Mitochondrial DNA point mutations are associated with various syndromes, among which mitochondrial myopathy, encephalopathy, lactic acidosis and stroke (MELAS) and myoclonic epilepsy with ragged red fibres (MERRF) are the most common.1 Here we present a case report of a young man with a mitochondrial encephalomyopathy caused by a mutation in the ND5 gene (13042A>G), which has been described previously.1

A 21-year-old man presented with a 2-year history of recurrent generalised epileptic seizures and myoclonus with no family history. Neurological examination revealed discrete nystagmus and multifocal myoclonus in the upper limbs, ataxia and focal dystonia presenting as writer's cramp. Neuropsychological examination showed slight working memory impairment and slowed learning of both verbal and visual material. Follow-up examinations conducted 18 and 33 months later did not show evidence of progression of the deficits. The patient exhibited anosodiaphoria throughout the observation period.

Laboratory studies showed a normal serum lactic acid level of 9.45 mg% with an abnormal lactic acid curve during ischaemic muscle exercise. Creatine kinase activity was within the normal range. A 50 min video EEG disclosed a pattern characteristic of myoclonic epilepsy. Visual evoked potentials showed delayed P100 responses. Electromyography and nerve conduction studies were within normal ranges. MRI showed hyperintensive lesions on T2 and FLAIR images in the right cerebral peduncle, close to the occipital horns of the lateral ventricles, in the left temporal lobe as well as in both frontal and parietal lobes (figure 1). Additionally, MR spectroscopy revealed elevated lactate levels within the previously specified areas. Open muscle biopsy of the left biceps brachii was performed under local anaesthesia. Treatment with valproate and oxcarbamazepine (not effective) was discontinued and levetiracetam in a daily dose of 2 g was introduced with good effect (no epileptic seizures and markedly reduced myoclonic jerks). Mitochondrial disease was suspected, but a PCR-RFLP test for common mitochondrial DNA (mtDNA) mutations (3243A>G, 8344A>G, 8993T>G/C) performed on blood leucocyte DNA gave negative results. Nevertheless, we continued investigations to confirm a mitochondrial disease as the clinical pattern was similar to MERRF syndrome.

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Figure 1

Axial FLAIR MRI showing multifocal bilateral cortical (frontal and parietal) hyperintensities.

Light microscopy performed on muscle tissue, processed using standard methods,2 showed muscle fibres of normal shape and diameter. Modified Gomori trichrome stain showed a slight increase in red-stained material in the subsarcolemmal region of several fibres (figure 2A). Oxidative enzyme reactions3 showed aggregation of enzymatic activity at the periphery of some fibres (figure 2B). COX-negative fibres were not detected (figure 2C). Electron microscopy revealed an increased number of mitochondria with irregular membranes but without crystalline inclusions (figure 2D). Biochemical studies of respiratory chain complexes showed a slight reduction in complex I activity.

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Figure 2

(A) Gomori trichrome stain ×400. Muscle fibre with an increase in red-stained material (arrow). (B) Succinic dehydrogenase ×400. Aggregation of enzyme activity at the periphery (arrow). (C) Cytochrome c oxidase ×200. (D) EM ×20 000. Delicate aggregation of mitochondria under the sarcolemma (black arrow). Lipid droplets (asterisk) and glycogen granules (white arrow). This figure is only reproduced in colour in the online version.

PCR and Southern tests for mtDNA deletions3 were performed on blood leucocyte DNA with negative results. Sequencing of the entire mtDNA4 revealed several common polymorphisms, one heteroplasmic pathogenic change (13042G>A) and two homoplasmic changes of unknown pathogenicity (4315A>G, 15917C>T). Sequence analysis of the mitochondrial MT-ND5 gene confirmed the presence of a heteroplasmic 13042G>A mutation in the patient's skeletal muscle, blood cells, hair and urinary epithelium which was absent from the mother's blood cells, hair and urinary epithelium (figure 3A,B).

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Figure 3

Electrophoregrams of the region encompassing the 13042G>A mutation. Samples from (A) the patient and (B) the patient's mother. (C) Analysis of the 13042G>A mutation level: restriction analysis of the last cycle hot PCR-RFLP product obtained with primers introducing the restriction site for BbvCI. The 13042G>A mutation eliminates this restriction site. U, uncut PCR product; C, control sample; PB, patient's blood; PM, patient's muscle; MB, mother's blood. This figure is only reproduced in colour in the online version.

Heteroplasmy analysis showed that the level of 13042G>A mutation was 68.9±1.78% in muscle and 29.8±1.65% in blood (figure 3C). The other two variants were found in all the tissues of the patient and his mother, suggesting they are not pathogenic.

13042G>A mtDNA point mutation in the MT-ND5 gene has been described in a patient with MELAS/MERRF overlap syndrome.1 This gene may be a hot spot for mutations in several overlap syndromes, including symptoms and signs of MELAS, MERRF, Leber's hereditary optic neuropathy (LHON) and Leigh syndromes. The case presented here corresponds with those described by Naini et al,1 although it resembles MERRF rather than MELAS (only MRI hyperintensities and abnormal lactic acid curve). Valentino et al 5 also did not find typical ragged red fibres but a decrease in complex I activity was observed. The 13042G>A mutation causes the substitution of a hydrophobic (alanine) by a hydrophilic (threonine) amino acid at highly conserved codon 236 (A236T).

The presented analysis suggests that the change 13042G>A in the patient's mtDNA is the result of a de novo mutation, in contrast to previously described cases which were due to germline mutations. The patient with LHON-like visual loss5 and the patient with a Leigh overlap syndrome6 had a family history compatible with maternal inheritance. For the patient with clinical features of MELAS and MERRF1 it was not possible to analyse tissues from the maternal relatives, but some symptoms found in that proband also appeared in his mother. In all cases described, the mutation 13042G>A occurred in the heteroplasmic state and the mutation load was always lower in the blood samples than in other tested tissues, which could suggest selection against mutant mtDNA in a tissue with fast turnover.

The assessment of mitochondrial disorders is still challenging. To our knowledge, our patient is the first with a de novo 13042G>A mutation. The mutation has a pleiotropic effect and carriers can present with symptoms from a wide spectrum of mitochondrial disorders.