Mutation analysis of genes related to methylmalonic acidemia: identification of eight novel mutations

Spectrum of genetic mutations in methylmalonic aciduria among Iranian patients

Methylmalonic aciduria (MMA) is described by high methylmalonic acid concentrations in the blood and urine. This condition can be isolated or in combination with homocystinuria. While variants in the MMUT, MMAA, MMAB, MMADHC and MCEE genes contribute to the pathogenesis of the isolated form, variants in MMACHC, MMADHC, LMBRD1, and ABCD4 genes, are responsible for diverse types of combined MMA and homocystinuria. In the current study, we report molecular tests of 15 Iranian patients who had mutations in MMA-related genes. Among the assessed patients, MMACHC gene was the most prevalently mutated gene (mutated in 7 patients). Each of MMAA, MMAB, and MMUT genes were mutated in 2 patients, respectively. Finally, we detected variants in each of ACSF3 and ABCD4 genes in one case, respectively. Among the identified variants, five variants were not reported before. Cumulatively, the current study provides some data about MMA-related variants among Iranian patients.

Identification of a biallelic MMUT variant (p.Thr230Arg) and its global perspective on clinical management

BACKGROUND

Methylmalonic acidemia (MMA), type mut (0) is a rare type of genetic inborn error of metabolism (IEM) that is caused by aberrant malonyl-CoA mutase activity. Diagnosing IEM can be challenging due to its inherited onset and varying degrees of severity.

METHODS AND RESULTS

In the present study, a consanguineous Pakistani family suspected of IEM was genetically analyzed using whole exome sequencing. A biallelic variant c.689 C > G (p.Thr230Arg) in MMUT was identified to be the causative factor of the disease, which helped in establishing the accurate diagnosis in the family to be MMA mut(0) type. On the basis of the genetic findings, the patient's condition was appropriately managed through a supportive nutrition plan and administration of oral L-carnitine.

CONCLUSIONS

Identification of MMUT mutation through whole exome sequencing was helpful in solving the family and devising targeted management strategies. This study highlights the utility of genetic analysis in diagnosing and treating metabolic disorders like MMA in Pakistani inbred population.

Associations between elevated uric acid and brain imaging abnormalities in pediatric patients with methylmalonic acidemia under 5 years of age

Methylmalonic acidemia (MMA) is the most common inborn organic acidemia, presenting multisystemic complications. Uric acid may have neurotoxic or neuroprotective effects due to its antioxidant or pro-inflammatory properties; however, its role in MMA brain injury remains unclear. We examined the correlation between the serum uric acid levels and brain imaging features of MMA. Data were collected from a cross-sectional study of 216 patients with MMA and 216 healthy matched controls aged 0-5 years in China. Serum uric acid levels were measured, and magnetic resonance imaging and computed tomography findings were retrieved from hospital records. Overall, 74.1% patients had brain abnormalities. Patients in the MMA group with abnormal brain imaging had higher serum uric acid levels than those in the MMA normal brain imaging and control groups. The area under the curve of serum uric acid was 0.74, 0.91, and 0.93 for MMA diagnosis with abnormal brain images, basal ganglia changes, and globus pallidus changes, respectively. Higher serum uric acid levels were independently associated with abnormal brain images. Children aged < 5 years with abnormal brain images in MMA exhibit elevated serum uric acid levels, serving as an effective auxiliary diagnostic indicator and independent risk factor for brain tissue injury.

Evaluation of the clinical, biochemical, and molecular spectrum of Cobalamin C (CblC) defect in 33 patients from Pakistan

BACKGROUND

Cobalamin C is the most common inborn error of intracellular cobalamin metabolism caused by biallelic pathogenic variants in the MMACHC gene, leading to impaired conversion of dietary vitamin B12 into its two metabolically active forms, methylcobalamin and adenosylcobalamin. Biochemical hallmarks are elevated plasma total homocysteine (HCYs) and low methionine accompanied by methylmalonic aciduria. This study aimed to evaluate the clinical, biochemical, and molecular analysis of Pakistani patients with CblC defect.

METHODS

Medical charts, urine organic acid (UOA) chromatograms, plasma amino acid levels, plasma tHcy and MMACHC gene results of patients presenting at the Biochemical Genetics Clinic, AKUH from 2013-2021 were reviewed. Details were collected on a pre-structured questionnaire. SPSS 22 was used for data analysis.

RESULTS

CblC was found in 33 cases (Male:Female 19:14). The median age of symptoms onset and diagnosis were 300 (IQR:135-1800) and 1380 (IQR: 240-2730) days. The most common clinical features were cognitive impairment (n = 29), seizures (n = 23), motor developmental delay (n = 20), hypotonia (n = 17), and sparse/hypopigmented scalp hair (n = 16). The MMACHC gene sequencing revealed homozygous pathogenic variant c.394C > T, (p.Arg132*) in 32 patients, whereas c.609G > A, (p.TRP203*) in one patient whose ancestors had settled in Pakistan from China decades ago. The median age of treatment initiation was 1530 (IQR: 240-2790). The median pre-treatment HCYs levels were 134 (IQR:87.2-155.5) compared to post-treatment levels of 33.3 (IQR: 27.3-44.95) umol/L.

CONCLUSIONS

Thirty-three cases of CblC defect from a single center underscores a significant number of the disorder within Pakistan. Late diagnosis emphasizes the need for increased clinical awareness and adequate diagnostic facilities.

Acylcarnitines: Nomenclature, Biomarkers, Therapeutic Potential, Drug Targets, and Clinical Trials

Acylcarnitines are fatty acid metabolites that play important roles in many cellular energy metabolism pathways. They have historically been used as important diagnostic markers for inborn errors of fatty acid oxidation and are being intensively studied as markers of energy metabolism, deficits in mitochondrial and peroxisomal β -oxidation activity, insulin resistance, and physical activity. Acylcarnitines are increasingly being identified as important indicators in metabolic studies of many diseases, including metabolic disorders, cardiovascular diseases, diabetes, depression, neurologic disorders, and certain cancers. The US Food and Drug Administration-approved drug L-carnitine, along with short-chain acylcarnitines (acetylcarnitine and propionylcarnitine), is now widely used as a dietary supplement. In light of their growing importance, we have undertaken an extensive review of acylcarnitines and provided a detailed description of their identity, nomenclature, classification, biochemistry, pathophysiology, supplementary use, potential drug targets, and clinical trials. We also summarize these updates in the Human Metabolome Database, which now includes information on the structures, chemical formulae, chemical/spectral properties, descriptions, and pathways for 1240 acylcarnitines. This work lays a solid foundation for identifying, characterizing, and understanding acylcarnitines in human biosamples. We also discuss the emerging opportunities for using acylcarnitines as biomarkers and as dietary interventions or supplements for many wide-ranging indications. The opportunity to identify new drug targets involved in controlling acylcarnitine levels is also discussed. SIGNIFICANCE STATEMENT: This review provides a comprehensive overview of acylcarnitines, including their nomenclature, structure and biochemistry, and use as disease biomarkers and pharmaceutical agents. We present updated information contained in the Human Metabolome Database website as well as substantial mapping of the known biochemical pathways associated with acylcarnitines, thereby providing a strong foundation for further clarification of their physiological roles.

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.

Proteinuria as a presenting sign of combined methylmalonic acidemia and homocysteinemia: case report

Background

Disorders of the metabolism and absorption of vitamin B12 can lead to decrease in activity of methionine synthetase and methylmalonate coenzyme A mutase (MMUT), which results in increased levels of methylmalonic acid and homocysteine in blood and urine. Often, combined methylmalonic acidemia (MMA) and homocysteinemia is misdiagnosed due to a lack of specific symptoms. The clinical manifestations are diverse, but proteinuria as the initial presentation is rare.

Case presentation

Two cases of MMA with homocysteinemia in children are reported. Proteinuria were a primary presenting symptom, followed by anemia and neurologic symptoms (frequent convulsions and unstable walking, respectively). Screening of amino acids and acyl carnitine in serum showed that the propionyl carnitine:acetylcarnitine ratio increased. Profiling of urinary organic acids by gas chromatography–mass spectrometry revealed high levels of methylmalonic acid. Homocysteine content in blood was increased. Comprehensive genetic analyses of peripheral blood-derived DNA demonstrated heterozygous variants of methylmalonic aciduria type C and homocystinuria (MMACHC) and amnionless (AMN) genes in our two patients, respectively. After active treatment, the clinical manifestations in Case 1 were relieved and urinary protein ceased to be observed; Case 2 had persistent proteinuria and was lost to follow-up.

Conclusions

Analyses of the organic acids in blood and urine suggested MMA combined with homocysteinemia. In such diseases, reports of renal damage are uncommon and proteinuria as the initial presentation is rare. Molecular analysis indicated two different genetic causes. Although the pathologic mechanisms were related to vitamin B12, the severity and prognosis of renal lesions were different. Therefore, gene detection provides new insights into inherited metabolic diseases.