Mitochondrial DNA depletion syndrome (MDS), characterized by succinate-CoA ligase deficiency and loss of mitochondrial DNA (mtDNA), is caused by specific variants in nuclear genes responsible for mtDNA maintenance. SUCLA2-related mitochondrial DNA depletion syndrome, type 5 (MTDPS-5), presents as a rare, severe early progressive encephalomyopathy. This report investigates a new family exhibiting clinical manifestations of MTDPS-5 and elucidates the genetic basis of this disorder. In two affected siblings, a novel maternally inherited nonsense variant [c.1234C>T (p.Arg412*)] in the SUCLA2 gene and a unique paternally inherited indel variant (g.48569263-48571020del1758insATGA) were identified. Additionally, the siblings exhibited blood mtDNA content lower than 33% compared to age-matched controls. These findings underscore the importance of assessing SUCLA2 variants in patients with severe early progressive encephalomyopathy, even in the absence of methylmalonic aciduria or mtDNA loss, thereby broaden the mutational spectrum of this gene.

Succinyl-CoA synthetase or succinate-CoA ligase deficiency can result from biallelic mutations in SUCLG1 gene that encodes for the alpha subunit of the succinyl-CoA synthetase. Mutations in this gene were initially associated with fatal infantile lactic acidosis. We describe an individual with a novel biallelic pathogenic mutation in SUCLG1 with a less severe phenotype dominated by behavioral problems. The mutation was identified to be c.512A>G corresponding to a p.Asn171Ser change in the protein. The liquid chromatography tandem mass spectrometry-based enzyme activity assay on cultured fibroblasts revealed a markedly reduced activity of succinyl-CoA synthetase enzyme when both ATP and GTP were substrates, affecting both ADP-forming and GDP-forming functions of the enzyme.

Succinate-CoA ligase (SUCL) is a heterodimer consisting of an alpha subunit encoded by SUCLG1, and a beta subunit encoded by either SUCLA2 or SUCLG2 catalyzing an ATP- or GTP-forming reaction, respectively, in the mitochondrial matrix. The deficiency of this enzyme represents an encephalomyopathic form of mtDNA depletion syndromes. We describe the fatal clinical course of a female patient with a pathogenic mutation in SUCLG1 (c.626C > A, p.Ala209Glu) heterozygous at the genomic DNA level, but homozygous at the transcriptional level. The patient exhibited early-onset neurometabolic abnormality culminating in severe brain atrophy and dystonia leading to death by the age of 3.5 years. Urine and plasma metabolite profiling was consistent with SUCL deficiency which was confirmed by enzyme analysis and lack of mitochondrial substrate-level phosphorylation (mSLP) in skin fibroblasts. Oxygen consumption- but not extracellular acidification rates were altered only when using glutamine as a substrate, and this was associated with mild mtDNA depletion and no changes in ETC activities. Immunoblot analysis revealed no detectable levels of SUCLG1, while SUCLA2 and SUCLG2 protein expressions were largely reduced. Confocal imaging of triple immunocytochemistry of skin fibroblasts showed that SUCLG2 co-localized only partially with the mitochondrial network which otherwise exhibited an increase in fragmentation compared to control cells. Our results outline the catastrophic consequences of the mutated SUCLG1 leading to strongly reduced SUCL activity, mSLP impairment, mislocalization of SUCLG2, morphological alterations in mitochondria and clinically to a severe neurometabolic disease, but in the absence of changes in mtDNA levels or respiratory complex activities.

To determine the frequency of disorders leading to methylmalonic acidurias. This cross-sectional study was conducted from January 2013 to April 2016 at the Aga Khan University Hospital, Karachi, and comprised patients diagnosed with methylmalonic acidurias based on urine organic acid analysis. Clinical history and biochemical data was collected from the biochemical genetics laboratory requisition forms. Organic acid chromatograms of all the subjects were critically reviewed by a biochemical pathologist and a metabolic physician. For assessing the clinical outcome, medical charts of the patients were reviewed. SPSS 19 was used for data analysis. Of the 1,778 patients 50(2.81%) were detected with methylmalonic acidurias. After excluding patients with non-significant peaks of methylmalonic acidemia, 41(2.31%) were included in the final analysis. Of these, 20(48.7%) were females, while the overall median age was 11.5 months (interquartile range: 6-41.5). On stratification by type of disorders leading to methylmalonic acidurias, 9(22%) had methylmalonic acidemia, 12(29%) had Cobalamin-related remethylation disorders, nonspecific methylmalonic acidurias in 16(39%), while 2(5%) each had succinyl coenzyme A synthetase and Vitamin B12 deficiency. respectively. Screening tests, including urine organic acid, provided valuable clues to the aetiology of methylmalonic acidurias.

To determine the frequency of organic acidurais (OA) and amino acidopathies (AA) in selected high-risk patients screened in two years. Retrospective Observational study. The Aga Khan University Hospital (AKUH), Karachi, from January 2013 to December 2014. Patients with OA and AA were included in the study and patients with IMDs other than OA and AA were excluded. Amino acids and organic acids were analyzed on high performance liquid chromatography and gas chromatography-mass spectrometry respectively. Clinical data and chromatograms of patients screened for IMDs were reviewed by chemical pathologist and metabolic physician. Eighty-eight cases (4.7%) were diagnosed including 41 OA (46.5%), 28 AA (31.8%) and 19 others (21.5%) from 1,866 specimens analyzed. Median age of the patients was 1.1 years, with high consanguinity rate (64.8%). Among OA, methyl CoA mutase deficiency was diagnosed in 9 (10.2%) and was suspected in 2 (2.3%) cases. Five (5.7%) cases of MHBD (2-methyl-3-hydroxybutyryl-CoA), 4 (4.5%) each of PPA (propionic aciduria) and HMG-CoA lyase deficiency, 3 (3.4%) cases each of IVA (isovaleric aciduria), multiple carboxylase deficiency, fructose-1, 6-biphosphatase deficiency, fumarase deficiency, GA-1 (glutaric aciduria type 1) and 2 (2.3%) cases of EMA (ethyl-malonic aciduria). AA included 8 (9.1%) cases of MSUD (maple syrup urine disease), 6 (6.8%) cases of CBS (cystathionine beta-synthetase) and UCDs (urea cycle disorders) each, 5 (5.7%) cases of hyperphenylalaninemia and 3 (3.4%) cases of hyperprolinemia were reported. Other inherited metabolic disorders included: 9 (10.2%) cases of intracellular cobalamin defects, 2 (2.3%) cases each of alkaptonuria, Canavan's disease, SUCL (succinate CoA ligase) deficiency, and 1 (1.1%) case each of DPD (dihydropyrimidine) deficiency, GA-2, NKH (non-ketotic hyperglycinemia), AADC (aromatic amino acid decarboxylase) deficiency. This study presents frequency of OA and AA in the high-risk Pakistani pediatric population analyzed locally.