Missense mutations have unexpected consequences: The McArdle disease paradigm

Dose-response effect of pre-exercise carbohydrates under muscle glycogen unavailability: Insights from McArdle disease

BACKGROUND

This study aimed to determine the effect of different carbohydrate (CHO) doses on exercise capacity in patients with McArdle disease-the paradigm of "exercise intolerance", characterized by complete muscle glycogen unavailability-and to determine whether higher exogenous glucose levels affect metabolic responses at the McArdle muscle cell (in vitro) level.

METHODS

Patients with McArdle disease (n = 8) and healthy controls (n = 9) underwent a 12-min submaximal cycling constant-load bout followed by a maximal ramp test 15 min after ingesting a non-caloric placebo. In a randomized, double-blinded, cross-over design, patients repeated the tests after consuming either 75 g or 150 g of CHO (glucose:fructose = 2:1). Cardiorespiratory, biochemical, perceptual, and electromyographic (EMG) variables were assessed. Additionally, glucose uptake and lactate appearance were studied in vitro in wild-type and McArdle mouse myotubes cultured with increasing glucose concentrations (0.35, 1.00, 4.50, and 10.00 g/L).

RESULTS

Compared with controls, patients showed the "classical" second-wind phenomenon (after prior disproportionate tachycardia, myalgia, and excess electromyographic activity during submaximal exercise, all p < 0.05) and an impaired endurance exercise capacity (-51% ventilatory threshold and -55% peak power output, both p < 0.001). Regardless of the CHO dose (p < 0.05 for both doses compared with the placebo), CHO intake increased blood glucose and lactate levels, decreased fat oxidation rates, and attenuated the second wind in the patients. However, only the higher dose increased ventilatory threshold (+27%, p = 0.010) and peak power output (+18%, p = 0.007). In vitro analyses revealed no differences in lactate levels across glucose concentrations in wild-type myotubes, whereas a dose-response effect was observed in McArdle myotubes.

CONCLUSION

CHO intake exerts beneficial effects on exercise capacity in McArdle disease, a condition associated with total muscle glycogen unavailability. Some of these benefits are dose dependent.

Acute ketone supplementation in the absence of muscle glycogen utilization: Insights from McArdle disease

BACKGROUND & AIMS

Ketone supplementation is gaining popularity. Yet, its effects on exercise performance when muscle glycogen cannot be used remain to be determined. McArdle disease can provide insight into this question, as these patients are unable to obtain energy from muscle glycogen, presenting a severely impaired physical capacity. We therefore aimed to assess the effects of acute ketone supplementation in the absence of muscle glycogen utilization (McArdle disease).

METHODS

In a randomized cross-over design, patients with an inherited block in muscle glycogen breakdown (i.e., McArdle disease, n = 8) and healthy controls (n = 7) underwent a submaximal (constant-load) test that was followed by a maximal ramp test, after the ingestion of a placebo or an exogenous ketone ester supplement (30 g of D-beta hydroxybutyrate/D 1,3 butanediol monoester). Patients were also assessed after carbohydrate (75 g) ingestion, which is currently considered best clinical practice in McArdle disease.

RESULTS

Ketone supplementation induced ketosis in all participants (blood [ketones] = 3.7 ± 0.9 mM) and modified some gas-exchange responses (notably increasing respiratory exchange ratio, especially in patients). Patients showed an impaired exercise capacity (-65 % peak power output (PPO) compared to controls, p < 0.001) and ketone supplementation resulted in a further impairment (-11.6 % vs. placebo, p = 0.001), with no effects in controls (p = 0.268). In patients, carbohydrate supplementation resulted in a higher PPO compared to ketones (+21.5 %, p = 0.001) and a similar response was observed vs. placebo (+12.6 %, p = 0.057).

CONCLUSIONS

In individuals who cannot utilize muscle glycogen but have a preserved ability to oxidize blood-borne glucose and fat (McArdle disease), acute ketone supplementation impairs exercise capacity, whereas carbohydrate ingestion exerts the opposite, beneficial effect.

Diagnosis and management of children with McArdle Syndrome (GSD V) in New South Wales

Glycogen storage type V (GSD V-McArdle Syndrome) is a rare neuromuscular disorder characterised by severe pain early after the onset of physical activity. A recent series indicated a diagnostic delay of 29 years; hence reports of children affected by the disorder are uncommon (Lucia et al., 2021, Neuromuscul Disord, 31, 1296-1310). This paper presents eight patients with a median onset age of 5.5 years and diagnosis of 9.5 years. Six patients had episodes of rhabdomyolysis with creatine kinase elevations >50 000 IU/L. Most episodes occurred in relation to eccentric non-predicted activities rather than regular exercise. One of the patients performed a non-ischaemic forearm test. One patient was diagnosed subsequent to a skeletal muscle biopsy, and all had confirmatory molecular genetic diagnosis. Three were homozygous for the common PYGM:c.148C > T (p.Arg50*) variant. All but one patient had truncating variants. All patients were managed with structured exercise testing to help them identify 'second-wind', and plan an exercise regimen. In addition all also had an exercise test with 25 g maltodextrin which had statistically significant effect on ameliorating ratings of perceived exertion. GSD V is under-recognised in paediatric practice. Genetic testing can readily diagnose the condition. Careful identification of second-wind symptomatology during exercise with the assistance of a multi-disciplinary team, allows children to manage activities and tolerate exercise. Maltodextrin can be used for structured exercise, but excessive utilisation may lead to weight gain. Early intervention and education may improve outcomes into adult life.

Creation of an iPSC-Based Skeletal Muscle Model of McArdle Disease Harbouring the Mutation c.2392T>C (p.Trp798Arg) in the PYGM Gene

McArdle disease is a rare autosomal recessive condition caused by mutations in the PYGM gene. This gene encodes the skeletal muscle isoform of glycogen phosphorylase or myophosphorylase. Patients with McArdle disease have an inability to obtain energy from their muscle glycogen stores, which manifests as a marked exercise intolerance. Nowadays, there is no cure for this disorder and recommendations are intended to prevent and mitigate symptoms. There is great heterogeneity among the pathogenic variants found in the PYGM gene, and there is no obvious correlation between genotypes and phenotypes. Here, we present the generation of the first human iPSC-based skeletal muscle model harbouring the second most frequent mutation in PYGM in the Spanish population: NM_005609.4: c.2392T>C (p.Trp798Arg). To this end, iPSCs derived from a McArdle patient and a healthy control were both successfully differentiated into skeletal muscle cells using a small molecule-based protocol. The created McArdle skeletal muscle model was validated by confirming distinctive biochemical aspects of the disease such as the absence of myophosphorylase, the most typical biochemical feature of these patients. This model will be very valuable for use in future high-throughput pharmacological screenings.

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.

Generation of the First Human In Vitro Model for McArdle Disease Based on iPSC Technology

McArdle disease is a rare autosomal recessive disorder caused by mutations in the PYGM gene. This gene encodes for the skeletal muscle isoform of glycogen phosphorylase (myophosphorylase), the first enzyme in glycogenolysis. Patients with this disorder are unable to obtain energy from their glycogen stored in skeletal muscle, prompting an exercise intolerance. Currently, there is no treatment for this disease, and the lack of suitable in vitro human models has prevented the search for therapies against it. In this article, we have established the first human iPSC-based model for McArdle disease. For the generation of this model, induced pluripotent stem cells (iPSCs) from a patient with McArdle disease (harbouring the homozygous mutation c.148C>T; p.R50* in the PYGM gene) were differentiated into myogenic cells able to contract spontaneously in the presence of motor neurons and generate calcium transients, a proof of their maturity and functionality. Additionally, an isogenic skeletal muscle model of McArdle disease was created. As a proof-of-concept, we have tested in this model the rescue of PYGM expression by two different read-through compounds (PTC124 and RTC13). The developed model will be very useful as a platform for testing drugs or compounds with potential pharmacological activity.

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.

Identification of Potential Muscle Biomarkers in McArdle Disease: Insights from Muscle Proteome Analysis

Glycogen storage disease type V (GSDV, McArdle disease) is a rare genetic myopathy caused by deficiency of the muscle isoform of glycogen phosphorylase (PYGM). This results in a block in the use of muscle glycogen as an energetic substrate, with subsequent exercise intolerance. The pathobiology of GSDV is still not fully understood, especially with regard to some features such as persistent muscle damage (i.e., even without prior exercise). We aimed at identifying potential muscle protein biomarkers of GSDV by analyzing the muscle proteome and the molecular networks associated with muscle dysfunction in these patients. Muscle biopsies from eight patients and eight healthy controls showing none of the features of McArdle disease, such as frequent contractures and persistent muscle damage, were studied by quantitative protein expression using isobaric tags for relative and absolute quantitation (iTRAQ) followed by artificial neuronal networks (ANNs) and topology analysis. Protein candidate validation was performed by Western blot. Several proteins predominantly involved in the process of muscle contraction and/or calcium homeostasis, such as myosin, sarcoplasmic/endoplasmic reticulum calcium ATPase 1, tropomyosin alpha-1 chain, troponin isoforms, and alpha-actinin-3, showed significantly lower expression levels in the muscle of GSDV patients. These proteins could be potential biomarkers of the persistent muscle damage in the absence of prior exertion reported in GSDV patients. Further studies are needed to elucidate the molecular mechanisms by which PYGM controls the expression of these proteins.

PYGM mRNA expression in McArdle disease: Demographic, clinical, morphological and genetic features

INTRODUCTION

McArdle disease presents clinical and genetic heterogeneity. There is no obvious association between genotype and phenotype. PYGM (muscle glycogen phosphorylase gene) mRNA expression and its association with clinical, morphological, and genetic aspects of the disease as a set have not been studied previously.

METHODS

We investigated genetic variation in PYGM considering the number of PTCs (premature termination codon) per sample and compared mRNA expression in skeletal muscle samples from 15 patients with McArdle disease and 16 controls to PTCs number and different aspects of the disease.

RESULTS

The main variant found was c.148C>T (PTC-premature termination codon). Patients with two PTCs showed 42% mRNA expression compared to the control group. Most cases showed an inversely proportional relation among PTCs and mRNA expression. Association between mRNA expression and other aspects of the disease showed no statistically significant difference (p> 0.05).

DISCUSSION

mRNA expression is not useful as a predictor factor for the prognosis and severity of the disease. Different mechanisms as post-transcriptional events, epigenetics factors or protein function may be involved.

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.

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.