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The mitochondrial DNA (mtDNA) transfer RNA (tRNA)Lys A→G(8344}8344 mutation was identified in seven patients. These patients and their relatives were assessed clinically; in one family the mutation was deduced to be present in four generations. The phenotype in index cases was consistent with the syndrome of myoclonus, epilepsy with ragged red fibres, with the core clinical features of myoclonus, ataxia and seizures. Amongst other features, progressive external ophthalmoplegia, Leighs syndrome and stroke-like episodes were observed, well recognized in mitochondrial myopathies but novel manifestations of this genotype. Samples of blood and muscle were analysed for the proportion of mutant mtDNA using an oligonucleotide hybridization technique. The proportion of mutant mtDNA in blood was significantly greater in symptomatic than asymptomatic cases. Furthermore, the proportion of mutant mtDNA in blood correlated with age of onset of disease and clinical severity assessed by a simple scale. Study of disease associated with the tRNALys A→G(8344]8344 mutation provides further insight into the pathogenesis and transmission of mitochondrial diseases. Quantification of the proportion of mtDNA in tissues demonstrates that this is a major factor determining the course of disease, but other, as yet unidentified factors are also likely to play a role.

The mitochondrial tRNALeu(UUR) A-→G(3243) mutation was identified in 22 unrelated patients. The probands and their relatives were assessed clinically and by quantitative mitochondrial DNA (mtDNA) analysis. While 10 probands had clinical features consistent with the syndrome of mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS), usually associated with this mutation, 12 probands had other phenotypes including other encephalopathies, chronic progressive external ophthalmoplegia (CPEO), myoclonic epilepsy and ragged red fibres (MERRF), myopathy alone and diabetes and deafness. Histochemical analyses of muscle biopsies showed a higher proportion of cytochrome oxidase (COX) negative fibres, but fewer strongly COX reactive fibres, in patients with CPEO compared with those with MELAS. The proportion of mutant mtDNA present in blood was significantly greater in symptomatic than asymptomatic subjects, and was correlated with age in both. This correlation was not observed in patients with the tRNALys A→G(8344) mutation. The proportion of mutant mtDNA A-→G(3243) in muscle was always greater than that in blood. Significant correlations between proportion of mutant mtDNA in blood and both age of onset of disease and a clinical severity score were observed. However, the proportions of mutant mtDNA in blood in affected and unaffected cases overlapped, preventing use of the genetic-clinical correlation for prognostic or predictive purposes. The presence of intrafamilial clustering of phenotypes and the imperfect relationship between proportion of mutant mtDNA and the presence or absence of disease suggests that other factors may determine the phenotype. To investigate this possibility further, the tRNALeu(UUR) gene was sequenced in 23 probands and six relatives. In 28 patients the sequence was normal apart from the 3243 mutation, but in members of one family there was a homoplasmic T→C transition at position 3290 which was not found in 140 controls or 50 other patients with mitochondrial myopathy. The family with this transition had high levels of mutant mtDNA A→G(3243), with a unique phenotype of predominant skeletal myopathy, suggesting that this second base change in tRNALeu(UUR) may influence the clinical phenotype.