Elsevier

Journal of the Neurological Sciences

Volume 379, 15 August 2017, Pages 137-143
Journal of the Neurological Sciences

Mitochondrial tRNA genes are hotspots for mutations in a cohort of patients with exercise intolerance and mitochondrial myopathy

https://doi.org/10.1016/j.jns.2017.05.056Get rights and content

Highlights

  • Our study revealed the role of mt-tRNA coding genes as hotspots for mutations in patients with mitochondrial myopathy.

  • Patients associated with the m.8344A > G mutation were prone to laryngopharyngeal muscle involvement.

  • The prognosis in our patients is relatively benign except one patient with the m.3302A > G mutation.

Abstract

Objective

Mitochondrial myopathy (MM) is a relatively rare type of mitochondrial disorder characterized by predominant skeletal muscle involvement. Both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) mutations have been reported as the genetic causes of this disease. Here, we described the clinical and genetic features of a cohort of patients with MM.

Methods

We conducted a retrospective, single center study enrolling 22 patients with clinically and myopathologically diagnosed MM. The clinical records and results of laboratory examinations were collected and analyzed. The follow-up was conducted by telephone interview in 12 patients. Muscle biopsy and gene analysis was performed in all patients. MtDNA mutation load was quantified in all available tissues.

Results

Muscle biopsy revealed ragged red fibers and/or cytochrome c oxidase deficient fibers in all patients. Mitochondrial DNA analysis identified pathogenic mutations in 11 patients, including four previously reported mutations (mt-tRNALeu(UUR) m.3243A > G in five patients, mt-tRNALys m.8344A > G in four patients, mt-tRNALeu(UUR) m.3302A > G in one patient, and mt-tRNALeu(UUR) m.3250T > C in one patient) and a novel possible pathogenic variant (MTND1 m.3437G > A) in one patient. The mtDNA mutation load was consistently higher in muscles than in blood. In the remaining 10 patients, there was no pathogenic mutation found either by the Sanger sequencing of entire mitochondrial genome or by the targeted next-generation sequencing which included 238 nuclear genes related to mitochondrial diseases. Clinically, the onset age of these 22 MM patients ranged from 1 to 51 years (mean = 21.1 ± 14.3 years), and the disease duration was between 3 and 44 years (mean = 14.1 ± 9.4 years). Proximal limb weakness with or without exercise intolerance was present in 21 patients, and one patient showed only exercise intolerance. Out of these 22 patients, dysphagia/dysarthria, neck flexor muscle weakness, dyspnea, cardiomyopathy and exercise induced myalgia were observed in five, two, four, one and four patients, respectively. Neither central nervous system manifestation nor brain MRI abnormality was present in these patients. Notably, three of the four patients carrying the m.8344A > G mutation presented with dysarthria. The follow-up of 12 patients revealed symptom improvements in four cases, stable conditions in two cases, and worsened conditions in five cases. The case with the m.3302A > G mutation died of respiratory failure.

Conclusions

Mitochondrial tRNA genes, as hotspots for mutations, accounted for 50% of MM in this cohort of patients. Patients associated with the m.8344A > G mutation were prone to laryngopharyngeal muscle involvement. The prognosis in our patients is relatively benign except one patient with the m.3302A > G mutation.

Introduction

Mitochondrial disorders are characterized by variable clinical manifestations usually involving multiple tissues and organs, such as skeletal muscle, brain and pancreas, with a significant energy demand. In most well defined phenotypes, such as mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), myoclonic epilepsy with ragged red fibers (MERRF) and chronic progressive external ophthalmoplegia/Kearns-Sayre syndrome (CPEO/KSS), limb muscle weakness occurs, associated with encephalopathies, seizures, diabetes mellitus or ophthalmoplegia ([1], [2]). Mitochondrial myopathy (MM), characterized by isolated or predominant skeletal muscle involvement, however, is relatively rare and frequently misdiagnosed as limb-girdle muscular dystrophy or lipid storage myopathy [39], [40]. For such patients, muscle biopsy can provide great value in the diagnostic work-up, showing ragged red fibers (RRFs) and/or cytochrome c oxidase (COX) deficient fibers ([1], [3]).

MM is heterogeneous in genetics. With the widespread availability of molecular genetic tests, both primary alterations of the mitochondrial DNA (mtDNA) and mutations in mitochondria-related nuclear DNA (nDNA) have been identified as genetic causes of MM [1]. Human mtDNA is a 16,569-kb circular, double-stranded molecule, encoding 13 subunits of the respiratory chain enzymes, 22 tRNA and two rRNA genes [4]. Although mutations in the mitochondrial-encoded cytochrome b gene have been defined as hot spot mutations in exercise intolerance or MM, other mtDNA mutations have also been reported ([5], [6], [7], [12]). Here, we present the clinical and genetic features of a cohort of patients with exercise intolerance and MM.

Section snippets

Patients

We retrospectively enrolled 22 patients with clinically and myopathologically suspected MM (8 female and 14 male) who were referred to the Department of Neurology, Peking University First Hospital from 2008 to 2015. All patients were Han Chinese and unrelated. Their clinical records and results of laboratory examinations were retrospectively collected and analyzed. All patients were interviewed and examined by at least two neurologists. Follow up data were available in 12 patients by telephone

Patients

The age of onset ranged from 1 to 51 years (mean = 21.1 ± 14.3 years). The disease duration ranged from 3 to 44 years (mean = 14.1 ± 9.4 years). Proximal limb weakness with or without exercise intolerance was present in 21 patients, and one patient showed only exercise intolerance. None of the patients showed ptosis, diplopia, epilepsy, dementia, stroke-like episodes, or other central nervous system symptoms. Physical examination showed proximal weakness in both upper and lower limbs in 15 patients and

Discussion

In 1973, H. Shibasaki first described a 60-year-old man with a 2-year history of proximal muscle weakness and atrophy with numerous giant and elongated mitochondria in muscle fibers under electron microscopy, indicating MM [10]. In 1999, Hirata K described seven patients with predominant proximal muscle weakness and atrophy caused by a mtDNA m.8291A > G mutation and suggested the concept of limb-girdle mitochondrial myopathy [11]. In the present study, we described 22 patients clinically

Conflicts of interest

The authors declared that they have no conflicts of interest to this work. Prof. Jan Smeitink is the founding CEO of Khondrion (www.khondrion.com).

Acknowledgments

The authors would like to thank the patients and their parents for cooperation. We thank Prof. Yinan Ma and Mrs. Ying Zhang for technical assistance in doing PCR-RFLP. This work was financially supported by grants from the Ministry of Science and Technology of China (No. 2011ZX09307-001-07) and Beijing Municipal Science and Technology Commission (No. Z151100003915126).

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