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

Late-onset methylmalonic acidemia and homocysteinemia (cblC disease): systematic review

INTRODUCTION

Combined methylmalonic acidemia and homocystinuria, cblC type is an inborn error of intracellular cobalamin metabolism and the most common one. The age of onset ranges from prenatal to adult. The disease is characterised by an elevation of methylmalonic acid (MMA) and homocysteine and a decreased production of methionine. The aim is to review existing scientific literature of all late onset cblC patients in terms of clinical symptoms, diagnosis, and outcome.

METHODS

A bibliographic database search was undertaken in PubMed (MEDLINE) complemented by a reference list search. We combined search terms regarding cblC disease and late onset. Two review authors performed the study selection, data extraction and quality assessment.

RESULTS

Of the sixty-five articles included in this systematic review, we collected a total of 199 patients. The most frequent clinical symptoms were neuropathy/myelopathy, encephalopathy, psychiatric symptoms, thrombotic microangiopathy, seizures, kidney disease, mild to severe pulmonary hypertension with heart failure and thrombotic phenomena. There were different forms of supplementation used in the different studies collected and, within these studies, some patients received several treatments sequentially and/or concomitantly. The general outcome was: 64 patients recovered, 78 patients improved, 4 patients did not improve, or the disease progressed, and 12 patients died.

CONCLUSIONS

Most scientific literature regarding the late onset cblC disease comes from case reports and case series. In most cases treatment initiation led to an improvement and even recovery of some patients. The lack of complete recovery underlines the necessity for increased vigilance in unclear clinical symptoms for cblC disease.

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.

Clinical, phenotypic and genetic landscape of case reports with genetically proven inherited disorders of vitamin B12 metabolism: A meta-analysis

Inherited disorders of B₁₂ metabolism produce a broad spectrum of manifestations, with limited knowledge of the influence of age and the function of related genes. We report a meta-analysis on 824 patients with a genetically proven diagnosis of an inherited disorder of vitamin B₁₂ metabolism. Gene clusters and age categories are associated with patients' manifestations. The "cytoplasmic transport" cluster is associated with neurological and ophthalmological manifestations, the "mitochondrion" cluster with hypotonia, acute metabolic decompensation, and death, and the "B₁₂ availability" and "remethylation" clusters with anemia and cytopenia. Hypotonia, EEG abnormalities, nystagmus, and strabismus are predominant in the younger patients, while neurological manifestations, such as walking difficulties, peripheral neuropathy, pyramidal syndrome, cerebral atrophy, psychiatric disorders, and thromboembolic manifestations, are predominant in the older patients. These results should prompt systematic checking of markers of vitamin B₁₂ status, including homocysteine and methylmalonic acid, when usual causes of these manifestations are discarded in adult patients.

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.

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.

Adult-onset CblC deficiency: a challenging diagnosis involving different adult clinical specialists

Background

Methylmalonic aciduria and homocystinuria, CblC type (OMIM #277400) is the most common disorder of cobalamin intracellular metabolism, an autosomal recessive disease, whose biochemical hallmarks are hyperhomocysteinemia, methylmalonic aciduria and low plasma methionine. Despite being a well-recognized disease for pediatricians, there is scarce awareness of its adult presentation. A thorough analysis and discussion of cobalamin C defect presentation in adult patients has never been extensively performed. This article reviews the published data and adds a new case of the latest onset of symptoms ever described for the disease.

Results

We present the emblematic case of a 45-year-old male, describing the diagnostic odyssey he ventured through to get to the appropriate treatment and molecular diagnosis. Furthermore, available clinical, biochemical and molecular data from 22 reports on cases and case series were collected, resulting in 45 adult-onset CblC cases, including our own. We describe the onset of the disease in adulthood, encompassing neurological, psychiatric, renal, ophthalmic and thromboembolic symptoms. In all cases treatment with intramuscular hydroxycobalamin was effective in reversing symptoms. From a molecular point of view adult patients are usually compound heterozygous carriers of a truncating and a non-truncating variant in the MMACHC gene.

Conclusion

Adult onset CblC disease is a rare disorder whose diagnosis can be delayed due to poor awareness regarding its presenting insidious symptoms and biochemical hallmarks. To avoid misdiagnosis, we suggest that adult onset CblC deficiency is acknowledged as a separate entity from pediatric late onset cases, and that the disease is considered in the differential diagnosis in adult patients with atypical hemolytic uremic syndromes and/or slow unexplained decline in renal function and/or idiopathic neuropathies, spinal cord degenerations, ataxias and/or recurrent thrombosis and/or visual field defects, maculopathy and optic disc atrophy. Plasma homocysteine measurement should be the first line for differential diagnosis when the disease is suspected. To further aid diagnosis, it is important that genes belonging to the intracellular cobalamin pathway are included within gene panels routinely tested for atypical hemolytic uremic syndrome and chronic kidney disorders.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02179-y.

Late-onset methylmalonic acidemia and homocysteinemia

Introduction

Introduction: cobalamin C (Cbl C) deficiency is the most common defect in intracellular cobalamin metabolism, associated with methylmalonic acidemia and homocystinuria. Its late clinical presentation is heterogeneous and may lead to a diagnostic delay. Case report: we report the case of a 45-year-old man with a 20-year history of chronic kidney disease and recently diagnosed spastic paraparesis, both of unknown origin. Metabolic studies revealed elevated levels of homocysteine and methylmalonic acid in the blood and urine. A genetic study confirmed cobalamin C deficiency. Treatment with hydroxocobalamin, betaine, carnitine, and folic acid was started. The patient eventually received a kidney transplant. Discussion: early diagnosis and appropriate treatment improve the clinical evolution of patients with Cbl C deficiency. Determination of homocysteine, organic acids, and other amino acids should be included in the differential diagnosis of patients with nephrological-neurological symptoms without a clear etiology.

Diagnosis of cobalamin C deficiency with renal abnormality from onset in a Chinese child by next generation sequencing: A case report

The aim of the present study was to present the diagnosis and treatment course of a patient with cobalamin C deficiency (cblC) hospitalized with renal function abnormality from the onset. A female, 7-year-old patient who presented with a cough and progressive dyspnea for 1 day was admitted to the Children's Hospital of Nanjing Medical University (Nanjing, China). A routine clinical examination was performed, including physical examination, routine blood and urine tests, blood gas analysis, computed tomography scans of the head, chest and abdomen, electrocardiogram, echocardiography and abdominal ultrasonography. In addition, laboratory tests were performed, including tests for viral infection and markers of autoimmunity, humoral immunity, myocardial enzymes and tumors. Tandem mass analysis and renal biopsy were conducted. Next generation sequencing (NGS) was performed to identify mutated genes, and structural investigation was conducted to identify the key residue mutations in the patient. Routine clinical examination revealed that the patient had multiple organ failure, indicating the presence of metabolic disease. Tandem mass analysis and renal biopsy also indicated that the patient had methylmalonic acidemia (MMA) and thrombotic microangiopathy combined with focal renal cortical necrosis. Furthermore, next-generation sequencing identified the presence of two heterozygous mutations in the MMA cblC type with homocystinuria (MMACHC) gene. Structural analysis demonstrated that the two mutations were in key components of the MMACHC protein. The patient was finally diagnosed with cblC according to the results obtained. In conclusion, NGS may aid in the diagnosis and therapeutic management of cblC with renal abnormality from the onset in children.

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.

Ocular disease in the cobalamin C defect: a review of the literature and a suggested framework for clinical surveillance

The association between combined methylmalonic acidemia and homocystinuria of cblC type (cobalamin C defect, cblC) and ocular disease is now well recognized, and is a significant component of morbidity and disability associated with the condition. In this review, through collation of historically reported cases of early- and late-onset cblC and previously unreported cases, we have attempted to characterize the epidemiology, clinical features, and pathomechanisms of individual ocular features of cblC. These data suggest that maculopathy and nystagmus with abnormal vision are extremely common and affect the majority of children with early-onset cblC, usually before school age; strabismus and optic atrophy are also seen at relatively high frequency. The timing of progression of macular disease may coincide with a critical period of postnatal foveal development. Maculopathy and retinal disease may be subclinical and show only partial correlation with the extent of visual deficits, and visual deterioration may be relentlessly progressive in spite of aggressive treatment of biochemical abnormalities. In later-onset forms of the disease, visual loss and ocular complications appear to be infrequent. Finally, we discuss investigational strategies in diagnosing and characterizing eye disease in individuals with cblC, explore possible therapeutic avenues that may attenuate progression and severity of eye disease, and propose a clinical surveillance guideline for monitoring progression of ocular disease in children and adults with cblC.

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.

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

Background

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

Methods

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

Results

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

Conclusions

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