Molecular genetic heterogeneity of myophosphorylase deficiency (McArdle's disease)

Predicting Pathology of Missense Mutations through Protein-Specific Evolutionary Pattern

Missense mutations, which are single base pair genetic alternation resulting in a different amino acid, are among the most common occurring variants in exon regions of the human genome and may lead to diseases. Thus to assess the effects of missense mutations, it is essential to investigate the evolutionary history of the protein under selection pressures. In this study, we employ a continuous-time Markov model to investigate the evolutionary patterns in protein sequences and a Bayesian Markov chain Monte Carlo method to estimate the substitution rates for protein of interest, from which we obtain scoring matrices. Specifically, we examined the evolutionary patterns of protein sequences containing missense mutations using a species tree to define the phylogeny of the protein of interest. We thoroughly studied the evolutionary pattern of human muscle glycogen phosphorylase containing 127 known missense mutations, and identified characteristic evolutionary patterns in 63 proteins with 2,238 missense mutations, including both deleterious and neutral effects. Our results show that the estimated protein-specific evolutionary pattern-based scoring matrices (PSM) lead to higher sensitivity in detecting the pathological effects of missense mutations, compared to the general evolutionary pattern-based scoring matrix of Blosum62 (BL62) matrix. By incorporating PSM, the performance of a recently released structure-based model SPRI for evaluating missense mutations is further improved.

McArdle disease in a patient with anorexia nervosa: a case report

BACKGROUND

McArdle disease is an autosomal recessive genetic disorder caused by a deficiency of the glycogen phosphorylase (myophosphorylase) enzyme, which muscles need to break down glycogen into glucose for energy. Symptoms include exercise intolerance, with fatigue, muscle pain, and cramps being manifested during the first few minutes of exercise, which may be accompanied by rhabdomyolysis.

CASE PRESENTATION

This case report describes for the first time the clinical features, diagnosis and management of a 20 year-old patient with anorexia nervosa and McArdle disease, documented by means of muscle biopsy.

CONCLUSION

Anorexia nervosa and McArdle disease interact in a detrimental bidirectional way. In addition, some laboratory parameter alterations (e.g., elevated values of creatine kinase) commonly attributed to the specific features of eating disorders (e.g., excessive exercising) may delay the diagnosis of metabolic muscle diseases. On the other hand, the coexistence of a chronic disease, such as McArdle disease, whose management requires the adoption of a healthy lifestyle, can help to engage patients in actively addressing their eating disorder.

Rare loss of function variants in the hepatokine gene INHBE protect from abdominal obesity

Identifying genetic variants associated with lower waist-to-hip ratio can reveal new therapeutic targets for abdominal obesity. We use exome sequences from 362,679 individuals to identify genes associated with waist-to-hip ratio adjusted for BMI (WHRadjBMI), a surrogate for abdominal fat that is causally linked to type 2 diabetes and coronary heart disease. Predicted loss of function (pLOF) variants in INHBE associate with lower WHRadjBMI and this association replicates in data from AMP-T2D-GENES. INHBE encodes a secreted protein, the hepatokine activin E. In vitro characterization of the most common INHBE pLOF variant in our study, indicates an in-frame deletion resulting in a 90% reduction in secreted protein levels. We detect associations with lower WHRadjBMI for variants in ACVR1C, encoding an activin receptor, further highlighting the involvement of activins in regulating fat distribution. These findings highlight activin E as a potential therapeutic target for abdominal obesity, a phenotype linked to cardiometabolic disease.

Molecular diagnosis of McArdle disease using whole-exome sequencing

Whole-exome sequencing (WES) analysis has been used recently as a diagnostic tool for finding molecular defects. In the present study, researchers attempted to analyze molecular defects through WES in a 13-year-old female patient who had not been diagnosed through a conventional genetic approach. DNA was extracted and subjected to WES analysis to identify the genetic defect. A total of 106,728 exons and splicing variants were selected, and synonymous single nucleotide variants (SNVs) and general single nucleotide polymorphisms (SNPs) were filtered out. Finally, nonsynonymous SNVs (c.C415T and c.C389T) of the PYGM gene were identified in nine compound heterozygous mutations. PYGM encodes myophosphorylase and degrades glycogen in the muscle to supply energy to muscle cells. The present study revealed that the patient's father had a c.C389T mutation and the mother had a c.C415T mutation, resulting in A130V and R139W missense mutations, respectively. To the best of our knowledge, the A130V variant in PYGM has not been reported in the common variant databases. All variations of the patient's family detected using WES were verified by Sanger sequencing. Because the patient had compound heterozygous mutations in the PYGM gene, the patient was presumed to exhibit markedly decreased muscle phosphorylase activity. To assess the function of myophosphorylase, an ischemic forearm exercise test was performed. The blood ammonia level sharply increased and the lactate level maintained a flat curve shape similar to the typical pattern of McArdle disease. Therefore, the diagnosis of the patient was confirmed to be McArdle disease, a glycogen storage disease. Through WES analysis, accurate and early diagnosis could be made in the present study. This report describes a novel compound heterozygous mutation of the PYGM gene in a Korean patient.

The phenotypic and genotypic features of a Scottish cohort with McArdle disease

This retrospective study evaluated the phenotypic and genotypic features of 14 patients with McArdle disease attending the West of Scotland adult muscle clinic. Although all patients experienced exercise-induced cramps, exercise intolerance and hyperCKaemia, only 71% (n = 10) experienced the second wind phenomenon, rhabdomyolysis and/or myoglobinuria. We observed a high rate of fixed muscle weakness (50%; n = 7), coronary artery disease (36%; n = 5), and psychological comorbidity (50%; n = 7). Although 79% had symptom onset in the first decade of life, the mean age at presentation and at genetic diagnosis was 43.8 years and 47.7 years, respectively. 93% had at least one copy of the common PYGM pathogenic variant, c.148C > T, p.(Arg50*), with 50% (n = 7) of the cohort being homozygous. Our cohort highlights the phenotypic variability seen in McArdle disease and underscores the potential for late-onset presentations. It emphasises the need for improved awareness and recognition of this condition amongst neurologists, rheumatologists and general physicians. A history of exercise intolerance and second wind phenomenon may not always be volunteered by the patient, underscoring the need to ask specific questions in clinic to extrapolate the relevant symptoms in this patient cohort.

McArdle disease and pregnancy: A case report and scoping review of pregnancy outcomes

McArdle disease is an autosomal recessive disorder affecting skeletal muscle glycogen metabolism. Limited data are available regarding pregnancy outcomes with this genetic condition. We present a recent case of a woman with McArdle disease, along with a scoping review of all published literature regarding pregnancy and delivery outcomes for women with McArdle disease. A total of 35 cases are summarised. Overall, pregnancy does not worsen or increase the risk for disease flare. Women can successfully deliver vaginally, with consideration of an assisted second stage recommended to reduce the risk of postpartum rhabdomyolysis.

A New Glycogen Storage Disease Caused by a Dominant PYGM Mutation

OBJECTIVE

Glycogen storage diseases (GSDs) are severe human disorders resulting from abnormal glucose metabolism, and all previously described GSDs segregate as autosomal recessive or X-linked traits. In this study, we aimed to molecularly characterize the first family with a dominant GSD.

METHODS

We describe a dominant GSD family with 13 affected members presenting with adult-onset muscle weakness, and we provide clinical, metabolic, histological, and ultrastructural data. We performed exome sequencing to uncover the causative gene, and functional experiments in the cell model and on recombinant proteins to investigate the pathogenic effect of the identified mutation.

RESULTS

We identified a heterozygous missense mutation in PYGM segregating with the disease in the family. PYGM codes for myophosphorylase, the enzyme catalyzing the initial step of glycogen breakdown. Enzymatic tests revealed that the PYGM mutation impairs the AMP-independent myophosphorylase activity, whereas the AMP-dependent activity was preserved. Further functional investigations demonstrated an altered conformation and aggregation of mutant myophosphorylase, and the concurrent accumulation of the intermediate filament desmin in the myofibers of the patients.

INTERPRETATION

Overall, this study describes the first example of a dominant glycogen storage disease in humans, and elucidates the underlying pathomechanisms by deciphering the sequence of events from the PYGM mutation to the accumulation of glycogen in the muscle fibers. ANN NEUROL 2020;88:274-282.

Single-centre experience on genotypic and phenotypic features of southern Brazilian patients with McArdle disease

McArdle disease (MD) is a metabolic myopathy caused by deficiency of the myophosphorylase enzyme. The aim of our study was to analyse a series of MD patients in Brazil and the correlation between clinical findings, laboratory data, electromyography, muscle biopsy and genetic features. The PYGM gene was analysed by PCR/RLFP and Sanger sequencing. The sample included 12 patients, aged 18-57 years, from unrelated families. Exercise intolerance was present in all cases. Serum creatine kinase levels at rest were increased in all patients. Forearm ischaemic exercise testing in five patients revealed no increase in venous lactate. Needle electromyography presented 'myopathic pattern' in six patients. Muscle biopsy showed vacuolar myopathy in 10 patients and deficiency of myophosphorylase enzyme in all patients. The genetic analysis showed p.R50X as the most common mutation (allelic frequency: 56.25%), other known mutations (p.Y574X, p.G205S, p.W798R, IVS14 + 1G > A and IVS19-1G > A) and a new mutation (p.Asn168Lysfs*15) were also identified. Several features of the disorder were similar to the vast majority of patients worldwide. The genetic findings of this study revealed a range of mutations that are quite similar to the European cohort. The discovery of one novel mutation increases the genotypic heterogeneity of PYGM gene.

Longitudinal case study and phenotypic multimodal characterization of McArdle disease-linked retinopathy: insight into pathomechanisms

Background: We present a longitudinal clinical characterization of PYGM-linked pattern dystrophy in an adult male patient.Materials and Methods: A patient affected by McArdle disease (glycogen storage disease type V) and homozygous for the nonsense variant PYGM c.148C>T p.(Arg50*) underwent ophthalmic examinations over a 9-year-interval, including fundus photography, fundus autofluorescence, optical coherence tomography (OCT), OCT-angiography and electroretinography (ERG).Results: At age 52, the patient was asymptomatic but yellow flecks were first observed in the macula of both eyes. This yellow flecks at the posterior pole progressed towards a pattern-like dystrophy over a 5-year-period. By fundus autofluorescence imaging the appearance of new hyperautofluorescent flecks and the extension of existing ones was observed over time. Concomitantly, a slow progression of the size of atrophic areas was seen at the posterior pole. Scotopic ERGs were within normal limits, but photopic Flicker responses were decreased, indicating reduced cone function.Conclusions: This additional case of PYGM-linked pattern dystrophy further confirms retinopathy as a clinical phenotype associated with McArdle disease. PYGM expression pattern suggests a disease mechanism involving impaired glycogen metabolism both in the retinal pigment epithelium and in cone photoreceptors.

McArdle Disease: Clinical, Biochemical, Histological and Molecular Genetic Analysis of 60 Patients

A clinical, biochemical, histological and molecular genetic analysis of 60 McArdle patients (33 males and 27 females; mean age at diagnosis: 37 years) was performed. The objective of this study was to identify a possible genotype–phenotype correlation in McArdle disease. All patients complained of exercise-induced myalgia and fatigue; permanent weakness was present in 47% of the patients. Five percent of patients conveyed of masticatory muscle weakness. Age of onset was <15 years in 92% patients. Serum creatine kinase was elevated 5 to13-fold. Forearm ischemic test showed decreased lactate production but excessively increased ammonia upon exercise (n = 16). Muscle biopsies revealed highly reduced or missing myophosphorylase activity (n = 20) (mean: 0.17 ± 0.35 U/g tissue; normal: 12–61) and histologically, sub-sarcolemmal glycogen accumulation (n = 9). Molecular genetic analysis revealed the common p.Arg50Ter mutation in 68% of the patients. Other rather frequent mutations were p.Arg270Ter (allele frequency: 5%) followed by c.2262delA and p.Met1Val (allele frequencies: 3%). Twenty-four other rare mutations were also identified. No genotype–phenotype correlation was observed. The analysis highlights that testing of the p.Arg50Ter mutation could be performed first in molecular genetic testing of patients with exercise intolerance possibly due to McArdle disease. However, there is enormous mutation heterogeneity in McArdle disease thus sequencing of the myophosphorylase gene is needed in patients highly suspicious of McArdle disease.

Investigating the genetic susceptibility to exertional heat illness

Background

We aimed to identify rare (minor allele frequency ≤1%), potentially pathogenic non-synonymous variants in a well-characterised cohort with a clinical history of exertional heat illness (EHI) or exertional rhabdomyolysis (ER). The genetic link between malignant hyperthermia (MH) and EHI was investigated due to their phenotypic overlap.

Methods

The coding regions of 38 genes relating to skeletal muscle calcium homeostasis or exercise intolerance were sequenced in 64 patients (mostly military personnel) with a history of EHI, or ER and who were phenotyped using skeletal muscle in vitro contracture tests. We assessed the pathogenicity of variants using prevalence data, in silico analysis, phenotype and segregation evidence and by review of the literature.

Results

We found 51 non-polymorphic, potentially pathogenic variants in 20 genes in 38 patients. Our data indicate that RYR1 p.T3711M (previously shown to be likely pathogenic for MH susceptibility) and RYR1 p.I3253T are likely pathogenic for EHI. PYGM p.A193S was found in 3 patients with EHI, which is significantly greater than the control prevalence (p=0.000025). We report the second case of EHI in which a missense variant at CACNA1S p.R498 has been found. Combinations of rare variants in the same or different genes are implicated in EHI.

Conclusion

We confirm a role of RYR1 in the heritability of EHI as well as ER but highlight the likely genetic heterogeneity of these complex conditions. We propose defects, or combinations of defects, in skeletal muscle calcium homeostasis, oxidative metabolism and membrane excitability are associated with EHI.

Absence of p.R50X Pygm read-through in McArdle disease cellular models

McArdle disease is an autosomal recessive disorder caused by the absence of muscle glycogen phosphorylase, which leads to blocked muscle glycogen breakdown. We used three different cellular models to evaluate the efficiency of different read-through agents (including amlexanox, Ataluren, RTC13 and G418) in McArdle disease. The first model consisted of HeLa cells transfected with two different GFP-PYGM constructs presenting the Pygm p.R50X mutation (GFP-PYGM p.R50X and PYGM Ex1-GFP p.R50X). The second cellular model was based on the creation of HEK293T cell lines stably expressing the PYGM Ex1-GFP p.R50X construct. As these plasmids encode murine Pygm cDNA without any intron sequence, their transfection in cells would allow for analysis of the efficacy of read-through agents with no concomitant nonsense-mediated decay interference. The third model consisted of skeletal muscle cultures derived from the McArdle mouse model (knock-in for the p.R50X mutation in the Pygm gene). We found no evidence of read-through at detectable levels in any of the models evaluated. We performed a literature search and compared the premature termination codon context sequences with reported positive and negative read-through induction, identifying a potential role for nucleotide positions −9, −8, −3, −2, +13 and +14 (the first nucleotide of the stop codon is assigned as +1). The Pygm p.R50X mutation presents TGA as a stop codon, G nucleotides at positions −1 and −9, and a C nucleotide at −3, which potentially generate a good context for read-through induction, counteracted by the presence of C at −2 and its absence at +4.

Summary: Here, we evaluated the efficiency of different read-through agents in McArdle disease cell culture models, revealing that read-through compounds do not restore full-length muscle glycogen phosphorylase.

McArdle Disease: New Insights into Its Underlying Molecular Mechanisms

McArdle disease, also known as glycogen storage disease type V (GSDV), is characterized by exercise intolerance, the second wind phenomenon, and high serum creatine kinase activity. Here, we recapitulate PYGM mutations in the population responsible for this disease. Traditionally, McArdle disease has been considered a metabolic myopathy caused by the lack of expression of the muscle isoform of the glycogen phosphorylase (PYGM). However, recent findings challenge this view, since it has been shown that PYGM is present in other tissues than the skeletal muscle. We review the latest studies about the molecular mechanism involved in glycogen phosphorylase activity regulation. Further, we summarize the expression and functional significance of PYGM in other tissues than skeletal muscle both in health and McArdle disease. Furthermore, we examine the different animal models that have served as the knowledge base for better understanding of McArdle disease. Finally, we give an overview of the latest state-of-the-art clinical trials currently being carried out and present an updated view of the current therapies.

The effect of muscle glycogen phosphorylase (Pygm) knockdown on zebrafish morphology

Muscle glycogen phosphorylase (PYGM) is a key enzyme in the first step of glycogenolysis. Mutation in the PYGM gene leads to autosomal recessive McArdle disease. Patients suffer from exercise intolerance with premature fatigue, muscle cramps and myalgia due to lack of available glucose in muscles. So far, no efficient treatment has been found. The zebrafish has many experimental advantages, and was successfully implemented as an animal model of human myopathies. Since zebrafish skeletal muscles share high similarity with human skeletal muscles, it is our animal of choice to investigate the impact of Pygm knockdown on skeletal muscle tissue. The two forms of the zebrafish enzyme, Pygma and Pygmb, share more than 80% amino acid sequence identity with human PYGM. We show that the Pygm level varies at both the mRNA and protein level in distinct stages of zebrafish development, which is correlated with glycogen level. The Pygm distribution in muscles varies from dispersed to highly organized at 72 hpf. The pygma and pygmb morpholino knockdown resulted in a reduced Pygm level in zebrafish morphants, which exhibited altered, disintegrated muscle structure and accumulation of glycogen granules in the subsarcolemmal region. Thus, lowering the Pygm level in zebrafish larvae leads to an elevated glycogen level and to morphological muscle changes mimicking the symptoms of human McArdle disease. The zebrafish model of this human disease might contribute to further understanding of its molecular mechanisms and to the development of appropriate treatment.

Next-generation sequencing approach to hyperCKemia: A 2-year cohort study

Objective

Next-generation sequencing (NGS) was applied in molecularly undiagnosed asymptomatic or paucisymptomatic hyperCKemia to investigate whether this technique might allow detection of the genetic basis of the condition.

Methods

Sixty-six patients with undiagnosed asymptomatic or paucisymptomatic hyperCKemia, referred to tertiary neuromuscular centers over an approximately 2-year period, were analyzed using a customized, targeted sequencing panel able to investigate the coding exons and flanking intronic regions of 78 genes associated with limb-girdle muscular dystrophies, rhabdomyolysis, and metabolic and distal myopathies.

Results

A molecular diagnosis was reached in 33 cases, corresponding to a positive diagnostic yield of 50%. Variants of unknown significance were found in 17 patients (26%), whereas 16 cases (24%) remained molecularly undefined. The major features of the diagnosed cases were mild proximal muscle weakness (found in 27%) and myalgia (in 24%). Fourteen patients with a molecular diagnosis and mild myopathic features on muscle biopsy remained asymptomatic at a 24-month follow-up.

Conclusions

This study of patients with undiagnosed hyperCKemia, highlighting the advantages of NGS used as a first-tier diagnostic approach in genetically heterogeneous conditions, illustrates the ongoing evolution of molecular diagnosis in the field of clinical neurology. Isolated hyperCKemia can be the sole feature alerting to a progressive muscular disorder requiring careful surveillance.

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.

Myophosphorylase (PYGM) mutations determined by next generation sequencing in a cohort from Turkey with McArdle disease

This study aimed to identify PYGM mutations in patients with McArdle disease from Turkey by next generation sequencing (NGS). Genomic DNA was extracted from the blood of the McArdle patients (n = 67) and unrelated healthy volunteers (n = 53). The PYGM gene was sequenced with NGS and the observed mutations were validated by direct Sanger sequencing. A diagnostic algorithm was developed for patients with suspected McArdle disease. A total of 16 deleterious PYGM mutations were identified, of which 5 were novel, including 1 splice-site donor, 1 frame-shift, and 3 non-synonymous variants. The p.Met1Val (27-patients/11-families) was the most common PYGM mutation, followed by p.Arg576* (6/4), c.1827+7A>G (5/4), c.772+2_3delTG (5/3), p.Phe710del (4/2), p.Lys754Asnfs (2/1), and p.Arg50* (1/1). A molecular diagnostic flowchart is proposed for the McArdle patients in Turkey, covering the 6 most common PYGM mutations found in Turkey as well as the most common mutation in Europe. The diagnostic algorithm may alleviate the need for muscle biopsies in 77.6% of future patients. A prevalence of any of the mutations to a geographical region in Turkey was not identified. Furthermore, the NGS approach to sequence the entire PYGM gene was successful in detecting a common missense mutation and discovering novel mutations in this population study.

Differential glucose metabolism in mice and humans affected by McArdle disease

McArdle disease (muscle glycogenosis type V) is a disease caused by myophosphorylase deficiency leading to "blocked" glycogen breakdown. A significant but varying glycogen accumulation in especially distal hind limb muscles of mice affected by McArdle disease has recently been demonstrated. In this study, we investigated how myophosphorylase deficiency affects glucose metabolism in hind limb muscle of 20-wk-old McArdle mice and vastus lateralis muscles from patients with McArdle disease. Western blot analysis and activity assay demonstrated that glycogen synthase was inhibited in glycolytic muscle from McArdle mice. The level and activation of proteins involved in contraction-induced glucose transport (AMPK, GLUT4) and glycogen synthase inhibition were increased in quadriceps muscle of McArdle mice. In addition, pCaMKII in quadriceps was reduced, suggesting lower insulin-induced glucose uptake, which could lead to lower glycogen accumulation. In comparison, tibialis anterior, extensor digitorum longus, and soleus had massive glycogen accumulation, but few, if any, changes or adaptations in glucose metabolism compared with wild-type mice. The findings suggest plasticity in glycogen metabolism in the McArdle mouse that is related to myosin heavy chain type IIB content in muscles. In patients, the level of GLUT4 was vastly increased, as were hexokinase II and phosphofructokinase, and glycogen synthase was more inhibited, suggesting that patients adapt by increasing capture of glucose for direct metabolism, thereby significantly reducing glycogen buildup compared with the mouse model. Hence, the McArdle mouse may be a useful tool for further comparative studies of disease mechanism caused by myophosphorylase deficiency and basic studies of metabolic adaptation in muscle.

Differential Muscle Involvement in Mice and Humans Affected by McArdle Disease

McArdle disease (muscle glycogenosis type V) is caused by myophosphorylase deficiency, which leads to impaired glycogen breakdown. We investigated how myophosphorylase deficiency affects muscle physiology, morphology, and glucose metabolism in 20-week-old McArdle mice and compared the findings to those in McArdle disease patients. Muscle contractions in the McArdle mice were affected by structural degeneration due to glycogen accumulation, and glycolytic muscles fatigued prematurely, as occurs in the muscles of McArdle disease patients. Homozygous McArdle mice showed muscle fiber disarray, variations in fiber size, vacuoles, and some internal nuclei associated with cytosolic glycogen accumulation and ongoing regeneration; structural damage was seen only in a minority of human patients. Neither liver nor brain isoforms of glycogen phosphorylase were upregulated in muscles, thus providing no substitution for the missing muscle isoform. In the mice, the tibialis anterior (TA) muscles were invariably more damaged than the quadriceps muscles. This may relate to a 7-fold higher level of myophosphorylase in TA compared to quadriceps in wild-type mice and suggests higher glucose turnover in the TA. Thus, despite differences, the mouse model of McArdle disease shares fundamental physiological and clinical features with the human disease and could be used for studies of pathogenesis and development of therapies.

Genes and exercise intolerance: insights from McArdle disease

McArdle disease (glycogen storage disease type V) is caused by inherited deficiency of a key enzyme in muscle metabolism, the skeletal muscle-specific isoform of glycogen phosphorylase, “myophosphorylase,” which is encoded by the PYGM gene. Here we review the main pathophysiological, genotypic, and phenotypic features of McArdle disease and their interactions. To date, moderate-intensity exercise (together with pre-exercise carbohydrate ingestion) is the only treatment option that has proven useful for these patients. Furthermore, regular physical activity attenuates the clinical severity of McArdle disease. This is quite remarkable for a monogenic disorder that consistently leads to the same metabolic defect at the muscle tissue level, that is, complete inability to use muscle glycogen stores. Further knowledge of this disorder would help patients and enhance understanding of exercise metabolism as well as exercise genomics. Indeed, McArdle disease is a paradigm of human exercise intolerance and PYGM genotyping should be included in the genetic analyses that might be applied in the coming personalized exercise medicine as well as in future research on genetics and exercise-related phenotypes.

McArdle Disease: Update of Reported Mutations and Polymorphisms in the PYGM Gene

McArdle disease is an autosomal-recessive disorder caused by inherited deficiency of the muscle isoform of glycogen phosphorylase (or "myophosphorylase"), which catalyzes the first step of glycogen catabolism, releasing glucose-1-phosphate from glycogen deposits. As a result, muscle metabolism is impaired, leading to different degrees of exercise intolerance. Patients range from asymptomatic to severely affected, including in some cases, limitations in activities of daily living. The PYGM gene codifies myophosphoylase and to date 147 pathogenic mutations and 39 polymorphisms have been reported. Exon 1 and 17 are mutational hot-spots in PYGM and 50% of the described mutations are missense. However, c.148C>T (commonly known as p.R50X) is the most frequent mutation in the majority of the studied populations. No genotype-phenotype correlation has been reported and no mutations have been described in the myophosphorylase domains affecting the phosphorylated Ser-15, the 280's loop, the pyridoxal 5'-phosphate, and the nucleoside inhibitor binding sites. A newly generated knock-in mouse model is now available, which renders the main clinical and molecular features of the disease. Well-established methods for diagnosing patients in laboratories around the world will shorten the frequent ∼20-year period stretching from first symptoms appearance to the genetic diagnosis.

The investigation and management of metabolic myopathies

Metabolic myopathies (MM) are rare inherited primary muscle disorders that are mainly due to abnormalities of muscle energy metabolism resulting in skeletal muscle dysfunction. These diseases include disorders of fatty acid oxidation, glyco(geno)lytic muscle disorders and mitochondrial respiratory chain (MRC) disease. Clinically these disorders present with a range of symptoms including infantile hypotonia, myalgia/exercise tolerance, chronic or acute muscle weakness, cramps/spasms/stiffness or episodic acute rhabdomyolysis. The precipitant may be fasting, infection, general anaesthesia, heat/cold or most commonly, exercise. However, the differential diagnosis includes a wide range of both acquired and inherited conditions and these include exposure to drugs/toxins, inflammatory myopathies, dystrophies and channelopathies. Streamlining of existing diagnostic protocols has now become a realistic prospect given the availability of second-generation sequencing. A diagnostic pathway using a ‘rhabdomyolysis’ gene panel at an early stage of the diagnostic process is proposed. Following detailed clinical evaluation and first-line investigations, some patients will be identified as candidates for McArdle disease/glycogen storage disease type V or MRC disease and these will be referred directly to the specialised services. However, for the majority of patients, second-line investigation is best undertaken through next-generation sequencing using a ‘rhabdomyolysis’ gene panel. Following molecular analysis and careful evaluation of the findings, some patients will receive a clear diagnosis. Further functional or specific targeted testing may be required in other patients to evaluate the significance of uncertain/equivocal findings. For patients with no clear diagnosis, further investigations will be required through a specialist centre.

Diagnostic evaluation of rhabdomyolysis

Rhabdomyolysis is characterized by severe acute muscle injury resulting in muscle pain, weakness, and/or swelling with release of myofiber contents into the bloodstream. Symptoms develop over hours to days after an inciting factor and may be associated with dark pigmentation of the urine. Serum creatine kinase and urine myoglobin levels are markedly elevated. Clinical examination, history, laboratory studies, muscle biopsy, and genetic testing are useful tools for diagnosis of rhabdomyolysis, and they can help differentiate acquired from inherited causes of rhabdomyolysis. Acquired causes include substance abuse, medication or toxic exposures, electrolyte abnormalities, endocrine disturbances, and autoimmune myopathies. Inherited predisposition to rhabdomyolysis can occur with disorders of glycogen metabolism, fatty acid β-oxidation, and mitochondrial oxidative phosphorylation. Less common inherited causes of rhabdomyolysis include structural myopathies, channelopathies, and sickle-cell disease. This review focuses on the differentiation of acquired and inherited causes of rhabdomyolysis and proposes a practical diagnostic algorithm. Muscle Nerve 51: 793-810, 2015.

Determining the prevalence of McArdle disease from gene frequency by analysis of next-generation sequencing data

Purpose

McArdle disease is one of the most common glycogen storage disorders. Although the exact prevalence is not known, it has been estimated to be 1 in 100,000 patients in the United States. More than 100 mutations in PYGM have been associated with this disorder. McArdle disease has significant clinical variability with some patients presenting with severe muscle pain and weakness while others have only mild, exercise-related symptoms.

Methods

Next-Generation sequencing data allow estimation of disease prevalence with minimal ascertainment bias. We analyzed gene frequencies in two cohorts of patients from exome sequencing results. We categorized variants into three groups: a curated set of published mutations, variants of uncertain significance, and likely benign variants.

Results

An initial estimate based on the frequency of six common mutations predicts a disease prevalence of 1/7,650 (95% CI 1/5,362 to 1/11,108), which greatly deviates from published estimates. A second method using the two most common mutations predicts a prevalence of 1/42,355 (95% CI 1/24,536 - 1/76,310) in Caucasians.

Conclusions

These results suggest that the currently accepted prevalence of McArdle disease is an underestimate and that some of the currently considered pathogenic variants are likely benign.

Diagnostic power of the non-ischaemic forearm exercise test in detecting glycogenosis type V

BACKGROUND AND PURPOSE

This was a retrospective study to assess the diagnostic value of the non-ischaemic forearm exercise test in detecting McArdle's disease.

METHODS

The study is a retrospective diagnostic study over 15 years (1999-2013) on a referred sample of patients suffering from exercise intolerance and various muscle complaints, generally with elevated creatine kinase (CK). In all, 1226 patients underwent the non-ischaemic forearm exercise test. Blood lactate, ammonia and CK levels were analyzed. DNA analyses and/or muscle biopsies were assessed to confirm the diagnosis of McArdle's disease. The results of 60 volunteers were used to compare with the results of study subjects.

RESULTS

In this cohort, 40 patients were finally diagnosed with McArdle's disease. Absolute values of lactate and ammonia rise were used to discriminate all McArdle patients from healthy patients. A sensitivity and specificity of respectively 100% and 99.7% were calculated. The 24-h CK level showed no significant difference from the CK level at the day of the test and confirms the safety of the test.

CONCLUSIONS

This study has formally assessed the diagnostic value of the non-ischaemic forearm exercise test in the detection of McArdle's disease. Very high sensitivity and specificity were observed. Furthermore, the test is easy to set up and to perform, it is non-traumatic and cost effective. It may circumvent a muscle biopsy in McArdle patients presenting the most common mutations. Hence, it is a perfect and safe screening instrument to detect patients with McArdle's disease. Glycogen storage disease type III patients, however, may show similar patterns to McArdle patients.

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.

McArdle Disease and Exercise Physiology

McArdle disease (glycogen storage disease Type V; MD) is a metabolic myopathy caused by a deficiency in muscle glycogen phosphorylase. Since muscle glycogen is an important fuel for muscle during exercise, this inborn error of metabolism provides a model for understanding the role of glycogen in muscle function and the compensatory adaptations that occur in response to impaired glycogenolysis. Patients with MD have exercise intolerance with symptoms including premature fatigue, myalgia, and/or muscle cramps. Despite this, MD patients are able to perform prolonged exercise as a result of the “second wind” phenomenon, owing to the improved delivery of extra-muscular fuels during exercise. The present review will cover what this disease can teach us about exercise physiology, and particularly focuses on the compensatory pathways for energy delivery to muscle in the absence of glycogenolysis.

Genomics and genetics in the biology of adaptation to exercise

This article is devoted to the role of genetic variation and gene-exercise interactions in the biology of adaptation to exercise. There is evidence from genetic epidemiology research that DNA sequence differences contribute to human variation in physical activity level, cardiorespiratory fitness in the untrained state, cardiovascular and metabolic response to acute exercise, and responsiveness to regular exercise. Methodological and technological advances have made it possible to undertake the molecular dissection of the genetic component of complex, multifactorial traits, such as those of interest to exercise biology, in terms of tissue expression profile, genes, and allelic variants. The evidence from animal models and human studies is considered. Data on candidate genes, genome-wide linkage results, genome-wide association findings, expression arrays, and combinations of these approaches are reviewed. Combining transcriptomic and genomic technologies has been shown to be more powerful as evidenced by the development of a recent molecular predictor of the ability to increase VO2max with exercise training. For exercise as a behavior and physiological fitness as a state to be major players in public health policies will require that the role of human individuality and the influence of DNA sequence differences be understood. Likewise, progress in the use of exercise in therapeutic medicine will depend to a large extent on our ability to identify the favorable responders for given physiological properties to a given exercise regimen.

Exertional rhabdomyolysis: a clinical review with a focus on genetic influences

In this review, the clinical and laboratory features of exertional rhabdomyolysis (ER) are discussed in detail, emphasizing the full clinical spectrum from physiological elevations of serum creatine kinase after exertion to life-threatening rhabdomyolysis with acute kidney injury and associated systemic complications. Laboratory markers used to diagnose both ER and rhabdomyolysis are very sensitive, but not very specific, and imperfectly distinguish "subclinical" or asymptomatic from severe, life-threatening illness. However, genetic factors, both recognized and yet to be discovered, likely influence this diverse clinical spectrum of disease and response to exercise. Genetic mutations causative for McArdle disease, carnitine palmitoyl transferase deficiency 2, myoadenylate deaminase deficiency, and malignant hyperthermia have all been associated with ER. Polymorphic variations in the myosin light chain kinase, α-actin 3, creatine kinase-muscle isoform, angiotensin I-converting enzyme, heat shock protein, and interleukin-6 genes have also been associated with either ER or exercise-induced serum creatine kinase elevations typical of ER. The prognosis for ER is significantly better than that for other etiologies of rhabdomyolysis, but the risk of recurrence after an initial episode is unknown. Guidelines for management are provided.

Cell models for McArdle disease and aminoglycoside-induced read-through of a premature termination codon

McArdle disease results from mutations in the gene encoding muscle glycogen phosphorylase (PYGM) protein and the two most common mutations are a premature termination codon (R50X) and a missense mutation (G205S). Myoblasts from patients cannot be used to create a cell model of McArdle disease because even normal myoblasts produce little or no PYGM protein in cell culture. We therefore created cell models by expressing wild-type or mutant (R50X or G205S) PYGM from cDNA integrated into the genome of Chinese hamster ovary cells. These cell lines enable the study of McArdle mutations in the absence of nonsense-mediated decay of mRNA transcripts. Although all cell lines produced stable mRNA, only wild-type produced detectable PYGM protein. Our data suggest that the G205S mutation affects PYGM by causing misfolding and accelerated protein turnover. Using the N-terminal region of PYGM containing the R50X mutation fused to green fluorescent protein, we were able to demonstrate both small amounts of truncated protein production and read-through of the R50X premature termination codon induced by the aminoglycoside, G418.

McArdle disease: a novel mutation in Jewish families from the Caucasus region

McArdle disease is caused by a myophosphorylase deficiency consequent to defects in the PYGM gene. A minority of the over-133 known mutations are associated with ethnicity, occurring mainly in patients from western Europe, the United States, and Japan. We identified a novel mutation, c.632delG, in three unrelated families of Jewish descent originating from the Caucasus region. This possibly ethnicity-associated mutation can significantly facilitate the diagnosis in Jews of the Caucasus and contribute to genetic consultations.

Confirmation of the efficacy of vitamin B6 supplementation for McArdle disease by follow-up muscle biopsy

No effective treatment for McArdle disease exists.We report a Japanese patient with McArdle disease who was treated with vitamin B(6) supplementation (60-90 mg/day). After treatment, increased muscle phosphorylase activity was confirmed by follow-up muscle biopsy (3.8 times higher than pretreatment levels). Increased lactate levels were seen on the forearm exercise test, and regular work activities could be resumed. Vitamin B(6) supplementation can enhance residual phosphorylase activity and improve insufficient anaerobic glycolysis of skeletal muscle.

Six novel mutations in the myophosphorylase gene in patients with McArdle disease and a family with pseudo-dominant inheritance pattern

McArdle disease is an autosomal recessive glycogenosis due to deficiency of the enzyme myophosphorylase. It results from homozygous or compound heterozygous mutations in the gene for this enzyme, PYGM. We report six novel mutations in the PYGM gene based upon sequencing data including three missense mutations (p.D51G, p.P398L, and p.N648Y), one nonsense mutation (p.Y75X), one frame-shift mutation (p.Y114SfsX181), and one amino acid deletion (p.Y53del) in six patients with McArdle disease. We also report on a Caucasian family that appeared to transmit McArdle disease in an autosomal dominant manner. In order to evaluate the potential pathogenicity of the sequence variants, we performed in silico analysis using PolyPhen-2 and SIFT BLink, along with species conservation analysis using UCSC Genome Browser. The above mutations were all predicted to be disease associated with high probability and with at least the same level of certainty as several confirmed mutations. The current data add to the list of pathogenic mutations in the PYGM gene associated with McArdle disease.

Metabolic disorders of fetal life: glycogenoses and mitochondrial defects of the mitochondrial respiratory chain

Two major groups of inborn errors of energy metabolism are reviewed -glycogenoses and defects of the mitochondrial respiratory chain - to see how often these disorders present in fetal life or neonatally. After some general considerations on energy metabolism in the pre- and postnatal development of the human infant, different glycogen storage diseases and mitochondrial encephalomyopathies are surveyed. General conclusions are that: (i) disorders of glycogen metabolism are more likely to cause 'fetal disease' than defects of the respiratory chain; (ii) mitochondrial encephalomyopathies, especially those due to defects of the nuclear genome, are frequent causes of neonatal or infantile diseases, typically Leigh syndrome, but usually do not cause fetal distress; (iii) notable exceptions include mutations in the complex III assembly gene BCS1L resulting in the GRACILE syndrome (growth retardation, aminoaciduria, cholestasis, iron overload, lactic acidosis, and early death), and defects of mitochondrial protein synthesis, which are the 'new frontier' in mitochondrial translational research.

Myophosphorylase deficiency (McArdle disease) in a patient with normal pregnancy and normal pregnancy outcome

McArdle disease is a rare, mostly autosomal recessive disorder of deficient myophosphorylation of glycogen in skeletal muscles. Recent knowledge regarding this condition means that women of childbearing age with McArdle disease can expect to labour normally without ill effect. We report a case of a 30-year-old woman in her first pregnancy who had an episode of exercise-induced myoglobinuria with a significant rise in serum creatine kinase (CK) levels in early pregnancy who then laboured normally but did require a caesarean section for a malposition of the fetal head.

Expression of glycogen phosphorylase isoforms in cultured muscle from patients with McArdle's disease carrying the p.R771PfsX33 PYGM mutation

Background

Mutations in the PYGM gene encoding skeletal muscle glycogen phosphorylase (GP) cause a metabolic disorder known as McArdle's disease. Previous studies in muscle biopsies and cultured muscle cells from McArdle patients have shown that PYGM mutations abolish GP activity in skeletal muscle, but that the enzyme activity reappears when muscle cells are in culture. The identification of the GP isoenzyme that accounts for this activity remains controversial.

Methodology/Principal Findings

In this study we present two related patients harbouring a novel PYGM mutation, p.R771PfsX33. In the patients' skeletal muscle biopsies, PYGM mRNA levels were ∼60% lower than those observed in two matched healthy controls; biochemical analysis of a patient muscle biopsy resulted in undetectable GP protein and GP activity. A strong reduction of the PYGM mRNA was observed in cultured muscle cells from patients and controls, as compared to the levels observed in muscle tissue. In cultured cells, PYGM mRNA levels were negligible regardless of the differentiation stage. After a 12 day period of differentiation similar expression of the brain and liver isoforms were observed at the mRNA level in cells from patients and controls. Total GP activity (measured with AMP) was not different either; however, the active GP activity and immunoreactive GP protein levels were lower in patients' cell cultures. GP immunoreactivity was mainly due to brain and liver GP but muscle GP seemed to be responsible for the differences.

Conclusions/Significance

These results indicate that in both patients' and controls' cell cultures, unlike in skeletal muscle tissue, most of the protein and GP activities result from the expression of brain GP and liver GP genes, although there is still some activity resulting from the expression of the muscle GP gene. More research is necessary to clarify the differential mechanisms of metabolic adaptations that McArdle cultures undergo in vitro.

Excessive skeletal muscle recruitment during strenuous exercise in McArdle patients

We compared the cardiorespiratory response and muscle recruitment [as determined by electromyography (EMG)] of 37 McArdle patients [19 males, 37.4 ± 2.8 years, body mass index (BMI): 25.1 ± 4.7 kg m(-2)] and 33 healthy controls (18 males, 36.4 ± 10.0 years, BMI: 25.7 ± 3.8 kg m(-2)) during cycle-ergometer exercise (an incremental test to exhaustion and a 12-min submaximal constant workload test). We obtained cardiorespiratory [oxygen uptake and heart rate (HR)] and EMG data (rectus femoris and vastus lateralis muscles). During the incremental test, the patients exhibited the expected hyperkinetic cardiovascular response shown by a marked increase in the slope of the HR:Power relationship (p < 0.001). Throughout the incremental test and at the point of fatigue, the patients produced significantly less power than the controls (peak power output: 67 ± 21 vs. 214 ± 56 watts respectively, p < 0.001), yet they demonstrated significantly higher levels of muscle activity for a given absolute power. During the constant workload test, patients displayed higher levels of EMG activity than the controls during the second half of the test, despite a lower power production (34 ± 13 vs. 94 ± 29 watts respectively, p < 0.001). In conclusion, since the McArdle patients required more motor unit recruitment for a given power output, our data suggest that the state of contractility of their muscles is reduced compared with healthy people. Excessive muscle recruitment for a given load could be one of the mechanisms explaining the exercise intolerance of these patients.

Sarcomeric α-actinins and their role in human muscle disease

In skeletal muscle, the sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-line and crosslink actin thin filaments to maintain the structure of the sarcomere. Based on their known protein binding partners, the sarcomeric α-actinins are likely to have a number of structural, signaling and metabolic roles in skeletal muscle. In addition, the α-actinins interact with many proteins responsible for inherited muscle disorders. In this paper, we explore the role of the sarcomeric α-actinins in normal skeletal muscle and in the pathogenesis of a range of neuromuscular disorders.

The human gene map for performance and health-related fitness phenotypes: the 2006-2007 update

This update of the human gene map for physical performance and health-related fitness phenotypes covers the research advances reported in 2006 and 2007. The genes and markers with evidence of association or linkage with a performance or a fitness phenotype in sedentary or active people, in responses to acute exercise, or for training-induced adaptations are positioned on the map of all autosomes and sex chromosomes. Negative studies are reviewed, but a gene or a locus must be supported by at least one positive study before being inserted on the map. A brief discussion on the nature of the evidence and on what to look for in assessing human genetic studies of relevance to fitness and performance is offered in the introduction, followed by a review of all studies published in 2006 and 2007. The findings from these new studies are added to the appropriate tables that are designed to serve as the cumulative summary of all publications with positive genetic associations available to date for a given phenotype and study design. The fitness and performance map now includes 214 autosomal gene entries and quantitative trait loci plus seven others on the X chromosome. Moreover, there are 18 mitochondrial genes that have been shown to influence fitness and performance phenotypes. Thus,the map is growing in complexity. Although the map is exhaustive for currently published accounts of genes and exercise associations and linkages, there are undoubtedly many more gene-exercise interaction effects that have not even been considered thus far. Finally, it should be appreciated that most studies reported to date are based on small sample sizes and cannot therefore provide definitive evidence that DNA sequence variants in a given gene are reliably associated with human variation in fitness and performance traits.

McArdle disease: what do neurologists need to know?

McArdle disease (also known as glycogen storage disease type V) is a pure myopathy caused by an inherited deficit of myophosphorylase, the skeletal muscle isoform of the enzyme glycogen phosphorylase. The disease exhibits clinical heterogeneity, but patients typically experience exercise intolerance, that is, reversible, acute crises (early fatigue and contractures, sometimes with rhabdomyolysis and myoglobinuria) triggered by static muscle contractions (e.g. lifting weights) or dynamic exercise (e.g. climbing stairs or running). In this Review, we discuss the main features of McArdle disease, with the aim of providing neurologists with up-to-date, useful information to assist their patients. The topics covered include diagnostic tools-for example, molecular genetic diagnosis, the classic ischemic forearm test and the so-called 'second wind' phenomenon-and current therapeutic options-for example, a carbohydrate-rich diet and carbohydrate ingestion shortly before strenuous exercise, in combination with medically supervised aerobic training of low to moderate intensity.

[McArdle disease (gycogenosis type V): analysis of clinical, biological and genetic features of five French patients]

INTRODUCTION

McArdle disease (glycogenosis type V) is an autosomal recessive metabolic myopathy. Defect in glycogen breakdown is due to mutations of the gene for myophosphorylase (PYGM). Among patients of the department, we searched for correlations between disease phenotype, biochemistry analysis of muscle samples and PYGM genotype.

METHODS

We included five patients whose muscle biopsy showed deposits of glycogen and negative histochemical staining for myophosphorylase.

RESULTS

All patients exhibited exercise intolerance and high serum CK levels (mean 4400). Two of them had an acute renal insufficiency caused by rhabdomyolysis. One patient developed moderate late-onset muscle weakness of the proximal part of upper limbs. Muscle glycogen concentration was high (three times the normal). Myophosphorylase activity was undetectable in four muscle samples out of five. Two patients were homozygous and two other heterozygous for the R50X mutation of PYGM. The other one had a novel missense mutation S814N. Patients homozygous for R50X mutation had higher CK levels (8080 versus 1457, p=0.046), but disease severity and muscle glycogen concentrations were equivalent.

CONCLUSIONS

Our patients had typical clinical and laboratory features of McArdle disease. Diagnosis was suggested by exercise intolerance with high CK levels. The R50X mutation was the most common (60% of the mutated alleles). We found no relationship between clinical severity, PYGM genotype and biochemistry analysis of muscle samples.

A novel PYGM mutation in a Korean patient with McArdle disease: the role of nonsense-mediated mRNA decay

We have identified a compound heterozygous mutation of PYGM in a Korean patient with McArdle disease, which is composed of a novel single codon deletion (p.779delE) and a common nonsense mutation (p.R50X). Our study also showed an evidence of nonsense-mediated mRNA decay (NMD) caused by p.R50X mutation, supporting the importance of RNA processing defects in the molecular pathology of McArdle disease.

Exercise capacity in a child with McArdle disease

We report the exercise capacity of an 8-year-old boy with clinical, histological, biochemical, and genetic evidence of McArdle disease. The patient presented with severe myalgia, proteinuria, hematuria, pyrexia, and elevated creatine kinase after swimming. After pre-exercise ingestion of sucrose, he performed treadmill exercise to symptom limitation. His peak oxygen uptake (18.8 mL/kg/min) and ventilatory threshold (16.0 mL/kg/min) were reduced by 40% and 20% compared with healthy age-matched and gender-matched controls. The results suggest that exercise capacity is reduced early in life in patients with McArdle disease and suggest the need for prophylactic exercise training (following pre-exercise feeding to prevent rhabdomyolysis) to minimize deconditioning.

Favorable responses to acute and chronic exercise in McArdle patients

OBJECTIVE

This study reports acute exercise responses in a large (N = 46) series of patients with McArdle disease and responses to exercise training in a smaller (n = 9) set of patients.

DESIGN

Patients were studied during both incremental and steady-state cycle ergometer exercise, using cardiopulmonary testing, and the patients were compared with age- and gender-matched controls.

SETTING

The study was performed in a university setting (clinical exercise physiology laboratory).

PARTICIPANTS

The 46 patients showed common features of McArdle disease. They were definitively diagnosed by histochemistry, biochemistry, and/or molecular genetic analysis. The 46 controls were healthy, sedentary individuals.

INTERVENTION

Nine patients were studied before and after an 8-month supervised aerobic exercise training program (including five weekly sessions of walking and/or cycling exercise with a duration no greater than 60 minutes).

MAIN OUTCOME MEASUREMENTS

The main indicators of exercise capacity that we measured were peak power output, peak oxygen uptake (VO2peak), and ventilatory threshold (VT).

RESULTS

Exercise capacity (peak power output, 35% control; VO2peak, 44% control; VT, 66% control) was markedly depressed in the patients. The patients who trained improved peak power output (25%), VO2peak (44%), and VT (27%), with no evidence of negative outcomes from training. Although not achieving normal values, the response to training put the patients into the lower limit of normal controls.

CONCLUSIONS

Under carefully controlled conditions, patients with McArdle disease may perform acute exercise safely, and they may respond favorably to training. This may offer an additional therapeutic option to help normalize the lifestyles of these patients.