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

Multiple origins and phenotypic implications of an extended human pseudoautosomal region shown by analysis of the UK Biobank

The 2.7-Mb major pseudoautosomal region (PAR1) on the short arms of the human X and Y chromosomes plays a critical role in meiotic sex chromosome segregation and male fertility and has been regarded as evolutionarily stable. However, some European Y chromosomes belonging to Y haplogroups (Y-Hgs) R1b and I2a carry an ∼115-kb extension (ePAR [extended PAR]) arising from X-Y non-allelic homologous recombination (NAHR). To investigate the diversity, history, and dynamics of ePAR formation, we screened for its presence, and that of the predicted reciprocal X chromosome deletion, among ∼218,300 46,XY males of the UK Biobank (UKB), a cohort associated with longitudinal clinical data. The UKB incidence of ePAR is ∼0.77%, and that of the deletion is ∼0.02%. We found that Y-Hg I2a sub-lineages accounted for nearly 90% of ePAR cases but, by Y haplotyping and breakpoint sequencing, determined that, in total, there have been at least 18 independent ePAR origins, associated with nine different Y-Hgs. We found examples of ePAR linked to Y-Hg K among men of self-declared Pakistani ancestry and Y-Hg E1, typical of men with African ancestry, showing that ePAR is not restricted to Europeans. ePAR formation is likely random, with high frequencies in some Y-Hgs arising through drift and male-mediated expansions. Sequencing recombination junction fragments identified likely reciprocal events, and the heterogeneity of ePAR and X-deletion junctions highlighted the recurrent nature of the NAHR events. A phenome-wide association study revealed an association between ePAR and elevated levels of circulating IGF-1 as well as musculoskeletal phenotypes.

Glycogen metabolism and structure: A review

Glycogen is a glucose polymer that plays a crucial role in glucose homeostasis by functioning as a short-term energy storage reservoir in animals and bacteria. Abnormalities in its metabolism and structure can cause several problems, including diabetes, glycogen storage diseases (GSDs) and muscular disorders. Defects in the enzymes involved in glycogen synthesis or breakdown, resulting in either excessive accumulation or insufficient availability of glycogen in cells seem to account for the most common pathogenesis. This review discusses glycogen metabolism and structure, including molecular architecture, branching dynamics, and the role of associated components within the granules. The review also discusses GSD type XV and Lafora disease, illustrating the broader implications of aberrant glycogen metabolism and structure. These conditions also impart information on important regulatory mechanisms of glycogen, which hint at potential therapeutic targets. Knowledge gaps and potential future research directions are identified.

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.

A century of exercise physiology: key concepts in regulation of glycogen metabolism in skeletal muscle

Glycogen is a branched, glucose polymer and the storage form of glucose in cells. Glycogen has traditionally been viewed as a key substrate for muscle ATP production during conditions of high energy demand and considered to be limiting for work capacity and force generation under defined conditions. Glycogenolysis is catalyzed by phosphorylase, while glycogenesis is catalyzed by glycogen synthase. For many years, it was believed that a primer was required for de novo glycogen synthesis and the protein considered responsible for this process was ultimately discovered and named glycogenin. However, the subsequent observation of glycogen storage in the absence of functional glycogenin raises questions about the true role of the protein. In resting muscle, phosphorylase is generally considered to be present in two forms: non-phosphorylated and inactive (phosphorylase b) and phosphorylated and constitutively active (phosphorylase a). Initially, it was believed that activation of phosphorylase during intense muscle contraction was primarily accounted for by phosphorylation of phosphorylase b (activated by increases in AMP) to a, and that glycogen synthesis during recovery from exercise occurred solely through mechanisms controlled by glucose transport and glycogen synthase. However, it now appears that these views require modifications. Moreover, the traditional roles of glycogen in muscle function have been extended in recent years and in some instances, the original concepts have undergone revision. Thus, despite the extensive amount of knowledge accrued during the past 100 years, several critical questions remain regarding the regulation of glycogen metabolism and its role in living muscle.

Myopathy can be a key phenotype of membrin (GOSR2) deficiency

T1-weighted, cross-sectional MR images showing shoulder girdle, abdominal, paraspinal, gluteal and thigh muscles almost completely replaced by fat, whereas lower leg muscles are almost unaffected i a patient who is compound heterozygous for pathogenic variants in GOSR2.

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.

Metastasis of Uveal Melanoma with Monosomy-3 Is Associated with a Less Glycogenetic Gene Expression Profile and the Dysregulation of Glycogen Storage

The prolonged storage of glucose as glycogen can promote the quiescence of tumor cells, whereas the accumulation of an aberrant form of glycogen without the primer protein glycogenin can induce the metabolic switch towards a glycolytic phenotype. Here, we analyzed the expression of n = 67 genes involved in glycogen metabolism on the uveal melanoma (UM) cohort of the Cancer Genome Atlas (TCGA) study and validated the differentially expressed genes in an independent cohort. We also evaluated the glycogen levels with regard to the prognostic factors via a differential periodic acid-Schiff (PAS) staining. UMs with monosomy-3 exhibited a less glycogenetic and more insulin-resistant gene expression profile, together with the reduction of glycogen levels, which were associated with the metastases. Expression of glycogenin-1 (Locus: 3q24) was lower in the monosomy-3 tumors, whereas the complementary isoform glycogenin-2 (Locus: Xp22.33) was upregulated in females. Remarkably, glycogen was more abundant in the monosomy-3 tumors of male versus female patients. We therefore provide the first evidence to the dysregulation of glycogen metabolism as a novel factor that may be aggravating the course of UM particularly in males.

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.

POPDC3 Gene Variants Associate with a New Form of Limb Girdle Muscular Dystrophy

OBJECTIVE

The Popeye domain containing 3 (POPDC3) gene encodes a membrane protein involved in cyclic adenosine monophosphate (cAMP) signaling. Besides gastric cancer, no disease association has been described. We describe a new muscular dystrophy associated with this gene.

METHODS

We screened 1,500 patients with unclassified limb girdle weakness or hyperCKemia for pathogenic POPDC3 variants. Five patients carrying POPDC3 variants were examined by muscle magnetic resonance imaging (MRI), muscle biopsy, and cardiac examination. We performed functional analyses in a zebrafish popdc3 knockdown model and heterologous expression of the mutant proteins in Xenopus laevis oocytes to measure TREK-1 current.

RESULTS

We identified homozygous POPDC3 missense variants (p.Leu155His, p.Leu217Phe, and p.Arg261Gln) in 5 patients from 3 ethnically distinct families. Variants affected highly conserved residues in the Popeye (p.Leu155 and p.Leu217) and carboxy-terminal (p.Arg261) domains. The variants were almost absent from control populations. Probands' muscle biopsies were dystrophic, and serum creatine kinase levels were 1,050 to 9,200U/l. Muscle weakness was proximal with adulthood onset in most patients and affected lower earlier than upper limbs. Muscle MRI revealed fat replacement of paraspinal and proximal leg muscles; cardiac investigations were unremarkable. Knockdown of popdc3 in zebrafish, using 2 different splice-site blocking morpholinos, resulted in larvae with tail curling and dystrophic muscle features. All 3 mutants cloned in Xenopus oocytes caused an aberrant modulation of the mechano-gated potassium channel, TREK-1.

INTERPRETATION

Our findings point to an important role of POPDC3 for skeletal muscle function and suggest that pathogenic variants in POPDC3 are responsible for a novel type of autosomal recessive limb girdle muscular dystrophy. ANN NEUROL 2019;86:832-843.

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.

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.