A homozygous nonsense mutation in the methylmalonyl-CoA epimerase gene (MCEE) results in mild methylmalonic aciduria

Methylmalonic aciduria (MMA-uria) is an autosomal recessive inborn error of amino acid metabolism, involving valine, threonine, isoleucine, and methionine. This organic aciduria may present in the neonatal period with life-threatening metabolic acidosis, hyperammonemia, feeding difficulties, pancytopenia, and coma. Most affected patients have mutations in the methylmalonyl-coenzyme A (methylmalonyl-CoA) mutase gene. Mildly affected patients may present in childhood with failure to thrive and recurrent attacks of metabolic acidosis. Both a higher residual activity of methylmalonyl-CoA mutase as well as the vitamin B12-responsive defects (cblA and cblB) may form the basis of the mild disorder. A few patients with moderate MMA-uria are known in whom no defect could be identified. Here we present a 16-year-old female patient with persisting moderate MMA-uria (approximately 50 mmol/mol creatinine). She was born to consanguineous Caucasian parents. Her fibroblast mutase activity was normal and no effect of vitamin B12 supplementation could be established. Reduced incorporation of 14C-propionate into macromolecules suggested a defect in the propionate-to-succinate pathway. We found a homozygous nonsense mutation (c.139C>T) in the methylmalonyl-CoA epimerase gene (MCEE), resulting in an early terminating signal (p.R47X). Both parents were heterozygous for this mutation; they were found to excrete normal amounts of methylmalonic acid (MMA). This is the first report of methylmalonyl-CoA epimerase deficiency, thereby unequivocally demonstrating the biochemical role of this enzyme in human metabolism.

Impact of cblB mutations on the function of ATP:cob(I)alamin adenosyltransferase in disorders of vitamin B12 metabolism

ATP:cob(I)alamin adenosyltransferase (MMAB protein; methylmalonic aciduria type B) is an enzyme of vitamin B(12) metabolism that converts reduced cob(I)alamin to the adenosylcobalamin co-factor required for the functional activity of methylmalonyl-CoA mutase. Mutations in the human MMAB gene result in a block in adenosylcobalamin synthesis and are responsible for the cblB complementation group of inherited vitamin B(12) disorders. In this study, we examined the impact of several mutations, previously identified in cblB patients and clustered within a small, highly conserved region in MMAB. We confirmed mitochondrial expression of MMAB in human cells and showed that two mutations, R186W and E193K, were associated with absent protein by Western blot, while one, R191W, coupled with another point mutation, produced a protein in patient fibroblasts. Wild type MMAB and all four mutant proteins were stably expressed at high level as GST-fusion proteins, but only the R191W protein was enzymatically active. It showed an elevated K(m) of 320 microM (vs 6.8 microM for wild type enzyme) for ATP and 60 microM (vs 3.7 microM) for cob(I)alamin, with a reduction in k(cat) for both substrates. Circular dichroism spectroscopy revealed that three mutant proteins examined retained a alpha-helical structure as for the wild type protein. Characterization of MMAB will contribute to our understanding of cobalamin processing in mammalian cells and of disease mechanisms in the genetic disorders.

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.

Late-onset thrombocytic microangiopathy caused by cblC disease: association with a factor H mutation

BACKGROUND

cblC disease is a cause of hemolytic uremic syndrome (HUS), which has been primarily described in neonates and infants with severe renal and neurological lesions.

PATIENTS

Two sisters aged 6 and 8.5 years presented with a latent hemolytic process characterized by undetectable or low plasma haptoglobin, respectively, associated with renal failure and gross proteinuria. Renal biopsies performed in both patients found typical findings of thrombotic microangiopathy suggesting the diagnosis of HUS. Both patients were free of neurologic signs.

RESULTS

Biochemical investigations found a cobalamin processing deficiency of the cblC type. Search for additional factors susceptible to worsen endothelial damage revealed homozygosity 677C--> T mutation in the methylenetetrahydrofolate reductase gene as well as heterozygosity for a 3254T--> C mutation in factor H in the patient with the most severe clinical presentation. Long-term subcutaneous administration of hydroxocobalamin in combination with oral betaine and folic acid resulted in clinical and biological improvement in both patients.

CONCLUSION

cblC disease may be a cause of chronic HUS with delayed onset in childhood. Superimposed mutation of factor H gene might influence clinical severity.

Prenatal diagnosis for methylmalonic acidemia and inborn errors of vitamin B12 metabolism and transport

Vitamin B12 (cobalamin) is an essential cofactor for two enzymes: methionine synthase (MS), which requires methylcobalamin (MeCbl), and methylmalonyl-CoA mutase (MUT), which requires adenosylcobalamin (AdoCbl). A number of individually rare inborn errors of cobalamin metabolism are known and are distinguished by complementation analysis (mut, cblA-cblH). From 1984 to 2005, we have performed prenatal diagnosis for 117 high-risk pregnancies. We identified a total of 21 affected pregnancies (18%): cblA, 2/8; cblB, 0/5; cblC, 10/52; cblE, 2/3; cblF, 0/5; cblG, 0/5; transcobalamin deficiency, 0/2; methylmalonyl-CoA mutase (mut) deficiency, 7/30; and unclassified MMA, 0/7. Studies were performed on amniotic fluid, cultured chorionic villus cells (CCVC), cultured amniocytes (CA), or various combinations of these three types of sample. Analyses done include propionate and methyltetrahydrofolate incorporation into protein and cobalamin cofactor levels (CA: 92%, CCVC: 18%), amniotic fluid metabolite measurement either by gas chromatography/mass spectrometry (GC/MS) or by liquid chromatography-tandem mass spectrometry (LC-MS/MS) (49%), and direct mutation analysis (5%). There was one false negative CCVC result in a pregnancy at risk for cblC and one false positive CCVC in a pregnancy at risk for mutase deficiency. One unaffected pregnancy at risk for an unclassified form of MMA and another unaffected pregnancy at risk for cblC, had higher than control MMA amniotic fluid levels. Our experience suggests that prenatal diagnosis for these disorders should be done by application of two independent methods, and that CA studies appear more reliable than CCVC studies.

Prenatal diagnosis for severe methylenetetrahydrofolate reductase deficiency by linkage analysis and enzymatic assay

Severe methylenetetrahydrofolate reductase (MTHFR) deficiency is characterized by varying degrees of developmental delay, motor and gait abnormalities, seizures, and thrombosis. Biochemical abnormalities include homocystinuria and hyperhomocysteinemia. Clinical severity correlates with MTHFR activity in cultured fibroblasts; activity can also be assayed in cultured amniocytes and chorionic villus cells (CVC). Forty-four private mutations have been identified, limiting the use of direct mutation detection for prenatal diagnosis. However, intragenic polymorphisms have been identified, making prenatal diagnosis by linkage analysis a possible option, even without knowledge of deleterious mutations. Prenatal diagnosis for severe MTHFR deficiency has been available by biochemical methodologies, but molecular genetic approaches have not yet been reported. We performed prenatal diagnosis for severe MTHFR deficiency in 11 at-risk pregnancies in seven families. A combined approach of linkage analysis and enzymatic assays was used in six pregnancies; linkage analysis alone was performed in one pregnancy. Linkage analysis for the 677C > T or 1298A > C polymorphisms predicted that all seven fetuses were unaffected. For six of these seven fetuses, enzymatic activities were also measured and demonstrated concordant results. Of the 10 pregnancies in which enzymatic assays were performed, activities in cultured amniocytes predicted six unaffected fetuses (1.4-7.1 nmol CHO/mg prot/h (U)) and one affected fetus (0.24 U [control 3.1-9.6 U]). Three pregnancies assessed via CVCs demonstrated two unaffected fetuses (3.6 and 7.7 U) and 1 affected fetus (0 U [control 4.5-7.8 U]). These values were compared to those of the probands (range = 0.02-0.7 U (control 2.4-11.7 U)) in cultured fibroblasts. Our findings suggest that linkage analysis for severe MTHFR deficiency can be a practical approach for prenatal diagnosis.

Homocysteine levels in A/J and C57BL/6J mice: genetic, diet, gender, and parental effects

Increased levels of homocysteine in the blood have been associated with various birth defects and adult diseases. However, the extent to which genetic factors control homocysteine levels in healthy individuals is unclear. Laboratory mice are valuable models for dissecting the genetic and environmental controls of total homocysteine (tHcy) levels. We assessed the inheritance of tHcy levels in two inbred strains, A/J and C57BL/6J (B6), under controlled physiological conditions and assessed the relative importance of genetic, diet, gender, and parental effects. Diet affected mean tHcy levels, whereas gender affected both the mean and variance of tHcy levels. Moreover, gender of the parents influenced mean tHcy levels in reciprocal F1 hybrids, suggesting maternal effects. Finally, gene-diet interactions affected heritability of mean tHcy levels. These studies showed that each of these factors contributes to tHcy levels and provided important clues to understanding homocysteine homeostasis in humans.

Homocysteine concentrations in adults with trisomy 21: effect of B vitamins and genetic polymorphisms

BACKGROUND

The effects of supplementation with B vitamins and of common polymorphisms in genes involved in homocysteine metabolism on plasma total homocysteine (tHcy) concentrations in trisomy 21 are unknown.

OBJECTIVES

We aimed to determine the effects of orally administered folic acid and of folic acid combined with vitamin B-12, vitamin B-6, or both on tHcy in adults with trisomy 21. The study was also intended to analyze the possible influence of gene polymorphisms.

DESIGN

One hundred sixty adults with trisomy 21 and 160 healthy, unrelated subjects aged 26 +/- 4 y were included. Plasma tHcy, red blood cell folate, serum folate, and vitamin B-12 were measured. Genotyping for the common methylenetetrahydrofolate reductase (MTHFR) 677C-->T, MTHFR 1298A-->C, cystathionine beta-synthase 844Ins68, methionine synthase 2756A-->C, methionine synthase reductase 66A-->G, and reduced folate carrier 80G-->A polymorphisms was carried out.

RESULTS

The mean tHcy concentration (9.8 +/- 0.7 micromol/L) of cases who did not use vitamins was not significantly different from that of controls (9.4 +/- 0.3 micromol/L). Plasma tHcy concentrations (7.6 +/- 0.3 mmol/L) in cases who used folic acid were significantly lower than in cases who did not. Folic acid combined with vitamin B-12 did not significantly change tHcy concentrations compared with those in cases who used only folic acid. Folic acid combined with vitamins B-6 and B-12 significantly lowered tHcy (6.5 +/- 0.5 micromol/L). The difference in tHcy according to MTHFR genotype was not significant. However, tHcy concentrations were slightly higher in TT homozygotes among the controls but not among the cases.

CONCLUSION

This study provides information on the relation between several polymorphisms in genes involved in homocysteine and folate metabolism in adults with trisomy 21.

Transcobalamin deficiency due to activation of an intra exonic cryptic splice site

Transcobalamin (TC), a vitamin B12 (cobalamin, Cbl) binding protein in plasma, promotes the cellular uptake of the vitamin by receptor-mediated endocytosis. Inherited TC deficiency is an autosomal recessive disorder characterized by megaloblastic anaemia caused by cellular vitamin B12 depletion. It may be accompanied by neurological complications, including a delay in psychomotor and mental development. This report describes three sisters with inherited TC deficiency resulting from a splicing defect in the TC gene. A point mutation was identified in intron 3 splice site of the TC gene that activates a cryptic splice site in exon 3. The transcript generated has an in-frame deletion of 81 nucleotides and the resulting truncated protein is unstable and not secreted by the cells. Until now, genetic studies have been reported in only five patients with TC deficiency and the molecular defect was different in each of them, which gives evidence for a genetic heterogeneity of the disease.

Potential for misdiagnosis due to lack of metabolic derangement in combined methylmalonic aciduria/hyperhomocysteinemia (cblC) in the neonate

We report two infants with an inborn error of cobalamin (vitamin B(12)) metabolism whose clinical presentation in the first month of life strongly suggested bacterial or viral sepsis. The absence of any acute metabolic derangement (acidosis, hyperammonemia, hypoglycemia, or ketosis) in association with clinical features suggesting sepsis (lethargy, obtundation) could impede the correct diagnosis of cobalamin C (cblC) disorder. In addition, this is the first documentation of cerebrospinal fluid hyperhomocysteinemia in cblC defect that was highly increased and is likely to be associated with neurotoxicity in cblC patients.

The molecular basis of glutamate formiminotransferase deficiency

Glutamate formiminotransferase deficiency, an autosomal recessive disorder and the second most common inborn error of folate metabolism, is presumed to be due to defects in the bifunctional enzyme glutamate formiminotransferase-cyclodeaminase (FTCD). Features of a severe phenotype, first identified in patients of Japanese descent, include elevated levels of formiminoglutamate (FIGLU) in the urine in response to histidine administration, megaloblastic anemia, and mental retardation. Features of a mild phenotype include high urinary excretion of FIGLU in the absence of histidine administration, mild developmental delay, and no hematological abnormalities. We found mutations in the human FTCD gene in three patients with putative glutamate formiminotransferase deficiency. Two siblings were heterozygous for missense mutations, c.457C>T (R135C) and c.940G>C (R299P). Mutagenesis of porcine FTCD and expression in E. coli showed that the R135C mutation reduced formiminotransferase activity to 61% of wild-type, whereas the R299P mutation reduced this activity to 57% of wild-type. The third patient was hemizygous for c.1033insG, with quantitative PCR indicating that the other allele contained a deletion. These mutations are the first identified in glutamate formiminotransferase deficiency and demonstrate that mutations in FTCD represent the molecular basis for the mild phenotype of this disease.

Characterization of mutations in severe methylenetetrahydrofolate reductase deficiency reveals an FAD-responsive mutation

Methylenetetrahydrofolate reductase (MTHFR) synthesizes 5-methyltetrahydrofolate, a major methyl donor for homocysteine remethylation to methionine. Severe MTHFR deficiency results in marked hyperhomocysteinemia and homocystinuria. Patients display developmental delay and a variety of neurological and vascular symptoms. Cloning of the human cDNA and gene has enabled the identification of 29 rare mutations in homocystinuric patients and two common variants [677C>T (A222V) and 1298A>C (E429A)] with mild enzymatic deficiency. Homozygosity for 677C>T or combined heterozygosity for both polymorphisms is associated with mild hyperhomocysteinemia. In this communication, we describe four novel mutations in patients with homocystinuria: two missense mutations (471C>G, I153M; 1025T>C, M338T), a nonsense mutation (1274G>A, W421X), and a 2-bp deletion (1553delAG). We expressed the 1025T>C mutation as well as two previously reported amino acid substitutions [983A>G (N324S) and 1027T>G (W339G)] and observed decreased enzyme activity at 10%, 36%, and 21% of control levels, respectively, with little or no effect on affinity for 5-methyltetrahydrofolate. One of these mutations, 983A>G (N324S), showed flavin adenine dinucleotide (FAD) responsiveness in vitro. Expression of these mutations in cis with the 677C>T polymorphism, as observed in the patients, resulted in an additional 50% decrease in enzyme activity. This report brings the total to 33 severe mutations identified in patients with severe MTHFR deficiency.

Update on Cobalamin, Folate, and Homocysteine

Three topics affecting cobalamin, folate, and homocysteine that have generated interest, activity, and advances in recent years are discussed. These are: (I) the application of an expanded variety of tools to the diagnosis of cobalamin deficiency, and how these affect and are affected by our current understanding of deficiency; (II) the nature of the interaction between homocysteine and vascular disease, and how the relationship is affected by vitamins; and (III) the improved understanding of relevant genetic disorders and common genetic polymorphisms, and how these interact with environmental influences.

The diagnostic approach to cobalamin deficiency now allows better diagnosis of difficult and atypical cases and more confident rejection of the diagnosis when deficiency does not exist. However, the process has also become a complex and sometimes vexing undertaking. Part of the difficulty derives from the lack of a diagnostic gold standard among the many available tests, part from the overwhelming numerical preponderance of patients with subclinical deficiency (in which isolated biochemical findings exist without clinical signs or symptoms) among the cobalamin deficiency states, and part from the decreased availability of reliable tests to identify the causes of a patient’s cobalamin deficiency and thus a growing deemphasis of that important part of the diagnostic process. In Section I, Dr. Carmel discusses the tests, the diagnostic issues, and possible approaches to the clinical evaluation. It is suggested no single algorithm fits all cases, some of which require more biochemical proof than others, and that differentiating between subclinical and clinical deficiency, despite their overlap, may be a helpful and practical point of departure in the evaluation of patients encountered in clinical practice. The arguments for and against a suggested expansion of the cobalamin reference range are also weighed.

The epidemiologic data suggest that homocysteine elevation is a risk factor for vascular and thrombotic disease. In Section II, Dr. Green notes that the interactions of metabolism and clinical risk are not well understood and a causative relationship remains unproven despite new reports that lowering homocysteine levels may reduce vascular complications. Genetic and acquired influences may interact in important ways that are still being sorted out. The use of vitamins, especially folate, often reduces homocysteine levels but also carries potential disadvantages and even risks. Folate fortification of the diet and supplement use have also markedly reduced the frequency of folate deficiency, and cobalamin deficiency is now the more common deficiency state, especially among the elderly.

Although genetic disorders are rare, they illuminate important metabolic mechanisms and pose diagnostic challenges, especially when clinical presentation occurs later in life. In Section III, Drs. Rosenblatt and Watkins use selected disorders to illustrate the subject. Imerslund-Gräsbeck syndrome, a hereditary disorder of cobalamin absorption at the ileal level, demonstrates genetic heterogeneity. Finnish patients show mutation of the gene for cubilin, the multiligand receptor for intrinsic factor. Surprisingly, Norwegian and other patients have been found recently to have mutations of the AMN (amnionless) gene, mutations that are lethal in mice at the embryonic stage. Two disorders of cobalamin metabolism, cblG and cblE, are now known to arise from mutations of the methionine synthase and methionine synthase reductase genes, respectively. These disorders feature megaloblastic anemia and neurologic manifestations. The folate disorder selected for illustration, methylenetetrahydrofolate reductase (MTHFR) deficiency, paradoxically causes neurological problems but no megaloblastic anemia. This rare deficiency is the most common inborn error of folate metabolism. It is distinct from the very common MTHFR gene polymorphisms, mutations that cause mild to moderate reductions in MTHFR activity but no direct clinical manifestations. The MTHFR polymorphisms, especially the 677C→T mutation, may contribute to vascular and birth defect risks, while reducing the risk of certain malignancies, such as colon cancer. These polymorphisms and those of genes for other enzymes and proteins related to cobalamin, folate, and homocysteine metabolism may be important role players in frequent interactions between genes and the environment.

Identification of the gene responsible for the cblB complementation group of vitamin B12-dependent methylmalonic aciduria

The methylmalonic acidurias are metabolic disorders resulting from deficient methylmalonyl-CoA mutase activity, a vitamin B(12)-dependent enzyme. We have cloned the gene for the cblB complementation group caused by deficient activity of a cob(I)alamin adenosyltransferase. This was accomplished by searching bacterial genomes for genes in close proximity to the methylmalonyl-CoA mutase gene that might encode a protein with the properties of an adenosyltransferase. A candidate was identified in the Archaeoglobus fulgidus genome and was used to probe the human genome database. It yielded a gene on chromosome 12q24 that encodes a predicted protein of 250 amino acids with 45% similarity to PduO in Salmonella enterica, a characterized cob(I)alamin adenosyltransferase. A northern blot revealed an RNA species of 1.1 kb predominating in liver and skeletal muscle. The gene was evaluated for deleterious mutations in cblB patient cell lines. Several mutations were identified including a 5 bp deletion (5del572gggcc576), two splice site mutations (IVS2-1G>T, IVS3-1G>A), andt several point mutations (A135T, R186W, R191W and E193K). Two additional amino acid substitutions (R19Q and M239K) were found in several patient cell lines but were found to be common polymorphisms (36% and 46%) in control alleles. The R186W mutation, which we suggest is disease-linked, is present in four of the six patient cell lines examined (homoallelic in two) and in 4 of 240 alleles in control samples. These data confirm that the identified gene, MMAB, corresponds to the cblB complementation group and has the appearance of a cob(I)alamin adenosyltransferase, as predicted from biochemical data.

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.

Hyperhomocysteinemia due to methionine synthase deficiency, cblG: structure of the MTR gene, genotype diversity, and recognition of a common mutation, P1173L

Mutations in the MTR gene, which encodes methionine synthase on human chromosome 1p43, result in the methylcobalamin deficiency G (cblG) disorder, which is characterized by homocystinuria, hyperhomocysteinemia, and hypomethioninemia. To investigate the molecular basis of the disorder, we have characterized the structure of the MTR gene, thereby identifying exon-intron boundaries. This enabled amplification of each of the 33 exons of the gene, from genomic DNA from a panel of 21 patients with cblG. Thirteen novel mutations were identified. These included five deletions (c.12-13delGC, c.381delA, c.2101delT, c.2669-2670delTG, and c.2796-2800delAAGTC) and two nonsense mutations (R585X and E1204X) that would result in synthesis of truncated proteins that lack portions critical for enzyme function. One mutation was identified that resulted in conversion of A to C of the invariant A of the 3' splice site of intron 9. Five missense mutations (A410P, S437Y, S450H, H595P, and I804T) were identified. The latter mutations, as well as the splice-site mutation, were not detected in a panel of 50 anonymous DNA samples, suggesting that these sequence changes are not polymorphisms present in the general population. In addition, a previously described missense mutation, P1173L, was detected in 16 patients in an expanded panel of 24 patients with cblG. Analysis of haplotypes constructed using sequence polymorphisms identified within the MTR gene demonstrated that this mutation, a C-->T transition in a CpG island, has occurred on at least two separate genetic backgrounds.

Genetic and molecular control of folate-homocysteine metabolism in mutant mice

Hyperhomocysteinemia adversely affects fundamental aspects of fetal development, adulthood, and aging, but the role of elevated homocysteine levels in these birth defects and adult diseases remains unclear. Mouse models are valuable for investigating the causes and consequences of hyperhomocysteinemia. We used a phenotype-based approach to identify mouse mutants for studying the relation between single gene mutations, homocysteine levels as a measure of the status of homocysteine metabolism, and gene expression profiles as a way to assess the impact of protein deficiency in mutant mice on steady-state transcription levels of genes in the folate-homocysteine pathways. These mutants were selected based on their propensity to produce phenotypes that are reminiscent of those associated with anomalies in folate-homocysteine metabolism in humans. We report identification of new, single-gene mouse models of homocysteinemia and characterization of their molecular and physiological impact on folate-homocysteine metabolism. Mutations in several genes involved in the hedgehog and WNT signal transduction pathways, as well as a gene involved in lipid metabolism, resulted in elevated homocysteine levels and altered expression profiles of folate-homocysteine metabolism genes. These results begin to unravel the complex relations between elevation of a single amino acid in the blood and the diverse birth defects and adult diseases associated with hyperhomocysteinemia.

Glycine N-methyltransferase deficiency: a novel inborn error causing persistent isolated hypermethioninaemia

This paper reports clinical and metabolic studies of two Italian siblings with a novel form of persistent isolated hypermethioninaemia, i.e. abnormally elevated plasma methionine that lasted beyond the first months of life and is not due to cystathionine beta-synthase deficiency, tyrosinaemia I or liver disease. Abnormal elevations of their plasma S-adenosylmethionine (AdoMet) concentrations proved they do not have deficient activity of methionine adenosyltransferase I/III. A variety of studies provided evidence that the elevations of methionine and AdoMet are not caused by defects in the methionine transamination pathway, deficient activity of methionine adenosyltransferase II, a mutation in methylenetetrahydrofolate reductase rendering this activity resistant to inhibition by AdoMet, or deficient activity of guanidinoacetate methyltransferase. Plasma sarcosine (N-methylglycine) is elevated, together with elevated plasma AdoMet in normal subjects following oral methionine loads and in association with increased plasma levels of both methionine and AdoMet in cystathionine beta-synthase-deficient individuals. However, plasma sarcosine is not elevated in these siblings. The latter result provides evidence they are deficient in activity of glycine N-methyltransferase (GNMT). The only clinical abnormalities in these siblings are mild hepatomegaly and chronic elevation of serum transaminases not attributable to conventional causes of liver disease. A possible causative connection between GNMT deficiency and these hepatitis-like manifestations is discussed. Further studies are required to evaluate whether dietary methionine restriction will be useful in this situation.

Neurological and neuropathologic heterogeneity in two brothers with cobalamin C deficiency

Two adult brothers, one documented to have methylmalonic acidemia with homocystinuria, or cobalamin C deficiency, after autopsy, displayed severe but divergent neurological presentations. One exhibited a myelopathy and the other chronic endocrine problems (Schmidt's syndrome) followed by a neuropsychiatric and dementing disorder owing to cerebral perivascular demyelination. The recognition of cobalamin C deficiency has practical implications because it is one of the few inherited diseases of central white matter that is treatable.