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

[Leber's hereditary optic neuropathy]

Leber's hereditary optic atrophy (LHON) is a genetic optic neuropathy that is more prevalent in young males but can occur from childhood to old age. The primary cause is mitochondrial genetic mutations, which are associated with dysfunction of mitochondrial electron transport chain complex I. It manifests as acute to subacute visual impairment, often starting unilaterally but progressing to involve both eyes within weeks to months. Visual loss is severe, with many patients having corrected visual acuity below 0.1. The differential diagnosis of optic neuritis is essential, and assessments such as pupillary light reflex, fluorescein fundus angiography, and magnetic resonance imaging can be useful for differentiation. LHON should be considered as one of the differential diagnoses for optic neuritis, and collaboration between neurologists and ophthalmologists is crucial for accurate diagnosis and appropriate treatment.

The Role of Optic Nerve Diffusion Restriction on Visual Prognosis in Methanol Poisoning

The aim of this study was to investigate the role of optic nerve diffusion status on cranio-orbital magnetic resonance imaging (MRI) in predicting visual prognosis in cases of methanol intoxication. Diffusion-weighted imaging (DWI) from 16 eyes of eight patients who were admitted to our clinic due to methanol intoxication was analysed retrospectively. The relationship between clinical and laboratory findings, treatment regimen, visual prognosis, and imaging findings was investigated. Diffusion restriction (DR) of the optic nerve on DWI was observed in seven (43%) eyes. Regardless of the clinical and laboratory characteristics and treatment regimen, visual acuity (VA) improved in eyes in which restricted diffusion regressed over the follow-up period. DWI of the optic nerve during the acute phase of methanol poisoning may provide prognostically important data. Improvement of DR during follow-up may be an indicator of an increase in VA.

Leber Hereditary Optic Neuropathy in a Family of Carriers of MT-ND5 m.13042G>T (A236S) Novel Variant

BACKGROUND

A Slovenian three-generation family with 3 individuals with bilateral optic neuropathy and 2 unaffected relatives with a novel homoplasmic missense variant m.13042G > T (A236S) in the ND5 gene is described. A detailed phenotype at initial diagnosis and a follow-up of bilateral optic neuropathy progression is presented for 2 affected individuals.

METHODS

A detailed phenotype analysis with clinical examination in the early and chronic phase with electrophysiology and OCT segmentation is presented. Genotype analysis with full mitochondrial genome sequencing was performed.

RESULTS

Two affected male individuals (maternal cousins) had a profound visual loss at an early age (11 and 20 years) with no recovery. The maternal grandmother exhibited bilateral optic atrophy with a history of visual loss at the age 58 years. The visual loss of both affected male individuals was characterized by centrocecal scotoma, abnormal color vision, abnormal PERG N95, and VEP. Later with disease progression, retinal nerve fiber layer thinning was observed on OCT. We observed no other extraocular clinical features. Mitochondrial sequencing identified a homoplasmic novel variant m.13042G > T (A236S) in the MT-ND5 gene, belonging to a haplogroup K1a.

CONCLUSIONS

Novel homoplasmic variant m.13042G > T (A236S) in the ND5 gene in our family was associated with Leber hereditary optic neuropathy-like phenotype. However, predicting the pathogenicity of a novel ultra-rare missense variant in the mitochondrial ND5 gene is challenging. Genetic counseling should consider genotypic and phenotypic heterogeneity, incomplete penetrance, haplogroup type, and tissue-specific thresholds.

Leber Hereditary Optic Neuropathy in Southwestern Ontario: A Growing List of Mutations

BACKGROUND

Leber hereditary optic neuropathy (LHON) is a rare but bilaterally blinding disease. Three characteristic disease-causing point mutations, and other less common mutations, are most often found on the mitochondrially encoded genes of NADH-ubiquinone oxidoreductase core subunits (MT-ND). The purpose of this study is to provide an overview of LHON mutations in Southwestern Ontario and to describe the associated demographic and clinical characteristics.

METHODS

A retrospective genetic and clinical chart review was performed from January 2015 to 2020. Patients were identified within a mitochondrial mutation database and included if a mutation was detected on the MT-ND1, -ND4, or -ND6 genes. A clinical chart review was done on all available patients.

RESULTS

Forty-five of 63 patients identified had classic disease-causing mutations (6.7% m.3460G>A, 44.4% m.11778G>A, and 48.9% m.14484T>C). Several of the remaining 18 patients had rare mutations previously documented in association with LHON. Of the 14 patients with clinical charts accessible for review, 12 had symptomatic disease, and all but one had bilateral optic neuropathies. Nine patients had classic LHON mutations and 3 had possible novel mutations; 7 were males; 9 had final visual acuity ≤ 20/200 in at least one eye; and 6 of those had ≤20/400 in both eyes.

CONCLUSIONS

This study adds to the literature on LHON in Canada, and specifically Southwestern Ontario. The demographic and clinical data regarding LHON in this geographic location, as well as possible novel disease-causing mutations, provide important information to aid clinicians in recognizing cases of LHON that may otherwise be disregarded.

Current and Future Landscape in Genetic Therapies for Leber Hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) is the most common primary mitochondrial genetic disease that causes blindness in young adults. Over 50 inherited mitochondrial DNA (mtDNA) variations are associated with LHON; however, more than 95% of cases are caused by one of three missense variations (m.11778 G > A, m.3460 G > A, and m.14484 T > C) encoding for subunits ND4, ND1, and ND6 of the respiration complex I, respectively. These variants remain silent until further and currently poorly understood genetic and environmental factors precipitate the visual loss. The clinical course that ensues is variable, and a convincing treatment for LHON has yet to emerge. In 2015, an antioxidant idebenone (Raxone) received European marketing authorisation to treat visual impairment in patients with LHON, and since then it was introduced into clinical practice in several European countries. Alternative therapeutic strategies, including gene therapy and gene editing, antioxidant and neurotrophic agents, mitochondrial biogenesis, mitochondrial replacement, and stem cell therapies are being investigated in how effective they might be in altering the course of the disease. Allotopic gene therapies are in the most advanced stage of development (phase III clinical trials) whilst most other agents are in phase I or II trials or at pre-clinical stages. This manuscript discusses the phenotype and genotype of the LHON disease with complexities and peculiarities such as incomplete penetrance and gender bias, which have challenged the therapies in development emphasising the most recent use of gene therapy. Furthermore, we review the latest results of the three clinical trials based on adeno-associated viral (AAV) vector-mediated delivery of NADH dehydrogenase subunit 4 (ND4) with mitochondrial targeting sequence, highlighting the differences in the vector design and the rationale behind their use in the allotopic transfer.

Rapid and Sensitive Diagnosis of Leber Hereditary Optic Neuropathy Variants Using CRISPR/Cas12a Detection

Leber hereditary optic neuropathy (LHON) is the most common maternally inherited mitochondrial disease, with >90% of cases harboring one of three point variants (m.3460G>A, m.11778G>A, and m.14484T>C). Rapid and sensitive diagnosis of LHON variants is urgently needed for early diagnosis and timely treatment after onset, which is currently limited. Herein, we adapted the Cas12a-based DNA detection platform for LHON mitochondrial variant diagnosis. Single-strand guide CRISPR RNAs and enzymatic recombinase amplification primers were first screened, the CRISPR/Cas12a system was then optimized with restriction enzymes, and finally compared with Sanger sequencing and next-generation sequencing (NGS) in multicenter clinical samples. This approach can be completed within 30 minutes using only one drop of blood and could reach a sensitivity of 1% of heteroplasmy. Among the 182 multicenter clinical samples, the CRISPR/Cas12a detection system showed high consistency with Sanger sequencing and NGS in both specificity and sensitivity. Notably, a sample harboring a de novo 3.78% m.11778G>A variant detected by NGS, but not by Sanger sequencing, was successfully confirmed using the CRISPR/Cas12a assay, which proved the effectiveness of our method. Overall, our CRISPR/Cas12a detection system provides an alternative for rapid, convenient, and sensitive detection of LHON variants, exhibiting great potential for clinical practice.

Advantages of mesenchymal stem cell over the other stem cells

A stem cell is a particular group of cells that has the extraordinary potential to convert within the body into particular cell types. They are used to regenerate tissues and cells in the body that have been damaged or destroyed by the disease. Stem cells come in three different varieties: adult stem cells, embryonic stem cells and induced pluripotent stem cells (iPSCs). Embryonic stem cells have a high chance of immune rejection and also have ethical dilemmas and iPSCs have genetic instability. Adult stem cells are difficult to analyze and extract for research since they are frequently insufficient in native tissues. However, mesenchymal stem cells (MSC) one of the categories of adult stem cells are stromal cells with a variety of potentials that can differentiate into a wide range of cell types. MSCs can be transplanted into a variety of people without worrying about rejection because they have demonstrated the ability to prevent an adverse reaction from the immune system. These transplants have powerful anti-inflammatory and immunosuppressive effects and greatly enhance the body's inherent healing capacity. While MSCs do not offer treatment for illnesses, the idea behind them is to enable the body to recover sufficiently for a protracted reduction in symptoms. In many cases, this is sufficient to significantly enhance the patient's well-being. Inspite of several advantages some potential long-term concerns connected to MSC therapy are maldifferentiation, immunosuppression and cancerous tumor growth. In this review, we will compare the mesenchymal stem cells with other stem cells with respect to the source of origin, their properties and therapeutic applications, and discuss the MSC's disadvantages.

[Rare pathogenic nucleotide variants of mitochondrial DNA associated with Leber's hereditary optic neuropathy]

Patients with Leber Hereditary Optic Neuropathy (LHON) in most cases have one of the three most common mutations: m.11778G>A in the ND4 gene, m.3460G>A in the ND1 gene, or m.14484T>C in the ND6 gene. According to the international Mitomap database, in addition to these three most common mutations, there are 16 other primary mutations that are even more rare. There are nucleotide substitutions that are classified as candidate or conditionally pathogenic mutations. Their involvement in the disease development is not proven due to insufficient research. Moreover, in many publications, the authors describe new primary and potential mitochondrial DNA mutations associated with LHON, which are not yet included in the genetic data bases. This makes it possible to expand the diagnostic spectrum during genetic testing in the future. The advancements in genetic diagnostic technologies allow confirmation of the clinical diagnosis of LHON. The importance of genetic verification of the disease is determined by the existing problem of differential diagnosis of hereditary optic neuropathies with optic neuropathies of a different origin.

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.

Mid51/Fis1 mitochondrial oligomerization complex drives lysosomal untethering and network dynamics

Lysosomes are highly dynamic organelles implicated in multiple diseases. Using live super-resolution microscopy, we found that lysosomal tethering events rarely undergo lysosomal fusion, but rather untether over time to reorganize the lysosomal network. Inter-lysosomal untethering events are driven by a mitochondrial Mid51/Fis1 complex that undergoes coupled oligomerization on the outer mitochondrial membrane. Importantly, Fis1 oligomerization mediates TBC1D15 (Rab7-GAP) mitochondrial recruitment to drive inter-lysosomal untethering via Rab7 GTP hydrolysis. Moreover, inhibiting Fis1 oligomerization by either mutant Fis1 or a Mid51 oligomerization mutant potentially associated with Parkinson’s disease prevents lysosomal untethering events, resulting in misregulated lysosomal network dynamics. In contrast, dominant optic atrophy–linked mutant Mid51, which does not inhibit Mid51/Fis1 coupled oligomerization, does not disrupt downstream lysosomal dynamics. As Fis1 conversely also regulates Mid51 oligomerization, our work further highlights an oligomeric Mid51/Fis1 mitochondrial complex that mechanistically couples together both Drp1 and Rab7 GTP hydrolysis machinery at mitochondria–lysosome contact sites. These findings have significant implications for organelle networks in cellular homeostasis and human disease.

Toxic and nutritional factors trigger leber hereditary optic neuropathy due to a mitochondrial tRNA mutation

Leber hereditary optic neuropathy is a mitochondrial disease mainly due to pathologic mutations in mitochondrial genes related to the respiratory complex I of the oxidative phosphorylation system. Genetic, physiological, and environmental factors modulate the penetrance of these mutations. We report two patients suffering from this disease and harboring a m.15950G > A mutation in the mitochondrial DNA‐encoded gene for the threonine transfer RNA. We also provide evidences supporting the pathogenicity of this mutation.

Clinical Overview of Leber Hereditary Optic Neuropathy

Leber hereditary ptic neuropathy (LHON) is a disease of young adults with bilateral, painless, subacute visual loss. The peak age of onset of LHON is in the second and third decades of life. Men are 4 times more likely to be affected than women. In about 25-50% of cases, both eyes are affected simultaneously. In unilateral cases, the other eye is usually affected 2 to 3 months later. Visual acuity deteriorates to counting fingers or worse with a dense central or centrocecal scotoma. In the subacute phase, the optic disc may appear hyperemic with swelling of the peripapillary retinal nerve fibre layer, peripapillary telangiectasias, and increased vascular tortuosity. Ocular coherence tomography of the macula shows marked thinning of the ganglion cell complex even at this stage. The diagnosis of LHON is made in a subject with a consistent clinical history and/or one of three common pathogenic mitochondrial DNA (mtDNA) variants identified by molecular genetic testing. Idebenone was approved by the European Medicines Agency under exceptional circumstances for the treatment of LHON. Current evidence suggests some benefit to vision in a subset of affected individuals treated with idebenone, particularly when treated within the first year of onset of vision loss. In this article, we discuss aetiology, clinical features, diagnosis, differential dignosis, prognosis and treatment.

Multishell Diffusion MR Tractography Yields Morphological and Microstructural Information of the Anterior Optic Pathway: A Proof-of-Concept Study in Patients with Leber’s Hereditary Optic Neuropathy

Tractography based on multishell diffusion-weighted magnetic resonance imaging (DWI) can be used to estimate the course of myelinated white matter tracts and nerves, yielding valuable information regarding normal anatomy and variability. DWI is sensitive to the local tissue microstructure, so tractography can be used to estimate tissue properties within nerve tracts at a resolution of millimeters. This study aimed to test the applicability of the method using a disease with a well-established pattern of myelinated nerve involvement. Eight patients with LHON and 13 age-matched healthy controls underwent tractography of the anterior optic pathway. Diffusion parameters were compared between groups, and for the patient group correlated with clinical/ophthalmological parameters. Tractography established the course of the anterior optic pathway in both patients and controls. Localized changes in fractional anisotropy were observed, and related to estimates of different tissue compartments within the nerve and tract. The proportion of different compartments correlated with markers of disease severity. The method described allows both anatomical localization and tissue characterization in vivo, permitting both visualization of variation at the individual level and statistical inference at the group level. It provides a valuable adjunct to ex vivo anatomical and histological study of normal variation and disease processes.

Gene therapy restores mitochondrial function and protects retinal ganglion cells in optic neuropathy induced by a mito-targeted mutant ND1 gene

Therapies for genetic disorders caused by mutated mitochondrial DNA are an unmet need, in large part due barriers in delivering DNA to the organelle and the absence of relevant animal models. We injected into mouse eyes a mitochondrially targeted Adeno-Associated-Virus (MTS-AAV) to deliver the mutant human NADH ubiquinone oxidoreductase subunit I (hND1/m.3460G>A) responsible for Leber’s hereditary optic neuropathy, the most common primary mitochondrial genetic disease. We show that the expression of the mutant hND1 delivered to retinal ganglion cells (RGC) layer colocalizes with the mitochondrial marker PORIN and the assembly of the expressed hND1 protein into host respiration complex I. The hND1 injected eyes exhibit hallmarks of the human disease with progressive loss of RGC function and number, as well as optic nerve degeneration. We also show that gene therapy in the hND1 eyes by means of an injection of a second MTS-AAV vector carrying wild type human ND1 restores mitochondrial respiratory complex I activity, the rate of ATP synthesis and protects RGCs and their axons from dysfunction and degeneration. These results prove that MTS-AAV is a highly efficient gene delivery approach with the ability to create mito-animal models and has the therapeutic potential to treat mitochondrial genetic diseases.

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.

DNAJC30 defect: a frequent cause of recessive Leber hereditary optic neuropathy and Leigh syndrome

The recent description of biallelic DNAJC30 variants in Leber hereditary optic neuropathy (LHON) and Leigh syndrome challenged the longstanding assumption for LHON to be exclusively maternally inherited and broadened the genetic spectrum of Leigh syndrome, the most frequent paediatric mitochondrial disease. Herein, we characterize 28 so far unreported individuals from 26 families carrying a homozygous DNAJC30 p.Tyr51Cys founder variant, 24 manifesting with LHON, two manifesting with Leigh syndrome, and two remaining asymptomatic. This collection of unreported variant carriers confirms sex-dependent incomplete penetrance of the homozygous variant given a significant male predominance of disease and the report of asymptomatic homozygous variant carriers. The autosomal recessive LHON patients demonstrate an earlier age of disease onset and a higher rate of idebenone-treated and spontaneous recovery of vision in comparison to reported figures for maternally inherited disease. Moreover, the report of two additional patients with childhood- or adult-onset Leigh syndrome further evidences the association of DNAJC30 with Leigh syndrome, previously only reported in a single childhood-onset case.

Methanol-induced optic neuropathy: a still-present problem

Methanol-induced optic neuropathy (Me-ION) is a serious condition that may result in long-term or irreversible visual impairment or even blindness secondary to damage and loss of function of the optic nerve and retina. Me-ION shows a tendency to occur as mass poisonings around the world with a clear predilection for poor societies in developing countries. The main mechanism underlying the molecular basis of Me-ION is the inhibition of the mitochondrial oxidative phosphorylation process through the binding of the toxic metabolite of methanol—formic acid—with the key enzyme of this process—cytochrome c oxidase. However, other mechanisms, including damage to the eye tissues by oxidative stress causing the intensification of the oxidative peroxidation process with the formation of cytotoxic compounds, as well as an increase in the synthesis of pro-inflammatory cytokines and influence on the expression of key proteins responsible for maintaining cell homeostasis, also play an important role in the pathogenesis of Me-ION. Histopathological changes in the eye tissues are mainly manifested as the degeneration of axons and glial cells of the optic nerve, often with accompanying damage of the retina that may involve all its layers. Despite the development of therapeutic approaches, persistent visual sequelae are seen in 30–40% of survivors. Thus, Me-ION continues to be an important problem for healthcare systems worldwide.

Diversified Treatment Options of Adult Stem Cells for Optic Neuropathies

Optic neuropathies refer to a group of ocular disorders with abnormalities or dysfunction of the optic nerve, sharing a common pathophysiology of retinal ganglion cell (RGC) death and axonal loss. RGCs, as the retinal neurons in the central nervous system, show limited capacity in regeneration or recovery upon diseases or after injuries. Critically, there is still no effective clinical treatment to cure most types of optic neuropathies. Recently, stem cell therapy was proposed as a potential treatment strategy for optic neuropathies. Adult stem cells, including mesenchymal stem cells and hematopoietic stem cells, have been applied in clinical trials based on their neuroprotective properties. In this article, the applications of adult stem cells on different types of optic neuropathies and the related mechanisms will be reviewed. Research updates on the strategies to enhance the neuroprotective effects of human adult stem cells will be summarized. This review article aims to enlighten the research scientists on the diversified functions of adult stem cells and consideration of adult stem cells as a potential treatment for optic neuropathies in future clinical practices.

Mitochondrial Genome Study Identifies Association Between Primary Open-Angle Glaucoma and Variants in MT-CYB, MT-ND4 Genes and Haplogroups

Background and purpose: Primary open-angle glaucoma (POAG) is an optic neuropathy characterized by death of retinal ganglion cells and atrophy of the optic nerve head. The susceptibility of the optic nerve to damage has been shown to be mediated by mitochondrial dysfunction. In this study, we aimed to determine a possible association between mitochondrial SNPs or haplogroups and POAG.

Methods: Mitochondrial DNA single nucleotide polymorphisms (mtSNPs) were genotyped using the Illumina Infinium Global Screening Array-24 (GSA) 700K array set. Genetic analyses were performed in a POAG case-control study involving the cohorts, Groningen Longitudinal Glaucoma Study-Lifelines Cohort Study and Amsterdam Glaucoma Study, including 721 patients and 1951 controls in total. We excluded samples not passing quality control for nuclear genotypes and samples with low call rate for mitochondrial variation. The mitochondrial variants were analyzed both as SNPs and haplogroups. These were determined with the bioinformatics software HaploGrep, and logistic regression analysis was used for the association, as well as for SNPs.

Results: Meta-analysis of the results from both cohorts revealed a significant association between POAG and the allele A of rs2853496 [odds ratio (OR) = 0.64; p = 0.006] within the MT-ND4 gene, and for the T allele of rs35788393 (OR = 0.75; p = 0.041) located in the MT-CYB gene. In the mitochondrial haplogroup analysis, the most significant p-value was reached by haplogroup K (p = 1.2 × 10⁻⁰⁵), which increases the risk of POAG with an OR of 5.8 (95% CI 2.7–13.1).

Conclusion: We identified an association between POAG and polymorphisms in the mitochondrial genes MT-ND4 (rs2853496) and MT-CYB (rs35788393), and with haplogroup K. The present study provides further evidence that mitochondrial genome variations are implicated in POAG. Further genetic and functional studies are required to substantiate the association between mitochondrial gene polymorphisms and POAG and to define the pathophysiological mechanisms of mitochondrial dysfunction in glaucoma.

Oxidative Stress in Optic Neuropathies

Increasing evidence indicates that changes in the redox system may contribute to the pathogenesis of multiple optic neuropathies. Optic neuropathies are characterized by the neurodegeneration of the inner-most retinal neurons, the retinal ganglion cells (RGCs), and their axons, which form the optic nerve. Often, optic neuropathies are asymptomatic until advanced stages, when visual impairment or blindness is unavoidable despite existing treatments. In this review, we describe systemic and, whenever possible, ocular redox dysregulations observed in patients with glaucoma, ischemic optic neuropathy, optic neuritis, hereditary optic neuropathies (i.e., Leber's hereditary optic neuropathy and autosomal dominant optic atrophy), nutritional and toxic optic neuropathies, and optic disc drusen. We discuss aspects related to anti/oxidative stress biomarkers that need further investigation and features related to study design that should be optimized to generate more valuable and comparable results. Understanding the role of oxidative stress in optic neuropathies can serve to develop therapeutic strategies directed at the redox system to arrest the neurodegenerative processes in the retina and RGCs and ultimately prevent vision loss.

Leber hereditary optic neuropathy-new insights and old challenges

Leber hereditary optic neuropathy (LHON) is the most common primary mitochondrial DNA (mtDNA) disorder with the majority of patients harboring one of three primary mtDNA point mutations, namely, m.3460G>A (MTND1), m.11778G>A (MTND4), and m.14484T>C (MTND6). LHON is characterized by bilateral subacute loss of vision due to the preferential loss of retinal ganglion cells (RGCs) within the inner retina, resulting in optic nerve degeneration. This review describes the clinical features associated with mtDNA LHON mutations and recent insights gained into the disease mechanisms contributing to RGC loss in this mitochondrial disorder. Although treatment options remain limited, LHON research has now entered an active translational phase with ongoing clinical trials, including gene therapy to correct the underlying pathogenic mtDNA mutation.

The Role of MicroRNAs in Mitochondria-Mediated Eye Diseases

The retina is among the most metabolically active tissues with high-energy demands. The peculiar distribution of mitochondria in cells of retinal layers is necessary to assure the appropriate energy supply for the transmission of the light signal. Photoreceptor cells (PRs), retinal pigment epithelium (RPE), and retinal ganglion cells (RGCs) present a great concentration of mitochondria, which makes them particularly sensitive to mitochondrial dysfunction. To date, visual loss has been extensively correlated to defective mitochondrial functions. Many mitochondrial diseases (MDs) show indeed neuro-ophthalmic manifestations, including retinal and optic nerve phenotypes. Moreover, abnormal mitochondrial functions are frequently found in the most common retinal pathologies, i.e., glaucoma, age-related macular degeneration (AMD), and diabetic retinopathy (DR), that share clinical similarities with the hereditary primary MDs. MicroRNAs (miRNAs) are established as key regulators of several developmental, physiological, and pathological processes. Dysregulated miRNA expression profiles in retinal degeneration models and in patients underline the potentiality of miRNA modulation as a possible gene/mutation-independent strategy in retinal diseases and highlight their promising role as disease predictive or prognostic biomarkers. In this review, we will summarize the current knowledge about the participation of miRNAs in both rare and common mitochondria-mediated eye diseases. Definitely, given the involvement of miRNAs in retina pathologies and therapy as well as their use as molecular biomarkers, they represent a determining target for clinical applications.

Novel mutations in malonyl-CoA-acyl carrier protein transacylase provoke autosomal recessive optic neuropathy

Inherited optic neuropathies are rare eye diseases of optic nerve dysfunction that present in various genetic forms. Previously, mutation in three genes encoding mitochondrial proteins has been implicated in autosomal recessive forms of optic atrophy that involve progressive degeneration of optic nerve and retinal ganglion cells (RGC). Using whole exome analysis, a novel double homozygous mutation p.L81R and pR212W in malonyl CoA-acyl carrier protein transacylase (MCAT), a mitochondrial protein involved in fatty acid biosynthesis, has now been identified as responsible for an autosomal recessive optic neuropathy from a Chinese consanguineous family. MCAT is expressed in RGC that are rich in mitochondria. The disease variants lead to structurally unstable MCAT protein with significantly reduced intracellular expression. RGC-specific knockdown of Mcat in mice, lead to an attenuated retinal neurofiber layer, that resembles the phenotype of optic neuropathy. These results indicated that MCAT plays an essential role in mitochondrial function and maintenance of RGC axons, while novel MCAT p.L81R and p.R212W mutations can lead to optic neuropathy.

The Role of Mitochondria in Carcinogenesis

The mitochondria are essential for normal cell functioning. Changes in mitochondrial DNA (mtDNA) may affect the occurrence of some chronic diseases and cancer. This process is complex and not entirely understood. The assignment to a particular mitochondrial haplogroup may be a factor that either contributes to cancer development or reduces its likelihood. Mutations in mtDNA occurring via an increase in reactive oxygen species may favour the occurrence of further changes both in mitochondrial and nuclear DNA. Mitochondrial DNA mutations in postmitotic cells are not inherited, but may play a role both in initiation and progression of cancer. One of the first discovered polymorphisms associated with cancer was in the gene NADH-ubiquinone oxidoreductase chain 3 (mt-ND3) and it was typical of haplogroup N. In prostate cancer, these mutations and polymorphisms involve a gene encoding subunit I of respiratory complex IV cytochrome c oxidase subunit 1 gene (COI). At present, a growing number of studies also address the impact of mtDNA polymorphisms on prognosis in cancer patients. Some of the mitochondrial DNA polymorphisms occur in both chronic disease and cancer, for instance polymorphism G5913A characteristic of prostate cancer and hypertension.

Humoral immune responses to AAV gene therapy in the ocular compartment

Viral vectors can be utilised to deliver therapeutic genes to diseased cells. Adeno-associated virus (AAV) is a commonly used viral vector that is favoured for its ability to infect a wide range of tissues whilst displaying limited toxicity and immunogenicity. Most humans harbour anti-AAV neutralising antibodies (NAbs) due to subclinical infections by wild-type virus during infancy and these pre-existing NAbs can limit the efficiency of gene transfer depending on the target cell type, route of administration and choice of serotype. Vector administration can also result in de novo NAb synthesis that could limit the opportunity for repeated gene transfer to diseased sites. A number of strategies have been described in preclinical models that could circumvent NAb responses in humans, however, the successful translation of these innovations into the clinical arena has been limited. Here, we provide a comprehensive review of the humoral immune response to AAV gene therapy in the ocular compartment. We cover basic AAV biology and clinical application, the role of pre-existing and induced NAbs, and possible approaches to overcoming antibody responses. We conclude with a framework for a comprehensive strategy for circumventing humoral immune responses to AAV in the future.

Hereditary Optic Neuropathies: Induced Pluripotent Stem Cell-Based 2D/3D Approaches

Inherited optic neuropathies share visual impairment due to the degeneration of retinal ganglion cells (RGCs) as the hallmark of the disease. This group of genetic disorders are caused by mutations in nuclear genes or in the mitochondrial DNA (mtDNA). An impaired mitochondrial function is the underlying mechanism of these diseases. Currently, optic neuropathies lack an effective treatment, and the implementation of induced pluripotent stem cell (iPSC) technology would entail a huge step forward. The generation of iPSC-derived RGCs would allow faithfully modeling these disorders, and these RGCs would represent an appealing platform for drug screening as well, paving the way for a proper therapy. Here, we review the ongoing two-dimensional (2D) and three-dimensional (3D) approaches based on iPSCs and their applications, taking into account the more innovative technologies, which include tissue engineering or microfluidics.

New avenues for therapy in mitochondrial optic neuropathies

Mitochondrial optic neuropathies are a group of optic nerve atrophies exemplified by the two commonest conditions in this group, autosomal dominant optic atrophy (ADOA) and Leber's hereditary optic neuropathy (LHON). Their clinical features comprise reduced visual acuity, colour vision deficits, centro-caecal scotomas and optic disc pallor with thinning of the retinal nerve fibre layer. The primary aetiology is genetic, with underlying nuclear or mitochondrial gene mutations. The primary pathology is owing to retinal ganglion cell dysfunction and degeneration. There is currently only one approved treatment and no curative therapy is available. In this review we summarise the genetic and clinical features of ADOA and LHON and then examine what new avenues there may be for therapeutic intervention. The therapeutic strategies to manage LHON and ADOA can be split into four categories: prevention, compensation, replacement and repair. Prevention is technically an option by modifying risk factors such as smoking cessation, or by utilising pre-implantation genetic diagnosis, although this is unlikely to be applied in mitochondrial optic neuropathies due to the non-life threatening and variable nature of these conditions. Compensation involves pharmacological interventions that ameliorate the mitochondrial dysfunction at a cellular and tissue level. Replacement and repair are exciting new emerging areas. Clinical trials, both published and underway, in this area are likely to reveal future potential benefits, since new therapies are desperately needed.

Plain language summary

Optic nerve damage leading to loss of vision can be caused by a variety of insults. One group of conditions leading to optic nerve damage is caused by defects in genes that are essential for cells to make energy in small organelles called mitochondria. These conditions are known as mitochondrial optic neuropathies and two predominant examples are called autosomal dominant optic atrophy and Leber's hereditary optic neuropathy. Both conditions are caused by problems with the energy powerhouse of cells: mitochondria. The cells that are most vulnerable to this mitochondrial malfunction are called retinal ganglion cells, otherwise collectively known as the optic nerve, and they take the electrical impulse from the retina in the eye to the brain. The malfunction leads to death of some of the optic nerve cells, the degree of vision loss being linked to the number of those cells which are impacted in this way. Patients will lose visual acuity and colour vision and develop a central blind spot in their field of vision. There is currently no cure and very few treatment options. New treatments are desperately needed for patients affected by these devastating diseases. New treatments can potentially arise in four ways: prevention, compensation, replacement and repair of the defects. Here we explore how present and possible future treatments might provide hope for those suffering from these conditions.

Real-World Clinical Experience With Idebenone in the Treatment of Leber Hereditary Optic Neuropathy

BACKGROUND

Leber hereditary optic neuropathy (LHON) leads to bilateral central vision loss. In a clinical trial setting, idebenone has been shown to be safe and to provide a trend toward improved visual acuity, but long-term evidence of effectiveness in real-world clinical practice is sparse.

METHODS

Open-label, multicenter, retrospective, noncontrolled analysis of long-term visual acuity and safety in 111 LHON patients treated with idebenone (900 mg/day) in an expanded access program. Eligible patients had a confirmed mitochondrial DNA mutation and had experienced the onset of symptoms (most recent eye) within 1 year before enrollment. Data on visual acuity and adverse events were collected as per normal clinical practice. Efficacy was assessed as the proportion of patients with either a clinically relevant recovery (CRR) or a clinically relevant stabilization (CRS) of visual acuity. In the case of CRR, time to and magnitude of recovery over the course of time were also assessed.

RESULTS

At time of analysis, 87 patients had provided longitudinal efficacy data. Average treatment duration was 25.6 months. CRR was observed in 46.0% of patients. Analysis of treatment effect by duration showed that the proportion of patients with recovery and the magnitude of recovery increased with treatment duration. Average gain in best-corrected visual acuity for responders was 0.72 logarithm of the minimal angle of resolution (logMAR), equivalent to more than 7 lines on the Early Treatment Diabetic Retinopathy Study (ETDRS) chart. Furthermore, 50% of patients who had a visual acuity below 1.0 logMAR in at least one eye at initiation of treatment successfully maintained their vision below this threshold by last observation. Idebenone was well tolerated, with most adverse events classified as minor.

CONCLUSIONS

These data demonstrate the benefit of idebenone treatment in recovering lost vision and maintaining good residual vision in a real-world setting. Together, these findings indicate that idebenone treatment should be initiated early and be maintained more than 24 months to maximize efficacy. Safety results were consistent with the known safety profile of idebenone.

Haploinsufficiency due to a novel ACO2 deletion causes mitochondrial dysfunction in fibroblasts from a patient with dominant optic nerve atrophy

ACO2 is a mitochondrial protein, which is critically involved in the function of the tricarboxylic acid cycle (TCA), the maintenance of iron homeostasis, oxidative stress defense and the integrity of mitochondrial DNA (mtDNA). Mutations in the ACO2 gene were identified in patients suffering from a broad range of symptoms, including optic nerve atrophy, cortical atrophy, cerebellar atrophy, hypotonia, seizures and intellectual disabilities. In the present study, we identified a heterozygous 51 bp deletion (c.1699_1749del51) in ACO2 in a family with autosomal dominant inherited isolated optic atrophy. A complementation assay using aco1-deficient yeast revealed a growth defect for the mutant ACO2 variant substantiating a pathogenic effect of the deletion. We used patient-derived fibroblasts to characterize cellular phenotypes and found a decrease of ACO2 protein levels, while ACO2 enzyme activity was not affected compared to two age- and gender-matched control lines. Several parameters of mitochondrial function, including mitochondrial morphology, mitochondrial membrane potential or mitochondrial superoxide production, were not changed under baseline conditions. However, basal respiration, maximal respiration, and spare respiratory capacity were reduced in mutant cells. Furthermore, we observed a reduction of mtDNA copy number and reduced mtDNA transcription levels in ACO2-mutant fibroblasts. Inducing oxidative stress led to an increased susceptibility for cell death in ACO2-mutant fibroblasts compared to controls. Our study reveals that a monoallelic mutation in ACO2 is sufficient to promote mitochondrial dysfunction and increased vulnerability to oxidative stress as main drivers of cell death related to optic nerve atrophy.

OXPHOS bioenergetic compensation does not explain disease penetrance in Leber hereditary optic neuropathy

Leber hereditary optic neuropathy (LHON) is one of the most common primary mitochondrial diseases. It is caused by point mutations in mitochondrial DNA (mtDNA) genes and in some cases, it can result in irreversible vision loss, primarily in young men. It is currently unknown why LHON mutations affect only some carriers and whether bioenergetic compensation enables unaffected carriers to overcome mitochondrial impairment and preserve cellular function. Here, we conducted bioenergetic metabolic assays and RNA sequencing to address this question using male-only, age-matched, m.11778G > A lymphoblasts and primary fibroblasts from both unaffected carriers and affected individuals. Our work indicates that OXPHOS bioenergetic compensation in LHON peripheral cells does not explain disease phenotype. We show that complex I impairment is similar in cells from unaffected carrier and affected patients, despite a transcriptional downregulation of metabolic pathways including glycolysis in affected cells relative to carriers detected by RNA sequencing. Although we did not detect OXPHOS bioenergetic compensation in carrier cells under basal conditions, our results indicate that cells from affected patients suffer a growth impairment under metabolic challenge compared to carrier cells, which were unaffected by metabolic challenge. If recapitulated in retinal ganglion cells, decreased susceptibility to metabolic challenge in unaffected carriers may help preserve metabolic homeostasis in the face of the mitochondrial complex I bioenergetic defect.

A Single-Arm, Prospective, Exploratory Study to Preliminarily Test Effectiveness and Safety of Skin Electrical Stimulation for Leber Hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) is an intractable disease associated with mitochondrial DNA (mtDNA) mutations. In this preliminary, single-arm, prospective, open-label exploratory trial, we investigated the effectiveness and safety of skin electrical stimulation (SES) for cases of LHON harboring the mtDNA 11,778 mutation. Of the 11 enrolled patients, 10 completed six sessions of SES once every two weeks over a 10-week period. The primary outcome measure was the change in logarithm of the minimum angle of resolution (logMAR)-converted best-corrected visual acuity (BCVA) at one week after the last session of SES. The main secondary outcome measures were the logMAR BCVA at four and eight weeks and Humphrey visual field test sensitivities at one, four, and eight weeks. At all follow-up points, the logMAR BCVA had improved significantly from baseline, [1.80 (1.70–1.80) at baseline, 1.75 (1.52–1.80) at one week, 1.75 (1.50–1.80) at four weeks, and 1.75 (1.52–1.80) at eight weeks; p < 0.05]. At eight weeks of follow-up, five patients showed >2-fold increase in the summed sensitivity at 52 measurement points from baseline. No adverse effects were observed. In conclusion, SES could be a viable treatment option for patients with LHON in the chronic phase harboring the mtDNA 11,778 mutation.

Esculetin and idebenone ameliorate galactose-induced cataract in a rat model

Cataract is the principal cause of blindness. The enzyme, aldose reductase (AR) is a key player in polyol pathway. Buildup of polyols and oxidative stress are the main causes of cataractogenesis. This study investigated the cytoprotective properties of esculetin and idebenone in galactose-induced cataract. Rats were partitioned into four groups each of ten rats. Control group was fed with normal diet; group 2 rats were fed with galactose diet (50%); groups 3, 4 rats were fed with galactose diet concurrently with either esculetin (50 mg/kg BW) or idebenone (100 mg/kg BW), for 20 days. The study revealed that esculetin and idebenone significantly reduced the elevated levels of Bax/Bcl-2 ratio, malondialdehyde, and DNA fragmentation and increased total antioxidant capacity level in lenses compared to the cataract-induced group. Only esculetin decreased AR, galactitol, and advanced glycated end products levels in lenses. Histopathological examinations supported the biochemical findings. Esculetin and idebenone may have chemopreventive effects for sugar cataract. PRACTICAL APPLICATIONS: Cataract is an age-related disease that might cause blindness in older adult people. Presently, no absolute pharmacological treatment is accessible for cataract. The use of natural products or their derivatives attract particular attention in modern medicines as they are believed to be safer with few or no side effects. Esculetin is a polyphenolic compound found in many medicinal plants. Idebenone is a synthetic analogue of coenzyme Q10. The current study is an approach to explore the anticataract effects of esculetin and idebenone in galactose-induced cataract in rats. Our study proved that both agents have anticataractogenic potentials due to their antioxidant and antiapoptotic properties.

Neurotrophin gene therapy to promote survival of spiral ganglion neurons after deafness

Hearing impairment is a major health and economic concern worldwide. Currently, the cochlear implant (CI) is the standard of care for remediation of severe to profound hearing loss, and in general, contemporary CIs are highly successful. But there is great variability in outcomes among individuals, especially in children, with many CI users deriving much less or even marginal benefit. Much of this variability is related to differences in auditory nerve survival, and there has been substantial interest in recent years in exploring potential therapies to improve survival of the cochlear spiral ganglion neurons (SGN) after deafness. Preclinical studies using osmotic pumps and other approaches in deafened animal models to deliver neurotrophic factors (NTs) directly to the cochlea have shown promising results, especially with Brain-Derived Neurotrophic Factor (BDNF). More recent studies have focused on the use of NT gene therapy to force expression of NTs by target cells within the cochlea. This could provide the means for a one-time treatment to promote long-term NT expression and improve neural survival after deafness. This review summarizes the evidence for the efficacy of exogenous NTs in preventing SGN degeneration after hearing loss and reviews the animal research to date suggesting that NT gene therapy can elicit long-term NT expression in the cochlea, resulting in significantly improved SGN and radial nerve fiber survival after deafness. In addition, we discuss NT gene therapy in other non-auditory applications and consider some of the remaining issues with regard to selecting optimal vectors, timing of treatment, and place/method of delivery, etc. that must be resolved prior to considering clinical application.

Electrophysiological and Structural Changes in Chinese Patients with LHON

OBJECTIVE

To review retrospectively the electrophysiological and structural changes in 13 Chinese patients with Leber hereditary optic neuropathy (LHON).

METHODS

26 eyes of 13 patients with a genetically confirmed diagnosis of LHON were categorized into two groups according to the duration of the disease: group 1 (duration less than 3 months) and group 2 (duration between 3 months and 18 years). Clinical history, comprehensive visual electrophysiology, optical coherence tomography (OCT), and color fundus photography were performed.

RESULTS

Fundoscopy showed optic disc hyperemia in group 1 and optic atrophy in group 2. OCT measures of retinal nerve fiber layer (RNFL) thickness around the optic disc and surrounding macula were normal in group 1 but reduced in group 2 (10 of 10 eyes). The thickness of the retinal ganglion cell layer (GCL) plus inner plexiform layer (IPL) surrounding the macula reduced significantly in group 1 and group 2 compared with a healthy control group. Pattern ERG (PERG) P50 amplitude was normal, but the N95/P50 ratio reduced in most of group 1 (4 of 5 eyes) and in all of group 2 (11 eyes). PERG P50 peak time was abnormally short in group 2. Multifocal electroretinography (mfERG) showed subnormal responses associated with ring 1 (the central area) and ring 2 in group 1 and reductions in rings 1, 2, and 3 in group 2.

CONCLUSION

The study highlights differences in retinal structure and function between the acute and chronic stages of LHON in a group of Chinese patients. There is PERG evidence of retinal ganglion cell dysfunction and OCT evidence of GCL + IPL thinning in both groups, but there is additional peripapillary RNFL loss in the chronic stage, associated with more severe RGC dysfunction. There is multifocal ERG evidence of localized macular dysfunction in both acute and chronic groups. The study highlights the importance of comprehensive electrophysiological and structural assessments of the retina in LHON and is pertinent to studies that aim to monitor disease progression or the effects of future therapeutic interventions.

Stem Cell Ophthalmology Treatment Study (SCOTS): bone marrow derived stem cells in the treatment of Dominant Optic Atrophy

Background

We report the results of 6 patients with Dominant Optic Atrophy (DOA) who met inclusion criteria and were treated in the Stem Cell Ophthalmology Treatment Study (SCOTS). SCOTS/SCOTS 2 is an Institutional Review Board approved and NIH registered (NCT03011541) clinical study that uses autologous bone marrow derived stem cells (BMSC) in the treatment of optic nerve and retinal disease.

Methods

This is an open label, non-randomized clinical study using natural history of the disease as the comparator. BMSC were separated from aspirated autologous bone marrow with minimal manipulation using an FDA cleared Class II medical device. Patients were treated with combinations of retrobulbar, subtenons, intravitreal or subretinal placement of BMSC followed by intravenous injection of BMSC depending on the arm of the study chosen. There were no surgical complications.

Results

Of the patients treated, 83.3% (5 of 6 patients) experienced visual improvements and in all of these cases both eyes improved. Ten eyes or 83.3% experienced gains in visual acuity with a median improvement of 2.125 Snellen lines, or approximately 10.63 letters. Two eyes were considered unchanged compared to longstanding measurements. Using LogMAR, the average improvement in vision for all eyes was 29.5%. The averagevisual acuity increasein eyes that improved was 33.3%. Findings were statistically significant with P<0.001.

Conclusions

Using autologous BMSC per protocols developed in the SCOTS/SCOTS 2 clinical studies resulted in statistically significant visual acuity improvements in patients with DOA or Kjers Optic Neuropathy. Improvements occurred in 83.3% of eyes and averaged 29.5%. Mitochondrial transfer and neuroprotective exosome secretions from the BMSC may have been key to the improvements observed in this mitochondrial disease.

Autosomal dominant optic atrophy with OPA1 gene mutations accompanied by auditory neuropathy and other systemic complications in a Japanese cohort

PURPOSE

This study aimed to describe the genetic and clinical characteristics of four Japanese patients with autosomal dominant optic atrophy (DOA) accompanied by auditory neuropathy and other systemic complications (i.e., DOA-plus disease).

METHODS

Four patients from four independent families underwent comprehensive ophthalmic and auditory examinations and were diagnosed with DOA-plus disease. The disease-causing gene variants in the OPA1 gene were identified by direct sequencing. The genetic and clinical data of 48 DOA patients without systemic complications-that is, with simple DOA-were compared to those of DOA-plus patients.

RESULTS

DOA-plus patients noticed a decrease in vision before the age of 14 and hearing impairment 3 to 13 years after the development of visual symptoms. Two patients had progressive external ophthalmoplegia, and one patient had vestibular dysfunction and ataxia. The DOA-plus phenotypes accounted for 13.3% (4/30) of the families with the OPA1 gene mutations. Each DOA-plus patient harbored one of the monoallelic mutations in the OPA1 gene: c.1334G>A, p.R445H, c.1618A>C, p.T540P, and c.892A>C, p.S298R. Missense mutations accounted for 100% (4/4) of the DOA-plus families and only 11.5% (3/26) of the families with simple DOA.

CONCLUSIONS

All the patients with the DOA-plus phenotype carried one of the missense mutations in the OPA1 gene. They all had typical ocular symptoms and signs of DOA in their first or second decade, and other systemic complications-such as auditory neuropathy, vestibular dysfunction, and ataxia-followed the ocular symptoms. We should consider the occurrence of extraocular complications in cases with DOA, especially when they carry the missense mutations in the OPA1 gene.

Treatment strategies for Leber hereditary optic neuropathy

PURPOSE OF REVIEW

Leber hereditary optic neuropathy (LHON) is the most common primary mitochondrial DNA (mtDNA) disorder in the population and it carries a poor visual prognosis. In this article, we review the development of treatment strategies for LHON, the evidence base and the areas of unmet clinical need.

RECENT FINDINGS

There is accumulating evidence that increasing mitochondrial biogenesis could be an effective strategy for protecting retinal ganglion cells in LHON. A number of clinical trials are currently investigating the efficacy of viral-based gene therapy for patients harbouring the m.11778G>A mtDNA mutation. For female LHON carriers of childbearing age, mitochondrial replacement therapy is being offered to prevent the maternal transmission of pathogenic mtDNA mutations.

SUMMARY

Although disease-modifying treatment options remain limited, a better understanding of the underlying disease mechanisms in LHON is paving the way for complementary neuroprotective and gene therapeutic strategies for this mitochondrial optic nerve disorder.

Whole Exome Sequencing Identifies Two Novel Mutations in the Reticulon 4-Interacting Protein 1 Gene in a Chinese Family with Autosomal Recessive Optic Neuropathies

Autosomal recessive optic neuropathies (IONs) are extremely rare disorders affecting retinal ganglion cells and the nervous system. RTN4IP1 has recently been identified as the third known gene associated with the autosomal recessive ION optic atrophy 10 (OPA10). Patients with RTN4IP1 mutations show early-onset optic neuropathy that can be followed by additional neurological symptoms such as seizures, ataxia, mental retardation, or even severe encephalopathy. Here, we report two siblings from a Chinese family who presented with early-onset optic neuropathy, epilepsy, and mild intellectual disability. Using whole exome sequencing combined with Sanger sequencing, we identified novel compound heterozygous RTN4IP1 mutations (c.646G > A, p.G216R and c.1162C > T, p.R388X) which both co-segregated with the disease phenotype and were predicted to be disease-causing by prediction software. An in vitro functional study in urine cells obtained from one of the patients revealed low expression of the RTN4IP1 protein. Our results identify novel compound heterozygous mutations in RTN4IP1 which are associated with OPA10, highlighting the frequency of RTN4IP1 mutations in human autosomal recessive IONs. To our knowledge, this is the first report of RTN4IP1 carriers from China.

The Mitochondrial Permeability Transition in Mitochondrial Disorders

Mitochondrial permeability transition pore (PTP), a (patho)physiological phenomenon discovered over 40 years ago, is still not completely understood. PTP activation results in a formation of a nonspecific channel within the inner mitochondrial membrane with an exclusion size of 1.5 kDa. PTP openings can be transient and are thought to serve a physiological role to allow quick Ca2+ release and/or metabolite exchange between mitochondrial matrix and cytosol or long-lasting openings that are associated with pathological conditions. While matrix Ca2+ and oxidative stress are crucial in its activation, the consequence of prolonged PTP opening is dissipation of the inner mitochondrial membrane potential, cessation of ATP synthesis, bioenergetic crisis, and cell death-a primary characteristic of mitochondrial disorders. PTP involvement in mitochondrial and cellular demise in a variety of disease paradigms has been long appreciated, yet the exact molecular entity of the PTP and the development of potent and specific PTP inhibitors remain areas of active investigation. In this review, we will (i) summarize recent advances made in elucidating the molecular nature of the PTP focusing on evidence pointing to mitochondrial FoF1-ATP synthase, (ii) summarize studies aimed at discovering novel PTP inhibitors, and (iii) review data supporting compromised PTP activity in specific mitochondrial diseases.

Simultaneous bilateral optic neuritis in China: clinical, serological and prognostic characteristics

PURPOSE

To analyse the clinical characteristics of simultaneous bilateral ON patients in China.

METHODS

This retrospective study was done on 51 primary bilateral ON patients between April 2008 and July 2016 at the Chinese People's Liberation Army General Hospital. Fifty eight primary unilateral ON patients formed the control group. Demographic data, clinical course, serum autoantibody status, connective tissue disorders, magnetic resonance imaging and visual functions were compared.

RESULTS

The mean age at disease onset in the bilateral group was younger than that of the unilateral group (p = 0.001). Cerebrospinal fluid (CSF) total cell count and CSF total protein were significantly higher in the bilateral group (p = 0.001, p = 0.025). Aquaporin-4 (AQP4) antibodies were detected in 39% and 21% of the bilateral and unilateral patients, respectively (p = 0.03). Twenty two percent of the bilateral patients fulfilled the diagnosis of neuromyelitis optica (NMO); 7% in the unilateral group did so (p = 0.03). Serum autoantibodies (ANA, SSA, SSB, etc.) were found in 49% of the bilateral patients and 29% of the unilateral patients (p = 0.035). After treatment, the bilateral patients were significantly more prone to severe visual disability eventually than their unilateral counterparts (p = 0.002). Patients with MOG-IgG (myelin oligodendrocyte glycoprotein-IgG) represented 26% of the patients negative for AQP4-IgG. Myelin oligodendrocyte glycoprotein-IgG (MOG-IgG) sero-positive patients were more likely to recover than the other patients (p < 0.001).

CONCLUSION

Simultaneous bilateral ON is a severe disorder closely related to serum AQP4-IgG and MOG-IgG, which are more likely to involve younger people and incur severe visual disability eventually. Myelin oligodendrocyte glycoprotein-IgG (MOG-IgG) sero-positive patients have higher risk of ON relapses and better visual prognosis.

Donation of mitochondria by iPSC-derived mesenchymal stem cells protects retinal ganglion cells against mitochondrial complex I defect-induced degeneration

Rationale: Retinal ganglion cell (RGC) degeneration is extremely hard to repair or regenerate and is often coupled with mitochondrial dysfunction. Mesenchymal stem cells (MSCs)-based treatment has been demonstrated beneficial for RGC against degeneration. However, underlying mechanisms of MSC-provided RGC protection are largely unknown other than neuroprotective paracrine actions. In this study, we sought to investigate whether mitochondrial donation from induced pluripotent stem cell-derived MSC (iPSC-MSCs) could preserve RGC survival and restore retinal function.

Methods: iPSC-MSCs were injected into the vitreous cavity of one eye in NADH dehydrogenase (ubiquinone) Fe-S protein 4 (Ndufs4) knockout (KO) and wild type mice. Phosphate buffer saline (PBS) or rotenone treated iPSC-MSCs were injected as control groups. Retinal function was detected by flash electroretinogram (ERG). Whole-mount immunofluorescence (IF), morphometric analysis, confocal microscopy imaging, polymerase chain reaction (PCR) of the retinas were conducted to investigate mitochondrial transfer from human iPSC-MSCs to mouse retina. Quantitative mouse cytokine arrays were carried out to measure retinal inflammatory response under difference treatments.

Results: RGC survival in the iPSC-MSC injected retina of Ndufs4 KO mice was significantly increased with improved retinal function. GFP labelled human mitochondria from iPSC-MSC were detected in the RGCs in the retina of Ndufs4 KO mice starting from 96 hours post injection. PCR result showed only human mitochondrial DNA without human nuclear DNA could be detected in the mouse retinas after iPSC-MSC treatment in Ndufs4 KO mice eye. Quantitative cytokine array analysis showed pro-inflammatory cytokines was also downregulated by this iPSC-MSC treatment.

Conclusion: Intravitreal transplanted iPSC-MSCs can effectively donate functional mitochondria to RGCs and protect against mitochondrial damage-induced RGC loss.

Mitophagy activation repairs Leber's hereditary optic neuropathy-associated mitochondrial dysfunction and improves cell survival

Leber's hereditary optic neuropathy (LHON) is a classical mitochondrial disease caused by mutations in the mitochondrial DNA encoding complex I subunits. Oxidative stress associated with complex I defect has been implicated in developing LHON phenotype such as retinal ganglion cell (RGC) death and loss of vision. However, the mechanism of LHON pathogenesis is still not very clear and thus no effective therapies are available to date. Using cybrid models for LHON, we show that autophagy is significantly compromised in cells carrying LHON-specific mtDNA mutations, which results in reduced clearance of dysfunctional mitochondria contributing to cell death. We further show that pharmacological activation of autophagy selectively clears the damaged mitochondria and thus repairs mitochondrial defects and improves overall cell survival in LHON cell models. Our results suggest that compromised autophagy is the missing link from oxidative stress to LHON pathogenesis. Activation of mitophagy ameliorates mitochondrial defects and exerts a protective role by improving cell survival in cells carrying LHON mutations that could be utilized as a potential therapeutic target for LHON treatment.

NLRP3 inflammasome in NMDA-induced retinal excitotoxicity

N-methyl-D-aspartate (NMDA)-induced excitotoxicity is an acute form of experimental retinal injury as a result of overactivation of glutamate receptors. NLRP3 (nucleotide-binding domain, leucine-rich-repeat containing family, pyrin domain containing-3) inflammasome, one of the most studied sensors of innate immunity, has been reported to play a critical role in retinal neurodegeneration with controversial implications regarding neuroprotection and cell death. Thus far, it has not been elucidated whether NMDA-mediated excitotoxicity can trigger NLRP3 inflammasome in vivo. Moreover, it is unknown if NLRP3 is beneficial or detrimental to NMDA-mediated retinal cell death. Here, we employed a murine model of NMDA-induced retinal excitotoxicity by administering 100 nmoles of NMDA intravitreally, which resulted in massive TUNEL⁺ (TdT-dUTP terminal nick-end labelling) cell death in all retinal layers and especially in retinal ganglion cells (RGCs) 24 h post injection. NMDA insult in the retina potentiates macrophage/microglia cell infiltration, primes the NLRP3 inflammasome in a transcription-dependent manner and induces the expression of interleukin-1β (IL-1β). However, despite NLRP3 inflammasome upregulation, systemic deletion of Nlrp3 or Casp1 (caspase-1) did not significantly alter the NMDA-induced, excitotoxicity-mediated TUNEL⁺ retinal cell death at 24 h (acute phase). Similarly, the deletion of the two aforementioned genes did not alter the survival of the Brn3a⁺ (brain-specific homeobox/POU domain protein 3A) RGCs in a significant way at 3- or 7-days post injection (long-term phase). Our results indicate that NMDA-mediated retinal excitotoxicity induces immune cell recruitment and NLRP3 inflammasome activity even though inflammasome-mediated neuroinflammation is not a leading contributing factor to cell death in this type of retinal injury.

Mitochondrial DNA Integrity: Role in Health and Disease

As the primary cellular location for respiration and energy production, mitochondria serve in a critical capacity to the cell. Yet, by virtue of this very function of respiration, mitochondria are subject to constant oxidative stress that can damage one of the unique features of this organelle, its distinct genome. Damage to mitochondrial DNA (mtDNA) and loss of mitochondrial genome integrity is increasingly understood to play a role in the development of both severe early-onset maladies and chronic age-related diseases. In this article, we review the processes by which mtDNA integrity is maintained, with an emphasis on the repair of oxidative DNA lesions, and the cellular consequences of diminished mitochondrial genome stability.