Low disease risk and penetrance in Leber hereditary optic neuropathy

Family Planning in Genetic Optic Atrophies in Israel, a Case Series and a Discussion of Ethical Considerations

BACKGROUND

Patients with genetic optic atrophies must navigate all stages of life with their visual impairment, including the important milestone of family planning. Advances in genetic testing now allows physicians and affected families to consider medical help with the aim of preventing blindness through preconception, preimplantation, and perinatal methods.

METHODS

This case series presents 4 patients with different genetic optic atrophies (Leber hereditary optic neuropathy [LHON], autosomal dominant optic atrophy, Wolfram syndrome, and papillorenal syndrome) who were followed by the Neuro-Ophthalmology Unit at a tertiary medical center between 2010 and 2023 and were of child-bearing age. The aim of this study was to increase understanding in family planning options for patients with optic atrophies, raise awareness of the solutions available, and provide guidance for clinicians to support their patients.

RESULTS

Advances in medicine, genetics, and medical technology allow multidisciplinary teams to assist patients in fulfilling their desire for a genetically healthy offspring. Customized solutions can be designed to meet the specific challenges posed by each type of genetic optic atrophy. The solutions proposed in this series are based on genetic testing done in the parents, which then allows to plan medical and genetic intervention individually. The solutions opted for in this series range from the decision to not have another child until PGD (Preimplantation genetic diagnosis).

CONCLUSIONS

We describe how genetic advancements have made it possible for patients with the 4 most common hereditary optic atrophies to fulfill their wish to have children without visually threatening genetic mutations. We also review the recent literature on the penetrance of optic atrophy in OA-mutation carriers and raise 2 significant ethical considerations: the reduction of a future life to a non-life-threatening impairment and that of public expenditure for non-life-threatening conditions.

Modifier variants in metabolic pathways are associated with an increased penetrance of Leber's Hereditary Optic Neuropathy

Leber's hereditary optic neuropathy (LHON) is a debilitating mitochondrial disease characterised by bilateral painless vision loss. Despite being the most prevalent mitochondrial disorder, the precise pathophysiological mechanisms underlying the penetrance of LHON remain poorly understood. Nuclear modifier genes have been long suspected to affect phenotype-severity, however, specific cellular pathways implicated in the disease penetrance have been only suggested recently. In recent years, autosomal recessive variants in nuclear genes involved in complex I function and metabolic pathways were recognised to cause a typical LHON phenotype. This was proposed as a new autosomal recessive disease mechanism for LHON (arLHON). The association between nuclear variants and the LHON phenotype makes the nuclear pathways disrupted in arLHON the strongest candidates to act as modifiers of mitochondrial LHON (mLHON). In this study we systematically investigated a large cohort of 23 symptomatic and 28 asymptomatic individuals carrying one of the three primary mitochondrial LHON variants. We identified several heterozygous pathogenic nuclear variants amongst the affected individuals that were consistently linked to metabolic and complex I related pathways, mirroring those disrupted in arLHON. Our findings are consistent with the presence of a second hit in specific biological pathways impairing ATP production. We propose that in addition to the primary mitochondrial variants, disruption in these nuclear-encoded pathways drives the clinical manifestation of LHON. Genes involved in the same pathways also emerge as exciting candidates for future association with arLHON. The present study deepens our understanding of LHON's pathophysiology and provides a new framework for identifying novel disease-modifying targets.

Family and genetic counseling in Leber hereditary optic neuropathy

AIM

Leber hereditary optic neuropathy (LHON) predominantly manifests during adolescence or young adulthood, resulting in sudden and profound vision loss in individuals who previously had normal vision. This abrupt change significantly impacts daily life, necessitating emotional support, counseling and low-vision rehabilitative services to help affected individuals cope with the shock and adapt to their residual vision. The psychosocial burden of dealing with vision loss extends beyond the individuals directly affected by LHON, affecting matrilineal relatives who face the dual challenges of grieving for their loved one's vision loss and managing their own uncertainty about potential vision loss and its familial implications.

METHOD

We reviewed key information that needs to be obtained prior to genetic counseling for LHON. We reviewed key counseling issues within LHON-affected families and the issues pending several subgroups of family members with distinct and varying genetic counseling needs.

RESULTS

Family subgroups requiring specific counseling issues include the individuals affected by LHON, their mother, siblings, father, partner, and children. Genetic counseling plays an integral part of clinical care in families affected by LHON, providing tailored support and information to each subgroup.

CONCLUSION

To provide accurate information to families and guide them toward potential supports, treatments and preventive measures, health professionals need to be aware of the factors influencing visual recovery and individual risk of vision loss.

Recognizing Leber's Hereditary Optic Neuropathy to avoid delayed diagnosis and misdiagnosis

Leber's Hereditary Optic Neuropathy (LHON) is a maternally inherited optic nerve disease primarily caused by mutations in mitochondrial DNA (mtDNA). The peak of onset is typically between 15 and 30 years, but variability exists. Misdiagnosis, often as inflammatory optic neuritis, delays treatment, compounded by challenges in timely genetic diagnosis. Given the availability of a specific treatment for LHON, its early diagnosis is imperative to ensure therapeutic appropriateness. This work gives an updated guidance about LHON differential diagnosis to clinicians dealing also with multiple sclerosi and neuromyelitis optica spectrtum disorders-related optic neuritis. LHON diagnosis relies on clinical signs and paraclinical evaluations. Differential diagnosis in the acute phase primarily involves distinguishing inflammatory optic neuropathies, considering clinical clues such as ocular pain, fundus appearance and visual recovery. Imaging analysis obtained with Optical Coherence Tomography (OCT) assists clinicians in early recognition of LHON and help avoiding misdiagnosis. Genetic testing for the three most common LHON mutations is recommended initially, followed by comprehensive mtDNA sequencing if suspicion persists despite negative results. We present and discuss crucial strategies for accurate diagnosis and management of LHON cases.

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.

Galactose-replacement unmasks the biochemical consequences of the G11778A mitochondrial DNA mutation of LHON in patient-derived fibroblasts

Leber's hereditary optic neuropathy (LHON) is a visual impairment associated with mutations of mitochondrial genes encoding elements of the electron transport chain. While much is known about the genetics of LHON, the cellular pathophysiology leading to retinal ganglion cell degeneration and subsequent vision loss is poorly understood. The impacts of the G11778A mutation of LHON on bioenergetics, redox balance and cell proliferation were examined in patient-derived fibroblasts. Replacement of glucose with galactose in the culture media reveals a deficit in the proliferation of G11778A fibroblasts, imparts a reduction in ATP biosynthesis, and a reduction in capacity to accommodate exogenous oxidative stress. While steady-state ROS levels were unaffected by the LHON mutation, cell survival was diminished in response to exogenous H2O2.

Digenic Inheritance in Rare Disorders and Mitochondrial Disease-Crossing the Frontier to a More Comprehensive Understanding of Etiology

Our understanding of rare disease genetics has been shaped by a monogenic disease model. While the traditional monogenic disease model has been successful in identifying numerous disease-associated genes and significantly enlarged our knowledge in the field of human genetics, it has limitations in explaining phenomena like phenotypic variability and reduced penetrance. Widening the perspective beyond Mendelian inheritance has the potential to enable a better understanding of disease complexity in rare disorders. Digenic inheritance is the simplest instance of a non-Mendelian disorder, characterized by the functional interplay of variants in two disease-contributing genes. Known digenic disease causes show a range of pathomechanisms underlying digenic interplay, including direct and indirect gene product interactions as well as epigenetic modifications. This review aims to systematically explore the background of digenic inheritance in rare disorders, the approaches and challenges when investigating digenic inheritance, and the current evidence for digenic inheritance in mitochondrial disorders.

The genetic puzzle of a SOD1-patient with ocular ptosis and a motor neuron disease: a case report

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with a complex genetic architecture, showing monogenic, oligogenic, and polygenic inheritance. In this study, we describe the case of a 71 years-old man diagnosed with ALS with atypical clinical features consisting in progressive ocular ptosis and sensorineural deafness. Genetic analyses revealed two heterozygous variants, in the SOD1 (OMIM*147450) and the TBK1 (OMIM*604834) genes respectively, and furthermore mitochondrial DNA (mtDNA) sequencing identified the homoplasmic m.14484T>C variant usually associated with Leber's Hereditary Optic Neuropathy (LHON). We discuss how all these variants may synergically impinge on mitochondrial function, possibly contributing to the pathogenic mechanisms which might ultimately lead to the neurodegenerative process, shaping the clinical ALS phenotype enriched by adjunctive clinical features.

Mitochondrial DNA Variants at Low-Level Heteroplasmy and Decreased Copy Numbers in Chronic Kidney Disease (CKD) Tissues with Kidney Cancer

The human mitochondrial genome (mtDNA) is a circular DNA molecule with a length of 16.6 kb, which contains a total of 37 genes. Somatic mtDNA mutations accumulate with age and environmental exposure, and some types of mtDNA variants may play a role in carcinogenesis. Recent studies observed mtDNA variants not only in kidney tumors but also in adjacent kidney tissues, and mtDNA dysfunction results in kidney injury, including chronic kidney disease (CKD). To investigate whether a relationship exists between heteroplasmic mtDNA variants and kidney function, we performed ultra-deep sequencing (30,000×) based on long-range PCR of DNA from 77 non-tumor kidney tissues of kidney cancer patients with CKD (stages G1 to G5). In total, this analysis detected 697 single-nucleotide variants (SNVs) and 504 indels as heteroplasmic (0.5% ≤ variant allele frequency (VAF) < 95%), and the total number of detected SNVs/indels did not differ between CKD stages. However, the number of deleterious low-level heteroplasmic variants (pathogenic missense, nonsense, frameshift and tRNA) significantly increased with CKD progression (p < 0.01). In addition, mtDNA copy numbers (mtDNA-CNs) decreased with CKD progression (p < 0.001). This study demonstrates that mtDNA damage, which affects mitochondrial genes, may be involved in reductions in mitochondrial mass and associated with CKD progression and kidney dysfunction.

Approaches to the epidemiology of NOHL in the region of Madrid: Survey of neuro-ophthalmologists

OBJECTIVE

To estimate the epidemiology of Leber's optic neuropathy (NOHL) in the Region of Madrid.

MATERIAL AND METHODS

The neuro-ophthalmologists who work at public hospitals of the CAM were interviewed by telephone. They were asked about the number of patients with NOHL that they had diagnosed during the time that they had been responsible for the neuro-ophthalmology department of that public hospital. The time worked and the population attended by the hospital were used to calculate the number of inhabitant-years in follow-up by each center during the corresponding period. The basic information of each case (date of birth, mutation, date of visual loss) was registered to avoid duplications.

RESULTS

Our work estimates a global incidence of 2.34 cases for 10,000,000 inhabitants-year and a prevalence estimated from incidence of one case for each 106.682 inhabitants. This prevalence was very similar in all the studied areas and considerably lower than that reported by other studies.

CONCLUSION

This work constitutes the first approach to the epidemiology of this disease in Spain. The prevalence of LHON in the region of Madrid, is probably lower than that reported in the literature in other regions. The prevalence and the incidence were homogeneously low in the 26 studied areas.

Oxidative Stress: A Suitable Therapeutic Target for Optic Nerve Diseases?

Optic nerve disorders encompass a wide spectrum of conditions characterized by the loss of retinal ganglion cells (RGCs) and subsequent degeneration of the optic nerve. The etiology of these disorders can vary significantly, but emerging research highlights the crucial role of oxidative stress, an imbalance in the redox status characterized by an excess of reactive oxygen species (ROS), in driving cell death through apoptosis, autophagy, and inflammation. This review provides an overview of ROS-related processes underlying four extensively studied optic nerve diseases: glaucoma, Leber's hereditary optic neuropathy (LHON), anterior ischemic optic neuropathy (AION), and optic neuritis (ON). Furthermore, we present preclinical findings on antioxidants, with the objective of evaluating the potential therapeutic benefits of targeting oxidative stress in the treatment of optic neuropathies.

Is the disease risk and penetrance in Leber hereditary optic neuropathy actually low?

Pedigree analysis showed that a large proportion of Leber hereditary optic neuropathy (LHON) family members who carry a mitochondrial risk variant never lose vision. Mitochondrial haplotype appears to be a major factor influencing the risk of vision loss from LHON. Mitochondrial variants, including m.14484T>C and m.11778G>A, have been added to gene arrays, and thus many patients and research participants are tested for LHON mutations. Analysis of the UK Biobank and Australian cohort studies found more than 1 in 1,000 people in the general population carry either the m.14484T>C or the m.11778G>A LHON variant. None of the subset of carriers examined had visual acuity at 20/200 or worse, suggesting a very low penetrance of LHON. Haplogroup analysis of m.14484T>C carriers showed a high rate of haplogroup U subclades, previously shown to have low penetrance in pedigrees. Penetrance calculations of the general population are lower than pedigree calculations, most likely because of modifier genetic factors. This Matters Arising Response paper addresses the Watson et al. (2022) Matters Arising paper, published concurrently in The American Journal of Human Genetics.