Mutation analysis in myophosphorylase deficiency (McArdle's disease)

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.

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.

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.

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.

GTG mutation in the start codon of the androgen receptor gene in a family of horses with 64,XY disorder of sex development

Genetic sex in mammals is determined by the sex chromosomal composition of the zygote. The X and Y chromosomes are responsible for numerous factors that must work in close concert for the proper development of a healthy sexual phenotype. The role of androgens in case of XY chromosomal constitution is crucial for normal male sex differentiation. The intracellular androgenic action is mediated by the androgen receptor (AR), and its impaired function leads to a myriad of syndromes with severe clinical consequences, most notably androgen insensitivity syndrome and prostate cancer. In this paper, we investigated the possibility that an alteration of the equine AR gene explains a recently described familial XY, SRY + disorder of sex development. We uncovered a transition in the first nucleotide of the AR start codon (c.1A>G). To our knowledge, this represents the first causative AR mutation described in domestic animals. It is also a rarely observed mutation in eukaryotes and is unique among the >750 entries of the human androgen receptor mutation database. In addition, we found another quiet missense mutation in exon 1 (c.322C>T). Transcription of AR was confirmed by RT-PCR amplification of several exons. Translation of the full-length AR protein from the initiating GTG start codon was confirmed by Western blot using N- and C-terminal-specific antibodies. Two smaller peptides (25 and 14 amino acids long) were identified from the middle of exon 1 and across exons 5 and 6 by mass spectrometry. Based upon our experimental data and the supporting literature, it appears that the AR is expressed as a full-length protein and in a functional form, and the observed phenotype is the result of reduced AR protein expression levels.

[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.

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.

Muscle pain in myophosphorylase deficiency (McArdle’s disease): The role of gender, genotype, and pain-related coping

Pain characteristics were examined in 24 patients with myophosphorylase deficiency (McArdle's disease). Pain parameters were related to mutation analyses as well as psychosocial data using a pain questionnaire including an assessment of psychosocial distress and coping measures (Beck Depression Inventory BDI; Kiel Pain Inventory KPI, Multidimensional Fatique Inventory MFI). Twenty-three patients complained of pain, which was intermittent and exercise-induced in 15 patients. Eight patients complained of permanent pain, which was superimposed by exercise-induced pain in 7 patients. Patients reported 3-7 different pain characters and various localisations. Patients with permanent pain were significantly more frequently female, experienced higher impact on general activities and sleep as well as higher scores on the MFI. Furthermore, these patients revealed higher scores regarding several psychosocial risk factors including avoidance behavior whereas patients with intermittent pain predominantly showed endurance coping. There was no correlation between age or disease duration, pain intensity as well as mutation type and development of permanent or intermittent pain. In addition, severity of the clinical phenotype did not correlate with ACE polymorphism. Although McArdle's disease is a muscle glycogenosis with marked biochemical homogeneity, the clinical presentation can be quite heterogeneous. A substantial number of patients revealed permanent pain as a major clinical symptom. As permanent pain is not related to age or disease duration, it might be a clinically important subgroup of McArdle's disease. Gender-related genetic factors as well as maladaptive pain-related coping may contribute to the development of such a chronic pain symptom.

Age-associated tyrosine nitration of rat skeletal muscle glycogen phosphorylase b: characterization by HPLC–nanoelectrospray–Tandem mass spectrometry

We identified age-dependent post-translational modifications of skeletal muscle glycogen phosphorylase b (Ph-b), isolated from F1 hybrids of Fisher 344 x Brown Norway rats. Ph-b isolated from 34 months old rats showed a statistically significant decrease in specific activity compared to 6 months old animals: 13.8+/-0.7 vs. 20.6+/-0.8 U mg(-1) protein, respectively. Western blot analysis of the purified Ph-b with anti-3-NT antibodies revealed an age-dependent accumulation of 3-nitrotyrosine (3-NT), quantified by reverse-phase HPLC-UV analysis to increase from 0.05+/-0.03 to 0.34+/-0.11 (mol 3-NT/mol Ph-b) for 6 vs. 34 months old rats, respectively. HPLC-nanoelectrospray ionization-tandem mass spectrometry revealed the accumulation of 3-NT on Tyr113, Tyr161 and Tyr573. While nitration of Tyr113 was detected for both young and old rats, 3-NT at positions 161 and 573 was identified only for Ph-b isolated from 34 months old rats. The sequence of the rat muscle Ph-b was corrected based on our protein sequence mapping and a custom rat PHS2 sequence containing 17 differently located amino acid residues was used instead of the database sequence. The in vitro reaction of peroxynitrite with Ph-b resulted in the nitration of multiple Tyr residues at positions 51, 52, 113, 155, 185, 203, 262, 280, 404, 473, 731, and 732. Thus, the in vitro nitration conditions only mimic the nitration of a single Tyr residue observed in vivo suggesting alternative pathways controlling the accumulation of 3-NT in vivo. Our data show a correlation of age-dependent 3-NT accumulation with Ph-b inactivation.

Variable presentation of the clinical phenotype of McArdle's disease in a kindred harbouring a novel compound genotype in the muscle glycogen phosphorylase gene

We report a Spanish family with muscle glycogen phosphorylase (PYGM) deficiency (McArdle's disease) harbouring a novel compound genotype (A659D/L586P). Four individuals who had the same genotype for PYGM, showed a wide variability in the presentation of the clinical phenotype, including one patient with a restrictive respiratory pattern, which is unusual in McArdle's disease. Moreover, these patients were studied for the insertion/deletion (I/D) trait in the angiotensin converting enzyme (ACE) which has been suggested to be a strong modulator of severity in McArdle's disease. Our results indicate no association of the I/D ACE trait in this family, suggesting that other factors would be more relevant in determining the severity of the clinical presentation.

Molecular analysis of myophosphorylase deficiency in Dutch patients with McArdle's disease

We report on 8 Dutch patients with McArdle's disease from 6 unrelated families. Molecular analysis revealed the presence of four previously described mutations: the common R49X mutation, the IVS14+1G>A mutation and the recently reported R269X and Y84X nonsense mutations; and two new molecular defects: a missense mutation R138W in the homozygous state in two siblings, and a frameshift mutation c.1797delT. This first genetic study of patients from The Netherlands with McArdle's disease confirms that the R49X mutation is also the most common in Dutch patients, and that there is genetic heterogeneity within this population. Moreover, our data support the hypothesis that the Y84X mutation is a relatively frequent mutation in McArdle's patients with a Central European background, and expand the already crowded map of mutations within the PYGM gene responsible for McArdle's disease.

Two novel mutations in the muscle glycogen phosphorylase gene in McArdle's disease

We report on a Spanish family with myophosphorylase (EC 2.4.1.1) deficiency (McArdle's disease). The proband and his symptomatic sister were compound heterozygous for two novel mutations: a T-to-G transversion in exon 14 (c1722 T>G) that changes a tyrosine to a stop codon (Y573X), and a G-to-A transition in exon 15 (c1827 G>A) that disrupts the consensus signal at the donor splicing site. These findings further expand knowledge of the genetic bases of muscle glycogen phosphorylase deficiency.

Muscle glycogenosis with low phosphorylase kinase activity: mutations in PHKA1, PHKG1 or six other candidate genes explain only a minority of cases

Muscle-specific deficiency of phosphorylase kinase (Phk) causes glycogen storage disease, clinically manifesting in exercise intolerance with early fatiguability, pain, cramps and occasionally myoglobinuria. In two patients and in a mouse mutant with muscle Phk deficiency, mutations were previously found in the muscle isoform of the Phk alpha subunit, encoded by the X-chromosomal PHKA1 gene (MIM # 311870). No mutations have been identified in the muscle isoform of the Phk gamma subunit (PHKG1). In the present study, we determined Q1the structure of the PHKG1 gene and characterized its relationship to several pseudogenes. In six patients with adult- or juvenile-onset muscle glycogenosis and low Phk activity, we then searched for mutations in eight candidate genes. The coding sequences of all six genes that contribute to Phk in muscle were analysed: PHKA1, PHKB, PHKG1, CALM1, CALM2 and CALM3. We also analysed the genes of the muscle isoform of glycogen phosphorylase (PYGM), of a muscle-specific regulatory subunit of the AMP-dependent protein kinase (PRKAG3), and the promoter regions of PHKA1, PHKB and PHKG1. Only in one male patient did we find a PHKA1 missense mutation (D299V) that explains the enzyme deficiency. Two patients were heterozygous for single amino-acid replacements in PHKB that are of unclear significance (Q657K and Y770C). No sequence abnormalities were found in the other three patients. If these results can be generalized, only a fraction of cases with muscle glycogenosis and a biochemical diagnosis of low Phk activity are caused by coding, splice-site or promoter mutations in PHKA1, PHKG1 or other Phk subunit genes. Most patients with this diagnosis probably are affected either by elusive mutations of Phk subunit genes or by defects in other, unidentified genes.

Breakdown of adenine nucleotide pool in fatiguing skeletal muscle in McArdle's disease: a noninvasive 31P-MRS and EMG study

Energy metabolism and electrical muscle activity were studied in the calf muscles of 19 patients with proven McArdle's disease and in 25 healthy subjects. Phosphorus magnetic resonance spectroscopy and surface electromyography (S-EMG) were performed during two isometric muscle contractions of 3 min at 30% maximum voluntary contraction, one performed during normal perfusion and the other during applied ischemia. After about 1 min of ischemic muscle contraction in diseased muscle a significant acceleration in phosphocreatine breakdown was observed, along with a significant decrease in adenosine triphosphate. During both contractions the absence of glycolysis was shown by a significant alkalinization. Furthermore, in patients we observed a greater increase in the S-EMG amplitude than in control subjects. We conclude that early on during moderate exercise, a small number of muscle fibers reach metabolic depletion, indicated by a reduction in the adenine nucleotide pool. An increasing number of motor units, which are still in a high-energy state, are continuously recruited to compensate for muscle fatigue. This functional compartmentation may contribute to the pathophysiology of exercise intolerance in McArdle's disease.

Phenotype modulators in myophosphorylase deficiency

Myophosphorylase deficiency is characterized by exercise intolerance, muscle cramps, and recurrent myoglobinuria. Some patients are severely affected, whereas others are minimally affected or asymptomatic. The molecular basis of the disease has been elucidated but does not provide an explanation for the clinical variability. In a large cohort of patients with myophosphorylase deficiency, we tested the hypothesis that polymorphic variants in either myoadenylate deaminase (MADA) or angiotensin-converting enzyme (ACE) could act as modulators of phenotype expression. Forty-seven patients were evaluated. Clinical severity was assessed according to a severity scale of four grades. MADA activity was studied by histochemical and biochemical analysis of muscle, and the Q12X mutation in the adenine monophosphate deaminase 1 gene (AMPD1) and the insertion/deletion polymorphism in the ACE gene were assessed genetically. A complete MADA defect together with the Q12X mutation was detected in one severely affected patient. Eleven patients were heterozygous for the Q12X mutation. There was no association between clinical grading and MADA status. In contrast, we found a highly significant (p < 0.01) association between ACE genotype and clinical severity, with strong correlation between severe phenotype and number of D alleles. We show that ACE insertion/deletion polymorphism may play a significant role as phenotype modulator in McArdle's disease.

Mutation and biochemical analysis in carnitine palmitoyltransferase type II (CPT II) deficiency

Carnitine palmitoyltransferase type II (CPT II) deficiency has three basic phenotypes, late-onset muscular (mild), infantile/juvenile hepatic (intermediate) and severe neonatal. We have measured fatty acid oxidation and CPT II activity and performed mutation studies in 24 symptomatic patients representing the full clinical spectrum of disease. Severe and intermediate phenotypes show a clear correlation with biochemical indices and genetic analysis revealed causative mutations in most patients. Studies of mild phenotypes suggest a more complex interaction, with higher residual fatty acid oxidation, a wider range of CPT II activity (10-60%) but little evidence of genotype-phenotype correlation. Residual CPT II mutant protein from myopathic patients shows thermal instability at 41 degrees C. The common 'polymorphisms' V3681 and M647V are strikingly overrepresented in the myopathic patients, the implication being that they may significantly influence the manifestation of clinical disease and could therefore potentially be considered as a susceptibility variants. Among myopathic individuals, males comprised 88% of patients, suggesting increased susceptibility to clinical disease. A small number of symptomatic patients appear to have significant residual CPT II activity (42-60%) The synergistic interaction of partial deficiencies of CPT II, muscle adenosine monophosphate deaminase and possibly other enzymes of muscle energy metabolism in the aetiology of episodic myopathy deserves wider consideration.

Two new mutations in the myophosphorylase gene in Italian patients with McArdle's disease

We report two new mutations in the myophosphorylase gene (PYGM) in two unrelated Italian patients with myophosphorylase deficiency (McArdle's disease). In one, we identified a missense C-to-T mutation at codon 269 in exon 7, changing CGA (arginine) to TGA (stop codon) (R269X). The second patient carried a G-to-C mutation, changing GCT (alanine) to CCT (proline) at codon 686 (A686P) in exon 17. Both were compound heterozygous, with the common mutation at codon 49 (R49X) on the other allele. Our data further expand the genetic heterogeneity in patients with McArdle's disease, suggesting that the possibility of novel mutations has to be taken into account when performing genetic analysis in distinct ethnic groups.

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.

A novel nonsense mutation (R269X) in the myophosphorylase gene in a patient with McArdle disease

We identified a novel nonsense mutation in the myophoshorylase gene in a patient of Italian origin with McArdle disease. This homozygous C-to-T transition (805C > T) results in the replacement of a arginine at amino acid position 269 with a stop codon (R269X). Our data further expand the genetic heterogeneity in patients with McArdle disease.

Muscle glycogenoses

There are 11 hereditary disorders of glycogen metabolism affecting muscle alone or together with other tissues, and they cause two main clinical syndromes: episodic, recurrent exercise intolerance with cramps, myalgia, and myoglobinuria; or fixed, often progressive weakness. Great strides have been made in our understanding of the molecular bases of these disorders, all of which show remarkable genetic heterogeneity. In contrast, the pathophysiological mechanisms underlying acute muscle breakdown and chronic weakness remain unclear. Although glycogen storage diseases have been studied for decades, new biochemical defects are still being discovered, especially in the glycolytic pathway. In addition, the pathogenesis of polyglucosan deposition is being clarified both in traditional glycogenoses and in disorders such as Lafora's disease. In some conditions, combined dietary and exercise regimens may be of help, and gene therapy, including recombinant enzyme replacement, is being actively pursued.

Resolution of a mispaired secondary structure intermediate could account for a novel micro-insertion/deletion (387 insA/del 8 bp) in the PYGM gene causing McArdle's disease

We report two siblings with McArdle's disease who are both compound heterozygotes for two non-identical frameshift mutations in the PYGM gene; a previously reported 753 delA in exon 18 and a novel 387 insA/del 8 bp in exon 10. The novel mutation is predicted to result in premature termination of translation 33 amino acids downstream of the site of mutation, potentially encoding a severely truncated protein of 419 amino acids instead of 841 amino acids. The complete lack of myophosphorylase activity observed in muscle derived from one sibling suggests that this mutation has deleterious functional consequences. The underlying mechanism of mutagenesis may have been slipped mispairing mediated by the formation of a Moebius loop-like secondary intermediate.

A homozygous missense mutation (A659D) in the myophosphorylase gene in a Spanish patient with McArdle's disease

We identified a novel missense mutation in the myophosphorylase gene (PYGM) in a Spanish patient with McArdle's disease. This homozygous C-to-A mutation results in the replacement of a highly conserved alanine at amino acid position 659 with an aspartic acid in the C-terminal domain of the myophosphorylase gene protein, near binding sites for pyridoxal phosphate and glucose. Our data further expand the genetic heterogeneity in patients with McArdle's disease.

McArdle's disease presenting with asymmetric, late-onset arm weakness

McArdle's disease or myophosphorylase deficiency is one of the most common muscle glycogenoses and typically presents in childhood or adolescence with exercise intolerance, myalgia, myoglobinuria, and cramps in exercising muscle. We describe an elderly man who developed asymmetric proximal arm weakness at age 73. He had no history of exercise-induced cramps, myalgias, or myoglobinuria. Creatine kinase levels were elevated, serum lactate did not rise on ischemic exercise testing, and muscle biopsy showed a vacuolar myopathy with absent myophosphorylase activity. This unusual case demonstrates that McArdle's disease may present with fixed, asymmetric proximal weakness at an advanced age and should be considered in this clinical setting, especially when a history of poor exercise tolerance can be elicited.

A missense mutation T487N in the myophosphorylase gene in a Spanish patient with McArdle's disease

A heterozygous C-to-A substitution at codon 487, changing a highly conserved threonine to an asparagine (T487N) was identified in two siblings with McArdle's disease who were also heterozygous for the nonsense mutation at codon 49 (R49X). Our data further expand the genetic heterogeneity in patients with McArdle's disease.

HyperCKemia as the only sign of McArdle's disease in a child

An asymptomatic 13-year-old boy, who never complained of exercise intolerance or myalgia, was found to have markedly elevated serum creatine kinase (CK) levels during a routine check-up. General physical and neurologic examinations were normal. Surprisingly, histochemical and biochemical analysis of muscle showed myophosphorylase deficiency and genetic analysis showed that the patient was homozygous for the most common mutation encountered in McArdle's disease (R49X). This case illustrates the fuzzy correlation between molecular defect and clinical phenotype in patients with McArdle's disease, and suggests that a thorough study of the muscle biopsy is important in patients with idiopathic hyperCKemia for correct diagnosis and careful follow-up.

Glycogen storage myopathies

The glycogen storage myopathies are caused by enzyme defects in the glycogenolytic or in the glycolytic pathway affecting skeletal muscle alone or in conjunction with other tissues. The authors review recent findings in this area, including a new entity, aldolase deficiency, and the wealth of molecular genetic data that are rapidly accumulating. Despite this progress, genotype-phenotyp3 correlations are still murky in most glycogen storage myopathies.

The molecular diagnosis of metabolic myopathies

The metabolic myopathies are distinguished by extensive clinical and genetic heterogeneity within and between individual disorders. There are a number of explanations for the variability observed that go beyond single gene mutations or degrees of heteroplasmy in the case of mitochondrial DNA mutations. Some of the contributing factors include protein subunit interactions, tissue-specificity, modifying genetic factors, and environmental triggers. Advances in the molecular analysis of metabolic myopathies during the last decade have not only improved the diagnosis of individual disorders but also helped to characterize the contributing factors that make these disorders so complex.

A missense mutation W797R in the myophosphorylase gene in a Spanish patient with McArdle's disease

We identified a novel missense mutation in the myophosphorylase gene (PYGM) in a Spanish patient with McArdle's disease. This homozygous T-to-C transition results in the replacement of a highly conserved tryptophan at amino acid position (aa) 797 with an arginine in the C-terminal domain of the PYGM protein. The lack of enzyme activity in the proband's muscle is consistent with a crucial role of the aa 797 in the normal function of the PYGM protein. Our data further expand the genetic heterogeneity in patients with McArdle's disease.

A new mutation in the regulatory domain of the myophosphorylase gene affecting protein dimer contact

We have identified a novel missense mutation in the myophosphorylase gene in a Spanish patient with McArdle's disease. The patient was homozygous for a T-to-C transition at codon 115 (L115P) in exon 3, which changed an encoded leucine (CUG) to a proline (CCG). This is the first mutation to be described in exon 3 and in a protein domain related to dimer contact. These data further emphasize the importance of private mutations in McArdle's disease, some of which are associated with specific ethnic groups.

McArdle's disease. The unsolved mystery of the reappearing enzyme

We assessed the frequency of muscle fibers showing histochemical phosphorylase activity in 27 muscle biopsies from 25 unrelated patients with McArdle's disease and studied by immunohistochemistry and in situ hybridization whether the muscle-specific isoform was expressed. Positive phosphorylase fibers were observed in 19% of our series of biopsies. We show that the enzyme isoform expressed in regenerating fibers differs according to the genotype of patients: the muscle-specific isoform is transcribed and translated in patients with none of the described mutations in at least one allele of the myophosphorylase gene, whereas it is neither transcribed nor translated in patients with identified mutations in both alleles.

Molecular characterization of McArdle's disease in two large Finnish families

We have studied two large unrelated Finnish families with myophosphorylase deficiency (McArdle's disease). In one, we identified a new nonsense mutation at codon 540 in exon 14 of the myophosphorylase gene, changing an encoded glutamic acid to a stop codon (E540X). The second family carried a splice-junction mutation at the 5' splice site of intron 14 (1844+G-->A), previously reported in one Caucasian patient and in a consanguineous Druze family. These data further enlarge the list of mutations associated with McArdle's disease and establish that McArdle's disease is genetically heterogeneous also within the Finnish population.

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.

McArdle's disease associated with homozygosity for the missense mutation Gly204Ser of the myophosphorylase gene in a Spanish patient

We studied a pateint whose clinical, morphological and biochemical findings were consistent with McArdle's disease. Molecular genetic studies revealed that the patient did not harbor the common Arg49Stop mutation and was homozygous for the Gly204Ser mutation. Until now, no patient having the missense mutation in the two alleles has been documented.

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.

Glycogen storage diseases of muscle

Ten specific enzyme defects of glycogen metabolism affect skeletal muscle alone or in combination with other tissues. The newest addition to this group of disorders is the defect of aldolase A (glycogenosis type XII), a block in terminal glycolysis associated with myopathy and a hemolytic trait. The muscle glycogenoses cause two major syndromes, one characterized by exercise intolerance, cramps, and myoglobinuria, and the other dominated by fixed, often progressive weakness. This review considers sequentially recent advances in the following: clinical features or clinical variants, including a brief description of glycogenosis type XII; animal models, both spontaneous and genetically engineered; physiopathologic mechanisms, especially of the exercise intolerance and myoglobinuria; biochemical and molecular features--molecular defects are just beginning to be discovered for some glycogenoses (e.g. phosphorylase-b-kinase deficiency or branching enzyme deficiency), whereas they form long lists for others, such as acid maltase deficiency and myophosphorylase deficiency; and therapeutic approaches, including enzyme replacement and gene therapy.

Mutations in the liver glycogen phosphorylase gene (PYGL) underlying glycogenosis type VI

Deficiency of glycogen phosphorylase in the liver gives rise to glycogen-storage disease type VI (Hers disease; MIM 232700). We report the identification of the first mutations in PYGL, the gene encoding the liver isoform of glycogen phosphorylase, in three patients with Hers disease. These are two splice-site mutations and two missense mutations. A mutation of the 5' splice-site consensus of intron 14 causes the retention of intron 14 and the utilization of two illegitimate 5' splice sites, whereas a mutation of the 3' splice-site consensus of intron 4 causes the skipping of exon 5. Two missense mutations, N338S and N376K, both cause nonconservative replacements of amino acids that are absolutely conserved even in yeast and bacterial phosphorylases. We also report corrections of the PYGL coding sequence, sequence polymorphisms, and a partial PYGL gene structure with introns in the same positions as in PYGM, the gene of the muscle isoform of phosphorylase. Our findings demonstrate that PYGL mutations cause Hers disease, and they may improve laboratory diagnosis of deficiencies of the liver phosphorylase system.

McArdle's disease in childhood: report of a new case

McArdle's disease (glycogenosis type V) is an inherited glycogen storage disease characterized clinically by myalgia, cramps and sometimes myoglobinuria, triggered by exercise. The onset of exercise intolerance is usually in late childhood or adolescence and diagnosis is exceptionally established during infancy. We report the case of a 6-year-old girl who had been complaining of aching muscles for a long time, and who presented after a near-drowning incident, with extensive muscle necrosis, probably secondary to myophosphorylase deficiency-induced cramps. These unusual manifestations led to the diagnosis of this rare disorder. We compare the clinical findings of this case to nine previous reports. This highlights the heterogeneous spectrum of this disease in childhood and supports the distinction of three clinical pictures in childhood: a neonatal form rapidly fatal, a milder form with congenital myopathic symptoms and a benign classical form with myalgia, cramps and pigmenturia.