A double mutation (A8296G and G8363A) in the mitochondrial DNA tRNA(Lys) gene associated with myoclonus epilepsy with ragged-red fibers

Objective: To define potential pathogenic mitochondrial DNA (mtDNA) point m utations in a patient with myoclonus epilepsy with ragged-red fibers (MERRF ) syndrome. Background: MERRF syndrome is typically associated with point m utations in the mtDNA tRNA(Lys) gene. Methods: We performed morphologic, bi ochemical, and genetic analysis of muscle samples from the patient and four relatives. Molecular genetic studies included sequencing, PCR, and restric tion enzyme analysis on whole muscle, blood, and single muscle fibers. Resu lts: Muscle biopsy showed cytochrome c oxidase (COX), negative ragged-red f ibers (RRF), and a defect of complex I of the mitochondrial respiratory cha in. We found an A8296G transition and a G83636 mutation in the mtDNA tRNA(L ys) gene. The A8296G was almost homoplasmic in muscle and blood from the pr opositus and his oligosymptomatic maternal relatives. The G8363A mutation w as heteroplasmic and more abundant in muscle than in blood, and its proport ion correlated with clinical severity. Single muscle fiber analysis showed significantly higher levels of G8363A genomes in COX-negative than in norma l fibers, and almost homoplasmic levels of mutant A8296G mtDNA in both COX- negative and normal fibers. The two mutations affect highly conserved nucle otides and were not found in controls. Conclusions: The G8363A mutation is pathogenic; the co-ocurrence of the A8296G mutation is of unclear significa nce and is likely to be a rare polymorphism.