Pure myopathy with enlarged mitochondria associated to a new mutation in MTND2 gene

Abstract

To date, only few mutations in the mitochondrial DNA (mtDNA)-encoded ND2 subunit of Complex I have been reported, usually presenting a severe phenotype characterized by early onset encephalomyopathy and early death. In this report, we describe a new mutation in the MTND2 gene in a 21-year-old man with a mild myopathic phenotype characterized by exercise intolerance and increased plasma lactate at rest. Electromyography and brain NMR were normal, and no cardiac involvement was present. Muscle biopsy showed a massive presence of ragged red - COX-positive fibres, with enlarged mitochondria containing osmiophilic inclusions. Biochemical assays revealed a severe isolated complex I deficiency. We identified a novel, heteroplasmic mutation m.4831G > A in the MTND2 gene, causing the p.Gly121Asp substitution in the ND2 protein. The mutation was present in the 95% of mitochondrial genomes from patient's muscle tissue, at a lower level in cells from the urinary tract and at a lowest level in lymphocytes from patient's blood; the base substitution was absent in fibroblasts and in the tissues from proband's healthy mother and brother. The specific skeletal muscle tissue involvement can explain the childhood-onset and the relatively benign, exclusively myopathic course of the disease.

Keywords: Complex I deficiency; Exercise intolerance; ND2.

Fig. 1

Muscle biopsy. A: Gomori trichrome stain showing numerous ragged red fibres. B: Cox staining showing that the ragged red fibres are COX-positive. C: Electron microscopy showing enlarged mitochondria with osmiophilic inclusions.

Fig. 2

A: Multi-alignment analysis. The comparison of the aminoacid sequences around Gly-121 in ND2 protein using clustalW between different species shows the high conservation of residue. B: Strategy of restriction fragment length polymorphism (RFLP) analysis with NlaIV enzyme. In the wt mtDNA molecules the NlaIV enzyme cut the 360 bp amplified fragment in two smaller fragment (192 bp and 168 bp), the mutation m.4831G > A abolishes the restriction site resulting the uncutted band. C: Result of RFLP analysis in patient and his family. The mutation is present in 95% of mtDNA from patient's muscle tissue (M), in 40% of genomes from urinary sediment (U), in < 5% from peripheral blood lymphocytes (L) and is absent in fibroblasts (F) as well as in tissues from the healthy mother and younger brother (L and U).