Patrick Yu-Wai-Man

Mitochondrial optic neuropathies – Disease mechanisms and therapeutic strategies

Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.

Inherited mitochondrial optic neuropathies

Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (DOA) are the two most common inherited optic neuropathies and they result in significant visual morbidity among young adults. Both disorders are the result of mitochondrial dysfunction: LHON from primary mitochondrial DNA (mtDNA) mutations affecting the respiratory chain complexes; and the majority of DOA families have mutations in the OPA1 gene, which codes for an inner mitochondrial membrane protein critical for mtDNA maintenance and oxidative phosphorylation. Additional genetic and environmental factors modulate the penetrance of LHON, and the same is likely to be the case for DOA which has a markedly variable clinical phenotype. The selective vulnerability of retinal ganglion cells (RGCs) is a key pathological feature and understanding the fundamental mechanisms that underlie RGC loss in these disorders is a prerequisite for the development of effective therapeutic strategies which are currently limited.

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

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.

Disturbed mitochondrial dynamics and neurodegenerative disorders.

Mitochondria form a highly interconnected tubular network throughout the cell via a dynamic process, with mitochondrial segments fusing and breaking apart continuously. Strong evidence has emerged to implicate disturbed mitochondrial fusion and fission as central pathological components underpinning a number of childhood and adult-onset neurodegenerative disorders. Several proteins that regulate the morphology of the mitochondrial network have been identified, the most widely studied of which are optic atrophy 1 and mitofusin 2. Pathogenic mutations that disrupt these two pro-fusion proteins cause autosomal dominant optic atrophy and axonal Charcot–Marie–Tooth disease type 2A, respectively. These disorders predominantly affect specialized neurons that require precise shuttling of mitochondria over long axonal distances. Considerable insight has also been gained by carefully dissecting the deleterious consequences of imbalances in mitochondrial fusion and fission on respiratory chain function, mitochondrial quality control (mitophagy), and programmed cell death. Interestingly, these cellular processes are also implicated in more-common complex neurodegenerative disorders, such as Alzheimer disease and Parkinson disease, indicating a common pathological thread and a close relationship with mitochondrial structure, function and localization. Understanding how these fundamental processes become disrupted will prove crucial to the development of therapies for the growing number of neurodegenerative disorders linked to disturbed mitochondrial dynamics.

A randomized placebo-controlled trial of idebenone in Leber’s hereditary optic neuropathy

Abstract Major advances in understanding the pathogenesis of inherited metabolic disease caused by mitochondrial DNA mutations have yet to translate into treatments of proven efficacy. Leber’s hereditary optic neuropathy is the most common mitochondrial DNA disorder causing irreversible blindness in young adult life. Anecdotal reports support the use of idebenone in Leber’s hereditary optic neuropathy, but this has not been evaluated in a randomized controlled trial. We conducted a 24-week multi-centre double-blind, randomized, placebo-controlled trial in 85 patients with Leber’s hereditary optic neuropathy due to m.3460G>A, m.11778G>A, and m.14484T>C or mitochondrial DNA mutations. The active drug was idebenone 900 mg/day. The primary end-point was the best recovery in visual acuity. The main secondary end-point was the change in best visual acuity. Other secondary end-points were changes in visual acuity of the best eye at baseline and changes in visual acuity for both eyes in each patient. Colour-contrast sensitivity and retinal nerve fibre layer thickness were measured in subgroups. Idebenone was safe and well tolerated. The primary end-point did not reach statistical significance in the intention to treat population. However, post hoc interaction analysis showed a different response to idebenone in patients with discordant visual acuities at baseline; in these patients, all secondary end-points were significantly different between the idebenone and placebo groups. This first randomized controlled trial in the mitochondrial disorder, Leber’s hereditary optic neuropathy, provides evidence that patients with discordant visual acuities are the most likely to benefit from idebenone treatment, which is safe and well tolerated.

Gene–environment interactions in Leber hereditary optic neuropathy

Leber hereditary optic neuropathy (LHON) is a genetic disorder primarily due to mutations of mitochondrial DNA (mtDNA). Environmental factors are thought to precipitate the visual failure and explain the marked incomplete penetrance of LHON, but previous small studies have failed to confirm this to be the case. LHON has no treatment, so identifying environmental triggers is the key to disease prevention, whilst potentially revealing new mechanisms amenable to therapeutic manipulation. To address this issue, we conducted a large, multicentre epidemiological study of 196 affected and 206 unaffected carriers from 125 LHON pedigrees known to harbour one of the three primary pathogenic mtDNA mutations: m.3460G>A, m.11778G>A and m.14484T>C. A comprehensive history of exposure to smoking, alcohol and other putative environmental insults was collected using a structured questionnaire. We identified a strong and consistent association between visual loss and smoking, independent of gender and alcohol intake, leading to a clinical penetrance of 93% in men who smoked. There was a trend towards increased visual failure with alcohol, but only with a heavy intake. Based on these findings, asymptomatic carriers of a LHON mtDNA mutation should be strongly advised not to smoke and to moderate their alcohol intake.

Prevalence of nuclear and mitochondrial DNA mutations related to adult mitochondrial disease

The prevalence of mitochondrial disease has proven difficult to establish, predominantly as a result of clinical and genetic heterogeneity. The phenotypic spectrum of mitochondrial disease has expanded significantly since the original reports that associated classic clinical syndromes with mitochondrial DNA (mtDNA) rearrangements and point mutations. The revolution in genetic technologies has allowed interrogation of the nuclear genome in a manner that has dramatically improved the diagnosis of mitochondrial disorders. We comprehensively assessed the prevalence of all forms of adult mitochondrial disease to include pathogenic mutations in both nuclear and mtDNA.

Universal heteroplasmy of human mitochondrial DNA

Mammalian cells contain thousands of copies of mitochondrial DNA (mtDNA). At birth, these are thought to be identical in most humans. Here, we use long read length ultra-deep resequencing-by-synthesis to interrogate regions of the mtDNA genome from related and unrelated individuals at unprecedented resolution. We show that very low-level heteroplasmic variance is present in all tested healthy individuals, and is likely to be due to both inherited and somatic single base substitutions. Using this approach, we demonstrate an increase in mtDNA mutations in the skeletal muscle of patients with a proofreading-deficient mtDNA polymerase γ due to POLG mutations. In contrast, we show that OPA1 mutations, which indirectly affect mtDNA maintenance, do not increase point mutation load. The demonstration of universal mtDNA heteroplasmy has fundamental implications for our understanding of mtDNA inheritance and evolution. Ostensibly de novo somatic mtDNA mutations, seen in mtDNA maintenance disorders and neurodegenerative disease and aging, will partly be due to the clonal expansion of low-level inherited variants.

OPA1 links human mitochondrial genome maintenance to mtDNA replication and distribution

Eukaryotic cells harbor a small multiploid mitochondrial genome, organized in nucleoids spread within the mitochondrial network. Maintenance and distribution of mitochondrial DNA (mtDNA) are essential for energy metabolism, mitochondrial lineage in primordial germ cells, and to prevent mtDNA instability, which leads to many debilitating human diseases. Mounting evidence suggests that the actors of the mitochondrial network dynamics, among which is the intramitochondrial dynamin OPA1, might be involved in these processes. Here, using siRNAs specific to OPA1 alternate spliced exons, we evidenced that silencing of the OPA1 variants including exon 4b leads to mtDNA depletion, secondary to inhibition of mtDNA replication, and to marked alteration of mtDNA distribution in nucleoid and nucleoid distribution throughout the mitochondrial network. We demonstrate that a small hydrophobic 10-kDa peptide generated by cleavage of the OPA1-exon4b isoform is responsible for this process and show that this peptide is embedded in the inner membrane and colocalizes and coimmunoprecipitates with nucleoid components. We propose a novel synthetic model in which a peptide, including two trans-membrane domains derived from the N terminus of the OPA1-exon4b isoform in vertebrates or from its ortholog in lower eukaryotes, might contribute to nucleoid attachment to the inner mitochondrial membrane and promotes mtDNA replication and distribution. Thus, this study places OPA1 as a direct actor in the maintenance of mitochondrial genome integrity.

The prevalence and natural history of dominant optic atrophy due to OPA1 mutations.

PURPOSE Autosomal dominant optic atrophy (DOA) is a major cause of visual impairment in young adults that is characterized by selective retinal ganglion cell loss. To define the prevalence and natural history of this optic nerve disorder, we performed a population-based epidemiologic and molecular study of presumed DOA cases in the north of England. DESIGN Case series. PARTICIPANTS Seventy-six affected probands with a clinical diagnosis of DOA were identified from our neuro-ophthalmology and neurogenetics database. METHODS OPA1 genetic testing was performed using a polymerase chain reaction-based sequencing strategy. OPA1-negative cases were then screened for large-scale OPA1 rearrangements and OPA3 mutations. Additional affected family members identified through contact tracing were examined, and longitudinal visual data were analyzed. MAIN OUTCOME MEASURES The prevalence and molecular characteristics of DOA in the north of England. Visual function and disease progression among patients with OPA1-positive mutations. RESULTS The detection rate of OPA1 mutations was 57.6% among probands with a positive family history of optic atrophy (19/33) and 14.0% among singleton cases (6/43). Approximately two thirds of our families with DOA harbored OPA1 mutations (14/22, 63.6%), and 5 novel OPA1 mutations were identified. Only 1 family carried a large-scale OPA1 rearrangement, and no OPA3 mutations were found in our optic atrophy cohort. The minimum point prevalence of DOA in the north of England was 2.87 per 100,000 (95% confidence interval [CI], 2.54-3.20), or 2.09 per 100,000 (95% CI, 1.95-2.23) when only OPA1-positive cases were considered. Snellen visual acuity varied markedly between OPA1-positive cases with a mean of 20/173 (range 20/20 to hand movements), and visual function worsened in 67.4% of patients during follow-up. The mean rate of visual loss was 0.032 logarithm of the minimum angle of resolution per year, but some patients experienced faster visual decline (range = 0-0.171 logarithm of the minimum angle of resolution/year). OPA1 missense mutations were associated with a significantly worse visual outcome compared with other mutational subtypes (P=0.0001). CONCLUSIONS Dominant optic atrophy causes significant visual morbidity and affects at least 1 in 35,000 of the general population.