Mitochondrial optic neuropathies.

Mitochondrial optic neuropathies have a leading role in the field of mitochondrial medicine ever since 1988, when the first mutation in mitochondrial DNA was associated with Leber's hereditary optic neuropathy (LHON). Autosomal dominant optic atrophy (DOA) was subsequently associated in 2000 with mutations in the nuclear DNA affecting the OPA1 gene. LHON and DOA are both characterized by selective neurodegeneration of retinal ganglion cells (RGCs) triggered by mitochondrial dysfunction. This is centered on respiratory complex I impairment in LHON and defective mitochondrial dynamics in OPA1-related DOA, leading to distinct clinical phenotypes. LHON is a subacute, rapid, severe loss of central vision involving both eyes within weeks or months, with age of onset between 15 and 35 years old. DOA is a more slowly progressive optic neuropathy, usually apparent in early childhood. LHON is characterized by marked incomplete penetrance and a clear male predilection. The introduction of next-generation sequencing has greatly expanded the genetic causes for other rare forms of mitochondrial optic neuropathies, including recessive and X-linked, further emphasizing the exquisite sensitivity of RGCs to compromised mitochondrial function. All forms of mitochondrial optic neuropathies, including LHON and DOA, can manifest either as pure optic atrophy or as a more severe multisystemic syndrome. Mitochondrial optic neuropathies are currently at the forefront of a number of therapeutic programs, including gene therapy, with idebenone being the only approved drug for a mitochondrial disorder.

Understanding the molecular basis and pathogenesis of hereditary optic neuropathies: towards improved diagnosis and management.

Hereditary optic neuropathies result from defects in the human genome, both nuclear and mitochondrial. The two main and most recognised phenotypes are dominant optic atrophy and Leber hereditary optic neuropathy. Advances in modern molecular diagnosis have expanded our knowledge of genotypes and phenotypes of inherited disorders that affect the optic nerve, either alone or in combination, with various forms of neurological and systemic degeneration. A unifying feature in the pathophysiology of these disorders appears to involve mitochondrial dysfunction, suggesting that the retinal ganglion cells and their axons are especially susceptible to perturbations in mitochondrial homoeostasis. As we better understand the pathogenesis behind these genetic diseases, aetiologically targeted therapies are emerging and entering into clinical trials, including treatments aimed at halting the cascade of neurodegeneration, replacing or editing the defective genes or their protein products, and potentially regenerating damaged optic nerves, as well as preventing generational disease transmission.

Current treatment options for treating OPA1-mutant dominant optic atrophy.

Dominant optic atrophy (DOA) is caused by OPA1 gene mutation, and it represents one of the most frequently diagnosed forms of hereditary optic neuropathies. This neurodegenerative disorder typically occurs in the first decades of life, and it is often associated with severe visual impairment. For this reason, several treatment options have been examined for the management of DOA, including vitamin supplements, ubiquinone analogues (in particular idebenone) and, more recently, gene therapy. Among them, idebenone has shown the most promising clinical outcomes in recent real-life studies. Furthermore, gene therapy represents also a promising therapeutic approach; however, more evidence in clinical trials is needed. In this review, we will summarize and discuss all the possible treatment options for DOA, in order to identify the current optimal management in these patients, whose visual prognosis remains unfortunately poor and unsatisfactory in the everyday clinical practice.

Induced Pluripotent Stem Cells for Inherited Optic Neuropathies-Disease Modeling and Therapeutic Development.

BACKGROUND: Inherited optic neuropathies (IONs) cause progressive irreversible visual loss in children and young adults. There are limited disease-modifying treatments, and most patients progress to become severely visually impaired, fulfilling the legal criteria for blind registration. The seminal discovery of the technique for reprogramming somatic nondividing cells into induced pluripotent stem cells (iPSCs) has opened several exciting opportunities in the field of ION research and treatment. EVIDENCE ACQUISITION: A systematic review of the literature was conducted with PubMed using the following search terms: autosomal dominant optic atrophy, ADOA, dominant optic atrophy, DOA, Leber hereditary optic neuropathy, LHON, optic atrophy, induced pluripotent stem cell, iPSC, iPSC derived, iPS, stem cell, retinal ganglion cell, and RGC. Clinical trials were identified on the ClinicalTrials.gov website. RESULTS: This review article is focused on disease modeling and the therapeutic strategies being explored with iPSC technologies for the 2 most common IONs, namely, dominant optic atrophy and Leber hereditary optic neuropathy. The rationale and translational advances for cell-based and gene-based therapies are explored, as well as opportunities for neuroprotection and drug screening. CONCLUSIONS: iPSCs offer an elegant, patient-focused solution to the investigation of the genetic defects and disease mechanisms underpinning IONs. Furthermore, this group of disorders is uniquely amenable to both the disease modeling capability and the therapeutic potential that iPSCs offer. This fast-moving area will remain at the forefront of both basic and translational ION research in the coming years, with the potential to accelerate the development of effective therapies for patients affected with these blinding diseases.

Red Light Irradiation In Vivo Upregulates DJ-1 in the Retinal Ganglion Cell Layer and Protects against Axotomy-Related Dendritic Pruning.

Retinal ganglion cells (RGCs) undergo dendritic pruning in a variety of neurodegenerative diseases, including glaucoma and autosomal dominant optic atrophy (ADOA). Axotomising RGCs by severing the optic nerve generates an acute model of RGC dendropathy, which can be utilized to assess the therapeutic potential of treatments for RGC degeneration. Photobiomodulation (PBM) with red light provided neuroprotection to RGCs when administered ex vivo to wild-type retinal explants. In the current study, we used aged (13-15-month-old) wild-type and heterozygous B6;C3-Opa1Q285STOP (Opa1+/-) mice, a model of ADOA exhibiting RGC dendropathy. These mice were pre-treated with 4 J/cm2 of 670 nm light for five consecutive days before the eyes were enucleated and the retinas flat-mounted into explant cultures for 0-, 8- or 16-h ex vivo. RGCs were imaged by confocal microscopy, and their dendritic architecture was quantified by Sholl analysis. In vivo 670 nm light pretreatment inhibited the RGC dendropathy observed in untreated wild-type retinas over 16 h ex vivo and inhibited dendropathy in ON-center RGCs in wild-type but not Opa1+/- retinas. Immunohistochemistry revealed that aged Opa1+/- RGCs exhibited increased nitrosative damage alongside significantly lower activation of NF-κB and upregulation of DJ-1. PBM restored NF-κB activation in Opa1+/- RGCs and enhanced DJ-1 expression in both genotypes, indicating a potential molecular mechanism priming the retina to resist future oxidative insult. These data support the potential of PBM as a treatment for diseases involving RGC degeneration.

Bioenergetic Crosstalk between Mesenchymal Stem Cells and various Ocular Cells through the intercellular trafficking of Mitochondria.

Rationale: Mitochondrial disorders preferentially affect tissues with high energy requirements, such as the retina and corneal endothelium, in human eyes. Mesenchymal stem cell (MSC)-based treatment has been demonstrated to be beneficial for ocular degeneration. However, aside from neuroprotective paracrine actions, the mechanisms underlying the beneficial effect of MSCs on retinal and corneal tissues are largely unknown. In this study, we investigated the fate and associated characteristics of mitochondria subjected to intercellular transfer from MSCs to ocular cells. Methods: MSCs were cocultured with corneal endothelial cells (CECs), 661W cells (a photoreceptor cell line) and ARPE-19 cells (a retinal pigment epithelium cell line). Immunofluorescence, fluorescence activated cell sorting and confocal microscopy imaging were employed to investigate the traits of intercellular mitochondrial transfer and the fate of transferred mitochondria. The oxygen consumption rate of recipient cells was measured to investigate the effect of intercellular mitochondrial transfer. Transcriptome analysis was performed to investigate the expression of metabolic genes in recipient cells with donated mitochondria. Results: Mitochondrial transport is a ubiquitous intercellular mechanism between MSCs and various ocular cells, including the corneal endothelium, retinal pigmented epithelium, and photoreceptors. Additionally, our results indicate that the donation process depends on F-actin-based tunneling nanotubes. Rotenone-pretreated cells that received mitochondria from MSCs displayed increased aerobic capacity and upregulation of mitochondrial genes. Furthermore, living imaging determined the ultimate fate of transferred mitochondria through either degradation by lysosomes or exocytosis as extracellular vesicles. Conclusions: For the first time, we determined the characteristics and fate of mitochondria undergoing intercellular transfer from MSCs to various ocular cells through F-actin-based tunneling nanotubes, helping to characterize MSC-based treatment for ocular tissue regeneration.

OPA1: How much do we know to approach therapy?

OPA1 is a GTPase that controls several functions, such as mitochondrial dynamics and energetics, mtDNA maintenance and cristae integrity. In the last years, there have been described other cellular pathways and mechanisms involving OPA1 directly or through its interaction. All this new information, by implementing our knowledge on OPA1 is instrumental to elucidating the pathogenic mechanisms of OPA1 mutations. Indeed, these are associated with dominant optic atrophy (DOA), one of the most common inherited optic neuropathies, and with an increasing number of heterogeneous neurodegenerative disorders. In this review, we overview all recent findings on OPA1 protein functions, on its dysfunction and related clinical phenotypes, focusing on the current therapeutic options and future perspectives to treat DOA and the other associated neurological disorders due to OPA1 mutations.

Eight human OPA1 isoforms, long and short: What are they for?

OPA1 is a dynamin-related GTPase that controls mitochondrial dynamics, cristae integrity, energetics and mtDNA maintenance. The exceptional complexity of this protein is determined by the presence, in humans, of eight different isoforms that, in turn, are proteolytically cleaved into combinations of membrane-anchored long forms and soluble short forms. Recent advances highlight how each OPA1 isoform is able to fulfill "essential" mitochondrial functions, whereas only some variants carry out "specialized" features. Long forms determine fusion, long or short forms alone build cristae, whereas long and short forms together tune mitochondrial morphology. These findings offer novel challenging therapeutic potential to gene therapy.

OPA1 gene therapy prevents retinal ganglion cell loss in a Dominant Optic Atrophy mouse model.

Dominant optic atrophy (DOA) is a rare progressive and irreversible blinding disease which is one of the most frequent forms of hereditary optic neuropathy. DOA is mainly caused by dominant mutation in the OPA1 gene encoding a large mitochondrial GTPase with crucial roles in membrane dynamics and cell survival. Hereditary optic neuropathies are commonly characterized by the degeneration of retinal ganglion cells, leading to the optic nerve atrophy and the progressive loss of visual acuity. Up to now, despite increasing advances in the understanding of the pathological mechanisms, DOA remains intractable. Here, we tested the efficiency of gene therapy on a genetically-modified mouse model reproducing DOA vision loss. We performed intravitreal injections of an Adeno-Associated Virus carrying the human OPA1 cDNA under the control of the cytomegalovirus promotor. Our results provide the first evidence that gene therapy is efficient on a mouse model of DOA as the wild-type OPA1 expression is able to alleviate the OPA1-induced retinal ganglion cell degeneration, the hallmark of the disease. These results displayed encouraging effects of gene therapy for Dominant Optic Atrophy, fostering future investigations aiming at clinical trials in patients.

Dominant Optic Atrophy and Leber's Hereditary Optic Neuropathy: Update on Clinical Features and Current Therapeutic Approaches.

Dominant optic atrophy (DOA) and Leber hereditary optic neuropathy (LHON) are the two most common inherited optic neuropathies encountered in clinical practice. This review provides a summary of recent advances in the understanding of the clinical manifestations, current treatments, and ongoing clinical trials of these two optic neuropathies. Substantial progress has been made in the understanding of the clinical, genetic, and pathophysiological basis of DOA and LHON. Pathogenic OPA1 gene mutations in DOA and 3 primary mutations of mitochondrial DNA in LHON-induced mitochondrial dysfunction, which in turn leads to increased reactive oxygen species levels in mitochondria and possibly insufficient ATP production. The pathologic hallmark of these inherited optic neuropathies is primary degeneration of retinal ganglion cells, preferentially in the papillomacular bundle, which results in temporal optic disc pallor and central or cecocentral visual loss. There are no effective treatments for patients with LHON and DOA, although clinical trials are underway for the former. Translational research for these diseases is entering an accelerated phase with the availability of animal models, and a variety of pharmacological and genetic therapies are being developed.

A Review of Mitochondrial Optic Neuropathies: From Inherited to Acquired Forms.

In recent years, the term mitochondrial optic neuropathy (MON) has increasingly been used within the literature to describe a group of optic neuropathies that exhibit mitochondrial dysfunction in retinal ganglion cells (RGCs). Interestingly, MONs include genetic aetiologies, such as Leber hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA), as well as acquired aetiologies resulting from drugs, nutritional deficiencies, and mixed aetiologies. Regardless of an inherited or acquired cause, patients exhibit the same clinical manifestations with selective loss of the RGCs due to mitochondrial dysfunction. Various novel therapies are being explored to reverse or limit damage to the RGCs. Here we review the pathophysiology, clinical manifestations, differential diagnosis, current treatment, and promising therapeutic targets of MON.

Emerging Mitochondrial Therapeutic Targets in Optic Neuropathies.

Optic neuropathies are an important cause of blindness worldwide. The study of the most common inherited mitochondrial optic neuropathies, Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (ADOA) has highlighted a fundamental role for mitochondrial function in the survival of the affected neuron-the retinal ganglion cell. A picture is now emerging that links mitochondrial dysfunction to optic nerve disease and other neurodegenerative processes. Insights gained from the peculiar susceptibility of retinal ganglion cells to mitochondrial dysfunction are likely to inform therapeutic development for glaucoma and other common neurodegenerative diseases of aging. Despite it being a fast-evolving field of research, a lack of access to human ocular tissues and limited animal models of mitochondrial disease have prevented direct retinal ganglion cell experimentation and delayed the development of efficient therapeutic strategies to prevent vision loss. Currently, there are no approved treatments for mitochondrial disease, including optic neuropathies caused by primary or secondary mitochondrial dysfunction. Recent advances in eye research have provided important insights into the molecular mechanisms that mediate pathogenesis, and new therapeutic strategies including gene correction approaches are currently being investigated. Here, we review the general principles of mitochondrial biology relevant to retinal ganglion cell function and provide an overview of the major optic neuropathies with mitochondrial involvement, LHON and ADOA, whilst highlighting the emerging link between mitochondrial dysfunction and glaucoma. The pharmacological strategies currently being trialed to improve mitochondrial dysfunction in these optic neuropathies are discussed in addition to emerging therapeutic approaches to preserve retinal ganglion cell function.

Therapeutic Approaches to Inherited Optic Neuropathies.

As a group, inherited optic neuropathies represent an important cause of severe irreversible visual loss among children and young adults. Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (DOA) are the two most common forms encountered in clinical practice and several shared disease pathways have emerged that contribute to retinal ganglion cell loss, and eventually visual failure. In this review, the author critically appraises the evidence base for the various therapeutic strategies that have been put forward to treat these two mitochondrially determined optic neuropathies, including future developments. Innovative in vitro fertilization techniques to prevent female carriers of childbearing age from transmitting pathogenic mitochondrial DNA mutations to their biological children will also be discussed.

Treatment strategies for inherited optic neuropathies: past, present and future.

Bilateral visual loss secondary to inherited optic neuropathies is an important cause of registrable blindness among children and young adults. The two prototypal disorders seen in clinical practice are Leber hereditary optic neuropathy (LHON) and autosomal dominant optic atrophy (DOA). About 90% of LHON cases are due to one of three mitochondrial DNA (mtDNA) point mutations: m.3460G>A, m.11778G>A, and m.14484T>C, which affect critical complex I subunits of the mitochondrial respiratory chain. The majority of patients with DOA harbour pathogenic mutations within OPA1, a nuclear gene that codes for a multifunctional inner mitochondrial membrane protein. Despite their contrasting genetic basis, LHON and DOA share overlapping pathological and clinical features that serve to highlight the striking tissue-specific vulnerability of the retinal ganglion cell (RGC) layer to disturbed mitochondrial function. In addition to severe visual loss secondary to progressive optic nerve degeneration, a subgroup of patients will also develop a more aggressive syndromic phenotype marked by significant neurological deficits. The management of LHON and DOA remains largely supportive, but major advances in our understanding of the mechanisms underpinning RGC loss in these two disorders are paving the way for novel forms of treatment aimed at halting or reversing visual deterioration at different stages of the disease process. In addition to neuroprotective strategies for rescuing RGCs from irreversible cell death, innovative in vitro fertilisation techniques are providing the tantalising prospect of preventing the germline transmission of pathogenic mtDNA mutations, eradicating in so doing the risk of disease in future generations.

Mitochondrial dysfunction in optic neuropathies: animal models and therapeutic options.

PURPOSE OF REVIEW: We review the recent advances in animal models generated to study the complexities of mitochondrial optic neuropathies and the therapeutic strategies proposed for these disorders. RECENT FINDINGS: We have recently witnessed a rapid proliferation of animal models attempting to recapitulate the clinical and pathogenic features of human genetic mitochondrial optic neuropathies, that is Leber's hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA). Although the generation of an animal model of disorders due to nuclear gene defects is well established and technically feasible, for mitochondrial DNA (mtDNA)-based diseases, there have been major limitations. Notwithstanding these difficulties, various approaches circumvented the problem by proposing biochemical or tissue-specific delivery models of mutant mtDNA able to induce retinal ganglion cell disease, contextually providing gene therapy solutions. Recently, the first mito-mice model of LHON has also been reported. In addition to gene therapy, new generation quinone-derived molecules and other strategies based on pharmacological activation of mitochondrial biogenesis are currently being tested, with the first clinical trials being initiated in humans. SUMMARY: Major advancements have been achieved in delivering mtDNA to mitochondria and generating faithful animal models of mtDNA-based optic neuropathy. The availability of these approaches, including animal models of nuclear-encoded optic neuropathies, provides unprecedented opportunities to test therapies, both genetic and pharmacological, paving the road to clinical trials in humans.

Treatment of hereditary optic neuropathies.

The hereditary optic neuropathies are inherited disorders in which optic nerve dysfunction is a prominent feature in the phenotypic expression of disease. Optic neuropathy may be primarily an isolated finding, such as in Leber hereditary optic neuropathy and dominant optic atrophy, or part of a multisystem disorder. The pathophysiological mechanisms underlying the hereditary optic neuropathies involve mitochondrial dysfunction owing to mutations in mitochondrial or nuclear DNA that encodes proteins essential to mitochondrial function. Effective treatments are limited, and current management includes therapies directed at enhancing mitochondrial function and preventing oxidative damage, as well as genetic counselling, and supportive and symptomatic measures. New therapies, including gene therapy, are emerging via animal models and human clinical trials. Leber hereditary optic neuropathy, in particular, provides a unique model for testing promising treatments owing to its characteristic sequential bilateral involvement and the accessibility of target tissue within the eye. Lessons learned from treatment of the hereditary optic neuropathies may have therapeutic implications for other disorders of presumed mitochondrial dysfunction. In this Review, the natural history of the common inherited optic neuropathies, the presumed pathogenesis of several of these disorders, and the literature to date regarding potential therapies are summarized.

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.