Late-onset polyglucosan body myopathy in five patients with a homozygous mutation in GYG1

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.

Glycogen storage diseases

Glycogen storage diseases (GSDs) are a group of rare, monogenic disorders that share a defect in the synthesis or breakdown of glycogen. This Primer describes the multi-organ clinical features of hepatic GSDs and muscle GSDs, in addition to their epidemiology, biochemistry and mechanisms of disease, diagnosis, management, quality of life and future research directions. Some GSDs have available guidelines for diagnosis and management. Diagnostic considerations include phenotypic characterization, biomarkers, imaging, genetic testing, enzyme activity analysis and histology. Management includes surveillance for development of characteristic disease sequelae, avoidance of fasting in several hepatic GSDs, medically prescribed diets, appropriate exercise regimens and emergency letters. Specific therapeutic interventions are available for some diseases, such as enzyme replacement therapy to correct enzyme deficiency in Pompe disease and SGLT2 inhibitors for neutropenia and neutrophil dysfunction in GSD Ib. Progress in diagnosis, management and definitive therapies affects the natural course and hence morbidity and mortality. The natural history of GSDs is still being described. The quality of life of patients with these conditions varies, and standard sets of patient-centred outcomes have not yet been developed. The landscape of novel therapeutics and GSD clinical trials is vast, and emerging research is discussed herein.

Diagnosis and management of glycogen storage disease type IV, including adult polyglucosan body disease: A clinical practice resource

Glycogen storage disease type IV (GSD IV) is an ultra-rare autosomal recessive disorder caused by pathogenic variants in GBE1 which results in reduced or deficient glycogen branching enzyme activity. Consequently, glycogen synthesis is impaired and leads to accumulation of poorly branched glycogen known as polyglucosan. GSD IV is characterized by a remarkable degree of phenotypic heterogeneity with presentations in utero, during infancy, early childhood, adolescence, or middle to late adulthood. The clinical continuum encompasses hepatic, cardiac, muscular, and neurologic manifestations that range in severity. The adult-onset form of GSD IV, referred to as adult polyglucosan body disease (APBD), is a neurodegenerative disease characterized by neurogenic bladder, spastic paraparesis, and peripheral neuropathy. There are currently no consensus guidelines for the diagnosis and management of these patients, resulting in high rates of misdiagnosis, delayed diagnosis, and lack of standardized clinical care. To address this, a group of experts from the United States developed a set of recommendations for the diagnosis and management of all clinical phenotypes of GSD IV, including APBD, to support clinicians and caregivers who provide long-term care for individuals with GSD IV. The educational resource includes practical steps to confirm a GSD IV diagnosis and best practices for medical management, including (a) imaging of the liver, heart, skeletal muscle, brain, and spine, (b) functional and neuromusculoskeletal assessments, (c) laboratory investigations, (d) liver and heart transplantation, and (e) long-term follow-up care. Remaining knowledge gaps are detailed to emphasize areas for improvement and future research.

GBE1-related disorders: Adult polyglucosan body disease and its neuromuscular phenotypes

Adult polyglucosan body disease (APBD) represents a complex autosomal recessive inherited neurometabolic disorder due to homozygous or compound heterozygous pathogenic variants in GBE1 gene, resulting in deficiency of glycogen-branching enzyme and secondary storage of glycogen in the form of polyglucosan bodies, involving the skeletal muscle, diaphragm, peripheral nerve (including autonomic fibers), brain white matter, spinal cord, nerve roots, cerebellum, brainstem and to a lesser extent heart, lung, kidney, and liver cells. The diversity of new clinical presentations regarding neuromuscular involvement is astonishing and transformed APBD in a key differential diagnosis of completely different clinical conditions, including axonal and demyelinating sensorimotor polyneuropathy, progressive spastic paraparesis, motor neuronopathy presentations, autonomic disturbances, leukodystrophies or even pure myopathic involvement with limb-girdle pattern of weakness. This review article aims to summarize the main clinical, biochemical, genetic, and diagnostic aspects regarding APBD with special focus on neuromuscular presentations.

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.

Sequential spontaneous compartment syndrome in multiple limbs in a young adult with GYG1 gene mutation

Compartment syndrome is a surgical emergency requiring immediate intervention. Majority of compartment syndromes are associated with trauma or surgery. Spontaneous compartment syndrome in multiple limbs is rare and alternative diagnosis should be sought. We report a young adult man who developed compartment syndrome in all four limbs sequentially over 4 years. On further evaluation, he was found to have a gene mutation in exon 3 of GYG1 gene. Spontaneous compartment syndrome in patients with GYG1 gene mutation does not appear to have been previously recognised. Although a direct causality cannot be confidently drawn, this gene is involved in muscle energy utilisation and is known to cause metabolic defect. Acute compartment syndrome, once diagnosed, warrants emergency surgical decompression. The subsequent management of spontaneous compartment syndrome demands a thorough medical assessment to identify any underlying metabolic or genetic predisposition.

Pulmonary glycogen deficiency as a new potential cause of respiratory distress syndrome

The glycogenin knockout mouse is a model of Glycogen Storage Disease type XV. These animals show high perinatal mortality (90%) due to respiratory failure. The lungs of glycogenin-deficient embryos and P0 mice have a lower glycogen content than that of wild-type counterparts. Embryonic lungs were found to have decreased levels of mature surfactant proteins SP-B and SP-C, together with incomplete processing of precursors. Furthermore, non-surviving pups showed collapsed sacculi, which may be linked to a significantly reduced amount of surfactant proteins. A similar pattern was observed in glycogen synthase1-deficient mice, which are devoid of glycogen in the lungs and are also affected by high perinatal mortality due to atelectasis. These results indicate that glycogen availability is a key factor for the burst of surfactant production required to ensure correct lung expansion at the establishment of air breathing. Our findings confirm that glycogen deficiency in lungs can cause respiratory distress syndrome and suggest that mutations in glycogenin and glycogen synthase 1 genes may underlie cases of idiopathic neonatal death.

Glycogenin-1 deficiency mimicking limb-girdle muscular dystrophy

Glycogen storage disease type XV (GSD XV) is a recently described muscle glycogenosis due to glycogenin-1 (GYG1) deficiency characterized by the presence of polyglucosan bodies on muscle biopsy (Polyglucosan body myopathy-2, PGBM2). Here we describe a 44 year-old man with limb-girdle muscle weakness mimicking a limb-girdle muscular dystrophy (LGMD), and early onset exertional myalgia. Neurologic examination revealed a waddling gait with hyperlordosis, bilateral asymmetric scapular winging, mild asymmetric deltoid and biceps brachii weakness, and pelvic-girdle weakness involving the gluteal muscles and, to a lesser extent, the quadriceps. Serum creatine kinase levels were slightly elevated. Electrophysiological examination showed a myopathic pattern. There was no cardiac or respiratory involvement. Whole-body muscle MRI revealed atrophy and fat replacement of the tongue, biceps brachii, pelvic girdle and erector spinae. A deltoid muscle biopsy showed the presence of PAS-positive inclusions that remained non-digested with alpha-amylase treatment. Electron microscopy studies confirmed the presence of polyglucosan bodies. A diagnostic gene panel designed by the Genetic Diagnosis Laboratory of Strasbourg University Hospital (France) for 210 muscular disorders genes disclosed two heterozygous, pathogenic GYG1 gene mutations (c.304G>C;p.(Asp102His) + c.164_165del). Considering the clinical heterogeneity found in the previously described 38 GYG-1 deficient patients, we suggest that GYG1 should be systematically included in targeted NGS gene panels for LGMDs, distal myopathies, and metabolic myopathies.

Glycogenin is Dispensable for Glycogen Synthesis in Human Muscle, and Glycogenin Deficiency Causes Polyglucosan Storage

CONTEXT

Glycogenin is considered to be an essential primer for glycogen biosynthesis. Nevertheless, patients with glycogenin-1 deficiency due to biallelic GYG1 (NM_004130.3) mutations can store glycogen in muscle. Glycogenin-2 has been suggested as an alternative primer for glycogen synthesis in patients with glycogenin-1 deficiency.

OBJECTIVE

The objective of this article is to investigate the importance of glycogenin-1 and glycogenin-2 for glycogen synthesis in skeletal and cardiac muscle.

DESIGN, SETTING, AND PATIENTS

Glycogenin-1 and glycogenin-2 expression was analyzed by Western blot, mass spectrometry, and immunohistochemistry in liver, heart, and skeletal muscle from controls and in skeletal and cardiac muscle from patients with glycogenin-1 deficiency.

RESULTS

Glycogenin-1 and glycogenin-2 both were found to be expressed in the liver, but only glycogenin-1 was identified in heart and skeletal muscle from controls. In patients with truncating GYG1 mutations, neither glycogenin-1 nor glycogenin-2 was expressed in skeletal muscle. However, nonfunctional glycogenin-1 but not glycogenin-2 was identified in cardiac muscle from patients with cardiomyopathy due to GYG1 missense mutations. By immunohistochemistry, the mutated glycogenin-1 colocalized with the storage of glycogen and polyglucosan in cardiomyocytes.

CONCLUSIONS

Glycogen can be synthesized in the absence of glycogenin, and glycogenin-1 deficiency is not compensated for by upregulation of functional glycogenin-2. Absence of glycogenin-1 leads to the focal accumulation of glycogen and polyglucosan in skeletal muscle fibers. Expression of mutated glycogenin-1 in the heart is deleterious, and it leads to storage of abnormal glycogen and cardiomyopathy.

From the seminal discovery of proteoglycogen and glycogenin to emerging knowledge and research on glycogen biology

Although the discovery of glycogen in the liver, attributed to Claude Bernard, happened more than 160 years ago, the mechanism involved in the initiation of glucose polymerization remained unknown. The discovery of glycogenin at the core of glycogen's structure and the initiation of its glucopolymerization is among one of the most exciting and relatively recent findings in Biochemistry. This review focuses on the initial steps leading to the seminal discoveries of proteoglycogen and glycogenin at the beginning of the 1980s, which paved the way for subsequent foundational breakthroughs that propelled forward this new research field. We also explore the current, as well as potential, impact this research field is having on human health and disease from the perspective of glycogen storage diseases. Important new questions arising from recent studies, their links to basic mechanisms involved in the de novo glycogen biogenesis, and the pervading presence of glycogenin across the evolutionary scale, fueled by high throughput -omics technologies, are also addressed.

Functional characterization of GYG1 variants in two patients with myopathy and glycogenin-1 deficiency

Glycogen storage disease XV is caused by variants in the glycogenin-1 gene, GYG1, and presents as a predominant skeletal myopathy or cardiomyopathy. We describe two patients with late-onset myopathy and biallelic GYG1 variants. In patient 1, the novel c.144-2A>G splice acceptor variant and the novel frameshift variant c.631delG (p.Val211Cysfs*30) were identified, and in patient 2, the previously described c.304G>C (p.Asp102His) and c.487delG (p.Asp163Thrfs*5) variants were found. Protein analysis showed total absence of glycogenin-1 expression in patient 1, whereas in patient 2 there was reduced expression of glycogenin-1, with the residual protein being non-functional. Both patients showed glycogen and polyglucosan storage in their muscle fibers, as revealed by PAS staining and electron microscopy. Age at onset of the myopathy phenotype was 53 years and 70 years respectively, with the selective pattern of muscle involvement on MRI corroborating the pattern of weakness. Cardiac evaluation of patient 1 and 2 did not show any specific abnormalities linked to the glycogenin-1 deficiency. In patient 2, who was shown to express the p.Asp102His mutated glycogenin-1, cardiac evaluation was still normal at age 77 years. This contrasts with the association of the p.Asp102His variant in homozygosity with a severe cardiomyopathy in several cases with an onset age between 30 and 50 years. This finding might indicate that the level of p.Asp102His mutated glycogenin-1 determines if a patient will develop a cardiomyopathy.

Myopathies Related to Glycogen Metabolism Disorders

Most of the glycogen metabolism disorders that affect skeletal muscle involve enzymes in glycogenolysis (myophosphorylase (PYGM), glycogen debranching enzyme (AGL), phosphorylase b kinase (PHKB)) and glycolysis (phosphofructokinase (PFK), phosphoglycerate mutase (PGAM2), aldolase A (ALDOA), β-enolase (ENO3)); however, 3 involve glycogen synthesis (glycogenin-1 (GYG1), glycogen synthase (GSE), and branching enzyme (GBE1)). Many present with exercise-induced cramps and rhabdomyolysis with higher-intensity exercise (i.e., PYGM, PFK, PGAM2), yet others present with muscle atrophy and weakness (GYG1, AGL, GBE1). A failure of serum lactate to rise with exercise with an exaggerated ammonia response is a common, but not invariant, finding. The serum creatine kinase (CK) is often elevated in the myopathic forms and in PYGM deficiency, but can be normal and increase only with rhabdomyolysis (PGAM2, PFK, ENO3). Therapy for glycogen storage diseases that result in exercise-induced symptoms includes lifestyle adaptation and carefully titrated exercise. Immediate pre-exercise carbohydrate improves symptoms in the glycogenolytic defects (i.e., PYGM), but can exacerbate symptoms in glycolytic defects (i.e., PFK). Creatine monohydrate in low dose may provide a mild benefit in PYGM mutations.

Polyglucosan myopathy and functional characterization of a novel GYG1 mutation

OBJECTIVES

Disorders of glycogen metabolism include rare hereditary muscle glycogen storage diseases with polyglucosan, which are characterized by storage of abnormally structured glycogen in muscle in addition to exercise intolerance or muscle weakness. In this study, we investigated the etiology and pathogenesis of a late-onset myopathy associated with glycogenin-1 deficiency.

MATERIALS AND METHODS

A family with two affected siblings, 64- and 66-year-olds, was studied. Clinical examination and whole-body MRI revealed weakness and wasting in the hip girdle and proximal leg muscles affecting ambulation in the brother. The sister had weakness and atrophy of hands and slight foot dorsiflexion difficulties. Muscle biopsy and whole-exome sequencing were performed in both cases to identify and characterize the pathogenesis including the functional effects of identified mutations.

RESULTS

Both siblings demonstrated storage of glycogen that was partly resistant to alpha-amylase digestion. Both were heterozygous for two mutations in GYG1, one truncating 1-base deletion (c.484delG; p.Asp163Thrfs*5) and one novel missense mutation (c.403G>A; p.Gly135Arg). The mutations caused reduced expression of glycogenin-1 protein, and the missense mutation abolished the enzymatic function as analyzed by an in vitro autoglucosylation assay.

CONCLUSION

We present functional evidence for the pathogenicity of a novel GYG1 missense mutation located in the substrate binding domain. Our results also demonstrate that glycogenin-1 deficiency may present with highly variable distribution of weakness and wasting also in the same family.

GYG1 causing progressive limb girdle myopathy with onset during teenage years (polyglucosan body myopathy 2)

An 84-year-old lady with slowly progressive limb and axial muscle weakness with onset in her teens was referred for genetic investigations. Targeted next generation sequencing (NGS) revealed a homozygous mutation GYG1 in exon5:c.487delG:p.D163fs, confirming the diagnosis of Polyglucosan Body Myopathy 2 (PGBM2). Retrospective review of muscle pathology revealed a florid vacuolar myopathy with histochemical and ultrastructural features consistent with a polyglucosan storage myopathy. No cardiac symptoms were reported. Our case is consistent with the core phenotype of GYG1-related PGBM2 apart from an early onset of weakness without cardiac symptoms. The presence of α-amylase resistant PAS-positive material in skeletal muscle biopsy of patients with slowly progressive limb girdle muscle weakness should prompt the search for GYG1 mutations. This case highlights the combined role of muscle pathology and NGS in the molecular resolution of patients with undiagnosed neuromuscular conditions.

Clinical heterogeneity and phenotype/genotype findings in 5 families with GYG1 deficiency

Objective

To describe the variability of muscle symptoms in patients carrying mutations in the GYG1 gene, encoding glycogenin-1, an enzyme involved in the biosynthesis of glycogen, and to discuss genotype-phenotype relations.

Methods

We describe 9 patients from 5 families in whom muscle biopsies showed vacuoles with an abnormal accumulation of glycogen in muscle fibers, partially α-amylase resistant suggesting polyglucosan bodies. The patients had either progressive early-onset limb-girdle weakness or late-onset distal or scapuloperoneal muscle affection as shown by muscle imaging. No clear definite cardiac disease was found. Histologic and protein analysis investigations were performed on muscle.

Results

Genetic analyses by direct or exome sequencing of the GYG1 gene revealed 6 different GYG1 mutations. Four of the mutations were novel. They were compound heterozygous in 3 families and homozygous in 2. Protein analysis revealed either the absence of glycogenin-1 or reduced glycogenin-1 expression with impaired glucosylation.

Conclusions

Our report extends the genetic and clinical spectrum of glycogenin-1-related myopathies to include scapuloperoneal and distal affection with glycogen accumulation.

Muscle glycogen synthesis and breakdown are both impaired in glycogenin-1 deficiency

OBJECTIVE

To study fat and carbohydrate metabolism during exercise in patients with glycogenin-1 (GYG1) deficiency, and to study whether IV glucose supplementation can alleviate exercise intolerance in these patients.

METHODS

This is a case-control study with 4 patients with GYG1 deficiency and 4 healthy controls. Patients performed 1 hour of cycling at 50% of their maximal workload capacity, while controls cycled at the same absolute workloads as patients. Heart rate was measured continuously, and production and utilization of fat and glucose was assessed by stable isotope technique. The following day, patients repeated the exercise, this time receiving an IV 10% glucose supplement.

RESULTS

Glucose utilization during exercise was similar in patients and controls, while palmitate utilization was greater in patients compared to controls. However, exercise-induced increases in lactate were attenuated to about half normal in patients. This was also the case during a handgrip exercise test. Glucose infusion improved exercise tolerance in patients, and lowered heart rate by on average 11 beats per minute during exercise.

CONCLUSIONS

The findings suggest that patients with GYG1 deficiency not only have abnormal formation of glycogen, but also have impaired muscle glycogenolysis, as suggested by impaired lactate production during exercise and improved exercise tolerance with glucose infusion.

Glycogen Synthesis in Glycogenin 1-Deficient Patients: A Role for Glycogenin 2 in Muscle

CONTEXT

Glycogen storage disease (GSD) type XV is a rare disease caused by mutations in the GYG1 gene that codes for the core molecule of muscle glycogen, glycogenin 1. Nonetheless, glycogen is present in muscles of glycogenin 1-deficient patients, suggesting an alternative for glycogen buildup. A likely candidate is glycogenin 2, an isoform expressed in the liver and heart but not in healthy skeletal muscle.

OBJECTIVE

We wanted to investigate the formation of glycogen and changes in glycogen metabolism in patients with GSD type XV.

DESIGN, SETTING, AND PATIENTS

Two patients with mutations in the GYG1 gene were investigated for histopathology, ultrastructure, and expression of proteins involved in glycogen synthesis and metabolism.

RESULTS

Apart from occurrence of polyglucosan (PG) bodies in few fibers, glycogen appeared normal in most cells, and the concentration was normal in patients with GSD type XV. We found that glycogenin 1 was absent, but glycogenin 2 was present in the patients, whereas the opposite was the case in healthy controls. Electron microscopy revealed that glycogen was present between and not inside myofibrils in type II fibers, compromising the ultrastructure of these fibers, and only type I fibers contained PG bodies. We also found significant changes to the expression levels of several enzymes directly involved in glycogen and glucose metabolism.

CONCLUSIONS

To our knowledge, this is the first report demonstrating expression of glycogenin 2 in glycogenin 1-deficient patients, suggesting that glycogenin 2 rescues the formation of glycogen in patients with glycogenin 1 deficiency.

Lack of Glycogenin Causes Glycogen Accumulation and Muscle Function Impairment

Glycogenin is considered essential for glycogen synthesis, as it acts as a primer for the initiation of the polysaccharide chain. Against expectations, glycogenin-deficient mice (Gyg KO) accumulate high amounts of glycogen in striated muscle. Furthermore, this glycogen contains no covalently bound protein, thereby demonstrating that a protein primer is not strictly necessary for the synthesis of the polysaccharide in vivo. Strikingly, in spite of the higher glycogen content, Gyg KO mice showed lower resting energy expenditure and less resistance than control animals when subjected to endurance exercise. These observations can be attributed to a switch of oxidative myofibers toward glycolytic metabolism. Mice overexpressing glycogen synthase in the muscle showed similar alterations, thus indicating that this switch is caused by the excess of glycogen. These results may explain the muscular defects of GSD XV patients, who lack glycogenin-1 and show high glycogen accumulation in muscle.

Cardiomyopathy as presenting sign of glycogenin-1 deficiency-report of three cases and review of the literature

We describe a new type of cardiomyopathy caused by a mutation in the glycogenin-1 gene (GYG1). Three unrelated male patients aged 34 to 52 years with cardiomyopathy and abnormal glycogen storage on endomyocardial biopsy were homozygous for the missense mutation p.Asp102His in GYG1. The mutated glycogenin-1 protein was expressed in cardiac tissue but had lost its ability to autoglucosylate as demonstrated by an in vitro assay and western blot analysis. It was therefore unable to form the primer for normal glycogen synthesis. Two of the patients showed similar patterns of heart dilatation, reduced ejection fraction and extensive late gadolinium enhancement on cardiac magnetic resonance imaging. These two patients were severely affected, necessitating cardiac transplantation. The cardiomyocyte storage material was characterized by large inclusions of periodic acid and Schiff positive material that was partly resistant to alpha-amylase treatment consistent with polyglucosan. The storage material had, unlike normal glycogen, a partly fibrillar structure by electron microscopy. None of the patients showed signs or symptoms of muscle weakness but a skeletal muscle biopsy in one case revealed muscle fibres with abnormal glycogen storage. Glycogenin-1 deficiency is known as a rare cause of skeletal muscle glycogen storage disease, usually without cardiomyopathy. We demonstrate that it may also be the cause of severe cardiomyopathy and cardiac failure without skeletal muscle weakness. GYG1 should be included in cardiomyopathy gene panels.