A clinical approach to diagnosis and management of mitochondrial myopathies.

This paper provides an overview of the different types of mitochondrial myopathies (MM), associated phenotypes, genotypes as well as a practical clinical approach towards disease diagnosis, surveillance, and management. nDNA-related MM are more common in pediatric-onset disease whilst mtDNA-related MMs are more frequent in adults. Genotype-phenotype correlation in MM is challenging due to clinical and genetic heterogeneity. The multisystemic nature of many MMs adds to the diagnostic challenge. Diagnostic approaches utilizing genetic sequencing with next generation sequencing approaches such as gene panel, exome and genome sequencing are available. This aids molecular diagnosis, heteroplasmy detection in MM patients and furthers knowledge of known mitochondrial genes. Precise disease diagnosis can end the diagnostic odyssey for patients, avoid unnecessary testing, provide prognosis, facilitate anticipatory management, and enable access to available therapies or clinical trials. Adjunctive tests such as functional and exercise testing could aid surveillance of MM patients. Management requires a multi-disciplinary approach, systemic screening for comorbidities, cofactor supplementation, avoidance of substances that inhibit the respiratory chain and exercise training. This update of the current understanding on MMs provides practical perspectives on current diagnostic and management approaches for this complex group of disorders.

Navigating Life With Primary Mitochondrial Myopathies: The Importance of the Patient Voice and Implications for Clinical Practice.

Primary mitochondrial myopathies (PMM) are rare disorders with diverse and progressive symptom presentations that cause a substantial, detrimental impact on the quality of life of patients and their caregivers. The burden of symptoms is compounded by their visibility and their unpredictable, progressive nature, leading to a sense of social stigmatization, limited autonomy, social isolation, and grief. There is also a lack of awareness and expertise in the medical community, which presents huge obstacles to diagnosis and provision of coordinated multidisciplinary care for these patients, along with a lack of disease-modifying treatments. The present commentary serves to raise awareness of the challenges faced by patients with PMM and their caregivers in their own words, including diagnostic delays, the burden of disease, and the need for further trials to develop disease-modifying treatments and improved understanding of the disease course. We also provide commentary on considerations for clinical practice, including the need for holistic care and multidisciplinary care teams, details of common 'red flag' symptoms, proposed diagnostic approaches, and suggested descriptions of multisystemic symptoms for physician-patient dialogue. In addition, we highlight the role patient advocacy and support groups play in supporting patients and providing access to reliable, up-to-date information and educational resources on these rare diseases.

Metabolic Myopathies.

PURPOSE OF REVIEW: Metabolic myopathies are disorders that affect skeletal muscle substrate oxidation. Although some drugs and hormones can affect metabolism in skeletal muscle, this review will focus on the genetic metabolic myopathies. RECENT FINDINGS: Impairments in glycogenolysis/glycolysis (glycogen storage disease), fatty acid transport/oxidation (fatty acid oxidation defects), and mitochondrial metabolism (mitochondrial myopathies) represent most metabolic myopathies; however, they often overlap clinically with structural genetic myopathies, referred to as pseudometabolic myopathies. Although metabolic myopathies can present in the neonatal period with hypotonia, hypoglycemia, and encephalopathy, most cases present clinically in children or young adults with exercise intolerance, rhabdomyolysis, and weakness. In general, the glycogen storage diseases manifest during brief bouts of high-intensity exercise; in contrast, fatty acid oxidation defects and mitochondrial myopathies usually manifest during longer-duration endurance-type activities, often with fasting or other metabolic stressors (eg, surgery, fever). The neurologic examination is often normal between events (except in the pseudometabolic myopathies) and evaluation requires one or more of the following tests: exercise stress testing, blood (eg, creatine kinase, acylcarnitine profile, lactate, amino acids), urine (eg, organic acids, myoglobin), muscle biopsy (eg, histology, ultrastructure, enzyme testing), and targeted (specific gene) or untargeted (myopathy panels) genetic tests. SUMMARY: Definitive identification of a specific metabolic myopathy often leads to specific interventions, including lifestyle, exercise, and nutritional modifications; cofactor treatments; accurate genetic counseling; avoidance of specific triggers; and rapid treatment of rhabdomyolysis.

Synergistic Deoxynucleoside and Gene Therapies for Thymidine Kinase 2 Deficiency.

OBJECTIVE: Autosomal recessive human thymidine kinase 2 (TK2) mutations cause TK2 deficiency, which typically manifests as a progressive and fatal mitochondrial myopathy in infants and children. Treatment with pyrimidine deoxynucleosides deoxycytidine and thymidine ameliorates mitochondrial defects and extends the lifespan of Tk2 knock-in mouse (Tk2KI ) and compassionate use deoxynucleoside therapy in TK2 deficient patients have shown promising indications of efficacy. To augment therapy for Tk2 deficiency, we assessed gene therapy alone and in combination with deoxynucleoside therapy in Tk2KI mice. METHODS: We generated pAAVsc CB6 PI vectors containing human TK2 cDNA (TK2). Adeno-associated virus (AAV)-TK2 was administered to Tk2KI , which were serially assessed for weight, motor functions, and survival as well as biochemical functions in tissues. AAV-TK2 treated mice were further treated with deoxynucleosides. RESULTS: AAV9 delivery of human TK2 cDNA to Tk2KI mice efficiently rescued Tk2 activity in all the tissues tested except the kidneys, delayed disease onset, and increased lifespan. Sequential treatment of Tk2KI mice with AAV9 first followed by AAV2 at different ages allowed us to reduce the viral dose while further prolonging the lifespan. Furthermore, addition of deoxycytidine and deoxythymidine supplementation to AAV9 + AAV2 treated Tk2KI mice dramatically improved mtDNA copy numbers in the liver and kidneys, animal growth, and lifespan. INTERPRETATION: Our data indicate that AAV-TK2 gene therapy as well as combination deoxynucleoside and gene therapies is more effective in Tk2KI mice than pharmacological alone. Thus, combination of gene therapy with substrate enhancement is a promising therapeutic approach for TK2 deficiency and potentially other metabolic disorders. ANN NEUROL 2021;90:640-652.

NAD+ Repletion Reverses Heart Failure With Preserved Ejection Fraction.

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Effects of fasting, feeding and exercise on plasma acylcarnitines among subjects with CPT2D, VLCADD and LCHADD/TFPD.

BACKGROUND: The plasma acylcarnitine profile is frequently used as a biochemical assessment for follow-up in diagnosed patients with fatty acid oxidation disorders (FAODs). Disease specific acylcarnitine species are elevated during metabolic decompensation but there is clinical and biochemical heterogeneity among patients and limited data on the utility of an acylcarnitine profile for routine clinical monitoring. METHODS: We evaluated plasma acylcarnitine profiles from 30 diagnosed patients with long-chain FAODs (carnitine palmitoyltransferase-2 (CPT2), very long-chain acyl-CoA dehydrogenase (VLCAD), and long-chain 3-hydroxy acyl-CoA dehydrogenase or mitochondrial trifunctional protein (LCHAD/TFP) deficiencies) collected after an overnight fast, after feeding a controlled low-fat diet, and before and after moderate exercise. Our purpose was to describe the variability in this biomarker and how various physiologic states effect the acylcarnitine concentrations in circulation. RESULTS: Disease specific acylcarnitine species were higher after an overnight fast and decreased by approximately 60% two hours after a controlled breakfast meal. Moderate-intensity exercise increased the acylcarnitine species but it varied by diagnosis. When analyzed for a genotype/phenotype correlation, the presence of the common LCHADD mutation (c.1528G > C) was associated with higher levels of 3-hydroxyacylcarnitines than in patients with other mutations. CONCLUSIONS: We found that feeding consistently suppressed and that moderate intensity exercise increased disease specific acylcarnitine species, but the response to exercise was highly variable across subjects and diagnoses. The clinical utility of routine plasma acylcarnitine analysis for outpatient treatment monitoring remains questionable; however, if acylcarnitine profiles are measured in the clinical setting, standardized procedures are required for sample collection to be of value.

Critical Illness and the Frailty Syndrome: Mechanisms and Potential Therapeutic Targets.

Frailty is a syndrome characterized by decreased reserves across multiple physiologic systems resulting in functional limitations and vulnerability to new stressors. Physical frailty develops over years in community-dwelling older adults but presents or worsens within days in the intensive care unit (ICU) because common mechanisms governing age-related physical frailty are often exacerbated by critical illness. The hallmark of physical frailty is a combined loss of muscle mass, force, and endurance. About one-third of ICU patients have frailty before hospitalization, which increases their risk for both short- and long-term disability and mortality. While there are several valid ways to measure clinical frailty in patients before or after an ICU admission, the mechanistic underpinnings of frailty in critically ill patients and ICU survivors have not been thoroughly investigated. Furthermore, therapeutic interventions to treat frailty during and after time in the ICU are lacking. In this narrative review, we examine studies that identify potential biological mechanisms underlying the development and propagation of physical frailty in both aging and critical illness (eg, inflammation, mitochondrial myopathy, and neuroendocrinopathy). We discuss specific aspects of these frailty mechanisms in older adults, critically ill patients, and ICU survivors that may represent therapeutic targets. Consistent with complexity underlying frailty, this syndrome is unlikely to result from an excess of a single harmful mediator or deficit of a single protective mediator. Rather, frailty occurs in the presence of an incompletely understood state of multisystem dysregulation. We further describe knowledge gaps that warrant clinical and translational research in frailty and critical care with an overall goal of developing effective frailty treatments in critically ill patients and ICU survivors.

Variable Cardiac Responses to Immunosuppressive Therapy in Anti-Mitochondrial Antibody-Positive Myositis.

We describe a case of anti-mitochondrial antibody-positive myositis associated with cardiovascular involvement. An electrophysiological study (EPS) showed binodal dysfunction, and cardiac magnetic resonance (CMR) imaging revealed left ventricular dysfunction with diffuse, patchy T2 high-intensity areas and late gadolinium enhancement indicative of inflammation and fibrosis. The left ventricular dysfunction was successfully treated with immunosuppressive therapy as documented by CMR. Persistence of conduction system dysfunction was confirmed by EPS, and a pacemaker was implanted. CMR and EPS concisely documented the variable cardiac response to treatment in anti-mitochondrial antibody-positive myositis. We demonstrate the utility of cardiac investigations in this rare disorder.

Case study of an inborn error manifested in the elderly: A woman with adult-onset mitochondrial disease mimicking systemic vasculitis.

Mitochondrial diseases are a group of disorders presenting mainly during infancy due to pathological dysfunction of the mitochondrial respiratory chain. We report a case of mitochondrial disease in an elderly woman complaining of generalized myalgia. A 69-year-old woman was admitted due to fatigue, general weakness, and a drowsy mental status. A brain magnetic resonance imaging (MRI) demonstrated multifocal lesions of increased T2 signal intensity, and laboratory findings were consistent with Fanconi syndrome. During her hospital course, she developed seizures, stress-induced cardiomyopathy, and respiratory failure. A muscle biopsy demonstrated ragged-red fibers in the muscle tissues seen in mitochondrial myopathy. We confirmed an 8 kb deletion in her mitochondrial DNA. Following treatment with l-carnitine, coenzyme Q10, and supportive measures, brain lesions on MRI scans disappeared, and the general symptoms gradually improved.

Exercise therapy for muscle and lower motor neuron diseases.

Muscle and lower motor neuron diseases share a common denominator of perturbed muscle function, most often related to wasting and weakness of muscles. This leads to a number of challenges, such as restricted mobility and respiratory difficulties. Currently there is no cure for these diseases. The purpose of this review is to present research that examines the effects of exercise in muscle and lower motor neuron diseases. Evidence indicates that moderate intensity aerobic- and strength exercise is advantageous for patients with muscle diseases, without causing harmful exercise-induced muscle damage. On the contrary, motor neuron diseases show a rather blunted response from exercise training. High-intensity training is a modality that seems safe and a promising exercise method, which may circumvent neural fatigue and provide effect to patients with motor neuron disease. Although we have come far in changing the view on exercise therapy in neuromuscular diseases to a positive one, much knowledge is still needed on what dose of time, intensity and duration should be implemented for different disease and how we should provide exercise therapy to very weak, non-ambulatory and wheelchair bound patients.

Diagnosis and Treatment of Mitochondrial Myopathies.

Mitochondrial myopathies are progressive muscle conditions caused primarily by the impairment of oxidative phosphorylation (OXPHOS) in the mitochondria. This causes a deficit in energy production in the form of adenosine triphosphate (ATP), particularly in skeletal muscle. The diagnosis of mitochondrial myopathy is reliant on the combination of numerous techniques including traditional histochemical, immunohistochemical, and biochemical testing combined with the fast-emerging molecular genetic techniques, namely next-generation sequencing (NGS). This has allowed for the diagnosis to become more effective in terms of determining causative or novel genes. However, there are currently no effective or disease-modifying treatments available for the vast majority of patients with mitochondrial myopathies. Existing therapeutic options focus on the symptomatic management of disease manifestations. An increasing number of clinical trials have investigated the therapeutic effects of various vitamins, cofactors, and small molecules, though these trials have failed to show definitive outcome measures for clinical practice thus far. In addition, new molecular strategies, specifically mtZFNs and mtTALENs, that cause beneficial heteroplasmic shifts in cell lines harboring varying pathogenic mtDNA mutations offer hope for the future. Moreover, recent developments in the reproductive options for patients with mitochondrial myopathies mean that for some families, the possibility of preventing transmission of the mutation to the next generation is now possible.

Outcome measures for children with mitochondrial disease: consensus recommendations for future studies from a Delphi-based international workshop.

Although there are no effective disease-modifying therapies for mitochondrial diseases, an increasing number of trials are being conducted in this rare disease group. The use of sensitive and valid endpoints is essential to test the effectiveness of potential treatments. There is no consensus on which outcome measures to use in children with mitochondrial disease. The aims of this two-day Delphi-based workshop were to (i) define the protocol for an international, multi-centre natural history study in children with mitochondrial myopathy and (ii) to select appropriate outcome measures for a validation study in children with mitochondrial encephalopathy. We suggest two sets of outcome measures for a natural history study in children with mitochondrial myopathy and for a proposed validation study in children with mitochondrial encephalopathy.

Renal Involvement in Neuropathy, Ataxia, Retinitis Pigmentosa (NARP) Syndrome: A Case Report.

We report a case of a patient who had the mitochondrial cytopathy complex of neuropathy, ataxia, and retinitis pigmentosa (NARP) syndrome diagnosed at age 11 years with a biopsy-proven kidney involvement that progressed to end-stage renal disease at age 21 years. Mutations of mitochondrial DNA (mtDNA) are maternally inherited and lead to mitochondrial cytopathies with predominant neurologic manifestations: psychomotor retardation, epilepsy, ataxia, neuropathy, and myopathy. Given the ubiquitous nature of mitochondria, cellular dysfunction can also appear in tissues with high metabolic turnover; thus, there can be cardiac, digestive, ophthalmologic, and kidney complications. Mutations in the MT-ATP6 gene of mtDNA have been shown to cause NARP syndrome without renal involvement. We report a patient who had NARP syndrome diagnosed at age 11 years in whom glomerular proteinuria was present very early after diagnosis. Although neurologic manifestations were stable over time, he developed worsening proteinuria and kidney function. He started dialysis therapy at age 21 years. Kidney biopsy confirmed the mitochondrial cytopathy histologically, with abnormal mitochondria seen on electron microscopy. The MT-ATP6 gene mutation was detected in the kidney biopsy specimen.

[MELAS: Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-Like Episodes].

Mitochondrial disease is caused by a deficiency in the energy supply to cells due to mitochondrial dysfunction. Mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) is a mitochondrial disease that presents with stroke-like episodes such as acute onset of neurological deficits and characteristic imaging findings. Stroke-like episodes in MELAS have the following features: 1) neurological deficits due to localization of lesions in the brain, 2) episodes often accompany epilepsy, 3) lesions do not follow the vascular supply area, 4) lesions are more often seen in the posterior brain than in the anterior brain, 5) lesions spread to an adjacent area in the brain, and 6) neurological symptoms often disappear together with imaging findings, but later relapse. About 80% of patients with MELAS have an A-to-G transition mutation at the nucleotide pair 3243 in the dihydrouridine loop of mitochondrial tRNALeu(UUR), which causes the absence of posttranscriptional taurine modification at the wobble nucleotide of mitochondrial tRNALeu(UUR) and disrupts protein synthesis. However, the precise pathophysiology of stroke-like episodes is under investigation, with possible hypotheses for these episodes including mitochondrial angiopathy, mitochondrial cytopathy, and neuron-astrocyte uncoupling. With regard to treatment, L-arginine and taurine have recently been suggested for relief of clinical symptoms.

MITOCHONDRIAL MYOPATHY: A NEW THERAPEUTIC APPROACH.

Restoration of deoxyribonucleic acid in mitochondrial myopathies may occur after a mechanical or chemical injury of striated muscle or by endurance training. Therapies with enzymes, gene therapies, or treatments with substances that stimulate mitochondrial biogenesis are used at the moment. Genesis of mitochondria may also come from myonuclei by releasing the nuclear respiratory factor-1/2 during muscle contractions. Multiplying of myonuclei depends on muscle satellite cell activation. Since the electromyostimulation increase the number of circulating stem cells that may participate in the genesis of new muscle fibers (adding to the deposit of specific stem cells of the muscle), and intermittent hypoxia stimulates the proliferation of muscle satellite cells, we propose to combine the two processes for the treatment of mitochondrial myopathies. Respective combined therapy may be useful for restoring damaged mitochondria by drug side effects.

Home-based aerobic exercise training improves skeletal muscle oxidative metabolism in patients with metabolic myopathies.

Aerobic training can be effective in patients with mitochondrial myopathies (MM) and McArdle's disease (McA). The aim of the study was to use noninvasive functional evaluation methods, specifically aimed at skeletal muscle oxidative metabolism, to evaluate the effects of an aerobic exercise training (cycle ergometer, 12 wk, 4 days/wk, ∼65-70% of maximal heart rate) in 6 MM and 7 McA. Oxygen uptake and skeletal muscle vastus lateralis fractional O2 extraction by near-infrared spectroscopy were assessed during incremental and low-intensity constant work rate (CWR) exercises before (BEFORE) and at the end (AFTER) of training. Peak O2 uptake increased significantly with training both in MM [14.7 ± 1.2 vs. 17.6 ± 1.4 ml·kg(-1)·min(-1) (mean ± SD)] and in McA (18.5 ± 1.8 ml·kg(-1)·min(-1) vs. 21.6 ± 1.9). Peak skeletal muscle fractional O2 extraction increased with training both in MM (22.0 ± 6.7 vs. 32.6 ± 5.9%) and in McA (18.5 ± 6.2 vs. 37.2 ± 7.2%). During low-intensity CWR in both MM and McA: V̇o2 kinetics became faster in AFTER, but only in the patients with slow V̇o2 kinetics in BEFORE; the transient overshoot in fractional O2 extraction kinetics disappeared. The level of habitual physical activity was not higher 3 mo after training (FOLLOW-UP vs. PRE). In MM and McA patients a home-based aerobic training program significantly attenuated the impairment of skeletal muscle oxidative metabolism and improved variables associated with exercise tolerance. Our findings indicate that in MM and McA patients near-infrared spectroscopy and V̇o2 kinetics can effectively detect the functional improvements obtained by training.

Megaconial muscular dystrophy caused by mitochondrial membrane homeostasis defect, new insights from skeletal and heart muscle analyses.

Megaconial congenital muscular dystrophy is a disease caused by pathogenic mutations in the gene encoding choline kinase beta (CHKB). Microscopically, the disease is hallmarked by the presence of enlarged mitochondria at the periphery of skeletal muscle fibres leaving the centre devoid of mitochondria. Clinical characteristics are delayed motor development, intellectual disability and dilated cardiomyopathy in half of reported cases. This study describes a patient presenting with the cardinal clinical features, in whom a homozygous nonsense mutation (c.248_249insT; p.Arg84Profs*209) was identified in CHKB and who was treated by heart transplantation. Microscopic evaluation of skeletal and heart muscles typically showed enlarged mitochondria. Spectrophotometric evaluation in both tissues revealed a mild decrease of all OXPHOS complexes. Using BN-PAGE analysis followed by activity staining subcomplexes of complex V were detected in both tissues, indicating incomplete complex V assembly. Mitochondrial DNA content was not depleted in analysed tissues. This is the first report describing the microscopic and biochemical abnormalities in the heart from an affected patient. A likely hypothesis is that the biochemical findings are caused by an abnormal lipid profile in the inner mitochondrial membrane resulting from a defective choline kinase B activity.