Pathogenic mitochondrial mt-tRNA Ala variants are uniquely associated with isolated myopathy

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Abstract

Pathogenic mitochondrial DNA (mtDNA) point mutations are associated with a wide range of clinical phenotypes, often involving multiple organ systems. We report two patients with isolated myopathy owing to novel mt-tRNA Ala variants. Muscle biopsy revealed extensive histopathological findings including cytochrome c oxidase (COX)-deficient fibres. Pyrosequencing confirmed mtDNA heteroplasmy for both mutations (m.5631G>A and m.5610G>A) whilst single-muscle fibre segregation studies (revealing statistically significant higher mutation loads in COX-deficient fibres than in COX-positive fibres), hierarchical mutation segregation within patient tissues and decreased steady-state mt-tRNA Ala levels all provide compelling evidence of pathogenicity. Interestingly, both patients showed very high-mutation levels in all tissues, inferring that the threshold for impairment of oxidative phosphorylation, as evidenced by COX deficiency, appears to be extremely high for these mt-tRNA Ala variants. Previously described mt-tRNA Ala mutations are also associated with a pure myopathic phenotype and demonstrate very high mtDNA heteroplasmy thresholds, inferring at least some genotype:phenotype correlation for mutations within this particular mt-tRNA gene.

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Lehmann, D., Schubert, K., Joshi, P. R., Hardy, S. A., Tuppen, H. A. L., Baty, K., … Taylor, R. W. (2015). Pathogenic mitochondrial mt-tRNA Ala variants are uniquely associated with isolated myopathy. European Journal of Human Genetics, 23(12), 1735–1738. https://doi.org/10.1038/ejhg.2015.73

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