Biochemical and clinical response to hydroxocobalamin versus cyanocobalamin treatment in patients with methylmalonic acidemia and homocystinuria (cblC)

Recognizing and Managing a Metabolic Crisis

In some relatively common inborn errors of metabolism there can be the accumulation of toxic compounds including ammonia and organic acids such as lactate and ketoacids, as well as energy deficits at the cellular level. The clinical presentation is often referred to as a metabolic emergency or crisis. Fasting and illness can result in encephalopathy within hours, and without appropriate recognition and intervention, the outcome may be permanent disability or death. This review outlines easy and readily available means of recognizing and diagnosing a metabolic emergency as well as general guidelines for management. Disease-specific interventions focus on parenteral nutrition to reverse catabolism, toxin removal strategies, and vitamin/nutrition supplementation.

Vitamin B12 in Foods, Food Supplements, and Medicines-A Review of Its Role and Properties with a Focus on Its Stability

Vitamin B₁₂, also known as the anti-pernicious anemia factor, is an essential micronutrient totally dependent on dietary sources that is commonly integrated with food supplements. Four vitamin B₁₂ forms-cyanocobalamin, hydroxocobalamin, 5'-deoxyadenosylcobalamin, and methylcobalamin-are currently used for supplementation and, here, we provide an overview of their biochemical role, bioavailability, and efficacy in different dosage forms. Since the effective quantity of vitamin B₁₂ depends on the stability of the different forms, we further provide a review of their main reactivity and stability under exposure to various environmental factors (e.g., temperature, pH, light) and the presence of some typical interacting compounds (oxidants, reductants, and other water-soluble vitamins). Further, we explore how the manufacturing process and storage affect B₁₂ stability in foods, food supplements, and medicines and provide a summary of the data published to date on the content-related quality of vitamin B₁₂ products on the market. We also provide an overview of the approaches toward their stabilization, including minimization of the destabilizing factors, addition of proper stabilizers, or application of some (innovative) technological processes that could be implemented and contribute to the production of high-quality vitamin B₁₂ products.

Differences in the Formation of Reactive Oxygen Species and Their Cytotoxicity between Thiols Combined with Aqua- and Cyanocobalamins

Cobalamin is an essential nutrient required for the normal functioning of cells. Its deficiency can lead to various pathological states. Hydroxocobalamin (HOCbl) and cyanocobalamin (CNCbl) are the forms of vitamin B12 that are most commonly used for supplementation. There is substantial evidence indicating that cobalamins can both suppress and promote oxidative stress; however, the mechanisms underlying these effects are poorly understood. Here, it was shown that the oxidation of thiols catalyzed by HOCbl and CNCbl is accompanied by reactive oxygen species (ROS) production and induces, under certain conditions, oxidative stress and cell death. The form of vitamin B12 and the structure of thiol play a decisive role in these processes. It was found that the mechanisms and kinetics of thiol oxidation catalyzed by HOCbl and CNCbl differ substantially. HOCbl increased the rate of oxidation of thiols to a greater extent than CNCbl, but quenched ROS in combination with certain thiols. Oxidation catalyzed by CNCbl was generally slower. Yet, the absence of ROS quenching resulted in their higher accumulation. The aforementioned results might explain a more pronounced cytotoxicity induced by combinations of thiols with CNCbl. On the whole, the data obtained provide a new insight into the redox processes in which cobalamins are involved. Our results might also be helpful in developing new approaches to the treatment of some cobalamin-responsive disorders in which oxidative stress is an important component.

Effect of oral or injectable supplementation with cobalamin in dogs with hypocobalaminemia caused by chronic enteropathy or exocrine pancreatic insufficiency

Background

Recent studies have shown similar efficacy of oral supplementation of cobalamin compared to injectable supplementation in dogs, but few prospective, randomized studies have been published.

Objectives

To evaluate efficacy of oral or injectable supplementation with cobalamin in normalizing serum cobalamin and methylmalonic acid (MMA) concentrations in dogs with hypocobalaminemia caused by either chronic enteropathy (CE) or exocrine pancreatic insufficiency (EPI).

Animals

Forty‐six client owned dogs with hypocobalaminemia.

Methods

Prospective randomized clinical trial. Dogs were divided into 2 groups (CE or EPI), and randomized to receive oral or injectable supplementation of cobalamin. Each dog had 3 visits and serum cobalamin and MMA concentrations were measured at each visit.

Results

In dogs with CE, serum cobalamin concentrations increased with oral (P = .02; median 149 [range 149‐231] to 733 [166‐1467] ng/L, median difference 552 [95% CI: 181‐899] ng/L) or injectable (P < .01; 168 [149‐233] to 563 [234‐965] ng/L, 367 [187‐623] ng/L) supplementation. In dogs with EPI, serum cobalamin concentrations increased with oral (P = .01; 162 [149‐214] to 919 [643‐3863] ng/L, 705 [503‐3356] ng/L) or injectable (P = .01; 177 [149‐217] to 390 [243‐907] ng/L, 192 [89‐361] ng/L) supplementation. Serum MMA concentrations decreased with oral or injectable supplementation in dogs with CE, but only with oral supplementation in dogs with EPI.

Conclusions and Clinical Importance

Oral supplementation is an alternative for cobalamin supplementation in dogs with hypocobalaminemia caused by CE or EPI.

A teenager with combined methylmalonic aciduria and homocystinuria (CblC type) presenting with neurological symptoms and congenital heart diseases: a case report

Combined methylmalonic acidemia and homocystinuria, is a rare autosomal recessive disorder due to defective intracellular cobalamin metabolism. We report an 18-year-old Chinese male who presented with hypermyotonia, seizures, and congenital heart diseases. Mutation analysis revealed c.365A>T and c.482 G>A mutations in the MMACHC gene, diagnosed with methylmalonic aciduria and homocystinuria (CblC type). After treatment with vitamin B₁₂, L-carnitine, betaine, and folate, which resulted in an improvement in his clinical symptoms and laboratory values. This case emphasizes that inborn errors of metabolism should be considered for a teenager presenting with challenging or neurologic symptoms, especially when combined with unexplained heart diseases.

Clinical and biochemical outcomes in cobalamin C deficiency with use of high-dose hydroxocobalamin in the early neonatal period

This case report describes a patient with early-onset cobalamin C deficiency who was started on treatment with high-dose parenteral hydroxocobalamin after diagnosis at 13 days of life. Prior to diagnosis, initial presenting symptoms included poor feeding, lethargy, apneic episodes, hypothermia, and hypotonia; these symptoms resolved after initiation of medication. Methylmalonic acid and homocysteine levels were trended and significantly improved with treatment. She was maintained on 2 mg/kg/day dosing of hydroxocobalamin. No adverse effects to treatment were observed. At the time of this report, the patient was 19 months of age; she had not manifested common findings of early-onset cobalamin C deficiency, including microcephaly, poor feeding, growth abnormalities, hypotonia, seizures, maculopathy, or neurodevelopmental delay. This report suggests that early initiation of high-dose hydroxocobalamin is safe and effective.

Causes and consequences of impaired methionine synthase activity in acquired and inherited disorders of vitamin B12 metabolism

Methyl-Cobalamin (Cbl) derives from dietary vitamin B₁₂ and acts as a cofactor of methionine synthase (MS) in mammals. MS encoded by MTR catalyzes the remethylation of homocysteine to generate methionine and tetrahydrofolate, which fuel methionine and cytoplasmic folate cycles, respectively. Methionine is the precursor of S-adenosyl methionine (SAM), the universal methyl donor of transmethylation reactions. Impaired MS activity results from inadequate dietary intake or malabsorption of B₁₂ and inborn errors of Cbl metabolism (IECM). The mechanisms at the origin of the high variability of clinical presentation of impaired MS activity are classically considered as the consequence of the disruption of the folate cycle and related synthesis of purines and pyrimidines and the decreased synthesis of endogenous methionine and SAM. For one decade, data on cellular and animal models of B₁₂ deficiency and IECM have highlighted other key pathomechanisms, including altered interactome of MS with methionine synthase reductase, MMACHC, and MMADHC, endoplasmic reticulum stress, altered cell signaling, and genomic/epigenomic dysregulations. Decreased MS activity increases catalytic protein phosphatase 2A (PP2A) and produces imbalanced phosphorylation/methylation of nucleocytoplasmic RNA binding proteins, including ELAVL1/HuR protein, with subsequent nuclear sequestration of mRNAs and dramatic alteration of gene expression, including SIRT1. Decreased SAM and SIRT1 activity induce ER stress through impaired SIRT1-deacetylation of HSF1 and hypomethylation/hyperacetylation of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α), which deactivate nuclear receptors and lead to impaired energy metabolism and neuroplasticity. The reversibility of these pathomechanisms by SIRT1 agonists opens promising perspectives in the treatment of IECM outcomes resistant to conventional supplementation therapies.

Report of rapid diagnosis and precise management of MMADHC-related intracellular cobalamin defect

Disorders of intracellular cobalamin metabolism are a group of metabolic disorders that lead to varied clinical presentation from intrauterine life to adulthood. We report a male infant with developmental regression, macrocytic anaemia and hyperpigmentation. Exome sequencing identified a homozygous pathogenic variant in the MMADHC gene, known to cause homocystinuria, cblD type (MIM #277410). We describe significant clinical improvement with targeted therapy and emphasise the relevance of genomic testing in accurate management of inherited metabolic disorders.

Efficacy of intramuscular hydroxocobalamin supplementation in cats with cobalamin deficiency and gastrointestinal disease

Background

In humans, absorption and tissue retention rates of intramuscularly administered hydroxocobalamin (OH‐Cbl) are superior compared to cyanocobalamin (CN‐Cbl). Supplementation with OH‐Cbl has not been described in cats.

Objectives

To evaluate effects of parenteral OH‐Cbl supplementation on clinical signs, serum Cbl and methylmalonic acid (MMA) concentrations in hypocobalaminemic cats with gastrointestinal disease.

Animals

Twenty‐three client‐owned cats.

Methods

Prospective study. Serum Cbl and MMA concentrations were determined at enrollment (t0), immediately before the 4th OH‐Cbl IM injection (300 μg, given q2 weeks) (t1), and 4 weeks after the 4th injection (t2). Severity of clinical signs (activity, appetite, vomiting, diarrhea, body weight) was graded at each time point and expressed as clinical disease activity score.

Results

Median clinical disease activity score decreased significantly from t0 (6; range, 2‐10) to t1 (1; range, 0‐6) and t2 (1; range, 0‐9). Median serum Cbl concentration increased significantly from 111 pmol/L (range, 111‐218; reference range, 225‐1451 pmol/L) at t0 to 1612 pmol/L (range, 526‐14 756) (P < .001) at t1, and decreased again significantly to 712 pmol/L (range, 205‐4265) (P < .01) at t2. Median baseline serum MMA concentration at t0 (802 nmol/L; range, 238‐151 000; reference range, 120‐420 nmol/L) decreased significantly (P < .001) to 199 nmol/L (range, 29‐478) at t1, and was 205 nmol/L (range, 88‐734) at t2. Serum MMA concentrations normalized in 22/23 cats at t1, and were not significantly higher at t2 compared to t1.

Conclusions and Clinical Importance

The herein described OH‐Cbl injection scheme appears efficacious for normalization of cellular Cbl deficiency in cats with gastrointestinal disease.

The vitamin B12 processing enzyme, mmachc, is essential for zebrafish survival, growth and retinal morphology

Cobalamin C (cblC) deficiency, the most common inborn error of intracellular cobalamin metabolism, is caused by mutations in MMACHC, a gene responsible for the processing and intracellular trafficking of vitamin B12. This recessive disorder is characterized by a failure to metabolize cobalamin into adenosyl- and methylcobalamin, which results in the biochemical perturbations of methylmalonic acidemia, hyperhomocysteinemia and hypomethioninemia caused by the impaired activity of the downstream enzymes, methylmalonyl-CoA mutase and methionine synthase. Cobalamin C deficiency can be accompanied by a wide spectrum of clinical manifestations, including progressive blindness, and, in mice, manifests with very early embryonic lethality. Because zebrafish harbor a full complement of cobalamin metabolic enzymes, we used genome editing to study the loss of mmachc function and to develop the first viable animal model of cblC deficiency. mmachc mutants survived the embryonic period but perished in early juvenile life. The mutants displayed the metabolic and clinical features of cblC deficiency including methylmalonic acidemia, severe growth retardation and lethality. Morphologic and metabolic parameters improved when the mutants were raised in water supplemented with small molecules used to treat patients, including hydroxocobalamin, methylcobalamin, methionine and betaine. Furthermore, mmachc mutants bred to express rod and/or cone fluorescent reporters, manifested a retinopathy and thin optic nerves (ON). Expression analysis using whole eye mRNA revealed the dysregulation of genes involved in phototransduction and cholesterol metabolism. Zebrafish with mmachc deficiency recapitulate the several of the phenotypic and biochemical features of the human disorder, including ocular pathology, and show a response to established treatments.

High-dose hydroxocobalamin achieves biochemical correction and improvement of neuropsychiatric deficits in adults with late onset cobalamin C deficiency

Cobalamin C (cblC) deficiency is the most common inborn error of intracellular cobalamin metabolism caused by pathogenic variant(s) in MMACHC and manifests with methylmalonic acidemia, hyperhomocysteinemia, and hypomethioninemia with a variable age of presentation. Individuals with late‐onset cblC may be asymptomatic until manifesting neuropsychiatric symptoms, thromboembolic events, and renal disease. Although hydroxocobalamin provides a foundation for therapy, optimal dose regimen for adult patients has not been systematically evaluated. We report three adult siblings with late‐onset cblC disease, and their biochemical and clinical responses to high‐dose hydroxocobalamin. The 28‐year‐old proband presented with severe psychosis, progressive neurological deterioration, and deep venous thrombosis complicated by a pulmonary embolism. MRI studies identified lesions in the spinal cord, periventricular white matter, and basal ganglia. Serum homocysteine and methylmalonic acid levels were markedly elevated. Hydroxocobalamin at standard dose (1 mg/day) initially resulted in partial metabolic correction. A regimen of high‐dose hydroxocobalamin (25 mg/day) together with betaine and folic acid resulted in rapid and sustainable biochemical correction, resolution of psychosis, improvement of neurological functions, and amelioration of brain and spinal cord lesions. Two siblings who did not manifest neuropsychiatric symptoms or thromboembolism achieved a satisfactory metabolic control with the same high‐dose regimen. Hydroxocobalamin injection was then spaced out to 25 mg weekly with good and sustainable metabolic control. All three patients are compound heterozygotes for c.271dupA p.Arg91LysfsX14 and c.389A > G p.Tyr130Cys. This study highlights the importance of evaluating intracellular cobalamin metabolism in adults with neuropsychiatric manifestations and/or thromboembolic events, and demonstrates that high‐dose hydroxocobalamin achieves rapid and sustainable metabolic control and improvement in neuropsychiatric outcomes in adults with late‐onset cblC disease.

Review of cobalamin status and disorders of cobalamin metabolism in dogs

Disorders of cobalamin (vitamin B₁₂) metabolism are increasingly recognized in small animal medicine and have a variety of causes ranging from chronic gastrointestinal disease to hereditary defects in cobalamin metabolism. Measurement of serum cobalamin concentration, often in combination with serum folate concentration, is routinely performed as a diagnostic test in clinical practice. While the detection of hypocobalaminemia has therapeutic implications, interpretation of cobalamin status in dogs can be challenging. The aim of this review is to define hypocobalaminemia and cobalamin deficiency, normocobalaminemia, and hypercobalaminemia in dogs, describe known cobalamin deficiency states, breed predispositions in dogs, discuss the different biomarkers of importance for evaluating cobalamin status in dogs, and discuss the management of dogs with hypocobalaminemia.

Prospective long-term evaluation of parenteral hydroxocobalamin supplementation in juvenile beagles with selective intestinal cobalamin malabsorption (Imerslund-Gräsbeck syndrome)

BACKGROUND

Prospective studies on maintenance treatment for Beagles with hereditary selective cobalamin (Cbl) malabsorption (Imerslund-Gräsbeck syndrome, IGS) are lacking. In our experience, measurement of methylmalonic acid (MMA), a Cbl-dependent metabolite, seems more helpful to monitor Cbl status as compared with serum Cbl concentrations.

OBJECTIVES

To evaluate a standardized Cbl supplementation scheme in Beagles with IGS. We hypothesized that a single parenteral dose of 1 mg hydroxocobalamin (OH-Cbl) would maintain clinical and metabolic remission for up to 2 months.

ANIMALS

Six client-owned juvenile Beagles with genetically confirmed IGS and 28 healthy control dogs.

METHODS

Prospective study. Monthly IM OH-Cbl (1 mg) supplementation was done over a median of 9 months (range, 6-13) in 6 dogs, followed by bimonthly (every 2 months) injections in 5 dogs over a median of 6 months (range, 3-10). Health status was assessed by routine clinical examinations at injection time points and owner observations. Voided urine samples were collected immediately before OH-Cbl injections for measurement of MMA-to-creatinine concentrations using a gas-liquid chromatography-tandem mass spectrometry (GC-MS) method.

RESULTS

All dogs were clinically healthy while receiving monthly and bimonthly OH-Cbl supplementation. Urinary MMA results in healthy dogs ranged from 1.3 to 76.5 mmol/mol creatinine (median, 2.9). Median urinary MMA concentrations did not differ between dogs with IGS receiving monthly (n = 49; 5.3 mmol/mol creatinine; range, 2.3-50.4) and bimonthly (n = 31; 5.3 mmol/mol creatinine; range, 1.6-50) injections.

CONCLUSIONS AND CLINICAL IMPORTANCE

A maintenance parenteral dose of 1 mg OH-Cbl monthly or bimonthly appears adequate in Beagles with IGS monitored by metabolic testing.

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.

Vitamin B12 among Vegetarians: Status, Assessment and Supplementation

Cobalamin is an essential molecule for humans. It acts as a cofactor in one-carbon transfers through methylation and molecular rearrangement. These functions take place in fatty acid, amino acid and nucleic acid metabolic pathways. The deficiency of vitamin B12 is clinically manifested in the blood and nervous system where the cobalamin plays a key role in cell replication and in fatty acid metabolism. Hypovitaminosis arises from inadequate absorption, from genetic defects that alter transport through the body, or from inadequate intake as a result of diet. With the growing adoption of vegetarian eating styles in Western countries, there is growing focus on whether diets that exclude animal foods are adequate. Since food availability in these countries is not a problem, and therefore plant foods are sufficiently adequate, the most delicate issue remains the contribution of cobalamin, which is poorly represented in plants. In this review, we will discuss the status of vitamin B12 among vegetarians, the diagnostic markers for the detection of cobalamin deficiency and appropriate sources for sufficient intake, through the description of the features and functions of vitamin B12 and its absorption mechanism.

Cobalamin coenzyme forms are not likely to be superior to cyano- and hydroxyl-cobalamin in prevention or treatment of cobalamin deficiency

Methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl) are coenzymes for methionine synthase and methylmalonyl-CoA mutase, respectively. Hydroxylcobalamin (HOCbl) and cyanocobalamin (CNCbl) are frequently used for supplementation. MeCbl and AdoCbl have recently emerged as alternative forms in supplements. In the light of metabolic transformation of Cbl into its cofactor forms, this review discusses current evidence on efficacy and utility of different Cbl forms in preventing or treating Cbl deficiency. Cbl-transporting proteins bind and mediate the uptake of all aforementioned forms of Cbl. After internalization and lysosomal release, Cbl binds to the cytosolic chaperon MMACHC that is responsible for (i) flavin-dependent decyanation of [CN-Co³⁺]Cbl to [Co²⁺]Cbl; (ii) glutathione-dependent dealkylation of MeCbl and AdoCbl to [Co^2+/1+]Cbl; and (iii) glutathione-dependent decyanation of CNCbl or reduction of HOCbl under anaerobic conditions. MMACHC shows a broad specificity for Cbl forms and supplies the Cbl²⁺ intermediate for synthesis of MeCbl and AdoCbl. Cobalamin chemistry, physiology, and biochemistry suggest that MeCbl and AdoCbl follow the same route of intracellular processing as CNCbl does. We conclude that supplementing MeCbl or AdoCbl is unlikely to be advantageous compared to CNCbl. On the other hand, there are obvious advantages of high parenteral doses (1–2 mg) of HOCbl in treating inborn errors of Cbl metabolism.

Pathogenic mutations differentially affect the catalytic activities of the human B12-processing chaperone CblC and increase futile redox cycling

Human CblC catalyzes the elimination of the upper axial ligand in cobalamin or B12 derivatives entering the cell from circulation. This processing step is critical for assimilation of dietary cobalamin into the active cofactor forms that support the B12-dependent enzymes, methionine synthase and methylmalonyl-CoA mutase. Using a modified nitroreductase scaffold tailored to bind cobalamin and glutathione, CblC exhibits versatility in the mechanism by which it removes cyano versus alkyl ligands in cobalamin. In this study, we have characterized the effects of two pathogenic missense mutations at the same residue, R161G and R161Q, which are associated with early and late onset of the CblC disorder, respectively. We find that the R161Q and R161G CblC mutants display lower protein stability and decreased dealkylation but not decyanation activity, suggesting that cyanocobalamin might be therapeutically useful for patients carrying mutations at Arg-161. The mutant proteins also exhibit impaired glutathione binding. In the presence of physiologically relevant glutathione concentrations, stabilization of the cob(II)alamin derivative is observed, which occurs at the expense of increased oxidation of glutathione. Futile redox cycling, which is suppressed in wild-type human CblC, explains the reported increase in oxidative stress levels associated with the CblC disorder.

A treatable metabolic cause of encephalopathy: cobalamin C deficiency in an 8-year-old male

Neurologic regression in a previously healthy child may be caused by metabolic or neurodegenerative disorders, many of which have no definitive treatment. We report a case of a previously healthy 8-year-old boy who presented with a month-long history of waxing and waning encephalopathy and acute regression, followed by seizures. Evaluation for a metabolic disorder revealed methylmalonic acidemia and hyperhomocysteinemia of the cobalamin C type due to a single, presumed homozygous pathogenic c.394 C>T mutation in the MMACHC gene. With the appropriate diet restrictions and vitamin replacement, he improved significantly and returned to his premorbid level of behavior. This case illustrates an unusual presentation of a treatable metabolic disorder and highlights the need to consider cobalamin defects in the differential diagnosis of healthy children with neurologic regression.

Clinical presentation and outcome in a series of 88 patients with the cblC defect

The cblC defect is the most common inborn error of vitamin B12 metabolism. Despite therapeutic measures, the long-term outcome is often unsatisfactory. This retrospective multicentre study evaluates clinical, biochemical and genetic findings in 88 cblC patients. The questionnaire designed for the study evaluates clinical and biochemical features at both initial presentation and during follow up. Also the development of severity scores allows investigation of individual disease load, statistical evaluation of parameters between the different age of presentation groups, as well as a search for correlations between clinical endpoints and potential modifying factors.

RESULTS

No major differences were found between neonatal and early onset patients so that these groups were combined as an infantile-onset group representing 88 % of all cases. Hypotonia, lethargy, feeding problems and developmental delay were predominant in this group, while late-onset patients frequently presented with psychiatric/behaviour problems and myelopathy. Plasma total homocysteine was higher and methionine lower in infantile-onset patients. Plasma methionine levels correlated with "overall impression" as judged by treating physicians. Physician's impression of patient's well-being correlated with assessed disease load. We confirmed the association between homozygosity for the c.271dupA mutation and infantile-onset but not between homozygosity for c.394C>T and late-onset. Patients were treated with parenteral hydroxocobalamin, betaine, folate/folinic acid and carnitine resulting in improvement of biochemical abnormalities, non-neurological signs and mortality. However the long-term neurological and ophthalmological outcome is not significantly influenced. In summary the survey points to the need for prospective studies in a large cohort using agreed treatment modalities and monitoring criteria.

Drug treatment of inborn errors of metabolism: a systematic review

BACKGROUND

The treatment of inborn errors of metabolism (IEM) has seen significant advances over the last decade. Many medicines have been developed and the survival rates of some patients with IEM have improved. Dosages of drugs used for the treatment of various IEM can be obtained from a range of sources but tend to vary among these sources. Moreover, the published dosages are not usually supported by the level of existing evidence, and they are commonly based on personal experience.

METHODS

A literature search was conducted to identify key material published in English in relation to the dosages of medicines used for specific IEM. Textbooks, peer reviewed articles, papers and other journal items were identified. The PubMed and Embase databases were searched for material published since 1947 and 1974, respectively. The medications found and their respective dosages were graded according to their level of evidence, using the grading system of the Oxford Centre for Evidence-Based Medicine.

RESULTS

83 medicines used in various IEM were identified. The dosages of 17 medications (21%) had grade 1 level of evidence, 61 (74%) had grade 4, two medications were in level 2 and 3 respectively, and three had grade 5.

CONCLUSIONS

To the best of our knowledge, this is the first review to address this matter and the authors hope that it will serve as a quickly accessible reference for medications used in this important clinical field.

Production and characterization of cyanocobalamin-enriched lettuce (Lactuca sativa L.) grown using hydroponics

When lettuces (Lactuca sativa L.) grown for 30 days in hydroponic culture were treated with various concentrations of cyanocobalamin for 24 h, its content in their leaves increased significantly from nondetectable to 164.6 ± 74.7 ng/g fresh weight. This finding indicated that consumption of only two or three of these fresh leaves is sufficient to meet the Recommended Dietary Allowance for adults of 2.4 μg/day. Analyses using a cobalamin-dependent Escherichia coli 215 bioautogram and LC/ESI-MS/MS demonstrated that the cyanocobalamin absorbed from the nutrient solutions by the leaves did not alter any other compounds such as coenzymes and inactive corrinoids. Gel filtration indicated that most (86%) of the cyanocobalamin in the leaves was recovered in the free cyanocobalamin fractions. These results indicated that cyanocobalamin-enriched lettuce leaves would be an excellent source of free cyanocobalamin, particularly for strict vegetarians or elderly people with food-bound cobalamin malabsorption.

Combined methylmalonic acidemia and homocystinuria, cblC type. I. Clinical presentations, diagnosis and management

Combined methylmalonic acidemia and homocystinuria, cblC type, is an inborn error of intracellular cobalamin metabolism with a wide spectrum of clinical manifestations that is stated to be the most common inherited disorder of cobalamin metabolism. This metabolic disease is caused by mutations in the MMACHC gene and results in impaired intracellular synthesis of adenosylcobalamin and methylcobalamin, cofactors for the methylmalonyl-CoA mutase and methionine synthase enzymes. Elevated methylmalonic acid and homocysteine with decreased methionine production are the biochemical hallmarks of this disorder. Awareness of the diverse clinical presentations associated with cblC disease is necessary to provide a timely diagnosis, to guide management of affected individuals and to establish a framework for the future treatment of individuals detected through expanded newborn screening. This article reviews the biochemistry, clinical presentations, genotype-phenotype correlations, diagnosis and management of cblC disease.

Combined methylmalonic aciduria and homocystinuria cblC type of a Taiwanese infant with c.609G>A and C.567dupT mutations in the MMACHC gene

Combined methylmalonic aciduria and homocystinuria, cobalamin (cbl)C type (cblC disease), the most common inborn error of vitamin B(12), is a rare disorder of intracellular cbl metabolism because of mutations in the MMACHC gene located in chromosome region 1p34.1. It has become possible to establish phenotype-genotype correlations and to observe ethnicity-related trends. This article provides detailed clinical manifestations and outcomes of a Taiwanese infant boy with early-onset cblC disease, heterozygous for c.609G>A and c.567dupT mutations, although there is limited information about cases with c.609G>A or c.567dupT mutation in the literature. He had no significant clinical abnormality during his neonatal period, whereas elevated C3 level was noted at newborn screening. He presented later with life-threatening manifestations and failure to thrive, which resolved through our treatment, although delayed development was still noted at 6 months of age. To date, all reported cblC patients with the c.609G>A mutation have been East Asians. Therefore, we suggest that c.609G>A should be included in the initial mutation screening tests for a cblC patient in East Asian populations.

The MMACHC proteome: hallmarks of functional cobalamin deficiency in humans

Cobalamin (Cbl, B(12)) is an essential micronutrient required to fulfill the enzymatic reactions of cytosolic methylcobalamin-dependent methionine synthase and mitochondrial adenosylcobalamin-dependent methylmalonyl-CoA mutase. Mutations in the MMACHC gene (cblC complementation group) disrupt processing of the upper-axial ligand of newly internalized cobalamins, leading to functional deficiency of the vitamin. Patients with cblC disease present with both hyperhomocysteinemia and methylmalonic acidemia, cognitive dysfunction, and megaloblastic anemia. In the present study we show that cultured skin fibroblasts from cblC patients export increased levels of both homocysteine and methylmalonic acid compared to control skin fibroblasts, and that they also have decreased levels of total intracellular folates. This is consistent with the clinical phenotype of functional cobalamin deficiency in vivo. The protein changes that accompany human functional Cbl deficiency are unknown. The proteome of control and cblC fibroblasts was quantitatively examined by two dimensional difference in-gel electrophoresis (2D-DIGE) and liquid chromatography-electrospray ionization-mass spectrometry (LC/ESI/MS). Major changes were observed in the expression levels of proteins involved in cytoskeleton organization and assembly, the neurological system and cell signaling. Pathway analysis of the differentially expressed proteins demonstrated strong associations with neurological disorders, muscular and skeletal disorders, and cardiovascular diseases in the cblC mutant cell lines. Supplementation of the cell cultures with hydroxocobalamin did not restore the cblC proteome to the patterns of expression observed in control cells. These results concur with the observed phenotype of patients with the cblC disorder and their sometimes poor response to treatment with hydroxocobalamin. Our findings could be valuable for designing alternative therapies to alleviate the clinical manifestation of the cblC disorder, as some of the protein changes detected in our study are common hallmarks of known pathologies such as Alzheimer's and Parkinson's diseases as well as muscular dystrophies.

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.

Interaction between MMACHC and MMADHC, two human proteins participating in intracellular vitamin B₁₂ metabolism

The identification of eight genes involved in inherited cobalamin (Cbl) disorders has provided insight into the complexity of the vitamin B₁₂ trafficking pathway. Detailed knowledge about the structure, interaction, and physiological functions for many of the gene products, including the MMACHC and MMADHC proteins, is lacking. Having cloned, expressed, and purified MMACHC in Escherichia coli, we demonstrated its monodispersity by dynamic light scattering and measured its hydrodynamic radius, either alone or in complex with each of four vitamin B₁₂ derivatives. Using solution-phase intrinsic fluorescence and label-free, real-time surface plasmon resonance (SPR), MMACHC bound cyanocobalamin and hydroxycobalamin with similar low micromolar affinities (K(D) 6.4 and 9.8 μM, respectively); adenosylcobalamin and methylcobalamin also shared similar binding affinities for MMACHC (K(D) 1.7 and 1.4 μM, respectively). To predict specific regions of interaction between MMACHC and the proposed partner protein MMADHC, MMACHC was subjected to phage display. Five putative MMACHC-binding sites were identified. Finally, MMADHC was confirmed as a binding partner for MMACHC both in vitro (SPR) and in vivo (bacterial two-hybrid system).

Cobalamin C defect: natural history, pathophysiology, and treatment

Cobalamin C (Cbl-C) defect is the most common inborn cobalamin metabolism error; it causes impaired conversion of dietary vitamin B12 into its two metabolically active forms, methylcobalamin and adenosylcobalamin. Cbl-C defect causes the accumulation of methylmalonic acid and homocysteine and decreased methionine synthesis. The gene responsible for the Cbl-C defect has been recently identified, and more than 40 mutations have been reported. MMACHC gene is located on chromosome 1p and catalyzes the reductive decyanation of CNCbl. Cbl-C patients present with a heterogeneous clinical picture and, based on their age at onset, can be categorized into two distinct clinical forms. Early-onset patients, presenting symptoms within the first year, show a multisystem disease with severe neurological, ocular, haematological, renal, gastrointestinal, cardiac, and pulmonary manifestations. Late-onset patients present a milder clinical phenotype with acute or slowly progressive neurological symptoms and behavioral disturbances. To improve clinical course and metabolic abnormalities, treatment of Cbl-C defect usually consists of a combined approach that utilizes vitamin B12 to increase intracellular cobalamin and to maximize deficient enzyme activities, betaine to provide a substrate for the conversion of homocysteine into methionine through betaine-homocysteine methyltransferase, and folic acid to enhance remethylation pathway. No proven efficacy has been demonstrated for carnitine and dietary protein restriction. Despite these measures, the long-term follow-up is unsatisfactory especially in patients with early onset, with frequent progression of neurological and ocular impairment. The unfavorable outcome suggests that better understanding of the pathophysiology of the disease is needed to improve treatment protocols and to develop new therapeutic approaches.

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.

Thermolability of mutant MMACHC protein in the vitamin B12-responsive cblC disorder

Methylmalonic aciduria and homocystinuria, cblC type, is the most common inborn error of cellular vitamin B12 metabolism. We previously showed that the protein carrying the mutation responsible for late-onset cblC (MMACHC-R161Q), treatable with high dose OHCbl, is able to bind OHCbl with wild-type affinity, leaving undetermined the disease mechanism involved [Froese et al., Mechanism of responsiveness, Mol. Genet. Metab. (2009).]. To assess whether the mutation renders the protein unstable, we investigated the thermostability of the wild-type and mutant MMACHC proteins, either unbound or bound to different cobalamins (Cbl), using differential scanning fluorimetry. We found that MMACHC-wt and MMACHC-R161Q are both very thermolabile proteins in their apo forms, with melting temperatures (T(m)) of 39.3+/-1.0 and 37.1+/-0.7 degrees C, respectively; a difference confirmed by unfolding of MMACHC-R161Q but not MMACHC-wt by isothermal denaturation at 35 degrees C over 120 min. However, with the addition of OHCbl, MMACHC-wt becomes significantly stabilized (Delta T(m max)=8 degrees C, half-maximal effective ligand concentration, AC(50)=3 microM). We surveyed the effect of different cobalamins on the stabilization of the wild-type protein and found that AdoCbl was the most stabilizing, exerting a maximum increase in T(m) of approximately 16 degrees C, followed by MeCbl at approximately 13 degrees C, each evaluated at 50 microM cofactor. The other cobalamins stabilized in the order (CN)(2)Cbi>OHCbl>CNCbl. Interestingly, the AC(50)'s for AdoCbl, MeCbl, (CN)(2)Cbi and OHCbl were similar and ranged from 1-3 microM, which compares well with the K(d) of 6 microM for OHCbl [Froese et al., Mechanism of responsiveness, Mol. Genet. Metab. (2009).]. Unlike MMACHC-wt, the mutant protein MMACHC-R161Q is only moderately stabilized by OHCbl (Delta T(m max)=4 degrees C). The dose-response curve also shows a lower effectivity of OHCbl with respect to stabilization, with an AC(50) of 7 microM. MMACHC-R161Q showed the same order of stabilization as MMACHC-wt, but each cobalamin stabilized this mutant protein less than its wild-type counterpart. Additionally, MMACHC-R161Q had a higher AC(50) for each cobalamin form compared to MMACHC-wt. Finally, we show that MMACHC-R161Q is able to support the base-off transition for AdoCbl and CNCbl, indicating this mutant is not blocked in that respect. Taken together, our results suggest that protein stability, as well as propensity for ligand-induced stabilization, contributes to the disease mechanism in late-onset cblC disorder. Our results underscore the importance of cofactor stabilization of MMACHC and suggest that even small increases in the concentration of cobalamin complexed with MMACHC may have therapeutic benefit in children with the late-onset, vitamin responsive cblC disease.

Mechanism of vitamin B12-responsiveness in cblC methylmalonic aciduria with homocystinuria

Patients with the cblC vitamin B(12) (cobalamin, cbl) disorder are defective in the intracellular synthesis of adenosylcobalamin and methylcobalamin and have combined homocystinuria and methylmalonic aciduria. While other vitamin B(12) disorders are treatable with high dose cyanocobalamin (CNCbl) or hydroxocobalamin (OHCbl), cblC patients respond well to OHCbl but not to CNCbl. Patient mutations were introduced into recombinant MMACHC (cblC) protein and the binding of CNCbl and OHCbl was examined. Three mutations were analyzed: G147D, associated with early onset, vitamin B(12) unresponsive disease; R161Q, associated with late onset disease that is highly responsive to OHCbl; and H122A, selected to test the hypothesis that H122 is central to a proposed vitamin B(12) binding motif on MMACHC. We report here that wild-type MMACHC binds both OHCbl and CNCbl with similar, tight affinity (K(d)=5.7 microM). We also report that MMACHC binds CNCbl in the base-off form, with the dimethylbenzimidazole (DMB) base of cobalamin displaced from coordination with the cobalt. In this form, wild-type MMACHC is able to reductively decyanate CNCbl to cob(II)alamin requiring only the presence of NADPH and FAD. We demonstrate that MMACHC with the G147D mutation is unable to bind either CNCbl or OHCbl, providing a straight forward explanation for the absence of response to either vitamin form. However, we show that MMACHC containing the R161Q mutation binds OHCbl with wild-type affinity, but is disturbed in binding CNCbl and has impaired decyanation. Finally, we show that H122A has reduced binding, but like R161Q, it binds OHCbl more tightly than CNCbl, suggesting that this histidine is not absolutely required for binding. These studies suggest that the ability of mutant MMACHC to respond to vitamin therapy depends on its ability to bind the vitamin with significant affinity, and for CNCbl, also on its ability to bind in the base-off form to facilitate reductive decyanation. These studies emphasize the continued use of OHCbl with cblC patients for maximum therapeutic effect.

A human vitamin B12 trafficking protein uses glutathione transferase activity for processing alkylcobalamins

Pathways for tailoring and processing vitamins into active cofactor forms exist in mammals that are unable to synthesize these cofactors de novo. A prerequisite for intracellular tailoring of alkylcobalamins entering from the circulation is removal of the alkyl group to generate an intermediate that can subsequently be converted into the active cofactor forms. MMACHC, a cytosolic cobalamin trafficking chaperone, has been shown recently to catalyze a reductive decyanation reaction when it encounters cyanocobalamin. In this study, we demonstrate that this versatile protein catalyzes an entirely different chemical reaction with alkylcobalamins using the thiolate of glutathione for nucleophilic displacement to generate cob(I)alamin and the corresponding glutathione thioether. Biologically relevant thiols, e.g. cysteine and homocysteine, cannot substitute for glutathione. The catalytic turnover numbers for the dealkylation of methylcobalamin and 5'-deoxyadenosylcobalamin by MMACHC are 11.7 +/- 0.2 and 0.174 +/- 0.006 h(-1) at 20 degrees C, respectively. This glutathione transferase activity of MMACHC is reminiscent of the methyltransferase chemistry catalyzed by the vitamin B(12)-dependent methionine synthase and is impaired in the cblC group of inborn errors of cobalamin disorders.

Spectrum of mutations in MMACHC, allelic expression, and evidence for genotype-phenotype correlations

Methylmalonic aciduria and homocystinuria, cblC type, is a rare disorder of intracellular vitamin B(12) (cobalamin [Cbl]) metabolism caused by mutations in the MMACHC gene. MMACHC was sequenced from the gDNA of 118 cblC individuals. Eleven novel mutations were identified, as well as 23 mutations that were observed previously. Six sequence variants capture haplotype diversity in individuals across the MMACHC interval. Genotype-phenotype correlations of common mutations were apparent; individuals with c.394C>T tend to present with late-onset disease whereas patients with c.331C>T and c.271dupA tend to present in infancy. Other missense variants were also associated with late- or early-onset disease. Allelic expression analysis was carried out on human cblC fibroblasts compound heterozygous for different combinations of mutations including c.271dupA, c.331C>T, c.394C>T, and c.482G>A. The early-onset c.271dupA mutation was consistently underexpressed when compared to control alleles and the late-onset c.394C>T and c.482G>A mutations. The early-onset c.331C>T mutation was also underexpressed when compared to control alleles and the c.394C>T mutation. Levels of MMACHC mRNA transcript in cell lines homozygous for c.271dupA, c.331C>T, and c.394C>T were assessed using quantitative real-time RT-PCR. Cell lines homozygous for the late onset c.394C>T mutation had significantly higher levels of transcript when compared to cell lines homozygous for the early-onset mutations. Differential or preferential MMACHC transcript levels may provide a clue as to why individuals carrying c.394C>T generally present later in life.

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.

Ocular phenotype in patients with methylmalonic aciduria and homocystinuria, cobalamin C type

PURPOSE

To assess and compare longitudinal visual function and retinal morphology in patients with methylmalonic aciduria with homocystinuria, cobalamin C type (cblC), and identified mutations in the MMACHC gene.

METHODS

Vision function, anterior segment, and fundi were evaluated in patients with homozygous or compound heterozygous MMACHC mutations. Best-corrected visual acuity, full-field electroretinogram (ERG), refractive error, and retinopathy were assessed and compared for different genotypes and ages at onset, defined as early (<1 year of age) or late (>5 years).

RESULTS

We identified 7 patients (homozygous mutation: 6 of 7; compound heterozygous mutations: 1 of 7) between the ages of 3 months and 20.6 years. Six patients were reexamined after 3.2 to 11.5 years (mean, 6.5) Ocular phenotype ranged from normal to severely compromised visual function. Visual acuity was reduced from 0.2 logMAR to counting fingers and from 0.0 to 0.3 logMAR in the early- (3 of 7) and in the late-onset group (4 of 7), respectively. No retinopathy was evident in the late-onset group. Only patients with the homozygous c.547_548 delGT mutations (n = 2) demonstrated advanced retinopathy associated with cone-rod or rod-cone dysfunction. Retinopathy occurred despite systemic treatment for cblC.

CONCLUSIONS

Ocular phenotype in patients with cblC is variable. Ocular involvement seems to be correlated with age at onset. Patients with early-onset cblC developed generally progressive retinal disease ranging from subtle retinal nerve fiber layer loss to advanced macular and optic atrophy with "bone spicule" pigmentation. Patients with late-onset disease showed no definite evidence of retinal degeneration.

Decyanation of vitamin B12 by a trafficking chaperone

The mystery of how the cyanide group in vitamin B(12) or cyanocobalamin, discovered 60 years ago, is removed, has been solved by the demonstration that the trafficking chaperone, MMACHC, catalyzes a reductive decyanation reaction. Electrons transferred from NADPH via cytosolic flavoprotein oxidoreductases are used to cleave the cobalt-carbon bond with reductive elimination of the cyanide ligand. The product, cob(II)alamin, is a known substrate for assimilation into the active cofactor forms, methylcobalamin and 5'-deoxyadenosylcobalamin, and is bound in the "base-off" state that is needed by the two B(12)-dependent target enzymes, methionine synthase and methylmalonyl-CoA mutase. Defects in MMACHC represent the most common cause of inborn errors of B(12) metabolism, and our results explain the observation that fibroblasts from these patients are poorly responsive to vitamin B(12) but show some metabolic correction with aquocobalamin, a cofactor form lacking the cyanide ligand, which is mirrored by patients showing poorer clinical responsiveness to cyano- versus aquocobalamin.

Neonatal vitamin B12 deficiency secondary to maternal subclinical pernicious anemia: identification by expanded newborn screening

A neonate with elevated propionylcarnitine on the newborn screen was found to have methylmalonic acidemia due to vitamin B(12) deficiency. The mother was also vitamin B(12)-deficient. This case illustrates the utility of expanded newborn screening for detection of vitamin B(12) deficiency, allowing prompt treatment and prevention of potential sequelae.

Hemolytic uremic syndrome (HUS) secondary to cobalamin C (cblC) disorder

Diarrhea-positive hemolytic uremic syndrome (HUS) is a common cause of acute renal failure in children. Diarrhea-negative (D-), or atypical HUS, is etiologically distinct. A Medline search identified seven previously reported D- cases of HUS secondary to cobalamin C (cblC) disease presenting in infancy. An infantile presentation is reported to be associated with a high mortality rate (6/7 cases). We describe the results of a 5-year longitudinal follow-up in a child diagnosed with D- HUS secondary to cblC disease in infancy. Mutation analysis in this patient identified homozygosity for the 271 dupA mutation (c.271 dupA) in the cblC MMACHC gene. We briefly review the published experience in cblC-associated HUS to highlight the clinical characteristics of this uncommon, but potentially treatable, condition.

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

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

Cobalamin C disease presenting with hemophagocytic lymphohistiocytosis

Cobalamin C disease is a rare genetic condition resulting in methylmalonic aciduria, homocystinuria, and hematologic abnormalities. Clinical characteristics include ophthalmologic findings and neurological abnormalities, such as microcephaly, seizure, and mental retardation. The authors report on a 4-month-old patient initially diagnosed with hemophagocytic lymphohistiocytosis (HLH), who was later diagnosed with cobalamin C disease.

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

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

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.

Cobalamin dose regimen for maximum homocysteine reduction in end-stage renal disease

Plasma total homocysteine (tHcy) concentrations are markedly increased in end-stage renal disease and only partially corrected by folic acid supplementation. We and others have reported that cobalamin, administered parenterally, reduces plasma tHcy substantially below the lowest concentrations attainable with folic acid. We have now carried out a randomized controlled clinical trial to compare the plasma Hcy-lowering effect of 3 intravenous cyanocobalamin dose regimens in maintenance hemodialysis patients: 1 mg postdialysis every 28, 14, and 7 days in addition to routine oral vitamin B supplementation. All patients in the hemodialysis unit where the study was carried out routinely received 1 mg intravenous cyanocobalamin every month, so participants who were randomized to receive the vitamin every 28 days simply continued with their existing treatment program. Serum cobalamin and plasma tHcy concentrations in the control group did not change over the course of the study. As measured after 8 weeks of therapy, intravenous cyanocobalamin every 14 days increased serum cobalamin approximately 2.5-fold and reduced plasma tHcy by 11.5% ( P = .035) below the concentration previously attained with monthly administration, whereas treatment every 7 days increased serum cobalamin concentrations approximately 5-fold and reduced plasma tHcy by 11.0% ( P = .013). These results show that intravenous cyanocobalamin at 7- or 14-day intervals reduces plasma tHcy concentrations of hemodialysis patients below the levels brought about by prior long-term administration every 4 weeks and confirms that plasma tHcy lowering with parenteral cobalamin is a true pharmacological effect and not merely correction of a latent deficiency state.

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.

Cobalamin C disease presenting as hemolytic-uremic syndrome in the neonatal period

Anew case of cobalamin C disease associated with hemolytic-uremic syndrome (HUS) in the neonatal period is described. A 28-day-old boy presented with failure to thrive, hypotonia, pancytopenia, and features of HUS (microangiopathic hemolytic anemia, thrombocytopenia, and renal failure). The possibility of the diagnosis of an underlying vitamin B12 disorder was prompted by evidence of megaloblastic changes on the peripheral smear and by finding in the literature a suggested association of neonatal HUS with this cobalamin-related metabolic disorder. Amino acid analysis showed elevated homocysteine levels in the plasma and increased levels of both homocysteine and methyl malonic acid in the urine. Diagnosis of cobalamin C disease was confirmed by complementation studies using skin fibroblasts. Therapy included parenteral hydroxocobalamin, carnitine, and leucovorin calcium (folinic acid). Cobalamin C disease should be considered in the diagnosis of patients presenting with HUS in infancy who have unexplained megaloblastosis, pancytopenia, neurologic impairment, and failure to thrive. Early diagnosis and institution of therapy may be effective in improving survival and quality of life.

Ventricular septal defect closure in a neonate with combined methylmalonic aciduria/homocystinuria

Methylmalonic acidemia with associated homocystinuria is a rare inborn error of amino acid metabolism affecting energy supply on the cellular level. Its effects on recovery from surgically induced organ ischemia are largely unknown. We report the successful closure of a nonrestrictive ventricular septal defect by following a normothermic strategy combined with ample metabolic substrate supply.

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

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

Long-term outcome in treated combined methylmalonic acidemia and homocystinemia

PURPOSE

To describe the clinical and biochemical features and long-term outcome of a cohort of eight patients with methylmalonic acidemia and homocystinuria (cblC).

METHODS

Documentation of clinical features at birth and longitudinal follow-up of the biochemical and clinical response to treatment with daily oral carnitine and intramuscular hydroxocobalamin observed during continuous follow-up for an average of 5.7 years.

RESULTS

Our patients had an increased incidence of congenital malformations including microcephaly (<5%) at birth (2 of 8), congenital heart disease (2 of 8), dysmorphic facial features (1 of 8), and thyroglossal duct cyst (1 of 8). Postnatal hydrocephalus (2 of 8) and hip dislocation caused by ligament laxity (1 of 8) were also noted. One patient had profound visual impairment before 6 months of age secondary to cblC retinopathy, and two patients had abnormal retinal pigmentation with normal visual function. All patients presented with poor growth, feeding problems, and/or seizures. No patients had acute acidotic crises before or after treatment. All patients had dramatic reduction of plasma free homocystine and urine methylmalonic acid excretion after initiation of therapy with carnitine, intramuscular (IM) hydroxocobalamin (OHcbl) and, in two cases, oral betaine. Growth was significantly improved in most cases after the initiation of therapy, and microcephaly was resolved in one patient. All patients were developmentally delayed regardless of age of treatment onset, although two patients had relatively mild developmental delay.

CONCLUSION

cblC patients may have an increased incidence of congenital malformations suggesting prenatal effects of abnormal cbl metabolism. Treatment with IM OHcbl and carnitine successfully corrects the biochemical abnormalities and improves growth. Developmental delay of variable severity is always present regardless of age at diagnosis or treatment onset.