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Editor,—Mitochondrial diseases manifest a variety of syndromic signs. Skeletal muscle, central nervous system, heart, eye, ear, liver, kidney, pancreas, bone marrow, and colon are the common target organs in mitochondrial diseases. The mitochondrial DNA (mtDNA) is responsible for the mitochondrial diseases through molecular defect of oxidative phosphorylation enzymes in conjunction with the nuclear genome.1 Optic neuropathy and cerebellar ataxia are a frequent association in mitochondrial diseases. We report a sporadic case of bilateral optic neuropathy, cerebellar ataxia, and peripheral neuropathy associated with a rare missense variation at np 14510 which replaced Val by Ala in the ND 6 coding sequence of mtDNA.
A 52 year old Japanese man first noted writing disturbance, tremor of hands, and mild gait disturbance at the age of 49 years. The patient had a 30 year history of drinking (daily alcohol consumption 125 g) and smoking (daily 20 cigarettes). Physical examination revealed a complex of neurological signs including cerebellar ataxia and peripheral neuropathy. The gait was broad based and ataxic. There was mild ataxia of the lower extremities on heel to knee test. Deep tendon reflexes were hyperactive with normal plantar responses. Peripheral nerve conduction velocity studies revealed sensory polyneuropathy in the upper and lower limbs. Muscular strength and volume of the limbs were normal. Magnetic resonance imaging showed cerebellar atrophy with dilatation of the fourth ventricle (Fig 1). The cerebrospinal fluid was normal without any inflammatory signs. Peripheral blood examination showed mild macrocytic anaemia. The serum levels of vitamin B-1, vitamin B-12, and folic acid were within the normal range. Red blood cell folate level was also normal. The patient also had suffered from insidious, progressive loss of central vision in both eyes since 51 years of age. At presentation, best corrected visual acuity was 0.1 in the right eye and 0.08 in the left. Pupils were round, isocoric, and sluggish to light. There was no blepharoptosis. Ocular motility was normal. Anterior segments and media were clear. Funduscopy disclosed dilatation of small retinal vessels neighbouring the optic nerve head in both eyes. Optic nerve heads were not hyperaemic but slightly atrophic with temporal pallor. Static and kinetic visual field testing showed central scotomas. Farnsworth's panel D-15 test showed red-green colour vision defects. Full field scotopic electroretinograms and visual evoked cortical potentials were normal. The patient was the fourth of five siblings of non-consanguineous parents. Because of the alcoholism, he was divorced and lived alone. Family members were not accessible, but according to his statement, there was no relative having similar disease.
Although the clinical signs in this patient were not inconsistent with the effects of chronic alcoholism and tobacco abuse alone, we could not dismiss the diagnosis of a mitochondrial disease subtype because of the bilateral optic neuropathy accompanied by cerebellar ataxia. Peripheral blood was obtained after informed consent and examined for common pathogenic mtDNA point mutations by polymerase chain reaction (PCR) restriction methods described elsewhere.2 The nucleotide sequences of sense and antisense strands of ND1, ND4, and ND6 genes of mtDNA from the patient were evaluated by autocycle sequencing methods. In the patient, no major pathological mutation was found by PCR restriction detection at np 3243 for mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), 3460 for Leber's hereditary optic neuropathy (LHON), 8344 for myoclonic epilepsy and ragged red fibres (MERRF), 8993 for neurogenic weakness, ataxia, and retinitis pigmentosa, 11778 for LHON, or 14484 for LHON. There was no known mutation in ND1, ND4, or ND6 gene of mtDNA by nucleotide sequencing. Instead, nucleotide sequencing andAlu I restriction detection confirmed a homoplasmic missense mutation at np 14510 (A14510G) that replaced Val by Ala in the ND 6 gene of mtDNA (Fig 2). The DNA samples reserved by us were examined for the relevant mtDNA mutation. The A14510G mutation was not detected in 468 Japanese individuals, including 39 LHON patients with 11778 mutation, 24 healthy carriers with 11778 mutation, one LHON patient with 3460 mutation, one LHON patient with 14484 mutation, 78 patients with aetiology undefined bilateral optic neuropathy, 239 patients with spinocerebellar ataxias who did not carry any abnormal trinucleotide repeats in the genes of spinocerebellar ataxia type 1, 2, 3, 6, 7, or 8, and 86 unrelated healthy individuals.
The nucleotide at np 14510 of mtDNA is most usually adenine (14510A). A literature survey reveals that 153 individuals in east Asia, 411 in an aboriginal Siberian population, 60 in a native American population, and 173 in a white population all had 14510A.3 Only one Australian among 147 individuals of African, Asian, white, aboriginal Australian, and aboriginal New Guinean was reported to have guanine at np 14510 (14510G).45 The A14510G mutation in the present patient was found in none of 468 Japanese patients with genetically proved LHON, unexplained bilateral optic neuropathy, cerebellar ataxia, or healthy individuals. Thus, grouped with previous data in other populations, the frequency of A14510G is estimated to be two in 1413, or approximately one in 700. Noticeably, this frequency is remarkably rare compared with commonly found mtDNA polymorphisms that are innocent or not pathogenic. Although clinical information from the Australian with A14510G is not available, it is possible to assume that the mtDNA plays a part in the aetiology of a syndromic clinical disorder. We could not confirm maternal inheritance or heteroplasmy of the A14510G mutation in our family. According to the statements from the patient, the family members had normal vision but clinical and molecular genetic assessments in them may give significant information for the genotype phenotype correlation of the A14510G mutation.
Mutational effect of A14510G is unknown. Two of the primary mtDNA mutations, G14459A (Ala72Val) for LHON plus dystonia and T14484C (Met64Val) for LHON, have been verified near A14510G (Val55Ala) in the ND6 gene.3 Another missense mutation (A14495G, Leu60Ser) has recently been found in the same gene in two LHON families, suggesting that the ND6 gene is a hot spot for LHON mutations.6 The amino acid sequence relevant to A14510G (VFLIYLGGMMVVFGYTTA; letters in bold are replaced by the mutations) is highly conserved in mammalia including human, bovine, and mouse, but not strictly conserved in xenopus, sea urchin, or drosophila. This region is a part of hydrophobic transmembrane helices of the ND enzyme. Mutations in the ND6 gene may disturb the enzymatic stability of NADH CoQ reductase.
Optic neuropathy is the predominant sign of LHON and may also be developed in patients with other mitochondrial disorders such as chronic progressive external ophthalmoplegia, MELAS, and MERRF, occasionally characterised by insidious, chronic progressive optic nerve disease.1 Our patient developed a late onset, insidious bilateral optic neuropathy with mildly atrophic optic nerve heads and tortuous retinal vessels, being compatible with features of LHON.7 Cerebellar ataxia and/or cerebellar atrophy are caused by mutations of mtDNA—for example, large scale deletions or tRNA mutations.3 An extensive review of the literature demonstrates a variety of neurological abnormalities in LHON patients, including cerebellar ataxias and peripheral nerve disorders.8 Our patient had cerebellar ataxia and sensory polyneuropathy, with evidence of cerebellar atrophy on magnetic resonance imaging. Similar neurological complications were rarely found in a LHON family with G11778A mutation.9
Although epigenic factors have been considered for the disease expression and visual outcome of LHON patients in association with mtDNA mutations, it has yet to be proved. A retrospective analysis of LHON sibships has failed to demonstrate a significant deleterious association between tobacco or alcohol consumption and vision loss among individuals at risk with the major mtDNA mutations.10 In the present clinical isolate, it remains unknown whether the malnutritional condition provided a potential risk factor in the clinical manifestation associated with the underlying mtDNA defect. The A14510G mutation of mtDNA is expected to be found in other independent patients especially with unknown optic neuropathy and cerebellar ataxia.
No proprietary interest.
This work was supported by grants in aid for scientific research (12877279, 12671715) from the Japanese Ministry of Education, Science and Culture.