isolated total RNA from the peripheral leukocytes of the propositus and b5R complementary DNA (cDNA) by reverse transcription– polymerase chain reaction (RT-PCR). The coding region of the b5R cDNA was analyzed by sequencing the cloned PCR products. The results showed that the propositus was homozygous for a G→A transition at codon 203 in exon 7, changing a cysteine to a tyrosine (Cys203Tyr). To characterize the mutant enzyme, both glutathione S-transferase (GST)-fused wild-type b5R and GST-fused mutant Cys203Tyr b5R were expressed in Escherichia coli and affinity purified. The results showed that the catalytic activity of the enzyme was not much affected by this amino acid substitution, but the mutant enzyme exhibited decreased heat stability and increased susceptibility to trypsin. These properties of the mutant enzyme would account for the restricted b5R deficiency and mild clinical manifestations of these type 1 patients. The finding of this novel mutation makes codon 203 the only position within the b5R gene at which more than 1 mutation has been found.

A novel mutation in the NADH-cytochrome b5 reductase gene of a Chinese patient with recessive congenital methemoglobinemia

Recessive congenital methemoglobinemia due to nicotinamide adenine dinucleotide (NADH)-cytochrome b5 reductase (b5R) deficiency is classified into 2 clinical types: type 1 (erythrocyte type) and type 2 (generalized type). We found a Chinese family with type 1 recessive congenital methemoglobinemia, the patients from which were diagnosed according to clinical symptoms and b5R enzyme activity in the blood cells. To learn the molecular basis of type 1 recessive congenital methemoglobinemia in this Chinese family, we isolated total RNA from the peripheral leukocytes of the propositus and b5R complementary DNA (cDNA) by reverse transcription– polymerase chain reaction (RT-PCR). The coding region of the b5R cDNA was analyzed by sequencing the cloned PCR products. The results showed that the propositus was homozygous for a G→A transition at codon 203 in exon 7, changing a cysteine to a tyrosine (Cys203Tyr). To characterize the mutant enzyme, both glutathione S-transferase (GST)-fused wild-type b5R and GST-fused mutant Cys203Tyr b5R were expressed in Escherichia coli and affinity purified. The results showed that the catalytic activity of the enzyme was not much affected by this amino acid substitution, but the mutant enzyme exhibited decreased heat stability and increased susceptibility to trypsin. These properties of the mutant enzyme would account for the restricted b5R deficiency and mild clinical manifestations of these type 1 patients. The finding of this novel mutation makes codon 203 the only position within the b5R gene at which more than 1 mutation has been found.