Pathogenic mitochondrial mt-tRNAAla variants are uniquely associated with isolated myopathy

Diana Lehmann,Robert W Taylor,Emma L Blakely,Stephan Zierz,Helen A L Tuppen,Christian Bamberg,Marcus Deschauer,Pushpa R Joshi,Kathrin Schubert,Karen Baty,Steven A Hardy

doi:10.1038/ejhg.2015.73

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-tRNAAla 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-tRNAAla 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-tRNAAla variants. Previously described mt-tRNAAla 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.

Highlights

We report two patients with isolated myopathy owing to novel mt-tRNAAla variants
Mitochondrial DNA disorders are associated with a wide range of different clinical phenotypes, from mild to severe.[1]
Mutations affecting mitochondrial-tRNA genes are prevalent amongst adults and usually associated with multisystemic disease presentations;[2] isolated organ involvement is rarely observed. It is unusual for mutations in one specific mt-tRNA gene to associate with a unique clinical phenotype, several mutations in mttRNALeu(UUR) and mt-tRNAIle are linked to MELAS and mitochondrial cardiomyopathy respectively.[3]

Summary

INTRODUCTION

Mitochondrial DNA (mtDNA) disorders are associated with a wide range of different clinical phenotypes, from mild to severe.[1]. Patient 1 Patient 1 is a young lady who presented at the age of 29 years with muscle weakness, initially involving her hands but spreading to her arms and legs progressively, resulting in her being non-ambulatory by the age of 40 years Her mother is healthy with no history of muscle disease, whereas the patient has no children or siblings. Long-range PCR of muscle DNA was undertaken to detect large-scale mtDNA rearrangements,[8] followed by sequencing of the entire mitochondrial genome in this tissue.[9,10] Analysis of mtDNA heteroplasmy was carried out by quantitative pyrosequencing including segregation studies within individual cytochrome c oxidase (COX)-positive and COX-deficient fibres.[11] High-resolution northern blotting to assess mt-tRNAAla steady-state levels in both patients was performed as described.[12]

RESULTS

DISCUSSION

Wong LJ