Genetic analysis of Japanese patients with myophosphorylase deficiency (McArdle's disease): single-codon deletion in exon 17 is the predominant mutation

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.

Creation and implementation of a European registry for patients with McArdle disease and other muscle glycogenoses (EUROMAC registry)

BACKGROUND

International patient registries are of particular importance for rare disorders, as they may contribute to overcome the lack of knowledge derived from low number of patients and limited awareness of these diseases, and help to learn more about their geographical or population-based specificities, which is relevant for research purposes and for promoting better standards of care and diagnosis. Our objective was to create and implement a European registry for patients with McArdle disease and other muscle glycogenoses (EUROMAC) and to disseminate the knowledge of these disorders.

RESULTS

Teams from nine different countries (United Kingdom, Spain, Italy, France, Germany, Denmark, Greece, Turkey and USA) created a consortium that developed the first European registry dedicated to rare muscle glycogenoses. A work plan was implemented to design the database and platform that constitute the registry, by choosing clinical, genetics and molecular variables of interest, based on experience gained from previous national registries for similar metabolic disorders. Among dissemination activities, several teaching events were organized in different countries, especially those where the consortium considered the awareness of these diseases needs to be promoted among health professionals and patients.

CONCLUSION

EUROMAC represents a step forward in the knowledge of those disorders to which it is dedicated, and will have relevant clinical outcomes at the diagnostic, epidemiological, clinical and research level.

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.

Clinical utility gene card for McArdle disease

Name of the disease (synonyms) McArdle disease (glycogenosis type V; glycogen storage disease V (GSDV); PYGM deficiency; muscle glycogen phosphorylase deficiency; myophosphorylase deficiency). OMIM# of the disease #232600. Name of the analysed genes or DNA/chromosome segments Muscle glycogen phosphoryalse (PYGM). OMIM# of the gene(s) #608455.Review of the analytical and clinical validity as well as of the clinical utility of DNA-based testing for variants in the PYGM gene(s) in⊠ diagnostic,⊠ predictive and⊠ prenatal settings and for⊠ risk assessment in relatives.

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.

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 significance of clinical and laboratory features in the diagnosis of glycogen storage disease type v: a case report

Glycogen storage disease type V (GSD-V) is the most common disorder of muscle glycogenosis with characteristic clinical and laboratory findings. A 32-yr-old woman complained of exercise intolerance and myoglobulinuria since early adolescence. She reported several episodes of second-wind phenomenon. Physical examination did not show any neurological abnormality, including fixed muscle weakness or atrophy. Serum creatine kinase level was 1,161 IU/L at rest. The result of the non-ischemic forearm exercise test was compatible with GSD-V. Mutation analysis identified the compound heterozygous mutations of the PYGM, p.D510fs and p.F710del, which has not yet been reported in Korea. The present case recognizes that detail clinical and laboratory analysis is the first step in the diagnosis of GSD-V.

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.

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.

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.

Analysis of spectrum and frequencies of mutations in McArdle disease. Identification of 13 novel mutations

BACKGROUND

McArdle disease, a common metabolic myopathy with autosomal recessive inheritance, is caused by a frequent R50X mutation and many rare mutations in the myophosphorylase gene.

OBJECTIVES

To identify spectrum and frequencies of myophosphorylase gene mutations in a large cohort of patients with McArdle disease, to discuss diagnostic implications, and to analyse genotype-phenotype relationship.

METHODS

Molecular genetic analysis of 56 index patients with muscle biopsy-proven myophosphorylase deficiency from Germany (n = 35), UK (n = 13), and several other countries (n = 8) was performed using direct sequencing.

RESULTS

Allele frequency of the R50X mutation was 58%, and 71% of the patients carried this mutation at least on one allele. We detected 26 other less common mutations, 13 of which are novel: G157V, R161C, Q337R, E384K, S450L, G486D, R570W, K575E, IVS6-2A>T, IVS10+1G>A, R650X, c.1354insC, c.1155_1156delGG. There was no genotype-phenotype correlation with respect to age of onset and severity. R270X was the most frequent mutation among the less common mutations reaching an allele frequency of 5% followed by R94W and G686R representing a frequency of 4% each.

CONCLUSIONS

The study further extends the genetic heterogeneity of myophosphorylase gene mutations showing no mutational hotspot and no genotype-phenotype correlation. Most novel missense mutations were located in secondary structures or active sites of the enzyme. Some of the less common mutations are recurrent with different frequencies within Europe. Ethnic origin and frequency of less common mutations must be considered to establish efficient strategies in molecular genetic testing. Performing molecular testing can avoid muscle biopsy.

Molecular genetics of McArdle's disease

This review highlights recent advances in our understanding of McArdle's disease, including the mechanisms involved in the regulation of the clinical phenotype. The latest molecular genetic studies have demonstrated the genetic heterogeneity of the disorder, with more than 65 mutations identified to date. There is not a specific treatment for McArdle's disease, but some nutritional treatments in combination with aerobic conditioning could improve the quality of life in most patients.

Molecular characterization of myophosphorylase deficiency (McArdle disease) in 34 patients from Southern France: identification of 10 new mutations. Absence of genotype-phenotype correlation

We report on 31 patients and 3 affected siblings (17 males and 17 females) from Southern France with McArdle disease (two from Spanish and three from Portuguese background). Molecular analysis revealed the presence of five previously described mutations: the common p.R50X nonsense mutation, the p.R94W and p.V456M missense mutations, the p.K609K conservative mutation which generates an aberrant splicing, and the p.K754fs frameshift mutation; and 10 new molecular defects: eight missense mutations at homozygous (p.G136D) or heterozygous state (p.T379M, p.G449R, p.T488I, p.R490Q, p.R570Q, p.R590H, and p.R715W), one nonsense mutation p.R650X and one deletion (p.delK170). Our results confirm that the p.R50X nonsense mutation is also the most common associated with myophosphorylase deficiency in the Southern French population: 21 of 25 French unrelated patients (15 homozygous and six heterozygous, i.e., 72% of the mutated alleles). Two patients, one from Algeria and one from Tunisia, were homozygous for a previously identified missense mutation p.V456M in a Moroccan subject. Our findings further demonstrate molecular heterogeneity of myophosphorylase deficiency, absence of genotype-phenotype correlation and expand the already crowded map of mutations within the myophosphorylase gene. Our study also provides evidence for increased medical interest of malignant hyperthermia susceptibility (MHS) because of 34 McArdle disease patients, three and two affected siblings were contracture-tested and found to be positive.

A 1-year-old infant with McArdle disease associated with hyper-creatine kinase-emia during febrile episodes

A 14-month-old girl was hospitalized due to repeated hyper-creatine kinase (CK)-emia during pyrexia. Mild hypotonia was observed, but other physical and neurological findings were unremarkable. The serum CK level was normal at rest or normothermia. Open muscle biopsy was performed on the rectus femoris, and showed glycogen storage and complete lack of phosphorylase activity histochemically and biochemically, establishing the diagnosis of McArdle disease. The diagnosis of McArdle disease in early infancy is uncommon. Until this study there have been no reports of clinical symptoms or muscle biopsy findings for McArdle disease in early childhood. This disease must be considered when transient hyper-CKemia is observed in infants, even if glycogen storage is unremarkable as compared with adult cases.

Two novel mutations in the myophosphorylase gene in a patient with McArdle disease

We identified two novel mutations in exon 2 of the myophosphorylase gene in a 33-year-old German women with McArdle disease. The patient was compound heterozygous for a novel nonsense mutation at codon 84 changing tyrosine to stop codon (Y84X) and for a novel missense mutation at codon 93 changing arginine to tryptophan (R93W). These mutations are the first to be described in exon 2 and expand the genetic heterogeneity in patients with McArdle disease.

A new stop codon mutation (Y52X) in the myophosphorylase gene in a Greek patient with McArdle's disease

We identified a novel stop codon mutation in the myophosphorylase gene in a Greek patient with typical symptoms of McArdle's disease. This is the first genetic study of myophosphorylase deficiency in a Greek family, showing that the proband was a compound heterozygous for the common "caucasian" mutation (R49X) and a new nonsense mutation (Y52X), both within exon 1. The new point mutation, a C-to-G transversion at codon 52, converts an encoded tyrosine to a stop codon. Our study confirms that the R49X is also present in the Greek population. The Y52X may represent a private mutation or a common mutation among Greeks. Our data further expand the already remarkable genetic heterogeneity of McArdle's disease. The prevalence of the Y52X mutation in Greek patients with McArdle's disease remains to be determined.

Molecular diagnosis of McArdle disease: revised genomic structure of the myophosphorylase gene and identification of a novel mutation

McArdle disease is a rare autosomal recessive disorder of the muscle glycogen metabolism caused by mutations in the muscle glycogen phosphorylase gene. Until now, a total number of 11 different mutations in the coding region or splice sites of the myophosphorylase gene have been identified. In contrast to a wealth of data on the RNA and protein level, little information is available on the genomic sequence of the corresponding gene. To facilitate molecular diagnosis of McArdle disease, we reinvestigated the genomic structure of the myophosphorylase gene and sequenced about 9.8 kilobases (kb) on the genomic level. By choosing 14 intronic primer pairs, we were able to amplify the complete human coding sequence as well as the adjacent splice sites of the 20 exons. Direct sequencing of the amplification products of a consanguineous Turkish family with typical McArdle disease revealed a novel single base pair deletion in exon 18, which predicts a frameshift and a premature termination of the protein. In summary, we established a system for molecular diagnosis of McArdle disease based on a revised genomic structure of the myophosphorylase gene and demonstrated its feasibility by identification of a novel mutation.

The molecular background of glycogen metabolism disorders

The molecular pathology of classical glycogen storage disorders, glycogen synthase deficiency and Fanconi-Bickel syndrome is reviewed. The isolation of the respective cDNAs, the chromosomal localization of the genes and the elucidation of the genomic organization enabled mutation analysis in most disorders. The findings have shed light on the multi-protein structure of the glucose-6-phosphatase system, the phosphorylase kinase enzymatic complex and the molecular background of the differential tissue expression in debranching enzyme deficiency. The immediate practical benefit of these studies is our extending ability to predict the outcome of clinical variants and to offer genetic counseling to most families. The elucidation of the tertiary structure of these proteins and their structure-function relationship poses major challenges for the future.

A new mutation in the myophosphorylase gene (Asn684Tyr) in a Spanish patient with McArdle's disease

We have identified a novel missense mutation, an A-T transition at codon 684 in exon 17, changing an encoded asparagine to a tyrosine (Asn684Tyr) in a Spanish patient with typical McArdle's disease. The patient was a compound heterozygote, with a previously-described mutation (Gly204Ser) on the other allele. This report expands the molecular genetic heterogeneity in McArdle's disease, emphasizes the presence of private mutations in specific ethnic groups, and indicates that geographic origin must be considered before undertaking DNA analysis for diagnosis.

A nonsense mutation in the myophosphorylase gene in a Japanese family with McArdle's disease

We identified a new mutation in the myophosphorylase gene in a Japanese family with McArdle's disease. This point mutation results in the replacement of a tryptophan at amino acid position 361 with a stop codon, the third nonsense mutation in this disorder. Our findings further expand the already wide spectrum of genetic lesions associated with McArdle's disease, and establish that molecular genetic heterogeneity is also present in the Japanese population.

Molecular characterization of myophosphorylase deficiency in a group of patients from northern Italy

We studied a group of 14 patients from Northern Italy with myophosphorylase deficiency. The disease presented considerable clinical and biochemical heterogeneity, which was reflected at the molecular level. The clinical presentation was typical in 3 patients, mild in 7 (exercise intolerance), and severe in 4 (fixed weakness). Enzyme activity was undetectable in 10 patients, below 3% of control in 3, and 13% of control in one. Enzymatic protein was detectable immunologically only in 1 patient. Myophosphorylase mRNA was present in 8 patients, but in 7 of them it was reduced in amount. Two patients were homozygous for the common nonsense R49X mutation, 5 were heterozygous. Two missense mutations not previously observed were identified in this group of patients. The frequency of alleles with the R49X mutation was significantly lower in this group of patients than in previously reported series. Myophosphorylase deficiency is genetically heterogeneous even among patients living in a small region and with a common ethnic background.