Complementation studies in the cblA class of inborn error of cobalamin metabolism: evidence for interallelic complementation and for a new complementation class (cblH)

Expression of the cobalamin transporters cubam and MRP1 in the canine ileum-Upregulation in chronic inflammatory enteropathy

Chronic inflammatory enteropathy (CIE) in dogs, a spontaneous model of human inflammatory bowel disease (IBD), is associated with a high rate of cobalamin deficiency. The etiology of hypocobalaminemia in human IBD and canine CIE remains unknown, and compromised intestinal uptake of cobalamin resulting from ileal cobalamin receptor deficiency has been proposed as a possible cause. Here, we evaluated the intestinal expression of the cobalamin receptor subunits, amnionless (AMN) and cubilin (CUBN), and the basolateral efflux transporter multi-drug resistance protein 1 (MRP1) in 22 dogs with CIE in comparison to healthy dogs. Epithelial CUBN and AMN levels were quantified by confocal laser scanning microscopy using immunohistochemistry in endoscopic ileal biopsies from dogs with (i) CIE and normocobalaminemia, (ii) CIE and suboptimal serum cobalamin status, (iii) CIE and severe hypocobalaminemia, and (iv) healthy controls. CUBN and MRP1 expression was quantified by RT-qPCR. Receptor expression was evaluated for correlation with clinical patient data. Ileal mucosal protein levels of AMN and CUBN as well as mRNA levels of CUBN and MRP1 were significantly increased in dogs with CIE compared to healthy controls. Ileal cobalamin receptor expression was positively correlated with age, clinical disease activity index (CCECAI) score, and lacteal dilation in the ileum, inversely correlated with serum folate concentrations, but was not associated with serum cobalamin concentrations. Cobalamin receptor downregulation does not appear to be the primary cause of hypocobalaminemia in canine CIE. In dogs of older age with severe clinical signs and/or microscopic intestinal lesions, intestinal cobalamin receptor upregulation is proposed as a mechanism to compensate for CIE-associated hypocobalaminemia. These results support oral supplementation strategies in hypocobalaminemic CIE patients.

Intracellular processing of vitamin B12 by MMACHC (CblC)

Vitamin B₁₂ (cobalamin, Cbl, B₁₂) is a water-soluble micronutrient synthesized exclusively by a group of microorganisms. Human beings are unable to make B₁₂ and thus obtain the vitamin via intake of animal products, fermented plant-based foods or supplements. Vitamin B₁₂ obtained from the diet comprises three major chemical forms, namely hydroxocobalamin (HOCbl), methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl). The most common form of B₁₂ present in supplements is cyanocobalamin (CNCbl). Yet, these chemical forms cannot be utilized directly as they come, but instead, they undergo chemical processing by the MMACHC protein, also known as CblC. Processing of dietary B₁₂ by CblC involves removal of the upper-axial ligand (beta-ligand) yielding the one-electron reduced intermediate cob(II)alamin. Newly formed cob(II)alamin undergoes trafficking and delivery to the two B₁₂-dependent enzymes, cytosolic methionine synthase (MS) and mitochondrial methylmalonyl-CoA mutase (MUT). The catalytic cycles of MS and MUT incorporate cob(II)alamin as a precursor to regenerate the coenzyme forms MeCbl and AdoCbl, respectively. Mutations and epimutations in the MMACHC gene result in cblC disease, the most common inborn error of B₁₂ metabolism, which manifests with combined homocystinuria and methylmalonic aciduria. Elevation of metabolites homocysteine and methylmalonic acid occurs because the lack of an active CblC blocks formation of the indispensable precursor cob(II)alamin that is necessary to activate MS and MUT. Thus, in patients with cblC disease, vitamin B₁₂ is absorbed and present in circulation in normal to high concentrations, yet, cells are unable to make use of it. Mutations in seemingly unrelated genes that modify MMACHC gene expression also result in clinical phenotypes that resemble cblC disease. We review current knowledge on structural and functional aspects of intracellular processing of vitamin B₁₂ by the versatile protein CblC, its partners and possible regulators.

Import of TAT-Conjugated Propionyl Coenzyme A Carboxylase Using Models of Propionic Acidemia

Propionic acidemia is caused by a deficiency of the enzyme propionyl coenzyme A carboxylase (PCC) located in the mitochondrial matrix. Cell-penetrating peptides, including transactivator of transcription (TAT), offer a potential to deliver a cargo into the mitochondrion. Here, we investigated the delivery of an α₆β₆ PCC enzyme into mitochondria using the HIV TAT peptide at several levels: into isolated mitochondria, in patient fibroblast cells, and in a mouse model. Results from Western blots and enzyme activity assays confirmed the import of TAT-PCC into mitochondria, as well as into patient fibroblasts, where the colocalization of imported TAT-PCC and mitochondria was also confirmed by confocal fluorescence microscopy. Furthermore, a single-dose intraperitoneal injection into PCC-deficient mice decreased the propionylcarnitine/acetylcarnitine (C3/C2) ratio toward the normal level. These results show that a cell-penetrating peptide can deliver active multimeric enzyme into mitochondria in vitro, in situ, and in vivo and push the size limit of intracellular delivery achieved so far. Our results are promising for other mitochondrion-specific deficiencies.

Methylmalonyl-coA epimerase deficiency: A new case, with an acute metabolic presentation and an intronic splicing mutation in the MCEE gene

Methylmalonyl-coA epimerase (MCE) follows propionyl-coA carboxylase and precedes methylmalonyl-coA mutase in the pathway converting propionyl-coA to succinyl-coA. MCE deficiency has previously been described in six patients, one presenting with metabolic acidosis, the others with nonspecific neurological symptoms or asymptomatic. The clinical significance and biochemical characteristics of this rare condition have been incompletely defined. We now describe a patient who presented acutely at 5 years of age with vomiting, dehydration, confusion, severe metabolic acidosis and mild hyperammonemia. At presentation, organic acid profiles were dominated by increased ketones and 3-hydroxypropionate, with moderately elevated methylcitrate and propionylglycine, and acylcarnitine profiles showed marked C3 (propionylcarnitine) elevation with normal C4DC (methylmalonylcarnitine + succinylcarnitine). Propionic acidemia was initially suspected, but it was subsequently noted that methylmalonic acid was mildly but persistently elevated in urine, and clearly elevated in plasma and cerebrospinal fluid. The overall biochemical profile prompted consideration of MCE deficiency. Studies on cultured fibroblasts showed moderately decreased propionate incorporation. Complementation analysis permitted assignment to the MCEE group. A heterozygous p.Arg47Ter (p.R47*) mutation in the MCEE gene was identified by sequencing of exons, and RNA studies identified a novel intronic splicing mutation, c.379-644A > G, confirming the diagnosis of MCE deficiency. Following the initial severe presentation, development has been normal and the clinical course over the subsequent six years has remained relatively uneventful on an essentially normal diet. This report contributes to the clinical and biochemical characterisation of this rare disorder, while highlighting potential causes of under-diagnosis or of diagnostic confusion.

Next generation sequencing of patients with mut methylmalonic aciduria: Validation of somatic cell studies and identification of 16 novel mutations

Mutations in the MUT gene, which encodes the mitochondrial enzyme methylmalonyl-CoA mutase, are responsible for the mut form of methylmalonic aciduria (MMA). In this study, a next generation sequencing (NGS) based gene panel was used to analyze 53 patients that had been diagnosed with mut MMA by somatic cell complementation analysis. A total of 54 different mutations in MUT were identified in 48 patients; 16 novel mutations were identified, including 1 initiation site mutation (c.2T>C [p.M1?]), 1 missense mutation (c.566A>T [p.N189I]), 2 nonsense mutations (c.129G>A [p.W43*] and c.1975C>T [p.Q659*]), 2 mutations affecting splice sites (c.753+3A>G and c.754-2A>G), 8 small insertions, deletions, and duplications (c.29dupT [p.L10Ffs*39], c.55dupG [p.V19Gfs*30], c.631_633delGAG [p.E211del], c.795_796insT [p.M266Yfs*7], c.1061delCinsGGA [p.S354Wfs*20], c.1065_1068dupATGG [p.S357Mfs*5], c.1181dupT [p.L394Ffs*30], c.1240delG [p.E414Kfs*17]), a large insertion (c.146_147ins279), and a large deletion involving exon 13. Phenotypic rescue and cDNA analysis were used to confirm that the c.146_147ins279 and c.631_633delGAG mutations were associated with the decreased methylmalonyl-CoA mutase function observed in the patient fibroblasts. In five patients, the NGS panel did not confirm the diagnosis made by complementation analysis. One of these patients was found to carry 2 novel mutations (c.433G > A [p.E145K] and c.511A>C [p.N171H]) in the SUCLG1 gene.

Inborn Error of Cobalamin Metabolism Associated with the Intracellular Accumulation of Transcobalamin-Bound Cobalamin and Mutations in ZNF143, Which Codes for a Transcriptional Activator

Vitamin B12 (cobalamin, Cbl) cofactors adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl) are required for the activity of the enzymes methylmalonyl-CoA mutase (MCM) and methionine synthase (MS). Inborn errors of Cbl metabolism are rare Mendelian disorders associated with hematological and neurological manifestations, and elevations of methylmalonic acid and/or homocysteine in the blood and urine. We describe a patient whose fibroblasts had decreased functional activity of MCM and MS and decreased synthesis of AdoCbl and MeCbl (3.4% and 1.0% of cellular Cbl, respectively). The defect in cultured patient fibroblasts complemented those from all known complementation groups. Patient cells accumulated transcobalamin-bound-Cbl, a complex which usually dissociates in the lysosome to release free Cbl. Whole-exome sequencing identified putative disease-causing variants c.851T>G (p.L284*) and c.1019C>T (p.T340I) in transcription factor ZNF143. Proximity biotinylation analysis confirmed the interaction between ZNF143 and HCFC1, a protein that regulates expression of the Cbl trafficking enzyme MMACHC. qRT-PCR analysis revealed low MMACHC expression levels both in patient fibroblasts, and in control fibroblasts incubated with ZNF143 siRNA.

Added value of next generation gene panel analysis for patients with elevated methylmalonic acid and no clinical diagnosis following functional studies of vitamin B12 metabolism

Next generation sequencing (NGS) based gene panel testing is increasingly available as a molecular diagnostic approach for inborn errors of metabolism. Over the past 40 years patients have been referred to the Vitamin B12 Clinical Research Laboratory at McGill University for diagnosis of inborn errors of cobalamin metabolism by functional studies in cultured fibroblasts. DNA samples from patients in which no diagnosis was made by these studies were tested by a NGS gene panel to determine whether any molecular diagnoses could be made. 131 DNA samples from patients with elevated methylmalonic acid and no diagnosis following functional studies of cobalamin metabolism were analyzed using the 24 gene extended cobalamin metabolism NGS based panel developed by Baylor Miraca Genetics Laboratories. Gene panel testing identified two or more variants in a single gene in 16/131 patients. Eight patients had pathogenic findings, one had a finding of uncertain significance, and seven had benign findings. Of the patients with pathogenic findings, five had mutations in ACSF3, two in SUCLG1 and one in TCN2. Thus, the NGS gene panel allowed for the presumptive diagnosis of 8 additional patients for which a diagnosis was not made by the functional assays.

Inborn Errors of Metabolism

Inborn errors of metabolism are single gene disorders resulting from the defects in the biochemical pathways of the body. Although these disorders are individually rare, collectively they account for a significant portion of childhood disability and deaths. Most of the disorders are inherited as autosomal recessive whereas autosomal dominant and X-linked disorders are also present. The clinical signs and symptoms arise from the accumulation of the toxic substrate, deficiency of the product, or both. Depending on the residual activity of the deficient enzyme, the initiation of the clinical picture may vary starting from the newborn period up until adulthood. Hundreds of disorders have been described until now and there has been a considerable clinical overlap between certain inborn errors. Resulting from this fact, the definite diagnosis of inborn errors depends on enzyme assays or genetic tests. Especially during the recent years, significant achievements have been gained for the biochemical and genetic diagnosis of inborn errors. Techniques such as tandem mass spectrometry and gas chromatography for biochemical diagnosis and microarrays and next-generation sequencing for the genetic diagnosis have enabled rapid and accurate diagnosis. The achievements for the diagnosis also enabled newborn screening and prenatal diagnosis. Parallel to the development the diagnostic methods; significant progress has also been obtained for the treatment. Treatment approaches such as special diets, enzyme replacement therapy, substrate inhibition, and organ transplantation have been widely used. It is obvious that by the help of the preclinical and clinical research carried out for inborn errors, better diagnostic methods and better treatment approaches will high likely be available.

High resolution melting analysis of the MMAB gene in cblB patients and in those with undiagnosed methylmalonic aciduria

Isolated methylmalonic aciduria (MMA) results either from a defect in the mitochondrial enzyme methylmalonylCoA mutase (MCM), or in the intracellular conversion of vitamin B12 (cobalamin) into its active coenzyme adenosylcobalamin (AdoCbl). Mutations in the MMAB gene affect the function of the enzyme ATP:cob(I)alamin adenosyltransferase (ATR) and the production of AdoCbl. Measurement of MCM function in cultured patient fibroblasts, followed by somatic cell complementation analysis in cases where MCM function is decreased, has classically been used to diagnose the cblB cobalamin disorder. A patient with persistent MMA, who could not be diagnosed using traditional somatic cell studies, was subsequently shown by sequencing in a clinical laboratory to contain two variants in the MMAB gene. This observation brings into question whether somatic cell studies have failed to diagnose other cblB patients with mild cellular phenotypes. A high resolution melting analysis (HRMA) assay was developed for the MMAB gene. It was used to scan 96 reference samples and two cohorts of patients: 42 patients diagnosed with cblB by complementation studies; and 181 patients with undiagnosed MMA. MMAB mutations, including one novel nonsense mutation (c.12 C>A [p.C4X]), were identified in all members of the cblB cohort. Four patients with undiagnosed MMA, including the index case described above, were found to contain variants in the MMAB gene: c.185C>T (p.T62M), c.394T>C (p.C132R), c.398C>T (p.S133F), c.521C>T (p.S174L), c.572G>A (p.R191Q). Only the index case was found to have two variants, suggesting that somatic cell studies diagnose almost all cblB patients.

High resolution melting analysis of the MMAA gene in patients with cblA and in those with undiagnosed methylmalonic aciduria

The gene product of MMAA is required for the intracellular metabolism of cobalamin (Cbl). Mutations in this gene lead to the cblA class of disorders, characterized by isolated methylmalonic aciduria. We have been concerned that somatic cell methods of diagnosis may miss patients with mild cellular phenotypes. A high resolution melting analysis (HRMA) assay was developed to rapidly scan the coding exons and flanking intronic regions of the MMAA gene for variants. DNA was scanned by HRMA from 96 unaffected reference individuals, 72 cblA patients confirmed by complementation, and 181 patients with isolated elevated methylmalonic acid, who could not be diagnosed using complementation analysis. Suspected variants were confirmed by Sanger sequencing. In the cblA cohort, HRMA correctly identified all previously known mutations as well as an additional 22 variants, 10 of which had not been previously reported. Novel variants included one duplication (c.551dupG, p.C187LfsX3), one deletion (c.387delC, p.Y129YfsX13), one splice site mutation (c.440-2A>G, splice site), 4 missense mutations (c.748G>A, p.E520K; c.820G>A, p.G274S; c.627G>T, p.R209S; c.826A>G, p.K276E), and 3 nonsense mutations (c.960G>A, p.W320X; c.1075C>T, p.E359X; c.1084C>T, p.Q362X). All novel missense variants affect highly conserved residues and are predicted to be damaging. Scanning of MMAA in the 181 undiagnosed samples revealed a single novel heterozygous missense change (c.821G>A, p.G274D).

Inborn errors of cobalamin absorption and metabolism

Derivatives of cobalamin (vitamin B(12)) are required for activity of two enzymes in humans. Adenosylcobalamin is required for activity of mitochondrial methylmalonylCoA mutase and methylcobalamin is required for activity of cytoplasmic methionine synthase. Deficiency in cobalamin, or inability to absorb cobalamin normally, can result in accumulation of methylmalonic acid and homocysteine in blood and urine. Methylmalonic acidemia can result in metabolic acidosis which in severe cases may be fatal. Hyperhomocysteinemia along with hypomethioninemia can result in hematologic (megaloblastic anemia, neutropenia, thrombocytopenia) and neurologic (subacute combined degeneration of the cord, dementia, psychosis) defects. Inborn errors affecting cobalamin absorption (inherited intrinsic factor deficiency, Imerslund–Gra¨ sbeck syndrome) and transport (transcobalamin deficiency) have been described. A series of inborn errors of intracellular cobalamin metabolism, designated cblA-cblG, have been differentiated by complementation analysis. These can give rise to isolated methylmalonic acidemia (cblA, cblB, cblD variant 2), isolated hyperhomocysteinemia (cblD variant 1, cblE, cblG) or combined methylmalonic acidemia and hyperhomocysteinemia (cblC, classic cblD, cblF). All these disorders are inherited as autosomal recessive traits. The genes underlying each of these disorders have been identified. Two other disorders, haptocorrin deficiency and transcobalamin receptor deficiency, have been described, but it is not clear that they have any consistent clinical phenotype.

Genetic disorders of vitamin B₁₂ metabolism: eight complementation groups--eight genes

Vitamin B12 (cobalamin, Cbl) is an essential nutrient in human metabolism. Genetic diseases of vitamin B12 utilisation constitute an important fraction of inherited newborn disease. Functionally, B12 is the cofactor for methionine synthase and methylmalonyl CoA mutase. To function as a cofactor, B12 must be metabolised through a complex pathway that modifies its structure and takes it through subcellular compartments of the cell. Through the study of inherited disorders of vitamin B12 utilisation, the genes for eight complementation groups have been identified, leading to the determination of the general structure of vitamin B12 processing and providing methods for carrier testing, prenatal diagnosis and approaches to treatment.

Clinical and molecular heterogeneity in patients with the cblD inborn error of cobalamin metabolism

OBJECTIVES

To describe 3 patients with the cblD disorder, a rare inborn error of cobalamin metabolism caused by mutations in the MMADHC gene that can result in isolated homocystinuria, isolated methylmalonic aciduria, or combined homocystinuria and methylmalonic aciduria.

STUDY DESIGN

Patient clinical records were reviewed. Biochemical and somatic cell genetic studies were performed on cultured fibroblasts. Sequence analysis of the MMADHC gene was performed on patient DNA.

RESULTS

Patient 1 presented with isolated methylmalonic aciduria, patient 3 with isolated homocystinuria, and patient 2 with combined methylmalonic aciduria and homocystinuria. Studies of cultured fibroblasts confirmed decreased synthesis of adenosylcobalamin in patient 1, decreased synthesis of methylcobalamin in patient 3, and decreased synthesis of both cobalamin derivatives in patient 2. The diagnosis of cblD was established in each patient by complementation analysis. Mutations in the MMADHC gene were identified in all patients.

CONCLUSIONS

The results emphasize the heterogeneous clinical, cellular and molecular phenotype of the cblD disorder. The results of molecular analysis of the MMADHC gene are consistent with the hypothesis that mutations affecting the N terminus of the MMADHC protein are associated with methylmalonic aciduria, and mutations affecting the C terminus are associated with homocystinuria.

Identification of a putative lysosomal cobalamin exporter altered in the cblF defect of vitamin B12 metabolism

Vitamin B(12) (cobalamin) is essential in animals for metabolism of branched chain amino acids and odd chain fatty acids, and for remethylation of homocysteine to methionine. In the cblF inborn error of vitamin B(12) metabolism, free vitamin accumulates in lysosomes, thus hindering its conversion to cofactors. Using homozygosity mapping in 12 unrelated cblF individuals and microcell-mediated chromosome transfer, we identified a candidate gene on chromosome 6q13, LMBRD1, encoding LMBD1, a lysosomal membrane protein with homology to lipocalin membrane receptor LIMR. We identified five different frameshift mutations in LMBRD1 resulting in loss of LMBD1 function, with 18 of the 24 disease chromosomes carrying the same mutation embedded in a common 1.34-Mb haplotype. Transfection of fibroblasts of individuals with cblF with wild-type LMBD1 rescued cobalamin coenzyme synthesis and function. This work identifies LMBRD1 as the gene underlying the cblF defect of cobalamin metabolism and suggests that LMBD1 is a lysosomal membrane exporter for cobalamin.

Causes of and diagnostic approach to methylmalonic acidurias

Several mutant genetic classes that cause isolated methylmalonic acidurias (MMAuria) are known based on biochemical, enzymatic and genetic complementation analysis. The mut(0) and mut(-) defects result from deficiency of MMCoA mutase apoenzyme which requires adenosyl-cobalamin (Ado-Cbl) as coenzyme. The cblA, cblB and the variant 2 form of cblD complementation groups are linked to processes unique to Ado-Cbl synthesis. The cblC, cblD and cblF complementation groups are associated with defective methyl-cobalamin synthesis as well. Mutations in the genes associated with most of these defects have been described. Recently a few patients have been described with mild MMAuria associated with mutations of the MMCoA epimerase gene or with neurological symptoms due to SUCL mutations. A comprehensive diagnostic approach involves investigations at the level of metabolites, genetic complementation analysis and enzymatic studies, and finally mutation analysis. MMA levels in urine range from 10-20 mmol/mol creatinine in mild disturbances of MMA metabolism to over 20000 mmol/mol creatinine in severe MMCoA mutase deficiency, but show considerable overlap and are of limited value for differential diagnosis. The underlying defect in isolated MMAuria can be characterized in cultured skin fibroblasts using several assays, e.g. conversion of propionate to succinate, specific activity of MMCoA, cobalamin adenosyltransferase assay, cellular uptake of CN-[(57)Co] cobalamin and its conversion to cobalamin coenzymes and complementation analysis. The reliable characterization of patients with isolated MMAuria pinpoints the correct gene for mutation analysis. Reliable classification of these patients is essential for ongoing and future prospective studies on treatment and outcome.

Gene identification for the cblD defect of vitamin B12 metabolism

BACKGROUND

Vitamin B12 (cobalamin) is an essential cofactor in several metabolic pathways. Intracellular conversion of cobalamin to its two coenzymes, adenosylcobalamin in mitochondria and methylcobalamin in the cytoplasm, is necessary for the homeostasis of methylmalonic acid and homocysteine. Nine defects of intracellular cobalamin metabolism have been defined by means of somatic complementation analysis. One of these defects, the cblD defect, can cause isolated methylmalonic aciduria, isolated homocystinuria, or both. Affected persons present with multisystem clinical abnormalities, including developmental, hematologic, neurologic, and metabolic findings. The gene responsible for the cblD defect has not been identified.

METHODS

We studied seven patients with the cblD defect, and skin fibroblasts from each were investigated in cell culture. Microcell-mediated chromosome transfer and refined genetic mapping were used to localize the responsible gene. This gene was transfected into cblD fibroblasts to test for the rescue of adenosylcobalamin and methylcobalamin synthesis.

RESULTS

The cblD gene was localized to human chromosome 2q23.2, and a candidate gene, designated MMADHC (methylmalonic aciduria, cblD type, and homocystinuria), was identified in this region. Transfection of wild-type MMADHC rescued the cellular phenotype, and the functional importance of mutant alleles was shown by means of transfection with mutant constructs. The predicted MMADHC protein has sequence homology with a bacterial ATP-binding cassette transporter and contains a putative cobalamin binding motif and a putative mitochondrial targeting sequence.

CONCLUSIONS

Mutations in a gene we designated MMADHC are responsible for the cblD defect in vitamin B12 metabolism. Various mutations are associated with each of the three biochemical phenotypes of the disorder.

Metabolic derangement of methionine and folate metabolism in mice deficient in methionine synthase reductase

Hyperhomocyst(e)inemia is a metabolic derangement that is linked to the distribution of folate pools, which provide one-carbon units for biosynthesis of purines and thymidylate and for remethylation of homocysteine to form methionine. In humans, methionine synthase deficiency results in the accumulation of methyltetrahydrofolate at the expense of folate derivatives required for purine and thymidylate biosynthesis. Complete ablation of methionine synthase activity in mice results in embryonic lethality. Other mouse models for hyperhomocyst(e)inemia have normal or reduced levels of methyltetrahydrofolate and are not embryonic lethal, although they have decreased ratios of AdoMet/AdoHcy and impaired methylation. We have constructed a mouse model with a gene trap insertion in the Mtrr gene specifying methionine synthase reductase, an enzyme essential for the activity of methionine synthase. This model is a hypomorph, with reduced methionine synthase reductase activity, thus avoiding the lethality associated with the absence of methionine synthase activity. Mtrr(gt/gt) mice have increased plasma homocyst(e)ine, decreased plasma methionine, and increased tissue methyltetrahydrofolate. Unexpectedly, Mtrr(gt/gt) mice do not show decreases in the AdoMet/AdoHcy ratio in most tissues. The different metabolite profiles in the various genetic mouse models for hyperhomocyst(e)inemia may be useful in understanding biological effects of elevated homocyst(e)ine.

Novel Mutations Found in Two Genes of Thai Patients with Isolated Methylmalonic Acidemia

Molecular genetic analysis of three patients diagnosed with isolated methylmalonic acidemia (MMA) revealed that one was mut (0) MMA, with a mutation in the MUT gene encoding the L: -methylmalonyl-CoA mutase (MCM), and two were cblB MMA, with mutations in the MMAB gene required for synthesizing the deoxyadenosylcobalamin cofactor of MCM. The mut (0) patient was homozygous for a novel nonsense mutation in MUT, p.R31X (c.167C --> T), and heterozygous for three previously described polymorphisms, p.K212K (c.712A --> G), p.H532R (c.1671A --> G), and p.V671I (c.2087G --> A). The new MMAB mutation, p.E152X (c.454G --> T), was found to be homozygous in one cblB patient and heterozygous in the other patient, who also had four intron polymorphisms in this gene.

Mitochondrial vitamin B12-binding proteins in patients with inborn errors of cobalamin metabolism

Inborn errors of vitamin B12 (cobalamin, Cbl) metabolism are autosomal recessive disorders and have been classified into nine distinct complementation classes (cblA-cblH and mut). Disorders affecting methylcobalamin metabolism cause megaloblastic anemia, which may be accompanied by leukopenia and thrombocytopenia, and a variety of neurological problems. Disorders affecting adenosylcobalamin cause methylmalonic acidemia and metabolic acidosis. Previous studies have shown that cobalamin binds to two enzymes in humans: methylmalonyl-CoA mutase in mitochondria and methionine synthase in the cytosol. In this study, cobalamin binding patterns were analyzed in crude mitochondrial fractions obtained from both control and patient fibroblasts that had been incubated with [57Co]cyanocobalamin. Crude mitochondrial fractions from control fibroblasts confirmed that the majority of [57Co]Cbl eluted with methylmalonyl-CoA mutase. However, in six of the nine disorders, at least one previously unidentified mitochondrial cobalamin binding protein was observed to bind [57Co]Cbl. The proportion of [57Co]Cbl that binds, is increased compared to controls when a deficiency in either adenosylcobalamin synthesis or utilization prevents binding to methylmalonyl-CoA mutase. Furthermore, unique cobalamin binding profiles emerged demonstrating how known mutations in these patients affect cobalamin binding to as yet unidentified proteins.

Spectrum of mutations in mut methylmalonic acidemia and identification of a common Hispanic mutation and haplotype

Cobalamin nonresponsive methylmalonic acidemia (MMA, mut complementation class) results from mutations in the nuclear gene MUT, which codes for the mitochondrial enzyme methylmalonyl CoA mutase (MCM). To better elucidate the spectrum of mutations that cause MMA, the MUT gene was sequenced in 160 patients with mut MMA. Sequence analysis identified mutations in 96% of disease alleles. Mutations were found in all coding exons, but predominantly in exons 2, 3, 6, and 11. A total of 116 different mutations, 68 of which were novel, were identified. Of the 116 different mutations, 53% were missense mutations, 22% were deletions, duplications or insertions, 16% were nonsense mutations, and 9% were splice-site mutations. Sixty-one of the mutations have only been identified in one family. A novel mutation in exon 2, c.322C>T (p.R108C), was identified in 16 of 27 Hispanic patients. SNP genotyping data demonstrated that Hispanic patients with this mutation share a common haplotype. Three other mutations were seen exclusively in Hispanic patients: c.280G>A (p.G94R), c.1022dupA, and c.970G>A (p.A324T). Seven mutations were seen almost exclusively in black patients, including the previously reported c.2150G>T (p.G717V) mutation, which was identified in 12 of 29 black patients. Two mutations were seen only in Asian patients. Some frequently identified mutations were not population-specific and were identified in patients of various ethnic backgrounds. Some of these mutations were found in mutation clusters in exons 2, 3, 6, and 11, suggesting a recurrent mutation.

Quantitative analysis of mitochondrial protein expression in methylmalonic acidemia by two-dimensional difference gel electrophoresis

Isolated methylmalonic acidemia (MMA) is a rare metabolic disease due to the deficient activity of L-methylmalonyl-CoA mutase (MCM). This mitochondrial enzyme converts L-methylmalonyl-CoA to succinyl-CoA using adenosylcobalamin (Adocbl) as cofactor. Isolated MMA is subdivided into five forms: mut MMA associated with MCM deficiency, three different defects related to mitochondrial Adocbl formation (cblA, cblB, and cblH), and cblD variant 2. We performed proteomic analysis on mitochondria from an individual with cblH/cblD disorder using 2-D DIGE to identify differentially expressed proteins in this disease. Comparative analysis of control/patient mitochondrial proteome allowed us to identify differential expression of 10 proteins. The most notable groups included proteins involved in apoptosis (cytochrome c), oxidative stress (manganese superoxide dismutase) and cell metabolism (succinyl-CoA ligase (GDP forming) and mitochondrial glycerophosphate dehydrogenase). Immunoblot analysis further validated 2-D DIGE results of two of these proteins in multiple MMA patients, suggesting that the differences in expression are a general effect in this disorder. It is feasible that the differential proteins identified in this study have a biological significance and might be related to the pathophysiology of MMA.

Homozygous nonsense mutation in the MCEE gene and siRNA suppression of methylmalonyl-CoA epimerase expression: a novel cause of mild methylmalonic aciduria

Methylmalonyl-CoA epimerase (MCE) catalyzes the interconversion of D- and L-methylmalonyl-CoA in the pathway responsible for the degradation of branched chain amino acids, odd chain-length fatty acids, and other metabolites. Despite the occurrence of metabolic disorders in the enzymatic step occurring immediately upstream of MCE (propionyl-CoA carboxylase) and downstream of MCE (adenosylcobalamin-dependent methylmalonyl-CoA mutase), no disease-causing mutations have been described affecting MCE itself. A patient, formerly identified as belonging to the cblA complementation group of vitamin B12 disorders but lacking mutations in the affected gene, MMAA, was tested for mutations in the MCEE gene. The patient's fibroblasts had normal levels of adenosylcobalamin compared to controls, whereas other cblA cell lines typically had reduced levels of the cofactor. As well, this patient had a milder form of methylmalonic aciduria than usually observed in cblA patients. The patient was found to be homozygous for a c.139C>T (p.R47X) mutation in MCEE by sequence analysis that was confirmed by restriction digestion of PCR products. One sibling, also with mild methylmalonic aciduria, was homozygous for the mutation. Both parents and one other sibling were heterozygous. A nearby insertion polymorphism, c.41-160_161insT, heterozygous in both parents, showed the wild-type configuration on the mutant alleles. To assess the impact of isolated MCE deficiency in cultured cells, HeLa cells were transfected with a selectable vector containing MCEE-specific small interfering RNA (siRNA) to suppress gene expression. The reduced level of MCEE mRNA resulted in the reduction of [14C]-propionate incorporation into cellular macromolecules. However, siRNA only led to a small reduction in pathway activity, suggesting that previously postulated non-enzymatic conversion of D- to L-methylmalonyl-CoA may contribute to some flux through the pathway. We conclude that the patient's MCEE defect was responsible for the mild methylmalonic aciduria, confirming a partial requirement for the enzymatic activity in humans.

Combined methylmalonic aciduria and homocystinuria (cblC): phenotype-genotype correlations and ethnic-specific observations

Methylmalonic aciduria and homocystinuria, cblC type (MIM 277400), is the most frequent inborn error of vitamin B12 (cobalamin, Cbl) metabolism, caused by an inability of the cell to convert Cbl to both of its active forms (MeCbl, AdoCbl). Although considered a disease of infancy, some patients develop symptoms in childhood, adolescence, or adulthood. The gene responsible for cblC, MMACHC, was recently identified. We studied phenotype-genotype correlations in 37 patients from published case-reports, representing most of the landmark descriptions of this disease. 25/37 had early-onset disease, presenting in the first 6 months of life: 17/25 were found to be either homozygous for the c.271dupA mutation (n=9) or for the c.331C>T mutation (n=3), or compound heterozygotes for these 2 mutations (n=5). 9/12 late-onset cases presented with acute neurological symptoms: 4/9 were homozygous for the c.394C>T mutation, 2/9 were compound heterozygotes for the c.271dupA and c.394C>T mutations, and 3/9, for the c.271dupA mutation and a missense mutation. Several observations on ethnic origins were noted: the c.331C>T mutation is seen in Cajun and French-Canadian patients and the c.394C>T mutation is common in the Asiatic-Indian/Pakistani/Middle Eastern populations. The recognition of phenotype-genotype correlations and the association of mutations with specific ethnicities will be useful for identification of disease-causing mutations in cblC patients, for carrier detection and prenatal diagnosis in families where mutations are known, and in setting up initial screening programs in molecular diagnostic laboratories. Further study into disease mechanism of specific mutations will help to understand phenotypic presentations and the overall pathogenesis in cblC patients.

Acquired and inherited disorders of cobalamin and folate in children

Cobalamin deficiency in the newborn usually results from cobalamin deficiency in the mother. Megaloblastic anaemia, pancytopenia and failure to thrive can be present, accompanied by neurological deficits if the diagnosis is delayed. Most cases of spina bifida and other neural tube defects result from maternal folate and/or cobalamin insufficiency in the periconceptual period. Polymorphisms in a number of genes involved in folate and cobalamin metabolism exacerbate the risk. Inborn errors of cobalamin metabolism affect its absorption, (intrinsic factor deficiency, Imerslund-Gräsbeck syndrome) and transport (transcobalamin deficiency) as well as its intracellular metabolism affecting adenosylcobalamin synthesis (cblA and cblB), methionine synthase function (cblE and cblG) or both (cblC, cblD and cblF). Inborn errors of folate metabolism include congenital folate malabsorption, severe methylenetetrahydrofolate reductase deficiency and formiminotransferase deficiency. The identification of disease-causing mutations in specific genes has improved our ability to diagnose many of these conditions, both before and after birth.

B12 trafficking in mammals: A for coenzyme escort service

Many coenzymes are vitamins that are assimilated in mammals into their active form from precursors obtained from the diet. They are often both rare and reactive rendering the likelihood low that the cell uses a collision-based strategy for their delivery to dependent enzymes. In humans, there are only two known B12 or cobalamin-dependent enzymes: methionine synthase and methylmalonyl-CoA mutase. However, the pathway for intracellular assimilation and utilization of this cofactor is complex as revealed by careful clinical analyses of fibroblasts from patients with disorders of cobalamin metabolism. In the recent past, six of the eight human genes involved in the B12 pathway have been identified and these have yielded important insights into their roles. The recent literature on the encoded proteins is reviewed, and a model for intracellular B12 trafficking is proposed in which B12 is escorted to its target proteins in the cytoplasmic and mitochondrial compartments in complex with chaperones, thereby averting problems of dilution and adventitious side reactions.

Mutation and biochemical analysis of patients belonging to the cblB complementation class of vitamin B12-dependent methylmalonic aciduria

Methylmalonic aciduria, cblB type (OMIM 251110) is an inborn error of vitamin B(12) metabolism that occurs due to mutations in the MMAB gene. MMAB encodes the enzyme ATP:cobalamin adenosyltransferase, which catalyzes the synthesis of the coenzyme adenosylcobalamin required for the activity of the mitochondrial enzyme methylmalonyl CoA mutase (MCM). MCM catalyzes the isomerization of methylmalonyl CoA to succinyl CoA. Deficient MCM activity results in methylmalonic aciduria and a susceptibility to life-threatening acidotic crises. The MMAB gene was sequenced from genomic DNA from a panel of 35 cblB patients, including five patients previously investigated. Nineteen MMAB mutations were identified, including 13 previously unknown mutations. These included 11 missense mutations, two duplications, one deletion, four splice-site mutations, and one nonsense mutation. None of these mutations was identified in 100 control alleles. Most of the missense mutations (9/11) were clustered in exon 7; many of these affected amino acid residues that are part of the probable active site of the enzyme. One previously described mutation, c.556C >T (p.R186W), was particularly common, accounting for 33% of pathogenic alleles. It was seen almost exclusively in patients of European background and was typically associated with presentation in the first year of life.

Identification of the gene responsible for methylmalonic aciduria and homocystinuria, cblC type

Methylmalonic aciduria and homocystinuria, cblC type (OMIM 277400), is the most common inborn error of vitamin B(12) (cobalamin) metabolism, with about 250 known cases. Affected individuals have developmental, hematological, neurological, metabolic, ophthalmologic and dermatologic clinical findings. Although considered a disease of infancy or childhood, some individuals develop symptoms in adulthood. The cblC locus was mapped to chromosome region 1p by linkage analysis. We refined the chromosomal interval using homozygosity mapping and haplotype analyses and identified the MMACHC gene. In 204 individuals, 42 different mutations were identified, many consistent with a loss of function of the protein product. One mutation, 271dupA, accounted for 40% of all disease alleles. Transduction of wild-type MMACHC into immortalized cblC fibroblast cell lines corrected the cellular phenotype. Molecular modeling predicts that the C-terminal region of the gene product folds similarly to TonB, a bacterial protein involved in energy transduction for cobalamin uptake.

Mutations in the MMAA gene in patients with the cblA disorder of vitamin B12 metabolism

Mutations in the MMAA gene on human chromosome 4q31.21 result in vitamin B12-responsive methylmalonic aciduria (cblA complementation group) due to deficiency in the synthesis of adenosylcobalamin. Genomic DNA from 37 cblA patients, diagnosed on the basis of cellular adenosylcobalamin synthesis, methylmalonyl-coenzyme A (CoA) mutase function, and complementation analysis, was analyzed for deleterious mutations in the MMAA gene by DNA sequencing of exons and flanking sequences. A total of 18 novel mutations were identified, bringing the total number of mutations identified in 37 cblA patients to 22. A total of 13 mutations result in premature stop codons; three are splice site defects; and six are missense mutations that occur at highly conserved residues. Eight of these mutations were common to two or more individuals. One mutation, c.433C>T (R145X), represents 43% of pathogenic alleles and a common haplotype was identified. Restriction endonuclease or heteroduplex diagnostic tests were designed to confirm mutations. None of the sequence changes identified in cblA patients were found in 100 alleles from unrelated control individuals.

Genetic and genomic systems to study methylmalonic acidemia

Methylmalonic acidemia (MMAemia) is the biochemical hallmark of a group of genetic metabolic disorders that share a common defect in the ability to convert methylmalonyl-CoA into succinyl-CoA. This disorder is due to either a mutant methylmalonyl-CoA mutase apoenzyme or impaired synthesis of adenosylcobalamin, the cofactor for this enzyme. In this article, we will provide an overview of the pathways disrupted in these disorders, discuss the known metabolic blocks with a particular focus on molecular genetics, and review the use of selected model organisms to study features of methylmalonic acidemia.

Genetic analysis of three genes causing isolated methylmalonic acidemia: identification of 21 novel allelic variants

Isolated methylmalonic aciduria (MMA) is an inborn error of metabolism due to the impaired isomerization of l-methylmalonyl-CoA to succinyl-CoA. This reaction is catalyzed by the mitochondrial protein methylmalonyl-CoA mutase (MCM, EC 5.4.99.2), an adenosylcobalamin-dependent enzyme. Four different forms of isolated MMA have been described: mut MMA associated with defects in the MCM apoenzyme, and phenotypically divided into two subtypes mut- and mut0 MMA, and three different defects involved in the synthesis of the active form of the cofactor adenosylcobalamin, termed cbl MMA, and classified into three different complementation groups cblA, cblB, and cblH associated with defects in the MMAA and MMAB genes and with an unidentified protein, respectively. In this work we describe the genetic analysis of 25 MMA patients, mainly from Spain. Using biochemical and cellular approaches our patients have been classified, identifying 13 mut MMA, 7 cblA, 2 cblB, and 3 noncblA, noncblB deficient patients. cDNA and genomic DNA sequence analysis of the MUT, MMAA, and MMAB genes have allowed us to identify 27 different changes, 21 novel ones. Among the missense mutations identified in the MUT gene only one, the c.970G>A (p.A324T) variant located in the substrate binding domain is likely a mut- mutation. The remaining missense mutations c.326A>G (p.Q109R), c.983T>C (p.L328P), c.1846C>T (p.R616C), and c.1850T>G (p.L617R) are probably mut0. In the MMAA patients analyzed, frameshift mutations are prevalent. We have explored the genotype-phenotype correlation for this clinically heterogeneous disease.

Towards a model to explain the intragenic complementation in the heteromultimeric protein propionyl-CoA carboxylase

Mutations in the PCCA or PCCB genes coding for alpha and beta subunits of propionyl CoA carboxylase can cause propionic acidemia. To understand the molecular basis of the intragenic complementation previously reported at the PCCB locus, we now examine the complementation behaviour of four carboxy-terminal and 11 amino-terminal naturally occurring mutant alleles both using cell fusion and reconstructing the complementation event by transfecting the mutant cDNAs to generate multimeric hybrid proteins. Alleles carrying mutations p.R410W and p.W531X are able to complement with 10 out of 11 amino-terminal mutations assayed. Only the unstable p.R512C, p.L519P and p.G112D mutants fail to complement. The results analyzed in the framework of the crystal structure of the homologous 12S transcarboxylase from Propionibacterium shermanii show that all mutant alleles studied are located at beta subunits interfaces, complementing alleles at the inter-trimer interface, where the catalysis probably happens, and non-complementing alleles at the intra-trimer interface, probably disrupting the trimer formation. Our results also show a remarkable stabilization effect when p.R410W is cotransfected with p.G246V. We propose a model for intragenic complementation requiring the production of two different beta subunits carrying carboxy and amino-terminal mutations that allow regenerating functional active sites and in which a stabilization effect between subunits could be relevant to ameliorate the biochemical phenotype of each mutation separately.

The cblD Defect Causes Either Isolated or Combined Deficiency of Methylcobalamin and Adenosylcobalamin Synthesis

Intracellular cobalamin is converted to adenosylcobalamin, coenzyme for methylmalonyl-CoA mutase and to methylcobalamin, coenzyme for methionine synthase, in an incompletely understood sequence of reactions. Genetic defects of these steps are defined as cbl complementation groups of which cblC, cblD (described in only two siblings), and cblF are associated with combined homocystinuria and methylmalonic aciduria. Here we describe three unrelated patients belonging to the cblD complementation group but with distinct biochemical phenotypes different from that described in the original cblD siblings. Two patients presented with isolated homocystinuria and reduced formation of methionine and methylcobalamin in cultured fibroblasts, defined as cblD-variant 1, and one patient with isolated methylmalonic aciduria and deficient adenosylcobalamin synthesis in fibroblasts, defined as cblD-variant 2. Cell lines from the cblD-variant 1 patients clearly complemented reference lines with the same biochemical phenotype, i.e. cblE and cblG, and the cblD-variant 2 cell line complemented cells from the mutant classes with isolated deficiency of adenosylcobalamin synthesis, i.e. cblA and cblB. Also, no pathogenic sequence changes in the coding regions of genes associated with the respective biochemical phenotypes were found. These findings indicate heterogeneity within the previously defined cblD mutant class and point to further complexity of intracellular cobalamin metabolism.

Human ATP:Cob(I)alamin adenosyltransferase and its interaction with methionine synthase reductase

The final step in the conversion of vitamin B(12) into coenzyme B(12) (adenosylcobalamin, AdoCbl) is catalyzed by ATP:cob(I)alamin adenosyltransferase (ATR). Prior studies identified the human ATR and showed that defects in its encoding gene underlie cblB methylmalonic aciduria. Here two common polymorphic variants of the ATR that are found in normal individuals are expressed in Escherichia coli, purified, and partially characterized. The specific activities of ATR variants 239K and 239M were 220 and 190 nmol min(-1) mg(-1), and their K(m) values were 6.3 and 6.9 mum for ATP and 1.2 and 1.6 mum for cob(I)alamin, respectively. These values are similar to those obtained for previously studied bacterial ATRs indicating that both human variants have sufficient activity to mediate AdoCbl synthesis in vivo. Investigations also showed that purified recombinant human methionine synthase reductase (MSR) in combination with purified ATR can convert cob(II)alamin to AdoCbl in vitro. In this system, MSR reduced cob(II)alamin to cob(I)alamin that was adenosylated to AdoCbl by ATR. The optimal stoichiometry for this reaction was approximately 4 MSR/ATR and results indicated that MSR and ATR physically interacted in such a way that the highly reactive reaction intermediate [cob(I)alamin] was sequestered. The finding that MSR reduced cob(II)alamin to cob(I)alamin for AdoCbl synthesis (in conjunction with the prior finding that MSR reduced cob(II)alamin for the activation of methionine synthase) indicates a dual physiological role for MSR.

Mutation analysis of the MMAA and MMAB genes in Japanese patients with vitamin B(12)-responsive methylmalonic acidemia: identification of a prevalent MMAA mutation

Methylmalonic acidemia (MMA) is caused by the deficient activity of l-methylmalonyl-CoA mutase, which is a vitamin B(12) (or cobalamin, Cbl)-dependent enzyme. MMA due to the effect of insufficient Cbl metabolism is classified into three forms (cblA, cblB, and cblH). Recently, the genes responsible for cblA and cblB were identified as MMAA and MMAB, respectively. The MMAA protein likely transports Cbl into the mitochondria for adenosylcobalamin synthesis, while the MMAB protein appears to be an adenosyltransferase. We performed a mutation analysis of 10 unrelated Japanese patients with vitamin B(12)-responsive MMA. Seven patients had mutations in MMAA, whereas the other three patients showed no disease-causing substitutions in either MMAA or MMAB. Five novel mutations were identified in MMAA (R22X, R145X, L217X, R359G, and 503delC). The 503delC mutation was observed in five of the seven MMAA patients, suggesting that the mutation is prevalent in Japanese patients. This finding may facilitate the DNA diagnosis of vitamin B(12)-responsive MMA within the Japanese population.

Identification of the molecular defect in patients with peroxisomal mosaicism using a novel method involving culturing of cells at 40°C: Implications for other inborn errors of metabolism

The peroxisome biogenesis disorders (PBDs), which comprise Zellweger syndrome (ZS), neonatal adrenoleukodystrophy, and infantile Refsum disease (IRD), represent a spectrum of disease severity, with ZS being the most severe, and IRD the least severe disorder. The PBDs are caused by mutations in one of the at least 12 different PEX genes encoding proteins involved in the biogenesis of peroxisomes. We report the biochemical characteristics and molecular basis of a subset of atypical PBD patients. These patients were characterized by abnormal peroxisomal plasma metabolites, but otherwise normal to very mildly abnormal peroxisomal parameters in cultured skin fibroblasts, including a mosaic catalase immunofluorescence pattern in fibroblasts. Since this latter feature made standard complementation analysis impossible, we developed a novel complementation technique in which fibroblasts were cultured at 40 degrees C, which exacerbates the defect in peroxisome biogenesis. Using this method, we were able to assign eight patients to complementation group 3 (CG3), followed by the identification of a single homozygous c.959C>T (p.S320F) mutation in their PEX12 gene. We also investigated various peroxisomal biochemical parameters in fibroblasts at 30 degrees C, 37 degrees C, and 40 degrees C, and found that all parameters showed a temperature-dependent behavior. The principle of culturing cells at elevated temperatures to exacerbate the defect in peroxisome biogenesis, and thereby preventing certain mutations from being missed, may well have a much wider applicability for a range of different inborn errors of metabolism.

MeaB is a component of the methylmalonyl-CoA mutase complex required for protection of the enzyme from inactivation

Adenosylcobalamin-dependent methylmalonyl-CoA mutase catalyzes the interconversion of methylmalonyl-CoA and succinyl-CoA. In humans, deficiencies in the mutase lead to methylmalonic aciduria, a rare disease that is fatal in the first year of life. Such inherited deficiencies can result from mutations in the mutase structural gene or from mutations that impair the acquisition of cobalamins. Recently, a human gene of unknown function, MMAA, has been implicated in methylmalonic aciduria (Dobson, C. M., Wai, T., Leclerc, D., Wilson, A., Wu, X., Dore, C., Hudson, T., Rosenblatt, D. S., and Gravel, R. A. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 15554-15559). MMAA orthologs are widespread in bacteria, archaea, and eukaryotes. In Methylobacterium extorquens AM1, a mutant defective in the MMAA homolog meaB was unable to grow on C(1) and C(2) compounds because of the inability to convert methylmalonyl-CoA to succinyl-CoA (Korotkova N., Chistoserdova, L., Kuksa, V., and Lidstrom, M. E. (2002) J. Bacteriol. 184, 1750-1758). Here we demonstrate that this defect is not due to the absence of adenosylcobalamin but due to an inactive form of methylmalonyl-CoA mutase. The presence of active mutase in double mutants defective in MeaB and in the synthesis of either R-methylmalonyl-CoA or adenosylcobalamin indicates that MeaB is necessary for protection of mutase from inactivation during catalysis. MeaB and methylmalonyl-CoA mutase from M. extorquens were cloned and purified in their active forms. We demonstrated that MeaB forms a complex with methylmalonyl-CoA mutase and stimulates in vitro mutase activity. These results support the hypothesis that MeaB functions to protect methylmalonyl-CoA mutase from irreversible inactivation.

Identification of the gene responsible for the cblA complementation group of vitamin B12-responsive methylmalonic acidemia based on analysis of prokaryotic gene arrangements

Vitamin B(12) (cobalamin) is an essential cofactor of two enzymes, methionine synthase and methylmalonyl-CoA mutase. The conversion of the vitamin to its coenzymes requires a series of biochemical modifications for which several genetic diseases are known, comprising eight complementation groups (cblA through cblH). The objective of this study was to clone the gene responsible for the cblA complementation group thought to represent a mitochondrial cobalamin reductase. Examination of bacterial operons containing genes in close proximity to the gene for methylmalonyl-CoA mutase and searching for orthologous sequences in the human genome yielded potential candidates. A candidate gene was evaluated for deleterious mutations in cblA patient cell lines, which revealed a 4-bp deletion in three cell lines, as well as an 8-bp insertion and point mutations causing a stop codon and an amino acid substitution. These data confirm that the identified gene, MMAA, corresponds to the cblA complementation group. It is located on chromosome 4q31.1-2 and encodes a predicted protein of 418 aa. A Northern blot revealed RNA species of 1.4, 2.6, and 5.5 kb predominating in liver and skeletal muscle. The deduced amino acid sequence reveals a domain structure, which belongs to the AAA ATPase superfamily that encompasses a wide variety of proteins including ATP-binding cassette transporter accessory proteins that bind ATP and GTP. We speculate that we have identified a component of a transporter or an accessory protein that is involved in the translocation of vitamin B(12) into mitochondria.

Transfection screening for defects in the PCCA and PCCB genes encoding propionyl-CoA carboxylase subunits

Propionic acidemia can result from mutations in the PCCA or PCCB genes encoding the alpha and beta subunits, respectively, of propionyl-CoA carboxylase. We have developed a method based on complementation of the enzyme defect using a lipid-mediated transient transfection of the normal human PCCA or PCCB cDNA into primary fibroblasts. We demonstrate the reliability of this method for identification of the defective PCC gene in order to unequivocally approach the mutational analysis in the corresponding PCCA and PCCB genes.

Methylmalonic Acid Measured in Plasma and Urine by Stable-Isotope Dilution and Electrospray Tandem Mass Spectrometry

Background: Liquid chromatography-tandem mass spectrometry (LC-MS/MS) with electrospray ionization is robust and allows accurate measurement of both low- and high-molecular weight components of complex mixtures. We developed a LC-MS/MS method for the analysis of methylmalonic acid (MMA), a biochemical marker for inherited disorders of propionate metabolism and acquired vitamin B12 deficiency.

Methods: We added 1 nmol of the internal standard MMA-d3 to 500 μL of plasma or 100 μL of urine before solid-phase extraction. After elution with 18 mol/L formic acid, the eluate was evaporated, and butyl ester derivatives were prepared with 3 mol/L HCl in n-butanol at 65 °C for 15 min. For separation, we used a Supelcosil LC-18, 33 × 4.6 mm column with 60:40 (by volume) acetonitrile:aqueous formic acid (1 g/L) as mobile phase. The transitions m/z 231 to m/z 119 and m/z 234 to m/z 122 were used in the selected reaction monitoring mode for MMA and MMA-d3, respectively. The retention time of MMA was 2.2 min in a 3.0-min analysis, without interference of a physiologically more abundant isomer, succinic acid.

Results: Daily calibrations between 0.25 and 8.33 nmol in 0.5 mL exhibited consistent linearity and reproducibility. At a plasma concentration of 0.12 μmol/L, the signal-to-noise ratio for MMA was 40:1. The regression equation for our previous gas chromatography-mass spectrometry (GC-MS) method (y) and the LC-MS/MS method (x) was: y = 1.030x − 0.032 (Sy|x = 1.03 μmol/L; n = 106; r = 0.994). Inter- and intraassay CVs were 3.8–8.5% and 1.3–3.4%, respectively, at mean concentrations of 0.13, 0.25, 0.60, and 2.02 μmol/L. Mean recoveries of MMA added to plasma were 96.9% (0.25 μmol/L), 96.0% (0.60 μmol/L), and 94.8% (2.02 μmol/L). One MS/MS system used only overnight (7.5 h) replaced two GC-MS systems (30 instrument-hours/day) to run 100–150 samples per day, with reductions of total cost (supplies plus equipment), personnel, and instrument time of 59%, 14%, and 75%, respectively.

Conclusions: This method is well suited for large-scale MMA testing (≥100 samples per day) where a shorter analytical time is highly desirable. Reagents are less expensive than the anion-exchange/cyclohexanol-HCl method, and sample preparation of batches up to 100 specimens is completed in less than 8 h and is automated.