Objectives: Many heteroplasmic point mutations in tRNA genes of mitochondrial DNA (mtDNA) have been associated with human diseases. We recently reported on a prospective 7-year study in which we enrolled 116 consecutive children with undefined encephalomyopathy. Seventeen of them were found to have both a defect in the mitochondrial respiratory chain and abnormal ultrastructure of muscle mitochondria, suggesting a clinically probable mitochondrial encephalopathy.
Methods: We determined the frequency of mtDNA mutations in these 17 children by analyzing the entire sequence of mtDNA by conformation-sensitive gel electrophoresis and sequencing.
Results: Three heteroplasmic tRNA mutations that were considered to be pathogenic were detected. Two of the mutations were novel transitions, 10438A>G in the tRNA(Arg) gene and 14696A>G in the tRNA(Glu) gene, whereas the third one was 3243A>G, the common MELAS mutation. The mutant load was very high in the blood and skeletal muscle of the patients and markedly lower in the blood of asymptomatic maternal relatives. The 10438A>G mutation changes the nucleotide flanking the anticodon, whereas 14696A>G changes a nucleotide in the stem of the pseudouridine loop, creating a novel base pair and reducing the wobble.
Conclusions: Our results emphasize that the analysis of the entire sequence of mtDNA is worthwhile in the diagnostic evaluation of patients with clinically probable mitochondrial encephalomyopathy. The frequency of pathogenic mtDNA mutations was found to be 18% among children with biochemically and histologically defined mitochondrial disease, suggesting that the likelihood of nuclear DNA mutations in such a group is several times higher than that of mtDNA mutations.