Methionine and serine formation in control and mutant human cultured fibroblasts: evidence for methyl trapping and characterization of remethylation defects

Decreased cortical Nrf2 gene expression in autism and its relationship to thiol and cobalamin status

Nuclear factor erythroid 2-related factor 2 (Nrf2) promotes expression of a large number of antioxidant genes and multiple studies have described oxidative stress and impaired methylation in autism spectrum disorder (ASD), including decreased brain levels of methylcobalamin(III) (MeCbl). Here we report decreased expression of the Nrf2 gene (NFE2L2) in frontal cortex of ASD subjects, as well as differences in other genes involved in redox homeostasis. In pooled control and ASD correlation analyses, hydroxocobalamin(III) (OHCbl) was inversely correlated with NFE2L2 expression, while MeCbl and total cobalamin abundance were positively correlated with NFE2L2 expression. Levels of methionine, S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH) and cystathionine were positively correlated with NFE2L2 expression, while homocysteine (HCY) was negatively correlated. The relationship between Nrf2 activity and cobalamin was further supported by a bioinformatics-based comparison of cobalamin levels in different tissues with expression of a panel of 40 Nrf2-regulated genes, which yielded a strong correlation. Lastly, Nrf2-regulated gene expression was also correlated with expression of intracellular cobalamin trafficking and processing genes, such as MMADHC and MTRR. These findings highlight a previously unrecognized relationship between the antioxidant-promoting role of Nrf2 and cobalamin status, which is dysfunctional in ASD.

MTHFR C677T polymorphism increases MTX sensitivity via the inhibition of S-adenosylmethionine and de novo purine synthesis

Objective: Currently, no guidelines are established for pharmacogenomic testing involving folate metabolic genes in long-term disease-modifying antirheumatic drugs' (DMARD) therapies. We carefully investigated how common genetic variations in methylenetetrahydrofolate reductase (MTHFR) influence cellular metabolic kinetics in response to methotrexate (MTX). Designs: Two distinct cell models: HepG2 with stabilized MTHFR inhibition using shRNA delivered by a Lentiviral vector; and Epstein-Barr virus transformed human lymphoblasts expressing MTHFR polymorphic allele 677C and 677T were used. Disease activity and DMARD use were compared between MTHFR-677CC, CT and TT rheumatoid arthritis (RA) patients in a cross-sectional study (n=120). Results: Compared with MTHFR-CC, MTHFR-TT carriers had lower mean weakly MTX dose (9.8 ± 3.3 compared with 12.1 ± 3.5, P<0.05). More MTHFR-TT carriers (8/11, 73%) reported MTX-related side effects compared with MTHFR-677CC (32/57, 56%) and MTHFR-677CT (30/51, 59%). No genotypic difference was found in other DMARDs. At the same dose of MTX, lymphoblasts were more sensitive in cell survival, protein and thymidine syntheses whereas HepG2 models were more susceptible to the inhibition of S-adenosylmethionine (adoMet) synthesis. MTHFR-C677T altered protein turnover and folate mediated 1-carbon metabolic fluxes in lymphoblasts with and without MTX. MTHFR function significantly affected transmethylation fluxes and adoMet homeostasis but not nucleotide biosyntheses in MTX-treated HepG2 cell-lines. Conclusion: Combining cell models, kinetic studies, and genetic tests in humans, the present study gives insight on how MTHFR effects hepatic transmethylation homeostasis during MTX therapy. We provide platforms that help predict the genetic impact on antifolate drugs, and further delineate tissue-specific target pathway in DMARD therapies. We suggest that genetic factors should be taken into account in clinical practice.

Characterization and review of MTHFD1 deficiency: four new patients, cellular delineation and response to folic and folinic acid treatment

In the folate cycle MTHFD1, encoded by MTHFD1, is a trifunctional enzyme containing 5,10-methylenetetrahydrofolate dehydrogenase, 5,10-methenyltetrahydrofolate cyclohydrolase and 10-formyltetrahydrofolate synthetase activity. To date, only one patient with MTHFD1 deficiency, presenting with hyperhomocysteinemia, megaloblastic anaemia, hemolytic uremic syndrome (HUS) and severe combined immunodeficiency, has been identified (Watkins et al J Med Genet 48:590-2, 2011). We now describe four additional patients from two different families. The second patient presented with hyperhomocysteinemia, megaloblastic anaemia, HUS, microangiopathy and retinopathy; all except the retinopathy resolved after treatment with hydroxocobalamin, betaine and folinic acid. The third patient developed megaloblastic anaemia, infection, autoimmune disease and moderate liver fibrosis but not hyperhomocysteinemia, and was successfully treated with a regime that included and was eventually reduced to folic acid. The other two, elder siblings of the third patient, died at 9 weeks of age with megaloblastic anaemia, infection and severe acidosis and had MTFHD1 deficiency diagnosed retrospectively. We identified a missense mutation (c.806C > T, p.Thr296Ile) and a splice site mutation (c.1674G > A) leading to exon skipping in the second patient, while the other three harboured a missense mutation (c.146C > T, p.Ser49Phe) and a premature stop mutation (c.673G > T, p.Glu225*), all of which were novel. Patient fibroblast studies revealed severely reduced methionine formation from [(14)C]-formate, which did not increase in cobalamin supplemented culture medium but was responsive to folic and folinic acid. These additional cases increase the clinical spectrum of this intriguing defect, provide in vitro evidence of disturbed methionine synthesis and substantiate the effectiveness of folic or folinic acid treatment.

Three new cases of late-onset cblC defect and review of the literature illustrating when to consider inborn errors of metabolism beyond infancy

Background

The cblC defect is a rare inborn error of intracellular cobalamin metabolism. Biochemical hallmarks are elevated homocysteine and low methionine in plasma accompanied by methylmalonic aciduria. Due to the heterogeneous clinical picture, patients with the late-onset form of the disease (onset >12 months) come to the attention of diverse medical specialists, e.g. paediatricians, neurologists, nephrologists, psychiatrists or haematologists. The report reviews the published clinical data and adds three new cases to raise awareness for this severe but often treatable disease.

Methods

The Pubmed and the Cochrane databases were searched for clinical reports on cblC patients and three unreported cases are presented to illustrate the clinical spectrum.

Results

Reports on 58 cases (30 females, 22 males, 6 = no information) and the three new cases underlined the clinical heterogeneity of the disease. Time between first symptoms and diagnosis ranged from three months to more than 20 years. Haemolytic uraemic syndrome and pulmonary hypertension were main presenting symptoms in preschool children. In older children/adolescents, psychiatric symptoms, cognitive impairment, ataxia and myelopathy were frequently observed while thromboembolic events and glomerulopathies were almost exclusively seen in adults. Brain atrophy, white matter lesions and myelopathy were frequently encountered. The majority of patients showed marked biochemical and clinical response to treatment with parenteral hydroxocobalamin combined with oral betaine, folate, carnitine and rarely methionine. The course was less favourable in late treated or untreated patients.

Conclusions

The late-onset cblC defect is a rare disease and unfortunately, diagnosis is often delayed. Raising awareness for this disorder can significantly improve patients’ outcome and perspective by timely initiation of targeted treatment. Newborn screening (NBS) for the cblC defect might be of benefit especially for late-onset patients since treatment seems efficient when initiated before irreversible organ damage. In general, inborn errors of metabolisms should be considered in unexplained medical cases at any age, especially in patients with multisystemic disease. More specifically, total homocysteine in plasma and methylmalonic acid in urine/plasma should be measured in unexplained neurologic, psychiatric, renal, haematologic and thromboembolic disease.

Novel Deletion Mutation Identified in a Patient with Late-Onset Combined Methylmalonic Acidemia and Homocystinuria, cblC Type

Combined methylmalonic aciduria and homocystinuria, cblC type (MMACHC), is the most common inborn error of cellular vitamin B12 metabolism and is caused by mutations in the MMACHC gene. This metabolic disease results in impaired intracellular synthesis of adenosylcobalamin and methylcobalamin, coenzymes for the methylmalonyl-CoA mutase and methionine synthase enzymes, respectively. The inability to produce normal levels of these two coenzymes leads to increased concentrations of methylmalonic acid and homocysteine in plasma and urine, together with normal or decreased concentration of methionine in plasma. Here, we report a novel homozygous deletion mutation (NM_015506.2:c.392_394del) resulting in an in-frame deletion of amino acid Gln131 and late-onset disease in a 23-year-old male. The patient presented with sensory and motoric disabilities, urine and fecal incontinence, and light cognitive impairment. There was an excessive urinary excretion of methylmalonic acid and greatly elevated plasma homocysteine. The clinical symptoms and the laboratory abnormalities responded partly to treatment with hydroxycobalamin, folinic acid, methionine, and betaine. Studies on patient fibroblasts together with spectroscopic activity assays on recombinant MMACHC protein reveal that Gln131 is crucial in order to maintain enzyme activity. Furthermore, structural analyses show that Gln131 is one of only two residues making hydrogen bonds to the tail of cobalamin. Circular dichroism spectroscopy indicates that the 3D structure of the deletion mutant is folded but perturbed compared to the wild-type protein.

A liquid chromatography mass spectrometry method for the measurement of cystathionine β-synthase activity in cell extracts

BACKGROUND

In order to correctly assess the efficacy of therapy or diet in intervention studies on the activity of cystathionine β-synthase (CBS) a sensitive analytical method is necessary.

METHODS

An electrospray LC-MS/MS method preceded by a solid phase extraction step was developed for the measurement of CBS activity in cell extracts. Nonafluoropentanoic acid was used as an ionpair to provide the underivatized cystathionine the desired retention on a C18 column.

RESULTS

A detection limit of 50pmol cystathionine/h/mg protein was achieved. In fibroblasts, intra- and inter-assay CVs for the CBS activity were 5.2% and 14.7%, respectively. A K(m) value of 8μmol/L for homocysteine, and 2.5μmol/L for serine was calculated. In fibroblasts wildtype, heterozygous, and homozygous CBS activity ranges measured were 8.5-27.0, 4.2-13.4, 0.0-0.7nmol/h×mg protein, respectively. The method was applied to a study where rats were fed 2 diets. Increase of dietary methionine (7.7 versus 3.8mg/kg methionine) significantly increased the CBS activity in rat liver lysates from a median of 58.0 to a median of 71.5 (P=0.037)nmol/h×mg protein. In a lymphoblasts cell culture experiment, the addition of Hcy to the culture media increased the activity of CBS 3 fold.

CONCLUSION

This LC-MS/MS is able to diagnose CBS deficiency at the enzyme level, and can accurately measure the effect diets or therapy might have on the CBS activity in a variety of cell types.

Mutations in ABCD4 cause a new inborn error of vitamin B12 metabolism

Inherited disorders of vitamin B12 (cobalamin) have provided important clues to how this vitamin, which is essential for hematological and neurological function, is transported and metabolized. We describe a new disease that results in failure to release vitamin B12 from lysosomes, which mimics the cblF defect caused by LMBRD1 mutations. Using microcell-mediated chromosome transfer and exome sequencing, we identified causal mutations in ABCD4, a gene that codes for an ABC transporter, which was previously thought to have peroxisomal localization and function. Our results show that ABCD4 colocalizes with the lysosomal proteins LAMP1 and LMBD1, the latter of which is deficient in the cblF defect. Furthermore, we show that mutations altering the putative ATPase domain of ABCD4 affect its function, suggesting that the ATPase activity of ABCD4 may be involved in intracellular processing of vitamin B12.

cblE-Type Homocystinuria Presenting with Features of Haemolytic-Uremic Syndrome in the Newborn Period

This study describes a cblE type of homocystinuria associated with haemolytic-uremic syndrome (HUS) features. We report on a male infant aged 43 days presenting with failure to thrive, hypotonia, pancytopaenia, HUS symptoms (microangiopathic haemolytic anaemia and thrombocytopaenia with signs of renal involvement) and fatal evolution. An underlying cobalamin disorder was diagnosed after a bone marrow examination revealed megaloblastic changes associated with hyperhomocysteinaemia. An urinary organic acid analysis revealed normal methylmalonic acid excretion. The cblE diagnosis was confirmed with a complementation analysis using skin fibroblasts and genetic studies of the MTRR gene. The patient treatment included parenteral hydroxocobalamin, carnitine, betaine and folinic acid, but there was no response. After the autopsy, the histopathological examination of the kidneys showed marked myointimal proliferation and narrowing of the vascular lumen. The central nervous system showed signs of haemorrhage that affected the putamen and the thalamus; diffuse white matter lesions with spongiosis, necrosis and severe astrogliosis were also observed. Microangiopathy was observed with an increase in vessel wall thickness, a reduction of the arterial inner diameter and capillary oedema. The signs of necrosis and haemorrhage were detected in the cerebellum, the cerebellar peduncles, the tegmentum and the bulbar olives.In conclusion, cblE should be considered when diagnosing patients presenting with HUS signs and symptoms during the newborn period. Despite early diagnosis, however, the specific treatment measures were not effective in this patient.

Genetic aspects of folate metabolism

The vitamin folate functions within the cell as a carrier of one-carbon units. The requirement for one-carbon transfers is ubiquitous and all mammalian cells carry out folate dependent reactions. In recent years, low folate status has been linked to risk of numerous adverse health conditions throughout life from birth defects and complications of pregnancy to cardiovascular disease, cancer and cognitive dysfunction in the elderly. In many instances inadequate intake of folate seems to be the primary contributor but there is also evidence that an underlying genetic susceptibility can play a modest role by causing subtle alterations in the availability, metabolism or distribution of intermediates in folate related pathways. Folate linked one-carbon units are essential for DNA synthesis and repair and as a source of methyl groups for biological methylation reactions. The notion of common genetic variants being linked to risk of disease was relatively novel in 1995 when the first functional folate-related polymorphism was discovered. Numerous polymorphisms have now been identified in folate related genes and have been tested for functionality either as a modifier of folate status or as being associated with risk of disease. Moreover, there is increasing research into the importance of folate-derived one-carbon units for DNA and histone methylation reactions, which exert crucial epigenetic control over cellular protein synthesis. It is thus becoming clear that genetic aspects of folate metabolism are wide-ranging and may touch on events as disparate as prenatal imprinting to cancer susceptibility. This chapter will review the current knowledge in this area.

Hyperpyrexia resulting in encephalopathy in a 14-month-old patient with cblC disease

Cobalamin C (cblC) defect, the most common inborn error of cobalamin metabolism, is a multisystem disorder usually presenting with progressive neurological, haematological and ophthalmological signs. We report on a cblC patient diagnosed in the newborn age who developed nearly normal during the first year of life. During an upper respiratory tract infection with severe hyperpyrexia at the age of 14months he developed an acute encephalopathic crisis resulting in severe mental retardation and marked internal and external cerebral atrophy. Hyperacute encephalopathic crises have not been observed so far in patients with cblC defect. It remains unclear, if this association is incidental or if the underlying metabolic defect may have predisposed the brain tissue to hyperpyrexia-induced damage.

Causes of and diagnostic approach to methylmalonic acidurias

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

Folate restriction and methylenetetrahydrofolate reductase 677T polymorphism decreases adoMet synthesis via folate-dependent remethylation in human-transformed lymphoblasts

The homozygous mutation (677TT) in the methylenetetrahydrofolate reductase (MTHFR) gene reduces enzyme activity and alters cellular folate composition. Previous epidemiological studies reported a potential protective effect of MTHFR677C --> T against acute lymphocytic leukemia and malignant lymphoma, but the mechanism remains to be determined. We investigated the biochemical impacts of MTHFR677C --> T on cellular S-adenosyl methionine (adoMet) synthesis, global DNA methylation, and de novo purine synthesis, all of which are potential regulatory pathways involved in tumorigenesis. Metabolic fluxes of homocysteine remethylation and de novo purine synthesis were compared between Epstein-Barr virus-transformed lymphoblasts expressing MTHFR 677C and MTHFR 677T using stable isotopic tracers and GCMS. MTHFR TT genotype significantly reduced folate-dependent remethylation under folate restriction, reflecting limited methylated folates under folate restriction. Data also suggested increased formylated folate pool and increased purine synthesis when folate is adequate. The impacts of MTHFR 677T polymorphism appeared closely related to folate status, and such alterations may modulate metabolic pathways involved in cancer onset/progression. The advantage of de novo purine synthesis found in the MTHFR TT genotype may account for the protective effect of MTHFR in hematological malignancies. These transformed cells are potential models for studying the consequences of human genetic variation and cancer pathogenesis.

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.

Prenatal and postnatal treatment in cobalamin C defect

OBJECTIVE

To evaluate prenatal treatment with hydroxycobalamin (OH-Cbl) in a pregnancy at risk for a severe form of the cobalamin C defect and postnatal treatment of the affected child.

STUDY DESIGN

Observational study with non-randomized intervention.

RESULTS

In contrast to reported pregnancies with affected fetuses in which maternal methylmalonic aciduria was found in the last trimester of pregnancy, there was no maternal methylmalonic aciduria in our case, given prenatal treatment with intramuscular OH-Cbl. We did not find that the concentration of odd long-chain fatty acids in cord blood erythrocytes reflects fetal methylmalonic academia. After birth, the infant was treated with intramuscular OH-Cbl and oral carnitine. Oral folate and betaine were added as adjunct therapy to decrease plasma total homocysteine. Because of inadequate metabolic control, a diet reduced in natural protein was introduced. The child had normal developmental milestones but had nystagmus, hyperpigmented retinopathy, and discrete truncal muscular hypotonia.

CONCLUSIONS

Despite prenatal and postnatal treatment, adequate metabolic control, absence of metabolic crises, and normal developmental milestones, this patient with the cobalamin C defect had characteristic symptoms of the disease.

Disturbed visual system function in methionine synthase deficiency

BACKGROUND

Isolated functional methionine synthase deficiency occurs in the cblE and cblG defects of methylcobalamin metabolism and is one of a number of causes of severely elevated plasma homocysteine. Clinical features are predominantly of a neurological nature but also include functional restriction of the visual system manifesting as loss of visual acuity and nystagmus. As yet, the origin and pathogenesis of impaired vision have not been explained.

MATERIALS AND METHODS

We investigated a patient who was proven by complementation analysis in cultured fibroblasts to belong to the cblG complementation group. Ganzfeld electroretinograms (ERG) and flash visual evoked potentials (VEP) were recorded over a period of 4 years.

RESULTS

Amplitudes of all International Society for Clinical Electrophysiology of Vision (ISCEV) standard responses were below normal. The greatest reductions were of rod response to 24 microV, of standard combined response (SC) b-wave to 120 microV, of oscillatory potentials (OP) to 5 microV, of cone response b-wave to 35 microV, and of 30 Hz flicker response to 8 microV. Except for SC and cone a-waves at age 2.5 and 3.5 years, as well as cone b-wave at 3.5 years, amplitudes remained at a subnormal level at follow-up examinations. Implicit times were slightly prolonged (SC b-wave 6 ms, OPs 2 ms, cone b-wave 2 ms, 30 Hz flicker 4 ms) or fell within the normal range. Responses of the flash VEP were severely deformed but reproducible.

CONCLUSIONS

This is the first report of detailed investigations of the visual system in a patient with isolated methionine synthase deficiency. Reduced oscillatory potentials suggest microvascular damage to the retina through homocysteine. Decreased photoreceptor function as well as ganglion cell loss as indicated by pathological flash VEPs may reflect a cytotoxic impact of homocysteine on neurons of the visual pathway.

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

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

Hypotension in the very low birthweight infant: the old, the new, and the uncertain

Hypotension occurs in up to 20% of very low birthweight infants, usually in the first 48 hours after birth. Its importance lies in its possible causal link with brain injury. Its management is controversial.

CblE type of homocystinuria: mild clinical phenotype in two patients homozygous for a novel mutation in the MTRR gene

Patients with the cblE type of homocystinuria usually present with megaloblastic anaemia, feeding difficulties, developmental delay and cerebral atrophy. We present a 14-year-old Spanish girl (patient 1) and a 10-year-old Portuguese boy (patient 2) with cblE disease and mild clinical phenotype. The main clinical feature in both patients was persistent megaloblastic anaemia observed at 3 years and at 2 months of age, respectively. Diagnosis was made at the ages of 9 and 7 years, respectively, owing to persistent macrocytosis despite cobalamin treatment. Plasma total homocysteine values at diagnosis were 91 micromol/L and 44 micromol/L, respectively, in the absence of methylmalonic aciduria. Neurological and neurophysiological examinations were normal except for two small lesions on brain MRI suggestive of ischaemia and slight abnormalities in somatosensitive evoked potentials. Enzymatic analysis, complementation studies and clearly reduced production of methylcobalamin from 57Co-labelled cyanocobalamin indicated functional methionine synthase reductase deficiency due to the cblE defect. Genetic analysis confirmed that both patients are homozygous for a novel mutation c.1361C>T in the methionine synthase reductase gene leading to a replacement of serine by leucine (S454L) in a highly conserved FAD-binding domain. We propose that homozygosity for this novel mutation may be associated with a mild phenotype, although its long-term deleterious neurological consequences remain possible. Furthermore, we propose that even in the absence of apparent neurological involvement, total homocysteine should be investigated in patients with resistant megaloblastic anaemia to detect possible mild forms of the cblE type of homocystinuria.

Cytoplasmic serine hydroxymethyltransferase mediates competition between folate-dependent deoxyribonucleotide and S-adenosylmethionine biosyntheses

Folate-dependent one-carbon metabolism is required for the synthesis of purines and thymidylate and for the remethylation of homocysteine to methionine. Methionine is subsequently adenylated to S-adenosylmethionine (SAM), a cofactor that methylates DNA, RNA, proteins, and many metabolites. Previous experimental and theoretical modeling studies have indicated that folate cofactors are limiting for cytoplasmic folate-dependent reactions and that the synthesis of DNA precursors competes with SAM synthesis. Each of these studies concluded that SAM synthesis has a higher metabolic priority than dTMP synthesis. The influence of cytoplasmic serine hydroxymethyltransferase (cSHMT) on this competition was examined in MCF-7 cells. Increases in cSHMT expression inhibit SAM concentrations by two proposed mechanisms: (1) cSHMT-catalyzed serine synthesis competes with the enzyme methylenetetrahydrofolate reductase for methylenetetrahydrofolate in a glycine-dependent manner, and (2) cSHMT, a high affinity 5-methyltetrahydrofolate-binding protein, sequesters this cofactor and inhibits methionine synthesis in a glycine-independent manner. Stable isotope tracer studies indicate that cSHMT plays an important role in mediating the flux of one-carbon units between dTMP and SAM syntheses. We conclude that cSHMT has three important functions in the cytoplasm: (1) it preferentially supplies one-carbon units for thymidylate biosynthesis, (2) it depletes methylenetetrahydrofolate pools for SAM synthesis by synthesizing serine, and (3) it sequesters 5-methyltetrahydrofolate and inhibits SAM synthesis. These results indicate that cSHMT is a metabolic switch that, when activated, gives dTMP synthesis higher metabolic priority than SAM synthesis.

CblE type of homocystinuria due to methionine synthase reductase deficiency: clinical and molecular studies and prenatal diagnosis in two families

The cblE type of homocystinuria is a rare autosomal recessive disorder, which manifests with megaloblastic anaemia and developmental delay in early childhood. This disease is caused by a defect in reductive activation of methionine synthase (MTR). Our study was directed at clinical, biochemical, enzymatic and molecular characterization of two Czech patients with the cblE type of homocystinuria. Case 1 involves a 20-year-old mentally retarded patient who presented with megaloblastic anaemia at 10 weeks of age. She was treated with folates and vitamin B12, and subsequent attempts to cease administration of folates led to recurrence of megaloblastic anaemia. Biochemical features included severe hyperhomocysteinaemia and hypomethioninaemia and in fibroblasts defective formation of methionine from formate, and no complementation with cblE cells. Subsequent molecular analysis of the methionine synthase reductase (MTRR) gene revealed compound heterozygosity for a transition c.1459G>A (G487R) and a 2bp insertion (c.1623-1624insTA). Case 2 involves an 8-year-old girl with nystagmus and developmental delay in whom megaloblastic anaemia was detected at 11 weeks of age. Severe hyperhomocysteinaemia with normal methionine levels was found and enzymatic and complementation studies confirmed the cblE defect. This patient is homozygous for a 140 bp insertion (c.903-904ins140). The insertion is caused by a T>C transition within intron 6 of the MTRR gene, which presumably leads to activation of an exon splicing enhancer. In the families of both patients, enzymatic and mutation analyses were successfully used for prenatal diagnosis. Our study expands the knowledge of the phenotypic and genotypic variability of the cblE type of homocystinuria and supports the concept that this disorder is caused by mutations in the MTRR gene.

Cobalamin disorder Cbl-C presenting with late-onset thrombotic microangiopathy

Two siblings, a boy age 12 and his sister age 4 years, presented with proteinuria and hematuria, hypertension, and chronic hemolytic anemia. At age 13 years, the boy developed an episode of severe hypertensive encephalopathy and transient renal failure. Both children are attending normal school, have no neurologic symptoms, and only minimal pigmentary retinal abnormalities. Renal biopsy showed a chronic thrombotic microangiopathic nephropathy. Both patients had hyperhomocysteinemia and mild methylmalonic aciduria. Fibroblasts showed decreased cobalamin uptake, reduced methyl- and adenosyl-cobalamin formation, and deficient incorporation of formate and propionate, compatible with the Cbl-C complementation group, but milder than that found in cells from most patients. Both patients and their father carry a balanced reciprocal translocation. Parenteral hydroxycobalamin treatment reduced the homocysteine levels, and methylmalonic acid disappeared. Increasing the dosage of hydroxycobalamin from 1 to 2.5, then 5 mg daily together with betaine, further reduced homocysteine levels (boy from 118 to 23 microM and girl from 59 to 14 microM). With this treatment, hemolysis has stopped, hematuria has disappeared, proteinuria has almost normalized, and creatinine clearance has been stable. Investigations for chronic thrombotic microangiopathy should include testing for this unusual but treatable disorder, regardless of age of presentation.

Early-onset cobalamin C/D deficiency: epilepsy and electroencephalographic features

PURPOSE

To describe epilepsy and EEG findings in the early-onset cobalamin (Cbl) C/D deficiency, an inborn error of intracellular Cbl metabolism characterized by high plasma levels of methylmalonic acid, homocystine, and homocysteine.

METHODS

Type and frequency of seizures were studied in 10 patients (six boys and four girls) who underwent waking and sleep EEG.

RESULTS

Half of patients had seizures in the first year of life (either concurrent with the other symptoms of disease or some months after the onset of disease); seizures occurred after 2 years in the other half of patients. Convulsive status epilepticus was the initial manifestation in three patients. During the follow-up, nine patients had seizures (mainly partial) despite specific treatment for Cbl C/D deficiency and antiepileptic drugs. Focal or multifocal epileptiform abnormalities during waking EEG that increased during sleep EEG were recorded in the majority of patients. Plasma levels of homocystine and homocysteine were constantly higher than normal, despite therapy institution.

CONCLUSIONS

Epilepsy and EEG abnormalities are prominent features in the early-onset type of combined methylmalonic aciduria and homocystinuria due to Cbl C/D deficiency, possibly related to the pathologically and persistently high levels of homocysteine, experimentally proven to induce seizures. Plasma amino acids evaluation and urinary acid organic analysis should be performed in any infant showing seizures associated with feeding difficulties and failure to thrive, at onset during the first year of life, as well as in any child with convulsive status epilepticus and a history of psychomotor developmental delay of unknown origin.

The role of serine hydroxymethyltransferase isozymes in one-carbon metabolism in MCF-7 cells as determined by (13)C NMR

The role of cytosolic and mitochondrial serine hydroxymethyltransferase in supplying one-carbon groups for purine and thymidylate biosynthesis in MCF-7 cells was investigated by observing folate-mediated one-carbon metabolism of l-[3-(13)C]serine, [2-(13)C]glycine, and [(13)C]formate. (13)C NMR was used to follow the incorporation of label into carbons 2 and 8 of purines and the methyl group attached to carbon 5 of thymidylate. The percentage enrichment of the (13)C label in purines was determined from the splitting patterns of the (1)H NMR spectra of C2 and C8 of adenine and C8 of guanine. The results show that formate is the major precursor in the cytosol of the one-carbon group in 10-formyltetrahydrofolate, which is used in purine biosynthesis, and the one-carbon group in 5,10-methylenetetrahydrofolate, which is used in thymidylate biosynthesis. Formate is formed in the mitochondria from carbon 3 of serine. The cleavage of serine to glycine and 5,10-methylenetetrahydrofolate by cytosolic serine hydroxymethyltransferase does not appear to be a major source of one-carbon groups for either purine or thymidylate biosynthesis. Carbon 3 of serine accounts for about 95% of the one-carbon pool, suggesting that other sources of one-carbon groups represent only minor pathways. [2-(13)C]Glycine is not a donor of one-carbons groups, confirming that MCF-7 cells lack a functional glycine cleavage system.

Mimosine is a cell-specific antagonist of folate metabolism

Iron deficiency and iron chelators are known to alter folate metabolism in mammals, but the underlying biochemical mechanisms have not been established. Although many studies have demonstrated that the iron chelators mimosine and deferoxamine inhibit DNA replication in mammalian cells, their mechanism of action remains controversial. The effects of mimosine on folate metabolism were investigated in human MCF-7 cells and SH-SY5Y neuroblastoma. Our findings indicate that mimosine is a folate antagonist and that its effects are cell-specific. MCF-7 cells cultured in the presence of 350 microm mimosine were growth-arrested, whereas mimosine had no effect on SH-SY5Y cell proliferation. Mimosine altered the distribution of folate cofactor forms in MCF-7 cells, indicating that mimosine targets folate metabolism. However, mimosine does not influence folate metabolism in SH-SY5Y neuroblastoma. The effect of mimosine on folate metabolism is associated with decreased cytoplasmic serine hydroxymethyltransferase (cSHMT) expression in MCF-7 cells but not in SH-SY5Y cells. MCF-7 cells exposed to mimosine for 24 h have a 95% reduction in cSHMT protein, and cSHMT promoter activity is reduced over 95%. Transcription of the cSHMT gene is also inhibited by deferoxamine in MCF-7 cells, indicating that mimosine inhibits cSHMT transcription by chelating iron. Analyses of mimosine-resistant MCF-7 cell lines demonstrate that although the effect of mimosine on cell cycle is independent of its effects on cSHMT expression, it inhibits both processes through a common regulatory mechanism.

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.