Mitochondrial DNA variants revealed by whole exome sequencing: from screening to diagnosis and follow-up.

Mutations in mitochondrial DNA play a crucial role in several diseases, but interpreting the clinical significance of mitochondrial DNA variants is challenging due to heteroplasmy, age-related loss of variants and evolving phenotypes. The aim of study was to identify mitochondrial pathogenic variants and explore their potential future association with specific phenotypes in patients during their lifetime, for both known and novel variants. We used a Python pipeline to analyse exome sequencing data from 418 patients (median age: 15 years; 52.9% males and 47.1% females), mostly diagnosed with neurological disorders, developmental and intellectual disabilities, behavioural and sensory disorders, cardiovascular and metabolic abnormalities, renal diseases and others. Screening identified 1,000 unique variants with heteroplasmy levels greater than 10% and 192 unique variants with 1-10% heteroplasmy, excluding hypervariable regions. Among these variants, four confirmed pathogenic variants were detected according to MITOMAP (m.1555 A > G, m.3243 A > G, m.9035T > C, and m.11778G > A), each identified in one patient. The application of pathogenicity and frequency criteria led to the identification of three unique variants and one in monozygotic twin sister with low levels of heteroplasmy, which were confirmed by next-generation sequencing. Finally, one of them, the variant m.15897G > A, was recognised as likely pathogenic (PP3, PS2). Our study highlights the complexity of diagnosing mitochondrial diseases associated with mtDNA mutations and emphasises the need for a comprehensive genotype-phenotype approach to correctly identify causal variants.