Mitochondrial Myopathy Research Articles

Exercise Testing, Physical Training and Fatigue in Patients with Mitochondrial Myopathy Related to mtDNA Mutations.

Mutations in mitochondrial DNA (mtDNA) cause disruption of the oxidative phosphorylation chain and impair energy production in cells throughout the human body. Primary mitochondrial disorders due to mtDNA mutations can present with symptoms from adult-onset mono-organ affection to death in infancy due to multi-organ involvement. The heterogeneous phenotypes that patients with a mutation of mtDNA can present with are thought, at least to some extent, to be a result of differences in mtDNA mutation load among patients and even among tissues in the individual. The most common symptom in patients with mitochondrial myopathy (MM) is exercise intolerance. Since mitochondrial function can be assessed directly in skeletal muscle, exercise studies can be used to elucidate the physiological consequences of defective mitochondria due to mtDNA mutations. Moreover, exercise tests have been developed for diagnostic purposes for mitochondrial myopathy. In this review, we present the rationale for exercise testing of patients with MM due to mutations in mtDNA, evaluate the diagnostic yield of exercise tests for MM and touch upon how exercise tests can be used as tools for follow-up to assess disease course or effects of treatment interventions.

DNA2 in Chromosome Stability and Cell Survival—Is It All about Replication Forks?

The conserved nuclease-helicase DNA2 has been linked to mitochondrial myopathy, Seckel syndrome, and cancer. Across species, the protein is indispensable for cell proliferation. On the molecular level, DNA2 has been implicated in DNA double-strand break (DSB) repair, checkpoint activation, Okazaki fragment processing (OFP), and telomere homeostasis. More recently, a critical contribution of DNA2 to the replication stress response and recovery of stalled DNA replication forks (RFs) has emerged. Here, we review the available functional and phenotypic data and propose that the major cellular defects associated with DNA2 dysfunction, and the links that exist with human disease, can be rationalized through the fundamental importance of DNA2-dependent RF recovery to genome duplication. Being a crucial player at stalled RFs, DNA2 is a promising target for anti-cancer therapy aimed at eliminating cancer cells by replication-stress overload.

Augmenter of liver regeneration regulates cellular iron homeostasis by modulating mitochondrial transport of ATP-binding cassette B8.

Chronic loss of Augmenter of Liver Regeneration (ALR) results in mitochondrial myopathy with cataracts; however, the mechanism for this disorder remains unclear. Here, we demonstrate that loss of ALR, a principal component of the MIA40/ALR protein import pathway, results in impaired cytosolic Fe/S cluster biogenesis in mammalian cells. Mechanistically, MIA40/ALR facilitates the mitochondrial import of ATP-binding cassette (ABC)-B8, an inner mitochondrial membrane protein required for cytoplasmic Fe/S cluster maturation, through physical interaction with ABCB8. Downregulation of ALR impairs mitochondrial ABCB8 import, reduces cytoplasmic Fe/S cluster maturation, and increases cellular iron through the iron regulatory protein-iron response element system. Our finding thus provides a mechanistic link between MIA40/ALR import machinery and cytosolic Fe/S cluster maturation through the mitochondrial import of ABCB8, and offers a potential explanation for the pathology seen in patients with ALR mutations.

The Diagnostic Yield of Electromyography at Detecting Abnormalities on Muscle Biopsy: A Single Center Experience

ABSTRACT Historically, electromyography (EMG) is utilized early in the diagnostic evaluation of neuromuscular disorders, but its importance may be diminishing with more sophisticated genetic, imaging and immunohistochemistry investigations now available. In the present study, the diagnostic yield of EMG at predicting pathological abnormalities confirmed by muscle biopsy was determined at our neuroscience center. A retrospective study of consecutive cases reviewed at neuromuscular multidisciplinary meetings between 2007 and 2016 identified patients who had EMG and muscle biopsy as part of their diagnostic evaluation. EMG and biopsy findings were categorized as myopathic, neurogenic or normal. The diagnostic accuracy was determined by calculating the concordance between EMG and pathological findings. Of the 175 cases included in the analysis, there was definite concordance between EMG and muscle biopsy findings in 134 cases (76.6%). Abnormal EMG produced sensitivity of 87% and specificity of 65% for abnormal muscle biopsy. Seventeen patients had a normal EMG and an abnormal muscle biopsy, of which 6 had histopathological findings consistent with mitochondrial myopathy, central core myopathy or glycogen storage disorder. Conflicting EMG and muscle biopsy findings were observed in 10 cases. Inclusion body myositis, chronic neuromuscular disorders and dual pathologies were associated with discordant findings. This study demonstrates that EMG has accurate predictive value in diagnosing neuromuscular disorders at our neuroscience center. EMG retains a vital role, particularly in initial diagnostic evaluations of neuromuscular disorders.

EP039 - Rare presentation of FDX2-related mitochondrial myopathy

eP039 - Rare presentation of FDX2-related mitochondrial myopathy

A study protocol for quantifying patient preferences in neuromuscular disorders: a case study of the IMI PREFER Project

Objectives: Patient preference studies are increasingly used to inform decision-making during the medical product lifecycle but are rarely used to inform early stages of drug development. The primary aim of this study is to quantify treatment preferences of patients with neuromuscular disorders, which represent serious and debilitating conditions with limited or no treatment options available.Methods: This quantitative patient preferences study was designed as an online survey, with a cross-over design. This study will target two different diseases from the neuromuscular disorders disease group, myotonic dystrophy type 1 (DM1) and mitochondrial myopathies (MM). Despite having different physio-pathological pathways both DM1 and MM manifest in a clinically similar manner and may benefit from similar treatment options. The sample will be stratified into three subgroups: two patient groups differentiated by age of symptom onset and one caregivers group. Each subgroup will be randomly assigned to complete two of three different preference elicitation methods at two different time points: Q-methodology survey, discrete choice experiment, and best-worst scaling type 2, allowing cross-comparisons of the results across each study time within participants and within elicitation methods. Additional variables such as sociodemographic, clinical and health literacy will be collected to enable analysis of potential heterogeneity.Ethics and Dissemination: This study protocol has undergone ethical review and approval by the Newcastle University R&D Ethics Committee (Ref: 15169/2018). All participants will be invited to give electronic informed consent to take part in the study prior accessing the online survey. All electronic data will be anonymised prior analysis. This study is part of the Patient Preferences in Benefit-Risk Assessments during the Drug Life Cycle (IMI-PREFER) project, a public-private collaborative research project aiming to develop expert and evidence-based recommendations on how and when patient preferences can be assessed and used to inform medical product decision making.

A C. elegans genome-wide RNAi screen for altered levamisole sensitivity identifies genes required for muscle function.

At the neuromuscular junction (NMJ), postsynaptic ionotropic acetylcholine receptors (AChRs) transduce a chemical signal released from a cholinergic motor neuron into an electrical signal to induce muscle contraction. To identify regulators of postsynaptic function, we conducted a genome-wide RNAi screen for genes required for proper response to levamisole, a pharmacological agonist of ionotropic L-AChRs at the Caenorhabditis elegans NMJ. A total of 117 gene knockdowns were found to cause levamisole hypersensitivity, while 18 resulted in levamisole resistance. Our screen identified conserved genes important for muscle function including some that are mutated in congenital myasthenic syndrome, congenital muscular dystrophy, congenital myopathy, myotonic dystrophy, and mitochondrial myopathy. Of the genes found in the screen, we further investigated those predicted to play a role in endocytosis of cell surface receptors. Loss of the Epsin homolog epn-1 caused levamisole hypersensitivity and had opposing effects on the levels of postsynaptic L-AChRs and GABAA receptors, resulting in increased and decreased abundance, respectively. We also examined other genes that resulted in a levamisole-hypersensitive phenotype when knocked down including gas-1, which functions in Complex I of the mitochondrial electron transport chain. Consistent with altered ATP synthesis impacting levamisole response, treatment of wild-type animals with levamisole resulted in L-AChR–dependent depletion of ATP levels. These results suggest that the paralytic effects of levamisole ultimately lead to metabolic exhaustion.

A novel exercise testing algorithm to diagnose mitochondrial myopathy.

Oxygen uptake efficiency slope (OUES) is a noninvasive cardiopulmonary exercise testing (CPET) measurement based on oxygen uptake (V˙O2 ) and minute ventilation (V˙E) and is a marker of the efficiency of oxygen utilization by the body. However, it has not been studied in mitochondrial disorders. We explored noninvasive CPET parameters, including OUES, as a way to reliably diagnose mitochondrial myopathy. We performed cycle ergometer maximal exercise testing on definite and suspected mitochondrial myopathy subjects (MM-D and MM-S) and their age- and sex-matched controls. OUES was corrected for body surface area (OUES/BSA) to eliminate the effect of body size. A total of 40 participants, including 20 MM-D (n = 13; 6 males; aged 14-64 years) and 7 MM-S (5 males, aged 11-30 years) subjects and 20 controls, completed the study. MM-D subjects showed lower aerobic fitness than controls. OUES/BSA was lower in MM-D subjects, suggesting inefficient oxygen utilization. Area under the curve (AUC) and 95% confidence interval (CI) for OUES/BSA (AUC, 0.91; 95% CI, 0.80-1.00), peak V˙O2 percent predicted (AUC, 0.95; 95% CI, 0.86-1.00), and V˙O2 /work slope (AUC, 0.94; 95% CI, 0.85-1.00) showed excellent ability to diagnose mitochondrial myopathy in MM-D subjects. We applied a diagnostic approach based on the parameters just noted to MM-S subjects and their controls and were able to support or disprove the diagnosis of mitochondrial myopathy. We proposed and applied an approach based on the aformentioned three CPET parameters to diagnose mitochondrial myopathy reliably and found it to be clinically useful.

Mitochondrial Myopathy, Encephalopathy, Lactic Acidosis, and Stroke-like Episodes(MELAS)

Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes(MELAS)is the most dominant form of mitochondrial diseases, presenting with headaches, seizures, and stroke-like episodes. Stroke-like episodes is a distinguishing feature of MELAS. Symptoms appear before the age of 20 years in 65-76% of patients. For the clinical diagnosis of MELAS, evidence of lactate accumulation in the central nervous system is important. The radiographic features of MELAS are stroke-like lesions in the affected brain areas, primarily the occipito-parietal or posterior temporal lobe. MRI shows high signal intensities on T2-weighted or FLAIR images. The cerebral blood flow in lesions can be increased in the acute phase. MR spectroscopy(MRS)shows a lactate peak in the brain lesions, which is important evidence of lactate accumulation. In pediatric or young adult patients with occipito-parietal stroke-like lesions, a prominent lactate peak in MRS is the key radiographic sign that supports the diagnosis of MELAS.

Taurine rescues mitochondria-related metabolic impairments in the patient-derived induced pluripotent stem cells and epithelial-mesenchymal transition in the retinal pigment epithelium

Mitochondria participate in various metabolic pathways, and their dysregulation results in multiple disorders, including aging-related diseases. However, the metabolic changes and mechanisms of mitochondrial disorders are not fully understood. Here, we found that induced pluripotent stem cells (iPSCs) from a patient with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) showed attenuated proliferation and survival when glycolysis was inhibited. These deficits were rescued by taurine administration. Metabolomic analyses showed that the ratio of the reduced (GSH) to oxidized glutathione (GSSG) was decreased; whereas the levels of cysteine, a substrate of GSH, and oxidative stress markers were upregulated in MELAS iPSCs. Taurine normalized these changes, suggesting that MELAS iPSCs were affected by the oxidative stress and taurine reduced its influence. We also analyzed the retinal pigment epithelium (RPE) differentiated from MELAS iPSCs by using a three-dimensional culture system and found that it showed epithelial mesenchymal transition (EMT), which was suppressed by taurine. Therefore, mitochondrial dysfunction caused metabolic changes, accumulation of oxidative stress that depleted GSH, and EMT in the RPE that could be involved in retinal pathogenesis. Because all these phenomena were sensitive to taurine treatment, we conclude that administration of taurine may be a potential new therapeutic approach for mitochondria-related retinal diseases.

Patient Preferences in Rare Diseases: A Qualitative Study in Neuromuscular Disorders to Inform a Quantitative Preference Study.

IntroductionIt has become increasingly important to include patient preference information in decision-making processes for drug development. As neuromuscular disorders represent multisystem, debilitating, and progressive rare diseases with few treatment options, this study aimed to explore unmet health care needs and patient treatment preferences for two neuromuscular disorders, myotonic dystrophy type 1 (DM1) and mitochondrial myopathies (MM) to inform early stages of drug development.MethodsFifteen semi-structured interviews and five focus group discussions (FGDs) were held with DM1 and MM adult patients and caregivers. Topics discussed included (1) reasons for study participation; (2) disease signs/symptoms and their impact on daily lives; (3) top desired benefits; and (4) acceptability of risks and tolerance levels for a hypothetical new treatment. Data were analyzed following a thematic ‘code’ approach.ResultsA total of 52 participants representing a wide range of disease severities participated. ‘Muscle strength’ and ‘energy and endurance’ were the disease-related unmet needs most often mentioned. Additionally, improved ‘balance’, ‘cognition’ and ‘gut function’ were the top desired treatment benefits, while ‘damage to the liver, kidneys or eyes’ was the most concerning risk. Factors influencing their tolerance to risks related to previously having experienced the risk and differentiation between permanent and temporary risks. A few differences were elicited between patients and caregivers.ConclusionsThis qualitative study provided an open forum to elicit treatment-desired benefits and acceptable risks to be established by patients themselves. These findings can inform decisions for developing new treatments and the design of clinical trials for DM1 and MM.Electronic supplementary materialThe online version of this article (10.1007/s40271-020-00482-z) contains supplementary material, which is available to authorized users.

Clinical and molecular characterization of mitochondrial DNA disorders in a group of Argentinian pediatric patients

ObjectiveTo describe the clinical and molecular features of a group of Argentinian pediatric patients with mitochondrial DNA (mtDNA) disorders, and to evaluate the results of the implementation of a classical approach for the molecular diagnosis of mitochondrial diseases. MethodsClinical data from 27 patients with confirmed mtDNA pathogenic variants were obtained from a database of 89 patients with suspected mitochondrial disease, registered from 2014 to 2020. Clinical data, biochemical analysis, neuroimaging findings, muscle biopsy and molecular studies were analyzed. ResultsPatients were 18 females and 9 males, with ages at onset ranging from 1 week to 14 years (median = 4 years). The clinical phenotypes were: mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome (n = 11), Leigh syndrome (n = 5), Kearns-Sayre syndrome (n = 3), Chronic Progressive External Ophthalmoplegia (n = 2), Leber hereditary optic neuropathy (n = 2), myoclonic epilepsy associated with ragged-red fibers (n = 1) and reversible infantile myopathy with cytochrome-C oxidase deficiency (n = 3). Most of the patients harbored pathogenic single nucleotide variants, mainly involving mt-tRNA genes, such as MT-TL1, MT-TE and MT-TK. Other point variants were found in complex I subunits, like MT-ND6, MT-ND4, MT-ND5; or in MT-ATP6. The m.13513G > A variant in MT-ND5 and the m.9185 T > C variant in MT-ATP6 were apparently de novo. The rest of the patients presented large scale-rearrangements, either the “common” deletion or a larger deletion. ConclusionsThis study highlights the clinical and genetic heterogeneity of pediatric mtDNA disorders. All the cases presented with classical phenotypes, being MELAS the most frequent. Applying classical molecular methods, it was possible to achieve a genetic diagnosis in 30% of the cases, suggesting that this is an effective first approach, especially for those centers from low-middle income countries, leaving NGS studies for those patients with inconclusive results.

Muscle fat replacement and modified ragged red fibers in two patients with reversible infantile respiratory chain deficiency

Reversible infantile respiratory chain deficiency is a severe neonatal mitochondrial myopathy that resolves spontaneously. It is caused by the homoplasmic m.14674T>C mtDNA mutation and additional nuclear variants in genes interacting with mt-tRNAGlu have been detected in some patients. We present detailed clinical, imaging, and muscle biopsy findings in a boy and a girl with neonatal hypotonia, feeding difficulties, lactic acidosis, and ragged red fibers. Both patients show fat replacement on muscle imaging, which was mild in the boy, but severe in the girl, affecting mostly the posterior leg muscles. In addition to the homoplasmic m.14674T>C, both patients carried heterozygous variants in QRSL1 (c. 686T>G; p.Val299Gly) and EARS2 (c.358C>T; p.Arg120Trp), respectively. It is very important to recognize the clinical and morphological signs of reversible infantile respiratory chain deficiency as patients should receive intensive supportive care in the first 6 months of life. Understanding the mechanism of the spontaneous recovery may lead to novel therapeutic perspectives in other mitochondrial diseases.

Unraveling an Unusual Phenocopy of Hypertrophic Cardiomyopathy: MELAS Syndrome.

The mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is an uncommon cause of cardiac hypertrophy, fibrosis, and dysfunction. It shares similar features to numerous other causes of left ventricular hypertrophy, and therefore, because of its rarity, may not be immediately considered as a diagnosis. Prompt recognition of clinical and cardiac imaging features may expedite diagnosis and management. We report the case of a 38-year-old man admitted with neurological symptoms and in whom the diagnostic workup led to the diagnosis of MELAS syndrome with cardiac involvement.

Clinico-pathological and Molecular Spectrum of Mitochondrial Polymerase γ Mutations in a Cohort from India.

Polymerase γ catalytic subunit (POLG), a nuclear gene, encodes the enzyme responsible for mitochondrial DNA (mtDNA) replication. POLG mutations are a major cause of inherited mitochondrial diseases. They present with varied phenotypes, age of onset, and severity. Reports on POLG mutations from India are limited. Hence, this study aimed to describe the clinico-pathological and molecular observations of POLG mutations. A total of 446 patients with clinical diagnosis of mitochondrial disorders were sequenced for all exons and intron-exon boundaries of POLG. Of these, 19 (4.26%) patients (M:F: 10:9) had POLG mutations. The age of onset ranged from 5 to 55years with an overlapping phenotypic spectrum. Ptosis, peripheral neuropathy, seizures, and ataxia were the common neurological features observed. The most common clinical phenotype was chronic progressive external ophthalmoplegia (CPEO) and CPEO plus (n=14). Muscle biopsy showed characteristic features of mitochondrial myopathy in fourteen patients (14/19) and respiratory chain enzyme deficiency in eleven patients (11/19). Multiple mtDNA deletions were seen in 47.36% (9/19) patients. Eight pathogenic POLG variations including two novel variations (p.G132R and p.V1106A) were identified. The common pathogenic mutation identified was p.L304R, being present in eight patients (42.1%) predominantly in the younger age group followed by p.W748S in four patients (21%). To the best of our knowledge, this is the first extensive study from India, highlights the clinico-pathological and molecular spectrum of POLG mutations.

Early death of ALS-linked CHCHD10-R15L transgenic mice with central nervous system, skeletal muscle, and cardiac pathology

SummaryMutations in coiled-coil-helix-coiled-coil-helix domain containing 10 (CHCHD10) have been identified in patients suffering from various degenerative diseases including mitochondrial myopathy, spinal muscular atrophy Jokela type, frontotemporal dementia, and/or amyotrophic lateral sclerosis (ALS). The pathogenic mechanism underlying CHCHD10-linked divergent disorders remains largely unknown. Here we show that transgenic mice overexpressing an ALS-linked CHCHD10 p.R15L mutation leads to an abbreviated lifespan compared with CHCHD10-WT transgenic mice. The occurrence and severity of the phenotype correlates to transgene copy number. Central nervous system (CNS), skeletal muscle, and cardiac pathology is apparent in CHCHD10-R15L transgenic mice. Despite the pathology, CHCHD10-R15L transgenic mice perform comparably to control mice in motor behavioral tasks until very close to death. Although paralysis is not observed, these models provide insight into the pleiotropic nature of CHCHD10 and suggest a contribution of CNS, skeletal muscle, and cardiac pathology to CHCHD10 p.R15L-ALS pathogenesis.

Recurrent rhabdomyolysis and exercise intolerance: A new phenotype of late-onset thymidine kinase 2 deficiency

A 29-year-old man developed, since the age of 18, exercise intolerance and exercise-induced rhabdomyolysis, with myoglobinuria. Muscle biopsy showed ragged-red fibers. Multiple mitochondrial DNA deletions were detected. The previously reported pathogenic homozygous mutation c.323C>T (p.Thr108Met) in TK2 was identified.This case expands the phenotypic spectrum of TK2 deficiency and indicates that it should be considered in the differential diagnosis of episodic rhabdomyolysis and exercise intolerance, along with other metabolic and mitochondrial myopathies. Since a new treatment is under development, it is essential improving knowledge of the natural history of TK2 deficiency.

Diagnostic yield of muscle biopsies in pediatric population: a tertiary center experience.

Muscle biopsy is still an important exam on the investigation of neuromuscular diseases although data regarding its diagnostic yield can be disappointing. We aimed to analyze the diagnostic yield of muscle biopsies in the pediatric population. We retrospectively analyzed a tertiary Neuropathology laboratory database to identify patients (<18 years old), submitted to muscle biopsy between January 2015 and August 2019. Demographics, clinical presentation, diagnosis, treatment, and follow-up were evaluated. Descriptive statistical analysis was performed. One-hundred and six patients were included, 52,8% (n = 56) were male. Median age at biopsy was 6 years (IQR 10 years). Patients were divided into 8 groups, according to clinical diagnostic suspicion: mitochondrial myopathies (n = 29), congenital myopathies (n = 9), inflammatory myopathies (n = 8), muscular dystrophies (n = 7), raised CK values in serum (n = 7), metabolic myopathies (n = 5), weakness /other neuromuscular symptoms (n = 30) and multiple clinical suspicions (n = 11). Biopsy was normal in 50 patients. Of the remaining, 27 displayed specific diagnostic features, with 88,9% (n = 24) allowing a definite diagnosis: muscular dystrophies (n = 7), metabolic myopathies (n = 5), congenital myopathies (n = 4), inflammatory myopathies (n = 4), mitochondrial myopathies (n = 3) and spinal muscular atrophy (n = 1). Histology led to a change of treatment in 4 patients, all diagnosed with inflammatory myopathies. Median length of follow-up was 1 year (IQR 2 years). Biopsy diagnostic yield was 22,6%, and it was useful either in diagnostic or therapeutic approaches in 35,8%. Although advances of molecular techniques led to a decrease in muscle biopsy indications, it remains an important tool on the diagnosis of neuromuscular diseases.

Mitochondrial G12630A variation is associated with statin-induced myalgia in Chinese patients with coronary artery disease

To identify mitochondrial gene variants associated with statin-induced myalgia in Chinese patients with coronary artery disease (CHD). This study was conducted in a cohort of 403 patients with CHD receiving rosuvastatin therapy, among whom 341 patients had complete follow-up data concerning myalgia and 389 patients had documented measurements of plasma creatine kinase (CK) level. All these patients underwent genetic analysis using GSA chip for detecting mitochondria gene variants associated with myalgia. A logistic regression model was used to assess the association between 69 mitochondrial single-nucleotide polymorphisms (SNPs) and myopathy in 341 patients. The impact of these mutation sites on CK levels in 389 patients was evaluated by linear regression analysis. G12630A variant was identified to correlate with an increased risk of myalgia in CHD patients (OR: 8.689, 95% CI: 1.586-47.6; P=0.01273), but CK levels did not differ significantly between patients with different genotypes of G12630A (P > 0.05). SNPs at T12285C and A13105G were found to significantly correlate with CK levels in these patients (P < 0.05). Mitochondrial G12630A variation is associated with statin-induced myalgia in patients with CHD, indicating the necessity of different treatment strategies for patients who carry this risk allele.

CREATINE SUPPLEMENTS, LACTATE VALUE AND PH IN DEVELOPMENT OF FATIGUE

The use of additional Cr intake is widespread among athletes, notonly among the elite, but also among people involved in sports at a recreational level.Cr supplements are also used to improve muscle function in conditions such asinflammatory and mitochondrial myopathies and muscular dystrophies.