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Mitochondrial diseases (MtDs) present diverse clinical phenotypes, yet large‐scale studies are hindered by their rarity. This retrospective, multicenter study, conducted across five Chinese hospitals' neurology departments from 2009 to 2019, aimed to address this gap. Nationwide, 1351 patients were enrolled, with a median onset age of 14.0 (18.5) years. The predominant phenotype was mitochondrial encephalomyopathy, lactic acidosis, and stroke‐like episodes (MELAS) (45.0%). Mitochondrial DNA (mtDNA) mutations were prevalent (87.4%), with m.3243A>G being the most common locus (48.7%). Meanwhile, POLG mutations in nuclear DNA (nDNA) accounted for 16.5%. Comparative analysis based on age groups (with a cut‐off at 14 years) revealed the highest prevalence of MELAS, with Leigh syndrome (LS) and chronic progressive external ophthalmoplegia (CPEO) being the second most common phenotypes in junior and senior groups, respectively. Notably, the most commonly mutated nuclear genes varied across age groups. In conclusion, MELAS predominated in this Chinese MtD cohort, underscored by m.3243A>G and POLG as principal mtDNA mutations and pathogenic nuclear genes. The phenotypic and genotypic disparities observed among different age cohorts highlight the complex nature of MtDs. [ABSTRACT FROM AUTHOR]

Objective: To explore the clinical characteristics of mitochondrial encephalomyopathy, lactic acidosis, and stroke‐like episodes (MELAS) caused by mitochondrial DNA‐encoded complex I subunit (mt‐ND) variants. Methods: In this retrospective study, the clinical, myopathological and brain MRI features of patients with MELAS caused by mt‐ND variants (MELAS‐mtND) were collected and compared with those of MELAS patients carrying the m.3243A > G variant (MELAS‐A3243G). Result: A total of 18 MELAS‐mtND patients (female: 7; median age: 24.5 years) represented 15.9% (n = 113) of all patients with MELAS caused by mtDNA variants in our neuromuscular center from January 2012 to June 2022. In this MELAS‐mtND cohort, the two most common variants were m.10191 T > C (4/18, 22.2%) and m.13513 G > A (3/18, 16.7%). The most frequent symptoms were seizures (14/18, 77.8%) and muscle weakness (11/18, 61.1%). Compared with 87 MELAS‐A3243G patients, MELAS‐mtND patients were significantly more likely to have a variant that was absent in blood cells (40% vs. 1.4%). Furthermore, MELAS‐mtND patients had a significantly lower MDC score (7.8 ± 2.7 vs. 9.8 ± 1.9); less hearing loss (27.8% vs. 54.0%), diabetes (11.1% vs. 37.9%), and migraine (33.3% vs. 62.1%); less short stature (males ≤ 165 cm; females ≤ 155 cm; 23.1% vs. 60.8%) and higher body mass index (20.4 ± 2.5 vs. 17.8 ± 2.7). MELAS‐mtND patients had significantly more normal muscle pathology (31.3% vs. 4.1%) and fewer RRFs/RBFs (62.5% vs. 91.9%), COX‐deficient fibers/blue fibers (25.0% vs. 85.1%) and SSVs (50.0% vs. 81.1%). Moreover, brain MRI evaluated at the first stroke‐like episode showed significantly more small cortical lesions in MELAS‐mtND patients (66.7% vs. 12.2%). Interpretation: Our results suggested that MELAS‐mtND patients have distinct clinical, myopathological and brain MRI features compared with MELAS‐A3243G patients. [ABSTRACT FROM AUTHOR]

The mitochondrial tRNALys (mt-tRNALys) mutation is initially associated with myoclonic epilepsy and ragged-red fibers (MERRF). The clinical, laboratory, morphologic and molecular findings from 22 mt-tRNALys mutation carriers from local database in East China were analyzed retrospectively. We identified 13 symptomatic and 9 asymptomatic individuals with a known pathogenic mitochondrial tRNALys mutation. The most common mutations were m.8344 A>G (81.8%), m.8363G>A (9.1%), m.8356 T>C (4.5%) and m.8356 T>G (4.5%). The degree of mutation heteroplasmy in blood was high both in symptomatic (mean 64.5%, range 41–82%) and asymptomatic individuals (mean 53.1%, range 21–78%). Age at onset ranged from 6 year-old to the age of 66 years (mean 35.8 ± 16.4 years old). The most frequent symptoms were muscle weakness (76.9%), exercise intolerance (76.9%), elevated creatine kinase levels (61.5%), peripheral neuropathy (69.2%) and cerebellar ataxia (61.5%), while myoclonus was only present in 23.1% of symptomatic patients. A diagnosis of mitochondrial myopathy (MM) and neuropathy ataxia and retinitis pigmentosa (NARP/NARP-like) syndrome was made in 77% of symptomatic patients, whereas the classic syndrome of myoclonic epilepsy with ragged-red fibers (MERRF) was rare (23%). In this cohort of patients with mt-tRNALys mutation, more than one third of our patients did not develop signs and symptoms of central nervous system involvement even in later stages of the disease, indicating the necessity to investigate the mt-tRNALys gene in 'pure' mitochondrial 'myo-neuropathy'. [ABSTRACT FROM AUTHOR]

Objective: Mitochondrial diseases are a group of genetic diseases caused by mutations in mitochondrial DNA and nuclear DNA, among which, mutations in mitochondrial tRNA genes possessing prominent status. In most of the cases, however, the detailed molecular pathogenesis of these tRNA gene mutations remains unclear. Methods: We performed the clinical emulation, muscle histochemistry, northern blotting analysis of tRNA levels, biochemical measurement of respiratory chain complex activities and mitochondrial respirations in muscle tissue and cybrid cells. Results: We found a novel m.4349C>T mutation in mitochondrial tRNAGln gene in a patient present with encephalopathy, epilepsy, and deafness. We demonstrated molecular pathomechanisms of this mutation. This mutation firstly disturbed the translation machinery of mitochondrial tRNAGlnand impaired mitochondrial respiratory chain complex activities, followed by remarkable mitochondrial dysfunction and ROS production. Interpretation. This study illustrated the pathogenicity of a novel m.4349C>T mutation and provided a better understanding of the phenotype associated with mutations in mitochondrial tRNAGln gene. [ABSTRACT FROM AUTHOR]

Objective: Riboflavin-responsive multiple acyl-coenzyme A dehydrogenation deficiency (RR-MADD) is an inherited fatty acid metabolism disorder mainly caused by genetic defects in electron transfer flavoprotein-ubiquinone oxidoreductase (ETF:QO). The variant ETF:QO protein folding deficiency, which can be corrected by therapeutic dosage of riboflavin supplement, has been identified in HEK-293 cells and is believed to be the molecular mechanism of this disease. To verify this hypothesis in vivo, we generated Etfdh (h)A84T knockin (KI) mice.Methods: Tissues from these mice as well as muscle biopsies and fibroblasts from 7 RR-MADD patients were used to examine the flavin adenine dinucleotide (FAD) concentration and ETF:QO protein amount.Results: All of the homozygous KI mice (Etfdh (h)A84T/(h)A84T , KI/KI) were initially normal. After being given a high-fat and vitamin B2 -deficient (HF-B2 D) diet for 5 weeks, they developed weight loss, movement ability defects, lipid storage in muscle and liver, and elevated serum acyl-carnitine levels, which are clinically and biochemically similar to RR-MADD patients. Both ETF:QO protein and FAD concentrations were significantly decreased in tissues of HF-B2 D-KI/KI mice and in cultured fibroblasts from RR-MADD patients. After riboflavin treatment, ETF:QO protein increased in proportion to elevated FAD concentrations, but not related to mRNA levels. These results were further confirmed in cultured fibroblasts from RR-MADD patients.Interpretation: For the first time, we successfully developed a RR-MADD mice model and confirmed that FAD homeostasis disturbances played a crucial role on the pathomechanism of RR-MADD in this mouse model and culture cells from patients. Supplementation of riboflavin may stabilize variant ETF:QO protein by rebuilding FAD homeostasis. Ann Neurol 2018;84:667-681. [ABSTRACT FROM AUTHOR]