Functional methionine synthase deficiency (cblE and cblG): clinical and biochemical heterogeneity

Vitamin B12 Metabolism: A Network of Multi-Protein Mediated Processes

The water-soluble vitamin, vitamin B12, also known as cobalamin, plays a crucial role in cellular metabolism, particularly in DNA synthesis, methylation, and mitochondrial functionality. Its deficiency can lead to hematological and neurological disorders; however, the manifestation of these clinical outcomes is relatively late. It leads to difficulties in the early diagnosis of vitamin B12 deficiency. A prolonged lack of vitamin B12 may have severe consequences including increased morbidity to neurological and cardiovascular diseases. Beyond inadequate dietary intake, vitamin B12 deficiency might be caused by insufficient bioavailability, blood transport disruptions, or impaired cellular uptake and metabolism. Despite nearly 70 years of knowledge since the isolation and characterization of this vitamin, there are still gaps in understanding its metabolic pathways. Thus, this review aims to compile current knowledge about the crucial proteins necessary to efficiently accumulate and process vitamin B12 in humans, presenting these systems as a multi-protein network. The epidemiological consequences, diagnosis, and treatment of vitamin B12 deficiency are also highlighted. We also discuss clinical warnings of vitamin B12 deficiency based on the ongoing test of specific moonlighting proteins engaged in vitamin B12 metabolic pathways.

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.

Inherited defects of cobalamin metabolism

Cobalamin (vitamin B₁₂) is required for activity of the enzymes methylmalonyl-CoA mutase and methionine synthase in human cells. Inborn errors affecting cobalamin uptake or metabolism are characterized by accumulation of the substrates for these enzymes, methylmalonic acid and homocysteine, in blood and urine. Inborn errors affecting synthesis of the adenosylcobalamin coenzyme required by methylmalonyl-CoA mutase (cblA and cblB) result in isolated methylmalonic aciduria; inborn errors affecting synthesis of the methylcobalamin coenzyme required by methionine synthase (cblE and cblG) result in isolated homocystinuria. Combined methylmalonic aciduria and homocystinuria is seen in patients with impaired intestinal cobalamin absorption (intrinsic factor deficiency, Imerslund-Gräsbeck syndrome) and with defects affecting synthesis of both cobalamin coenzymes (cblC, cblD, cblF and cblJ). A series of disorders caused by pathogenic variant mutations affecting gene regulators (transcription factors) of the MMACHC gene have recently been described (HCFC1 [cblX disorder] and deficiencies of THAP11, and ZNF143 [the cblK disorder]).

Methyl Metabolism and the Clock: An Ancient Story With New Perspectives

Methylation, that is, the transfer or synthesis of a –CH₃ group onto a target molecule, is a pervasive biochemical modification found in organisms from bacteria to humans. In mammals, a complex metabolic pathway powered by the essential nutrients vitamin B9 and B12, methionine and choline, synthesizes S-adenosylmethionine, the methyl donor in the methylation of nucleic acids, proteins, fatty acids, and small molecules by over 200 substrate-specific methyltransferases described so far in humans. Methylations not only play a key role in scenarios for the origin and evolution of life, but they remain essential for the development and physiology of organisms alive today, and methylation deficiencies contribute to the etiology of many pathologies. The methylation of histones and DNA is important for circadian rhythms in many organisms, and global inhibition of methyl metabolism similarly affects biological rhythms in prokaryotes and eukaryotes. These observations, together with various pieces of evidence scattered in the literature on circadian gene expression and metabolism, indicate a close mutual interdependence between biological rhythms and methyl metabolism that may originate from prebiotic chemistry. This perspective first proposes an abiogenetic scenario for rhythmic methylations and then outlines mammalian methyl metabolism, before reanalyzing previously published data to draw a tentative map of its profound connections with the circadian clock.

Methionine synthase deficiency: Variable clinical presentation and benefit of early diagnosis and treatment

Methionine synthase deficiency (cblG complementation group) is a rare inborn error of metabolism affecting the homocysteine re-methylation pathway. It leads to a biochemical phenotype of hyperhomocysteinemia and hypomethioninemia. The clinical presentation of cblG is variable, ranging from seizures, encephalopathy, macrocytic anemia, hypotonia, and feeding difficulties in the neonatal period to onset of psychiatric symptoms or acute neurologic changes in adolescence or adulthood. Given the variable and nonspecific symptoms seen in cblG, the diagnosis of affected patients is often delayed. Medical management of cblG includes the use of hydroxocobalamin, betaine, folinic acid, and in some cases methionine supplementation. Treatment has been shown to lead to improvement in the biochemical profile of affected patients, with lowering of total homocysteine levels and increasing methionine levels. However, the published literature contains differing conclusions on whether treatment is effective in changing the natural history of the disease. Herein, we present five patients with cblG who have shown substantial clinical benefit from treatment with objective improvement in their neurologic outcomes. We demonstrate more favorable outcomes in our patients who were treated early in life, especially those who were treated before neurologic symptoms manifested. Given improved outcomes from treatment of presymptomatic patients, cblG warrants inclusion in newborn screening.

Ocular manifestations in patients with inborn errors of intracellular cobalamin metabolism: a systematic review

Inherited disorders of cobalamin (cbl) metabolism (cblA-J) result in accumulation of methylmalonic acid (MMA) and/or homocystinuria (HCU). Clinical presentation includes ophthalmological manifestations related to retina, optic nerve and posterior visual alterations, mainly reported in cblC and sporadically in other cbl inborn errors.We searched MEDLINE EMBASE and Cochrane Library, and analyzed articles reporting ocular manifestations in cbl inborn errors. Out of 166 studies a total of 52 studies reporting 163 cbl and 24 mut cases were included. Ocular manifestations were found in all cbl defects except for cblB and cblD-MMA; cblC was the most frequent disorder affecting 137 (84.0%) patients. The c.271dupA was the most common pathogenic variant, accounting for 70/105 (66.7%) cases. One hundred and thirty-seven out of 154 (88.9%) patients presented with early-onset disease (0-12 months). Nystagmus and strabismus were observed in all groups with the exception of MMA patients while maculopathy and peripheral retinal degeneration were almost exclusively found in MMA-HCU patients. Optic nerve damage ranging from mild temporal disc pallor to complete atrophy was prevalent in MMA-HCU.and MMA groups. Nystagmus was frequent in early-onset patients. Retinal and macular degeneration worsened despite early treatment and stabilized systemic function in these patients. The functional prognosis remains poor with final visual acuity < 20/200 in 55.6% (25/45) of cases. In conclusion, the spectrum of eye disease in Cbl patients depends on metabolic severity and age of onset. The development of visual manifestations over time despite early metabolic treatment point out the need for specific innovative therapies.

Neuroprotective Roles of the Reverse Transsulfuration Pathway in Alzheimer's Disease

The reverse transsulfuration pathway has emerged as a central hub that integrates the metabolism of sulfur-containing amino acids and redox homeostasis. Transsulfuration involves the transfer of sulfur from homocysteine to cysteine. Cysteine serves as the precursor for several sulfur-containing molecules, which play diverse roles in cellular processes. Recent evidence shows that disruption of the flux through the pathway has deleterious consequences. In this review article, I will discuss the actions and regulation of the reverse transsulfuration pathway and its links to other metabolic pathways, which are disrupted in Alzheimer’s disease (AD). The potential nodes of therapeutic intervention are also discussed, which may pave the way for the development of novel treatments.

Naturally occurring cobalamin (B12) analogs can function as cofactors for human methylmalonyl-CoA mutase

Cobalamin, commonly known as vitamin B₁₂, is an essential micronutrient for humans because of its role as an enzyme cofactor. Cobalamin is one of over a dozen structurally related compounds - cobamides - that are found in certain foods and are produced by microorganisms in the human gut. Very little is known about how different cobamides affect B₁₂-dependent metabolism in human cells. Here, we test in vitro how diverse cobamide cofactors affect the function of methylmalonyl-CoA mutase (MMUT), one of two cobalamin-dependent enzymes in humans. We find that, although cobalamin is the most effective cofactor for MMUT, multiple cobamides support MMUT function with differences in binding affinity (K_d), binding kinetics (k_on), and concentration dependence during catalysis (K_{M, app}). Additionally, we find that six disease-associated MMUT variants that cause cobalamin-responsive impairments in enzymatic activity also respond to other cobamides, with the extent of catalytic rescue dependent on the identity of the cobamide. Our studies challenge the exclusive focus on cobalamin in the context of human physiology, indicate that diverse cobamides can support the function of a human enzyme, and suggest future directions that will improve our understanding of the roles of different cobamides in human biology.

Genes and genetics in hyperhomocysteinemia and the "1-carbon metabolism": implications for retinal structure and eye functions

Homocysteine (Hcy), a sulfur-containing nonproteinogenic amino acid, is generated as a metabolic intermediate. Hcy constitutes an important part of the "1-carbon metabolism" during methionine turnover. Elevated levels of Hcy known as hyperhomocysteinemia (HHcy) results from vitamin B deficiency, lack of exercise, smoking, excessive alcohol intake, high-fat and methionine-rich diet, and the underlying genetic defects. These factors directly affect the "1-carbon metabolism (methionine-Hcy-folate)" of a given cell. In fact, the Hcy levels are determined primarily by dietary intake, vitamin status, and the genetic blueprint of the susceptible individual. Although Hcy performs an important role in cellular functions, genetic alterations in any of the key enzymes responsible for the "1-carbon metabolism" could potentially upset the metabolic cycle, thus causing HHcy environment in susceptible people. As such, HHcy relates to several clinical conditions like atherosclerosis, myocardial infarction, stroke, cognitive impairment, dementia, Parkinson's disease, multiple sclerosis, epilepsy, and ocular disorders, among others. This article summarizes the findings from our laboratory and public database regarding genetics of HHcy and its effects on ocular disorders, their respective management during dysregulation of the 1-carbon metabolism.

Genetic variants of the folate metabolic system and mild hyperhomocysteinemia may affect ADHD associated behavioral problems

An etiologically complex disorder, Attention Deficit Hyperactivity Disorder (ADHD), is often associated with various levels of cognitive deficit. Folate/vitamin B₉ is crucial for numerous biochemical pathways including neural stem cell proliferation and differentiation, regulation of gene expression, neurotransmitter synthesis, myelin synthesis and repair, etc. and a scarcity has often been linked to cognitive deficit. Our pilot study in the field revealed significant association of few genetic variants with ADHD. Mild hyperhomocysteinemia and vitamin B₁₂ deficiency was also noticed in the probands. In the present study additional genetic variants, folate and vitamin B₆, which may affect folate-homocysteine metabolic pathway, were investigated in 866 individuals including nuclear families with ADHD probands (N=221) and ethnically matched controls (N=286) to find out whether ADHD associated traits are affected by these factors. Population based analysis revealed significant over representation of MTRR rs1801394 "G" allele and "GG" genotype in all as well as male probands. Stratified analysis showed significantly higher frequency of RFC1 rs1051266 and BHMT rs3733890 "AG" genotypes in full term and prematurely delivered ADHD probands respectively. Probands with rs1801394 "GG" genotype and BHMT rs3733890 "G" allele showed association with hyperhomocysteinemia. MTHFR rs1801131, MTR rs1805087 and BHMT rs3733890 also showed association with ADHD index. While rs1051266, rs1801131, and rs1805087 showed association with behavioral problems, rs3733890 was associated with ODD score. Conduct problem exhibited association with RFC1 rs1051266, MTHFR rs1801133 and MTRR rs1801394. Gene-gene interaction analysis revealed positive synergistic interactions between rs1051266, rs1801131 and rs1801394 in the probands as compared to the controls. It can be inferred from the data obtained that folate system genetic variants and mild hyperhomocysteimenia may affect ADHD associated traits by attenuating folate metabolism.

Guidelines for diagnosis and management of the cobalamin-related remethylation disorders cblC, cblD, cblE, cblF, cblG, cblJ and MTHFR deficiency

BACKGROUND

Remethylation defects are rare inherited disorders in which impaired remethylation of homocysteine to methionine leads to accumulation of homocysteine and perturbation of numerous methylation reactions.

OBJECTIVE

To summarise clinical and biochemical characteristics of these severe disorders and to provide guidelines on diagnosis and management.

DATA SOURCES

Review, evaluation and discussion of the medical literature (Medline, Cochrane databases) by a panel of experts on these rare diseases following the GRADE approach.

KEY RECOMMENDATIONS

We strongly recommend measuring plasma total homocysteine in any patient presenting with the combination of neurological and/or visual and/or haematological symptoms, subacute spinal cord degeneration, atypical haemolytic uraemic syndrome or unexplained vascular thrombosis. We strongly recommend to initiate treatment with parenteral hydroxocobalamin without delay in any suspected remethylation disorder; it significantly improves survival and incidence of severe complications. We strongly recommend betaine treatment in individuals with MTHFR deficiency; it improves the outcome and prevents disease when given early.

Homocysteine and disease: Causal associations or epiphenomenons?

Nutritional and genetic deficiencies of folate and vitamin B₁₂ lead to elevation of cellular homocysteine (Hcy), which translates in increased plasma Hcy. The sources and role of elevated plasma Hcy in pathology continues to be a subject of intense scientific debate. Whether a cause, mediator or marker, little is known about the molecular mechanisms and interactions of Hcy with cellular processes that lead to disease. The use of folic acid reduces the incidence of neural tube defects, but the effect of Hcy-lowering interventions with folic acid in cardiovascular disease and cognitive impairment remains controversial. The fact that levels of Hcy in plasma do not always reflect cellular status of this amino acid may account for the substantial gaps that exist between epidemiological, intervention and basic research studies. Understanding whether plasma Hcy is a mechanistic player or an epiphenomenon in pathogenesis requires further investigation, and this research is essential to improve the assessment and potential treatment of hyperhomocysteinemias.

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.

Quantitative assessment of the association between MS gene polymorphism and colorectal cancer risk

Accumulating genetic association studies have investigated the risk of colorectal cancer (CRC) in relation to MS gene polymorphism with uncertain conclusions. In the current study, we sought to assess the association between MS gene and CRC. We performed an updated meta-analysis including 18 case-control studies with a total of 10, 303 CRC patients and 15, 389 CRC-free controls to estimate the strength of the association using odds ratios with the corresponding 95 % confidence intervals. Overall, no CRC risk associated with the genotypes of MS gene polymorphism was indicated in our meta-analysis. Similarly, the stratified analysis according to ethnicity and control source did not show any evident association either. The results of our updated meta-analysis suggest that MS gene polymorphism may not serve as a biomarker for the CRC risk. Future large-scale and well-designed studies are required to clarify the association identified in the present meta-analysis.

Novel insights in folate receptors and transporters: implications for disease and treatment of immune diseases and cancer

Folate receptors and transporters as well as folate enzymes play an essential role in human disease and form important targets for the treatment of immune diseases and cancer. To discuss new developments in this area, every 2 years a multidisciplinary meeting is held, which aims to be an informal forum for fundamental scientists and clinicians. During this meeting, the regulation of folate transporters and folate enzymes is discussed at the level of expression, transcription, translation, post-translational modification, and splicing and enzyme regulation. Importantly, this knowledge is applied and translated into exciting clinical applications by clinicians with various backgrounds, such as surgeons, nephrologists, rheumatologists and oncologists. Moreover, the meeting provides an excellent forum for a scientific interaction between academia and industry.

Methionine synthase reductase deficiency (CblE): A report of two patients and a novel mutation

IMPORTANCE

Functional methionine synthase reductase deficiency, also known as cobalamin E disorder, is a rare autosomal recessive inherited disease that results in an impaired remethylation of homocysteine to methionine. It presents with macrocytic anemia, hyperhomocysteinemia, and hypomethioninemia, and may also be accompanied with neurological impairment.

CLINICAL PRESENTATION

We describe two new cases of unrelated girls with megaloblastic anemia misclassified at first as congenital dyserythropoietic anemia with development of neurologic dysfunction in one of them.

INTERVENTION

The posterior finding of biochemical features (hyperhomocysteinemia and hypomethioninemia) focused the diagnosis on the inborn errors of intracellular vitamin B12. Subsequent molecular analysis of the methionine synthase reductase (MTRR) gene revealed compound heterozygosity for a transition c.1361C > T (p.Ser454Leu) and another, not yet described in literature, c.1677-1G > A (p.Glu560fs) in one patient, and a single homozygosis mutation, c.1361C > T (p.Ser545Leu) in the other one. These mutations confirmed the diagnosis of cobalamin E deficiency.

CONCLUSION

Treatment with hydroxocobalamin in combination with betaine appears to be useful for hematological improvement and prevention of brain disabilities in CblE-affected patients. Our study widens the clinical, molecular, metabolic, and cytological knowledge of deficiency MTRR enzyme.

Sulfur as a signaling nutrient through hydrogen sulfide

Hydrogen sulfide (H₂S) has emerged as an important signaling molecule with beneficial effects on various cellular processes affecting, for example, cardiovascular and neurological functions. The physiological importance of H₂S is motivating efforts to develop strategies for modulating its levels. However, advancement in the field of H₂S-based therapeutics is hampered by fundamental gaps in our knowledge of how H₂S is regulated, its mechanism of action, and its molecular targets. This review provides an overview of sulfur metabolism; describes recent progress that has shed light on the mechanism of H₂S as a signaling molecule; and examines nutritional regulation of sulfur metabolism, which pertains to health and disease.

Mouse model for deficiency of methionine synthase reductase exhibits short-term memory impairment and disturbances in brain choline metabolism

Hyperhomocysteinaemia can contribute to cognitive impairment and brain atrophy. MTRR (methionine synthase reductase) activates methionine synthase, which catalyses homocysteine remethylation to methionine. Severe MTRR deficiency results in homocystinuria with cognitive and motor impairments. An MTRR polymorphism may influence homocysteine levels and reproductive outcomes. The goal of the present study was to determine whether mild hyperhomocysteinaemia affects neurological function in a mouse model with Mtrr deficiency. Mtrr+/+, Mtrr+/gt and Mtrrgt/gt mice (3 months old) were assessed for short-term memory, brain volumes and hippocampal morphology. We also measured DNA methylation, apoptosis, neurogenesis, choline metabolites and expression of ChAT (choline acetyltransferase) and AChE (acetylcholinesterase) in the hippocampus. Mtrrgt/gt mice exhibited short-term memory impairment on two tasks. They had global DNA hypomethylation and decreased choline, betaine and acetylcholine levels. Expression of ChAT and AChE was increased and decreased respectively. At 3 weeks of age, they showed increased neurogenesis. In the cerebellum, mutant mice had DNA hypomethylation, decreased choline and increased expression of ChAT. Our work demonstrates that mild hyperhomocysteinaemia is associated with memory impairment. We propose a mechanism whereby a deficiency in methionine synthesis leads to hypomethylation and compensatory disturbances in choline metabolism in the hippocampus. This disturbance affects the levels of acetylcholine, a critical neurotransmitter in learning and memory.

Navigating the B(12) road: assimilation, delivery, and disorders of cobalamin

The reactivity of the cobalt-carbon bond in cobalamins is the key to their chemical versatility, supporting both methyl transfer and isomerization reactions. During evolution of higher eukaryotes that utilize vitamin B12, the high reactivity of the cofactor coupled with its low abundance pressured development of an efficient system for uptake, assimilation, and delivery of the cofactor to client B12-dependent enzymes. Although most proteins suspected to be involved in B12 trafficking were discovered by 2009, the recent identification of a new protein reveals that the quest for elucidating the intracellular B12 highway is still far from complete. Herein, we review the biochemistry of cobalamin trafficking.

Update and new concepts in vitamin responsive disorders of folate transport and metabolism

Derivatives of folic acid are involved in transfer of one-carbon units in cellular metabolism, playing a role in synthesis of purines and thymidylate and in the remethylation of homocysteine to form methionine. Five inborn errors affecting folate transport and metabolism have been well studied: hereditary folate malabsorption, caused by mutations in the gene encoding the proton-coupled folate transporter (SLC46A1); glutamate formiminotransferase deficiency, caused by mutations in the FTCD gene; methylenetetrahydrofolate reductase deficiency, caused by mutations in the MTHFR gene; and functional methionine synthase deficiency, either as the result of mutations affecting methionine synthase itself (cblG, caused by mutations in the MTR gene) or affecting the accessory protein methionine synthase reductase (cblE, caused by mutations in the MTRR gene). Recently additional inborn errors have been identified. Cerebral folate deficiency is a clinically heterogeneous disorder, which in a few families is caused by mutations in the FOLR1 gene. Dihydrofolate reductase deficiency is characterized by megaloblastic anemia and cerebral folate deficiency, with variable neurological findings. It is caused by mutations in the DHFR gene. Deficiency in the trifunctional enzyme containing methylenetetrahydrofolate dehydrogenase, methenyltetrahydrofolate cyclohydrolase and formyltetrahydrofolate synthetase activities, has been identified in a single patient with megaloblastic anemia, atypical hemolytic uremic syndrome and severe combined immune deficiency. It is caused by mutations in the MTHFD1 gene.

Importance of gene variants and co-factors of folate metabolic pathway in the etiology of idiopathic intellectual disability

Different components of the folate metabolic cycle are crucial for maintaining integrity of DNA. The present study was aimed at exploring the role of some important constituents of the folate cycle in the etiology of idiopathic intellectual disability (IID). Nuclear families with IID probands (n=226) and ethnically matched controls (n=181) were recruited for micronucleus, karyotype, genetic polymorphism (MTR rs1805087, MTRR rs1801394, and DHFR rs70991108), folate, vitamin B6, vitamin B12, and cysteine analysis. Significant difference in genotype frequencies in IID probands and their parents were observed for rs1805087 (P=0.03, 0.02), rs1801394 (P=0.03, 0.001), and rs70991108 ((P=0.03, 0.02) as compared to controls. IID probands showed significantly higher micronucleus frequency (P=0.01) and decreased vitamin B6 level (P=0.002). A strong correlation between rs1801394 'G' allele and micronucleus was also noticed. From the present investigation, a role of genetic polymorphisms and vitamin B6 levels could be hypothesized in the etiology of IID.

Inborn errors of cobalamin absorption and metabolism

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

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

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

Two-tier approach to the newborn screening of methylenetetrahydrofolate reductase deficiency and other remethylation disorders with tandem mass spectrometry

OBJECTIVE

To validate a 2-tier approach for newborn screening (NBS) of remethylation defects.

STUDY DESIGN

The original NBS dried blood spots of 5 patients with a proven diagnosis of a remethylation disorder and 1 patient with biochemical evidence of such disorder were analyzed retrospectively to determine disease ranges for methionine (Met; 4.7-8.1 micromol/L; 1 percentile of healthy population, 11.1 micromol/L), the methionine/phenylalanine ratio (Met/Phe; 0.09-0.16; 1 percentile of healthy population, 0.22), and total homocysteine (tHcy; 42-157 micromol/L; 99 percentile of normal population, 14.7 micromol/L). These preliminary disease ranges showed a sufficient degree of segregation from healthy population data, allowing the selection of cutoff values. A simple algorithm was then developed to reflex cases to a second-tier testing for tHcy, which has been applied prospectively for 14 months.

RESULTS

A total of 86 333 NBS samples were tested between January 2007 and March 2008, and 233 of them (0.27%) met the criteria for second-tier testing of tHcy. All cases revealed concentrations of tHcy <15 micromol/L and were considered unaffected. No false-negative results have been reported with a state-wide system based on 2 combined metabolic clinics and laboratories that cover the entire Minnesota population and border areas of neighboring states.

CONCLUSIONS

Pending more conclusive evidence from the prospective identification of additional true-positive cases, NBS for remethylation disorders appears to be feasible with existing methodologies, with only a marginal increase of the laboratory workload.

Global analysis of transcriptional regulation by poly(ADP-ribose) polymerase-1 and poly(ADP-ribose) glycohydrolase in MCF-7 human breast cancer cells

Poly(ADP-ribose) polymerase-1 (PARP-1) and poly(ADP-ribose) glycohydrolase (PARG) are enzymes that modify target proteins by the addition and removal, respectively, of ADP-ribose polymers. Although a role for PARP-1 in gene regulation has been well established, the role of PARG is less clear. To investigate how PARP-1 and PARG coordinately regulate global patterns of gene expression, we used short hairpin RNAs to stably knock down PARP-1 or PARG in MCF-7 cells followed by expression microarray analyses. Correlation analyses showed that the majority of genes affected by the knockdown of one factor were similarly affected by the knockdown of the other factor. The most robustly regulated common genes were enriched for stress-response and metabolic functions. In chromatin immunoprecipitation assays, PARP-1 and PARG localized to the promoters of positively and negatively regulated target genes. The levels of chromatin-bound PARG at a given promoter generally correlated with the levels of PARP-1 across the subset of promoters tested. For about half of the genes tested, the binding of PARP-1 at the promoter was dependent on the binding of PARG. Experiments using stable re-expression of short hairpin RNA-resistant catalytic mutants showed that PARP-1 and PARG enzymatic activities are required for some, but not all, target genes. Collectively, our results indicate that PARP-1 and PARG, nuclear enzymes with opposing enzymatic activities, localize to target promoters and act in a similar, rather than antagonistic, manner to regulate gene expression.

The tinker, tailor, soldier in intracellular B12 trafficking

The recognition of eight discrete genetic complementation groups among patients with inherited cobalamin disorders provided early insights into the complexity of a cofactor-processing pathway that supports only two known B(12)-dependent enzymes in mammals. With the identification of all eight genes now completed, biochemical interrogations of their functions have started and are providing novel insights into a trafficking pathway involving porters that tinker with and tailor the active cofactor forms and editors that ensure the fidelity of the cofactor loading process. The principles of sequestration and escorted delivery of a rare and reactive organometallic cofactor that are emerging from studies on B(12) might be of general relevance to other cofactor trafficking pathways.

Homocysteine-NMDA receptor-mediated activation of extracellular signal-regulated kinase leads to neuronal cell death

Hyperhomocysteinemia is an independent risk factor for stroke and neurological abnormalities. However, the underlying cellular mechanisms by which elevated homocysteine can promote neuronal death is not clear. In the present study we have examined the role of NMDA receptor-mediated activation of the extracellular signal-regulated kinase-mitogen-activated protein (ERK-MAP) kinase pathway in homocysteine-dependent neurotoxicity. The study demonstrates that in neurons l-homocysteine-induced cell death was mediated through activation of NMDA receptors. The study also shows that homocysteine-dependent NMDA receptor stimulation and resultant Ca2+ influx leads to rapid and sustained phosphorylation of ERK-MAP kinase. Inhibition of ERK phosphorylation attenuates homocysteine-mediated neuronal cell death thereby demonstrating that activation of ERK-MAP kinase signaling pathway is an intermediate step that couples homocysteine-mediated NMDA receptor stimulation to neuronal death. The findings also show that cAMP response-element binding protein (CREB), a pro-survival transcription factor and a downstream target of ERK, is only transiently activated following homocysteine exposure. The sustained activation of ERK but a transient activation of CREB together suggest that exposure to homocysteine initiates a feedback loop that shuts off CREB signaling without affecting ERK phosphorylation and thereby facilitates homocysteine-mediated neurotoxicity.

Severe skin necrosis after breast reconstruction with a transverse rectus abdominis musculocutaneous flap in methylenetetrahydrofolate reductase deficiency

We report a case of unexpected severe skin necrosis after autologous transverse rectus abdominis musculocutaneous flap breast reconstruction in a patient with homozygosis for the 5,10- methylenetetrahydrofolate reductase (MTHFR) gene. This genetic deficiency is hypothesised as the cause of this exceptional skin necrosis. A 46-year-old woman underwent a radical mastectomy and immediate rectus abdominis musculocutaneous flap breast reconstruction. At the end of surgery, the blood supply to the flap and the abdominal wall was excellent. On the 5th postoperative day, the patient developed an extensive abdominal skin necrosis and a partial flap necrosis on the reconstructed breast. The rectus abdominis musculocutaneous flap breast reconstruction can be proposed to patients without any haemostatic defect in order to avoid life-threatening complications and unaesthetic results. This procedure requires careful patient selection, detailed preoperative planning, and complete laboratory investigations: However, mutation of the 5,10- methylenetetrahydrofolate reductase gene is too exceptional to form part of the routine preoperative investigation and can be looked for in these cases of extensive necrosis.

Restricted role for methionine synthase reductase defined by subcellular localization

Methionine synthase reductase (MSR; gene name MTRR) is responsible for the reductive activation of methionine synthase. Cloning of the MTRR gene had revealed two major transcription start sites which, by alternative splicing, allows for two potential translation products of 698 and 725 amino acids. While the shorter protein was expected to target the cytosol where methionine synthase is located, the additional sequence in the longer protein was consistent with a role as a mitochondrial leader sequence. The possibility that MSR might target mitochondria was also suggested by the work of Leal et al. [N.A. Leal, H. Olteanu, R. Banerjee, T.A. Bobik, Human ATP:Cob(I)alamin adenosyltransferase and its interaction with methionine synthase reductase, J. Biol. Chem. 279 (2004) 47536-47542.] who showed that it can act as the reducing enzyme in combination with MMAB (ATP:Cob(I)alamin adenosyltransferase) to generate adenosylcobalamin from cob(II)alamin in vitro. Here we examined directly whether MSR protein is found in mitochondria. We show that, while two transcripts are produced by alternative splicing, the N-terminal segment of the putative mitochondrial form of MSR fused to GFP does not contain a sufficiently strong mitochondrial leader sequence to direct the fusion protein to the mitochondria of human fibroblasts. Further, antibodies to MSR protein localized MSR to the cytosol, but not to the mitochondria of human fibroblasts or the human hepatoma line Huh-1, as determined by Western blot analysis and immunofluorescence of cells in situ. These data confirm that MSR protein is restricted to the cytosol but, based on the Leal study, suggest that a similar protein may interact with MMAB to reduce the mitochondrial cobalamin substrate in the generation of adenosylcobalamin.

Ginsenosides: are any of them candidates for drugs acting on the central nervous system?

The last two decades have shown a marked expansion in the number of publications regarding the effects of Panax ginseng. Ginsenosides, which are unique saponins isolated from Panax ginseng, are the pharmacologically active ingredients in ginseng, responsible for its effects on the central nervous system (CNS) and the peripheral nervous system. Recent studies have shown that ginsenosides regulate various types of ion channels, such as voltage-dependent and ligand-gated ion channels, in neuronal and heterologously expressed cells. Ginsenosides inhibit voltage-dependent Ca(2+), K(+), and Na(+) channel activities in a stereospecific manner. Ginsenosides also inhibit ligand-gated ion channels such as N-methyl-d-aspartate, some subtypes of nicotinic acetylcholine, and 5-hydroxytryptamine type 3 receptors. Competition and site-directed mutagenesis experiments revealed that ginsenosides interact with ligand-binding sites or channel pore sites and inhibit open states of ion channels. This review will introduce recent findings and advances on ginsenoside-induced regulation of ion channel activities in the CNS, and will further expand the possibilities that ginsenosides may be useful and potentially therapeutic choices in the treatment of neurodegenerative disorders.

Crystal structure and solution characterization of the activation domain of human methionine synthase

Human methionine synthase (hMS) is a multidomain cobalamin-dependent enzyme that catalyses the conversion of homocysteine to methionine by methyl group transfer. We report here the 1.6 A crystal structure of the C-terminal activation domain of hMS. The structure is C-shaped with the core comprising mixed alpha and beta regions, dominated by a twisted antiparallel beta sheet with a beta-meander region. These features, including the positions of the active-site residues, are similar to the activation domain of Escherichia coli cobalamin-dependent MS (MetH). Structural and solution studies suggest a small proportion of hMS activation domain exists in a dimeric form, which contrasts with the monomeric form of the E. coli homologue. Fluorescence studies show that human activation domain interacts with the FMN-binding domain of human methionine synthase reductase (hMSR). This interaction is enhanced in the presence of S-adenosyl-methionine. Binding of the D963E/K1071N mutant activation domain to the FMN domain of MSR is weaker than with wild-type activation domain. This suggests that one or both of the residues D963 and K1071 are important in partner binding. Key differences in the sequences and structures of hMS and MetH activation domains are recognized and include a major reorientation of an extended 3(10)-containing loop in the human protein. This structural alteration might reflect differences in their respective reactivation complexes and/or potential for dimer formation. The reported structure is a component of the multidomain hMS : MSR complex, and represents an important step in understanding the impact of clinical mutations and polymorphisms in this key electron transfer complex.

Homocysteine and Parkinson's disease: a dangerous liaison?

Homocysteine, a sulphur-containing amino acid formed by demethylation of methionine, is involved in numerous processes of methyl group transfer, all playing pivotal roles in the biochemistry of the human body. Increased levels of plasma homocysteine (hyperhomocysteinemia) - which may result from a deficiency of folate, vitamin B6 or B12 or mutations in enzymes regulating the catabolism of homocysteine - are associated with a wide range of clinical manifestations, mostly affecting the central nervous system (e.g., mental retardation, cerebral atrophy and epileptic seizures). Recent evidence suggests that changes in the metabolic fate of homocysteine, leading to hyperhomocysteinemia, may also play a role in the pathophysiology of neurodegenerative disorders, particularly Parkinson's disease (PD). The nervous system might be particularly sensitive to homocysteine, due to the excitotoxic-like properties of the amino acid. However, experimental findings have shown that homocysteine does not seem to posses direct, cytotoxic activity, while the amino acid has proven able to synergize with more specific neurotoxic insults. Hyperhomocysteinemia has been repeatedly reported in PD patients; the increase, however, seems mostly related to the methylated catabolism of l-Dopa, the main pharmacological treatment of PD. Therefore, hyperhomocysteinemia may not be specific to movement disorders or other neurological diseases, the condition being, in fact, rather the result of the combinations of different factors, mainly metabolic, but also genetic and pharmacological, intervening in the neurodegenerative process.

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

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

Neuroprotective effects of ginsenoside Rg3 against homocysteine-induced excitotoxicity in rat hippocampus

We previously demonstrated that ginsenoside Rg(3) (Rg(3)), one of the active ingredients in Panax ginseng, attenuates NMDA receptor-mediated currents and NMDA-induced neurotoxicity (Kim, S., Kim, T., Ahn, K., Park, W.K., Nah, S.Y., Rhim, H., 2004. Ginsenoside Rg(3) antagonizes NMDA receptors through a glycine modulatory site in rat cultured hippocampal neurons. Biochem. Biophys. Res. Commun. 323, 416-424). Accumulating evidence suggests that homocysteine (HC), a metabolite of methionine, exerts its excitotoxicity through NMDA receptor activation. In the present study, we examined the neuroprotective effects of Rg(3) on HC-induced hippocampal excitotoxicity in vitro and in vivo. Our in vitro studies using rat cultured hippocampal neurons revealed that Rg(3) treatment significantly and dose-dependently inhibited HC-induced hippocampal cell death, with an EC(50) value of 28.7+/-7.5 muM. Rg(3) treatment not only significantly reduced HC-induced DNA damage, but also dose-dependently attenuated HC-induced caspase-3 activity in vitro. Our in vivo studies revealed that intracerebroventricular (i.c.v.) pre-administration of Rg(3) significantly and dose-dependently reduced i.c.v. HC-induced hippocampal damage in rats. To examine the mechanisms underlying the in vitro and in vivo neuroprotective effects of Rg(3) against HC-induced hippocampal excitotoxicity, we examined the effect of Rg(3) on HC-induced intracellular Ca(2+) elevations in cultured hippocampal cells and found that Rg(3) treatment dose-dependently inhibited HC-induced intracellular Ca(2+) elevation, with an IC(50) value of 41.5+/-17.5 muM. In addition, Rg(3) treatment dose-dependently inhibited HC-induced currents in Xenopus oocytes expressing the NMDA receptor, with an IC(50) of 47.3+/-14.2 muM. These results collectively indicate that Rg(3)-induced neuroprotection against HC in rat hippocampus might be achieved via inhibition of HC-mediated NMDA receptor activation.

Expression and activity of methionine cycle genes are altered following folate and vitamin E deficiency under oxidative challenge: modulation by apolipoprotein E-deficiency

Folate deficiency increases neuronal oxidative damage and potentiates the deleterious effects of apolipoprotein E (ApoE) deficiency. Mice lacking ApoE (ApoE -/- mice) upregulate the expression and activity of another enzyme, glutathione synthase (GS), when deprived of folate, in an apparent attempt to compensate for increased oxidative damage. Herein, we examined the influence of ApoE and folate deficiency on expression and activity of several enzymes of the methionine cycle. Expression and activity of methylene tetrahydrofolate reductase was increased in the order ApoE +/+ < ApoE +/- < ApoE -/- in response to folate deprivation. Expression of cystathione beta synthase followed a similar pattern. By contrast, expression and activity of methionine synthase decreased following folate deprivation in the order ApoE +/+ < ApoE +/- < ApoE -/-. These studies demonstrate that folate deficiency induces compensatory regulation of methionine cycle genes, and that these effects are potentiated by ApoE deficiency in a gene-dosage manner. They further support the notion that latent genetic deficiencies, including those of methionine cycle, may contribute to Alzheimer's disease, especially in concert with age-related nutritional deficiencies.

Subacute combined degeneration of the spinal cord in cblC disorder despite treatment with B12

Subacute combined degeneration (SCD) of the spinal cord is a characteristic complication of vitamin B12 deficiency, but it has never been neuropathologically demonstrated in a B12-inborn error of metabolism. In this report SCD is documented in a 15-year-old boy with early-onset cobalamin C (cblC) disorder. The neuropathologic findings included multifocal demyelination and vacuolation with predilection for the dorsal and lateral columns at the mid-thoracic level of the spinal cord, confirming the similarity of SCD in cblC disorder to the classic adult SCD due to vitamin B12 deficiency. SCD developed in this boy despite treatment for cblC disorder that began at 3 months of age. There is clinical and experimental evidence to suggest that a deficiency in remethylation with concomitant reduction in brain methionine may be the cause of SCD. In this patient plasma methionine levels were low without betaine and/or l-methionine supplementation and in the normal range for only a 2-year period during compliance with therapy. In cblC disorder, a consistent increase in blood methionine to high normal or above normal levels by the use of betaine and l-methionine supplementation may be helpful in preventing SCD. This is especially important now that the presymptomatic detection of cblC disorder is possible through the expansion of newborn screening.

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.

B12 trafficking in mammals: A for coenzyme escort service

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

Production of homocysteine in serum-starved apoptotic PC12 cells depends on the activation and modification of Ras

PC12 pheochromocytoma cells expressing a dominant inhibitory mutant of Ha-Ras (M-M17-26) and PC12 cells transfected with normal c-RasH (M-CR3B) have been used to investigate the role of nitrosylation and farnesylation of Ras on the production of homocysteine and the activities of the redox-sensitive transcription factors NF-kappaB and c-Fos. We found that under serum and nerve growth factor withdrawal conditions undifferentiated apoptotic M-CR3B cells accumulated more homocysteine than M-M17-26 cells, and the production of homocysteine decreased in the presence of manumycin and increased in the presence of l-NAME. Furthermore, we have shown that manumycin increased the activity of c-Fos in the M-CR3B cells and decreased the activity of NF-kappaB, while l-NAME decreased the activities of both transcription factors, and accelerated apoptosis of M-CR3B cells. In contrast, in M-M17-26 cells manumycin did not change the activity of c-Fos, nor the activity of NF-kappaB. We conclude that trophic factor withdrawal stimulates Ras, which apparently through the Rac/NADPH oxidase system induces permanent oxidative stress, modulates the activities of NF-kappaB and c-Fos, induces production of homocysteine and accelerates apoptosis. Nitrosylation of Ras is necessary for maintaining the survival of PC12 cells, while farnesylation of Ras stimulates apoptosis under withdrawal conditions.

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.

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.