Cloning of bovine muscle glycogen phosphorylase cDNA and identification of a mutation in cattle with myophosphorylase deficiency, an animal model for McArdle's disease

Gene therapy for glycogen storage diseases

Glycogen storage disorders (GSDs) are inherited disorders of metabolism resulting from the deficiency of individual enzymes involved in the synthesis, transport, and degradation of glycogen. This literature review summarizes the development of gene therapy for the GSDs. The abnormal accumulation of glycogen and deficiency of glucose production in GSDs lead to unique symptoms based upon the enzyme step and tissues involved, such as liver and kidney involvement associated with severe hypoglycemia during fasting and the risk of long-term complications including hepatic adenoma/carcinoma and end stage kidney disease in GSD Ia from glucose-6-phosphatase deficiency, and cardiac/skeletal/smooth muscle involvement associated with myopathy +/- cardiomyopathy and the risk for cardiorespiratory failure in Pompe disease. These symptoms are present to a variable degree in animal models for the GSDs, which have been utilized to evaluate new therapies including gene therapy and genome editing. Gene therapy for Pompe disease and GSD Ia has progressed to Phase I and Phase III clinical trials, respectively, and are evaluating the safety and bioactivity of adeno-associated virus vectors. Clinical research to understand the natural history and progression of the GSDs provides invaluable outcome measures that serve as endpoints to evaluate benefits in clinical trials. While promising, gene therapy and genome editing face challenges with regard to clinical implementation, including immune responses and toxicities that have been revealed during clinical trials of gene therapy that are underway. Gene therapy for the glycogen storage diseases is under development, addressing an unmet need for specific, stable therapy for these conditions.

Discovery and Biotechnological Exploitation of Glycoside-Phosphorylases

Among carbohydrate active enzymes, glycoside phosphorylases (GPs) are valuable catalysts for white biotechnologies, due to their exquisite capacity to efficiently re-modulate oligo- and poly-saccharides, without the need for costly activated sugars as substrates. The reversibility of the phosphorolysis reaction, indeed, makes them attractive tools for glycodiversification. However, discovery of new GP functions is hindered by the difficulty in identifying them in sequence databases, and, rather, relies on extensive and tedious biochemical characterization studies. Nevertheless, recent advances in automated tools have led to major improvements in GP mining, activity predictions, and functional screening. Implementation of GPs into innovative in vitro and in cellulo bioproduction strategies has also made substantial advances. Herein, we propose to discuss the latest developments in the strategies employed to efficiently discover GPs and make the best use of their exceptional catalytic properties for glycoside bioproduction.

Preclinical Research in McArdle Disease: A Review of Research Models and Therapeutic Strategies

McArdle disease is an autosomal recessive disorder of muscle glycogen metabolism caused by pathogenic mutations in the PYGM gene, which encodes the skeletal muscle-specific isoform of glycogen phosphorylase. Clinical symptoms are mainly characterized by transient acute “crises” of early fatigue, myalgia and contractures, which can be accompanied by rhabdomyolysis. Owing to the difficulty of performing mechanistic studies in patients that often rely on invasive techniques, preclinical models have been used for decades, thereby contributing to gain insight into the pathophysiology and pathobiology of human diseases. In the present work, we describe the existing in vitro and in vivo preclinical models for McArdle disease and review the insights these models have provided. In addition, despite presenting some differences with the typical patient’s phenotype, these models allow for a deep study of the different features of the disease while representing a necessary preclinical step to assess the efficacy and safety of possible treatments before they are tested in patients.

Systemic AAV8-mediated delivery of a functional copy of muscle glycogen phosphorylase (Pygm) ameliorates disease in a murine model of McArdle disease

McArdle disease is a disorder of carbohydrate metabolism that causes painful skeletal muscle cramps and skeletal muscle damage leading to transient myoglobinuria and increased risk of kidney failure. McArdle disease is caused by recessive mutations in the muscle glycogen phosphorylase (PYGM) gene leading to absence of PYGM enzyme in skeletal muscle and preventing access to energy from muscle glycogen stores. There is currently no cure for McArdle disease. Using a preclinical animal model, we aimed to identify a clinically translatable and relevant therapy for McArdle disease. We evaluated the safety and efficacy of recombinant adeno-associated virus serotype 8 (rAAV8) to treat a murine model of McArdle disease via delivery of a functional copy of the disease-causing gene, Pygm. Intraperitoneal injection of rAAV8-Pygm at post-natal day 1-3 resulted in Pygm expression at 8 weeks of age, accompanied by improved skeletal muscle architecture, reduced accumulation of glycogen and restoration of voluntary running wheel activity to wild-type levels. We did not observe any adverse reaction to the treatment at 8 weeks post-injection. Thus, we have investigated a highly promising gene therapy for McArdle disease with a clear path to the ovine large animal model endemic to Western Australia and subsequently to patients.

Preclinical Research in Glycogen Storage Diseases: A Comprehensive Review of Current Animal Models

GSD are a group of disorders characterized by a defect in gene expression of specific enzymes involved in glycogen breakdown or synthesis, commonly resulting in the accumulation of glycogen in various tissues (primarily the liver and skeletal muscle). Several different GSD animal models have been found to naturally present spontaneous mutations and others have been developed and characterized in order to further understand the physiopathology of these diseases and as a useful tool to evaluate potential therapeutic strategies. In the present work we have reviewed a total of 42 different animal models of GSD, including 26 genetically modified mouse models, 15 naturally occurring models (encompassing quails, cats, dogs, sheep, cattle and horses), and one genetically modified zebrafish model. To our knowledge, this is the most complete list of GSD animal models ever reviewed. Importantly, when all these animal models are analyzed together, we can observe some common traits, as well as model specific differences, that would be overlooked if each model was only studied in the context of a given GSD.

McArdle Disease: New Insights into Its Underlying Molecular Mechanisms

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

Preclinical Development of New Therapy for Glycogen Storage Diseases

Glycogen storage disease (GSD) consists of more than 10 discrete conditions for which the biochemical and genetic bases have been determined, and new therapies have been under development for several of these conditions. Gene therapy research has generated proof-of-concept for GSD types I (von Gierke disease) and II (Pompe disease). Key features of these gene therapy strategies include the choice of vector and regulatory cassette, and recently adeno-associated virus (AAV) vectors containing tissue-specific promoters have achieved a high degree of efficacy. Efficacy of gene therapy for Pompe disease depend upon the induction of immune tolerance to the therapeutic enzyme. Efficacy of von Gierke disease is transient, waning gradually over the months following vector administration. Small molecule therapies have been evaluated with the goal of improving standard of care therapy or ameliorating the cellular abnormalities associated with specific GSDs. The receptor-mediated uptake of the therapeutic enzyme in Pompe disease was enhanced by administration of β2 agonists. Rapamycin reduced the liver fibrosis observed in GSD III. Further development of gene therapy could provide curative therapy for patients with GSD, if efficacy from preclinical research is observed in future clinical trials and these treatments become clinically available.

Phenotype consequences of myophosphorylase dysfunction: insights from the McArdle mouse model

KEY POINTS

This is the first study to analyse the effect of muscle glycogen phosphorylase depletion in metabolically different muscle types. In McArdle mice, muscle glycogen phosphorylase is absent in both oxidative and glycolytic muscles. In McArdle mice, the glycogen debranching enzyme (catabolic) is increased in oxidative muscles, whereas the glycogen branching enzyme (anabolic) is increased in glycolytic muscles. In McArdle mice, total glycogen synthase is decreased in both oxidative and glycolytic muscles, whereas the phosphorylated inactive form of the enzyme is increased in both oxidative and glycolytic enzymes. In McArdle mice, glycogen content is higher in glycolytic muscles than in oxidative muscles. Additionally, in all muscles analysed, the glycogen content is higher in males than in females. The maximal endurance capacity of the McArdle mice is significantly lower compared to heterozygous and wild-type mice.

ABSTRACT

McArdle disease, caused by inherited deficiency of the enzyme muscle glycogen phosphorylase (GP-MM), is arguably the paradigm of exercise intolerance. The recent knock-in (p.R50X/p.R50X) mouse disease model allows an investigation of the phenotypic consequences of muscle glycogen unavailability and the physiopathology of exercise intolerance. We analysed, in 2-month-old mice [wild-type (wt/wt), heterozygous (p.R50X/wt) and p.R50X/p.R50X)], maximal endurance exercise capacity and the molecular consequences of an absence of GP-MM in the main glycogen metabolism regulatory enzymes: glycogen synthase, glycogen branching enzyme and glycogen debranching enzyme, as well as glycogen content in slow-twitch (soleus), intermediate (gastrocnemius) and glycolytic/fast-twitch (extensor digitorum longus; EDL) muscles. Compared with wt/wt, exercise capacity (measured in a treadmill test) was impaired in p.R50X/p.R50X (∼48%) and p.R50X/wt mice (∼18%). p.R50X/p.R50X mice showed an absence of GP-MM in the three muscles. GP-MM was reduced in p.R50X/wt mice, especially in the soleus, suggesting that the function of 'slow-twitch' muscles is less dependent on glycogen catabolism. p.R50X/p.R50X mice showed increased glycogen debranching enzyme in the soleus, increased glycogen branching enzyme in the gastrocnemius and EDL, as well as reduced levels of mucle glycogen synthase protein in the three muscles (mean ∼70%), reflecting a protective mechanism for preventing deleterious glycogen accumulation. Additionally, glycogen content was highest in the EDL of p.R50X/p.R50X mice. Amongst other findings, the present study shows that the expression of the main muscle glycogen regulatory enzymes differs depending on the muscle phenotype (slow- vs. fast-twitch) and that even partial GP-MM deficiency affects maximal endurance capacity. Our knock-in model might help to provide insights into the importance of glycogen on muscle function.

Large animal models and new therapies for glycogen storage disease

Glycogen storage diseases (GSD), a unique category of inherited metabolic disorders, were first described early in the twentieth century. Since then, the biochemical and genetic bases of these disorders have been determined, and an increasing number of animal models for GSD have become available. At least seven large mammalian models have been developed for laboratory research on GSDs. These models have facilitated the development of new therapies, including gene therapy, which are undergoing clinical translation. For example, gene therapy prolonged survival and prevented hypoglycemia during fasting for greater than one year in dogs with GSD type Ia, and the need for periodic re-administration to maintain efficacy was demonstrated in that dog model. The further development of gene therapy could provide curative therapy for patients with GSD and other inherited metabolic disorders.

The pathogenomics of McArdle disease--genes, enzymes, models, and therapeutic implications

Numerous biomedical advances have been made since Carl and Gerty Cori discovered the enzyme phosphorylase in the 1940s and the Scottish physician Brian McArdle reported in 1951 a previously 'undescribed disorder characterized by a gross failure of the breakdown in muscle of glycogen'. Today we know that this disorder, commonly known as 'McArdle disease', is caused by inherited deficiency of the muscle isoform of glycogen phosphorylase (GP). Here we review the main aspects of the 'pathogenomics' of this disease including, among others: the spectrum of mutations in the gene (PYGM) encoding muscle GP; the interplay between the different tissue GP isoforms in cellular cultures and in patients; what can we learn from naturally occurring and recently laboratory-generated animal models of the disease; and potential therapies.

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

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

Myophosphorylase deficiency (glycogen storage disease Type V) in a herd of Charolais cattle in New Zealand: confirmation by PCR-RFLP testing

AIM

To describe a disease of muscle in Charolais calves and confirm the putative diagnosis of inherited myophosphorylase deficiency.

METHODS

Variously stained paraffin sections of muscle prepared from affected calves were used to describe the lesions. A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) test was developed and applied to affected calves, their sires, dams and other individuals.

RESULTS

The lesions were those of rhabdomyolysis of skeletal muscles and sub-sarcolemmal spaces in normal fibres. The PCRRFLP test confirmed the expected mutation for phosphorylase deficiency of Charolais cattle in two affected calves. In addition, sires, dams and other closely-related individuals of four affected calves tested as heterozygous for the mutation. Other apparently unrelated animals also tested as heterozygous.

CONCLUSIONS

The diagnosis of myophosphorylase deficiency was confirmed. The PCR-RFLP test is suitable for use in controlling this recessively-inherited disorder as it can diagnose heterozygous individuals that are otherwise clinically normal.

A critical analysis of disease-associated DNA polymorphisms in the genes of cattle, goat, sheep, and pig

Genetic variations through their effects on gene expression and protein function underlie disease susceptibility in farm animal species. The variations are in the form of single nucleotide polymorphisms, deletions/insertions of nucleotides or whole genes, gene or whole chromosomal rearrangements, gene duplications, and copy number polymorphisms or variants. They exert varying degrees of effects on gene action, such as substitution of an amino acid for another, shift in reading frame and premature termination of translation, and complete deletion of entire exon(s) or gene(s) in diseased individuals. These factors influence gene function by affecting mRNA splicing pattern or by altering/eliminating protein function. Elucidating the genetic bases of diseases under the control of many genes is very challenging, and it is compounded by several factors, including host x pathogen x environment interactions. In this review, the genetic variations that underlie several diseases of livestock (under monogenic and polygenic control) are analyzed. Also, factors hampering research efforts toward identification of genetic influences on animal disease identification and control are highlighted. A better understanding of the factors analyzed could be better harnessed to effectively identify and control, genetically, livestock diseases. Finally, genetic control of animal diseases can reduce the costs associated with diseases, improve animal welfare, and provide healthy animal products to consumers, and should be given more attention.

Genotyping glycogen storage disease type II and type V in cattle reared in the Czech Republic

Genotyping was carried out for glycogen storage disease type II and type V in seven cattle breeds. The analysis was carried out using the polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) method. In the breeds analysed [Charolais, Czech Spotted (Czech Simmental), Belgian Blue, Limousine, Blonde d'Aquitaine, Aberdeen Angus, and Beef Simmental sires reared in the Czech Republic], the recessive allele was not found in the PYGM (phosphorylase glycogen, muscle) responsible for the glycogen storage disease type V. In the same panel, the recessive allele in exon 7, exon 9 and exon 13 of the GAA (glucosidase alpha, acid), causing the glycogen storage disease type II was not found. Therefore, we have not revealed the recessives outside previous reported breeds. The knowledge of the breed-specific occurrence of inherited disorders facilitates focusing and reduces the costs of detecting the heterozygous carriers of recessive inherited disorders.

Internal restriction sites: quality assurance aids in genotyping

Improvements to restriction fragment length polymorphism (RFLP)-based genotyping assays currently used for detection of mutations responsible for bovine ferrochelatase and myophosphorylase deficiencies, and equine hyperkalemic periodic paralysis (HYPP) are described. Reports of sporadic inhibition of restriction enzyme activity suggest a critical factor in RFLP-based genotyping assays should be assurance that restriction enzymes perform to specification with every sample. The RFLP genotyping assays that use either a mismatched recognition sequence in one or both of the oligonucleotides, or incorporate a second native site within the PCR amplicon, provide the mechanism by which efficiency of restriction enzymes can be assessed with every sample. The outcome is confirmation of the activity of the discriminating enzyme regardless of genotype.

A direct StyI polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) test for the myophosphorylase mutation in cattle

Myophosphorylase deficiency in cattle is a muscle disease induced by a C-->T point mutation in codon 489 of the myophosphorylase gene, which until now has only been diagnosed in the Charolais breed. The disease seems to be inherited in an autosomal monogenic recessive manner. A calf of double muscled phenotype was suspected of suffering from myophosphorylase deficiency based on typical symptoms, i.e. brown-coloured, transparent urine, occurring after exercise; exercise intolerance; symptoms of pain; and an elevated level of plasma creatine kinase. The presence of the previously described mutation was excluded using a newly developed, improved polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) procedure to identify easily heterozygous carriers and homozygous affected animals.

A deletion of the paracellin-1 gene is responsible for renal tubular dysplasia in cattle

Various hereditary diseases analogous to particular human heritable diseases have been identified in cattle. Investigation of these cattle diseases will provide useful information regarding the pathogenesis of the corresponding human diseases. Renal tubular dysplasia is an autosomal recessive disease of Japanese black cattle characterized by renal failure and growth retardation. We have previously mapped the locus responsible for the disease within a region on bovine chromosome 1. In the present study, we further typed additional markers in this region and found that a genomic segment of bovine chromosome 1 including the microsatellite marker BMS4009 was deleted in the affected animals. Construction of a physical map covering this region with BAC clones and comparison of the nucleotide sequences of this region between normal and affected animals revealed that a region of 37 kb including exons 1 to 4 of the bovine paracellin-1 gene was deleted in the affected animals. The paracellin-1 gene, which is the causative gene for human renal hypomagnesemia with hypercaciuria and nephrocalcinosis, encodes a tight junction protein of renal epithelial cells. Therefore, we concluded that deletion of the paracellin-1 gene is responsible for renal tubular dysplasia of cattle, and the cattle disease could be a good model for the human disease.

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.

Structural and functional characterization of the Drosophila glycogen phosphorylase gene

We identified a P element insertional mutant of the Drosophila glycogen phosphorylase (DGPH) gene. Glycogen phosphorylase protein concentration and enzyme activity are decreased while glycogen content is increased in flies homozygous for the mutant allele. The DGPH gene has been cloned and sequenced; its open reading frame codes for a protein of 844 amino acids with a predicted molecular mass of 97 kDa. Comparison of the conceptual amino acid sequence of the Drosophila glycogen phosphorylase with glycogen phosphorylase sequences from other organisms shows a high degree of homology to mammalian enzymes. All the residues of the allosteric effector binding sites, the active site, and the site of phosphorylation are exactly conserved, but some of the residues of the glycogen storage site are not.

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.