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.

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.

Peripheral Nervous System Involvement in Late-Onset Cobalamin C Disease?

Background: Cobalamin C (cblC) has a fundamental role in both central and peripheral nervous system function at any age. Neurologic manifestations may be the earliest and often the only manifestation of hereditary or acquired cblC defect. Peripheral neuropathy remains a classical but underdiagnosed complication of cblC defect, especially in late-onset cblC disease caused by mutations in the methylmalonic aciduria type C and homocysteinemia (MMACHC) gene. So the clinical, electrophysiological, and pathological characteristics of late-onset cblC disease are not well-known.

Methods: A retrospective study of patients with late-onset cblC disease was conducted at our hospital on a 3-year period. The neuropathy was confirmed by the nerve conduction study. Sural biopsies were performed in 2 patients.

Results: Eight patients were identified, with a mean onset age of 16.25 ± 6.07 years. All patients had methylmalonic aciduria, homocysteinemia, compound heterozygous MMACHC gene mutations were detected in all patients, and 7/8 patients with c.482G>A mutation. One patient concomitant with homozygote c.665C>T mutation in 5,10-methylenetetrahydrofolate reductase (MTHFR) gene. All patients showed limb weakness and cognitive impairment. Five patients had possible sensorimotor axonal polyneuropathy predominantly in the distal lower limbs. Sural biopsies showed loss of myelinated and unmyelinated fibers. Electro microscopy revealed crystalline-like inclusions bodies in Schwann cells and axonal degeneration.

Conclusion: Late-onset cblC disease had possible heterogeneous group of distal axonal neuropathy. c.482G>A mutation is a hot spot mutation in late-onset cblC disease.

Do not Miss Rare and Treatable Cause of Early-Onset Hemolytic Uremic Syndrome: Cobalamin C Deficiency

The most common disorder of vitamin B12 metabolism is methylmalonic aciduria and homocystinuria type cobalamin C (cblC), which accounts for most of the cases is referred to as cblC deficiency. Cobalamin deficiency is one of the causes of early-onset hemolytic uremic syndrome (HUS). Here, we present the cases of 2 infants with cobalamin deficiency who presented with early-onset HUS. The first patient was a 5-month-old female who was admitted to the hospital with seizure, pallor, and yellow-colored diarrheal stools. Initial laboratory examination showed direct Coombs test-negative hemolytic anemia. Later, she developed acute kidney injury and thrombocytopenia. Bone marrow aspiration showed megaloblastic features, and urinary examination showed elevated levels of methylmalonic acid (MMA), -methyl citrate, and 3-hydroxypropionic acid. Methionine-restricted diet, parenteral hydroxocobalamine, folinic acid, carnitine, and betaine were initiated. Hemolytic activity was -controlled with this treatment. Genetic screening showed homozygous mutation on the MMACHCgene (p.R161*[c.481C>T]). The second patient was a 3-month-old male infant who was admitted to the hospital with malaise and diarrhea. Laboratory examination showed direct Coombs test-negative hemolytic anemia with leukopenia. Later he developed acute kidney injury and thrombocytopenia. Bone marrow aspiration revealed megaloblastic changes. Urine organic acid test showed increased levels of MMA. Parenteral hydroxocobalamine, folinic acid, carnitine, and betaine were initiated. He died due to respiratory failure and cardiac arrest. Direct sequencing of MMACHC identified homozygous mutation, c.271dup A. CblC deficiency is an important cause of early-onset HUS and prompt diagnosis will provide specific treatment modalities.

Do not Miss Rare and Treatable Cause of Early-Onset Hemolytic Uremic Syndrome: Cobalamin C Deficiency

The most common disorder of vitamin B12 metabolism is methylmalonic aciduria and homocystinuria type cobalamin C (cblC), which accounts for most of the cases is referred to as cblC deficiency. Cobalamin deficiency is one of the causes of early-onset hemolytic uremic syndrome (HUS). Here, we present the cases of 2 infants with cobalamin deficiency who presented with early-onset HUS. The first patient was a 5-month-old female who was admitted to the hospital with seizure, pallor, and yellow-colored diarrheal stools. Initial laboratory examination showed direct Coombs test-negative hemolytic anemia. Later, she developed acute kidney injury and thrombocytopenia. Bone marrow aspiration showed megaloblastic features, and urinary examination showed elevated levels of methylmalonic acid (MMA), -methyl citrate, and 3-hydroxypropionic acid. Methionine-restricted diet, parenteral hydroxocobalamine, folinic acid, carnitine, and betaine were initiated. Hemolytic activity was -controlled with this treatment. Genetic screening showed homozygous mutation on the MMACHCgene (p.R161*[c.481C>T]). The second patient was a 3-month-old male infant who was admitted to the hospital with malaise and diarrhea. Laboratory examination showed direct Coombs test-negative hemolytic anemia with leukopenia. Later he developed acute kidney injury and thrombocytopenia. Bone marrow aspiration revealed megaloblastic changes. Urine organic acid test showed increased levels of MMA. Parenteral hydroxocobalamine, folinic acid, carnitine, and betaine were initiated. He died due to respiratory failure and cardiac arrest. Direct sequencing of MMACHC identified homozygous mutation, c.271dup A. CblC deficiency is an important cause of early-onset HUS and prompt diagnosis will provide specific treatment modalities.