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

Severe heart failure in a unique case of cobalamin-C-deficiency resolved with LVAD implantation and subsequent heart transplantation

Introduction Cobalamin c deficiency (cblC), an inborn error of vitamin B12 metabolism, is caused by mutations of the MMACHC gene. It usually leads to a multisystemic disease; 50% of all patients with cblC have various structural heart defects. Severe congestive heart failure (HF) may also occur and its prognosis is poorly documented. Case report We present the case of a young man who had been diagnosed with cblC due to C331T mutation in the MMACHC gene at the age of 3 days and had been treated with substitution therapy (OH-Cbl, mecobalamine, carnitine, betaine, and calcium folinate) since then. He had mildly impaired cognitive function; an ectopic hypophysis/pituitary insufficiency, with adequate hormone replacement therapy; obstructive sleep apnea syndrome, treated with CPAP, bronchial asthma, and obesity (BMI of 30). The liver and kidney functions were normal. He developed severe dilated cardiomyopathy and HF at the age of 12y. With medical treatment, his condition improved and he was stable (NYHA class II) for several years. Six years later, his status deteriorated rapidly, as he developed advanced HF, INTERMACS 3. The cardiac ultrasound revealed dilated ventricles with severely depressed ejection fraction (EF), increased filling pressures, and pulmonary hypertension (sPAP 60 mmHg). Cardiac MRI showed extremely dilated chambers (LVedv 609 mL, RVedv 398 mL) with pronounced non-compaction, and a left ventricle EF of 13%. A primary prophylactic ICD and a left ventricular assist device (LVAD/HM3) were implanted, and the patient was subsequently listed for heart transplantation (HTx). After 25 months on the waiting list, he underwent an uncomplicated HTx. However postoperatively, he got two episodes of cardiac tamponade, as well as mediastinitis, treated with antibiotics and vaccum assisted closure. He developed severe kidney failure, which fully recovered after two months, and was treated successfully for an early moderate allograft rejection (ISHT 2). At the latest outward visit, twelve months after HTx, the patient was doing excellent. Summary To the best of our knowledge, this is the first ever reported case of a patient with CblC undergoing an LVAD implantation and subsequently a HTx. Although both interventions were complicated with bleeding events, this seems to be a treatment option for advanced HF in patients with CblC.

Clinical and genetic analysis of methylmalonic aciduria in 60 patients from Southern China: a single center retrospective study

BACKGROUND

Methylmalonic aciduria (MMA) is a group of rare genetic metabolic disorders resulting from defects in methylmalonyl coenzyme A mutase (MCM) or intracellular cobalamin (cbl) metabolism. MMA patients show diverse clinical and genetic features across different subtypes and populations.

METHODS

We retrospectively recruited 60 MMA patients from a single center and diagnosed them based on their clinical manifestations and biochemical assays. We then performed genetic analysis to confirm the diagnosis and identify the causal variants.

RESULTS

We confirmed the common clinical manifestations of MMA reported previously. We also described four rare MMA cases with unusual symptoms or genetic variants, such as pulmonary hypertension or limb weakness in late-onset patients. We identified 15 MMACHC and 26 MMUT variants in 57 patients, including 6 novel MMUT variants. Two patients had only one MMAA variant each, and one patient had mild MMA due to mitochondrial DNA depletion syndrome caused by a SUCLA2 variant. Among 12 critically ill patients, isolated MMA was associated with higher C3, blood ammonia, and acidosis, while combined MMA was linked to hydrocephalus on skull MRI. MMACHC c.658-660delAAG and MMUT c.1280G > A variants were correlated with more severe phenotypes.

CONCLUSIONS

Our study demonstrates the clinical and genotypic heterogeneity of MMA patients and indicates that metabolic screening and genetic analysis are useful tools to identify rare cases.

The MMACHC variant c.158T>C: Mild clinical and biochemical phenotypes and marked hydroxocobalamin response in cblC patients

Mutations in MMACHC cause cobalamin C disease (cblC, OMIM 277400), the commonest inborn error of vitamin B12 metabolism. In cblC, deficient activation of cobalamin results in methylcobalamin and adenosylcobalamin deficiency, elevating methylmalonic acid (MMA) and total plasma homocysteine (tHcy). We retrospectively reviewed the medical files of seven cblC patients: three compound heterozygotes for the MMACHC (NM_015506.3) missense variant c.158T>C p.(Leu53Pro) in trans with the common pathogenic mutation c.271dupA (p.(Arg91Lysfs*14), "compounds"), and four c.271dupA homozygotes ("homozygotes"). Compounds receiving hydroxocobalamin intramuscular injection monotherapy had age-appropriate psychomotor performance and normal ophthalmological examinations. In contrast, c.271dupA homozygotes showed marked psychomotor retardation, retinopathy and feeding problems despite penta-therapy (hydroxocobalamin, betaine, folinic acid, l-carnitine and acetylsalicylic acid). Pretreatment levels of plasma and urine MMA and tHcy were higher in c.271dupA homozygotes than in compounds. Under treatment, levels of the compounds approached or entered the reference range but not those of c.271dupA homozygotes (tHcy: compounds 9.8-32.9 μM, homozygotes 41.6-106.8 (normal (N) < 14); plasma MMA: compounds 0.14-0.81 μM, homozygotes, 10.4-61 (N < 0.4); urine MMA: compounds 1.75-48 mmol/mol creatinine, homozygotes 143-493 (N < 10)). Patient skin fibroblasts all had low cobalamin uptake, but this was milder in compound cells. Also, the distribution pattern of cobalamin species was qualitatively different between cells from compounds and from homozygotes. Compared to the classic cblC phenotype presented by c.271dupA homozygous patients, c.[158T>C];[271dupA] compounds had mild clinical and biochemical phenotypes and responded strikingly to hydroxocobalamin monotherapy.

Clinical and electroencephalogram characteristics of methylmalonic acidemia with MMACHC and MUT gene mutations

OBJECTIVE

This study investigated the clinical, imaging, and electroencephalogram (EEG) characteristics of methylmalonic acidemia (MMA) with nervous system damage as the primary manifestation.

METHODS

From January 2017 to November 2022, patients with nervous system injury as the main clinical manifestation, diagnosed with methylmalonic acidemia by metabolic and genetic testing, were enrolled and analyzed. Their clinical, imaging, and electroencephalogram data were analyzed.

RESULTS

A total of 18 patients were enrolled, including 15 males and 3 females. The clinical symptoms were convulsions, poor feeding, growth retardation, disorder of consciousness, developmental delay, hypotonia, and blood system changes. There were 6 cases (33%) of hydrocephalus, 9 (50%) of extracerebral space widened, 5 (27%) of corpus callosum thinning, 3 (17%) of ventricular dilation, 3 (17%) of abnormal signals in the brain parenchyma (frontal lobe, basal ganglia region, and brain stem), and 3 (17%) of abnormal signals in the lateral paraventricular. In addition, there were 3 cases (17%) of cerebral white matter atrophy and 1 (5%) of cytotoxic edema in the basal ganglia and cerebral peduncle. EEG data displayed 2 cases (11%) of hypsarrhythmia, 3 (17%) of voltage reduction, 12(67%) of abnormal discharge, 13 (72%) of abnormal sleep physiological waves or abnormal sleep structure, 1 (5%) of immature (delayed) EEG development, and 8 (44%) of slow background. There were 2 cases (11%) of spasms, 1 (5%) of atonic seizures, and 1 (5%) of myoclonic seizures. There were 16 patients (89%) with hyperhomocysteinemia. During follow-up, 1 patient was lost to follow-up, and 1 died. In total, 87.5% (14/16) of the children had varying developmental delays. EEG was re-examined in 11 cases, of which 8 were normal, and 3 were abnormal. Treatments included intramuscular injections of vitamin B12, L-carnitine, betaine, folic acid, and oral antiepileptic therapy. Acute treatment included anti-infective, blood transfusion, fluid replacement, and correcting acidosis. The other treatments included low-protein diets and special formula milk powder.

CONCLUSION

Methylmalonic acidemia can affect the central nervous system, leading to structural changes or abnormal signals on brain MRI. Metabolic screening and genetic testing help clarify the diagnosis. EEG can reflect changes in brain waves during the acute phase.

Would, early, versus late hydroxocobalamin dose intensification treatment, prevent cognitive decline, macular degeneration and ocular disease, in 5 patients with early-onset cblC deficiency?

In early-onset (EO) cblC deficiency (MMACHC), hydroxocobalamin dose-intensification (OHCBL-DI) improved biochemical and clinical outcome. In mammals, Cobalamin is reduced, in a reaction mediated by MMACHC. Pathogenic variants in MMACHC disrupt the synthesis pathway of methyl-cobalamin (MetCbl) and 5'-deoxy-adenosyl-cobalamin (AdoCbl), cofactors for both methionine synthase (MS) and methyl-malonyl-CoA mutase (MCM) enzymes. In 5 patients (pts.), with EO cblC deficiency, biochemical and clinical responses were studied following OHCbl-DI (mean ± SD 6,5 ± 3,3 mg/kg/day), given early, before age 5 months (pts. 1, 2, 3 and 4) or lately, at age 5 years (pt. 5). In all pts., total homocysteine (tHcy), methyl-malonic acid (MMA) and Cob(III)alamin levels were measured. Follow-up was performed during 74/12 years (pts. 1, 2, 3), 33/12 years (pt. 4) and 34/12 years (pt. 5). OHCbl was delivered intravenously or subcutaneously. Mean ± SD serum Cob(III)alamin levels were 42,2 × 106 ± 28, 0 × 106 pg/ml (normal: 200-900 pg/ml). In all pts., biomarkers were well controlled. All pts., except pt. 5, who had poor vision, had central vision, mild to moderate nystagmus, and with peri-foveolar irregularity in pts. 1, 2 and 4, yet none had the classic bulls' eye maculopathy and retinal degeneration characteristic of pts. with EO cblC deficiency. Only pt. 5, had severe cognitive deficiency. Both visual and cognitive functions were better preserved with early than with late OHCBL-DI. OHCBL-DI is suggested to bypass MMACHC, subsequently to be rescued by methionine synthase reductase (MSR) and adenosyl-transferase (ATR) to obtain Cob(I)alamin resulting in improved cognitive and retinal function in pts. with EO cblC deficiency.

Late-onset cblC defect: clinical, biochemical and molecular analysis

BACKGROUND

cblC defect is the most common type of methylmalonic acidemia in China. Patients with late-onset form (>1 year) are often misdiagnosed due to heterogeneous symptoms. This study aimed to describe clinical characteristics and evaluate long-term outcomes of Chinese patients with late-onset cblC defect.

METHODS

A total of 85 patients with late-onset cblC defect were enrolled. Clinical data, including manifestations, metabolites, molecular diagnosis, treatment and outcome, were summarized and analyzed.

RESULTS

The age of onset ranged from 2 to 32.8 years old (median age 8.6 years, mean age 9.4 years). The time between first symptoms and diagnosis ranged from a few days to 20 years (median time 2 months, mean time 20.7 months). Neuropsychiatric symptoms were presented as first symptoms in 68.2% of cases, which were observed frequently in schoolchildren or adolescents. Renal involvement and cardiovascular disease were observed in 20% and 8.2% of cases, respectively, which occurred with the highest prevalence in preschool children. Besides the initial symptoms, the disease progressed in most patients and cognitive decline became the most frequent symptom overall. The levels of propionylcarnitine, propionylcarnitine / acetylcarnitine ratio, methylmalonic acid, methylcitric acid and homocysteine, were decreased remarkably after treatment (P<0.001). Twenty-four different mutations of MMACHC were identified in 78 patients, two of which were novel. The c.482G>A variant was the most frequent mutated allele in this cohort (25%). Except for 16 patients who recovered completely, the remaining patients were still left with varying degrees of sequelae in a long-term follow-up. The available data from 76 cases were analyzed by univariate analysis and multivariate logistic regression analysis, and the results showed that the time from onset to diagnosis (OR = 1.025, P = 0. 024) was independent risk factors for poor outcomes.

CONCLUSIONS

The diagnosis of late-onset cblC defect is often delayed due to poor awareness of its various and nonspecific symptoms, thus having an adverse effect on the prognosis. It should be considered in patients with unexplained neuropsychiatric and other conditions such as renal involvement, cardiovascular diseases or even multiple organ damage. The c.482G>A variant shows the highest frequency in these patients. Prompt treatment appears to be beneficial.

Diagnosis and management of pulmonary veno-occlusive disease

INTRODUCTION

Pulmonary veno-occlusive disease (PVOD) is an orphan disease and uncommon etiology of pulmonary arterial hypertension (PAH) characterized by substantial small pulmonary vein and capillary involvement.

AREAS COVERED

PVOD, also known as 'PAH with features of venous/capillary involvement' in the current ESC/ERS classification.

EXPERT OPINION

In recent years, particular risk factors for PVOD have been recognized, including genetic susceptibilities and environmental factors (such as exposure to occupational organic solvents, chemotherapy, and potentially tobacco). The discovery of biallelic mutations in the EIF2AK4 gene as the cause of heritable PVOD has been a breakthrough in understanding the molecular basis of PVOD. Venous and capillary involvement (PVOD-like) has also been reported to be relatively common in connective tissue disease-associated PAH (especially systemic sclerosis), and in rare pulmonary diseases like sarcoidosis and pulmonary Langerhans cell granulomatosis. Although PVOD and pulmonary arterial hypertension (PAH) exhibit similarities, including severe precapillary PH, it is essential to differentiate between them since PVOD has a worse prognosis and requires specific management. Indeed, PVOD patients are characterized by poor response to PAH-approved drugs, which can lead to pulmonary edema and clinical deterioration. Due to the lack of effective treatments, early referral to a lung transplantation center is crucial.

Case report: An asymptomatic mother with an inborn error of cobalamin metabolism (cblC) detected through high homocysteine levels during prenatal diagnosis

Background

The most common disorder of the intracellular cobalamin metabolism pathway is the combined methylmalonic acidemia and homocysteinemia, cblC type (cblC). There is a variation in its clinical spectrum ranging from severe neonatal-onset forms that are highly fatal to later-onset forms which are milder. In this study, the first case of an asymptomatic Chinese woman with a defect in congenital cobalamin (cblC type) metabolism at prenatal diagnosis due to elevated homocysteine level is identified.

Case presentation

The proband, a male child born to a 29-year-old G1P0 mother, admitted to local hospital with feeding disorder, intellectual disability, seizures, microcephaly, as well as heterophthalmos. The level of the urine methylmalonic was elevated. Equally found were increased blood propionylcarnitine (C3) and propionylcarnitine/free carnitine ratio (C3/C0) and decreased methionine levels. The plasma total homocysteine level was elevated at 101.04 μmol/L (normal < 15 μmol/L). The clinical diagnosis of combined methylmalonic acidemia and homocysteinemia was supported. Four years later, the mother of the boy married again and came to us for prenatal diagnosis exactly 15 weeks after her last menstrual period. Subsequently, there is an increase in the amniotic fluid methylmalonate. The level of the amniotic fluid total homocysteine was marginally high. A considerably elevated amniotic fluid C3 was equally observed. In addition, there is a respective significant increase in the plasma and urine total homocysteine at 31.96 and 39.35 μmol/L. After the sequencing of MMACHC genes, it is found that the boy, a proband carried a homozygous mutation of the MMACHC at c.658_660delAAG. While the boy's mother, she carries two mutations in MMACHC: c.658_660delAAG and c.617G>A. The fetus is a carrier of the MMACHC gene. Following the administration of routine treatment, the mother remained symptom-free in the course of pregnancy, and she gave birth to a healthy boy.

Conclusion

Variable and nonspecific symptoms characterized the cblC type of methylmalonic acidemia combined with homocysteinemia. Both biochemical assays and mutation analysis are recommended as crucial complementary techniques.

Abnormal chondrocyte development in a zebrafish model of cblC syndrome restored by an MMACHC cobalamin binding mutant

Variants in the MMACHC gene cause combined methylmalonic acidemia and homocystinuria cblC type, the most common inborn error of intracellular cobalamin (vitamin B12) metabolism. cblC is associated with neurodevelopmental, hematological, ocular, and biochemical abnormalities. In a subset of patients, mild craniofacial dysmorphia has also been described. Mouse models of Mmachc deletion are embryonic lethal but cause severe craniofacial phenotypes such as facial clefts. MMACHC encodes an enzyme required for cobalamin processing and variants in this gene result in the accumulation of two metabolites: methylmalonic acid (MMA) and homocysteine (HC). Interestingly, other inborn errors of cobalamin metabolism, such as cblX syndrome, are associated with mild facial phenotypes. However, the presence and severity of MMA and HC accumulation in cblX syndrome is not consistent with the presence or absence of facial phenotypes. Thus, the mechanisms by which mutations in MMACHC cause craniofacial defects are yet to be completely elucidated. Here we have characterized the craniofacial phenotypes in a zebrafish model of cblC (hg13) and performed restoration experiments with either a wildtype or a cobalamin binding deficient MMACHC protein. Homozygous mutants did not display gross morphological defects in facial development but did have abnormal chondrocyte nuclear organization and an increase in the average number of neighboring cell contacts, both phenotypes were fully penetrant. Abnormal chondrocyte nuclear organization was not associated with defects in the localization of neural crest specific markers, sox10 (RFP transgene) or barx1. Both nuclear angles and the number of neighboring cell contacts were fully restored by wildtype MMACHC and a cobalamin binding deficient variant of the MMACHC protein. Collectively, these data suggest that mutation of MMACHC causes mild to moderate craniofacial phenotypes that are independent of cobalamin binding.

The complex machinery of human cobalamin metabolism

Vitamin B₁₂ (cobalamin, Cbl) is required as a cofactor by two human enzymes, 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR) and methylmalonyl-CoA mutase (MMUT). Within the body, a vast array of transporters, enzymes and chaperones are required for the generation and delivery of these cofactor forms. How they perform these functions is dictated by the structure and interactions of the proteins involved, the molecular bases of which are only now being elucidated. In this review, we highlight recent insights into human Cbl metabolism and address open questions in the field by employing a protein structure and interactome based perspective. We discuss how three very similar proteins-haptocorrin, intrinsic factor and transcobalamin-exploit slight structural differences and unique ligand receptor interactions to effect selective Cbl absorption and internalisation. We describe recent advances in the understanding of how endocytosed Cbl is transported across the lysosomal membrane and the implications of the recently solved ABCD4 structure. We detail how MMACHC and MMADHC cooperate to modify and target cytosolic Cbl to the client enzymes MTR and MMUT using ingenious modifications to an ancient nitroreductase fold, and how MTR and MMUT link with their accessory enzymes to sustainably harness the supernucleophilic potential of Cbl. Finally, we provide an outlook on how future studies may combine structural and interactome based approaches and incorporate knowledge of post-translational modifications to bring further insights.

Abnormal chondrocyte intercalation in a zebrafish model of cblC syndrome restored by an MMACHC cobalamin binding mutant

Variants in the MMACHC gene cause combined methylmalonic acidemia and homocystinuria cblC type, the most common inborn error of intracellular cobalamin (vitamin B12) metabolism. cblC is associated with neurodevelopmental, hematological, ocular, and biochemical abnormalities. In a subset of patients, mild craniofacial dysmorphia has also been described. Mouse models of Mmachc deletion are embryonic lethal but cause severe craniofacial phenotypes such as facial clefts. MMACHC encodes an enzyme required for cobalamin processing and variants in this gene result in the accumulation of two metabolites: methylmalonic acid (MMA) and homocysteine (HC). Interestingly, other inborn errors of cobalamin metabolism, such as cblX syndrome, are associated with mild facial phenotypes. However, the presence and severity of MMA and HC accumulation in cblX syndrome is not consistent with the presence or absence of facial phenotypes. Thus, the mechanisms by which mutation of MMACHC cause craniofacial defects have not been completely elucidated. Here we have characterized the craniofacial phenotypes in a zebrafish model of cblC ( hg13 ) and performed restoration experiments with either wildtype or a cobalamin binding deficient MMACHC protein. Homozygous mutants did not display gross morphological defects in facial development, but did have abnormal chondrocyte intercalation, which was fully penetrant. Abnormal chondrocyte intercalation was not associated with defects in the expression/localization of neural crest specific markers, sox10 or barx1 . Most importantly, chondrocyte organization was fully restored by wildtype MMACHC and a cobalamin binding deficient variant of MMACHC protein. Collectively, these data suggest that mutation of MMACHC causes mild to moderate craniofacial phenotypes that are independent of cobalamin binding.

Haemolytic uraemic syndrome

Haemolytic uraemic syndrome (HUS) is a heterogeneous group of diseases that result in a common pathology, thrombotic microangiopathy, which is classically characterised by the triad of non-immune microangiopathic haemolytic anaemia, thrombocytopenia, and acute kidney injury. In this Seminar, different causes of HUS are discussed, the most common being Shiga toxin-producing Escherichia coli HUS. Identifying the underlying thrombotic microangiopathy trigger can be challenging but is imperative if patients are to receive personalised disease-specific treatment. The quintessential example is complement-mediated HUS, which once carried an extremely high mortality but is now treated with anti-complement therapies with excellent long-term outcomes. Unfortunately, the high cost of anti-complement therapies all but precludes their use in low-income countries. For many other forms of HUS, targeted therapies are yet to be identified.

Case report: A late-onset cobalamin C defect first presenting as a depression in a teenager

Background: The cobalamin C (cblC) defect, a common inborn disorder of cobalamin metabolism due to a genetic mutation in MMACHC, can cause combined methylmalonic acid and homocysteine accumulation in blood, urine, or both. In this article, a late-onset case was reported, and the patient first presented with depression identified with the MMACHC gene. We summarized the clinical features of the cblC defect, the relationship between genotype and phenotype, and the clinical experience concerning the diagnosis and treatment of the cblC defect.

Case presentation: Initially presented with depression, the 16-year-old female patient showed progressive abnormal gait and bilateral lower limb weakness after 3 months. Blood routine examination suggested severe hyperhomocysteinemia, and screening for urine organic acids found elevated methylmalonic acid. Family gene sequencing showed mutations detected in MMACHC. She had a compound heterozygous mutation, while the c.271dupA (p.R91Kfs∗14) was only detected in her father and the c.482 G>A (p.R161Q) was only detected in her mother. Hence, she was diagnosed with a cblC defect and treated with B vitamin supplements. The muscle strength of both lower limbs improved notably.

Conclusion: This case indicated that depression could be a presenting sign of cblC-type methylmalonic aciduria and homocysteinemia, and enhanced the genotype–phenotype relationship of the cblC defect, which will contribute to further understanding of this emerging disease.

Late-onset cblC deficiency around puberty: a retrospective study of the clinical characteristics, diagnosis, and treatment

Background

cblC deficiency is the most common type of methylmalonic aciduria in China. Late-onset patients present with various non-specific symptoms and are usually misdiagnosed. The purpose of this study is to investigate the clinical features of patients with late-onset cblC deficiency and explore diagnosis and management strategies around puberty.

Results

This study included 56 patients (35 males and 21 females) with late-onset cblC deficiency who were admitted to our clinic between 2002 and September 2021. The diagnosis was confirmed by metabolic and genetic tests. The clinical and biochemical features, disease triggers, outcome, and associated genetic variants were examined. The onset age ranged from 10 to 20 years (median age, 12 years). Fifteen patients (26.8%) presented with symptoms after infection or sports training. Further, 46 patients (82.1%) had neuropsychiatric diseases; 11 patients (19.6%), cardiovascular diseases; and 6 patients (10.7%), pulmonary hypertension. Renal damage was observed in 6 cases (10.7%). Genetic analysis revealed 21 variants of the MMACHC gene in the 56 patients. The top five common variants detected in 112 alleles were c.482G > A (36.6%), c.609G > A (16.1%), c.658_660delAAG (9.8%), c.80A > G (8.0%), and c.567dupT (6.3%). Thirty-nine patients carried the c.482G > A variant. Among 13 patients who exhibited spastic paraplegia as the main manifestation, 11 patients carried c.482G > A variants. Six patients who presented with psychotic disorders and spastic paraplegia had compound heterozygotic c.482G > A and other variants. All the patients showed improvement after metabolic treatment with cobalamin, l-carnitine, and betaine, and 30 school-aged patients returned to school. Two female patients got married and had healthy babies.

Conclusions

Patients with late-onset cblC deficiency present with a wide variety of neuropsychiatric symptoms and other presentations, including multiple organ damage. As a result, cb1C deficiency can easily be misdiagnosed as other conditions. Metabolic and genetic studies are important for accurate diagnosis, and metabolic treatment with cobalamin, l-carnitine, and betaine appears to be beneficial.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02471-x.

Versatile Enzymology and Heterogeneous Phenotypes in Cobalamin Complementation Type C Disease

Nutritional deficiency and genetic errors that impair the transport, absorption, and utilization of vitamin B₁₂ (B₁₂) lead to hematological and neurological manifestations. The cblC disease (cobalamin complementation type C) is an autosomal recessive disorder caused by mutations and epi-mutations in the MMACHC gene and the most common inborn error of B₁₂ metabolism. Pathogenic mutations in MMACHC disrupt enzymatic processing of B₁₂, an indispensable step before micronutrient utilization by the two B₁₂-dependent enzymes methionine synthase (MS) and methylmalonyl-CoA mutase (MUT). As a result, patients with cblC disease exhibit plasma elevation of homocysteine (Hcy, substrate of MS) and methylmalonic acid (MMA, degradation product of methylmalonyl-CoA, substrate of MUT). The cblC disorder manifests early in childhood or in late adulthood with heterogeneous multi-organ involvement. This review covers current knowledge on the cblC disease, structure–function relationships of the MMACHC protein, the genotypic and phenotypic spectra in humans, experimental disease models, and promising therapies.

Identification of MMACHC and ZEB2 mutations causing coexistent cobalamin C disease and Mowat-Wilson syndrome in a 2-year-old girl

Cobalamin C (cblC) disease and Mowat-Wilson syndrome (MWS) are rare hereditary diseases. To date, there have been no reports of people suffering from these two genetic diseases, or whether there is any correlation between the two diseases. We reported a 2-year-old girl with both cblC disease and MWS. The patient initially manifested as slow weight gain, hypotonia, broad nasal bridge, high forehead, high palate arch, ear crease, patent ductus arteriosus, atrial and ventricular septal defect and bilateral mild ventriculomegaly in the neonatal period. However, as the baby grew older, the typical facial features became more prominent, and overall developmental delays were noted at the subsequent follow-up, with the motor and cognitive development significantly lagging behind that of other children of the same age. At 26 days old, laboratory tests revealed remarkably elevated levels of serum homocysteine, C3/C2 and urine organic acid. Whole-exome sequencing detected compound heterozygous variants in MMACHC, including one previously reported mutation [c.609G > A (p.W203X) and a novel missense mutation[ c.643 T > C (p.Y215H)]. The computer simulations of the protein structure analysis of the novel missense mutation showed the variant p.Y215H replaced a neutral amino acid with a strongly basic lysine, which broken the local structure by changing the carbon chain skeleton and decreasing the interaction with adjacent amino acid. This is expected to damage the utilization of vitamin B12 and influence the synthesis of AdoCbl and MeCbl, contributing to its pathogenicity. Thus, clinical and genetic examinations confirmed the cblC disease. Another heterozygous variant in ZEB2 [NM_014795; loss1(exon:2-10)(all); 127901 bp] was detected by whole-exome sequencing. The heterozygous 3.04 Mb deletion in EB2 [GRCH37]del(2)(q22.2q22.3) (chr2:142237964-145274917) was also confirmed by genome-wide copy number variations (CNVs) scan, which was pathogenic and led to the diagnosis of Mowat-Wilson syndrome. The biochemical indicators associated with cblC disease in the patient were well controlled after treatment with vitamin B12 and betaine. Here, a patient with coexisting cblC disease and MWS caused by different pathogenic genes was reported, which enriched the clinical research on these two rare genetic diseases.

Investigation on a MMACHC mutant from cblC disease: The c.394C>T variant

The cblC disease is an inborn disorder of the vitamin B12 (cobalamin, Cbl) metabolism characterized by methylmalonic aciduria and homocystinuria. The clinical consequences of this disease are devastating and, even when early treated with current therapies, the affected children manifest symptoms involving vision, growth, and learning. The illness is caused by mutations in the gene codifying for MMACHC, a 282aa protein that transports and transforms the different Cbl forms. Here we present data on the structural properties of the truncated protein p.R132X resulting from the c.394C > T mutation that, along with c.271dupA and c.331C > T, is among the most common mutations in cblC. Although missing part of the Cbl binding domain, p.R132X is associated to late-onset symptoms and, therefore, it is supposed to retain residual function. However, to our knowledge structural-functional studies on c.394C > T mutant aimed at verifying this hypothesis are still lacking. By using a biophysical approach including Circular Dichroism, fluorescence, Small Angle X-ray Scattering, and Molecular Dynamics, we show that the mutant protein MMACHC-R132X retains secondary structure elements and remains compact in solution, partly preserving its binding affinity for Cbl. Insights on the fragile stability of MMACHC-R132X-Cbl are provided.

A case of vitamin B12 deficiency neurological syndrome in a young adult due to late-onset cobalamin C (CblC) deficiency: a diagnostic challenge

Vitamin B12 deficiency can present with neurologic and psychiatric symptoms without macrocytic anaemia. We describe a case of late-onset cobalamin C deficiency which typically presents with normal serum vitamin B12 concentrations, posing an additional diagnostic challenge. A 23-year-old woman with decreased muscle strength and hallucinations was diagnosed with ‘catatonic depression’ and admitted to a residential mental health facility. She was referred to our hospital for further investigation 3 months later. Heteroanamnesis revealed that the symptoms had been evolving progressively over several months. Magnetic resonance imaging (MRI) of the brain showed diffuse symmetrical white matter lesions in both hemispheres. Routine laboratory tests including vitamin B12 and folic acid were normal except for a slight normocytic, normochromic anaemia. Over the next 6 weeks her symptoms deteriorated, and she became unresponsive to stimuli. A new MRI scan showed progression of the white matter lesions. The neurologist requested plasma homocysteine (Hcys) which was more than 8 times the upper limit of normal. Further testing revealed increased methylmalonic acid and the patient was diagnosed with adult-onset cobalamin C deficiency. This case illustrates that Hcys and/or methylmalonic acid should be determined in patients presenting with neuropsychiatric symptoms suggestive of vitamin B12 deficiency with a normal serum vitamin B12 to rule out a late-onset cobalamin C deficiency.

The genotype analysis and prenatal genetic diagnosis among 244 pedigrees with methylmalonic aciduria in China

OBJECTIVES

To investigate the phenotypes, biochemical features and genotypes for 244 pedigrees with methylmalonic aciduria (MMA) in China, and to perform the prenatal genetic diagnosis by chorionic villus for these pedigrees.

MATERIALS AND METHODS

Gene analyses were performed for 244 pedigrees. There are 130 pedigrees, chorionic villus sampling was performed on the pregnant women to conduct the prenatal diagnosis.

RESULTS

Among 244 patients, 168 (68.9%) cases were combined methylmalonic aciduria and homocystinuria, 76 (31.1%) cases were isolated methylmalonic aciduria. All the patients were diagnosed with MMA by their clinical manifestation, elevated blood propionylcarnitine, propionylcarnitine to acetylcarnitine ratio, and/or urine/blood methylmalonic acid with or without homocysteine. MMACHC, MMUT, SUCLG1 and LMBRD1 gene variants were found in 236 (96.7%) pedigrees included 6 probands with only one heterozygous variant out of 244 cases. For the 130 pedigrees who received a prenatal diagnosis, 22 fetuses were normal, 69 foetuses were carriers of heterozygous variants, and the remaining 39 foetuses harboured compound heterozygous variants or homozygous variants. The follow-up results were consistent with the prenatal diagnosis.

CONCLUSION

The present study indicates genetic heterogeneity in MMA patients. Genetic analysis is a convenient method for prenatal diagnosis that will aid in avoiding the delivery of MMA patients.

Structural Study of the Complex of cblC Methylmalonic Aciduria and Homocystinuria-Related Protein MMACHC with Cyanocobalamin

MMACHC is an essential protein for the body to metabolise vitamin B12, and its deficiency will cause cblC-type methylmalonic aciduria and homocystinuria. MMACHC can interact with cyanocobalamin (a type of vitamin B12) cofactor and plays an important role in targeting cyanocobalamin to the enzyme of interest. In this paper, the GST-tag fusion-tagged MMACHC protein was successfully expressed by Escherichia coli (E. coli) low-temperature induction, and the high-purity MMACHC protein was successfully purified by affinity chromatography and gel filtration. Further, the crystal structure of MMACHC and cyanocobalamin complex was obtained with a resolution of 1.93 Å using X-ray diffraction. By analysing the complex structure of MMACHC and cyanocobalamin, we revealed the reasons for the diversity of MMACHC substrates and explained the reasons for the differences in disease conditions caused by different MMACHC site mutations. The acquisition of the complex structure of MMACHC and cyanocobalamin will play a significant role in promoting research on the metabolic pathway of vitamin B12.

Intracellular processing of vitamin B12 by MMACHC (CblC)

Vitamin B₁₂ (cobalamin, Cbl, B₁₂) is a water-soluble micronutrient synthesized exclusively by a group of microorganisms. Human beings are unable to make B₁₂ and thus obtain the vitamin via intake of animal products, fermented plant-based foods or supplements. Vitamin B₁₂ obtained from the diet comprises three major chemical forms, namely hydroxocobalamin (HOCbl), methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl). The most common form of B₁₂ present in supplements is cyanocobalamin (CNCbl). Yet, these chemical forms cannot be utilized directly as they come, but instead, they undergo chemical processing by the MMACHC protein, also known as CblC. Processing of dietary B₁₂ by CblC involves removal of the upper-axial ligand (beta-ligand) yielding the one-electron reduced intermediate cob(II)alamin. Newly formed cob(II)alamin undergoes trafficking and delivery to the two B₁₂-dependent enzymes, cytosolic methionine synthase (MS) and mitochondrial methylmalonyl-CoA mutase (MUT). The catalytic cycles of MS and MUT incorporate cob(II)alamin as a precursor to regenerate the coenzyme forms MeCbl and AdoCbl, respectively. Mutations and epimutations in the MMACHC gene result in cblC disease, the most common inborn error of B₁₂ metabolism, which manifests with combined homocystinuria and methylmalonic aciduria. Elevation of metabolites homocysteine and methylmalonic acid occurs because the lack of an active CblC blocks formation of the indispensable precursor cob(II)alamin that is necessary to activate MS and MUT. Thus, in patients with cblC disease, vitamin B₁₂ is absorbed and present in circulation in normal to high concentrations, yet, cells are unable to make use of it. Mutations in seemingly unrelated genes that modify MMACHC gene expression also result in clinical phenotypes that resemble cblC disease. We review current knowledge on structural and functional aspects of intracellular processing of vitamin B₁₂ by the versatile protein CblC, its partners and possible regulators.

The Follow-Up of Chinese Patients in cblC Type Methylmalonic Acidemia Identified Through Expanded Newborn Screening

Objective: The cblC type of combined methylmalonic acidemia and homocystinuria, an inherited disorder with variable phenotypes, is included in newborn screening (NBS) programs at multiple newborn screening centers in China. The present study aimed to investigate the long-term clinical benefits of screening individual.

Methods: A national, retrospective multi-center study of infants with confirmed cblC defect identified by NBS between 2004 and 2020 was conducted. We collected a large cohort of 538 patients and investigated their clinical data in detail, including disease onset, biochemical metabolites, and gene variation, and explored different factors on the prognosis.

Results: The long-term outcomes of all patients were evaluated, representing 44.6% for poor outcomes. In our comparison of patients with already occurring clinical signs before treatment to asymptomatic ones, the incidence of intellectual impairment, movement disorders, ocular complications, hydrocephalus, and death were significantly different (p < 0.01). The presence of disease onset [Odd ratio (OR) 12.39, 95% CI 5.15–29.81; p = 0.000], variants of c.609G>A (OR 2.55, 95% CI 1.49–4.35; p = 0.001), and c.567dupT (OR 2.28, 95% CI 1.03–5.05; p = 0.042) were independently associated with poor outcomes, especially for neurodevelopmental deterioration.

Conclusion: NBS, avoiding major disease-related events and allowing an earlier treatment initiation, appeared to have protective effects on the prognosis of infants with cblC defect.

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.

Case Report: Membranous Nephropathy Secondary to Cobalamin C Disease

Background

Mutation of MMACHC causes inherited cobalamin C disease with methylmalonic academia (MMA) and homocysteinemia. Renal complications may also be present in patients with this deficiency. However, membranous nephropathy secondary to cobalamin C disease has not been reported to date.

Case Presentation

We encountered a 17-year-old female patient with a trans-compound mutation of MMACHC who presented with membranous nephropathy, MMA, homocysteinemia, and hyperuricemia. The mutations of c.80A>G (chr1:45966084) and c.482G>A (chr1:45974520) (predicting p.Gln27Arg and p.Arg161Gln missense changes at the amino acid level) had been inherited from her father and mother, respectively. Hydroxocobalamin, betaine, and L-carnitine were administered. The patient achieved complete remission of the membranous nephropathy and resolution of the MMA, homocysteinemia, and hyperuricemia.

Conclusion

Membranous nephropathy secondary to cobalamin C disease is reversible with timely intervention.

Prominent renal complications associated with MMACHC pathogenic variant c.80A > G in Chinese children with cobalamin C deficiency

OBJECTIVE

CblC deficiency, the most common cobalamin metabolic abnormality, is caused by pathogenic variants in the MMACHC gene. The renal complications of this disease have been described only in a small number of cases. This study aimed to better delineate renal phenotype and genetic characteristics in Chinese children with cblC defect.

METHODS

Children with cblC deficiency who manifested as kidney damage were enrolled. Clinical, renal pathological, and genetic data were reviewed in detail.

RESULTS

Seven cases were enrolled. Ages at disease onset ranged from 9 months to 5 years. All patients presented with hematuria and proteinuria, and 2/7 cases presented with nephrotic syndrome. Renal dysfunction was observed in 4/7 cases. Renal biopsy was performed in 5/7 cases, and all of them had renal thrombotic microangiopathy. Macrocytic anemia was detected in all seven patients. Six out of seven cases had hypertension, and 2/7 cases presented with pulmonary hypertension. Two of them had a mild intellectual disability, and one suffered from epilepsy. Increased urine methylmalonic acid and plasma homocysteine were detected in seven cases, while two patients had normal levels of urine methylmalonic acid at the initial evaluation. After diagnosis, all seven cases were treated with hydroxocobalamin IM. Six cases were followed-up for 3-8 years. After treatments, anemia was the first to be recovered, followed by proteinuria. Renal function recovered after 1 year in two cases, whereas patient 2 progressed to stage 2 chronic kidney disease 13 years after onset. While a case presented with end-stage kidney disease because of late diagnosis, one case died 3 months after disease onset due to giving up treatment. Three MMACHC pathogenic variants c.80A > G (8/14), c.609G > A (4/14), and c.658_660delAAG (2/14) were detected in all seven children.

CONCLUSION

MMACHC variant c.80A > G may be associated with prominent renal complications in Chinese cblC patients. Macrocytic anemia and hyperhomocysteinemia are useful clues for patients with hematuria and proteinuria caused by cblC defect. The most frequent renal pathological manifestation is thrombotic microangiopathy. Early diagnosis and treatment resulted in improving renal and hematological signs.

Combined Methylmalonic Aciduria and Homocystinuria Presenting as Pulmonary Hypertension

Combined methylmalonic aciduria and homocystinuria, cblC type, (MAHCC) is a rare autosomal recessive metabolic disorder of remethylation caused due to mutations in the MMACHC (metabolism of cobalamin associated C) gene with predominant neurological involvement. Microvascular, renal, and cardiovascular complications are also known to occur. However, the disease presenting primarily with a cardiovascular phenotype without any neurological involvement is a rare entity. We report a case of developmentally normal 23-mo-old female child, who presented with pulmonary arterial hypertension (PAH) and succumbed to cardiac failure. Extensive workup for PAH was inconclusive. Posthumous trio whole-exome sequencing revealed pathogenic compound heterozygous variants in the MMACHC. Diagnosis of MAHCC should be considered as a differential diagnosis for unexplained PAH in children. An elevated plasma homocysteine level can serve as a simple screening modality for this disorder. Accurate diagnosis has paramount therapeutic implications, as management with hydroxocobalamin and betaine may lead to partial or complete remission of PAH in these patients.

Spectrum of clinical manifestation of methylmalonic acidemia and homocystinuria in a family of six siblings: novel combination of transcobalamin receptor defect (CD320) and cblC deficiency (MMACHC)

Background

Methylmalonic acidemia with homocystinuria is caused by a rare inborn error of vitamin B12 (cobalamin) metabolism. There are four complementation classes of cobalamin defects cblC, cblD, cblF, and cblJ that are responsible for combined methylmalonic acidemia and homocystinuria.

Case presentation

We report a case of a Pakistani family composed of six children diagnosed with methylmalonic acidemia and homocystinuria (MMA + HCU). Mutation analysis of siblings revealed a variable combination of c.394C>T mutation in the MMACHC gene and c.262_264del in CD320 gene. All siblings had variable age of onset, initial symptomatology, the severity of disease, and response to treatment. The maximum age of presentation was 6.5 years and the minimum age was at birth. The spectrum of symptoms ranged from a subtle learning disability to global developmental delay within the same family. None of them had a seizure disorder, megaloblastic anemia, visual disturbance, and vascular events.CD320 defect itself is very rare, and only 12 cases have been reported so far. We report six cases, four of them had homozygous MMACHC variant c.394C>T and the other two had heterozygous MMACHC mutations in c.394C>T and c.262_264del in CD 320 genes. To date, neither such case has been reported in the literature or this compound heterozygous mutation.

Conclusion

Our case report emphasizes that the diagnosis of inherited metabolic disorder in a child obviates the need to screen all siblings for the same disorder.

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.

Clinical features and outcomes of patients with cblC type methylmalonic acidemia carrying gene c.609G>A mutation

To explore the clinical features and long-term outcomes of patients with cblC type methylmalonic acidemia (MMA) carrying c.609G>A (p.W203X) mutation of gene. The clinical and laboratory findings of 720 patients with MMA carrying the c.609G>A mutation were retrospectively analyzed. There were 172 cases carrying homozygous mutations of c.609G>A (group A), 169 cases carrying compound heterozygous mutations of c.609G>A with c.482G>A (p.R161Q), c.80A>G or c.394C>T (p.R132X) (group B), and 379 cases carrying compound heterozygous mutations of c.609G>A with c.658_660delAAG(p.K220del), c.315A>Tor c.567dupT(p.I190fs13)(group C).The clinical manifestations, the level of blood acylcarnitine, homocysteine and urinary organic acid, and the therapeutic efficacy were compared among groups. Logistic regression was used to analyze the factors influencing the prognosis of patients. There were 306 patients (42.5%) detected from newborn screening, including 156 cases with disease onset; and 414 patients were not detected from the screening, among whom 10 cases were diagnosed by testing after the sibling confirmed, and the remaining 404 were clinical cases. In 560 patients with disease onset, the median onset age is (3 days to 20 years). The onset age of patients in group B was later than that in group A and group C (<0.01). Patients aged mostly manifested as vomiting, diarrhea, feeding difficulties and convulsions, while those year mostly manifested as movement disorders and mental retardation. Patients with renal disease all carried mutations of c.80A>G or c.482G>A, and patients with pulmonary hypertension all carried c.80A>G mutations. A total of 621 cases had long-term follow-up, 156 cases (25.1%) developed well, 433 cases (69.7%) had development delay and 32 cases (5.2%) died. The available data of 559 cases were analyzed by logistic regression, and the results showed that the neonatal screening, disease onset, age of onset and gene mutation site were significantly associated with the prognosis of patients (<0.05 or <0.01). The c.609G>A mutation in gene is associated with early-onset MMA, and most patients, clinical onset occurred within 1 month after birth. The neonatal screening and early treatment can improve the prognosis of patients,whereas clinical onset is unfavorable for prognosis. Patients with c.609G>A homozygous mutation have a worse prognosis than those with the compound heterozygous mutation of c.609G>A with other mutations.

PRDX1 gene-related epi-cblC disease is a common type of inborn error of cobalamin metabolism with mono- or bi-allelic MMACHC epimutations

Background

The role of epigenetics in inborn errors of metabolism (IEMs) is poorly investigated. Epigenetic changes can contribute to clinical heterogeneity of affected patients but could also be underestimated determining factors in the occurrence of IEMs. An epigenetic cause of IEMs has been recently described for the autosomal recessive methylmalonic aciduria and homocystinuria, cblC type (cblC disease), and it has been named epi-cblC. Epi-cblC has been reported in association with compound heterozygosity for a genetic variant and an epimutation at the MMACHC locus, which is secondary to a splicing variant (c.515-1G > T or c.515-2A > T) at the adjacent PRDX1 gene. Both these variants cause aberrant antisense transcription and cis-hypermethylation of the MMACHC gene promotor with subsequent silencing. Until now, only nine epi-cblC patients have been reported.

Methods

We report clinical/biochemical assessment, MMACHC/PRDX1 gene sequencing and genome-wide DNA methylation profiling in 11 cblC patients who had an inconclusive MMACHC gene testing. We also compare clinical phenotype of epi-cblC patients with that of canonical cblC patients.

Results

All patients turned out to have the epi-cblC disease. One patient had a bi-allelic MMACHC epimutation due to the homozygous PRDX1:c.515-1G > T variant transmitted by both parents. We found that the bi-allelic epimutation produces the complete silencing of MMACHC in the patient’s fibroblasts. The remaining ten patients had a mono-allelic MMACHC epimutation, due to the heterozygous PRDX1:c.515-1G > T, in association with a mono-allelic MMACHC genetic variant. Epi-cblC disease has accounted for about 13% of cblC cases diagnosed by newborn screening in the Tuscany and Umbria regions since November 2001. Comparative analysis showed that clinical phenotype of epi-cblC patients is similar to that of canonical cblC patients.

Conclusions

We provide evidence that epi-cblC is an underestimated cause of inborn errors of cobalamin metabolism and describe the first instance of epi-cblC due to a bi-allelic MMACHC epimutation. MMACHC epimutation/PRDX1 mutation analyses should be part of routine genetic testing for all patients presenting with a metabolic phenotype that combines methylmalonic aciduria and homocystinuria.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13148-021-01117-2.

Absence of MMACHC in peripheral retinal cells does not lead to an ocular phenotype in mice

Combined methylmalonic aciduria with homocystinuria (cblC type) is a rare disease caused by mutations in the MMACHC gene. MMACHC encodes an enzyme crucial for intracellular vitamin B₁₂ metabolism, leading to the accumulation of toxic metabolites e.g. methylmalonic acid (MMA) and homocysteine (Hcy), and secondary disturbances in folate and one-carbon metabolism when not fully functional. Patients with cblC deficiency often present in the neonatal or early childhood period with a severe multisystem pathology, which comprises a broad spectrum of treatment-resistant ophthalmological phenotypes, including retinal degeneration, impaired vision, and vascular changes. To examine the potential function of MMACHC in the retina and how its loss may impact disease, we performed gene expression studies in human and mouse, which showed that local expression of MMACHC in the retina and retinal pigment epithelium is relatively stable over time. To study whether functional MMACHC is required for retinal function and tissue integrity, we generated a transgenic mouse lacking Mmachc expression in cells of the peripheral retina. Characterization of this mouse revealed accumulation of cblC disease related metabolites, including MMA and the folate-dependent purine synthesis intermediates AICA-riboside and SAICA-riboside in the retina. Nevertheless, fundus appearance, morphology, vasculature, and cellular composition of the retina, as well as ocular function, remained normal in mice up to 6 or 12 months of age. Our data indicates that peripheral retinal neurons do not require intrinsic expression of Mmachc for survival and function and questions whether a local MMACHC deficiency is responsible for the retinal phenotypes in patients.

Preimplantation Genetic Testing for Rare Inherited Disease of MMA-CblC: an Unaffected Live Birth

Methylmalonic acidemia combined with homocysteinemia and cobalamin C type (MMA-CblC, MIM # 277400) is a rare inherited disease with cobalamin metabolic disorder, which are caused by deficiency in the MMACHC gene. A couple with a proband child carried with compound heterozygous mutations of MMACHC (c.609G>A and c.567 dup T, NM_015506) sought for assisted reproductive technology to avoid the transmission of pathogenic genetic variants and unnecessary induction of labor. Thus, in vitro fertilization (IVF), preimplantation genetic testing (PGT), and prenatal genetic diagnosis were applied to fulfill this clinical demand. In this study, seven embryos were biopsied and carried out whole-genome amplification using multiple annealing and looping-based amplification cycle (MALBAC) method. Sanger sequencing together with copy number variation (CNV) analysis and single-nucleotide polymorphism (SNP) haplotyping was conducted to detect the mutated alleles and chromosomal abnormalities simultaneously. Three embryos (E07, E06, and E02) were confirmed without CNVs and inherited mutations at MMACHC gene. Embryo E07 with the best embryo ranking of 5BB was selected preferentially to transfer which led to a successful pregnancy and an unaffected live birth. Prenatal genetic diagnosing with amniotic fluid cells, Sanger sequencing with cord blood cells, and neonate MMA screening further verified our successful application of PGT in preventing mutated allele transmission for this rare inherited disease.

Screening for inborn errors of metabolism in psychotic patients using Next Generation Sequencing

Inborn errors of metabolism (IEMs) are a group of rare genetic disorders which, when emerging later in life, are often characterized by neuropsychiatric manifestations including psychosis. This study aimed to determine whether it would be useful to screen patients presenting with a psychotic disorder for IEMs by a single blood sample using Next Generation Sequencing (NGS), in order to detect rare, treatable causes of psychotic disorders. Blood was drawn from 60 patients with a psychotic disorder, with a duration of illness of less than 5 years. Blood samples were screened for 67 genes using NGS (Illumina® MiSeq sequencing technique). The results were compared to the human reference genome (GoNL, n = 498). The identified variants were classified according to the ACMG classification. For the psychotic patients, 6 variants of a likely pathogenic (class 4, n = 2) or pathogenic (class 5, n = 4) origin were found. As all variants were heterozygous, no patients were considered to be affected by an IEM. For the GoNL control group, 73 variants of a likely pathogenic (class 4, n = 31) or pathogenic (class 5, n = 42) origin were found. All of these found variants were heterozygous. Therefore, these individuals from the control group were considered to be a carrier only. Thus, no patients were identified to have an IEM as an underlying disease using this approach. However, NGS may be useful to detect variants of genes associated with IEMs in an enriched subgroup of psychotic patients.

Redox-Linked Coordination Chemistry Directs Vitamin B12 Trafficking

Metals are partners for an estimated one-third of the proteome and vary in complexity from mononuclear centers to organometallic cofactors. Vitamin B₁₂ or cobalamin represents the epitome of this complexity and is the product of an assembly line comprising some 30 enzymes. Unable to biosynthesize cobalamin, mammals rely on dietary provision of this essential cofactor, which is needed by just two enzymes, one each in the cytoplasm (methionine synthase) and the mitochondrion (methylmalonyl-CoA mutase). Brilliant clinical genetics studies on patients with inborn errors of cobalamin metabolism spanning several decades had identified at least seven genetic loci in addition to the two encoding B₁₂ enzymes. While cells are known to house a cadre of chaperones dedicated to metal trafficking pathways that contain metal reactivity and confer targeting specificity, the seemingly supernumerary chaperones in the B₁₂ pathway had raised obvious questions as to the rationale for their existence.With the discovery of the genes underlying cobalamin disorders, our laboratory has been at the forefront of ascribing functions to B₁₂ chaperones and elucidating the intricate redox-linked coordination chemistry and protein-linked cofactor conformational dynamics that orchestrate the processing and translocation of cargo along the trafficking pathway. These studies have uncovered novel chemistry that exploits the innate chemical versatility of alkylcobalamins, i.e., the ability to form and dismantle the cobalt-carbon bond using homolytic or heterolytic chemistry. In addition, they have revealed the practical utility of the dimethylbenzimidazole tail, an appendage unique to cobalamins and absent in the structural cousins, porphyrin, chlorin, and corphin, as an instrument for facilitating cofactor transfer between active sites.In this Account, we navigate the chemistry of the B₁₂ trafficking pathway from its point of entry into cells, through lysosomes, and into the cytoplasm, where incoming cobalamin derivatives with a diversity of upper ligands are denuded by the β-ligand transferase activity of CblC to the common cob(II)alamin intermediate. The broad reaction and lax substrate specificity of CblC also enables conversion of cyanocobalamin (technically, vitamin B₁₂, i.e., the form of the cofactor in one-a-day supplements), to cob(II)alamin. CblD then hitches up with CblC via a unique Co-sulfur bond to cob(II)alamin at a bifurcation point, leading to the cytoplasmic methylcobalamin or mitochondrial 5'-deoxyadenosylcobalamin branch. Mutations at loci upstream of the junction point typically affect both branches, leading to homocystinuria and methylmalonic aciduria, whereas mutations in downstream loci lead to one or the other disease. Elucidation of the biochemical penalties associated with individual mutations is providing molecular insights into the clinical data and, in some instances, identifying which cobalamin derivative(s) might be therapeutically beneficial.Our studies on B₁₂ trafficking are revealing strategies for cofactor sequestration and mobilization from low- to high-affinity and low- to high-coordination-number sites, which in turn are regulated by protein dynamics that constructs ergonomic cofactor binding pockets. While these B₁₂ lessons might be broadly relevant to other metal trafficking pathways, much remains to be learned. This Account concludes by identifying some of the major gaps and challenges that are needed to complete our understanding of B₁₂ trafficking.

Hemolytic Uremic Syndrome Due to Methylmalonic Acidemia and Homocystinuria in an Infant: A Case Report and Literature Review

Methylmalonic acidemia and homocystinuria cobalamin C (cblC) type is the most common inborn error of the intracellular cobalamin metabolism, associated with multisystem involvement and high mortality rates, especially in the early-onset form of the disease. Hemolytic uremic syndrome (HUS) is a rare manifestation and needs to be distinguished from other causes of renal thrombotic microangiopathy. We describe a case of a 3-month-old infant, with failure to thrive, hypotonia and pallor, who developed HUS in the setting of cblC deficit, along with dilated cardiomyopathy, and presented delayed response to optic stimulation in visual evoked potentials, as well as enlarged bilateral subarachnoid spaces and delayed myelination in brain magnetic resonance imaging. Renal damage was reversed, while neurodevelopmental profile and eye contact improved after supplementation with parenteral hydroxycobalamin, oral folic acid, betaine and levocarnitine. Homozygous mutation of c.271dupA in the MMACHC gene was ultimately detected. In this report, we highlight the diagnostic challenges as well as the significance of early recognition and multidisciplinary management of this unusual condition. A brief review of published case reports of early-onset cblC deficit and related HUS is depicted, pointing out the initial clinical presentation, signs of renal damage and outcome, MMACHC gene type of mutations and accompanying extra-renal manifestations.

A high frequency and geographical distribution of MMACHC R132* mutation in children with cobalamin C defect

Cobalamin C defect is caused by pathogenic variants in the MMACHC gene leading to impaired conversion of dietary vitamin B₁₂ into methylcobalamin and adenosylcobalamin. Variants in the MMACHC gene cause accumulation of methylmalonic acid and homocysteine along with decreased methionine synthesis. The spectrum of MMACHC gene variants differs in various populations. A total of 19 North Indian children (age 0-18 years) with elevated methylmalonic acid and homocysteine were included in the study, and their DNA samples were subjected to Sanger sequencing of coding exons with flanking intronic regions of MMACHC gene. The genetic analysis resulted in the identification of a common pathogenic nonsense mutation, c.394C > T (R132*) in 85.7% of the unrelated cases with suspected cobalamin C defect. Two other known mutations c.347T > C (7%) and c.316G > A were also detected. Plasma homocysteine was significantly elevated (> 100 µmol/L) in 75% of the cases and methionine was decreased in 81% of the cases. Propionyl (C3)-carnitine, the primary marker for cobalamin C defect, was found to be elevated in only 43.75% of cases. However, the secondary markers such as C3/C2 and C3/C16 ratios were elevated in 87.5% and 100% of the cases, respectively. Neurological manifestations were the most common in our cohort. Our findings of the high frequency of a single MMACHC R132* mutation in cases with combined homocystinuria and methylmalonic aciduria may be proven helpful in designing a cost-effective and time-saving diagnostic strategy for resource-constraint settings. Since the R132* mutation is located near the last exon-exon junction, this is a potential target for the read-through therapeutics.

Outcomes of patients with cobalamin C deficiency: A single center experience

Biallelic variants in MMACHC results in the combined methylmalonic aciduria and homocystinuria, called cobalamin (cbl) C (cblC) deficiency. We report 26 patients with cblC deficiency with their phenotypes, genotypes, biochemical parameters, and treatment outcomes, who were diagnosed and treated at our center. We divided all cblC patients into two groups: group 1: SX group: identified after manifestations of symptoms (n = 11) and group 2: NB group: identified during the asymptomatic period via newborn screening (NBS) or positive family history of cblC deficiency (n = 15). All patients in the SX group had global developmental delay and/or cognitive dysfunction at the time of the diagnosis and at the last assessment. Seizure, stroke, retinopathy, anemia, cerebral atrophy, and thin corpus callosum in brain magnetic resonance imaging (MRI) were common in patients in the SX group. Global developmental delay and cognitive dysfunction was present in nine patients in the NB group at the last assessment. Retinopathy, anemia, and cerebral atrophy and thin corpus callosum in brain MRI were less frequent. We report favorable outcomes in patients identified in the neonatal period and treated pre-symptomatically. Identification of cblC deficiency by NBS is crucial to improve neurodevelopmental outcomes.

Analysis of 70 patients with hydrocephalus due to cobalamin C deficiency

OBJECTIVE

To analyze the clinical characteristics of patients with hydrocephalus secondary to cobalamin C (cblC) deficiency and to discuss the optimal strategies for assessing and treating such patients by performing clinical and laboratory studies in 70 patients.

METHODS

A total of 1,211 patients were clinically diagnosed with methylmalonic acidemia (MMA) from 1998 to 2019. Among them, cblC deficiency was confirmed in 70 patients with hydrocephalus by brain imaging and biochemical and genetic analysis.

RESULTS

Of the 70 patients, 67 (95.7%) had early-onset MMA and homocystinuria. The patients typically had high blood propionylcarnitine and total homocysteine, low methionine, and methylmalonic aciduria. Signs of intracranial hypertension were relatively rare. We measured ventricular dilatation early in the disease by cranial ultrasound and MRI and/or CT. Eighteen different MMACHC mutations, including 4 novel mutations (c.427C>T, c.568insT, c.599G>A, and c.615C>A), were identified biallelically in all 70 patients. c.609G>A was the most frequent mutation, followed by c.658_660del, c.217C>T, and c.567dupT. Three cases were diagnosed by postmortem study. Metabolic therapy, including cobalamin injections supplemented with oral l-carnitine and betaine, was administered in the remaining 67 cases. A ventriculoperitoneal shunt was performed in 36 cases. During the follow-up, psychomotor development, nystagmus, impaired vision, and sunset eyes improved gradually.

CONCLUSION

Hydrocephalus is a severe condition with several different causes. In this study, ventriculomegaly was found in 70 patients with cblC deficiency. Early diagnosis, etiologic treatment, and prompt surgical intervention are crucial to improve the prognosis of patients.

Chlorocob(II)alamin Formation Which Enhances the Thiol Oxidase Activity of the B12-Trafficking Protein CblC

CblC is a chaperone that catalyzes removal of the β-axial ligand of cobalamin (or B₁₂), generating cob(II)alamin in an early step in the cofactor trafficking pathway. Cob(II)alamin is subsequently partitioned to support cellular needs for the synthesis of active cobalamin cofactor derivatives. In addition to the β-ligand transferase activity, the Caenorhabdiitis elegans CblC (ceCblC) and clinical R161G/Q variants of the human protein exhibit robust thiol oxidase activity, converting glutathione to glutathione disulfide while concomitantly reducing O₂ to H₂O₂. The chemical efficiency of the thiol oxidase side reaction during ceCblC-catalyzed dealkylation of alkylcobalamins is noteworthy in that it effectively scrubs ambient oxygen from the reaction mixture, leading to air stabilization of the highly reactive cob(I)alamin product. In this study, we report that the enhanced thiol oxidase activity of ceCblC requires the presence of KCl, which explains how the wasteful thiol oxidase activity is potentially curtailed inside cells where the chloride concentration is low. We have captured an unusual chlorocob(II)alamin intermediate that is formed in the presence of potassium chloride, a common component of the reaction buffer, and have characterized it by electron paramagnetic resonance, magnetic circular dichroism, and computational analyses. The ability to form a chlorocob(II)alamin intermediate could represent an evolutionary vestige in ceCblC, which is structurally related to bacterial B₁₂-dependent reductive dehalogenases that have been proposed to form halogen cob(II)alamin intermediates in their catalytic cycle.

Clinical features and underlying genetic causes in neonatal encephalopathy: A large cohort study

This study aimed to investigate the potential genetic causes of neonatal encephalopathy (NE) in a large cohort of Chinese patients. We included 366 neonates with encephalopathy. Whole exome sequencing was performed to assess the potential molecular defects. In this study, 43 patients (11.7%) were identified with pathogenic or likely pathogenic variants and 10 patients (2.7%) carried variants with unknown significance. Compared with patients without genetic findings (28.9%), patients with genetic findings (96.2%) displayed a significant higher incidence of seizure (P = .0009); however, a lower frequency of abnormal magnetic resonance imaging (MRI) results (P < .0001). Epileptic encephalopathy related genes account for nearly half (46.4%) of all genetic defects of NE with seizures. Follow-up results revealed genetic diagnosis, seizure and severe abnormal electroencephalograph results were significantly associated with high risk of developmental delay (P < .05). This study increases the understanding of genetic contribution to NE. Our findings suggest that the full-term NE patients with seizure, the greater the possibility of genetic diseases. However, for newborns especially the preterm babies with abnormal MRI findings, there is smaller possibility of genetic diseases. NE caused from genetic diseases have poor prognosis, and intensive intervention and follow-up is necessary for these newborns.

Development of infantile tremor syndrome after initiation of hydroxycobalamin treatment in an infant with a late diagnosis of cobalamin C disorder

Combined methylmalonic aciduria and homocystinuria (cobalamin C deficiency, cblC) is a well-described disorder of vitamin B12 metabolism caused by mutations in the MMACHC gene with multisystemic manifestations. While there is no cure, combined treatment with intramuscular hydroxycobalamin and oral betaine may reduce the severity of symptoms and improve clinical outcome. We report a female patient diagnosed with late-onset cobalamin C deficiency at the age of 8 months who presented with developmental regression and severe dermatitis. She developed a movement disorder after initiation of hydroxycobalamin treatment. Similar movement disorders have been described in patients with nutritional vitamin B12 deficiencies following cobalamin supplementation but have not previously been reported in patients with cobalamin C disorder. The movement disorder in our patient gradually resolved with clonazepam treatment, despite no seizure activity detected on EEG. She was eventually weaned off the clonazepam and the abnormal movements have not recurred. The patient remains developmentally delayed but is showing no other symptoms related to cobalamin C deficiency. The patient has a younger affected sibling who was treated from birth and who is physically and developmentally entirely normal; she did not have abnormal movements after treatment with hydroxycobalamin was initiated. There is no clear consensus on the cause of movement disorders that develop following initiation of intramuscular vitamin B12 treatment.

Variable phenotypes and outcomes associated with the MMACHC c.609G>A homologous mutation: long term follow-up in a large cohort of cases

Background

Cobalamin C deficiency (cblC) caused by the MMACHC mutations is the most common type of the disorders of intracellular cobalamin metabolism. While the c.609G > A mutation is most frequent in Chinese cblC patients, its correlation with phenotype has not been delineated. Here we aim to investigate the factors affecting variable phenotypes and outcomes associated with the MMACHC c.609G > A homologous mutation in 149 Chinese cases to have implications for treatment and prevention.

Methods

We assessed 149 cblC patients caused by MMACHC c.609G > A homozygous mutation. The clinical manifestations, complications, treatment, and outcomes were evaluated; 120 patients were followed-up till December 2019.

Results

Two patients (1.3%) were prenatally diagnosed, treated after birth and consequently showed normal development. In 15 patients (10.1%) detected by newborn screening, 10 were treated at the age of 2 weeks and showed normal development, while the other 5 were treated after onset and showed neurologic disorders. All 132 clinically diagnosed patients (88.6%) developed symptoms at age from few minutes after birth to 72 months. Among them, 101 (76.5%) had early-onset (before the age of 12 months) and 31 (23.5%) had late-onset (after the age of 12 months). Totally 5 patients died and 24 were lost to follow-up. Of the 132 clinical diagnosed patients, 92 (69.7%) presented with developmental delay, 65 (49.2%) had seizures, 37 (28.0%) had anemia, 24 (18.2%) had feeding difficulty, 23 (17.4%) had ocular problems, and 22 (16.7%) had hydrocephalus. Compared with the non-developmental delay group, the onset age, the age at treatment initiation and the time from onset to treatment initiation were later in the developmental delay group. Seizure group showed significantly higher urinary methylmalonic acid concentration. During long-term follow-up, plasma total homocysteine (tHcy) levels were significantly higher in patients in the uncontrolled group than those in the seizure-free group.

Conclusions

Most cblC patients caused by MMACHC c.609G > A homozygous mutation showed early-onset. The clinically diagnosed patients usually showed the presence of irreversible brain disorders. Patients treated from the pre-symptomatic stage showed favorable outcomes. Therefore, newborn screening, prenatal diagnosis and early treatment are crucial and the c.609G > A mutant allele should be listed in the pre-pregnancy carrier screening panel in China.

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.

Diagnostic Challenges Using a 2-Tier Strategy for Methylmalonic Acidurias: Data from 1.2 Million Dried Blood Spots

BACKGROUND

The detection of methylmalonic acid (MMA) by second-tier analysis has been shown to reduce the number of false positives in newborn screening (NBS) for genetically determined methylmalonic acidurias (MMAuria). In addition to genetic conditions, MMA is an indicator of vitamin B12 status, thus applicable to detect maternal vitamin B12 deficiency in the newborns screened.

METHODS

Biochemical and clinical follow-up data of a 7.5-year pilot study with 1.2 million newborns screened were reviewed.

RESULTS

Among 1,195,850 NBS samples, 3,595 (0.3%) fulfilled criteria for second-tier analysis of MMA. In 37 (0.003%; 1/32,000) samples, elevated concentrations of MMA were detected, resulting in diagnostic workup at a metabolic center in 21 newborns. In 6 infants (1/199,000), genetic conditions were established, 1 infant with cobalamin C deficiency (CblC) showed only a moderate elevation of MMA. The remaining 15 newborns (1/79,000) displayed significantly lower concentrations of MMA and were evaluated for maternal vitamin B12 deficiency. In 9 mothers, vitamin B12 deficiency was verified, and 6 showed no indication for vitamin B12 deficiency. Treatment with vitamin B12 normalized biochemical parameters in all 15 infants.

CONCLUSIONS

Applying a 2-tier strategy measuring MMA in NBS identified genetic conditions of MMAuria. It was possible to separate severe, early-onset phenotypes from maternal vitamin B12 deficiency. However, the detection of CblC deficiency with mildly elevated MMA interferes with impaired vitamin B12 status of unknown relevance and thus burdens possibly healthy newborns. Regarding maternal vitamin B12 deficiency, testing and supplementing mothers-to-be is preferable. This might decrease straining follow-up of newborns and improve quality and overall perception of NBS.

A case report of recurrent acute pancreatitis associated with life threatening atypical hemolytic uremic syndrome

INTRODUCTION

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy defined by the sudden onset of hemolytic anemia, thrombocytopenia, and acute kidney injury (AKI). HUS is categorized as either typical, caused by Shiga toxin-producing Escherichia coli infection, or atypical HUS (aHUS), usually complement mediated or secondary to systemic disease. We describe a rare case of aHUS in an adult male patient with recurrent acute pancreatitis.

PATIENT CLINICAL FINDINGS

A 32-year-old Caucasian male presented to our institution for his third episode of alcohol-induced pancreatitis. He presented with abdominal pain, elevated lipase and pancreatic inflammation on computed tomography consistent with acute pancreatitis. While admitted, he developed sudden onset severe thrombocytopenia, AKI and hemolytic anemia.

DIAGNOSIS, THERAPEUTIC INTERVENTIONS, OUTCOMES

Peripheral blood smear, haptoglobin and hemoglobin level confirmed microangiopathic hemolytic anemia. Worsening anemia, thrombocytopenia and AKI were consistent with the diagnosis of aHUS. The patient's pancreatitis resolved with supportive measures, but resolution of significant thrombocytopenia and AKI was not achieved until administration of eculizumab, a complement inhibiting therapy. Eculizumab therapy provided dramatic improvement in this patient, with platelet count increasing from a low of 11,000 to >100,000 within 48 hours of therapy. Creatinine and hemoglobin levels returned to baseline within 3 weeks.

CONCLUSION

Recurrent pancreatitis is suggested as the etiology of atypical HUS in this patient and this condition should be recognized and treated in a timely manner for optimal clinical outcomes.

The human B12 trafficking protein CblC processes nitrocobalamin

In humans, cobalamin or vitamin B₁₂ is delivered to two target enzymes via a complex intracellular trafficking pathway comprising transporters and chaperones. CblC (or MMACHC) is a processing chaperone that catalyzes an early step in this trafficking pathway. CblC removes the upper axial ligand of cobalamin derivatives, forming an intermediate in the pathway that is subsequently converted to the active cofactor derivatives. Mutations in the cblC gene lead to methylmalonic aciduria and homocystinuria. Here, we report that nitrosylcobalamin (NOCbl), which was developed as an antiproliferative reagent, and is purported to cause cell death by virtue of releasing nitric oxide, is highly unstable in air and is rapidly oxidized to nitrocobalamin (NO₂Cbl). We demonstrate that CblC catalyzes the GSH-dependent denitration of NO₂Cbl forming 5-coordinate cob(II)alamin, which had one of two fates. It could be oxidized to aquo-cob(III)alamin or enter a futile thiol oxidase cycle forming GSH disulfide. Arg-161 in the active site of CblC suppressed the NO₂Cbl-dependent thiol oxidase activity, whereas the disease-associated R161G variant stabilized cob(II)alamin and promoted futile cycling. We also report that CblC exhibits nitrite reductase activity, converting cob(I)alamin and nitrite to NOCbl. Finally, the denitration activity of CblC supported cell proliferation in the presence of NO₂Cbl, which can serve as a cobalamin source. The newly described nitrite reductase and denitration activities of CblC extend its catalytic versatility, adding to its known decyanation and dealkylation activities. In summary, upon exposure to air, NOCbl is rapidly converted to NO₂Cbl, which is a substrate for the B₁₂ trafficking enzyme CblC.

Rapid screening of MMACHC gene mutations by high-resolution melting curve analysis

Background

Cobalamin (cbl) C is a treatable rare hereditary disorder of cbl metabolism with autosomal recessive inheritance. It is the most common organic acidemia, manifested as methylmalonic academia combined with homocysteinemia. Early screening and diagnosis are important. The mutation spectrum of the MMACHC gene causing cblC varies among populations. The mutation spectrum in Chinese population is notably different from that in other populations.

Methods

A PCR followed by high‐resolution melting curve analysis (PCR‐HRM) method covering all coding exons of MMACHC gene was designed to verify 14 pathogenic MMACHC gene variants found in patients with cblC, including all common mutations in Chinese patients with cblC.

Result

By PCR‐HRM analysis, 14 pathogenic variants of MMACHC showed distinctly different melting curves, which were consistent with Sanger sequencing. The homozygous type of the most common mutation c.609G > A (p.Trp203Ter) can also be analyzed by specially designed PCR‐HRM.

Conclusion

The established PCR‐HRM method for screening common pathogenic MMACHC variants in Chinese patients with cblC has the advantages of high accuracy, high throughput, low cost, and high speed. It is suitable for the large‐sample screening of suspected children with methylmalonic acidemia and carriers in population.

Cobalamin c deficiency associated with antifactor h antibody-associated hemolytic uremic syndrome in a young adult

Background

Thrombotic microangiopathy (TMA) syndromes are characterized by the association of hemolytic anemia, thrombocytopenia and organ injury due to arteriolar and capillary thrombosis.

Case presentation

We report the first case of adult onset cobalamin C (Cbl C) disease associated with anti-factor H antibody-associated hemolytic uremic syndrome (HUS).

A 19-year-old woman was admitted to the nephrology department owing to acute kidney failure, proteinuria, and hemolytic anemia with schizocytes.

TMA was diagnosed and plasma exchanges were started in emergency.

Exhaustive analyses showed 1) circulating anti factor H antibody and 2) hyperhomocysteinemia, hypomethioninemia and high levels of methylmalonic aciduria pointing towards Clb C disease. Cbl C disease has been confirmed by methylmalonic aciduria and homocystinuria type C protein gene sequencing revealing two heterozygous pathogenic variants.

The kidney biopsy showed 1) intraglomerular and intravascular thrombi 2) noticeable thickening of the capillary wall with a duplication aspect of the glomerular basement membrane and a glomerular capillary wall IgM associated with Cbl C disease related TMA.

We initiated treatment including hydroxycobalamin, folinic acid, betaine and levocarnitine and Eculizumab. Rituximab infusions were performed allowing a high decrease in anti-factor H antibody rate.

Six month after the disease onset, Eculizumab was weaning and vitaminotherapy continued. Outcome was favorable with a dramatic improvement in kidney function.

Conclusion

TMA with renal involvement can have a complex combination of risk factors including anti-FH autoantibody in the presence of cblC deficiency.