Objective
The prevalence of mitochondrial disease has proven difficult to establish, predominantly as a result of clinical and genetic heterogeneity. The phenotypic spectrum of mitochondrial disease ...has expanded significantly since the original reports that associated classic clinical syndromes with mitochondrial DNA (mtDNA) rearrangements and point mutations. The revolution in genetic technologies has allowed interrogation of the nuclear genome in a manner that has dramatically improved the diagnosis of mitochondrial disorders. We comprehensively assessed the prevalence of all forms of adult mitochondrial disease to include pathogenic mutations in both nuclear and mtDNA.
Methods
Adults with suspected mitochondrial disease in the North East of England were referred to a single neurology center from 1990 to 2014. For the midyear period of 2011, we evaluated the minimum prevalence of symptomatic nuclear DNA mutations and symptomatic and asymptomatic mtDNA mutations causing mitochondrial diseases.
Results
The minimum prevalence rate for mtDNA mutations was 1 in 5,000 (20 per 100,000), comparable with our previously published prevalence rates. In this population, nuclear mutations were responsible for clinically overt adult mitochondrial disease in 2.9 per 100,000 adults.
Interpretation
Combined, our data confirm that the total prevalence of adult mitochondrial disease, including pathogenic mutations of both the mitochondrial and nuclear genomes (≈1 in 4,300), is among the commonest adult forms of inherited neurological disorders. These figures hold important implications for the evaluation of interventions, provision of evidence‐based health policies, and planning of future services. Ann Neurol 2015 Ann Neurol 2015;77:753–759
ABSTRACT
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.
Mutations in OPA1 cause autosomal dominant optic atrophy (DOA) as well as DOA+, a phenotype characterized by more severe neurological deficits. OPA1 deficiency causes mitochondrial fragmentation and ...also disrupts cristae, respiration, mitochondrial DNA (mtDNA) maintenance, and cell viability. It has not yet been established whether phenotypic severity can be modulated by genetic modifiers of OPA1. We screened the entire known mitochondrial proteome (1,531 genes) to identify genes that control mitochondrial morphology using a first‐in‐kind imaging pipeline. We identified 145 known and novel candidate genes whose depletion promoted elongation or fragmentation of the mitochondrial network in control fibroblasts and 91 in DOA+ patient fibroblasts that prevented mitochondrial fragmentation, including phosphatidyl glycerophosphate synthase (PGS1). PGS1 depletion reduces CL content in mitochondria and rebalances mitochondrial dynamics in OPA1‐deficient fibroblasts by inhibiting mitochondrial fission, which improves defective respiration, but does not rescue mtDNA depletion, cristae dysmorphology, or apoptotic sensitivity. Our data reveal that the multifaceted roles of OPA1 in mitochondria can be functionally uncoupled by modulating mitochondrial lipid metabolism, providing novel insights into the cellular relevance of mitochondrial fragmentation.
Synopsis
Phenotypic screening of OPA1 patient fibroblasts identifies multiple genetic suppressors of mitochondrial fragmentation including PGS1, a key enzyme in cardiolipin biosynthesis. PGS1 depletion reduces mitochondrial fission and restores normal mitochondrial morphology to OPA1‐deficient fibroblasts.
Mitochondrial morphology defects in human fibroblasts can be automatically imaged and quantified by supervised machine learning, allowing for imaging‐based screening.
High‐throughput screening identifies new genes required or the maintenance of mitochondrial morphology in fibroblasts from healthy individuals.
High‐throughput screening of OPA1 patient fibroblasts identifies genetic modifiers of mitochondrial fragmentation not previously linked to Dominant Optic Atrophy.
Loss of PGS1 in OPA1‐deficient fibroblasts restores mitochondrial morphology and respiration, but not cristae dysmorphology, apoptotic sensitivity, nor mtDNA content.
Mitochondrial morphology defects can be functionally uncoupled from other pleiotropic effects of OPA1 loss.
Phenotypic screening of OPA1 patient fibroblasts identifies multiple genetic suppressors of mitochondrial fragmentation including PGS1, a key enzyme in cardiolipin biosynthesis. PGS1 depletion reduces mitochondrial fission and restores normal mitochondrial morphology to OPA1‐deficient fibroblasts.
Objective
Autosomal dominant optic atrophy (ADOA) starts in early childhood with loss of visual acuity and color vision deficits. OPA1 mutations are responsible for the majority of cases, but in a ...portion of patients with a clinical diagnosis of ADOA, the cause remains unknown. This study aimed to identify novel ADOA‐associated genes and explore their causality.
Methods
Linkage analysis and sequencing were performed in multigeneration families and unrelated patients to identify disease‐causing variants. Functional consequences were investigated in silico and confirmed experimentally using the zebrafish model.
Results
We defined a new ADOA locus on 7q33‐q35 and identified 3 different missense variants in SSBP1 (NM_001256510.1; c.113G>A p.(Arg38Gln), c.320G>A p.(Arg107Gln) and c.422G>A p.(Ser141Asn)) in affected individuals from 2 families and 2 singletons with ADOA and variable retinal degeneration. The mutated arginine residues are part of a basic patch that is essential for single‐strand DNA binding. The loss of a positive charge at these positions is very likely to lower the affinity of SSBP1 for single‐strand DNA. Antisense‐mediated knockdown of endogenous ssbp1 messenger RNA (mRNA) in zebrafish resulted in compromised differentiation of retinal ganglion cells. A similar effect was achieved when mutated mRNAs were administered. These findings point toward an essential role of ssbp1 in retinal development and the dominant‐negative nature of the identified human variants, which is consistent with the segregation pattern observed in 2 multigeneration families studied.
Interpretation
SSBP1 is an essential protein for mitochondrial DNA replication and maintenance. Our data have established pathogenic variants in SSBP1 as a cause of ADOA and variable retinal degeneration. ANN NEUROL 2019;86:368–383
Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking ...pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.
Management of LHON ‐ An update Yu‐Wai‐Man, Patrick
Acta ophthalmologica (Oxford, England),
January 2024, 2024-01-00, 20240101, Volume:
102, Issue:
S279
Journal Article
Peer reviewed
Leber hereditary optic neuropathy (LHON) is a devastating cause of blindness that predominantly affects young adult men. Over 90% of patients harbour one of three mitochondrial DNA point mutations, ...namely, m.3460G>A (MT‐ND1), m.11778G>A (MT‐ND4) and m.144484T>C (MT‐ND6). Idebenone is the only approved treatment for LHON. New evidence has refined the management guidelines for idebenone, in particular the duration of treatment and the therapeutic window of opportunity. Gene therapy using allotopic expression is an attractive strategy for LHON as it corrects the underlying mitochondrial DNA defect. Promising results have been obtained for the m.1178G>A (MT‐ND4) mutation for patients treated within 1 year of disease onset with an intravitreal injection of a modified adeno‐associated viral vector carrying the replacement gene.
Three decades have elapsed since the first reports of human disease being directly caused by defects within the mitochondrial genome, including the identification of the m.11778G > A mitochondrial ...DNA (mtDNA) mutation in patients with Leber hereditary optic neuropathy (LHON). LHON causes severe bilateral visual loss among young adults with an estimated prevalence of 1 in 30 000 in the general population. Most patients carry point mutations in mtDNA with the three most common pathogenic variants being m.3460G > A (MT‐ND1), m.11778G > A (MT‐ND4) and m.14484 T > G (MT‐ND6). LHON carries a poor visual prognosis and patient management remains largely supportive. However, major advances in our understanding of the mechanisms underpinning retinal ganglion cell loss in this mitochondrial disorder are paving the way for novel forms of treatment aimed at halting or reversing visual deterioration at different stages of the disease process. The eye is an ideal target organ for gene therapy given its relative ease of anatomical access, but some key issues need to be explored further, in particular, the most efficient gene delivery systems and the optimal window for therapeutic intervention in LHON.
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