The homoplasmic MT-TK m.8357T > C mtDNA variant as a cause of multiorgan mitochondrial disease

The diagnosis of disorders associated with mitochondrial DNA (mtDNA) variants presents substantial complexity due to their genetic and clinical heterogeneity, which is largely influenced by mtDNA heteroplasmy. However, the level of heteroplasmy alone is often not sufficient to predict the clinical phenotype including its severity and progression. This study concerns the characterization of the m.8357T > C variant in the MT-TK gene, encoding for mt-tRNA-Lys found in two pediatric siblings. Both had symptoms suggestive of a mitochondrial disease, including severe hearing loss, easy fatigability, decreased activity of mitochondrial complex I in muscle samples, epilepsy, metabolic acidosis with hyperkalemia, and mild kidney impairment. The m.8357T > C mtDNA variant was homoplasmic in muscle, blood, urine and fibroblasts. Immortalized fibroblasts from the patients showed reduced activity of mitochondrial complexes I, III and IV, decreased mitochondrial respiration, and abnormal depolarization of the mitochondrial membrane potential. The mt-tRNA-Lys levels were reduced as compared to the mt-tRNA-Leu (UUR) or the snRNA encoded by RNU6B nuclear gene; the level of three mitochondrial DNA encoded proteins was decreased, altogether suggesting a defective translation machinery in cells carrying the variant. Consistently, fibroblasts from the mother, who had only mild hearing loss, despite high level of heteroplasmy, showed some biochemical abnormalities, however milder than in her daughter and son. Contrariwise, their maternal aunt, who showed intellectual disability, mild hearing loss, easy fatigability and weakness was also virtually homoplasmic for the m.8357T > C in blood and urinary sediment cells. These findings suggest the pathogenicity of the m.8357T > C variant but only in condition of homoplasmy.