Phenotype and genotype of twelve Chinese children with mitochondrial DNA depletion syndromes

Objective To explore the phenotype and genotype of mitochondrial DNA depletion syndromes (MDS) in Chinese children. Methods The clinical and genetic data of 12 MDS patients (8 were boys and 4 were girls) diagnosed in the Department of Neurology in Beijing Children′s Hospital, Capital Medical University from October 2010 to April 2018 were retrospectively collected and analyzed. Results The developmental milestones were normal or mildly retardated before disease onset. The age of onset ranged from 0 to 2.9-year-old. Most cases developed postnatal or after infection. The most common initial symptoms were feeding difficulty, seizure, muscle weakness, psychomotor regression and hepatic dysfunction. At the last evaluation, all the patients had developmental retardation, failure to thrive, muscle weakness, and dysphagia. Other clinical features were weight loss (9 cases), hearing impairment (7 cases), ptosis (6 cases), seizure (5 cases), dyspnea (4 cases), visual impairment (1 case), hirsutism (1 case), lactic acidosis (7 cases), elevated hepatic enzymes (4 cases) and creatine kinase (2 cases), elevated protein in cerebrospinal fluid (3 cases), abnormalities on screening for inborn error of metabolism (10 cases) and brain magnetic resonance imaging (MRI) (10 cases), abnormal electromyogram (including neurogenic or myogenic injury) (5 cases). Five patients died of infection or multiple organ failure. A total of 18 novel mutations presented below were detected in these patients. Among the 6 cases of encephalomyopathy, there were 3 with SUCLG1 mutation (c. 916G>T, c. 619T>C, c. 980dupT were novel), 2 with SUCLA2 mutation (c. 851G>A, c.971G>A were novel), and one with RRM2B mutation (c.456-2A>G, c.212T>C were novel). All the cases of hepatic encephalopathy all had POLG mutations (c. 3151G>A, c. 2294C>T, c. 2858G>C, c. 680G>A and c. 150_158delGCAGCAGCA were novel). Two cases of infantile-onset spinocerebellar ataxia had TWNK mutations (c. 1163C>T, c. 1319T>C, c. 1388G>A and c. 257_258delAG were novel). One case of myopathy had TK2 mutations (c.557C>G and c.341A>T were novel). Conclusions The clinical and genetic features of MDS were heterogeneous. Eighteen novel mutations in six MDS related genes were reported, which expanded the genetic spectrum of MDS in Chinese children. Key words: DNA, mitochondrial; Genes; Retrospective studies