Multi-system neurological disease is common in patients with OPA1 mutations

Abstract

Additional neurological features have recently been described in seven families transmitting pathogenic mutations in OPA1, the most common cause of autosomal dominant optic atrophy. However, the frequency of these syndromal 'dominant optic atrophy plus' variants and the extent of neurological involvement have not been established. In this large multi-centre study of 104 patients from 45 independent families, including 60 new cases, we show that extra-ocular neurological complications are common in OPA1 disease, and affect up to 20% of all mutational carriers. Bilateral sensorineural deafness beginning in late childhood and early adulthood was a prominent manifestation, followed by a combination of ataxia, myopathy, peripheral neuropathy and progressive external ophthalmoplegia from the third decade of life onwards. We also identified novel clinical presentations with spastic paraparesis mimicking hereditary spastic paraplegia, and a multiple sclerosis-like illness. In contrast to initial reports, multi-system neurological disease was associated with all mutational subtypes, although there was an increased risk with missense mutations [odds ratio = 3.06, 95% confidence interval = 1.44-6.49; P = 0.0027], and mutations located within the guanosine triphosphate-ase region (odds ratio = 2.29, 95% confidence interval = 1.08-4.82; P = 0.0271). Histochemical and molecular characterization of skeletal muscle biopsies revealed the presence of cytochrome c oxidase-deficient fibres and multiple mitochondrial DNA deletions in the majority of patients harbouring OPA1 mutations, even in those with isolated optic nerve involvement. However, the cytochrome c oxidase-deficient load was over four times higher in the dominant optic atrophy + group compared to the pure optic neuropathy group, implicating a causal role for these secondary mitochondrial DNA defects in disease pathophysiology. Individuals with dominant optic atrophy plus phenotypes also had significantly worse visual outcomes, and careful surveillance is therefore mandatory to optimize the detection and management of neurological disability in a group of patients who already have significant visual impairment.

Figure 1

Diagrammatic representation of the OPA1 gene with the location of mutations resulting in DOA+ syndromes. The mutation type is indicated by (i) red stars (missense); (ii) grey squares (nonsense); (iii) blue circles (splice site); and (iv) green triangles (deletion). CC = coiled-coil domain; GE = GTPase effector domain; UTR = untranslated region.

Figure 2

(A) Cumulative distribution curve of age on onset of visual failure in pure DOA and DOA+, and (B) comparison of mean LogMAR visual acuity between patients with pure DOA and DOA+ features (P = 0.0170). The whiskers represent the minimum and maximum LogMAR values, the ends of the boxes are the upper and lower quartiles, the vertical length of the boxes indicate the interquartile range, and the line within the boxes represent the median LogMAR values for each group.

Figure 3

Molecular investigations and MRI neuroimaging of a 58-year-old female presenting with a multiple sclerosis-like illness and harbouring a c.2613 + 1g>a splice-site mutation within intron 25 of the OPA1 gene (UK-6). (A) Family tree of the affected proband (II-3). DNA samples were obtained from her 62-year-old (II-1) and 59-year-old (II-2) brothers, who both had a pure DOA phenotype, and OPA1 sequencing confirmed the presence of the same heterozygous c.2613 + 1g>a variant. The asterisk indicates family members where DNA samples were available for molecular analysis. (B) A 16 kb long-range PCR assay was performed on homogenate DNA extracted from the patient’s skeletal muscle biopsy, and multiple deletion bands were identified. Lane a: 1 kb DNA ladder with the adjacent numbers indicating the size of the band; lane b: patient; lane c: wild-type control. (C) Axial T₂-weighted fluid attenuated inversion recovery and (D) sagittal T₂-weighted turbo spin echo slices showing cortical atrophy and characteristic white matter lesions in the periventricular areas and more peripherally near to grey matter.

Figure 4

Mean level of COX-deficiency in skeletal muscle biopsies: (A) pure DOA versus DOA+ phenotypes (P = 0.0226) and (B) pure DOA versus age-matched controls (P < 0.0001). The error bars represent the standard error of the mean.

Figure 5

Clinical, histochemical and molecular features observed in a 59-year-old female with isolated optic nerve involvement due to an exon 27 c.2713C>T (p.R905X) OPA1 missense mutation. (A) Right, and (B) left optic discs displaying generalized pallor of the neuro-retinal rim and pathological excavation. (C) Dual COX-SDH histochemistry performed on 20 µm thick cryostat sections showing the presence of COX-deficient muscle fibres (33/1100, 3.00%). (D) Long-range PCR of the patient’s homogenate skeletal muscle DNA revealing multiple deletion bands in addition to the wild-type 16-kb fragment. Lane a: 1-kb DNA ladder with the adjacent numbers indicating the size of the band; lane b: patient; lane c: wild-type control.

Figure 6

Comparison of mtDNA copy number in homogenate skeletal muscle DNA from patients with OPA1 mutations (mean = 5144, SD = 2843, n = 24) with age-matched healthy controls (mean = 4669, SD = 2640, n = 20, P = 0.5719), and patients with mtDNA depletion syndromes (mean = 524, SD = 345, n = 6, P = 0.0005). The error bars represent the standard error of the mean.

Figure 7

Evolution of the major clinical features observed in DOA+ syndromes.

Figure 8

In silico prediction of coiled-coil regions within the segment of 37 amino acids encoded by exon 5b: (A) wild-type, and (B) p.S256R substitution. The modelling accuracy increases with larger scanning windows (28 > 21 > 14), and probabilities > 0.5 indicate a high likelihood of coiled-coil conformation. The p.S256R substitution is predicted to reduce the coiled-coil forming potential by ∼45%. (http://www.ch.embnet.org/software/COILS_form.html). Green line = window 14; Blue line = window 21; Red line = window 28.