Idebenone treatment in patients with OPA1-mutant dominant optic atrophy

Idebenone Treatment in Patients with OPA1-Dominant Optic Atrophy: A Prospective Phase 2 Trial

The aim of this study was to evaluate the therapeutic effect of idebenone in patients with OPA1-dominant optic atrophy (DOA). Sixteen patients with genetically confirmed OPA1-DOA were treated with 900 mg idebenone daily for 12 months. The primary endpoint was the best recovery/least deterioration of visual acuity. Secondary endpoints were the changes of visual acuity, colour vision, contrast sensitivity, visual field, peripapillary retinal nerve fibre layer thickness (pRNFLT), and visual-related quality of life. For the primary endpoint, a significant increase was observed for the right eye (p = .0027), for the left eye (p = .0111) and for the better-seeing eye (p = .0152). For visual fields, a significant improvement was observed for the left eye between baseline and 9 months (p = .0038). Regarding pRNFLT, a significant decrease was found for the left eye between baseline and 3 months (p = .0413) and between baseline and 6 months (p = .0448). In the visual function questionnaire, a significant improvement was observed in the subscale general vision (p = .0156) and in the composite score (p = .0256). In conclusion, best recovery of visual acuity improved, even though the amount of improvement was small. Furthermore, a maintenance of visual function after 12 months of idebenone intake could be observed as well as a significant improvement in vision-related quality of life.Whether this effect is due to idebenone treatment, the placebo effect, or is explainable by the natural progression of DOA, remains unclear. Trial registration: EU Clinical Trials Register, EudraCT Number: 2019-001493-28.

OPA1 deficiency impairs oxidative metabolism in cycling cells, underlining a translational approach for degenerative diseases

Dominant optic atrophy is an optic neuropathy with varying clinical symptoms and progression. A severe disorder is associated with certain OPA1 mutations and includes additional symptoms for >20% of patients. This underscores the consequences of OPA1 mutations in different cellular populations, not only retinal ganglionic cells. We assessed the effects of OPA1 loss of function on oxidative metabolism and antioxidant defences using an RNA-silencing strategy in a human epithelial cell line. We observed a decrease in the mitochondrial respiratory chain complexes, associated with a reduction in aconitase activity related to an increase in reactive oxygen species (ROS) production. In response, the NRF2 (also known as NFE2L2) transcription factor was translocated into the nucleus and upregulated SOD1 and GSTP1. This study highlights the effects of OPA1 deficiency on oxidative metabolism in replicative cells, as already shown in neurons. It underlines a translational process to use cycling cells to circumvent and describe oxidative metabolism. Moreover, it paves the way to predict the evolution of dominant optic atrophy using mathematical models that consider mitochondrial ROS production and their detoxifying pathways.

Drug Drop Test: How to Quickly Identify Potential Therapeutic Compounds for Mitochondrial Diseases Using Yeast Saccharomyces cerevisiae

Mitochondrial diseases (MDs) refer to a group of clinically and genetically heterogeneous pathologies characterized by defective mitochondrial function and energy production. Unfortunately, there is no effective treatment for most MDs, and current therapeutic management is limited to relieving symptoms. The yeast Saccharomyces cerevisiae has been efficiently used as a model organism to study mitochondria-related disorders thanks to its easy manipulation and well-known mitochondrial biogenesis and metabolism. It has been successfully exploited both to validate alleged pathogenic variants identified in patients and to discover potential beneficial molecules for their treatment. The so-called "drug drop test", a phenotype-based high-throughput screening, especially if coupled with a drug repurposing approach, allows the identification of molecules with high translational potential in a cost-effective and time-saving manner. In addition to drug identification, S. cerevisiae can be used to point out the drug's target or pathway. To date, drug drop tests have been successfully carried out for a variety of disease models, leading to very promising results. The most relevant aspect is that studies on more complex model organisms confirmed the effectiveness of the drugs, strengthening the results obtained in yeast and demonstrating the usefulness of this screening as a novel approach to revealing new therapeutic molecules for MDs.

OPA1 Dominant Optic Atrophy: Diagnostic Approach in the Pediatric Population

A clinical and genetic study was conducted with pediatric patients and their relatives with optic atrophy 1 (OPA1) mutations to establish whether there is a genotype-phenotype correlation among the variants detected within and between families. Eleven children with a confirmed OPA1 mutation were identified during the study period. The main initial complaint was reduced visual acuity (VA), present in eight patients of the cohort. Eight of eleven patients had a positive family history of optic atrophy. The mean visual acuity at the start of the study was 0.40 and 0.44 LogMAR in the right and left eye, respectively. At the end of the study, the mean visual acuity was unchanged. Optical coherence tomography during the first visit showed a mean retinal nerve fiber layer thickness of 81.6 microns and 80.5 microns in the right and left eye, respectively; a mean ganglion cell layer of 52.5 and 52.4 microns, respectively, and a mean central macular thickness of 229.5 and 233.5 microns, respectively. The most common visual field defect was a centrocecal scotoma, and nine out of eleven patients showed bilateral temporal disc pallor at baseline. Sequencing of OPA1 showed seven different mutations in the eleven patients, one of which, NM_130837.3: c.1406_1407del (p.Thr469LysfsTer16), has not been previously reported. Early diagnosis of dominant optic atrophy is crucial, both for avoiding unnecessary consultations and/or treatments and for appropriate genetic counseling.

Mitochondrial Fission and Fusion: Molecular Mechanisms, Biological Functions, and Related Disorders

Mitochondria are dynamic organelles that undergo fusion and fission. These active processes occur continuously and simultaneously and are mediated by nuclear-DNA-encoded proteins that act on mitochondrial membranes. The balance between fusion and fission determines the mitochondrial morphology and adapts it to the metabolic needs of the cells. Therefore, these two processes are crucial to optimize mitochondrial function and its bioenergetics abilities. Defects in mitochondrial proteins involved in fission and fusion due to pathogenic variants in the genes encoding them result in disruption of the equilibrium between fission and fusion, leading to a group of mitochondrial diseases termed disorders of mitochondrial dynamics. In this review, the molecular mechanisms and biological functions of mitochondrial fusion and fission are first discussed. Then, mitochondrial disorders caused by defects in fission and fusion are summarized, including disorders related to MFN2, MSTO1, OPA1, YME1L1, FBXL4, DNM1L, and MFF genes.

First Description of Inheritance of a Postzygotic OPA1 Mosaic Variant

Optic atrophy 1 (MIM #165500) is caused by pathogenic variants in the gene OPA1 (OPA1 MITOCHONDRIAL DYNAMIN-LIKE GTPase, MIM *605290) and is inherited in an autosomal dominant manner. We describe a 6-year-old male patient with severe early onset manifestation of optic atrophy, whose parents are subjectively asymptomatic. OPA1-sequence analysis revealed the heterozygous missense variant NM_015560.3:c.806C>T, p.(Ser269Phe) in the patient. Segregation analysis of the parents showed that the mother carried a low-grade postzygotic mosaic of this variant, which apparently also involves germline cells. In line with this, ophthalmological investigation of the mother showed subclinical manifestation of optic atrophy 1. This is the first report of an OPA1 postzygotic mosaic that was inherited to offspring.

Role of Oxidative Stress in Ocular Diseases Associated with Retinal Ganglion Cells Degeneration

Ocular diseases associated with retinal ganglion cell (RGC) degeneration is the most common neurodegenerative disorder that causes irreversible blindness worldwide. It is characterized by visual field defects and progressive optic nerve atrophy. The underlying pathophysiology and mechanisms of RGC degeneration in several ocular diseases remain largely unknown. RGCs are a population of central nervous system neurons, with their soma located in the retina and long axons that extend through the optic nerve to form distal terminals and connections in the brain. Because of this unique cytoarchitecture and highly compartmentalized energy demand, RGCs are highly mitochondrial-dependent for adenosine triphosphate (ATP) production. Recently, oxidative stress and mitochondrial dysfunction have been found to be the principal mechanisms in RGC degeneration as well as in other neurodegenerative disorders. Here, we review the role of oxidative stress in several ocular diseases associated with RGC degenerations, including glaucoma, hereditary optic atrophy, inflammatory optic neuritis, ischemic optic neuropathy, traumatic optic neuropathy, and drug toxicity. We also review experimental approaches using cell and animal models for research on the underlying mechanisms of RGC degeneration. Lastly, we discuss the application of antioxidants as a potential future therapy for the ocular diseases associated with RGC degenerations.

Leigh Syndrome: A Tale of Two Genomes

Leigh syndrome is a rare, complex, and incurable early onset (typically infant or early childhood) mitochondrial disorder with both phenotypic and genetic heterogeneity. The heterogeneous nature of this disorder, based in part on the complexity of mitochondrial genetics, and the significant interactions between the nuclear and mitochondrial genomes has made it particularly challenging to research and develop therapies. This review article discusses some of the advances that have been made in the field to date. While the prognosis is poor with no current substantial treatment options, multiple studies are underway to understand the etiology, pathogenesis, and pathophysiology of Leigh syndrome. With advances in available research tools leading to a better understanding of the mitochondria in health and disease, there is hope for novel treatment options in the future.

The Present and Future of Mitochondrial-Based Therapeutics for Eye Disease

Translational Relevance

Mitochondria are viable therapeutic targets for a broad spectrum of ocular diseases.

Therapeutic Options in Hereditary Optic Neuropathies

Options for the effective treatment of hereditary optic neuropathies have been a long time coming. The successful launch of the antioxidant idebenone for Leber's Hereditary Optic Neuropathy (LHON), followed by its introduction into clinical practice across Europe, was an important step forward. Nevertheless, other options, especially for a variety of mitochondrial optic neuropathies such as dominant optic atrophy (DOA), are needed, and a number of pharmaceutical agents, acting on different molecular pathways, are currently under development. These include gene therapy, which has reached Phase III development for LHON, but is expected to be  developed also for DOA, whilst most of the other agents (other antioxidants, anti-apoptotic drugs, activators of mitobiogenesis, etc.) are almost all at Phase II or at preclinical stage of research. Here, we review proposed target mechanisms, preclinical evidence, available clinical trials with primary endpoints and results, of a wide range of tested molecules, to give an overview of the field, also providing the landscape of future scenarios, including gene therapy, gene editing, and reproductive options to prevent transmission of mitochondrial DNA mutations.

Drug repositioning as a therapeutic strategy for neurodegenerations associated with OPA1 mutations

OPA1 mutations are the major cause of dominant optic atrophy (DOA) and the syndromic form DOA plus, pathologies for which there is no established cure. We used a ‘drug repurposing’ approach to identify FDA-approved molecules able to rescue the mitochondrial dysfunctions induced by OPA1 mutations. We screened two different chemical libraries by using two yeast strains carrying the mgm1^I322M and the chim3^P646L mutations, identifying 26 drugs able to rescue their oxidative growth phenotype. Six of them, able to reduce the mitochondrial DNA instability in yeast, have been then tested in Opa1 deleted mouse embryonic fibroblasts expressing the human OPA1 isoform 1 bearing the R445H and D603H mutations. Some of these molecules were able to ameliorate the energetic functions and/or the mitochondrial network morphology, depending on the type of OPA1 mutation. The final validation has been performed in patients’ fibroblasts, allowing to select the most effective molecules. Our current results are instrumental to rapidly translating the findings of this drug repurposing approach into clinical trial for DOA and other neurodegenerations caused by OPA1 mutations.

Therapeutic Approaches to Treat Mitochondrial Diseases: "One-Size-Fits-All" and "Precision Medicine" Strategies

Primary mitochondrial diseases (PMD) refer to a group of severe, often inherited genetic conditions due to mutations in the mitochondrial genome or in the nuclear genes encoding for proteins involved in oxidative phosphorylation (OXPHOS). The mutations hamper the last step of aerobic metabolism, affecting the primary source of cellular ATP synthesis. Mitochondrial diseases are characterized by extremely heterogeneous symptoms, ranging from organ-specific to multisystemic dysfunction with different clinical courses. The limited information of the natural history, the limitations of currently available preclinical models, coupled with the large variability of phenotypical presentations of PMD patients, have strongly penalized the development of effective therapies. However, new therapeutic strategies have been emerging, often with promising preclinical and clinical results. Here we review the state of the art on experimental treatments for mitochondrial diseases, presenting "one-size-fits-all" approaches and precision medicine strategies. Finally, we propose novel perspective therapeutic plans, either based on preclinical studies or currently used for other genetic or metabolic diseases that could be transferred to PMD.

Nutritional Optic Neuropathies: State of the Art and Emerging Evidences

Nutritional optic neuropathy is a cause of bilateral, symmetrical, and progressive visual impairment with loss of central visual acuity and contrast sensitivity, dyschromatopsia, and a central or centrocecal scotoma. The clinical features are not pathognomonic, since hereditary and toxic forms share similar signs and symptoms. It is becoming increasingly common due to the widespread of bariatric surgery and strict vegetarian or vegan diets, so even the scientific interest has recently increased. In particular, recent studies have focused on possible pathogenetic mechanisms, and on novel diagnostic and therapeutic strategies in order to prevent the onset, make a prompt diagnosis and an accurate nutritional supplementation, and to avoid irreversible optic nerve atrophy. Nowadays, there is clear evidence of the role of cobalamin, folic acid, thiamine, and copper, whereas further studies are needed to define the role of niacin, riboflavin, and pyridoxine. This review aims to summarize the etiology, diagnosis, and treatment of nutritional optic neuropathy, and it is addressed not only to ophthalmologists, but to all physicians who could come in contact with a patient with a possible nutritional optic neuropathy, being a fundamental multidisciplinary approach.

Comparative In Vitro Toxicology of Novel Cytoprotective Short-Chain Naphthoquinones

Short-chain quinones (SCQs) have been identified as potential drug candidates against mitochondrial dysfunction, which largely depends on the reversible redox characteristics of the active quinone core. We recently identified 11 naphthoquinone derivatives, 1–11, from a library of SCQs that demonstrated enhanced cytoprotection and improved metabolic stability compared to the clinically used benzoquinone idebenone. Since the toxicity properties of our promising SCQs were unknown, this study developed multiplex methods and generated detailed toxicity profiles from 11 endpoint measurements using the human hepatocarcinoma cell line HepG2. Overall, the toxicity profiles were largely comparable across different assays, with simple standard assays showing increased sensitivity compared to commercial toxicity assays. Within the 11 naphthoquinones tested, the L-phenylalanine derivative 4 consistently demonstrated the lowest toxicity across all assays. The results of this study not only provide useful information about the toxicity features of SCQs but will also enable the progression of the most promising drug candidates towards their clinical use.

Idebenone increases chance of stabilization/recovery of visual acuity in OPA1-dominant optic atrophy

We previously documented that idebenone treatment in OPA1‐Dominant Optic Atrophy (OPA1‐DOA) led to some degrees of visual improvement in seven patients. We here present the results of a cohort study, which investigated the effect of off‐label idebenone administration in a larger OPA1‐DOA group compared with untreated patients. Inclusion criteria were: OPA1‐DOA clinical and molecular diagnosis, baseline visual acuity (VA) greater than/equal to counting fingers and treatment duration greater than 7 months. We found a significant difference between the last visit and baseline VA in favor of stabilization/recovery in idebenone‐treated as compared to untreated patients. This effect was retained after controlling for confounders.

Mitochondrial disorders and the eye

Mitochondria are cellular organelles that play a key role in energy metabolism and oxidative phosphorylation. Malfunctioning of mitochondria has been implicated as the cause of many disorders with variable inheritance, heterogeneity of systems involved, and varied phenotype. Metabolically active tissues are more likely to be affected, causing an anatomic and physiologic disconnect in the treating physicians' mind between presentation and underlying pathophysiology. We shall focus on disorders of mitochondrial metabolism relevant to an ophthalmologist. These disorders can affect all parts of the visual pathway (crystalline lens, extraocular muscles, retina, optic nerve, and retrochiasm). After the introduction reviewing mitochondrial structure and function, each disorder is reviewed in detail, including approaches to its diagnosis and most current management guidelines.

Functional wiring of proteostatic and mitostatic modules ensures transient organismal survival during imbalanced mitochondrial dynamics

Being an assembly of protein machines, cells depend on adequate supply of energetic molecules for retaining their homeodynamics. Consequently, mitochondria functionality is ensured by quality control systems and mitochondrial dynamics (fusion/fission). Similarly, proteome stability is maintained by the machineries of the proteostasis network. We report here that reduced mitochondrial fusion rates in Drosophila caused developmental lethality or if induced in the adult accelerated aging. Imbalanced mitochondrial dynamics were tolerable for various periods in young flies, where they caused oxidative stress and proteome instability that mobilized Nrf2 and foxo to upregulate cytoprotective antioxidant/proteostatic modules. Consistently, proteasome inhibition or Nrf2, foxo knock down in young flies exaggerated perturbed mitochondrial dynamics toxicity. Neither Nrf2 overexpression (with concomitant proteasome activation) nor Atg8a upregulation suppressed the deregulated mitochondrial dynamics toxicity, which was mildly mitigated by antioxidants. Thus, despite extensive functional wiring of mitostatic and antioxidant/proteostatic modules, sustained loss-of mitostasis exhausts adaptation responses triggering premature aging.

•

Reduced mitochondrial fusion rates cause severe organismal toxicity and progeria.

•

Perturbed mitostasis activates cytoprotective antioxidant and proteostatic modules.

•

Nrf2 or Foxo KD exaggerates the imbalanced mitochondrial dynamics induced toxicity.

•

Antioxidants mildly alleviate loss-of mitochondrial dynamics-mediated progeria.

The idebenone metabolite QS10 restores electron transfer in complex I and coenzyme Q defects

Idebenone is a hydrophilic short-chain coenzyme (Co) Q analogue, which has been used as a potential bypass of defective complex I in both Leber Hereditary Optic Neuropathy and OPA1-dependent Dominant Optic Atrophy. Based on its potential antioxidant effects, it has also been tested in degenerative disorders such as Friedreich's ataxia, Huntington's and Alzheimer's diseases. Idebenone is rapidly modified but the biological effects of its metabolites have been characterized only partially. Here we have studied the effects of quinones generated during in vivo metabolism of idebenone with specific emphasis on 6-(9-carboxynonyl)-2,3-dimethoxy-5-methyl-1,4-benzoquinone (QS10). QS10 partially restored respiration in cells deficient of complex I or of CoQ without inducing the mitochondrial permeability transition, a detrimental effect of idebenone that may offset its potential benefits [Giorgio et al. (2012) Biochim. Biophys. Acta 1817: 363-369]. Remarkably, respiration was largely rotenone-insensitive in complex I deficient cells and rotenone-sensitive in CoQ deficient cells. These findings indicate that, like idebenone, QS10 can provide a bypass to defective complex I; and that, unlike idebenone, QS10 can partially replace endogenous CoQ. In zebrafish (Danio rerio) treated with rotenone, QS10 was more effective than idebenone in allowing partial recovery of respiration (to 40% and 20% of the basal respiration of untreated embryos, respectively) and allowing zebrafish survival (80% surviving embryos at 60 h post-fertilization, a time point at which all rotenone-treated embryos otherwise died). We conclude that QS10 is potentially more active than idebenone in the treatment of diseases caused by complex I defects, and that it could also be used in CoQ deficiencies of genetic and acquired origin.

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.

Mitochondrial disorders of the retinal ganglion cells and the optic nerve

OBJECTIVES

To summarise and discuss recent findings and future perspectives concerning mitochondrial disorders (MIDs) affecting the retinal ganglion cells and the optic nerve (mitochondrial optic neuropathy. MON).

METHOD

Literature review.

RESULTS

MON in MIDs is more frequent than usually anticipated. MON may occur in specific as well as non-specific MIDs. In specific and non-specific MIDs, MON may be a prominent or non-prominent phenotypic feature and due to mutations in genes located either in the mitochondrial DNA (mtDNA) or the nuclear DNA (nDNA). Clinically, MON manifests with painless, bilateral or unilateral, slowly or rapidly progressive visual impairment and visual field defects. In some cases, visual impairment may spontaneously recover. The most frequent MIDs with MON include LHON due to mutations in mtDNA-located genes and autosomal dominant optic atrophy (ADOA) or autosomal recessive optic atrophy (AROA) due to mutations in nuclear genes. Instrumental investigations for diagnosing MON include fundoscopy, measurement of visual acuity, visual fields, and color vision, visually-evoked potentials, optical coherence tomography, fluorescein angiography, electroretinography, and MRI of the orbita and cerebrum. In non-prominent MON, work-up of the muscle biopsy with transmission electron microscopy may indicate mitochondrial destruction. Treatment is mostly supportive but idebenone has been approved for LHON and experimental approaches are promising.

CONCLUSIONS

MON needs to be appreciated, requires extensive diagnostic work-up, and supportive treatment should be applied although loss of vision, as the most severe outcome, can often not be prevented.

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.

Incomplete penetrance in mitochondrial optic neuropathies

Incomplete penetrance characterizes the two most frequent inherited optic neuropathies, Leber's Hereditary Optic Neuropathy (LHON) and dominant optic atrophy (DOA), due to genetic errors in the mitochondrial DNA (mtDNA) and the nuclear DNA (nDNA), respectively. For LHON, compelling evidence has accumulated on the complex interplay of mtDNA haplogroups and environmental interacting factors, whereas the nDNA remains essentially non informative. However, a compensatory mechanism of activated mitochondrial biogenesis and increased mtDNA copy number, possibly driven by a permissive nDNA background, is documented in LHON; when successful it maintains unaffected the mutation carriers, but in some individuals it might be hampered by tobacco smoking or other environmental factors, resulting in disease onset. In females, mitochondrial biogenesis is promoted and maintained within the compensatory range by estrogens, partially explaining the gender bias in LHON. Concerning DOA, none of the above mechanisms has been fully explored, thus mtDNA haplogroups, environmental factors such as tobacco and alcohol, and further nDNA variants may all participate as protective factors or, on the contrary, favor disease expression and severity. Next generation sequencing, complemented by transcriptomics and proteomics, may provide some answers in the next future, even if the multifactorial model that seems to apply to incomplete penetrance in mitochondrial optic neuropathies remains problematic, and careful stratification of patients will play a key role for data interpretation. The deep understanding of which factors impinge on incomplete penetrance may shed light on the pathogenic mechanisms leading to optic nerve atrophy, on their possible compensation and, thus, on development of therapeutic strategies.

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.

Characterization of two novel intronic OPA1 mutations resulting in aberrant pre-mRNA splicing

BACKGROUND

We report two novel splice region mutations in OPA1 in two unrelated families presenting with autosomal-dominant optic atrophy type 1 (ADOA1) (ADOA or Kjer type optic atrophy). Mutations in OPA1 encoding a mitochondrial inner membrane protein are a major cause of ADOA.

METHODS

We analyzed two unrelated families including four affected individuals clinically suspicious of ADOA. Standard ocular examinations were performed in affected individuals of both families. All coding exons, as well as exon-intron boundaries of the OPA1 gene were sequenced. In addition, multiplex ligation-dependent probe amplification (MLPA) was performed to uncover copy number variations in OPA1. mRNA processing was monitored using RT-PCR and subsequent cDNA analysis.

RESULTS

We report two novel splice region mutations in OPA1 in two unrelated individuals and their affected relatives, which were previously not described in the literature. In one family the heterozygous insertion and deletion c.[611-37_611-38insACTGGAGAATGTAAAGGGCTTT;611-6_611-16delCATATTTATCT] was found in all investigated family members leading to the activation of an intronic cryptic splice site. In the second family sequencing of OPA1 disclosed a de novo heterozygous deletion c.2012+4_2012+7delAGTA resulting in exon 18 and 19 skipping, which was not detected in healthy family members.

CONCLUSION

We identified two novel intronic mutations in OPA1 affecting the correct OPA1 pre-mRNA splicing, which was confirmed by OPA1 cDNA analysis. This study shows the importance of transcript analysis to determine the consequences of unclear intronic mutations in OPA1 in proximity to the intron-exon boundaries.

Optic Nerve

Optic nerve diseases arise from many different etiologies including inflammatory, neoplastic, genetic, infectious, ischemic, and idiopathic. Understanding some of the characteristics of the most common optic neuropathies along with therapeutic approaches to these diseases is helpful in designing recommendations for individual patients. Although many optic neuropathies have no specific treatment, some do, and it is those potentially treatable or preventable conditions which need to be recognized in order to help patients regain their sight or develop a better understanding of their own prognosis. In this chapter several diseases are discussed including idiopathic intracranial hypertension, optic neuritis, ischemic optic neuropathies, hereditary optic neuropathies, trauma, and primary tumors of the optic nerve. For each condition there is a presentation of the signs and symptoms of the disease, in some conditions the evaluation and diagnostic criteria are highlighted, and where possible, current therapy or past trials are discussed.

Loss of functional OPA1 unbalances redox state: implications in dominant optic atrophy pathogenesis

Objective

OPA1 mutations cause protein haploinsufficiency leading to dominant optic atrophy (DOA), an incurable retinopathy with variable severity. Up to 20% of patients also develop extraocular neurological complications. The mechanisms that cause this optic atrophy or its syndromic forms are still unknown. After identifying oxidative stress in a mouse model of the pathology, we sought to determine the consequences of OPA1 dysfunction on redox homeostasis.

Methods

Mitochondrial respiration, reactive oxygen species levels, antioxidant defenses, and cell death were characterized by biochemical and in situ approaches in both in vitro and in vivo models of OPA1 haploinsufficiency.

Results

A decrease in aconitase activity suggesting an increase in reactive oxygene species and an induction of antioxidant defenses was observed in cortices of a murine model as well as in OPA1 downregulated cortical neurons. This increase is associated with a decline in mitochondrial respiration in vitro. Upon exogenous oxidative stress, OPA1‐depleted neurons did not further exhibit upregulated antioxidant defenses but were more sensitive to cell death. Finally, low levels of antioxidant enzymes were found in fibroblasts from patients supporting their role as modifier factors.

Interpretation

Our study suggests that the pro‐oxidative state induced by OPA1 loss may contribute to DOA pathogenesis and that differences in antioxidant defenses can explain the variability in expressivity. Furthermore, antioxidants may be used as therapy as they could prevent or delay DOA symptoms in patients.

A randomized, placebo-controlled trial of the benzoquinone idebenone in a mouse model of OPA1-related dominant optic atrophy reveals a limited therapeutic effect on retinal ganglion cell dendropathy and visual function

Dominant optic atrophy (DOA) arises from mutations in the OPA1 gene that promotes fusion of the inner mitochondrial membrane and plays a role in maintaining ATP levels. Patients display optic disc pallor, retinal ganglion cell (RGC) loss and bilaterally reduced vision. We report a randomized, placebo-controlled trial of idebenone at 2000 mg/kg/day in 56 Opa1 mutant mice (B6;C3-Opa1(Q285STOP)), with RGC dendropathy and visual loss, and 63 wildtype mice. We assessed cellular responses in the retina, brain and liver and RGC morphology, by diolistic labeling, Sholl analysis and quantification of dendritic morphometric features. Vision was assessed by optokinetic responses. ATP levels were raised by 0.57 nmol/mg (97.73%, p=0.035) in brain from idebenone-treated Opa1 mutant mice, but in the liver there was an 80.35% (p=0.011) increase in oxidative damage. NQO1 expression in Opa1 mutant mice was reduced in the brain (to 30.5%, p=0.002) but not in retina, and neither expression level was induced by idebenone. ON-center RGCs failed to show major recovery, other than improvements in secondary dendritic length (by 53.89%, p=0.052) and dendritic territory (by 2.22 × 10(4) μm(2) or 90.24%, p=0.074). An improvement in optokinetic response was observed (by 12.2 ± 3.2s, p=0.003), but this effect was not sustained over time. OFF-center RGCs from idebenone-treated wildtype mice showed shrinkage in total dendritic length by 2.40 mm (48.05%, p=0.025) and a 47.37% diminished Sholl profile (p=0.029). Visual function in wildtype idebenone-treated mice was impaired (2.9 fewer head turns than placebo, p=0.007). Idebenone appears largely ineffective in protecting Opa1 heterozygous RGCs from dendropathy. The detrimental effect of idebenone in wildtype mice has not been previously observed and raises some concerns.

Targeting mitochondrial function to protect against vision loss

INTRODUCTION

Mitochondria, essential to multicellular life, convert food into ATP to satisfy cellular energy demands. Since different tissues have different energy requirements, mitochondrial density is high in tissues with high metabolic needs, such as the visual system, which is therefore highly susceptible to limited energy supply as a result of mitochondrial dysfunction.

AREAS COVERED

Vision impairment is a common feature of most mitochondrial diseases. At the same time, there is mounting evidence that mitochondrial impairment contributes to the pathogenesis of major eye diseases such as glaucoma and might also be involved in the reported vision impairment in neurodegenerative disorders such as Alzheimer's disease.

EXPERT OPINION

Rather than relying on symptomatic treatment, acknowledging the mitochondrial origin of visual disorders in mitochondrial, neurodegenerative and ocular diseases could lead to novel therapeutics that aim to modulate mitochondrial function in order to protect against vision loss. This approach has already shown some promising clinical results in inherited retinal disorders, which supports the idea that targeting mitochondria could also be a treatment option for other optic neuropathies.

Oxidative stress, mitochondrial and proteostasis malfunction in adrenoleukodystrophy: A paradigm for axonal degeneration

Peroxisomal and mitochondrial malfunction, which are highly intertwined through redox regulation, in combination with defective proteostasis, are hallmarks of the most prevalent multifactorial neurodegenerative diseases-including Alzheimer's (AD) and Parkinson's disease (PD)-and of the aging process, and are also found in inherited conditions. Here we review the interplay between oxidative stress and axonal degeneration, taking as groundwork recent findings on pathomechanisms of the peroxisomal neurometabolic disease adrenoleukodystrophy (X-ALD). We explore the impact of chronic redox imbalance caused by the excess of very long-chain fatty acids (VLCFA) on mitochondrial respiration and biogenesis, and discuss how this impairs protein quality control mechanisms essential for neural cell survival, such as the proteasome and autophagy systems. As consequence, prime molecular targets in the pathogenetic cascade emerge, such as the SIRT1/PGC-1α axis of mitochondrial biogenesis, and the inhibitor of autophagy mTOR. Thus, we propose that mitochondria-targeted antioxidants; mitochondrial biogenesis boosters such as the antidiabetic pioglitazone and the SIRT1 ligand resveratrol; and the autophagy activator temsirolimus, a derivative of the mTOR inhibitor rapamycin, hold promise as disease-modifying therapies for X-ALD.

Diffusion Tensor Imaging Mapping of Brain White Matter Pathology in Mitochondrial Optic Neuropathies

BACKGROUND AND PURPOSE

Brain white matter is frequently affected in mitochondrial diseases; optic atrophy gene 1-autosomal dominant optic atrophy and Leber hereditary optic neuropathy are the most frequent mitochondrial monosymptomatic optic neuropathies. In this observational study, brain white matter microstructure was characterized by DTI in patients with optic atrophy gene 1-autosomal dominant optic atrophy and Leber hereditary optic neuropathy, in relation to clinical and genetic features.

MATERIALS AND METHODS

Nineteen patients with optic atrophy gene 1-autosomal dominant optic atrophy and 17 with Leber hereditary optic neuropathy older than 18 years of age, all genetically diagnosed, and 19 healthy volunteers underwent DTI by using a 1.5T MR imaging scanner and neurologic and ophthalmologic assessments. Brain white matter DTI metrics were calculated for all participants, and, in patients, their correlations with genetics and clinical findings were calculated.

RESULTS

Compared with controls, patients with optic atrophy gene 1-autosomal dominant optic atrophy had an increased mean diffusivity in 29.2% of voxels analyzed within major white matter tracts distributed throughout the brain, while fractional anisotropy was reduced in 30.3% of voxels. For patients with Leber hereditary optic neuropathy, the proportion of altered voxels was only 0.5% and 5.5%, respectively, of which half was found within the optic radiation and 3.5%, in the smaller acoustic radiation. In almost all regions, fractional anisotropy diminished with age in patients with optic atrophy gene 1-autosomal dominant optic atrophy and correlated with average retinal nerve fiber layer thickness in several areas. Mean diffusivity increased in those with a missense mutation. Patients with Leber hereditary optic neuropathy taking idebenone had slightly milder changes.

CONCLUSIONS

Patients with Leber hereditary optic neuropathy had preferential involvement of the optic and acoustic radiations, consistent with trans-synaptic degeneration, whereas patients with optic atrophy gene 1-autosomal dominant optic atrophy presented with widespread involvement suggestive of a multisystemic, possibly a congenital/developmental, disorder. White matter changes in Leber hereditary optic neuropathy and optic atrophy gene 1-autosomal dominant optic atrophy may be exploitable as biomarkers.

Strategies for treating mitochondrial disorders: an update

Mitochondrial diseases are a heterogeneous group of disorders resulting from primary dysfunction of the respiratory chain due to both nuclear and mitochondrial DNA mutations. The wide heterogeneity of biochemical dysfunctions and pathogenic mechanisms typical of this group of diseases has hindered therapy trials; therefore, available treatment options remain limited. Therapeutic strategies aimed at increasing mitochondrial functions (by enhancing biogenesis and electron transport chain function), improving the removal of reactive oxygen species and noxious metabolites, modulating aberrant calcium homeostasis and repopulating mitochondrial DNA could potentially restore the respiratory chain dysfunction. The challenge that lies ahead is the translation of some promising laboratory results into safe and effective therapies for patients. In this review we briefly update and discuss the most feasible therapeutic approaches for mitochondrial diseases.

Cell biology. Metabolic control of cell death

Beyond their contribution to basic metabolism, the major cellular organelles, in particular mitochondria, can determine whether cells respond to stress in an adaptive or suicidal manner. Thus, mitochondria can continuously adapt their shape to changing bioenergetic demands as they are subjected to quality control by autophagy, or they can undergo a lethal permeabilization process that initiates apoptosis. Along similar lines, multiple proteins involved in metabolic circuitries, including oxidative phosphorylation and transport of metabolites across membranes, may participate in the regulated or catastrophic dismantling of organelles. Many factors that were initially characterized as cell death regulators are now known to physically or functionally interact with metabolic enzymes. Thus, several metabolic cues regulate the propensity of cells to activate self-destructive programs, in part by acting on nutrient sensors. This suggests the existence of "metabolic checkpoints" that dictate cell fate in response to metabolic fluctuations. Here, we discuss recent insights into the intersection between metabolism and cell death regulation that have major implications for the comprehension and manipulation of unwarranted cell loss.

Early macular retinal ganglion cell loss in dominant optic atrophy: genotype-phenotype correlation

PURPOSE

To assess the peripapillary retinal nerve fiber and macular retinal ganglion cell (RGC) loss in patients with dominant optic atrophy (DOA) stratified by OPA1 mutation type.

DESIGN

Cross-sectional study.

METHODS

We studied 39 patients from 28 pedigrees with DOA harboring heterozygous mutations in the OPA1 gene along with 45 age-matched healthy subjects. The retinal nerve fiber layer (RNFL) and ganglion cell-inner plexiform layer (GC-IPL) of patients with DOA were evaluated by optical coherence tomography (OCT) and compared to those of controls. Patients' eyes were divided into 4 groups based on increasing severity of visual loss (DOA1 to DOA4) and were stratified by OPA1 mutation type.

RESULTS

The average thicknesses of the RNFL and GC-IPL were smaller in patients with DOA than in healthy controls (P < 0.0001). RNFL analysis showed a significant reduction of the average, superior and inferior quadrants thicknesses in the DOA4 group compared to the DOA1 group (P = 0.001, P = 0.002 and P = 0.001, respectively). GC-IPL analysis showed a significant thinning in the superotemporal and superior sectors in the patients with DOA2 compared to those with DOA1 (P = 0.046 and P = 0.04, respectively). Stratifying by mutation type, average, superior and nasal RNFL thinning was significantly more severe in missense mutations and had a presumed dominant-negative effect compared to mutations causing haploinsufficiency.

CONCLUSIONS

The present study demonstrates that in DOA, loss of macular RGCs is the earliest pathologic event, better reflected by GC-IPL measurements, whereas RNFL thickness is a measure of spared axons in late stages of the disease. Thus, mild cases (DOA2) show significant macular RGC loss as opposed to substantial maintenance of RNFL thickness, which is significantly decreased only in severe cases (DOA4). A clear genotype/phenotype correlation emerged, stratifying OCT measures by OPA1 mutation type, missense mutations being the most severe.

Medical management of hereditary optic neuropathies

Hereditary optic neuropathies are diseases affecting the optic nerve. The most common are mitochondrial hereditary optic neuropathies, i.e., the maternally inherited Leber's hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA). They both share a mitochondrial pathogenesis that leads to the selective loss of retinal ganglion cells and axons, in particular of the papillo-macular bundle. Typically, LHON is characterized by an acute/subacute loss of central vision associated with impairment of color vision and swelling of retinal nerve fibers followed by optic atrophy. DOA, instead, is characterized by a childhood-onset and slowly progressive loss of central vision, worsening over the years, leading to optic atrophy. The diagnostic workup includes neuro-ophthalmologic evaluation and genetic testing of the three most common mitochondrial DNA mutations affecting complex I (11778/ND4, 3460/ND1, and 14484/ND6) for LHON and sequencing of the nuclear gene OPA1 for DOA. Therapeutic strategies are still limited including agents that bypass the complex I defect and exert an antioxidant effect (idebenone). Further strategies are aimed at stimulating compensatory mitochondrial biogenesis. Gene therapy is also a promising avenue that still needs to be validated.

Genetics and ocular disorders: a focused review

Increasingly accurate phenotyping leads to better genetic evaluation. Genetic eye conditions may be due to a common cellar defect (eg, ciliopathies or RASopathies). Early-onset retinal dystrophies may be associated with renal disease. An understanding of genetic testing helps clinicians identify shortcomings in testing which may lead to a better understanding of the most appropriate test for a given ocular condition. Dedicated genetic counselors within ophthalmic and pediatric clinics are likely to improve the delivery of clinical care in these settings.

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.

Peripheral neuropathy in mitochondrial disorders

Why is peripheral neuropathy common but mild in many mitochondrial disorders, and why is it, in some cases, the predominant or only manifestation? Although this question remains largely unanswered, recent advances in cellular and molecular biology have begun to clarify the importance of mitochondrial functioning and distribution in the peripheral nerve. Mutations in proteins involved in mitochondrial dynamics (ie, fusion and fission) frequently result in a Charcot-Marie-Tooth phenotype. Peripheral neuropathies with different phenotypic presentations occur in mitochondrial diseases associated with abnormalities in mitochondrial DNA replication and maintenance, or associated with defects in mitochondrial respiratory chain complex V. Our knowledge of mitochondrial disorders is rapidly growing as new nuclear genes are identified and new phenotypes described. Early diagnosis of mitochondrial disorders, essential to provide appropriate genetic counselling, has become crucial in a few treatable conditions. Recognising and diagnosing an underlying mitochondrial defect in patients presenting with peripheral neuropathy is therefore of paramount importance.

Mitochondrial optic neuropathies: our travels from bench to bedside and back again

The standard scientific method requires that you make an interesting observation, generate a hypothesis and then design an experiment to test the hypothesis. In ophthalmology, as in most fields of medicine, the observations and hypotheses tend to have more degrees of freedom, and the interpretation of experiments is also more complicated and often indeterminate. But sometimes it works out, going back and forth from bench to bedside to bench, in reiterative cycles. A successful example of alternating bench and bedside studies was presented (AAS) at the 2012 Alper Memorial given at the Washington Hospital Medical Center, illustrating a series of questions and investigations that pertain to mitochondrial optic neuropathies, beginning two decades ago, before the concept of mitochondrial optic neuropathies had much meaning. Basic science questions are often best answered by that extraordinary experiment of nature that we call clinical disease, and clinical questions are often best tested in the laboratory.