The clinical maze of mitochondrial neurology
TL;DR: Current knowledge on the genetics and pathogenesis of the six categories of mitochondrial disorders is reviewed, focusing on their salient clinical manifestations and highlighting novel clinical entities.
Abstract: Mitochondrial diseases involve the respiratory chain, which is under the dual control of nuclear and mitochondrial DNA (mtDNA). The complexity of mitochondrial genetics provides one explanation for the clinical heterogeneity of mitochondrial diseases, but our understanding of disease pathogenesis remains limited. Classification of Mendelian mitochondrial encephalomyopathies has been laborious, but whole-exome sequencing studies have revealed unexpected molecular aetiologies for both typical and atypical mitochondrial disease phenotypes. Mendelian mitochondrial defects can affect five components of mitochondrial biology: subunits of respiratory chain complexes (direct hits); mitochondrial assembly proteins; mtDNA translation; phospholipid composition of the inner mitochondrial membrane; or mitochondrial dynamics. A sixth category-defects of mtDNA maintenance-combines features of Mendelian and mitochondrial genetics. Genetic defects in mitochondrial dynamics are especially important in neurology as they cause optic atrophy, hereditary spastic paraplegia, and Charcot-Marie-Tooth disease. Therapy is inadequate and mostly palliative, but promising new avenues are being identified. Here, we review current knowledge on the genetics and pathogenesis of the six categories of mitochondrial disorders outlined above, focusing on their salient clinical manifestations and highlighting novel clinical entities. An outline of diagnostic clues for the various forms of mitochondrial disease, as well as potential therapeutic strategies, is also discussed.
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TL;DR: The demonstration of a mammalian mtDNA genetic bottleneck explains how new germline variants can increase to high levels within a generation, and the ultimate fixation of less-severe mutations that escape germline selection explains how they can contribute to the risk of late-onset disorders.
Abstract: Mitochondrial DNA (mtDNA) mutations have been associated with numerous human diseases, from severe inherited disorders to common late-onset diseases. In this Review, the authors consider the origins of these mtDNA mutations in a single cell, their spread across populations and their contributions to disease risk. Common genetic variants of mitochondrial DNA (mtDNA) increase the risk of developing several of the major health issues facing the western world, including neurodegenerative diseases. In this Review, we consider how these mtDNA variants arose and how they spread from their origin on one single molecule in a single cell to be present at high levels throughout a specific organ and, ultimately, to contribute to the population risk of common age-related disorders. mtDNA persists in all aerobic eukaryotes, despite a high substitution rate, clonal propagation and little evidence of recombination. Recent studies have found that de novo mtDNA mutations are suppressed in the female germ line; despite this, mtDNA heteroplasmy is remarkably common. The demonstration of a mammalian mtDNA genetic bottleneck explains how new germline variants can increase to high levels within a generation, and the ultimate fixation of less-severe mutations that escape germline selection explains how they can contribute to the risk of late-onset disorders.
638 citations
TL;DR: This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates.
Abstract: Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and ex...
630 citations
TL;DR: In this paper, the authors evaluated the minimum prevalence of symptomatic nuclear DNA mutations and symptomatic and asymptomatic mtDNA mutations causing mitochondrial diseases, and found that the mtDNA mutation rate was 1 in 5,000 (20 per 100,000), comparable with the previously published prevalence rates.
Abstract: 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
591 citations
01 Jan 2015
TL;DR: This work comprehensively assessed the prevalence of all forms of adult mitochondrial disease to include pathogenic mutations in both nuclear and mtDNA.
Abstract: 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.
512 citations
Journal Article•
TL;DR: In this paper, mutations in the OPA1 gene can also be responsible for a syndromic form of DOA associated with sensorineural deafness, ataxia, axonal sensory-motor polyneuropathy, chronic progressive external ophthalmoplegia and mitochondrial myopathy.
Abstract: Mutations in OPA1, a dynamin-related GTPase involved in mitochondrial fusion, cristae organization and control of apoptosis, have been linked to non-syndromic optic neuropathy transmitted as an autosomal-dominant trait (DOA). We here report on eight patients from six independent families showing that mutations in the OPA1 gene can also be responsible for a syndromic form of DOA associated with sensorineural deafness, ataxia, axonal sensory-motor polyneuropathy, chronic progressive external ophthalmoplegia and mitochondrial myopathy with cytochrome c oxidase negative and Ragged Red Fibres. Most remarkably, we demonstrate that these patients all harboured multiple deletions of mitochondrial DNA (mtDNA) in their skeletal muscle, thus revealing an unrecognized role of the OPA1 protein in mtDNA stability. The five OPA1 mutations associated with these DOA 'plus' phenotypes were all mis-sense point mutations affecting highly conserved amino acid positions and the nuclear genes previously known to induce mtDNA multiple deletions such as POLG1, PEO1 (Twinkle) and SLC25A4 (ANT1) were ruled out. Our results show that certain OPA1 mutations exert a dominant negative effect responsible for multi-systemic disease, closely related to classical mitochondrial cytopathies, by a mechanism involving mtDNA instability.
394 citations
References
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TL;DR: This finding demonstrated that a nucleotide change in a mitochondrial DNA energy production gene can result in a neurological disease.
Abstract: Leber's hereditary optic neuropathy is a maternally inherited disease resulting in optic nerve degeneration and cardiac dysrhythmia. A mitochondrial DNA replacement mutation was identified that correlated with this disease in multiple families. This mutation converted a highly conserved arginine to a histidine at codon 340 in the NADH dehydrogenase subunit 4 gene and eliminated an Sfa NI site, thus providing a simple diagnostic test. This finding demonstrated that a nucleotide change in a mitochondrial DNA energy production gene can result in a neurological disease.
2,230 citations
TL;DR: Observations demonstrate that mtDNA heteroplasmy can occur in man and that human disease may be associated with defects of the mitochondrial genome.
Abstract: In vitro studies of muscle mitochondrial metabolism in patients with mitochondrial myopathy have identified a variety of functional defects of the mitochondrial respiratory chain, predominantly affecting complex I (NADH-CoQ reductase) or complex III (ubiquinol-cytochrome c reductase) in adult cases. These two enzymes consist of approximately 36 subunits, eight of which are encoded by mitochondrial DNA (mtDNA). The increased incidence of maternal, as opposed to paternal, transmission in familial mitochondrial myopathy suggests that these disorders may be caused by mutations of mtDNA. Multiple restriction endonuclease analysis of leukocyte mtDNA from patients with the disease, and their relatives, showed no differences in cleavage patterns between affected and unaffected individuals in any single maternal line. When muscle mtDNA was studied, nine of 25 patients were found to have two populations of muscle mtDNA, one of which had deletions of up to 7 kilobases in length. These observations demonstrate that mtDNA heteroplasmy can occur in man and that human disease may be associated with defects of the mitochondrial genome.
1,700 citations
TL;DR: Transformants obtained with various mitochondrial donors exhibited a respiratory phenotype that was in most cases distinct from that of the rho 0 parent or the donor, indicating that the genotypes of the mitochondrial and nuclear genomes as well as their specific interactions play a role in the respiratory competence of a cell.
Abstract: Two human cell lines (termed rho 0), which had been completely depleted of mitochondrial DNA (mtDNA) by long-term exposure to ethidium bromide, were found to be dependent on uridine and pyruvate for growth because of the absence of a functional respiratory chain. Loss of either of these two metabolic requirements was used as a selectable marker for the repopulation of rho 0 cells with exogenous mitochondria by complementation. Transformants obtained with various mitochondrial donors exhibited a respiratory phenotype that was in most cases distinct from that of the rho 0 parent or the donor, indicating that the genotypes of the mitochondrial and nuclear genomes as well as their specific interactions play a role in the respiratory competence of a cell.
1,654 citations
TL;DR: The core components of the evolutionarily conserved fusion and fission machineries have now been identified, and mechanistic studies have revealed the first secrets of the complex processes that govern fusion andfission of a double membrane-bound organelle.
Abstract: Mitochondria are dynamic organelles that constantly fuse and divide. These processes (collectively termed mitochondrial dynamics) are important for mitochondrial inheritance and for the maintenance of mitochondrial functions. The core components of the evolutionarily conserved fusion and fission machineries have now been identified, and mechanistic studies have revealed the first secrets of the complex processes that govern fusion and fission of a double membrane-bound organelle. Mitochondrial dynamics was recently recognized as an important constituent of cellular quality control. Defects have detrimental consequences on bioenergetic supply and contribute to the pathogenesis of neurodegenerative diseases. These findings open exciting new directions to explore mitochondrial biology.
1,637 citations
TL;DR: It is concluded that the primary gene mutated in CMT2A is MFN2, and seven large pedigrees affected with Charcot-Marie-Tooth neuropathy type 2A are concluded.
Abstract: We report missense mutations in the mitochondrial fusion protein mitofusin 2 (MFN2) in seven large pedigrees affected with Charcot-Marie-Tooth neuropathy type 2A (CMT2A). Although a mutation in kinesin family member 1B-β (KIF1B) was associated with CMT2A in a single Japanese family, we found no mutations in KIF1B in these seven families. Because these families include all published pedigrees with CMT2A and are ethnically diverse, we conclude that the primary gene mutated in CMT2A is MFN2.
1,479 citations