Abstract

Purpose: To elucidate the biochemical and molecular genetic defect in a 16-year-old patient presenting with apical hypertrophic cardiomyopathy and neuropathy suspected for a mitochondrial disorder. Methods: Measurement of the mitochondrial energy-generating system (MEGS) capacity in muscle and enzyme analysis in muscle and fibroblasts were performed. Relevant parts of the mitochondrial DNA were analysed by sequencing. Transmitochondrial cybrids were obtained by fusion of 143B206 TK- rho zero cells with patient-derived enucleated fibroblasts. Immunoblotting techniques were applied to study the complex V assembly. Results: A homoplasmic nonsense mutation m.8529G>A (p.Trp55X) was detected in the mitochondrial ATP8 gene in the patient’s fibroblasts and muscle tissue. A decreased complex V activity was measured in the patient’s fibroblasts and muscle tissue, and was confirmed in cybrid clones containing patient derived mitochondrial DNA. Immunoblotting after Blue Native Polyacrylamide Gel Electrophoresis (BN-PAGE) revealed a lack of holocomplex V and increased amounts of subcomplexes of mitochondrial ATP synthase. In-gel activity assay of ATP hydrolysis showed activity of free F1-ATPase in the patient’s muscle tissue and cybrid clones. Conclusion: We describe the first pathogenic mutation in the mitochondrial ATP8 gene, resulting in an improper assembly and a decreased activity of the complex V holoenzyme.