Case reportA novel mutation (8342G→A) in the mitochondrial tRNALys gene associated with progressive external ophthalmoplegia and myoclonus
Introduction
Mutations of mitochondrial DNA (mtDNA) are responsible for a large number of neuromuscular disorders. Large-scale rearrangements are usually sporadic, while point mutations are maternally inherited, and may involve either structural genes or genes encoding the RNA components of the mitochondrial translational apparatus [1]. Some mutations are frequent, and often associated with nosologically well-defined syndromes [2]. However, the molecular basis of several mitochondrial encephalomyopathy (MEM) cases remain undetermined. In addition, the `normal' mtDNA sequence is very variable among different individuals, due to the presence of population-specific and `private' polymorphisms [3]. Therefore, the pathogenicity of a given sequence change must be validated on the basis of rigorous pathogenetic criteria such as an absolute segregation with the disease, obvious impairment of gene function (e.g. frameshift mutations in structural genes), and the presence of mutation heteroplasmy in good correlation with clinical and biochemical findings. This holds true also for sporadic patients with no evidence of maternal inheritance. We have been performing a systematic search for mtDNA mutations in the tRNA genes of a series of patients with atypical or incomplete `mitochondrial' presentations. As an example, we report here the case of a 39-year-old woman affected by an apparently sporadic neuromuscular disorder. Morphological, biochemical and molecular studies led us to the identification of a new heteroplasmic point mutation in the mitochondrial tRNALys gene in this patient.
Section snippets
Case report
The proband was a 39-year-old woman born from healthy, non-consanguineous parents. At 10-years of age she underwent a `surgical procedure' for bilateral strabismus. During the second decade of life she developed progressive muscle weakness and fatigability, with worsening of her ptosis and ophthalmoparesis. Rapid, but transient worsening of her muscle symptoms occurred at the age of 29 years, causing transient paralysis of limbs, dysphagia, and dyspnoea, and, in addition, nausea and vomiting.
Morphological and biochemical analyses
Morphological analysis of skeletal muscle and biochemical assays of the individual respiratory complexes on muscle homogenate were carried out as described 4, 5. Specific activities of each complex were normalized to that of citrate synthase (CS), an indicator of the number of mitochondria.
Silver-staining single-stranded conformation polymorphism and sequencing analysis
Twenty-two suitable pairs of PCR primers were designed, on the sequences flanking each mt-tRNA gene [6]. PCR was performed on 300 ng of genomic DNA following standard cycles at 56°C of annealing temperature.
Morphological, histochemical, and biochemical investigations
As shown in Fig. 1, a needle muscle biopsy performed on the left quadriceps revealed no typical ragged-red fibers (1A), the presence of lipid storage in numerous fibers (1B), a diffuse, severe decrease of the histochemical reaction to cytochrome c oxidase (COX) (1C), and subsarcolemmal hyperreactivity to succinate dehydrogenase (1D). The latter finding demonstrates that, in spite of the absence of ragged-red fibers, a marked proliferation of mitochondria occurred in the skeletal muscle of the
Discussion
From a clinical point of view, our patient had symptoms suggestive of a mitochondrial disorder [1], such as PEO, muscle weakness, and myoclonus. Numerous COX-negative fibers were detected in the muscle biopsy. Typical ragged-red fibers, however, were absent. This is an unusual finding in cases associated with mutations in mitochondrial tRNA genes. The pathogenic significance of the new 8342G→A transition found in our patient is supported by the following evidence. First, this change disrupts a
Acknowledgements
We are indebted to Ms. B. Geehan for revising the manuscript and to Dr. Luca Roz for technical advice. Supported by Fondazione Telethon-Italy (grant no. 767 to M.Z.) and EU Human Capital and Mobility network grant on `Mitochondrial Biogenesis in Development and Disease'.
References (17)
- et al.
Myoclonic epilepsy and ragged-red fibers disease (MERRF) is associated with a mitochondrial DNA tRNALys mutation
Cell
(1990) - et al.
Atypical presentations associated with the MELAS mutation at position 3243 of human mtDNA
Neuromuscular Disord
(1993) - et al.
Automatic sequencing of mitochondrial tRNA genes in patients with mitochondrial encephalomyopathy
Biochim Biophys Acta
(1994) - et al.
Mitochondrial disorders
Medicine
(1998) - et al.
Neurological presentations of mitochondrial disorders
J Inher Metab Dis
(1996) - et al.
Mitochondrial DNA variation in human populations and implications for detection of mitochondrial DNA mutations of pathological significance
J Bioenerg Biomembr
(1994) - Dubowitz V. Muscle biopsy: a practical approach. London: Bailliere–Tindall,...
- Darley-Usmar VM, Rickwood D, Wilson MT, editors. Mitochondria, a practical approach. Washington DC:IRL Press,...
Cited by (38)
Progressive external ophthalmoplegia
2023, Handbook of Clinical NeurologyCytochrome c oxidase deficiency
2021, Biochimica et Biophysica Acta - BioenergeticsCitation Excerpt :In the last 20 years, many mutations affecting mt-tRNAs have been linked to different forms of mitochondrial diseases that variably affect the activities of the MRC complexes, some typically resulting in variable severity of predominant or exclusive COX deficiency, depending on the levels of heteroplasmy. For instance, mutations in mt-tRNALys ([40,41–46,263]), mt-tRNAAla [47–50], mt-tRNAPhe ([51–53]; Almalki et al., 2014), mt-tRNALeu [54–58], mt-tRNATrp [59–66], mt-tRNAAsp [67] cause respiratory defective phenotypes with COX deficiency. But, as seen above, while the MERRF mutation is clearly and consistently associated with COX deficiency, the same is not true for the m.3243 MELAS mutation, which is rather associated with predominant complex I deficiency.
MERRF Classification: Implications for Diagnosis and Clinical Trials
2018, Pediatric NeurologyDouble trouble progressive external ophthalmoplegia and Huntington's disease
2016, Molecular Genetics and Metabolism ReportsClinical and molecular findings in eight Egyptian patients with suspected mitochondrial disorders and optic atrophy
2013, Egyptian Journal of Medical Human GeneticsCitation Excerpt :Nystagmus is one of the clinical features of OXPHOS defects presenting in the neonatal period [58], commonly associated with LS [9,40] and was found in LHON patients [59]. Ptosis was reported in probands with MELAS/MERRF overlap disease [60], LS [56,61] and non-classical mitochondrial disease [53,62]. All of our patients had high blood lactate and pyruvate levels.
Strabismus
2013, Emery and Rimoin's Principles and Practice of Medical Genetics