McArdle disease: what do neurologists need to know?

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.

Comparative gene expression profiling between human cultured myotubes and skeletal muscle tissue

Background

A high-sensitivity DNA microarray platform requiring nanograms of RNA input facilitates the application of transcriptome analysis to individual skeletal muscle (SM) tissue samples. Culturing myotubes from SM-biopsies enables investigating transcriptional defects and assaying therapeutic strategies. This study compares the transcriptome of aneurally cultured human SM cells versus that of tissue biopsies.

Results

We used the Illumina expression BeadChips to determine the transcriptomic differences between tissue and cultured SM samples from five individuals. Changes in the expression of several genes were confirmed by QuantiGene Plex assay or reverse transcription real-time PCR. In cultured myotubes compared to the tissue, 1216 genes were regulated: 583 down and 633 up. Gene ontology analysis showed that downregulated genes were mainly associated with cytoplasm, particularly mitochondria, and involved in metabolism and the muscle-system/contraction process. Upregulated genes were predominantly related to cytoplasm, endoplasmic reticulum, and extracellular matrix. The most significantly regulated pathway was mitochondrial dysfunction. Apoptosis genes were also modulated. Among the most downregulated genes detected in this study were genes encoding metabolic proteins AMPD1, PYGM, CPT1B and UCP3, muscle-system proteins TMOD4, MYBPC1, MYOZ1 and XIRP2, the proteolytic CAPN3 and the myogenic regulator MYF6. Coordinated reduced expression of five members of the GIMAP gene family, which form a cluster on chromosome 7, was shown, and the GIMAP4-reduction was validated. Within the most upregulated group were genes encoding senescence/apoptosis-related proteins CDKN1A and KIAA1199 and potential regulatory factors HIF1A, TOP2A and CCDC80.

Conclusions

Cultured muscle cells display reductive metabolic and muscle-system transcriptome adaptations as observed in muscle atrophy and they activate tissue-remodeling and senescence/apoptosis processes.

Rhabdomyolysis: a review, with emphasis on the pediatric population

Rhabdomyolysis is a common clinical syndrome and accounts for 7% of all cases of acute kidney injury (AKI) in the USA. It can result from a wide variety of disorders, such as trauma, exercise, medications and infection, but in the pediatric population, infection and inherited disorders are the most common causes of rhabdomyolysis. Approximately half of patients with rhabdomyolysis present with the triad of myalgias, weakness and dark urine. The clinical suspicion, especially in the setting of trauma or drugs, is supported by elevated creatinine kinase levels and confirmed by the measurement of myoglobin levels in serum or urine. Muscle biopsy and genetic testing should be performed if rhabdomyolysis is recurrent or metabolic myopathy is suspected. Early recognition is important to prevent AKI through the use of aggressive hydration. Prevention is important in patients with inherited forms, but novel therapies may be developed with the better understanding of the pathophysiology and genetics of rhabdomyolysis.

The role of muscle biopsy in the age of genetic testing

PURPOSE OF REVIEW

The purpose of this review is to discuss the role of muscle biopsy in the current age of genetic testing.

RECENT FINDINGS

The diagnostic approach to patients with suspected genetically determined myopathies has been altered by recent advances in molecular diagnostic technologies and by the increased number of conditions for which the genetic alterations have been identified. Myopathological aspects can narrow down the differential diagnosis when the clinical phenotype is not informative enough and can help guide the molecular investigation.

SUMMARY

Here, we review genetic and myopathological aspects of selected genetically determined myopathies.

[McArdle disease revealed by exercise intolerance associated with severe rhabdomyolysis]

McArdle's disease (MAD) is a rare hereditary myopathy secondary to a deficit in myophosphorylase, an essential enzyme for the use of muscular glycogen reserves. Exercise intolerance to a variable degree is the fundamental manifestation. Muscular enzymes are usually normal or slightly elevated, except during episodes of rhabdomyolysis. Generally, the electromyogram has poor sensitivity for the diagnosis of exercise myopathies. The muscular biopsy can be misleadingly normal. The role of MRI in the diagnosis of MAD is not well clarified in the literature. We report the case of a 16-year-old patient, hospitalized in July 2008 for exercise intolerance. On admission, he was asymptomatic and the physical examination was non contributive. Serum creatine kinase levels and renal function measures were normal. Cycloergometer exercise testing unmasked the disease. EMG and muscular biopsies were normal. During the second hospitalization, this time for rhabdomyolysis, T2 weighted MRI of the thighs showed high intensity signals from the gracilis muscles. The control MRI, made after 2 weeks of rest, was normal. Right gracilis muscle biopsy demonstrated excess glycogen with myophosphorylase deficiency, establishing the diagnosis of MAD. MAD is a rare metabolic myopathy to consider in patients with a history of exercise intolerance. The muscle biopsy can be misleadingly normal and should be, to our opinion, be guided by MRI findings.

[Exercise therapy and myopathies]

Since the first consensus papers published early in the 2000s, a growing number of recent publications has shown that adapted physical activity is not only safe in the context of myopathy but also potentially effective as a therapeutic tool. After a short recall of the different exercise modalities, the mechanical strain they induce and the expected muscular benefits, the present paper reviews the different studies related to exercise therapy in myopathic patients and provides a critical analysis of the topic. Myopathies are rare diseases with many different etiologies and a large number of training modalities which could be useful for the different muscular challenges have been proposed. We have chosen to focus on several specific training modalities and to discuss the results from the most recent papers. The purpose of this review is to, firstly, update physical training guidelines for patients with myopathy and, secondly, highlight some common pitfalls associated with this strategy. This is particularly important for medical and allied professionals involved in prescribing and managing exercise therapy protocols.

The second wind phenomenon in very young McArdle's patients

We investigated the phenomenon of second wind in four patients with McArdle's disease: a brother and sister (aged 4 and 12 years respectively) and two unrelated patients, a boy of 14 and a 17-year-old girl. We also studied the siblings' healthy 6-year-old sister. Each patient performed a 15-min exercise test at a constant workload and a subsequent graded exercise test until exhaustion. Overall the healthy girl and the youngest McArdle's patient, the 4-year-old boy, did not show a second wind phenomenon. Further, the peak cardio-respiratory capacity of the young McArdle's boy was normal for his age (32.3 mL 02/kg/min) and he did not report any function limitations during physical education classes.

Effects of aging and calorie restriction on rat skeletal muscle glycogen synthase and glycogen phosphorylase

Calorie restriction's (CR) effects on age-associated changes in glycogen-metabolizing enzymes were studied in rat soleus (SOL) and tibialis anterior (TA) muscles. Old (24 months) compared to young (6 months) rats maintained ad libitum on a standard diet had reduced glycogen synthase (GS) activity, lower muscle GS protein levels, increased phosphorylation of GS at site 3a with less activation in SOL. Age-associated impairments in GS protein and activation-phosphorylation were also shown in TA. There was an age-associated reduction in glycogen phosphorylase (GP) activity level in SOL, while brain/muscle isoforms (B/M) of GP protein levels were higher. GP activity and protein levels were preserved, but GP was inactivated in TA with age. Glycogen content was unchanged in both muscles. CR did not alter GS or GP activity/protein levels in young rats. CR hindered age-related decreases in GS activity/protein, unrelated to GS mRNA levels, and GS inactivation-phosphorylation; not on GP. In older rats, CR enhanced glycogen accumulation in SOL. Short-term fasting did not recapitulate CR effects in old rats. Thus, the predominant age-associated impairments on skeletal muscle GS and GP activities occur in the oxidative SOL muscle of rats, and CR can attenuate the loss of GS activity/activation and stimulate glycogen accumulation.