Work-Up for Pediatric Mitochondrial Disorder by Means of Next Generation Sequencing

We read with interest the article by Öncül et al. about 16 pediatric patients with suspected mitochondrial disorder (MID) undergoing next generation sequencing for mtDNA variants [1]. Thirteen patients underwent mtDNA sequencing, 2 patients whole exome sequencing (WES), and 1 patient whole genome sequencing (WGS) [1]. Seven variants were found in protein-coding genes, 4 in tRNA genes, and 1 in an rRNA gene [1]. Only two variants were classified as pathogenic, three as likely pathogenic, and 7 seven as variants of unknown significance(VUS) [1]. The study is appealing but raises concerns that require discussion. A limitation of the study is that the pathogenicity of new mtDNA variants was assessed by in silico bioinformatic methods using the Protter and Phyre2 programs but not by functional tests. Pathogenicity of mtDNA tRNA variants is commonly assessed by application of the Yarham score relying on the number of independent reports, presence of heteroplasmy, segregation of the disease with the variant, biochemical investigations, segregation of the variant with biochemical investigations in single fiber studies, pathogenicity on trans-mitochondrial cybrid studies, normality on trans-mitochondrial cybrid studies, and the evolutionary conservation of the variant [2].