Altered vitamin B12 metabolism in fibroblasts from a patient with megaloblastic anemia and homocystinuria due to a new defect in methionine biosynthesis

Homocystinuria diagnosis and management: it is not all classical

Homocystinuria (HCU) refers to a group of inherited disorders of homocysteine metabolism associated with high blood homocysteine concentration, thromboembolic tendency and neurocognitive symptoms. The most common causes of a high blood homocysteine relate to underlying vitamin B12 or folate deficiency which must be excluded first. Thereafter, an inherited metabolic condition can be considered.The most prevalent inherited disorder of homocysteine metabolism is classical HCU caused by deficiency of the pyridoxine-dependent enzyme, cystathione beta-synthase, which converts homocysteine to cystathionine in the transsulphuration pathway. An alternative route for homocysteine metabolism is its remethylation to methionine by the cobalamin-dependent enzyme, methionine synthase, using the folate derivative, methyltetrahydrofolate, as a methyl donor. Remethylation defects are caused by impaired activity of methionine synthase itself, of an enzyme required to generate its methylcobalamin cofactor from dietary vitamin B12, or of the enzyme methyltetrahydrofolate reductase (MTHFR), which generates the methyl donor.The correct diagnosis can be inferred from additional laboratory investigations including a complete blood count and quantitation of methionine and methylmalonic acid. Methionine is high/normal in HCU and low in the remethylation disorders. In the latter, cobalamin defects are readily distinguished from MTHFR by a coexisting macrocytic anaemia and further delineated by presence or absence of methylmalonic acid in urine or plasma.Lowering homocysteine reverses thromboembolic risk. In HCU, this may be achieved with pyridoxine alone or with betaine as an alternative methyl donor. Some patients additionally follow a methionine-restricted diet. Betaine is the primary treatment for MTHFR and cobalamin disorders are managed with high-dose hydroxocobalamin.

Long-term Outcome of 4 Patients With Transcobalamin Deficiency Caused by 2 Novel TCN2 Mutations

Cobalamin (vitamin B12 [Cbl]) is an essential cofactor for many biochemical pathways. Transcobalamin (TC) is required to internalize Cbl into the cells through membrane receptor-mediated endocytosis. Cbl is then processed in the cytoplasm and mitochondria by complementation factors leading to its active metabolites; methylcobalamin and 5-deoxyadenosyl-cobalamin. Deficiency of TC results in an elevation in methylmalonic acid and homocysteine. Patients usually present with macrocytic anemia, pancytopenia, failure to thrive, gastrointestinal symptoms, and neurological dysfunction. In this study, we report 4 patients from 2 unrelated families, with confirmed diagnosis of TC deficiency. Patients initially had a typical presentation of TC deficiency: severe diarrhea and vomiting, recurrent infections, stomatitis, macrocytic anemia, and neutropenia. Interestingly one of the patients was diagnosed at 3 months of age and developed ataxic gait related to cerebellar atrophy at the age of 14 months. His elder affected sibling was diagnosed at 5 months of age was completely normal. Two sibs, diagnosed at 2 months of age and immediately after birth, had autism spectrum disorder. Molecular investigations showed 2 novel mutations in TCN2 gene. Patients were treated and stayed stable on weekly injection of Cbl. In conclusion, TC deficiency has a wide heterogeneity in clinical phenotype, genotype, laboratory, and radiologic findings. Early detection of the disease and early initiation of aggressive parenteral treatment is probably associated with better prognosis and disease control.

Megaloblastic Anemias: Nutritional and Other Causes

Vitamin B₁₂ and folate deficiencies are major causes of megaloblastic anemia. Causes of B₁₂ deficiency include pernicious anemia, gastric surgery, intestinal disorders, dietary deficiency, and inherited disorders of B₁₂ transport or absorption. The prevalence of folate deficiency has decreased because of folate fortification, but deficiency still occurs from malabsorption and increased demand. Other causes include drugs and inborn metabolic errors. Clinical features of megaloblastic anemia include anemia, cytopenias, jaundice, and megaloblastic marrow morphology. Neurologic symptoms occur in B₁₂ deficiency, but not in folate deficiency. Management includes identifying any deficiency, establishing its cause, and replenishing B₁₂ or folate parenterally or orally.

The Neurology of Cobalamin

The following review indicates that the impact of cobalamin on neurologic disease extends far beyond the traditional myelopathy of classical pernicious anemia. The delineation of a broad spectrum of inherited disorders of cobalamin processing has served to illustrate and precisely define each step in the normal absorption, transport and intracellular metabolism of this essential vitamin. Recent clinical work has extended the boundaries of acquired cobalamin deficiency to encompass a variety of neuropsychiatric disturbances without identifiable concomitant hematologic derangements and emphasized the utility and sensitivity of new laboratory tests. These findings will demand increased vigilance from clinicians so that atypical and subtle cobalamin deficiency states will be readily diagnosed. The wide range of neurologic dysfunction observed in both inherited and acquired disorders of cobalamin metabolism challenges basic scientists to delineate cobalamin’s presumed important role in the normal development and homeostasis of the nervous system.

Lessons in biology from patients with inborn errors of vitamin B12 metabolism

BACKGROUND

Since 1975 cells lines from patients with suspected inborn errors of vitamin B12 metabolism have been referred to our laboratory because of elevations of homocysteine, methylmalonic acid, or both.

DESIGN

Cultured fibroblasts from patients were subjected to a battery of tests: incorporation of labelled propionate and methyltetrahydrofolate into cellular macromolecules, to test the functional integrity of methylmalonyl-CoA mutase and methionine synthase, respectively; uptake of labelled cyanocobalamin and synthesis of adenosylcobalamin and methylcobalamin; and, where applicable, complementation analysis.

RESULTS

This approach has allowed for the discovery of novel steps in the cellular transport and metabolism of vitamin B12, including those involving cellular uptake, the efflux of vitamin B12 from lysosomes, and the synthesis of adenosylcobalamin and methylcobalamin. For all of these disorders, the responsible genes have been discovered.

CONCLUSION

The study of highly selected patients with suspected inborn errors of metabolism has consistently resulted in the discovery of previously unknown metabolic steps and has provided new lessons in biology.

Functional variant in methionine synthase reductase intron-1 significantly increases the risk of congenital heart disease in the Han Chinese population

BACKGROUND

Homocysteine is known to be an independent risk factor for congenital heart disease (CHD). Methionine synthase reductase (MTRR) is essential for the adequate remethylation of homocysteine, which is the dominant pathway for homocysteine removal during early embryonic development.

METHODS AND RESULTS

Here, we report that the c.56+781 A>C (rs326119) variant of intron-1 of MTRR significantly increases the risk of CHD in the Han Chinese population. In 3 independent case-control studies involving a total of 2340 CHD patients and 2270 healthy control participants from different geographic areas, we observed that patients carrying the heterozygous AC and homozygous CC genotype had a 1.40-fold (odds ratio=1.40; P=2.32×10(-7)) and 1.84-fold (odds ratio=1.84; P=2.3×10(-11)) increased risk, respectively, of developing CHD than those carrying the wild-type AA genotype. Both in vivo quantitative real-time polymerase chain reaction analysis of MTRR mRNA in cardiac tissue samples from CHD patients and in vitro luciferase assays in transfected cells demonstrated that the c.56+781 C allele profoundly decreased MTRR transcription. Further analysis demonstrated that the c.56+781 C allele manifested reduced CCAAT/enhancer binding protein-α binding affinity. In addition, healthy individuals with the homozygous CC genotype had significantly elevated levels of plasma homocysteine compared with the wild-type AA carriers.

CONCLUSIONS

We have demonstrated that the MTRR c.56+781 A>C variant is an important genetic marker for increased CHD risk because this variant results in functionally reduced MTRR expression at the transcriptional level. Our results accentuate the significance of functional single-nucleotide polymorphisms in noncoding regions of the homocysteine/folate metabolism pathway core genes for their potential contributions to the origin of CHD.

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.

Positive newborn screen for methylmalonic aciduria identifies the first mutation in TCblR/CD320, the gene for cellular uptake of transcobalamin-bound vitamin B(12)

Elevated methylmalonic acid in five asymptomatic newborns whose fibroblasts showed decreased uptake of transcobalamin-bound cobalamin (holo-TC), suggested a defect in the cellular uptake of cobalamin. Analysis of TCblR/CD320, the gene for the receptor for cellular uptake of holo-TC, identified a homozygous single codon deletion, c.262_264GAG (p.E88del), resulting in the loss of a glutamic acid residue in the low-density lipoprotein receptor type A-like domain. Inserting the codon by site-directed mutagenesis fully restored TCblR function.

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.

Gene-nutrient interactions: importance of folic acid and vitamin B12 during early embryogenesis

The role that nutritional factors play in mammalian development has received renewed attention over the past two decades as the scientific literature has exploded with reports that folic acid supplementation in the periconceptional period can protect embryos from a number of highly significant malformations. As is often the case, the relationship between B vitamin supplementation and improved pregnancy outcomes is more complicated than initially perceived, as the interaction between nutritional factors and selected genes must be considered. In this review, we attempt to summarize the complex clinical and experimental literature on nutritional factors, their biological transport mechanisms, and interactions with genetic polymorphisms that impact early embryogenesis. While not exhaustive, our goal was to provide an overview of important gene-nutrient interactions, focusing on folic acid and vitamin B12, to serve as a framework for understanding the multiple roles they play in early embryogenesis.

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.

Polymorphic background of methionine synthase reductase modulates the phenotype of a disease-causing mutation

Methionine synthase reductase (MTRR) is the locus of the cblE class of inborn errors of cobalamin metabolism that is characterized by megaloblastic anemia and homocystinuria. Two highly prevalent SNPs, c.66A>G (p.Ile22Met) and c.524C>T (p.Ser175Leu), are found in the MTRR gene. On the basis of the allele frequency of these amino acids and sequence comparison with members of the same family of proteins, the p.Ile22/p.Ser175 sequence is designated as wild type. While characterizing a pathogenic methionine synthase reductase (MSR) mutation, c.166G>A (p.Val56Met), we discovered an interaction between the mutation and one of the polymorphic sites. Thus, when the p.Val56Met mutant was initially expressed in the p.Ile22/p.Ser175 background, we were surprised to find that kinetically, it was virtually indistinguishable from wild-type protein. To determine if the polymorphisms interacted with the p.Val56Met mutation, it was expressed in all four possible genetic backgrounds. We found that in the p.Ile22Met background, the p.Val56Met mutation impacted the kinetics of MSR and an approximately three- to 10-fold higher concentration of the p.Ile22Met/p.Val56Met mutant was required for maximal activation of methionine synthase vs. the range seen with wild-type MSR variants. A comparable (three- to seven-fold) diminution in MSR activity was observed in extracts of fibroblast cells from patients carrying the p.Val56Met mutation on one MSR allele and a null mutation on the other. These results predicted that the patient allele encodes the p.Val56Met mutation and the p.Ile22Met variation, which was confirmed by sequence analysis. This study reveals how a genetic variation can modulate phenotypic expression of a disease-causing mutation.

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.

cblE type of homocystinuria due to methionine synthase reductase deficiency: functional correction by minigene expression

The cblE type of homocystinuria is a rare autosomal recessive disorder caused by impaired reductive activation of methionine synthase. Although earlier biochemical studies proposed that the methionine synthase enzyme might be activated by two different reducing systems, mutations were reported in only the methionine synthase reductase gene (MTRR) in cblE patients. The pathogenicity of MTRR mutations, however, has not yet been tested functionally. We report on nine patients of European origin affected by the cblE type of homocystinuria. They presented between 2 weeks and 3 years of age (median age 4 weeks) with anemia, which was macrocytic in only three patients, and with neurological involvement in all but two cases. Bone marrow examination performed in seven patients showed megaloblastic changes in all but one of them. All patients exhibited moderate to severe hyperhomocysteinemia (median plasma total homocysteine [Hcy] 92 mumol/L, range 44-169), while clearly reduced methionine was observed only in four cases. Pathogenic mutations were identified in both parental alleles of the MTRR gene in all patients. Five known (c.903+469T>C, c.1361C>T, c.1459G>A, c.1557-4_1557+3del7, and c.1622_1623dupTA) and three novel mutations (c.7A>T, c.1573C>T, and c.1953-6_1953-2del5) were detected. Importantly, transfection of fibroblasts of cblE patients with a wild-type MTRR minigene expression construct resulted in a significant approximately four-fold increase of methionine synthesis, indicating correction of the enzyme defect. Our study shows a link between a milder predominantly hematological presentation and homozygosity for the c.1361C>T mutation, but no other obvious genotype-phenotype correlation. The identification of mutations in the MTRR gene, together with restoration of methionine synthesis following MTRR minigene expression in cblE cells confirms that this disease is caused by defects in the MTRR gene.

Polymorphisms of methionine synthase and methionine synthase reductase and risk of lung cancer: a case-control analysis

Although tobacco is the major lung cancer risk factor, folate deficiency has also been implicated as a risk. Methionine synthase (MS; gene symbol, MTR) and methionine synthase reductase (MSR; gene symbol, MTRR) play important roles in the folate metabolism pathway. It was hypothesized that polymorphisms of MTR and MTRR are associated with lung cancer risk and interact with dietary intake of folate-related nutrients in lung cancer etiology. In a hospital-based, case-control study of 1,035 lung cancer cases and 1,148 controls of non-Hispanic whites, frequency matched by age, sex, ethnicity and smoking status, the MTR 2756A>G and MTRR 66A>G polymorphisms were genotyped. It was found that the MTRRG allele was associated with a significantly increased lung cancer risk [adjusted odd ratio (OR)=1.34, 95% confidence interval (CI)=1.06-1.70 for the AG genotype and OR=1.39, 95% CI=1.08-1.78 for the GG genotype compared to the AA genotype]. Further analysis suggested some evidence of gene-diet interactions between the MTRR 66A>G polymorphism and dietary intake of total folate and vitamin B12. When the two polymorphisms were evaluated together by the number of the variant alleles (i.e. the MTR2756G and MTRR66A), lung cancer risk was significantly increased in a dose-dependent manner (Ptrend=0.045). The risk of lung cancer was 1.29 (0.98-1.69) for one variant allele, and 1.36 (1.04-1.77) for two or more variant alleles compared to the wild-type (0 variant allele) genotype. In conclusion, our data provide evidence supporting the association between the MTR 2756A>G and MTRR 66A>G polymorphisms and lung cancer risk, which may be modulated by dietary nutrient intake.

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.

Adenosyltransferase: an enzyme and an escort for coenzyme B12?

Many organic cofactors are both rare and reactive. They are usually in low abundance, which poses problems for efficient collision-based targeting to dependent enzymes, whereas their reactivity is problematic for side reactions. Sequestration and escorted delivery presents one solution to this conundrum, but such porters, if they exist, are mostly unknown. In humans, the mitochondrial enzyme methylmalonyl-coenzyme A mutase uses coenzyme B(12) (adenosylcobalamin) but would be inactive if bound to the cofactor precursor that is delivered to the mitochondrion. Adenosyltransferase converts cob(II)alamin to coenzyme B(12). Based on kinetic evidence for interaction between the two enzymes, the 40-fold greater affinity for coenzyme B(12) and the higher coordination number for cobalt in the mutase, we propose that the adenosyltransferase is a dual-function protein: an enzyme that synthesizes coenzyme B(12) and a chaperone that delivers it.

Pathobiology and genetics of neural tube defects

PURPOSE

Neural tube defects (NTDs), including spina bifida and anencephaly, are common congenital malformations that occur when the neural tube fails to achieve proper closure during early embryogenesis. Based on epidemiological and clinical data obtained over the last few decades, it is apparent that these multifactorial defects have a significant genetic component to their etiology that interacts with specific environmental risk factors. The purpose of this review article is to synthesize the existing literature on the genetic factors contributing to NTD risk.

RESULTS

To date, there is evidence that closure of the mammalian neural tube initiates and fuses intermittently at four discrete locations. Disruption of this process at any of these four sites may lead to an NTD, possibly arising through closure site-specific genetic mechanisms. Candidate genes involved in neural tube closure include genes of the folate metabolic pathway, as well as those involved in folate transport.

CONCLUSIONS

Although extensive efforts have focused on elucidating the genetic risk factors contributing to the etiology of NTDs, the population burden for these malformations remains unknown. One group at high risk for having children with NTDs is epileptic women receiving antiepileptic medications during pregnancy. Efforts to better understand the genetic factors that may contribute to their heightened risk, as well as the pathogenesis of neural tube closure defects, are reviewed herein.

Modulation of intracellular Ca(2+) concentration by vitamin B12 in rat thymocytes

We have studied several novel effects of vitamin B12 (cyanocobalamin) on cellular Ca(2+) homeostasis in rat thymocytes. We determined the effect of various concentrations of vitamin B12 on intracellular Ca(2+) concentration ([Ca(2+)]i) and parameters of Ca(2+)in signaling using the fluorescent dye Fura-2. The basal [Ca(2+)]i in Ca(2+)-containing media was 115 +/- 5 nM but in vitamin B12 (10 nM)-treated thymocytes [Ca(2+)]i was decreased to 60 +/- 15 nM (mean +/- SEM) during the first 5 min. The decline in [Ca(2+)]i was accompanied by an increase in the endoplasmic reticulum Ca(2+) store, presumably as a result of Ca-ATPase activation. At the same time 100 nM-10 mM B12 induced the accumulation of Ca(2+) in mitochondria. Somewhat higher concentrations of B12 (1-10 microM) had no effect on [Ca(2+)]i. A further increase in B12 concentration with range from 50 microM to 1 mM caused a dose-dependent elevation of [Ca(2+)]i from the basal level (115 +/- 5 nM) up to 200 +/- 50 nM in thymocytes, and this elevation was partially blocked in Ca(2+)-free media. This high concentration of vitamin B12 caused a gradual decrease of endoplasmic reticulum Ca(2+) stores by means of Ca-ATPase inhibition. The B12-induced increase in [Ca(2+)]i was not observed after depletion of intracellular Ca(2+) stores, induced by addition of 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ), an inhibitor of endoplasmic reticulum Ca (2+)-ATPase, concanavalin A, or arachidonic acid. These studies show that vitamin B12 regulates [Ca(2+)]i via several different mechanisms at different B12 concentrations. Participation of G proteins and calmodulin activity in B12-mediated [Ca(2+)]i increase is discussed.

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

AIM

To investigate genetic heterogeneity within the cblA class of inborn error of cobalamin metabolism.

CONTEXT

The cblA disorder is characterised by vitamin B12 (cobalamin) responsive methylmalonic aciduria and deficient synthesis of adenosylcobalamin, required for activity of the mitochondrial enzyme methylmalonyl CoA mutase. The cblA gene has not been identified or cloned. We have previously described a patient with the clinical and biochemical phenotype of the cblA disorder whose fibroblasts complemented cells from patients with all known types of inborn error of adenosylcobalamin synthesis, including cblA.

METHODS

We have performed somatic cell complementation analysis of the cblA variant fibroblast line with a panel of 28 cblA lines. We have also performed detailed complementation analysis on a panel of 10 cblA fibroblast lines, not including the cblA variant line.

RESULTS

The cblA variant line complemented all 28 cell lines of the panel. There was evidence for interallelic complementation among the 10 cblA lines used for detailed complementation analysis; no cell line in this panel complemented all other members.

CONCLUSIONS

These results strongly suggest that the cblA variant represents a novel complementation class, which we have designated cblH and which represents a mutation at a distinct gene. They also suggest that the cblA gene encodes a protein that functions as a multimer, allowing for extensive interallelic complementation.

Lethal late onset cblB methylmalonic aciduria

OBJECTIVE

To alert the physicians to the possibility of a late-onset inborn error of metabolism in an apparently previously healthy patient with acute clinical presentation.

DESIGN

Case report.

SETTING

Pediatric unit and general intensive care unit.

PATIENT

An apparently previously healthy 12-yr-old female presented acutely with vomiting, fever, bronchopneumonia, and progressive loss of consciousness associated with ketoacidosis, hyperglycemia, and hyperammonemia. She died 3 days later with a diagnosis of insulin-dependent diabetes mellitus.

INTERVENTIONS

Intravenous hydration, glucose and insulin, mechanical ventilation.

MEASUREMENTS AND MAIN RESULTS

Organic acid analysis on a postmortem sample of aqueous humor revealed high levels of methylmalonic acid. Enzymatic studies on cultured fibroblasts were consistent with the diagnosis of cblB methylmalonic aciduria.

CONCLUSIONS

The diagnosis of cblB methylmalonic aciduria was made in a postmortem patient who died with a misdiagnosis of insulin-dependent diabetes mellitus. Unclear biochemical findings and positive family history should strongly lead to suspicion of an inborn error of metabolism in an apparently previously healthy critically ill patient.

Molecular cloning, expression and physical mapping of the human methionine synthase reductase gene

Methionine synthase reductase (EC 2.1.1.135) is a flavoprotein essential for maintenance of methionine synthase in an active state. We characterized the human gene for methionine synthase reductase (MTRR). The gene is approximately 34kb and comprises 15 exons, varying in size from 43 to 1213bp, and 14 introns whose sizes vary from 108bp to 5kb. The positions of several junctions are conserved between the MTRR gene and the C. elegans ortholog, as well as with the rat cytochrome P450 reductase gene. A 1.3kb CpG island encompasses the 5'-flanking region and exon 1 and extends into intron 1. A short region including the transcription start site is sufficient to confer promoter activity, with a better outcome when accompanied by intron 1. The promoter region contains putative binding sites for Sp1, AP-1, AP-2 as well as CAAT motifs, but no consensus TATA box. Primer extension analysis revealed a single major transcription start site, located 137bp upstream of the previously reported initiator ATG. An alternative splicing event involving a portion of exon 1 predicts that translation can potentially be initiated at two different ATG codons. The gene was physically assigned to a narrow area between markers WI1755 and D5S1957.

Transcobalamin II deficiency with methylmalonic aciduria in three sisters

Transcobalamin II (TC II) is a plasma protein that binds vitamin B12 (cobalamin, Cbl) and facilitates cellular Cbl uptake by receptor-mediated endocytosis. In autosomal recessive TC II deficiency, intracellular Cbl deficiency results in an early onset of megaloblastic anaemia that may be accompanied by neurological abnormalities. Inadequate treatment may lead to neurological abnormalities. We describe three sisters, the daughters of first cousins of Moroccan origin, with TC II deficiency requiring continuous and long-term vitamin B12 treatment. The diagnosis was suspected from the finding of low unsaturated vitamin B12 binding capacity and confirmed by absence of detectable TC II by radioimmunoassay and by inability of cultured fibroblasts to synthesize TC II.

Progressive neurological deterioration and MRI changes in cblC methylmalonic acidaemia treated with hydroxocobalamin

Cobalamin C (cblC) defects result in decreased activity of both methylmalonyl-CoA mutase and N5-methyltetrahydrofolate:homocysteine methyltransferase (methionine synthase), with subsequent methylmalonic acid-uria and homocystinuria. Patients typically show failure to thrive, developmental delay and megaloblastic anaemia. Vitamin B12 therapy has been beneficial in some cases. We report a now 4-year-old Hispanic girl with cblC disease documented by complementation analysis, with progressive neurological deterioration and worsening head MRI changes while on intramuscular hydroxocobalamin begun at age 3 weeks. Oral carnitine and folic acid were added at age 1 year. Blood levels of methylmalonic acid were reduced to treatment ranges. In the absence of acute metabolic crises, she developed microcephaly, progressive hypotonia and decreased interactiveness. Funduscopic examination was normal at age 13 months. At age 19 months, she developed nystagmus, and darkly pigmented fundi and sclerotic retinal vessels were observed on examination. Her neonatal head MRI was normal. By age 1 year, the MRI showed diffuse white-matter loss with secondary third and lateral ventricle enlargement, a thin corpus callosum, and normal basal ganglia. At age 15 months, progression of the white-matter loss, as well as hyperintense globi pallidi, were present. Interval progression of both grey- and white-matter loss was seen at age 27 months. We therefore caution that progressive neurological deterioration and head MRI abnormalities may still occur in cblC disease, despite early initiation of hydroxocobalamin therapy and improvement in toxic metabolite concentrations in physiological fluids.

Outcome of individuals with low-moderate methylmalonic aciduria detected through a neonatal screening program

BACKGROUND

The clinical spectrum of methylmalonic aciduria (MMAuria) ranges from severe, neonatal acidosis to benign asymptomatic organic aciduria. In 1975, screening for MMAuria was established in the province of Quebec. Although newborn screening programs facilitate presymptomatic detection and treatment and also detect asymptomatic variants, uncertainties about potential long-term hazards of mild to moderate elevations of MMA create concern. The objective of this study was to examine the outcome of individuals excreting low to intermediate quantities of MMA, ascertained by a newborn screening program.

RESULTS AND STUDY DESIGN

One hundred and thirty-six individuals with elevations of urinary MMA were initially identified by the screening program; 122 individuals were noted to have excretion of urinary MMA <1400 micromol/mmol creatinine. At follow-up assessment at 1 year of age, in 65 of these 122 individuals, the MMA excretion had resolved. Of the remaining individuals, 9 were lost to follow-up, 13 had symptoms, and the remaining 35 were free of symptoms. Among the 35 individuals with asymptomatic persistent MMAuria, MMA excretion has resolved in 13 over 1 year; 22 individuals exhibit persistent low-moderate MMAuria (range, 210 to 1133 micromol/mmol creatinine).

CONCLUSION

Follow-up examination of individuals in the latter asymptomatic cohort with persistent low-moderate MMAuria indicates normal somatic and cognitive outcomes.

Purification of soluble cytochrome b5 as a component of the reductive activation of porcine methionine synthase

In mammals, methionine synthase plays a central role in the detoxification of the rogue metabolite homocysteine. It catalyzes a transmethylation reaction in which a methyl group is transferred from methyltetrahydrofolate to homocysteine to generate tetrahydrofolate and methionine. The vitamin B12 cofactor cobalamin plays a direct role in this reaction by alternately accepting and donating the methyl group that is in transit from one substrate (methyltetrahydrofolate) to another (homocysteine). The reactivity of the cofactor intermediate cob(I)alamin renders the enzyme susceptible to oxidative damage. The oxidized enzyme may be returned to the catalytic turnover cycle via a reductive methylation reaction that requires S-adenosylmethionine as a methyl group donor, and a source of electrons. In this study, we have characterized an NADPH-dependent pathway for the reductive activation of porcine methionine synthase. Two proteins are required for the transfer of electrons from NADPH, one of which is microsomal and the other cytoplasmic. The cytoplasmic protein has been purified to homogeneity and is soluble cytochrome b5. It supports methionine synthase activity in the presence of NADPH and the microsomal component in a saturable manner. In addition, purified microsomal cytochrome P450 reductase and soluble cytochrome b5 reconstitute the activity of the porcine methionine synthase. Identification of soluble cytochrome b5 as a member of the reductive activation system for methionine synthase describes a function for this protein in non-erythrocyte cells. In erythrocytes, soluble cytochrome b5 functions in methemoglobin reduction. In addition, it identifies an additional locus in which genetic polymorphisms may play a role in the etiology of hyperhomocysteinemia, which is correlated with cardiovascular diseases.

Homocysteine and vitamins in cardiovascular disease

On the basis of recent retrospective and prospective studies, it is now widely accepted that increased total plasma homocysteine is a risk factor for cardiovascular disease. Impaired enzyme function as a result of genetic mutation or deficiency of the essential B vitamins folic acid, B12, and B6 can lead to hyperhomocysteinemia. Oxidized forms of homocysteine account for 98–99% of total plasma homocysteine. Although there is uncertainty as to whether increased homocysteine is causal or merely a proxy for cardiovascular disease, several lines of evidence suggest that it may play a role in atherothrombotic disease. Homocysteine appears to alter the anticoagulant properties of endothelial cells to a procoagulant phenotype. Mildly increased homocysteine causes dysfunction of the vascular endothelium. Folic acid effectively lowers homocysteine concentration in the plasma. Intervention studies are urgently needed to determine if lowering homocysteine is effective in decreasing the morbidity and mortality of cardiovascular disease.

Cloning and mapping of a cDNA for methionine synthase reductase, a flavoprotein defective in patients with homocystinuria

Methionine synthase catalyzes the remethylation of homocysteine to methionine via a reaction in which methylcobalamin serves as an intermediate methyl carrier. Over time, the cob(I)alamin cofactor of methionine synthase becomes oxidized to cob(II)alamin rendering the enzyme inactive. Regeneration of functional enzyme requires reductive methylation via a reaction in which S-adenosylmethionine is utilized as a methyl donor. Patients of the cblE complementation group of disorders of folate/cobalamin metabolism who are defective in reductive activation of methionine synthase exhibit megaloblastic anemia, developmental delay, hyperhomocysteinemia, and hypomethioninemia. Using consensus sequences to predicted binding sites for FMN, FAD, and NADPH, we have cloned a cDNA corresponding to the "methionine synthase reductase" reducing system required for maintenance of the methionine synthase in a functional state. The gene MTRR has been localized to chromosome 5p15.2-15.3. A predominant mRNA of 3.6 kb is detected by Northern blot analysis. The deduced protein is a novel member of the FNR family of electron transferases, containing 698 amino acids with a predicted molecular mass of 77,700. It shares 38% identity with human cytochrome P450 reductase and 43% with the C. elegans putative methionine synthase reductase. The authenticity of the cDNA sequence was confirmed by identification of mutations in cblE patients, including a 4-bp frameshift in two affected siblings and a 3-bp deletion in a third patient. The cloning of the cDNA will permit the diagnostic characterization of cblE patients and investigation of the potential role of polymorphisms of this enzyme as a risk factor in hyperhomocysteinemia-linked vascular disease.

Folate-responsive homocystinuria and megaloblastic anaemia in a female patient with functional methionine synthase deficiency (cblE disease)

This first detailed report of a female patient with functional methionine synthase deficiency due to the cblE defect describes treatment with several vitamins and cofactors and clinical progress for 17 years. Before treatment, major findings were microcephaly, psychomotor retardation, episodic reduced consciousness, megaloblastic anaemia, increased plasma free homocystine (> 20 mumol/L), low plasma methionine (< 10 mumol/L) and increased excretion of formiminoglutamate. On high-dose folic acid, biochemical abnormalities such as formiminoglutamate excretion and homocystinuria nearly normalized, but clinical and haematological abnormalities remained. On replacement of folate with methylcobalamin, alertness, motor function, speech and the electroencephalogram improved, biochemical features were similar, but the mean corpuscular volume increased. The best control was observed on a combination of folate and methylcobalamin. At 17 years of age she remains severely mentally retarded. In cultured fibroblasts methionine synthesis was reduced (0.03 nmol/mg/per 16 h, controls 2.4-6.9); methionine synthase activity was normal under high reducing conditions but decreased on limiting the reducing agent, dithiothreitol, to 5 mmol/L (18% of total, controls 51-81%); formation of methylcobalamin was low (4.5% of total cobalamins, control 57.5%) and complementation studies indicated the cblE defect. Methionine formation showed only minor increases in cells grown in folate- or cobalamin-supplemented medium. Serine synthesis, which was low in normal medium, increased with cobalamin supplementation. These studies suggest further heterogeneity within cblE mutants, show the difficulty of establishing the enzyme defect in vitro, and indicate a role for folate in addition to cobalamin in treatment.

Functional methionine synthase deficiency due to cblG disorder: a report of two patients and a review

Functional methionine synthase deficiency due to abnormal methylcobalamin metabolism causes megaloblastic anemia, moderate to severe developmental delay, lethargy, and anorexia in association with homocystinuria. Patients with this disorder of cobalamin metabolism can be classified into two separate groups, cblE or cblG, primarily on the basis of complementation analysis with cultured skin fibroblasts. We describe two unrelated boys, ages 3 and 5 years, with the cblG defect in methylcobalamin synthesis. Both children presented with severe developmental delay, lethargy, anorexia, and megaloblastic anemia. The diagnosis of homocystinuria was delayed in each case due to difficulties with detection of small amounts of homocystine in physiologic samples. The clinical course of cblG disease is favorably altered by treatment with intramuscular hydroxycobalamin. Megaloblastosis in the presence of adequate supplies of cobalamin and folate in the blood must alert the clinician to the possibility of functional methionine synthase deficiency and should prompt a careful search for associated biochemical hallmarks, including homocystinuria/emia.

Defects in auxiliary redox proteins lead to functional methionine synthase deficiency

Methionine synthase catalyzes a methyl transfer reaction from methyltetrahydrofolate to homocysteine to form methionine and tetrahydrofolate and is dependent on methylcobalamin, a derivative of vitamin B12, for activity. Due to the lability of the intermediate, cob(I)alamin, the activity of methionine synthase is additionally dependent on a redox activation system. In bacteria, two flavoproteins, NADPH-flavodoxin reductase and flavodoxin, shuttle electrons from NADPH to methionine synthase. Their mammalian counterparts are unknown, and a putative intrinsic thiol oxidase activity of the mammalian methionine synthase has been proposed to be involved. We demonstrate that the mammalian methionine synthase can be activated in an NADPH-dependent reaction and requires a minimum of two redox proteins. This model is consistent with our results from biochemical complementation studies between cblG and cblE cell lines and mutation detection analysis in cblG cell lines. These demonstrate that the cblG cell line has defects affecting methionine synthase directly, whereas the cblE cell line has defects in the redox proteins. We have also identified a P1173L mutation in the activation domain of methionine synthase in the cblG cell line WG1505.

Response of the methionine synthase system to short-term culture with homocysteine and nitrous oxide and its relation to methionine dependence

We compared the metabolic response of a methionine(Met)-dependent (P60) human glioma cell line with that of a Met-independent variant (P60H) when cultured in a homocysteine (Hcy) medium and exposed to N2O. In Hcy medium (without Met), remethylation of Hcy in P60H cells was enhanced and supported growth, whereas remethylation was low in P60 cells, which failed to thrive under these conditions. Both cell types seemed to contain adequate amounts of folates and total cobalamin (Cbl). P60 cells showed increased total and methylcobalamin (CH3Cbl) content after the shift to a Hcy medium, but the high, stable level of CH3Cbl detected in P60H cells was not attained. Further metabolic differences were induced by N2O exposure, which markedly reduced Met-synthase activity in cell-free extracts in both cell lines and completely blocked intact-cell Hcy remethylation in P60, whereas Hcy remethylation was only partly inhibited in P60H cells cultured in Met medium. The residual Hcy remethylation in P60H cells may be related to only a moderate depletion of CH3Cbl. The resulting high CH3Cbl level relative to Met-synthase activity during N2O exposure was even higher in Hcy medium. These findings in P60H cells probably reflect increased provision of Cbl to support Hcy remethylation under metabolic strain. The inability of P60 to furnish CH3Cbl to the enzyme may explain both the Met-dependent phenotype and the increased sensitivity of Hcy remethylation to N2O exposure in these cells.

Varying neurological phenotypes among muto and mut- patients with methylmalonylCoA mutase deficiency

MethylmalonylCoA mutase (MCM) is a mitochondrial homodimer responsible for the isomerization of methylmalonylCoA to succinylCoA. Apomutase defects are traditionally divided into muto and mut- classes on the basis of residual mutase activity. Clinical findings were reviewed in 20 patients with methylmalonic aciduria secondary to MCM deficiency. All 11 muto patients had an early neonatal presentation; 6 of these patients died in infancy and 3 of 5 survivors had a poor neurological outcome as evidenced by severe delay or spastic quadriparesis with dystonia. The 2 other survivors include a 27-month-old child with a mild delay in verbal and fine motor skills and an adolescent with low normal intelligence. Of the 9 mut- patients, 7 became symptomatic in late infancy or childhood and 2 were picked up on screening. Two of the 9 patients have never had an episode of metabolic decompensation yet both are neurologically compromised; one severely retarded and autistic, the other mildly delayed. Four mut- patients have had episodic acidosis and are neurologically moderately affected, while 3 have had episodic acidosis and are neurologically intact. These results confirm phenotypic pleomorphism without a consistent pattern of neurological injury and suggest some broad correlation between mutase class and phenotype. Survival with good outcome is possible among muto patients as is significant morbidity among mut- patients. Acidosis and metabolic imbalance are not necessary preconditions for significant morbidity.

The methylcobalamin metabolism of cultured human fibroblasts

The effect of supplying exogenous methylcobalamin (MeCbl), a methyl donor to methionine synthase (MS), on the cellular metabolism of MeCbl was tested in cultured fibroblasts from healthy persons and from a subject with an inherited defect in the synthesis of MeCbl. MeCbl bound to transcobalamin II (TCII) was taken up in larger amounts than cyanocobalamin (CN-Cbl), but was equal to the uptake of hydroxocobalamin (OH-Cbl). The form of Cbl in the lysosomes persisted as the same form, bound to TCII, to which the cells were exposed in the medium. Once released from the lysosomes, both MeCbl and OH-Cbl were converted in the same proportions to coenzyme forms, suggesting equivalent entry into common cellular pools of Cbl from which active forms are synthesized. Exogenous MeCbl enjoyed no advantage in binding to MS, in synthesis of MeCbl, and in supporting cell division in the absence of methionine. All evidence supported the concept that in human cells the active MeCbl on MS forms de novo on the enzyme. It appeared unlikely that therapeutic MeCbl would have any advantage over OH-Cbl in the treatment of MeCbl deficiency or Cbl deficiency in general.

Heterogeneity in cblG: differential retention of cobalamin on methionine synthase

Cultured fibroblasts from patients with functional methionine synthase deficiency have been shown to belong to two complementation classes, cblE and cblG. Both are associated with decreased intracellular levels of methylcobalamin (MeCbl) and decreased incorporation of label from 5-methyltetrahydrofolate into macromolecules. Methionine synthase specific activity is normal or near normal in cell extracts from cblE patients under standard reducing conditions, whereas specific activity is low in cblG extracts. Seven of 10 cblG cell lines accumulated [57Co]CN-Cbl equivalent to control cells and showed similar proportions of label associated with the two intracellular cobalamin binders, methionine synthase and methylmalonyl-CoA mutase. The remaining three cblG lines showed reduced accumulation of labeled Cbl and virtually none associated with methionine synthase. The specific activity of methionine synthase was decreased in cell extracts from both cblG subgroups, being almost undetectable in extracts from the latter three lines. Incorporation of label from [14C]MeTHF into either macromolecules or into methionine was decreased in both cblG groups, but was paradoxically higher in the three lines with very low in vitro methionine synthase activity. These results demonstrate further heterogeneity within cblG and suggest that the defect in the three variant lines affects the ability of methionine synthase to retain Cbl.

Methylenetetrahydrofolate reductase (MR) deficiency: thermolability of residual MR activity, methionine synthase activity, and methylcobalamin levels in cultured fibroblasts

Methylenetetrahydrofolate reductase (MR) deficiency is the most common inborn error of folate metabolism with more than two dozen patients described. The phenotypic spectrum ranges from severe neurological deterioration and early death to asymptomatic adults. Some patients with a severe deficiency of MR have been shown to have thermolabile reductase at 55 degrees C. Since methyltetrahydrofolate, the product of MR, is a methyl donor for methylcobalamin (MeCbl), the cofactor for methionine synthase (MS), we have looked at MeCbl accumulation and MS activity in fibroblasts from 15 patients with MR deficiency. Thermolabile MR was most often but not always seen in later onset disease. MeCbl levels were often lowest in the patients with early onset disease. All but two patients had levels of methionine synthase within the control range.

Cloning and expression of a mutant methylmalonyl coenzyme A mutase with altered cobalamin affinity that causes mut- methylmalonic aciduria

Distinct genotypic and phenotypic forms of methylmalonyl CoA mutase (MCM) apoenzyme deficiency can be delineated by biochemical analysis of mutant fibroblasts. One form, designated mut-, expresses a phenotype in which residual enzyme activity is evident in cultured cells exposed to high concentrations of hydroxycobalamin. We describe cloning of an MCM cDNA from cells exhibiting a mut- phenotype and characterization of the mutant gene product overexpressed in primary muto human fibroblasts and Saccharomyces cerevisiae. Three novel base changes were observed. Recombinant clones containing one of these base changes (G717V) express four characteristics of the mut- phenotype: failure to constitute [14C]propionate incorporation activity in fibroblasts assayed under basal cell culture conditions, constitution of [14C]propionate incorporation activity in fibroblasts stimulated with 0.1-1.0 micrograms/ml hydroxycobalamin, interallelic complementation with alleles bearing an R93H mutation, and an apparent Km (adenosylcobalamin) 1,000-fold higher than normal. These results demonstrate that the G717V mutation produces the mut- phenotype and localizes determinants for adenosylcobalamin binding near the carboxyl terminus of MCM.

Clinical and biochemical observations in a patient with combined Pompe disease and cblC mutation

Metabolic studies are described in a patient who presented at 3 weeks of age with severe anaemia, hyperbilirubinaemia and hypotonicity. Clinically, glycogen storage disease type II (Pompe disease) was suspected because of a massively enlarged heart and hepatosplenomegaly. This was confirmed biochemically by the demonstration of glycogen accumulation in skeletal muscle and undetectable acid alpha-1,4-glucosidase activity in fibroblasts. Further biochemical studies in this patient surprisingly revealed homocystinuria and methylmalonic aciduria, suggesting a defect in the uptake, transport or intracellular metabolism of vitamin B12. Studies in cultured fibroblasts from the patient revealed a low uptake of [57Co]cyanocobalamin and an impaired intracellular conversion to both 5'-deoxyadenosylcobalamin and methylcobalamin. Moreover, the incorporation of labelled propionate into proteins as well as the formation of labelled methionine from labelled 5-methyltetrahydrofolate was deficient in fibroblasts from the patient. Complementation studies revealed the presence of the cblC mutation in this patient. No treatment was initiated and the patient died at the age of 31 days. We conclude that the patient was affected by both glycogen storage disease type II and cblC disease. The remarkable combination of these two rare inborn errors can be the result of the consanguinity of the parents.

A nonradioactive assay for N5-methyltetrahydrofolate-homocysteine methyltransferase (methionine synthase) based on o-phthaldialdehyde derivatization of methionine and fluorescence detection

The enzyme N5-methyltetrahydrofolate-homocysteine methyltransferase (methionine synthase, EC 2.1.1.13) catalyzes the conversion of homocysteine to methionine in the presence of a reducing system. N5-Methyltetrahydrofolate serves as a methyl donor in this reaction. An assay for the enzyme is described, which is based on methionine quantitation by o-phthaldialdehyde (OPA) derivatization and reversed-phase liquid chromatography. The enzymatic reaction is linear for at least 120 min under reducing conditions (125 mM 2-mercaptoethanol) and running the assay below an oil layer. This reducing system does not interfere with formation of the methionine-OPA adduct, which is separated from interfering compounds and an internal standard (norvaline) by a mobile phase adjusted to pH 5.0. The inclusion of internal standard increases the precision of the assay and corrects for the variable fluorescence yield due to occasional inaccurate pH adjustment before the derivatization step. Norvaline was suitable for this purpose because it elutes close to methionine and is not a natural amino acid present in biological extracts. This nonradioactive assay for methionine synthase was evaluated by comparison with a conventional method based on isolation of radioactive methionine by anion-exchange chromatography and by determination of enzyme activity in extract from cultured cells and liver.

Changes in cobalamin metabolism are associated with the altered methionine auxotrophy of highly growth autonomous human melanoma cells

Our aim was to identify the biochemical defect responsible for the inability of highly growth autonomous human tumor cells to proliferate in culture medium devoid of methionine, but containing homocysteine and 5-methyletrahydrofolic acid. We have adopted the terms "homocysteine-responsive" and "homocysteine-nonresponsive" to describe cells which can or cannot proliferate in methionine-free homocysteine-supplemented medium. Using a panel of genetically related homocysteine-responsive and -nonresponsive human melanoma cell lines, the results from a number of experiments indicate that acquisition of the "homocysteine-nonresponsive phenotype" is associated with the reduced intracellular accumulation of methyl-cobalamin, a critical cofactor of the methionine synthase enzyme. When in vitro methionine synthase assays were performed in the presence of exogenously added methyl-cobalamin, specific methionine synthase activity in extracts obtained from homocysteine-responsive cells was only twofold greater than that observed with extracts prepared from homocysteine-nonresponsive cells. However, when exogenous methyl-cobalamin was omitted from the enzyme assays, methionine synthase activity in extracts derived from homocysteine-nonresponsive cells was dramatically reduced, compared with the small decrease observed with homocysteine-responsive cell extracts. Compared with their homocysteine-responsive counterparts, homocysteine-nonresponsive cells exhibited increased levels of cobalamin efflux and decreased intracellular accumulation of methyl-cobalamin. There was a clear relationship between the abilities of these related melanoma cell lines to proliferate in methionine-free homocysteine-supplemented medium, and the extent of cobalamin loss and capacity of exogenously added methyl-cobalamin to stimulate in vitro methionine synthase activity. These results indicate a link between alterations in the intracellular trafficking and/or metabolism of cobalamin and the increased growth autonomy of human melanoma cells.

Genetic characterization of a MUT locus mutation discriminating heterogeneity in mut0 and mut- methylmalonic aciduria by interallelic complementation

Genetic complementation of fibroblasts from patients with methylmalonic aciduria (MMA) defines a unique class of allelic mutations arising from mutations at the locus encoding the methylmalonyl coenzyme A (CoA) mutase apoenzyme. Various phenotypes of MMA have been delineated including complete absence of enzyme activity (mut0) and abnormal enzyme activity with an elevated Km for adenosylcobalamin (mut-). We describe genetic studies on a cell line (WG1130) from a patient with mut0 MMA which exhibited an unusual complementation phenotype, complementing with three of nine mut0 cell lines and four of five mut- cell lines. This suggests that interallelic complementation occurs between mutant alleles in WG1130 and subsets of alleles associated with both mut0 and mut- phenotypes. The methylmalonyl CoA mutase cDNA was cloned from WG1130 and found to contain a G354----A (Arg93----His) mutation. Gene transfer of this mutant clone into primary fibroblasts from patients with MMA confirms that this mutation expresses a mut0 phenotype when transferred into a mut0 cell line with low levels of mRNA but can contribute to apoenzyme function when transferred into mut cell lines which show correction with WG1130 by somatic cell complementation. These results point to further heterogeneity within both mut0 and mut- and may enable identification of mutations affecting discrete components of apoenzyme function.

Heterozygous mutations at the mut locus in fibroblasts with mut0 methylmalonic acidemia identified by polymerase-chain-reaction cDNA cloning

Genetic defects in the enzyme methylmalonyl CoA mutase cause a disorder of organic acid metabolism termed "mut methylmalonic acidemia." Various phenotypes of mut methylmalonic acidemia are distinguished by the presence (mut-) or absence (mut0) of residual enzyme activity. The recent cloning and sequencing of a cDNA for human methylmalonyl CoA mutase enables molecular characterization of mutations underlying mut phenotypes. We identified compound heterozygous mutations in a mut0 fibroblast cell (MAS) line by cloning the methylmalonyl CoA mutase cDNA by using the polymerase chain reaction (PCR), sequencing with internal primers, and confirming the pathogenicity of observed mutations by DNA-mediated gene transfer. Both mutations alter amino acids common to the normal human, mouse, and Propionibacterium shermanii enzymes. This analysis points to evolutionarily preserved determinants critical for enzyme structure or function. The application and limitation of cDNA cloning by PCR for the identification of mutations are discussed.