Fatal mitochondrial myopathy, lactic acidosis, and complex I deficiency associated with a heteroplasmic A --> G mutation at position 3251 in the mitochondrial tRNALeu(UUR) gne.

A girl, who died at 14 years of age from a rapidly progressive mitochondrial myopathy, was found to be heteroplasmic for a mutation in the mitochondrial tRNALeu(UUR) gene at position 3251. A large proportion of muscle fibres contained accumulations of abnormal mitochondria but no cytochrome c oxidase deficient fibres were present. Polarographic and enzymatic measurements on isolated muscle mitochondria revealed a profound isolated complex I deficiency. A high percentage of mutant mtDNA was found in muscle (94%), fibroblasts (93%), brain (90%), liver (80%), and heart (79%). The family was not available for investigation. For genotype to phenotype correlation studies, we investigated the proportion of mutated mtDNA in single muscle fibres of normal appearance and muscle fibres with accumulations of mitochondria. The proportion of mutant mtDNA was 28% (range <0.3%-86%) in normal appearing fibres and 61% (range 15%-88%) in abnormal fibres. The difference in the proportion of mutant mtDNA was highly significant (P < 0.001) between the two groups of fibres.