Aerobic conditioning: An effective therapy in McArdle's disease

Cellular mechanisms of acute rhabdomyolysis in inherited metabolic diseases

Acute rhabdomyolysis (RM) constitutes a life-threatening emergency resulting from the (acute) breakdown of skeletal myofibers, characterized by a plasma creatine kinase (CK) level exceeding 1000 IU/L in response to a precipitating factor. Genetic predisposition, particularly inherited metabolic diseases, often underlie RM, contributing to recurrent episodes. Both sporadic and congenital forms of RM share common triggers. Considering the skeletal muscle's urgent need to rapidly adjust to environmental cues, sustaining sufficient energy levels and functional autophagy and mitophagy processes are vital for its preservation and response to stressors. Crucially, the composition of membrane lipids, along with lipid and calcium transport, and the availability of adenosine triphosphate (ATP), influence membrane biophysical properties, membrane curvature in skeletal muscle, calcium channel signaling regulation, and determine the characteristics of autophagic organelles. Consequently, a genetic defect involving ATP depletion, aberrant calcium release, abnormal lipid metabolism and/or lipid or calcium transport, and/or impaired anterograde trafficking may disrupt autophagy resulting in RM. The complex composition of lipid membranes also alters Toll-like receptor signaling and viral replication. In response, infections, recognized triggers of RM, stimulate increased levels of inflammatory cytokines, affecting skeletal muscle integrity, energy metabolism, and cellular trafficking, while elevated temperatures can reduce the activity of thermolabile enzymes. Overall, several mechanisms can account for RMs and may be associated in the same disease-causing RM.

McArdle Disease: A Diagnostic Challenge Due to Nonspecific Clinical Manifestations

McArdle disease is a rare myopathy caused by hereditary myophosphorylase deficiency. It presents nonspecific symptoms, such as intolerance to physical exercise, early fatigue, and myalgias, and represents a paradigmatic example of one of the main challenges in clinical practice: the recognition of nonspecific and common symptoms as clinically relevant manifestations of rare diseases. The nonspecificity of symptoms leads to a frequent delay from the onset of first clinical signs to diagnosis. We present a case of McArdle disease diagnosed in a Portuguese adult patient. We believe this report will highlight the need to enhance knowledge and awareness about this rare pathology to diagnose and manage these patients promptly.

Metabolic aspects of glycogenolysis with special attention to McArdle disease

The physiological function of muscle glycogen is to meet the energy demands of muscle contraction. The breakdown of glycogen occurs through two distinct pathways, primarily cytosolic and partially lysosomal. To obtain the necessary energy for their function, skeletal muscles utilise also fatty acids in the β-oxidation. Ketogenesis is an alternative metabolic pathway for fatty acids, which provides an energy source during fasting and starvation. Diseases arising from impaired glycogenolysis lead to muscle weakness and dysfunction. Here, we focused on the lack of muscle glycogen phosphorylase (PYGM), a rate-limiting enzyme for glycogenolysis in skeletal muscles, which leads to McArdle disease. Metabolic myopathies represent a group of genetic disorders characterised by the limited ability of skeletal muscles to generate energy. Here, we discuss the metabolic aspects of glycogenosis with a focus on McArdle disease, offering insights into its pathophysiology. Glycogen accumulation may influence the muscle metabolic dynamics in different ways. We emphasize that a proper treatment approach for such diseases requires addressing three important and interrelated aspects, which include: symptom relief therapy, elimination of the cause of the disease (lack of a functional enzyme) and effective and early diagnosis.

McArdle's, a Rare Disease That Every Family Doctor Can Manage: A Case Report

McArdle's disease is a rare autosomal recessive disorder that affects glycogen storage. It typically manifests in adolescence or early adulthood with presenting symptoms, such as fatigue, myalgia, exercise intolerance, and cramps, which can be easily overlooked. This case report seeks to offer a comprehensive overview of the perspective of a patient living with McArdle's disease, emphasizing the importance of treatment encouragement. The report documents the case of a 25-year-old male presenting with myalgia and fatigue exacerbated by strenuous exercise, illustrating the diagnostic challenges associated with McArdle's disease, primarily attributable to clinician unawareness. Furthermore, the case highlights the importance of adhering to lifestyle modifications to mitigate symptoms and prevent flare-ups, as well as the crucial role of the family doctor in such lifestyle maintenance.

Repeated oral sucrose dosing after the second wind is unnecessary in patients with McArdle disease: Results from a randomized, placebo-controlled, double-blind, cross-over study

It is well-established that oral sucrose ingested shortly before exercise improves early exercise tolerance in individuals with McArdle disease. This is by supplying blood-borne glucose for muscle metabolism to compensate for the blocked glycogenolysis. The present study investigated if individuals with McArdle disease could benefit further from repeated sucrose ingestion during prolonged exercise. In this double-blind, placebo-controlled, cross-over study, the participants were randomized to ingest either sucrose or placebo first and subsequently the opposite on two separate days. The participants ingested the drink 10 min before and thrice (after 10, 25, and 40 min) during a 60-min submaximal exercise test on a cycle ergometer. The primary outcome was exercise capacity as indicated by heart rate (HR) and perceived exertion (PE) responses to exercise. Secondary outcomes included changes in blood metabolites, insulin and carbohydrate, and fatty acid oxidation rates during exercise. Nine participants with McArdle disease were included in the study. We confirmed improvement of exercise capacity with oral sucrose vs. placebo during early exercise (pre-second wind) indicated by lower peak HR and PE (p < 0.02). We found no further beneficial effect with repeated sucrose versus placebo ingestion during prolonged exercise, as indicated by no difference in HR or PE post-second wind (p > 0.05). Glucose, lactate, insulin, and carbohydrate oxidation rates increased, and fatty acid oxidation decreased with sucrose versus placebo (p ≤ 0.0002). We can conclude that repeated sucrose ingestion is not recommended during prolonged exercise. This finding can prevent excessive caloric intake and reduce the risk of obesity and insulin resistance.

Can a modified ketogenic diet be a nutritional strategy for patients with McArdle disease? Results from a randomized, single-blind, placebo-controlled, cross-over study

BACKGROUND

McArdle disease is caused by myophosphorylase deficiency leading to blocked glycogenolysis in skeletal muscle. Consequently, individuals with McArdle disease have intolerance to physical activity, muscle fatigue, and pain. These symptoms vary according to the availability of alternative fuels for muscle contraction. In theory, a modified ketogenic diet (mKD) can provide alternative fuels in the form of ketone bodies and potentially boost fat oxidation.

METHODS

This randomized, single-blind, placebo-controlled, cross-over study aimed to investigate if a mKD improves exercise capacity in individuals with McArdle disease. Participants were randomized to follow a mKD (75-80% fat, 15% protein, 5-10% carbohydrates) or placebo diet (PD) first for three weeks, followed by a wash-out period, and then the opposite diet. The primary outcome was change in heart rate during constant-load cycling. Secondary outcomes included change in plasma metabolites, perceived exertion, indirect calorimetry measures, maximal exercise capacity, and patient-reported outcomes.

RESULTS

Fifteen out of 20 patients with genetically verified McArdle disease completed all study visits, and 14 were included in the data analyses. We found that the mKD induced a metabolic shift towards increased fat oxidation (∼60% increase), and a 19-fold increase in plasma β-hydroxybutyrate (p < 0.05). The mKD did not improve heart rate responses during constant-load cycling but did improve patient-reported outcomes and maximal exercise capacity (∼20% increase) compared to the PD.

CONCLUSION

The mKD did not alleviate all McArdle disease-related symptoms but did induce some positive changes. To date, no satisfactory treatment options exist other than exercise training. To that end, a mKD can be a possible nutritional strategy for some individuals with McArdle disease who are motivated to undertake a restrictive diet.

CLINICAL TRIAL REGISTRATION

clinical trials.gov: NCT04044508.

Diagnosis and management of metabolic myopathies

Metabolic myopathies are a set of rare inborn errors of metabolism leading to disruption in energy production. Relevant to skeletal muscle, glycogen storage disease and fatty acid oxidation defects can lead to exercise intolerance, rhabdomyolysis, and weakness in children and adults, distinct from the severe forms that involve multiple-organ systems. These nonspecific, dynamic symptoms along with conditions that mimic metabolic myopathies can make diagnosis challenging. Clinicians can shorten the time to diagnosis by recognizing the typical clinical phenotypes and performing next generation sequencing. With improved access and affordability of molecular testing, clinicians need to be well-versed in resolving variants of uncertain significance relevant to metabolic myopathies. Once identified, patients can improve quality of life, safely engage in exercise, and reduce episodes of rhabdomyolysis by modifying diet and lifestyle habits.

A Case Report of McArdle Disease Diagnosed Following Statin-Induced Myositis

McArdle disease is a rare condition, characterized by a deficiency of phosphorylase muscle isoform, an enzyme responsible for the breaking down of glycogen, necessary for obtaining energy. Patients typically present with exercise intolerance, myalgias, fatigue, cramps, muscle stiffness, and/or weakness induced by physical activity. The diagnosis is generally established late, with a median delay of about 29 years. We present the case of a female patient with a long history of myalgias, muscle weakness, and exercise intolerance, diagnosed with McArdle disease by the age of 74, after statin-induced myopathy. We aim to review the diagnosis and treatment of this disease, as a way to raise awareness among the medical community.

Exercise testing and prescription in patients with inborn errors of muscle energy metabolism

Skeletal muscle is a dynamic organ requiring tight regulation of energy metabolism in order to provide bursts of energy for effective function. Several inborn errors of muscle energy metabolism (IEMEM) affect skeletal muscle function and therefore the ability to initiate and sustain physical activity. Exercise testing can be valuable in supporting diagnosis, however its use remains limited due to the inconsistency in data to inform its application in IEMEM populations. While exercise testing is often used in adults with IEMEM, its use in children is far more limited. Once a physiological limitation has been identified and the aetiology defined, habitual exercise can assist with improving functional capacity, with reports supporting favourable adaptations in adult patients with IEMEM. Despite the potential benefits of structured exercise programs, data in paediatric populations remain limited. This review will focus on the utilisation and limitations of exercise testing and prescription for both adults and children, in the management of McArdle Disease, long chain fatty acid oxidation disorders, and primary mitochondrial myopathies.

Development of Continuum of Care for McArdle disease: A practical tool for clinicians and patients

McArdle disease (glycogen storage disease type V; GSDV) is a rare genetic disease caused by the inability to break down glycogen in skeletal muscle due to a deficiency in myophosphorylase. Glycolysis is only partially blocked in GSDV, as muscle fibres can take up circulating glucose and convert it to glucose-6-phosphate downstream of the metabolic block. Because skeletal muscle predominantly relies on anaerobic energy during the first few minutes of transition from rest to activity, and throughout more intense activities, individuals with GSDV experience muscle fatigue/pain, tachypnea, and tachycardia during these activities. If warning signs are not heeded, a muscle contracture may rapidly occur, and if significant, may lead to acute rhabdomyolysis. Without a cure or treatment, individuals with GSDV must be consistent in employing proper management techniques; however, this can be challenging due to the nuances inherent in this metabolic myopathy. The International Association for Muscle Glycogen Storage Disease collaborated with an international team of five expert clinicians to identify areas of learning to achieve an optimal state. A Continuum of Care model was developed that outlines five pivotal steps (diagnosis; understanding; acceptance; learning and exercise) to streamline assessments and more succinctly assist clinicians in determining patient-specific learning needs. This model serves as a translational tool to help optimize care for this patient population.

Muscle glycogen unavailability and fat oxidation rate during exercise: Insights from McArdle disease

Carbohydrate availability affects fat metabolism during exercise; however, the effects of complete muscle glycogen unavailability on maximal fat oxidation (MFO) rate remain unknown. Our purpose was to examine the MFO rate in patients with McArdle disease, comprising an inherited condition caused by complete blockade of muscle glycogen metabolism, compared to healthy controls. Nine patients (three women, aged 36 ± 12 years) and 12 healthy controls (four women, aged 40 ± 13 years) were studied. Several molecular markers of lipid transport/metabolism were also determined in skeletal muscle (gastrocnemius) and white adipose tissue of McArdle (Pygm p.50R*/p.50R*) and wild-type male mice. Peak oxygen uptake ( V ̇ O 2 peak ${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$ ), MFO rate, the exercise intensity eliciting MFO rate (FATmax) and the MFO rate-associated workload were determined by indirect calorimetry during an incremental cycle-ergometer test. Despite having a much lower V ̇ O 2 peak ${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$ (24.7 ± 4 vs. 42.5 ± 11.4 mL kg^-1  min^-1 , respectively; P < 0.0001), patients showed considerably higher values for the MFO rate (0.53 ± 0.12 vs. 0.33 ± 0.10 g min^-1 , P = 0.001), and for the FATmax (94.4 ± 7.2 vs. 41.3 ± 9.1 % of V ̇ O 2 peak ${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$ , P < 0.0001) and MFO rate-associated workload (1.33 ± 0.35 vs. 0.81 ± 0.54 W kg^-1 , P = 0.020) than controls. No between-group differences were found overall in molecular markers of lipid transport/metabolism in mice. In summary, patients with McArdle disease show an exceptionally high MFO rate, which they attained at near-maximal exercise capacity. Pending more mechanistic explanations, these findings support the influence of glycogen availability on MFO rate and suggest that these patients develop a unique fat oxidation capacity, possibly as an adaptation to compensate for the inherited blockade in glycogen metabolism, and point to MFO rate as a potential limiting factor of exercise tolerance in this disease. KEY POINTS: Physically active McArdle patients show an exceptional fat oxidation capacity. Maximal fat oxidation rate occurs near-maximal exercise capacity in these patients. McArdle patients' exercise tolerance might rely on maximal fat oxidation rate capacity. Hyperpnoea might cloud substrate oxidation measurements in some patients. An animal model revealed overall no higher molecular markers of lipid transport/metabolism.

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.

Long-Term Exercise Intervention in Patients with McArdle Disease: Clinical and Aerobic Fitness Benefits

INTRODUCTION

The long-term effects of exercise in patients with McArdle disease-the paradigm of "exercise intolerance"-are unknown. This is an important question because the severity of the disease frequently increases with time.

PURPOSE

This study aimed to study the effects of a long-term exercise intervention on clinical and fitness-related outcomes in McArdle patients.

METHODS

Seventeen patients (exercise group: n = 10, 6 male, 38 ± 18 yr; control: n = 7, 4 male, 38 ± 18 yr) participated in a 2-yr unsupervised intervention including moderate-intensity aerobic (cycle-ergometer exercise for 1 h) and resistance (high load-low repetition circuit) training on 5 and 2-3 d·wk -1 , respectively. Patients were assessed at baseline and postintervention. Besides safety, outcomes included clinical severity (e.g., exercise intolerance features) on a 0-3 scale (primary outcome), and aerobic fitness, gross muscle efficiency, and body composition (total/regional fat, muscle, and bone mass; secondary outcomes).

RESULTS

The exercise program was safe and resulted in a reduction of 1 point (-1.0; 95% confidence interval, -1.6 to -0.5; P = 0.025) in clinical severity versus the control group, with 60% of participants in the exercise group becoming virtually asymptomatic and with no functional limitation in daily life activities. Compared with controls, the intervention induced significant and large benefits (all P < 0.05) in the workload eliciting the ventilatory threshold (both in absolute (watts, +37%) and relative units (watts per kilogram of total body mass or of lower-limb muscle mass, +44%)), peak oxygen uptake (in milliliters per kilogram per minute, +28%), and peak workload (in absolute (+27%) and relative units (+33%)). However, no significant changes were found for muscle efficiency or for any measure of body composition.

CONCLUSIONS

A 2-yr unsupervised intervention including aerobic and resistance exercise is safe and induces major benefits in the clinical course and aerobic fitness of patients with McArdle disease.

A Systematic Review investigating the Effectiveness of Exercise training in Glycogen Storage Diseases

Introduction

Glycogen storage diseases (GSDs) are rare inborn errors of carbohydrate metabolism typically with skeletal muscle and liver involvement. In those with skeletal muscle involvement, the majority display symptoms of exercise intolerance which can cause profound exercise limitation and impair everyday living and quality of life (QoL). There are no curative treatments for GSDs, thus therapeutic options, such as exercise training, are aimed at improving QoL by alleviating signs and symptoms. In order to investigate the effectiveness of exercise training in adults with GSDs, we systematically reviewed the literature.

Methods

In this review we conducted searches within SCOPUS and MEDLINE to identify potential papers for inclusion. These papers were independently assessed for inclusion and quality by two authors. We identified 23 studies which included aerobic training, strength training or respiratory muscle training in patients with McArdles (n = 41) and Pompe disease (n = 139).

Results

In McArdle disease, aerobic exercise training improved aerobic capacity (VO2 peak) by 14-111% with further benefits to functional capacity and well-being. Meanwhile, strength training increased muscle peak power by 100-151% and reduced disease severity. In Pompe disease, a combination of aerobic and strength training improved VO2 peak by 9-10%, muscle peak power by 64%, functional capacity and well-being. Furthermore, respiratory muscle training (RMT) improved respiratory muscular strength [maximum inspiratory pressure (MIP) increased by up to 65% and maximum expiratory pressure (MEP) by up to 70%], with additional benefits shown in aerobic capacity, functional capacity and well-being.

Conclusion

This adds to the growing body of evidence which suggests that supervised exercise training is safe and effective in improving aerobic capacity and muscle function in adults with McArdle or Pompe disease. However, the literature base is limited in quality and quantity with a dearth of literature regarding exercise training in other GSD subtypes.

The Second Wind in McArdle Patients: Fitness Matters

Background: The “second wind” (SW) phenomenon—commonly referring to both an initial period of marked intolerance to dynamic exercise (e.g., brisk walking) that is not followed by perceived improvement and disappearance of previous tachycardia (i.e., the actual “SW”) until 6–10 min has elapsed—is an almost unique feature of McArdle disease that limits adherence to an active lifestyle. In this regard, an increase in the workload eliciting the SW could potentially translate into an improved patients’ exercise tolerance in daily life. We aimed to determine whether aerobic fitness and physical activity (PA) levels are correlated with the minimum workload eliciting the SW in McArdle patients—as well as with the corresponding heart rate value. We also compared the SW variables and aerobic fitness indicators in inactive vs. active patients.

Methods: Fifty-four McArdle patients (24 women, mean ± SD age 33 ± 12 years) performed 12-min constant-load and maximum ramp-like cycle-ergometer tests for SW detection and aerobic fitness [peak oxygen uptake (VO_2peak) and workload and ventilatory threshold] determination, respectively. They were categorized as physically active/inactive during the prior 6 months (active = reporting ≥150 min/week or ≥75 min/week in moderate or vigorous-intensity aerobic PA, respectively) and were also asked on their self-report of the SW.

Results: Both peak and submaximal indicators of aerobic fitness obtained in the ramp tests were significantly correlated with the workload of the SW test, with a particularly strong correlation for the VO_2peak and peak workload attained by the patients (both Pearson’s coefficients > 0.70). Twenty (seven women) and 24 patients (18 women) were categorized as physically active and inactive, respectively. Not only the aerobic fitness level [∼18–19% higher values of VO_2peak (ml⋅kg^–1⋅min^–1)] but also the workload of the SW tests was significantly higher in active than in inactive patients. All the inactive patients reported that they experienced the SW during walking/brisk walking in daily life, whereas active patients only reported experiencing this phenomenon during more strenuous activities (very brisk walking/jogging and bicycling).

Conclusion: A higher aerobic fitness and an active lifestyle are associated with a higher workload eliciting the so-called SW phenomenon in patients with McArdle disease, which has a positive impact on their exercise tolerance during daily living.

Targeted Therapies for Metabolic Myopathies Related to Glycogen Storage and Lipid Metabolism: a Systematic Review and Steps Towards a 'Treatabolome'

BACKGROUND

Metabolic myopathies are a heterogenous group of muscle diseases typically characterized by exercise intolerance, myalgia and progressive muscle weakness. Effective treatments for some of these diseases are available, but while our understanding of the pathogenesis of metabolic myopathies related to glycogen storage, lipid metabolism and β-oxidation is well established, evidence linking treatments with the precise causative genetic defect is lacking.

OBJECTIVE

The objective of this study was to collate all published evidence on pharmacological therapies for the aforementioned metabolic myopathies and link this to the genetic mutation in a format amenable to databasing for further computational use in line with the principles of the "treatabolome" project.

METHODS

A systematic literature review was conducted to retrieve all levels of evidence examining the therapeutic efficacy of pharmacological treatments on metabolic myopathies related to glycogen storage and lipid metabolism. A key inclusion criterion was the availability of the genetic variant of the treated patients in order to link treatment outcome with the genetic defect.

RESULTS

Of the 1,085 articles initially identified, 268 full-text articles were assessed for eligibility, of which 87 were carried over into the final data extraction. The most studied metabolic myopathies were Pompe disease (45 articles), multiple acyl-CoA dehydrogenase deficiency related to mutations in the ETFDH gene (15 articles) and systemic primary carnitine deficiency (8 articles). The most studied therapeutic management strategies for these diseases were enzyme replacement therapy, riboflavin, and carnitine supplementation, respectively.

CONCLUSIONS

This systematic review provides evidence for treatments of metabolic myopathies linked with the genetic defect in a computationally accessible format suitable for databasing in the treatabolome system, which will enable clinicians to acquire evidence on appropriate therapeutic options for their patient at the time of diagnosis.

Effects of acute nutritional ketosis during exercise in adults with glycogen storage disease type IIIa are phenotype-specific: An investigator-initiated, randomized, crossover study

Glycogen storage disease type IIIa (GSDIIIa) is an inborn error of carbohydrate metabolism caused by a debranching enzyme deficiency. A subgroup of GSDIIIa patients develops severe myopathy. The purpose of this study was to investigate whether acute nutritional ketosis (ANK) in response to ketone-ester (KE) ingestion is effective to deliver oxidative substrate to exercising muscle in GSDIIIa patients. This was an investigator-initiated, researcher-blinded, randomized, crossover study in six adult GSDIIIa patients. Prior to exercise subjects ingested a carbohydrate drink (~66 g, CHO) or a ketone-ester (395 mg/kg, KE) + carbohydrate drink (30 g, KE + CHO). Subjects performed 15-minute cycling exercise on an upright ergometer followed by 10-minute supine cycling in a magnetic resonance (MR) scanner at two submaximal workloads (30% and 60% of individual maximum, respectively). Blood metabolites, indirect calorimetry data, and in vivo ³¹ P-MR spectra from quadriceps muscle were collected during exercise. KE + CHO induced ANK in all six subjects with median peak βHB concentration of 2.6 mmol/L (range: 1.6-3.1). Subjects remained normoglycemic in both study arms, but delta glucose concentration was 2-fold lower in the KE + CHO arm. The respiratory exchange ratio did not increase in the KE + CHO arm when workload was doubled in subjects with overt myopathy. In vivo ³¹ P MR spectra showed a favorable change in quadriceps energetic state during exercise in the KE + CHO arm compared to CHO in subjects with overt myopathy. Effects of ANK during exercise are phenotype-specific in adult GSDIIIa patients. ANK presents a promising therapy in GSDIIIa patients with a severe myopathic phenotype. TRIAL REGISTRATION NUMBER: ClinicalTrials.gov identifier: NCT03011203.

Diagnostic challenges in metabolic myopathies

INTRODUCTION

Metabolic myopathies comprise a clinically etiological diverse group of disorders caused by defects in cellular energy metabolism including the breakdown of carbohydrates and fatty acids, which include glycogen storage diseases and fatty acid oxidation disorders. Their wide clinical spectrum ranges from infantile severe multisystemic disorders to adult-onset myopathies. To suspect in adults these disorders, clinical features such as exercise intolerance and recurrent myoglobinuria need investigation while another group presents fixed weakness and cardiomyopathy as a clinical pattern.

AREAS COVERED

In metabolic myopathies, clinical manifestations are important to guide diagnostic tests used in order to lead to the correct diagnosis. The authors searched in literature the most recent techniques developed. The authors present an overview of the most common phenotypes of Pompe disease and what is currently known about the mechanism of ERT treatment. The most common disorders of lipid metabolism are overviewed, with their possible dietary or supplementary treatments.

EXPERT COMMENTARY

The clinical suspicion is the clue to conduct in-depth investigations in suspected cases of metabolic myopathies that lead to the final diagnosis with biochemical molecular studies and often nowadays by the use of Next Generation Sequencing (NGS) to determine gene mutations.

Titrating a modified ketogenic diet for patients with McArdle disease: A pilot study

Glycogen storage disease type V (GSDV) is a rare inborn error of carbohydrate metabolism. Patients present with exercise intolerance due to blocked glycogen breakdown in skeletal muscle. Introducing alternative fuel substrates, such as ketone bodies (KBs), could potentially alleviate muscle symptoms. This pilot study investigates which of three different modified ketogenic diet regimes is optimal for GSDV-patients to follow in a future large-scale study. Participants were randomised to follow one of three diet regimes for 3 weeks (#1: 65%/15%/20%; #2: 75%/15%/10%, or #3: 80%/15%/5%, fat/protein/carbohydrate). The primary outcome was exercise tolerance assessed by heart rate (HR) changes during constant load cycling. Secondary outcomes included levels of ketosis, and changes in perceived exertion and indirect calorimetry measures during exercise. Ten GSDV-patients were included. Eight completed the study. The other two were excluded. Diet #3 showed the highest average KB level (1.1 mmol/L) vs #2 (0.5 mmol/L) and #1 (0.3 mmol/L). Five patients reported subjective symptom relief, all of whom were on diets #2 and #3. All diet regimes seemed to improve fatty acid oxidation rates and exercise capacity as indicated by a small decrease in HR and perceived exertion. The results of this open-label pilot study show that diets #2 and #3 induce ketosis and improve symptoms and exercise capacity in GSDV-patients. Diet #2 had the highest acceptability score and was superior or equal to diet #3 in all other parameters, except level of ketosis. Based on this, we suggest testing diet #2 in a large-scale, placebo-controlled study in GSDV.

Sex Differences and the Influence of an Active Lifestyle on Adiposity in Patients with McArdle Disease

McArdle disease (glycogenosis-V) is associated with exercise intolerance, however, how it affects an important marker of cardiometabolic health as it is adiposity remains unknown. We evaluated the association between physical activity (PA) and adiposity in patients with McArdle disease. We assessed 199 adults of both sexes (51 McArdle patients (36 ± 11 years) and 148 healthy controls (35 ± 10 years)). Body fat (BF) was determined using dual-energy X-ray absorptiometry (DXA) method and each patient’s PA was assessed with the International PA Questionnaire (IPAQ). Although body mass index values did not differ between patients and controls, McArdle patients had significantly higher values of BF in all body regions (p < 0.05) and higher risk of suffering obesity (odds ratio (OR): 2.54, 95% confidence interval (95% CI): 1.32–4.88). Male patients had higher BF and obesity risk (OR: 3.69, 95% CI: 1.46−9.34) than their sex-matched controls, but no differences were found within the female sex (p < 0.05). In turn, active female patients had lower trunk BF than their inactive peers (p < 0.05). Males with McArdle seem to have adiposity problems and a higher risk of developing obesity than people without the condition, while female patients show similar or even better levels in the trunk region with an active lifestyle. Therefore, special attention should be given to decrease adiposity and reduce obesity risk in males with McArdle disease.

Impaired lipolysis in propionic acidemia: A new metabolic myopathy?

The objective of this study was to investigate the fat and carbohydrate metabolism in a patient with propionic acidemia (PA) during exercise by means of indirect calorimetry and stable isotope technique. A 34-year-old patient with PA performed a 30-minute submaximal cycle ergometer test. Data were compared to results from six gender- and age-matched healthy controls. Main findings are that the patient with PA had impaired lipolysis, blunted fatty acid oxidation, compensatory increase in carbohydrate utilization, and low work capacity. Our findings indicate that PA should be added to the list of metabolic myopathies.

Beneficial Effects of Ketogenic Diet on Phosphofructokinase Deficiency (Glycogen Storage Disease Type VII)

Background: A deficiency of muscle phosphofructokinase (PFKM) causes a rare metabolic muscle disease, the Tarui disease (Glycogen storage disease type VII, GSD VII) characterized by exercise intolerance with myalgia due to an inability to use glucose as an energy resource. No medical treatment for GSD VII currently exists. The aim of this study was to determine whether a dietary intervention with excessive fat intake would benefit GSD VII.

Patient and Methods: A ketogenic diet (KD) intervention implemented as a modified Atkins diet was established for one patient with PFKM deficiency, with a low late lactate response and very high ammonia levels associated with exercise. We recorded the KD intervention for a total of 5 years with clinical and physiotherapeutic evaluations and regular laboratory parameters. Cardiopulmonary exercise testing, including breath gas analysis and venous lactate and ammonia measurements, was performed before KD and at 3, 8 months and 5 years after initiation of KD.

Results: During the 5 years on KD, the patient's muscle symptoms had alleviated and exercise tolerance had improved. In exercise testing, venous ammonia had normalized, the lactate profile remained similar, but oxygen uptake and mechanical efficiency had increased and parameters showing ventilation had improved.

Conclusions: This study is the first to show a long-term effect of KD in GSD VII with an alleviation of muscle symptoms, beneficial effects on breathing, and improvement in exercise performance and oxygen uptake. Based on these findings, KD can be recommended under medical and nutritional supervision for selected patients with GSD VII, although further research of this rare disease is warranted.

No effect of triheptanoin on exercise performance in McArdle disease

Objective

To study if treatment with triheptanoin, a 7‐carbon triglyceride, improves exercise tolerance in patients with McArdle disease. McArdle patients have a complete block in glycogenolysis and glycogen‐dependent expansion of tricarboxylic acid cycle (TCA), which may restrict fat oxidation. We hypothesized that triheptanoin metabolism generates substrates for the TCA, which potentially boosts fat oxidation and improves exercise tolerance in McArdle disease.

Methods

Double‐blind, placebo‐controlled, crossover study in patients with McArdle disease completing two treatment periods of 14 days each with a triheptanoin or placebo diet (1 g/kg/day). Primary outcome was change in mean heart rate during 20 min submaximal exercise on a cycle ergometer. Secondary outcomes were change in peak workload and oxygen uptake along with changes in blood metabolites and respiratory quotients.

Results

Nineteen of 22 patients completed the trial. Malate levels rose on triheptanoin treatment versus placebo (8.0 ± SD2.3 vs. 5.5 ± SD1.8 µmol/L, P < 0.001), but dropped from rest to exercise (P < 0.001). There was no difference in exercise heart rates between triheptanoin (120 ± SD16 bpm) and placebo (121 ± SD16 bpm) treatments. Compared with placebo, triheptanoin did not change the submaximal respiratory quotient (0.82 ± SD0.05 vs. 0.84 ± SD0.03), peak workload (105 ± SD38 vs. 102 ± SD31 Watts), or peak oxygen uptake (1938 ± SD499 vs. 1977 ± SD380 mL/min).

Interpretation

Despite increased resting plasma malate with triheptanoin, the increase was insufficient to generate a normal TCA turnover during exercise and the treatment has no effect on exercise capacity or oxidative metabolism in patients with McArdle disease.

Nutrition and exercise in Pompe disease

The current standard of care for Pompe disease (PD) is the administration of enzyme replacement therapy (ERT). Exercise and nutrition are often considered as complementary strategies rather than "treatments" per se. Nutritional assessment is important in patients with locomotor disability because the relative hypodynamia limits energy expenditure and thus the total amount of energy must be reduced to avoid obesity. A lower total energy intake often leads to lower protein and micronutrient intake. Consequently, ensuring that Pompe patients are tested for and replaced for deficiencies (protein, vitamin D, vitamin B12, etc.) is an important aspect of care. Furthermore, given the role of autophagy in the pathophysiology of PD and the fact that fasting induces autophagy, it is important that strategies such as nutritional timing and amino acid intake (L-arginine, L-leucine) be evaluated as therapies. Exercise interventions have been shown to improve six-minute walk testing distance by more than what was seen in the seminal ERT study in late-onset PD. Exercise therapy can also activate autophagy, and this is likely another component of its efficacy. The current review will evaluate the theoretical and practical aspects of nutrition and exercise as therapies for patients with PD.

Glycogen metabolism and glycogen storage disorders

Glucose is the main energy fuel for the human brain. Maintenance of glucose homeostasis is therefore, crucial to meet cellular energy demands in both - normal physiological states and during stress or increased demands. Glucose is stored as glycogen primarily in the liver and skeletal muscle with a small amount stored in the brain. Liver glycogen primarily maintains blood glucose levels, while skeletal muscle glycogen is utilized during high-intensity exertion, and brain glycogen is an emergency cerebral energy source. Glycogen and glucose transform into one another through glycogen synthesis and degradation pathways. Thus, enzymatic defects along these pathways are associated with altered glucose metabolism and breakdown leading to hypoglycemia ± hepatomegaly and or liver disease in hepatic forms of glycogen storage disorder (GSD) and skeletal ± cardiac myopathy, depending on the site of the enzyme defects. Overall, defects in glycogen metabolism mainly present as GSDs and are a heterogenous group of inborn errors of carbohydrate metabolism. In this article we review the genetics, epidemiology, clinical and metabolic findings of various types of GSD, and glycolysis defects emphasizing current treatment and implications for future directions.

Myopathies Related to Glycogen Metabolism Disorders

Most of the glycogen metabolism disorders that affect skeletal muscle involve enzymes in glycogenolysis (myophosphorylase (PYGM), glycogen debranching enzyme (AGL), phosphorylase b kinase (PHKB)) and glycolysis (phosphofructokinase (PFK), phosphoglycerate mutase (PGAM2), aldolase A (ALDOA), β-enolase (ENO3)); however, 3 involve glycogen synthesis (glycogenin-1 (GYG1), glycogen synthase (GSE), and branching enzyme (GBE1)). Many present with exercise-induced cramps and rhabdomyolysis with higher-intensity exercise (i.e., PYGM, PFK, PGAM2), yet others present with muscle atrophy and weakness (GYG1, AGL, GBE1). A failure of serum lactate to rise with exercise with an exaggerated ammonia response is a common, but not invariant, finding. The serum creatine kinase (CK) is often elevated in the myopathic forms and in PYGM deficiency, but can be normal and increase only with rhabdomyolysis (PGAM2, PFK, ENO3). Therapy for glycogen storage diseases that result in exercise-induced symptoms includes lifestyle adaptation and carefully titrated exercise. Immediate pre-exercise carbohydrate improves symptoms in the glycogenolytic defects (i.e., PYGM), but can exacerbate symptoms in glycolytic defects (i.e., PFK). Creatine monohydrate in low dose may provide a mild benefit in PYGM mutations.

Perspectives on Exertional Rhabdomyolysis

Exertional (exercise-induced) rhabdomyolysis is a potentially life threatening condition that has been the subject of research, intense discussion, and media attention. The causes of rhabdomyolysis are numerous and can include direct muscle injury, unaccustomed exercise, ischemia, extreme temperatures, electrolyte abnormalities, endocrinologic conditions, genetic disorders, autoimmune disorders, infections, drugs, toxins, and venoms. The objective of this article is to review the literature on exertional rhabdomyolysis, identify precipitating factors, and examine the role of the dietary supplement creatine monohydrate. PubMed and SPORTDiscus databases were searched using the terms rhabdomyolysis, muscle damage, creatine, creatine supplementation, creatine monohydrate, and phosphocreatine. Additionally, the references of papers identified through this search were examined for relevant studies. A meta-analysis was not performed. Although the prevalence of rhabdomyolysis is low, instances still occur where exercise is improperly prescribed or used as punishment, or incomplete medical history is taken, and exertional rhabdomyolysis occurs. Creatine monohydrate does not appear to be a precipitating factor for exertional rhabdomyolysis. Healthcare professionals should be able to recognize the basic signs of exertional rhabdomyolysis so prompt treatment can be administered. For the risk of rhabdomyolysis to remain low, exercise testing and prescription must be properly conducted based on professional standards.

A New Condition in McArdle Disease: Poor Bone Health-Benefits of an Active Lifestyle

INTRODUCTION-PURPOSE

McArdle disease (muscle glycogen phosphorylase deficiency) is a genetic condition associated with exercise intolerance, but how it affects lean mass (LM) and bone mineral content (BMC) and density (BMD) in patients is unknown. We compared these variables between McArdle patients and age-/sex-matched healthy controls and assessed their potential association with physical activity levels in patients.

METHODS

A case-control, cross-sectional design was used to examine LM, BMC, and BMD by using dual-energy x-ray absorptiometry in 136 young adults of both sexes (36 McArdle patients (33 ± 15 yr) and 103 controls (34 ± 11 yr)). Physical activity was assessed using the International Physical Activity Questionnaire.

RESULTS

McArdle patients had significantly lower LM values in whole-body and regional sites compared with their corresponding controls, whereas no differences were found (except for the trunk) when physically active patients (n = 23) were compared with controls. All bone-related variables were significantly lower in patients than in controls (average difference of 13% for BMC and 7.6% for BMD). By contrast, no significant differences at the lumbar spine, pelvis, and femur sites were found between physically active patients and controls.

CONCLUSIONS

We report on a previously undescribed condition in McArdle patients, poor bone health, which warrants further attention because it can occur in relatively young adults. An active lifestyle can at least partly alleviate this disorder presumably because of its beneficial effect on LM.

Muscle molecular adaptations to endurance exercise training are conditioned by glycogen availability: a proteomics-based analysis in the McArdle mouse model

KEY POINTS

Although they are unable to utilize muscle glycogen, McArdle mice adapt favourably to an individualized moderate-intensity endurance exercise training regime. Yet, they fail to reach the performance capacity of healthy mice with normal glycogen availability. There is a remarkable difference in the protein networks involved in muscle tissue adaptations to endurance exercise training in mice with and without glycogen availability. Indeed, endurance exercise training promoted the expression of only three proteins common to both McArdle and wild-type mice: LIMCH1, PARP1 and TIGD4. In turn, trained McArdle mice presented strong expression of mitogen-activated protein kinase 12 (MAPK12).

ABSTRACT

McArdle's disease is an inborn disorder of skeletal muscle glycogen metabolism that results in blockade of glycogen breakdown due to mutations in the myophosphorylase gene. We recently developed a mouse model carrying the homozygous p.R50X common human mutation (McArdle mouse), facilitating the study of how glycogen availability affects muscle molecular adaptations to endurance exercise training. Using quantitative differential analysis by liquid chromatography with tandem mass spectrometry, we analysed the quadriceps muscle proteome of 16-week-old McArdle (n = 5) and wild-type (WT) (n = 4) mice previously subjected to 8 weeks' moderate-intensity treadmill training or to an equivalent control (no training) period. Protein networks enriched within the differentially expressed proteins with training in WT and McArdle mice were assessed by hypergeometric enrichment analysis. Whereas endurance exercise training improved the estimated maximal aerobic capacity of both WT and McArdle mice as compared with controls, it was ∼50% lower than normal in McArdle mice before and after training. We found a remarkable difference in the protein networks involved in muscle tissue adaptations induced by endurance exercise training with and without glycogen availability, and training induced the expression of only three proteins common to McArdle and WT mice: LIM and calponin homology domains-containing protein 1 (LIMCH1), poly (ADP-ribose) polymerase 1 (PARP1 - although the training effect was more marked in McArdle mice), and tigger transposable element derived 4 (TIGD4). Trained McArdle mice presented strong expression of mitogen-activated protein kinase 12 (MAPK12). Through an in-depth proteomic analysis, we provide mechanistic insight into how glycogen availability affects muscle protein signalling adaptations to endurance exercise training.

Wave of renal impairment

We present a case of a 51-year-old man who went to the emergency department after an almost-drowning episode, presenting with muscular weakness, myalgia and dark urine. Laboratory data showed a severe rhabdomyolysis (creatine kinase 497 510 U/L). Despite aggressive fluid therapy, an oliguric acute kidney injury was established with temporary need of haemodialysis. The patient had a longtime history of exercise intolerance and family history of a metabolic myopathy, namely a sister with McArdle's disease. The genetic test was positive. McArdle's disease is an autosomal recessive disorder caused by mutations in the muscle glycogen phosphorylase gene that encodes the myophosphorylase. The main symptom consists in exercise intolerance and the most severe complication is rhabdomyolysis with acute renal failure. Metabolic myopathies, such as McArdle's disease, should be considered in patients with acute renal failure due to unexplained severe rhabdomyolysis, especially if there are chronic complaints of exercise intolerance and positive family history.

Metabolic Myopathies

PURPOSE OF REVIEW

Metabolic myopathies are genetic disorders that impair intermediary metabolism in skeletal muscle. Impairments in glycolysis/glycogenolysis (glycogen-storage disease), fatty acid transport and oxidation (fatty acid oxidation defects), and the mitochondrial respiratory chain (mitochondrial myopathies) represent the majority of known defects. The purpose of this review is to develop a diagnostic and treatment algorithm for the metabolic myopathies.

RECENT FINDINGS

The metabolic myopathies can present in the neonatal and infant period as part of more systemic involvement with hypotonia, hypoglycemia, and encephalopathy; however, most cases present in childhood or in adulthood with exercise intolerance (often with rhabdomyolysis) and weakness. The glycogen-storage diseases present during brief bouts of high-intensity exercise, whereas fatty acid oxidation defects and mitochondrial myopathies present during a long-duration/low-intensity endurance-type activity or during fasting or another metabolically stressful event (eg, surgery, fever). The clinical examination is often normal between acute events, and evaluation involves exercise testing, blood testing (creatine kinase, acylcarnitine profile, lactate, amino acids), urine organic acids (ketones, dicarboxylic acids, 3-methylglutaconic acid), muscle biopsy (histology, ultrastructure, enzyme testing), MRI/spectroscopy, and targeted or untargeted genetic testing.

SUMMARY

Accurate and early identification of metabolic myopathies can lead to therapeutic interventions with lifestyle and nutritional modification, cofactor treatment, and rapid treatment of rhabdomyolysis.

Genotypic and phenotypic features of all Spanish patients with McArdle disease: a 2016 update

BACKGROUND

We recently described the genotype/phenotype features of all Spanish patients diagnosed with McArdle disease as of January 2011 (n = 239, prevalence of ~1/167,000) (J Neurol Neurosurg Psychiatry 2012;83:322-8). Several caveats were however identified suggesting that the prevalence of the disease is actually higher.

METHODS

We have now updated main genotype/phenotype data, as well as potential associations within/between them, of all Spanish individuals currently diagnosed with McArdle disease (December 2016).

RESULTS

Ninety-four new patients (all Caucasian) have been diagnosed, yielding a prevalence of ~1/139,543 individuals. Around 55% of the mutated alleles have the commonest PYGM pathogenic mutation p.R50X, whereas p.W798R and p.G205S account for 10 and 9% of the allelic variants, respectively. Seven new mutations were identified: p.H35R, p.R70C, p.R94Q, p.L132WfsX163, p.Q176P, p.R576Q, and c.244-3_244-2CA. Almost all patients show exercise intolerance, the second wind phenomenon and high serum creatine kinase activity. There is, however, heterogeneity in clinical severity, with 8% of patients being asymptomatic during normal daily life, and 21% showing limitations during daily activities and fixed muscle weakness. A major remaining challenge is one of diagnosis, which is often delayed until the third decade of life in 72% of new patients despite the vast majority (86%) reporting symptoms before 20 years. An important development is the growing proportion of those reporting a 4-year improvement in disease severity (now 34%) and following an active lifestyle (50%). Physically active patients are more likely to report an improvement after a 4-year period in the clinical course of the disease than their inactive peers (odds ratio: 13.98; 95% confidence interval: 5.6, 34.9; p < 0.001). Peak oxygen uptake is also higher in the former (20.7 ± 6.0 vs. 16.8 ± 5.3 mL/kg/min, p = 0.0013). Finally, there is no association between PYGM genotype and phenotype manifestation of the disease.

CONCLUSIONS

The reported prevalence of McArdle disease grows exponentially despite frequent, long delays in genetic diagnosis, suggesting that many patients remain undiagnosed. Until a genetic cure is available (which is not predicted in the near future), current epidemiologic data support that adoption of an active lifestyle is the best medicine for these patients.

Equine atypical myopathy in the UK: Epidemiological characteristics of cases reported from 2011 to 2015 and factors associated with survival

BACKGROUND

Equine atypical myopathy (AM) is a toxic rhabdomyolysis associated with ingestion of hypoglycin A, derived typically in Europe, from Acer pseudoplatanus tree. Despite the wide distribution of this tree species in the UK, the number of cases reported annually varies, and there has been an apparent increase in prevalence in recent years. Although AM was first recognised in the UK, epidemiological studies have never been conducted focused solely on this country.

OBJECTIVES

To describe the spatiotemporal distribution, presentation, treatment and outcome of AM cases reported in the UK.

STUDY DESIGN

Retrospective case series.

METHODS

British AM cases reported to the atypical myopathy alert website, between 2011 and 2015 were included (n = 224). Data were obtained via standardised epidemiological questionnaires from owners and veterinarians. Factors associated with survival were assessed using logistic regression.

RESULTS

Most cases reported were from England (87.9%). Survival was 38.6% (n = 73/189). Clinical factors associated with reduced odds of survival included, hypothermia (odds ratio [OR] 0.18; 95% confidence interval [CI] 0.06-0.57; P = 0.01), bladder distension (OR 0.11; CI 0.02-0.59; P = 0.01), tachycardia (OR 0.97; CI 0.94-0.99; P = 0.04) and serum creatine kinase activity >100,000 IU/L (OR 0.17; CI 0.04-0.68; P = 0.01) in the univariable analysis as well as recumbency. The latter was the only sign retained in multivariable analysis (OR = 0.19; CI 0.06-0.62; P = 0.006). Administration of vitamins during the disease was associated with survival (OR 3.75; CI 1.21-11.57; P = 0.02).

MAIN LIMITATIONS

Reporting cases to the Atypical Myopathy Alert Group is voluntary; therefore, under-reporting will result in underestimation of AM cases; furthermore, direct owner-reporting could have introduced misdiagnosis bias.

CONCLUSION

Some areas of the UK reported AM cases more commonly. Clinical signs such as recumbency, rectal temperature, distended bladder and serum creatine kinase activity might be useful prognostic indicators though should be considered in the context of the clinical picture. Treatment with vitamins increases odds of survival.

Higher oxidative stress in skeletal muscle of McArdle disease patients

McArdle disease (MCD) is an autosomal recessive condition resulting from skeletal muscle glycogen phosphorylase deficiency. The resultant block in glycogenolysis leads to an increased flux through the xanthine oxidase pathway (myogenic hyperuricemia) and could lead to an increase in oxidative stress. We examined markers of oxidative stress (8-isoprostane and protein carbonyls), NAD(P)H-oxidase, xanthine oxidase and antioxidant enzyme (superoxide dismutase, catalase and glutathione peroxidase) activity in skeletal muscle of MCD patients (N = 12) and controls (N = 12). Eight-isoprostanes and protein carbonyls were higher in MCD patients as compared to controls (p < 0.05). There was a compensatory up-regulation of catalase protein content and activity (p < 0.05), mitochondrial superoxide dismutase (MnSOD) protein content (p < 0.01) and activity (p < 0.05) in MCD patients, yet this increase was not sufficient to protect the muscle against elevated oxidative damage. These results suggest that oxidative stress in McArdle patients occurs and future studies should evaluate a potential role for oxidative stress contributing to acute pathology (rhabdomyolysis) and possibly later onset fixed myopathy.

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.

Single-Blind, Randomized, Controlled Clinical Trial of Exercise in Ambulatory Spinal Muscular Atrophy: Why are the Results Negative?

BACKGROUND

The benefits of exercise on long-term health and well-being are well established. The possible benefits of exercise in Spinal Muscular Atrophy (SMA) have not been explored in a controlled clinical trial format.

OBJECTIVE

To assess the effects of exercise on measures of function, strength, and exercise capacity in ambulatory SMA patients.

METHODS

Fourteen participants, ages 10-48 years, were randomized to control and exercise cohorts after a 1 month lead-in period. The exercise group received 6 months of intervention. Thereafter, both groups received the intervention for the remaining 12 months. Participants were monitored for a total of 19 months. Exercise included individualized home-based cycling and strengthening. The primary outcome measure was distance walked during the six-minute walk test (6MWT). Secondary outcomes included strength, function, exercise capacity, quality of life and fatigue.

RESULTS

Twelve participants completed the first 7 months of the study, and 9 completed all 19 months. At baseline, the groups were similar on all clinical variables. There were no group changes at any time point in the 6MWT, fatigue, or function. Percent-predicted VO2 max improved 4.9% in all participants in 6 months (p = 0.036) (n = 10).

CONCLUSION

Daily exercise is safe in ambulatory SMA and should be encouraged. We did not uncover any deleterious effects on strength, function, or fatigue. Our study documented a reduction in oxidative capacity and a blunted conditioning response to exercise possibly representing an important insight into underlying pathophysiological mechanisms. These findings also may be linked causally to mitochondrial depletion in SMA and warrant further study.

Rodent models for resolving extremes of exercise and health

The extremes of exercise capacity and health are considered a complex interplay between genes and the environment. In general, the study of animal models has proven critical for deep mechanistic exploration that provides guidance for focused and hypothesis-driven discovery in humans. Hypotheses underlying molecular mechanisms of disease and gene/tissue function can be tested in rodents to generate sufficient evidence to resolve and progress our understanding of human biology. Here we provide examples of three alternative uses of rodent models that have been applied successfully to advance knowledge that bridges our understanding of the connection between exercise capacity and health status. First we review the strong association between exercise capacity and all-cause morbidity and mortality in humans through artificial selection on low and high exercise performance in the rat and the consequent generation of the "energy transfer hypothesis." Second we review specific transgenic and knockout mouse models that replicate the human disease condition and performance. This includes human glycogen storage diseases (McArdle and Pompe) and α-actinin-3 deficiency. Together these rodent models provide an overview of the advancements of molecular knowledge required for clinical translation. Continued study of these models in conjunction with human association studies will be critical to resolving the complex gene-environment interplay linking exercise capacity, health, and disease.

McArdle disease: a unique study model in sports medicine

McArdle disease is arguably the paradigm of exercise intolerance in humans. This disorder is caused by inherited deficiency of myophosphorylase, the enzyme isoform that initiates glycogen breakdown in skeletal muscles. Because patients are unable to obtain energy from their muscle glycogen stores, this disease provides an interesting model of study for exercise physiologists, allowing insight to be gained into the understanding of glycogen-dependent muscle functions. Of special interest in the field of muscle physiology and sports medicine are also some specific (if not unique) characteristics of this disorder, such as the so-called 'second wind' phenomenon, the frequent exercise-induced rhabdomyolysis and myoglobinuria episodes suffered by patients (with muscle damage also occurring under basal conditions), or the early appearance of fatigue and contractures, among others. In this article we review the main pathophysiological features of this disorder leading to exercise intolerance as well as the currently available therapeutic possibilities. Patients have been traditionally advised by clinicians to refrain from exercise, yet sports medicine and careful exercise prescription are their best allies at present because no effective enzyme replacement therapy is expected to be available in the near future. As of today, although unable to restore myophosphorylase deficiency, the 'simple' use of exercise as therapy seems probably more promising and practical for patients than more 'complex' medical approaches.

Exercise in muscle glycogen storage diseases

Glycogen storage diseases (GSD) are inborn errors of glycogen or glucose metabolism. In the GSDs that affect muscle, the consequence of a block in skeletal muscle glycogen breakdown or glucose use, is an impairment of muscular performance and exercise intolerance, owing to 1) an increase in glycogen storage that disrupts contractile function and/or 2) a reduced substrate turnover below the block, which inhibits skeletal muscle ATP production. Immobility is associated with metabolic alterations in muscle leading to an increased dependence on glycogen use and a reduced capacity for fatty acid oxidation. Such changes may be detrimental for persons with GSD from a metabolic perspective. However, exercise may alter skeletal muscle substrate metabolism in ways that are beneficial for patients with GSD, such as improving exercise tolerance and increasing fatty acid oxidation. In addition, a regular exercise program has the potential to improve general health and fitness and improve quality of life, if executed properly. In this review, we describe skeletal muscle substrate use during exercise in GSDs, and how blocks in metabolic pathways affect exercise tolerance in GSDs. We review the studies that have examined the effect of regular exercise training in different types of GSD. Finally, we consider how oral substrate supplementation can improve exercise tolerance and we discuss the precautions that apply to persons with GSD that engage in exercise.

Sodium valproate increases the brain isoform of glycogen phosphorylase: looking for a compensation mechanism in McArdle disease using a mouse primary skeletal-muscle culture in vitro

McArdle disease, also termed ‘glycogen storage disease type V’, is a disorder of skeletal muscle carbohydrate metabolism caused by inherited deficiency of the muscle-specific isoform of glycogen phosphorylase (GP-MM). It is an autosomic recessive disorder that is caused by mutations in the PYGM gene and typically presents with exercise intolerance, i.e. episodes of early exertional fatigue frequently accompanied by rhabdomyolysis and myoglobinuria. Muscle biopsies from affected individuals contain subsarcolemmal deposits of glycogen. Besides GP-MM, two other GP isoforms have been described: the liver (GP-LL) and brain (GP-BB) isoforms, which are encoded by the PYGL and PYGB genes, respectively; GP-BB is the main GP isoform found in human and rat foetal tissues, including the muscle, although its postnatal expression is dramatically reduced in the vast majority of differentiated tissues with the exception of brain and heart, where it remains as the major isoform. We developed a cell culture model from knock-in McArdle mice that mimics the glycogen accumulation and GP-MM deficiency observed in skeletal muscle from individuals with McArdle disease. We treated mouse primary skeletal muscle cultures in vitro with sodium valproate (VPA), a histone deacetylase inhibitor. After VPA treatment, myotubes expressed GP-BB and a dose-dependent decrease in glycogen accumulation was also observed. Thus, this in vitro model could be useful for high-throughput screening of new drugs to treat this disease. The immortalization of these primary skeletal muscle cultures could provide a never-ending source of cells for this experimental model. Furthermore, VPA could be considered as a gene-expression modulator, allowing compensatory expression of GP-BB and decreased glycogen accumulation in skeletal muscle of individuals with McArdle disease.

Summary: Use of this in vitro model showed that sodium valproate (VPA) can reverse the muscle phenotype from a McArdle-like to a normal histological and biochemical profile.

Metabolic and mitochondrial myopathies

Metabolic and mitochondrial myopathies encompass a heterogeneous group of disorders that result in impaired energy production in skeletal muscle. Symptoms of premature muscle fatigue, sometimes leading to myalgia, rhabdomyolysis, and myoglobinuria, typically occur with exercise that would normally depend on the defective metabolic pathway. But in another group of these disorders, the dominant muscle symptom is weakness. This article reviews the clinical features, diagnosis, and management of these diseases with emphasis on the recent literature.

The pathogenomics of McArdle disease--genes, enzymes, models, and therapeutic implications

Numerous biomedical advances have been made since Carl and Gerty Cori discovered the enzyme phosphorylase in the 1940s and the Scottish physician Brian McArdle reported in 1951 a previously 'undescribed disorder characterized by a gross failure of the breakdown in muscle of glycogen'. Today we know that this disorder, commonly known as 'McArdle disease', is caused by inherited deficiency of the muscle isoform of glycogen phosphorylase (GP). Here we review the main aspects of the 'pathogenomics' of this disease including, among others: the spectrum of mutations in the gene (PYGM) encoding muscle GP; the interplay between the different tissue GP isoforms in cellular cultures and in patients; what can we learn from naturally occurring and recently laboratory-generated animal models of the disease; and potential therapies.