Distribution and clinical expression of the tRNA(Lys) mutation in mitochondrial DNA in MERRF syndrome

The distribution and clinical expression of mutant mtDNA with an A-to-G transition at the 8,344th nucleotide position in the tRNA(Lys) gene were studied in different tissues of the probands and relatives in a Taiwanese family with myoclonic epilepsy with ragged-red fibers (MERRF) syndrome. The proportions of mutant mtDNA were 89% to 95% for muscles and 78% to 99% for blood cells in the symptomatic probands except for one who had no mutant mtDNA in the muscles. In asymptomatic maternal relatives, these were 1% to 80% for muscles and 54% to 78% for blood cells. However, there was a lack of correlation between the clinical severity and the proportion of mutant mtDNA. Furthermore, among the maternal relatives studied, some asymptomatic family members had proportions of mutant mtDNA > 80% in muscles and > 53% in blood cells. Although these findings may suggest a threshold effect of mtDNA in MERRF syndrome, clinical manifestations are difficult to predict from the proportions of mutant mtDNA at the tRNA(Lys) gene only.