Rare primary mitochondrial DNA mutations and probable synergistic variants in Leber's hereditary optic neuropathy

Mitochondrial retinopathies and optic neuropathies: The impact of retinal imaging on modern understanding of pathogenesis, diagnosis, and management

Advancements in ocular imaging have significantly broadened our comprehension of mitochondrial retinopathies and optic neuropathies by examining the structural and pathological aspects of the retina and optic nerve in these conditions. This article aims to review the prominent imaging characteristics associated with mitochondrial retinopathies and optic neuropathies, aiming to deepen our insight into their pathogenesis and clinical features. Preceding this exploration, the article provides a detailed overview of the crucial genetic and clinical features, which is essential for the proper interpretation of in vivo imaging. More importantly, we will provide a critical analysis on how these imaging modalities could serve as biomarkers for characterization and monitoring, as well as in guiding treatment decisions. However, these imaging methods have limitations, which will be discussed along with potential strategies to mitigate them. Lastly, the article will emphasize the potential advantages and future integration of imaging techniques in evaluating patients with mitochondrial eye disorders, considering the prospects of emerging gene therapies.

A computational study to assess the pathogenicity of single or combinations of missense variants on respiratory complex I

Variants found in the respiratory complex I (CI) subunit genes encoded by mitochondrial DNA can cause severe genetic diseases. However, it is difficult to establish a priori whether a single or a combination of CI variants may impact oxidative phosphorylation. Here we propose a computational approach based on coarse-grained molecular dynamics simulations aimed at investigating new CI variants. One of the primary CI variants associated with the Leber hereditary optic neuropathy (m.14484T>C/MT-ND6) was used as a test case and was investigated alone or in combination with two additional rare CI variants whose role remains uncertain. We found that the primary variant positioned in the E-channel region, which is fundamental for CI function, stiffens the enzyme dynamics. Moreover, a new mechanism for the transition between π- and α-conformation in the helix carrying the primary variant is proposed. This may have implications for the E-channel opening/closing mechanism. Finally, our findings show that one of the rare variants, located next to the primary one, further worsens the stiffening, while the other rare variant does not affect CI function. This approach may be extended to other variants candidate to exert a pathogenic impact on CI dynamics, or to investigate the interaction of multiple variants.

Mitochondrial complex I subunit MT-ND1 mutations affect disease progression

Mitochondrial respiratory chain complex I is an important component of the oxidative respiratory chain, with the mitochondrially encoded NADH:ubiquinone oxidoreductase core subunit 1 (MT-ND1) being one of the core subunits. MT-ND1 plays a role in the assembly of complex I and its enzymatic function. MT-ND1 gene mutation affects pathophysiological processes, such as interfering with the early assembly of complex I, affecting the ubiquinone binding domain and proton channel of complex I, and affecting oxidative phosphorylation, thus leading to the occurrence of diseases. The relationship between MT-ND1 gene mutation and disease has been has received increasing research attention. Therefore, this article reviews the impact of MT-ND1 mutations on disease progression, focusing on the impact of such mutations on diseases and their possible mechanisms, as well as the application of targeting MT-ND1 gene mutations in disease diagnosis and treatment. We aim to provide a new perspective leading to a more comprehensive understanding of the relationship between MT-ND1 gene mutations and diseases.

Identification and characterization of a new pathologic mutation in a large Leber hereditary optic neuropathy pedigree

BACKGROUND

Most patients suffering from Leber hereditary optic neuropathy carry one of the three classic pathologic mutations, but not all individuals with these genetic alterations develop the disease. There are different risk factors that modify the penetrance of these mutations. The remaining patients carry one of a set of very rare genetic variants and, it appears that, some of the risk factors that modify the penetrance of the classical pathologic mutations may also affect the phenotype of these other rare mutations.

RESULTS

We describe a large family including 95 maternally related individuals, showing 30 patients with Leber hereditary optic neuropathy. The mutation responsible for the phenotype is a novel transition, m.3734A > G, in the mitochondrial gene encoding the ND1 subunit of respiratory complex I. Molecular-genetic, biochemical and cellular studies corroborate the pathogenicity of this genetic change.

CONCLUSIONS

With the study of this family, we confirm that, also for this very rare mutation, sex and age are important factors modifying penetrance. Moreover, this pedigree offers an excellent opportunity to search for other genetic or environmental factors that additionally contribute to modify penetrance.

A challenging differential diagnosis - Leber's Hereditary Optic Neuropathy

Leber's hereditary optic neuropathy (LHON) is the most common maternally inherited disease linked to mitochondrial DNA (mtDNA). The patients present with subacute asymmetric bilateral vision loss. Approximately 95% of the LHON cases are caused by m.3460G>A (MTND1), m.11778G>A (MTND4), and m.14484T>C (MTND6) mutations. The hallmark of hereditary optic neuropathies determined by mitochondrial dysfunction is the vulnerability and degeneration of retinal ganglion cells (RGC). We present the case of a 28-year-old man who came to our clinic complaining of a subacute decrease in visual acuity of his left eye. From his medical history, we found out that one month before he had the same symptoms in the right eye. From the family history, we noted that an uncle has had vision problems since childhood. We carried out complete blood tests, including specific antibodies for autoimmune and infectious diseases. Laboratory tests and MRI were within normal limits. A blood test of the mtDNA showed the presence of 11778 G>A mutation on the mtND6 gene. The medical history, the fundus appearance, the OCT, and the paraclinical investigations, made us diagnose our patient with Leber's hereditary optic neuropathy. As soon as possible, we began the treatment with systemic idebenone, 900 mg/day. We examined the patient 2, 6, and 10 weeks after initiating the treatment. Abbreviations: LHON = Leber's Hereditary Optic Neuropathy, mtDNA = mitochondrial DNA, VA = visual acuity, RE = right eye, LE = left eye, OCT = Optical coherence tomography, pRNFL = peripapillary retinal nerve fiber layer, GCL = retinal ganglion cells layer, MRI = magnetic resonance imaging, VEP = visual evoked potentials, VEP IT = VEP implicit time, VEP A = VEP amplitude.

[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.

Compact zinc finger base editors that edit mitochondrial or nuclear DNA in vitro and in vivo

DddA-derived cytosine base editors (DdCBEs) use programmable DNA-binding TALE repeat arrays, rather than CRISPR proteins, a split double-stranded DNA cytidine deaminase (DddA), and a uracil glycosylase inhibitor to mediate C•G-to-T•A editing in nuclear and organelle DNA. Here we report the development of zinc finger DdCBEs (ZF-DdCBEs) and the improvement of their editing performance through engineering their architectures, defining improved ZF scaffolds, and installing DddA activity-enhancing mutations. We engineer variants with improved DNA specificity by integrating four strategies to reduce off-target editing. We use optimized ZF-DdCBEs to install or correct disease-associated mutations in mitochondria and in the nucleus. Leveraging their small size, we use a single AAV9 to deliver into heart, liver, and skeletal muscle in post-natal mice ZF-DdCBEs that efficiently install disease-associated mutations. While off-target editing of ZF-DdCBEs is likely too high for therapeutic applications, these findings demonstrate a compact, all-protein base editing research tool for precise editing of organelle or nuclear DNA without double-strand DNA breaks.

DNAJC30 disease-causing gene variants in a large Central European cohort of patients with suspected Leber's hereditary optic neuropathy and optic atrophy

BACKGROUND

Leber's hereditary optic neuropathy (LHON) has been considered a prototypical mitochondriopathy and a textbook example for maternal inheritance linked to certain disease-causing variants in the mitochondrial genome. Recently, an autosomal recessive form of LHON (arLHON) has been described, caused by disease-causing variants in the nuclear encoded gene DNAJC30.

METHODS AND RESULTS

In this study, we screened the DNAJC30 gene in a large Central European cohort of patients with a clinical diagnosis of LHON or other autosomal inherited optic atrophies (OA). We identified likely pathogenic variants in 35/1202 patients, corresponding to a detection rate of 2.9%. The previously described missense variant c.152A>G;p.(Tyr51Cys) accounts for 90% of disease-associated alleles in our cohort and we confirmed a strong founder effect. Furthermore, we identified two novel pathogenic variants in DNAJC30: the nonsense variant c.610G>T;p.(Glu204*) and the in-frame deletion c.230_232del;p.(His77del). Clinical investigation of the patients with arLHON revealed a younger age of onset, a more frequent bilateral onset and an increased clinically relevant recovery compared with LHON associated with disease-causing variants in the mitochondrial DNA.

CONCLUSION

This study expands previous findings on arLHON and emphasises the importance of DNAJC30 in the genetic diagnostics of LHON and OA in European patients.

Mitochondrial respiratory complexes: Significance in human mitochondrial disorders and cancers

Mitochondria are pivotal organelles that govern cellular energy production through the oxidative phosphorylation system utilizing five respiratory complexes. In addition, mitochondria also contribute to various critical signaling pathways including apoptosis, damage-associated molecular patterns, calcium homeostasis, lipid, and amino acid biosynthesis. Among these diverse functions, the energy generation program oversee by mitochondria represents an immaculate orchestration and functional coordination between the mitochondria and nuclear encoded molecules. Perturbation in this program through respiratory complexes' alteration results in the manifestation of various mitochondrial disorders and malignancy, which is alarmingly becoming evident in the recent literature. Considering the clinical relevance and importance of this emerging medical problem, this review sheds light on the timing and nature of molecular alterations in various respiratory complexes and their functional consequences observed in various mitochondrial disorders and human cancers. Finally, we discussed how this wealth of information could be exploited and tailored to develop respiratory complex targeted personalized therapeutics and biomarkers for better management of various incurable human mitochondrial disorders and cancers.

Mitochondrial DNA variation in Parkinson’s disease: Analysis of “out-of-place” population variants as a risk factor

Mitochondrial DNA (mtDNA), a potential source of mitochondrial dysfunction, has been implicated in Parkinson’s disease (PD). However, many previous studies investigating associations between mtDNA population variation and PD reported inconsistent or contradictory findings. Here, we investigated an alternative hypothesis to determine whether mtDNA variation could play a significant role in PD risk. Emerging evidence suggests that haplogroup-defining mtDNA variants may have pathogenic potential if they occur “out-of-place” on a different maternal lineage. We hypothesized that the mtDNA of PD cases would be enriched for out-of-place variation in genes encoding components of the oxidative phosphorylation complexes. We tested this hypothesis with a unique dataset comprising whole mitochondrial genomes of 70 African ancestry PD cases, two African ancestry control groups (n = 78 and n = 53) and a replication group of 281 European ancestry PD cases and 140 controls from the Parkinson’s Progression Markers Initiative cohort. Significantly more African ancestry PD cases had out-of-place variants than controls from the second control group (P < 0.0125), although this association was not observed in the first control group nor the replication group. As the first mtDNA study to include African ancestry PD cases and to explore out-of-place variation in a PD context, we found evidence that such variation might be significant in this context, thereby warranting further replication in larger cohorts.

A Systematic Review of the Impact of Mitochondrial Variations on Male Infertility

According to current estimates, infertility affects one in four couples trying to conceive. Primary or secondary infertility can be due either to both partners or only to the man or the woman. Up to 15% of infertility cases in men can be attributed to genetic factors that can lead to irreversible partial or complete spermatogenic arrest. The increased use of assisted reproductive technology (ART) has provided not only insights into the causes of male infertility but also afforded a diagnostic tool to detect and manage this condition among couples. Genes control a variety of physiological attributes, such as the hypothalamic–pituitary–gonadal axis, development, and germ cell differentiation. In the era of ART, it is important to understand the genetic basis of infertility so as to provide the most tailored therapy and counseling to couples. Genetic factors involved in male infertility can be chromosome abnormalities or single-gene disorders, mitochondrial DNA (mtDNA) mutations, Y-chromosome deletions, multifactorial disorders, imprinting disorders, or endocrine disorders of genetic origin. In this review, we discuss the role of mitochondria and the mitochondrial genome as an indicator of sperm quality and fertility.

Mitochondrial Retinopathies

The retina is an exquisite target for defects of oxidative phosphorylation (OXPHOS) associated with mitochondrial impairment. Retinal involvement occurs in two ways, retinal dystrophy (retinitis pigmentosa) and subacute or chronic optic atrophy, which are the most common clinical entities. Both can present as isolated or virtually exclusive conditions, or as part of more complex, frequently multisystem syndromes. In most cases, mutations of mtDNA have been found in association with mitochondrial retinopathy. The main genetic abnormalities of mtDNA include mutations associated with neurogenic muscle weakness, ataxia and retinitis pigmentosa (NARP) sometimes with earlier onset and increased severity (maternally inherited Leigh syndrome, MILS), single large-scale deletions determining Kearns-Sayre syndrome (KSS, of which retinal dystrophy is a cardinal symptom), and mutations, particularly in mtDNA-encoded ND genes, associated with Leber hereditary optic neuropathy (LHON). However, mutations in nuclear genes can also cause mitochondrial retinopathy, including autosomal recessive phenocopies of LHON, and slowly progressive optic atrophy caused by dominant or, more rarely, recessive, mutations in the fusion/mitochondrial shaping protein OPA1, encoded by a nuclear gene on chromosome 3q29.

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.

Leber's hereditary optic neuropathy: Current approaches and future perspectives on Mesenchymal stem cell-mediated rescue

Leber's Hereditary Optic Neuropathy (LHON) is an inherited optic nerve disorder. It is a mitochondrially inherited disease due to point mutation in the MT-ND1, MT-ND4, and MT-ND6 genes of mitochondrial DNA (mtDNA) coding for complex I subunit proteins. These mutations affect the assembly of the mitochondrial complex I and hence the electron transport chain leading to mitochondrial dysfunction and oxidative damage. Optic nerve cells like retinal ganglion cells (RGCs) are more sensitive to mitochondrial loss and oxidative damage which results in the progressive degeneration of RGCs at the axonal region of the optic nerve leading to bilateral vision loss. Currently, gene therapy using Adeno-associated viral vector (AAV) is widely studied for the therapeutics application in LHON. Our review highlights the application of cell-based therapy for LHON. Mesenchymal stem cells (MSCs) are known to rescue cells from the pre-apoptotic stage by transferring healthy mitochondria through tunneling nanotubes (TNT) for cellular oxidative function. Empowering the transfer of healthy mitochondria using MSCs may replace the mitochondria with pathogenic mutation and possibly benefit the cells from progressive damage. This review discusses the ongoing research in LHON and mitochondrial transfer mechanisms to explore its scope in inherited optic neuropathy.

Leber's Hereditary Optic Neuropathy: A Report on Novel mtDNA Pathogenic Variants

Leber's hereditary optic neuropathy (LHON) is due to missense point mutations affecting mitochondrial DNA (mtDNA); 90% of cases harbor the m.3460G>A, m.11778G>A, and m.14484T>C primary mutations. Here, we report and discuss five families with patients affected by symptomatic LHON, in which we found five novel mtDNA variants. Remarkably, these mtDNA variants are located in complex I genes, though without strong deleterious effect on respiration in cellular models: this finding is likely linked to the tissue specificity of LHON. This study observes that in the case of a strong clinical suspicion of LHON, it is recommended to analyze the whole mtDNA sequence, since new rare mtDNA pathogenic variants causing LHON are increasingly identified.

Leber Hereditary Optic Neuropathy: Review of Treatment and Management

Leber hereditary optic neuropathy (LHON) is a maternally inherited mitochondrial disease that specifically targets the retinal ganglion cells by reducing their ability to produce enough energy to sustain. The mutations of the mitochondrial DNA that cause LHON are silent until an unknown trigger causes bilateral central visual scotoma. After the onset of loss of vision, most patients experience progressive worsening within the following months. Few of them regain some vision after a period of ~1 year. Management of LHON patients has been focused on understanding the triggers of the disease and its pathophysiology to prevent the onset of visual loss in a carrier. Medical treatment is recommended once visual loss has started in at least one eye. Research evaluated drugs that are thought to be able to restore the mitochondrial electron transport chain of the retinal ganglion cells. Significant advances were made in evaluating free radical cell scavengers and gene therapy as potential treatments for LHON. Although encouraging the results of clinical trial have been mixed in stopping the worsening of visual loss. In patients with chronic disease of over 1 year, efficient treatment that restores vision is yet to be discovered. In this review, we summarize the management strategies for patients with LHON before, during, and after the loss of vision, explain the rationale and effectiveness of previous and current treatments, and report findings about emerging treatments.

Impaired complex I repair causes recessive Leber's hereditary optic neuropathy

Leber’s hereditary optic neuropathy (LHON) is the most frequent mitochondrial disease and was the first to be genetically defined by a point mutation in mitochondrial DNA (mtDNA). A molecular diagnosis is achieved in up to 95% of cases, the vast majority of which are accounted for by 3 mutations within mitochondrial complex I subunit–encoding genes in the mtDNA (mtLHON). Here, we resolve the enigma of LHON in the absence of pathogenic mtDNA mutations. We describe biallelic mutations in a nuclear encoded gene, DNAJC30, in 33 unsolved patients from 29 families and establish an autosomal recessive mode of inheritance for LHON (arLHON), which to date has been a prime example of a maternally inherited disorder. Remarkably, all hallmarks of mtLHON were recapitulated, including incomplete penetrance, male predominance, and significant idebenone responsivity. Moreover, by tracking protein turnover in patient-derived cell lines and a DNAJC30-knockout cellular model, we measured reduced turnover of specific complex I N-module subunits and a resultant impairment of complex I function. These results demonstrate that DNAJC30 is a chaperone protein needed for the efficient exchange of complex I subunits exposed to reactive oxygen species and integral to a mitochondrial complex I repair mechanism, thereby providing the first example to our knowledge of a disease resulting from impaired exchange of assembled respiratory chain subunits.

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.

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.

A hospital-based five-year prospective study on the prevalence of Leber's hereditary optic neuropathy with genetic confirmation

Purpose

To estimate the prevalence of Leber hereditary optic neuropathy (LHON) along with genetic screening at a tertiary eye care center in southern India.

Methods

Patients with LHON were identified at the Neuro-Ophthalmology Clinic, Aravind Eye Hospital (AEH; Madurai, India) from 2015 to 2019. Clinical data were collected along with blood samples. Genetic testing was performed for the confirmation of LHON using a multiplex PCR restriction fragment length polymorphism (RFLP) approach to detect the primary mutations 3460A, 11778A, and 14484C in mitochondrial DNA (mtDNA).

Results

During the study period, 1,598,441 outpatients attended AEH of whom 40,527 were referred to the Neuro-Ophthalmology Clinic. Among them, 55 patients were diagnosed with LHON. The male to female ratio was 8.2:1.0, and the mean age at onset was 20.95 years (SD 8.940). The estimated prevalence was 1:737 or 13.57 per 10,000 (95% confidence intervals [CI] 10.23-17.66) at the Neuro-Ophthalmology Clinic. The frequency of primary mutations in the patients with LHON was determined as 43.6% (24/55), giving a prevalence of 1:1689 or 5.92 per 10,000 (95% CI 3.78-8.81).

Conclusions

The high prevalence of LHON observed at a single hospital highlights the impact of the disease in southern India. As the epidemiology of LHON remains unexplored in this region, these findings will pave the way to evaluate the national prevalence. Further, screening the whole mitochondrial genome may help to increase the detection of mutations to estimate the accurate prevalence of the disease.

Genotypic and phenotypic characteristics of Korean children with childhood-onset Leber's hereditary optic neuropathy

PURPOSE

We sought to identify the phenotypic and genotypic characteristics of Korean children with genetically confirmed Leber's hereditary optic neuropathy (LHON).

METHODS

The medical records of 64 genetically confirmed LHON patients were reviewed. Seventeen patients aged 13 years or younger with optic atrophy with positive mitochondrial DNA (mtDNA) mutations were considered to demonstrate childhood-onset LHON. The non-childhood-onset group included 47 patients with genetically confirmed LHON who experienced disease onset later than 13 years of age. The type of mtDNA mutation, visual acuity (VA), color vision, fundus photography, retinal nerve fiber layer (RNFL) thickness, and visual field were investigated.

RESULTS

Sequence analysis of the mitochondrial genome revealed five different kinds of LHON-associated mtDNA mutations among our childhood-onset patients, including m.11778G>A (58.8%), m.3496G>T (11.8%), m.3497C>T (5.9%), m.11696G>A (5.9%), and m.14502T>C (5.9%). The mean final best-corrected VA in the childhood-onset group was better than that in the non-childhood-onset group with the value of logMAR 0.29 (0.09-0.75) vs. 0.55 (0.27-1.29) (expressed as median (interquartile range); p = 0.05). Spontaneous visual recovery was observed in 35.3% of the childhood-onset group but in only 12.8% of the non-childhood-onset group (p = 0.04). Eight patients (47.1%) showed interocular asymmetry of the disease, with two presenting true unilateral involvement of the optic nerve and the other six patients demonstrating unilateral subclinical manifestations with bilateral optic atrophy.

CONCLUSION

Involvement of secondary mitochondrial mutations was confirmed in patients with childhood-onset LHON. Characteristic clinical features of childhood-onset LHON included a higher proportion of subacute or insidious onset of symptoms, better VA, higher spontaneous recovery, and asymmetrical ocular involvement.

Mitochondrial DNA Variation of Leber's Hereditary Optic Neuropathy in Western Siberia

Our data first represent the variety of Leber's hereditary optic neuropathy (LHON) mutations in Western Siberia. LHON is a disorder caused by pathogenic mutations in mitochondrial DNA (mtDNA), inherited maternally and presents mainly in young adults, predominantly males. Clinically, LHON manifests itself as painless central vision loss, resulting in early onset of disability. The epidemiology of LHON has not been fully investigated yet. In this study, we report 44 genetically unrelated families with LHON manifestation. We performed whole mtDNA genome sequencing and provided genealogical and molecular genetic data on mutations and haplogroup background of LHON patients. Known "primary" pathogenic mtDNA mutations (MITOMAP) were found in 32 families: m.11778G>A represents 53.10% (17/32), m.3460G>A-21.90% (7/32), m.14484T>C-18.75% (6/32), and rare m.10663T>C and m.3635G>A represent 6.25% (2/32). We describe potentially pathogenic m.4659G>A in one subject without known pathogenic mutations, and potentially pathogenic m.6261G>A, m.8412T>C, m.8551T>C, m.9444C>T, m.9921G>A, and m.15077G>A in families with known pathogenic mutations confirmed. We suppose these mutations could contribute to the pathogenesis of optic neuropathy development. Our results indicate that haplogroup affiliation and mutational spectrum of the Western Siberian LHON cohort substantially deviate from those of European populations.

Leber's Hereditary Optic Neuropathy-Specific Heteroplasmic Mutation m.14495A>G Found in a Chinese Family

Purpose

Leber's hereditary optic neuropathy (LHON) is a mitochondrial DNA (mtDNA)-associated, maternally inherited eye disease. Mutation heteroplasmy level is one of the leading causes to trigger LHON manifestation. In this study, we aimed to identify the causative mutation in a large Han Chinese family with LHON and explore the underlying pathogenic mechanism in this LHON family.

Methods

The whole-mtDNA sequence was amplified by long-range PCR. Mutations were subsequently identified by next-generation sequencing (NGS) and validated by Sanger sequencing. The heteroplasmy rates of those family members were determined by digital PCR (dPCR). Mitochondrial haplogroups were assigned based on mtDNA tree build 17.

Results

The m.14495A>G mutation was identified as causative due to its higher heteroplasmy level (>50%) in patients than in their unaffected relatives. All mutation carriers belong to M7b1a1 and are assigned to Asian mtDNA lineage. Interestingly, our result revealed that high mtDNA copy number in carrier might prevent LHON manifestation.

Conclusions

This is the first report of m.14495A>G mutation in Asian individuals with LHON. Our study shows that dPCR technology can provide more reliable results in mutation heteroplasmy assay and determination of the cellular mtDNA content, making it a potentially promising tool for clinical precise diagnosis of LHON. Furthermore, our results also add evidence to the opinion that higher mtDNA content may protect mutation carriers from LHON.

Translational Relevance

dPCR can be used for the assessment of LHON disease, and a new genetic-based diagnostic strategy has been proposed for LHON patients with the m.14495A>G mutation.

Cardiovascular Manifestations of Mitochondrial Disease

Genetic mitochondrial cardiomyopathies are uncommon causes of heart failure that may not be seen by most physicians. However, the prevalence of mitochondrial DNA mutations and somatic mutations affecting mitochondrial function are more common than previously thought. In this review, the pathogenesis of genetic mitochondrial disorders causing cardiovascular disease is reviewed. Treatment options are presently limited to mostly symptomatic support, but preclinical research is starting to reveal novel approaches that may lead to better and more targeted therapies in the future. With better understanding and clinician education, we hope to improve clinician recognition and diagnosis of these rare disorders in order to improve ongoing care of patients with these diseases and advance research towards discovering new therapeutic strategies to help treat these diseases.

Subclinical Leber's hereditary optic neuropathy with pediatric acute spinal cord onset: more than meets the eye

Background

Leber’s hereditary optic neuropathy (LHON) is a mitochondrial disease characterized by visual loss consequent to optic nerve atrophy. In some cases, LHON is associated with heterogeneous neurological extraocular manifestations and is referred to as “Leber plus disease”; rarely it is associated with a multiple sclerosis (MS)-like syndrome known as Harding disease, but no pediatric extraocular acute spinal onset is reported.

Case presentation

We describe the case of a 5-year-old girl carrying the G3460A mtDNA mutation who was referred to clinical examination for bilateral upper and lower limb weakness with no sign of optic neuropathy. Spinal cord MRI showed hyperintense signal alterations in T2-weighted and restricted diffusion in DWI sequences in the anterior portion of the cervical and dorsal spinal cord resembling a spinal cord vascular injury. No association between this mutation and pediatric spinal cord lesions has previously been reported. Alternative diagnostic hypotheses, including infective, ischemic and inflammatory disorders, were not substantiated by clinical and instrumental investigations.

Conclusions

Our case reports a novel pediatric clinical manifestation associated with the m.3460G > A mtDNA mutation, broadening the clinical spectrum of this disease. Early identification of new cases and monitoring of carriers beginning in childhood is important to prevent neurological deterioration and preserve long-term function.

Oldies but Goldies mtDNA Population Variants and Neurodegenerative Diseases

mtDNA is transmitted through the maternal line and its sequence variability, which is population specific, is assumed to be phenotypically neutral. However, several studies have shown associations between the variants defining some genetic backgrounds and the susceptibility to several pathogenic phenotypes, including neurodegenerative diseases. Many of these studies have found that some of these variants impact many of these phenotypes, including the ones defining the Caucasian haplogroups H, J, and Uk, while others, such as the ones defining the T haplogroup, have phenotype specific associations. In this review, we will focus on those that have shown a pleiotropic effect in population studies in neurological diseases. We will also explore their bioenergetic and genomic characteristics in order to provide an insight into the role of these variants in disease. Given the importance of mitochondrial population variants in neurodegenerative diseases a deeper analysis of their effects might unravel new mechanisms of disease and help design new strategies for successful treatments.

Clinical syndromes associated with mtDNA mutations: where we stand after 30 years

The landmark year 1988 can be considered as the birthdate of mitochondrial medicine, when the first pathogenic mutations affecting mtDNA were associated with human diseases. Three decades later, the field still expands and we are not ‘scraping the bottom of the barrel’ yet. Despite the tremendous progress in terms of molecular characterization and genotype/phenotype correlations, for the vast majority of cases we still lack a deep understanding of the pathogenesis, good models to study, and effective therapeutic options. However, recent technological advances including somatic cell reprogramming to induced pluripotent stem cells (iPSCs), organoid technology, and tailored endonucleases provide unprecedented opportunities to fill these gaps, casting hope to soon cure the major primary mitochondrial phenotypes reviewed here. This group of rare diseases represents a key model for tackling the pathogenic mechanisms involving mitochondrial biology relevant to much more common disorders that affect our currently ageing population, such as diabetes and metabolic syndrome, neurodegenerative and inflammatory disorders, and cancer.

Peculiar combinations of individually non-pathogenic missense mitochondrial DNA variants cause low penetrance Leber's hereditary optic neuropathy

We here report on the existence of Leber’s hereditary optic neuropathy (LHON) associated with peculiar combinations of individually non-pathogenic missense mitochondrial DNA (mtDNA) variants, affecting the MT-ND4, MT-ND4L and MT-ND6 subunit genes of Complex I. The pathogenic potential of these mtDNA haplotypes is supported by multiple evidences: first, the LHON phenotype is strictly inherited along the maternal line in one very large family; second, the combinations of mtDNA variants are unique to the two maternal lineages that are characterized by recurrence of LHON; third, the Complex I-dependent respiratory and oxidative phosphorylation defect is co-transferred from the proband’s fibroblasts into the cybrid cell model. Finally, all but one of these missense mtDNA variants cluster along the same predicted fourth E-channel deputed to proton translocation within the transmembrane domain of Complex I, involving the ND1, ND4L and ND6 subunits. Hence, the definition of the pathogenic role of a specific mtDNA mutation becomes blurrier than ever and only an accurate evaluation of mitogenome sequence variation data from the general population, combined with functional analyses using the cybrid cell model, may lead to final validation. Our study conclusively shows that even in the absence of a clearly established LHON primary mutation, unprecedented combinations of missense mtDNA variants, individually known as polymorphisms, may lead to reduced OXPHOS efficiency sufficient to trigger LHON. In this context, we introduce a new diagnostic perspective that implies the complete sequence analysis of mitogenomes in LHON as mandatory gold standard diagnostic approach.

Leber’s hereditary optic neuropathy (LHON) is a common cause of maternally inherited vision loss. In the large majority of cases LHON is due to mitochondrial DNA (mtDNA) point mutations, clearly distinct from common polymorphisms normally found in the general population, affecting the mitochondrial function, thus defined as pathogenic. For the first time, we here demonstrate, on the genetic and functional ground, that unusual combinations of otherwise polymorphic and non-pathogenic mtDNA variants are sufficient for causing low-penetrance maternally inherited optic neuropathy in pedigrees fitting the LHON clinical diagnosis. Our findings bridge the blurry border between “pathogenic” and “neutral” mutations in an overall continuum that truly depends on the specific and sometime unique combination of variants characterizing each mitogenome. As a result, we conclude that, for an accurate diagnosis of LHON and possibly of other mitochondrial diseases, the only approach that can disclose all possible causative sources is complete mitogenome sequencing.

Frequency of primary mutations of Leber's hereditary optic neuropathy patients in North Indian population

Purpose:

Leber's hereditary optic neuropathy (LHON) is an inherited optic neuropathy characterized by subacute painless vision loss. The majority of LHON is caused due to one of the three primary mutations in the mitochondrial DNA (m.G3460A, m.G11778A, and m.T14484C). The frequency of these mutations differs in different populations. The purpose of this study is to observe the frequency of three common primary mutations in the North Indian population.

Methods:

Forty LHON patients within the age group of 10–50 years underwent molecular testing for primary mutations. For two patients, testing for mother and other siblings was also carried out, using bidirectional sequencing.

Results:

A total of 11 out of 40 (27.5%) patients were found to be carrying m.G11778A mutation. Siblings of two probands were also positive for the same mutation. In one family, two primary mutations (m.G11778A and m.T14484C) were found in the proband and in the mother as well.

Conclusion:

In this study, 27.5% mutation was detected in North Indian LHON families. These results suggest that m.G11778A mutation is more frequent in this population. The results of the present study are compatible with studies of an Asian population and Northern European population.

International Consensus Statement on the Clinical and Therapeutic Management of Leber Hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) is currently estimated as the most frequent mitochondrial disease (1 in 27,000-45,000). Its molecular pathogenesis and natural history is now fairly well understood. LHON also is the first mitochondrial disease for which a treatment has been approved (idebenone-Raxone, Santhera Pharmaceuticals) by the European Medicine Agency, under exceptional circumstances because of the rarity and severity of the disease. However, what remains unclear includes the optimal target population, timing, dose, and frequency of administration of idebenone in LHON due to lack of accepted definitions, criteria, and general guidelines for the clinical management of LHON. To address these issues, a consensus conference with a panel of experts from Europe and North America was held in Milan, Italy, in 2016. The intent was to provide expert consensus statements for the clinical and therapeutic management of LHON based on the currently available evidence. We report the conclusions of this conference, providing the guidelines for clinical and therapeutic management of LHON.

The metabolomic signature of Leber's hereditary optic neuropathy reveals endoplasmic reticulum stress

Leber's hereditary optic neuropathy (MIM#535000), the commonest mitochondrial DNA-related disease, is caused by mutations affecting mitochondrial complex I. The clinical expression of the disorder, usually occurring in young adults, is typically characterized by subacute, usually sequential, bilateral visual loss, resulting from the degeneration of retinal ganglion cells. As the precise action of mitochondrial DNA mutations on the overall cell metabolism in Leber's hereditary optic neuropathy is unknown, we investigated the metabolomic profile of the disease. High performance liquid chromatography coupled with tandem mass spectrometry was used to quantify 188 metabolites in fibroblasts from 16 patients with Leber's hereditary optic neuropathy and eight healthy control subjects. Latent variable-based statistical methods were used to identify discriminating metabolites. One hundred and twenty-four of the metabolites were considered to be accurately quantified. A supervised orthogonal partial least squares discriminant analysis model separating patients with Leber's hereditary optic neuropathy from control subjects showed good predictive capability (Q 2cumulated = 0.57). Thirty-eight metabolites appeared to be the most significant variables, defining a Leber's hereditary optic neuropathy metabolic signature that revealed decreased concentrations of all proteinogenic amino acids, spermidine, putrescine, isovaleryl-carnitine, propionyl-carnitine and five sphingomyelin species, together with increased concentrations of 10 phosphatidylcholine species. This signature was not reproduced by the inhibition of complex I with rotenone or piericidin A in control fibroblasts. The importance of sphingomyelins and phosphatidylcholines in the Leber's hereditary optic neuropathy signature, together with the decreased amino acid pool, suggested an involvement of the endoplasmic reticulum. This was confirmed by the significantly increased phosphorylation of PERK and eIF2α, as well as the greater expression of C/EBP homologous protein and the increased XBP1 splicing, in fibroblasts from affected patients, all these changes being reversed by the endoplasmic reticulum stress inhibitor, TUDCA (tauroursodeoxycholic acid). Thus, our metabolomic analysis reveals a pharmacologically-reversible endoplasmic reticulum stress in complex I-related Leber's hereditary optic neuropathy fibroblasts, a finding that may open up new therapeutic perspectives for the treatment of Leber's hereditary optic neuropathy with endoplasmic reticulum-targeting drugs.

Mutation analysis of Leber's hereditary optic neuropathy using a multi-gene panel

The present study investigates the spectrum and incidence of mitochondrial DNA (mtDNA) mutations associated with Leber's hereditary optic neuropathy (LHON) in a Han population using a multi-gene panel with 46 LHON-associated mutations among 13 mitochondrial genes. A total of 23 mutations were observed in a cohort of 275 patients and 281 control subjects using multi-gene panel analysis. The causative mutations associated with LHON were identified to be m.11778G>A, m.14484T>C, m.3460 G>A, m.3635G>A, m.3866T>C and m.3733G>A, responsible for 70.55% cases in the patient cohort. The secondary mutations in the Chinese LHON population were m.12811T>C, m.11696 G>A, m.3316G>A, m.3394T>C, m.14502T>C, m.3497C>T, m.3571C>T, m.12338T>C, m.14693A>G, m.4216T>C and m.15951A>G, with incidences of 5.09, 4.36, 4.00, 4.00, 4.00, 2.55, 1.82, 1.82, 1.45, 1.09 and 1.09%, respectively. Besides three hotspot genes, MT-ND1, MT-ND4 and MT-ND6, MT-ND5 also had a high incidence of secondary mutations. Those mutations reported as rare causative mutations in a European LHON population, m.3376G>A, m.3700G>A and m.4171C>A, m.10663T>C, m.13051G>A, m.14482C>G/A, m.14495A>G and m.14568C>T were undetected in the present study. The primary and secondary mutations associated with LHON in the present multi-gene panel will advance the current understanding of the clinical phenotype of LHON, and provide useful information for early diagnosis.

Mitochondrial genetics and therapeutic overview of Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) is a common inherited mitochondrial disorder that is characterized by the degeneration of the optic nerves, leading to vision loss. The major mutations in the mitochondrial genes ND1, ND4, and ND6 of LHON subjects are found to increase the oxidative stress experienced by the optic nerve cell, thereby leading to nerve cell damage. Accurate treatments are not available and drugs that are commercially available like Idebenone, EPI-743, and Bendavia with their antioxidant role help in reducing the oxidative stress experienced by the cell thereby preventing the progression of the disease. Genetic counseling plays an effective role in making the family members aware of the inheritance pattern of the disease. Gene therapy is an alternative for curing the disease but is still under study. This review focuses on the role of mitochondrial genes in causing LHON and therapeutics available for treating the disease. A systematic search has been adopted in various databases using the keywords "LHON," "mitochondria," "ND1," "ND4," "ND6," and "therapy" and the following review on mitochondrial genetics and therapeutics of LHON has been developed with obtained articles from 1988 to 2017.

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.

Mild phenotypes and proper supercomplex assembly in human cells carrying the homoplasmic m.15557G > A mutation in cytochrome b gene

Respiratory complex III (CIII) is the first enzymatic bottleneck of the mitochondrial respiratory chain both in its native dimeric form and in supercomplexes. The mammalian CIII comprises 11 subunits among which cytochrome b is central in the catalytic core, where oxidation of ubiquinol occurs at the Qo site. The Qo- or PEWY-motif of cytochrome b is the most conserved through species. Importantly, the highly conserved glutamate at position 271 (Glu271) has never been studied in higher eukaryotes so far and its role in the Q-cycle remains debated. Here, we showed that the homoplasmic m.15557G > A/MT-CYB, which causes the p.Glu271Lys amino acid substitution predicted to dramatically affect CIII, induces a mild mitochondrial dysfunction in human transmitochondrial cybrids. Indeed, we found that the severity of such mutation is mitigated by the proper assembly of CIII into supercomplexes, which may favor an optimal substrate channeling and buffer superoxide production in vitro.

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.

Disease-associated mitochondrial mutations and the evolution of primate mitogenomes

Several human diseases have been associated with mutations in mitochondrial genes comprising a set of confirmed and reported mutations according to the MITOMAP database. An analysis of complete mitogenomes across 139 primate species showed that most confirmed disease-associated mutations occurred in aligned codon positions and gene regions under strong purifying selection resulting in a strong evolutionary conservation. Only two confirmed variants (7.1%), coding for the same amino acids accounting for severe human diseases, were identified without apparent pathogenicity in non-human primates, like the closely related Bornean orangutan. Conversely, reported disease-associated mutations were not especially concentrated in conserved codon positions, and a large fraction of them occurred in highly variable ones. Additionally, 88 (45.8%) of reported mutations showed similar variants in several non-human primates and some of them have been present in extinct species of the genus Homo. Considering that recurrent mutations leading to persistent variants throughout the evolutionary diversification of primates are less likely to be severely damaging to fitness, we suggest that these 88 mutations are less likely to be pathogenic. Conversely, 69 (35.9%) of reported disease-associated mutations occurred in extremely conserved aligned codon positions which makes them more likely to damage the primate mitochondrial physiology.

Idebenone: A Review in Leber's Hereditary Optic Neuropathy

Idebenone (Raxone(®)), a short-chain benzoquinone, is the only disease-specific drug approved to treat visual impairment in adolescents and adults with Leber's hereditary optic neuropathy (LHON), a rare genetic mitochondrial disease that causes rapid and progressive bilateral vision loss. The mechanism of action of idebenone involves its antioxidant properties and ability to act as a mitochondrial electron carrier. Idebenone overcomes mitochondrial complex I respiratory chain deficiency in patients with LHON by transferring electrons directly to mitochondrial complex III (by-passing complex I), thereby restoring cellular energy (ATP) production and re-activating inactive-but-viable retinal ganglion cells, which ultimately prevents further vision loss and promotes vision recovery. The approval of idebenone in the treatment of LHON was based on the overall data from a randomized clinical trial, a follow-up study and real-world data. Taken together, these studies provide convincing evidence that oral idebenone 900 mg/day for 24 weeks has persistent beneficial effects in preventing further vision impairment and promoting vision recovery in patients with LHON relative to the natural course of the disease. Therefore, idebenone is a valuable agent to treat visual impairment in adolescents and adults with LHON.

Leber Hereditary Optic Neuropathy: A Mitochondrial Disease Unique in Many Ways

Leber hereditary optic neuropathy (LHON) was the first mitochondrial disease to be identified as being caused by mutations in the mitochondrial DNA (mtDNA). This disease has been studied extensively in the past two decades, particularly in Brazilian, Chinese and European populations; and many primary mutations have been reported. However, the disease is enigmatic with many unique features, and there still are several important questions to be resolved. The incomplete penetrance, the male-biased disease expression and the prevalence in young adults all defy a proper explanation. It has been reported that the development of LHON is affected by the interaction between mtDNA mutations, mtDNA haplogroup background, nuclear genes, environmental factors and epigenetics. Furthermore, with the help of new animal models for LHON that have been created in recent years, we are continuing to learn more about the mechanism of this disease. The stage has now been reached at which there is a good understanding of both the genetic basis of the disease and its epidemiology, but just how the blindness that follows from the death of cells in the optic nerve can be prevented remains to be a pharmacological challenge. In this chapter, we summarize the progress that has been made in various recent studies on LHON, focusing on the molecular pathogenic mechanisms, clinical features, biochemical effects, the pharmacology and its treatment.

Animal inference on human mitochondrial diseases

Several pathological mutations in the human mitochondrial genome have been characterized based on medical, genetic and biochemical evidence. The observation that the structure and core functions of the mitochondrial genome are conserved from animals to man suggests that the analysis of animal variation may be informative to further characterize, and possibly predict, human pathological variants. We studied the distribution of sequence site-wise diversity and structural heterogeneity (based on several scales of hydrophobicity and supercomplex classification of mitochondrial genes) at different taxonomic levels in ∼15,000 human and animal genomes. We found that human pathological mutations tend to lay in regions of low diversity and that states that are pathological in humans appear to be extremely rare in animals, with two noticeable exceptions (T10663C and C14568T). Focusing on hydrophobicity, as possibly the most general site-wise functional parameter of a protein, we deploy the observed range of hydrophobicity in mammals as a proxy for the range of permissible states compatible with an efficient functioning of the mitochondrial machinery. We show that, while non pathological human variants tend to fall within the hypothesized range, pathological mutations generally fall outside this range. We further analyzed this distribution quantitatively to show that the estimated probability of observed states can indeed be used to predict the pathogenicity of a mutation in humans. This study provides a proof of principle that animal data can indeed be informative to predict the pathogenicity of a human mutation alongside, or in the absence of, additional evidence.

Functional Characterization of Three Concomitant MtDNA LHON Mutations Shows No Synergistic Effect on Mitochondrial Activity

The presence of more than one non-severe pathogenic mutation in the same mitochondrial DNA (mtDNA) molecule is very rare. Moreover, it is unclear whether their co-occurrence results in an additive impact on mitochondrial function relative to single mutation effects. Here we describe the first example of a mtDNA molecule harboring three Leber's hereditary optic neuropathy (LHON)-associated mutations (m.11778G>A, m.14484T>C, m.11253T>C) and the analysis of its genetic, biochemical and molecular characterization in transmitochondrial cells (cybrids). Extensive characterization of cybrid cell lines harboring either the 3 mutations or the single classic m.11778G>A and m.14484T>C mutations revealed no differences in mitochondrial function, demonstrating the absence of a synergistic effect in this model system. These molecular results are in agreement with the ophthalmological characteristics found in the triple mutant patient, which were similar to those carrying single mtDNA LHON mutations.

A multi-parametric workflow for the prioritization of mitochondrial DNA variants of clinical interest

Assigning a pathogenic role to mitochondrial DNA (mtDNA) variants and unveiling the potential involvement of the mitochondrial genome in diseases are challenging tasks in human medicine. Assuming that rare variants are more likely to be damaging, we designed a phylogeny-based prioritization workflow to obtain a reliable pool of candidate variants for further investigations. The prioritization workflow relies on an exhaustive functional annotation through the mtDNA extraction pipeline MToolBox and includes Macro Haplogroup Consensus Sequences to filter out fixed evolutionary variants and report rare or private variants, the nucleotide variability as reported in HmtDB and the disease score based on several predictors of pathogenicity for non-synonymous variants. Cutoffs for both the disease score as well as for the nucleotide variability index were established with the aim to discriminate sequence variants contributing to defective phenotypes. The workflow was validated on mitochondrial sequences from Leber’s Hereditary Optic Neuropathy affected individuals, successfully identifying 23 variants including the majority of the known causative ones. The application of the prioritization workflow to cancer datasets allowed to trim down the number of candidate for subsequent functional analyses, unveiling among these a high percentage of somatic variants. Prioritization criteria were implemented in both standalone (http://sourceforge.net/projects/mtoolbox/) and web version (https://mseqdr.org/mtoolbox.php) of MToolBox.