A number sign (#) is used with this entry because of evidence that neurodegeneration with ataxia and late-onset optic atrophy (NDAXOA) is caused by heterozygous mutation in the SDHA gene (600857) on chromosome 5p15.

Biallelic mutation in the SDHA gene causes autosomal recessive mitochondrial complex II deficiency (MC2DN1; 252011).

Neurodegeneration with ataxia and late-onset optic atrophy (NDAXOA) is an autosomal dominant disorder with somewhat variable manifestations. Most affected individuals present in mid-adulthood with slowly progressive cerebellar and gait ataxia, optic atrophy, and myopathy or myalgia. Some patients may have a childhood history of neurologic features, including limited extraocular movements. Additional features can include cardiomyopathy, psychiatric disturbances, and peripheral sensory impairment (summary by Taylor et al., 1996 and Courage et al., 2017).

Taylor et al. (1996) reported 2 adult sisters, aged 56 and 62 years, who presented in their mid-forties with progressive neurologic decline, including imbalance, ataxia, vertigo, headaches, visual disturbances, limb weakness, and sensory dysesthesia. Physical examination showed optic atrophy and limited extraocular movements. Brain imaging was normal in one sister and showed atrophic changes in the brainstem and cerebellum in the other sister. Muscle biopsies were normal morphologically, but showed about a 50% isolated decrease in total mitochondrial complex II as well as SDH activity. Immunoblot analysis of patient platelet mitochondria showed a 6.5-fold increase in the 70-kD Fp subunit in the soluble fraction compared to controls, suggesting a compensatory response and synthesis of an aberrant protein. Cultured patient cells did not have complex II deficiency, which the authors postulated was due to a high content of riboflavin in the growth medium; however, treatment of the patients with riboflavin did not result in biochemical or clinical improvement.

Courage et al. (2017) reported a family in which a father and his 2 children had isolated complex II deficiency with variable manifestations. The proband was a 60-year-old man with neurologic symptoms since early childhood, when he presented with clumsiness, nystagmus in one eye, cramps in the feet, and speech impediment. At age 15 years, he had worsening ocular paresis, pyramidal signs, and ataxia. He was also found to have cardiomegaly and cardiomyopathy. At age 47, a myocardial biopsy showed increased mitochondria; he was also diagnosed with optic atrophy at this time. The cardiac and neurologic abnormalities were progressive. The patient also had recurrent depression with suicide attempts. Brain imaging was normal. The proband's affected son was a 30-year-old man diagnosed with cardiomyopathy in early childhood. As an adult, he presented with stable cardiomyopathy and developed progressive optic atrophy. Another child of the proband died at 7 months of age due to cardiac insufficiency associated with increased serum lactate and methylglutaconic aciduria; postmortem examination of cardiac muscle showed abnormal mitochondrial accumulation. Fibroblasts derived from all patients showed isolated complex II deficiency at about 50% of normal values. The authors emphasized the intrafamilial variability, with onset of various symptoms from infancy to late adulthood.

The transmission pattern of NDAXOA in the families reported by Taylor et al. (1996) and Courage et al. (2017) was consistent with autosomal dominant inheritance.

In 2 adult sisters with NDAXOA, who were originally reported by Taylor et al. (1996), Birch-Machin et al. (2000) identified a heterozygous missense mutation in the SDHA gene (R451C; 600857.0011). The mutation, which was found by direct sequencing, was not found in an unaffected sister or in 80 control samples. In vitro functional expression studies in E. coli showed almost no mitochondrial complex II or SDH activity compared to controls; there was decreased protein expression at the membrane. The evidence suggested that the mutation interfered with association of the FAD cofactor. Patient myoblasts showed normal membrane content of SDHA immunoreactive protein, but complex II and SDH activity were 50% of normal values.

In 3 affected members of a 2-generation family with NDAXOA, Courage et al. (2017) identified a heterozygous R451C mutation in the SDHA gene. Molecular modeling predicted that the mutation interfered with succinate binding, causing a loss of succinate dehydrogenase activity. Functional studies of the variant were not performed, but patient cells showed about a 50% decrease in complex II activity. The patients in this family also developed cardiomyopathy, including an infant who died at 7 months of age.