Immunoblot analyses of glycogen debranching enzyme in different subtypes of glycogen storage disease type III

Glycogen storage diseases: An update

Glycogen storage diseases (GSDs), also referred to as glycogenoses, are inherited metabolic disorders of glycogen metabolism caused by deficiency of enzymes or transporters involved in the synthesis or degradation of glycogen leading to aberrant storage and/or utilization. The overall estimated GSD incidence is 1 case per 20000-43000 live births. There are over 20 types of GSD including the subtypes. This heterogeneous group of rare diseases represents inborn errors of carbohydrate metabolism and are classified based on the deficient enzyme and affected tissues. GSDs primarily affect liver or muscle or both as glycogen is particularly abundant in these tissues. However, besides liver and skeletal muscle, depending on the affected enzyme and its expression in various tissues, multiorgan involvement including heart, kidney and/or brain may be seen. Although GSDs share similar clinical features to some extent, there is a wide spectrum of clinical phenotypes. Currently, the goal of treatment is to maintain glucose homeostasis by dietary management and the use of uncooked cornstarch. In addition to nutritional interventions, pharmacological treatment, physical and supportive therapies, enzyme replacement therapy (ERT) and organ transplantation are other treatment approaches for both disease manifestations and long-term complications. The lack of a specific therapy for GSDs has prompted efforts to develop new treatment strategies like gene therapy. Since early diagnosis and aggressive treatment are related to better prognosis, physicians should be aware of these conditions and include GSDs in the differential diagnosis of patients with relevant manifestations including fasting hypoglycemia, hepatomegaly, hypertransaminasemia, hyperlipidemia, exercise intolerance, muscle cramps/pain, rhabdomyolysis, and muscle weakness. Here, we aim to provide a comprehensive review of GSDs. This review provides general characteristics of all types of GSDs with a focus on those with liver involvement.

A retrospective longitudinal study and comprehensive review of adult patients with glycogen storage disease type III

INTRODUCTION

A deficiency of glycogen debrancher enzyme in patients with glycogen storage disease type III (GSD III) manifests with hepatic, cardiac, and muscle involvement in the most common subtype (type a), or with only hepatic involvement in patients with GSD IIIb.

OBJECTIVE AND METHODS

To describe longitudinal biochemical, radiological, muscle strength and ambulation, liver histopathological findings, and clinical outcomes in adults (≥18 years) with glycogen storage disease type III, by a retrospective review of medical records.

RESULTS

Twenty-one adults with GSD IIIa (14 F & 7 M) and four with GSD IIIb (1 F & 3 M) were included in this natural history study. At the most recent visit, the median (range) age and follow-up time were 36 (19-68) and 16 years (0-41), respectively. For the entire cohort: 40% had documented hypoglycemic episodes in adulthood; hepatomegaly and cirrhosis were the most common radiological findings; and 28% developed decompensated liver disease and portal hypertension, the latter being more prevalent in older patients. In the GSD IIIa group, muscle weakness was a major feature, noted in 89% of the GSD IIIa cohort, a third of whom depended on a wheelchair or an assistive walking device. Older individuals tended to show more severe muscle weakness and mobility limitations, compared with younger adults. Asymptomatic left ventricular hypertrophy (LVH) was the most common cardiac manifestation, present in 43%. Symptomatic cardiomyopathy and reduced ejection fraction was evident in 10%. Finally, a urinary biomarker of glycogen storage (Glc4) was significantly associated with AST, ALT and CK.

CONCLUSION

GSD III is a multisystem disorder in which a multidisciplinary approach with regular clinical, biochemical, radiological and functional (physical therapy assessment) follow-up is required. Despite dietary modification, hepatic and myopathic disease progression is evident in adults, with muscle weakness as the major cause of morbidity. Consequently, definitive therapies that address the underlying cause of the disease to correct both liver and muscle are needed.

Narrative review of glycogen storage disorder type III with a focus on neuromuscular, cardiac and therapeutic aspects

Glycogen storage disorder type III (GSDIII) is a rare inborn error of metabolism due to loss of glycogen debranching enzyme activity, causing inability to fully mobilize glycogen stores and its consequent accumulation in various tissues, notably liver, cardiac and skeletal muscle. In the pediatric population, it classically presents as hepatomegaly with or without ketotic hypoglycemia and failure to thrive. In the adult population, it should also be considered in the differential diagnosis of left ventricular hypertrophy or hypertrophic cardiomyopathy, myopathy, exercise intolerance, as well as liver cirrhosis or fibrosis with subsequent liver failure. In this review article, we first present an overview of the biochemical and clinical aspects of GSDIII. We then focus on the recent findings regarding cardiac and neuromuscular impairment associated with the disease. We review new insights into the pathophysiology and clinical picture of this disorder, including symptomatology, imaging and electrophysiology. Finally, we discuss current and upcoming treatment strategies such as gene therapy aimed at the replacement of the malfunctioning enzyme to provide a stable and long-term therapeutic option for this debilitating disease.

Metabolic Profiling in Human Fibroblasts Enables Subtype Clustering in Glycogen Storage Disease

Glycogen storage disease subtypes I and III (GSD I and GSD III) are monogenic inherited disorders of metabolism that disrupt glycogen metabolism. Unavailability of glucose in GSD I and induction of gluconeogenesis in GSD III modify energy sources and possibly, mitochondrial function. Abnormal mitochondrial structure and function were described in mice with GSD Ia, yet significantly less research is available in human cells and ketotic forms of the disease. We hypothesized that impaired glycogen storage results in distinct metabolic phenotypes in the extra- and intracellular compartments that may contribute to pathogenesis. Herein, we examined mitochondrial organization in live cells by spinning-disk confocal microscopy and profiled extra- and intracellular metabolites by targeted LC-MS/MS in cultured fibroblasts from healthy controls and from patients with GSD Ia, GSD Ib, and GSD III. Results from live imaging revealed that mitochondrial content and network morphology of GSD cells are comparable to that of healthy controls. Likewise, healthy controls and GSD cells exhibited comparable basal oxygen consumption rates. Targeted metabolomics followed by principal component analysis (PCA) and hierarchical clustering (HC) uncovered metabolically distinct poises of healthy controls and GSD subtypes. Assessment of individual metabolites recapitulated dysfunctional energy production (glycolysis, Krebs cycle, succinate), reduced creatinine export in GSD Ia and GSD III, and reduced antioxidant defense of the cysteine and glutathione systems. Our study serves as proof-of-concept that extra- and intracellular metabolite profiles distinguish glycogen storage disease subtypes from healthy controls. We posit that metabolite profiles provide hints to disease mechanisms as well as to nutritional and pharmacological elements that may optimize current treatment strategies.

Uniparental isodisomy of chromosome 1 results in glycogen storage disease type III with profound growth retardation

Background

Glycogen storage disease type III (GSDIII) is caused by mutations of AGL gene with debranching enzyme deficiency. Patients with GSDIII manifest fasting hypoglycemia, hepatomegaly, hepatopathy, myopathy, and cardiomyopathy. We report on an 18‐year‐old boy with a profound growth retardation (<3 SD) besides typical clinical features of GSDIII, whereby endocrinological studies were negative.

Methods and Results

Molecular analysis of AGL gene revealed the homozygous reported variant c.3903_3904insA. Since discordant results from segregation studies showed the carrier status in one parent only, SNP array and short tandem repeats analyses were performed, revealing a paternal disomy of chromosome 1 (UPD1).

Conclusion

This study describes the first case of GSDIII resulting from UPD1. UPD can play an important role even in case of imprinted genes. DIRAS3 is a maternally imprinted tumor suppressor gene, located on chromosome 1p31, and implicated in growth and oncogenesis. It can be speculated that DIRAS3 overexpression might have a role in the severe short stature of our patient. The study emphasizes the importance of parental segregation analysis especially in patients with recessive conditions to look for specific genetic causes of disease and to estimate properly the risk of family recurrence.

[Molecular and clinical characterization of Colombian patients suffering from type III glycogen storage disease]

INTRODUCTION

Type III glycogen storage disease (GSD III) is an autosomal recessive disorder in which a mutation in the AGL gene causes deficiency of the glycogen debranching enzyme. The disease is characterized by fasting hypoglycemia, hepatomegaly and progressive myopathy. Molecular analyses of AGL have indicated heterogeneity depending on ethnic groups. The full spectrum of AGL mutations in Colombia remains unclear.

OBJECTIVE

To describe the clinical and molecular characteristics of ten Colombian patients diagnosed with GSD III.

MATERIALS AND METHODS

We recruited ten Colombian children with a clinical and biochemical diagnosis of GSD III to undergo genetic testing. The full coding exons and the relevant exon-intron boundaries of the AGL underwent Sanger sequencing to identify mutation.

RESULTS

All patients had the classic phenotype of the GSD III. Genetic analysis revealed a mutation p.Arg910X in two patients. One patient had the mutation p.Glu1072AspfsX36, and one case showed a compound heterozygosity with p.Arg910X and p.Glu1072AspfsX36 mutations. We also detected the deletion of AGL gene 3, 4, 5, and 6 exons in three patients. The in silico studies predicted that these defects are pathogenic. No mutations were detected in the amplified regions in three patients.

CONCLUSION

We found mutations and deletions that explain the clinical phenotype of GSD III patients. This is the first report with a description of the clinical phenotype and the spectrum of AGL mutations in Colombian patients. This is important to provide appropriate prognosis and genetic counseling to the patient and their relatives.

A Novel Nonsense Mutation of the AGL Gene in a Romanian Patient with Glycogen Storage Disease Type IIIa

Background. Glycogen storage disease type III (GSDIII) is a rare metabolic disorder with autosomal recessive inheritance, caused by deficiency of the glycogen debranching enzyme. There is a high phenotypic variability due to different mutations in the AGL gene. Methods and Results. We describe a 2.3-year-old boy from a nonconsanguineous Romanian family, who presented with severe hepatomegaly with fibrosis, mild muscle weakness, cardiomyopathy, ketotic fasting hypoglycemia, increased transaminases, creatine phosphokinase, and combined hyperlipoproteinemia. GSD type IIIa was suspected. Accordingly, genomic DNA of the index patient was analyzed by next generation sequencing of the AGL gene. For confirmation of the two mutations found, genetic analysis of the parents and grandparents was also performed. The patient was compound heterozygous for the novel mutation c.3235C>T, p.Gln1079(⁎) (exon 24) and the known mutation c.1589C>G, p.Ser530(⁎) (exon 12). c.3235 >T, p.Gln1079(⁎) was inherited from the father, who inherited it from his mother. c.1589C>G, p.Ser530(⁎) was inherited from the mother, who inherited it from her father. Conclusion. We report the first genetically confirmed case of a Romanian patient with GSDIIIa. We detected a compound heterozygous genotype with a novel mutation, in the context of a severe hepatopathy and an early onset of cardiomyopathy.

Exercise in muscle glycogen storage diseases

Glycogen storage diseases (GSD) are inborn errors of glycogen or glucose metabolism. In the GSDs that affect muscle, the consequence of a block in skeletal muscle glycogen breakdown or glucose use, is an impairment of muscular performance and exercise intolerance, owing to 1) an increase in glycogen storage that disrupts contractile function and/or 2) a reduced substrate turnover below the block, which inhibits skeletal muscle ATP production. Immobility is associated with metabolic alterations in muscle leading to an increased dependence on glycogen use and a reduced capacity for fatty acid oxidation. Such changes may be detrimental for persons with GSD from a metabolic perspective. However, exercise may alter skeletal muscle substrate metabolism in ways that are beneficial for patients with GSD, such as improving exercise tolerance and increasing fatty acid oxidation. In addition, a regular exercise program has the potential to improve general health and fitness and improve quality of life, if executed properly. In this review, we describe skeletal muscle substrate use during exercise in GSDs, and how blocks in metabolic pathways affect exercise tolerance in GSDs. We review the studies that have examined the effect of regular exercise training in different types of GSD. Finally, we consider how oral substrate supplementation can improve exercise tolerance and we discuss the precautions that apply to persons with GSD that engage in exercise.

Inherent lipid metabolic dysfunction in glycogen storage disease IIIa

We studied two patients from a nonconsanguineous family with life-long abnormal liver function, hepatomegaly and abnormal fatty acid profiles. Abnormal liver function, hypoglycemia and muscle weakness are observed in various genetic diseases, including medium-chain acyl-CoA dehydrogenase (MCAD) deficiency and glycogen storage diseases. The proband showed increased free fatty acids, mainly C8 and C10, resembling fatty acid oxidation disorder. However, no mutation was found in ACADM and ACADL gene. Sequencing of theamylo-alpha-1, 6-glucosidase, 4-alpha-glucanotransferase (AGL) gene showed that both patients were compound heterozygotes for c.118C > T (p.Gln40X) and c.753_756 del CAGA (p.Asp251Glufsx29), whereas their parents were each heterozygous for one of these mutations. The AGL protein was undetectable in EBV-B cells from the two patients. Transcriptome analysis demonstrated a significant different pattern of gene expression in both of patients’ cells, including genes involving in the PPAR signaling pathway, fatty acid biosynthesis, lipid synthesis and visceral fat deposition and metabolic syndrome. This unique gene expression pattern is probably due to the absence of AGL, which potentially accounts for the observed clinical phenotypes of hyperlipidemia and hepatocyte steatosis in glycogen storage disease type IIIa.

Exercise intolerance in Glycogen Storage Disease Type III: weakness or energy deficiency?

Myopathic symptoms in Glycogen Storage Disease Type IIIa (GSD IIIa) are generally ascribed to the muscle wasting that these patients suffer in adult life, but an inability to debranch glycogen likely also has an impact on muscle energy metabolism. We hypothesized that patients with GSD IIIa can experience exercise intolerance due to insufficient carbohydrate oxidation in skeletal muscle. Six patients aged 17-36-years were studied. We determined VO 2peak (peak oxygen consumption), the response to forearm exercise, and the metabolic and cardiovascular responses to cycle exercise at 70% of VO 2peak with either a saline or a glucose infusion. VO 2peak was below normal. Glucose improved the work capacity by lowering the heart rate, and increasing the peak work rate by 30% (108 W with glucose vs. 83 W with placebo, p=0.018). The block in muscle glycogenolytic capacity, combined with the liver involvement caused exercise intolerance with dynamic skeletal muscle symptoms (excessive fatigue and muscle pain), and hypoglycemia in 4 subjects. In this study we combined anaerobic and aerobic exercise to systematically study skeletal muscle metabolism and exercise tolerance in patients with GSD IIIa. Exercise capacity was significantly reduced, and our results indicate that this was due to a block in muscle glycogenolytic capacity. Our findings suggest that the general classification of GSD III as a glycogenosis characterized by fixed symptoms related to muscle wasting should be modified to include dynamic exercise-related symptoms of muscle fatigue. A proportion of the skeletal muscle symptoms in GSD IIIa, i.e. weakness and fatigue, may be related to insufficient energy production in muscle.

Mutation Analysis in Glycogen Storage Disease Type III Patients in the Netherlands: Novel Genotype-Phenotype Relationships and Five Novel Mutations in the AGL Gene

Glycogen Storage Disease type III (GSD III) is an autosomal recessive disorder in which a mutation in the AGL gene causes deficiency of the glycogen debranching enzyme. In childhood, it is characterized by hepatomegaly, keto-hypoglycemic episodes after short periods of fasting, and hyperlipidemia. In adulthood, myopathy, cardiomyopathy, and liver cirrhosis are the main complications. To determine the genotype of the GSD III patients (n = 14) diagnosed and treated in our center, mutation analysis was performed by either denaturing gradient gel electrophoresis or full gene sequencing. We developed, validated and applied both methods, and in all patients a mutation was identified on both alleles. Five novel pathogenic mutations were identified in seven patients, including four missense mutations (c.643G>A, p.Asp215Asn; c.655A>G, p.Asn219Asp; c.1027C>T, p.Arg343Trp; c.1877A>G, p.His626Arg) and one frameshift mutation (c.3911delA, p.Asn1304fs). The c.643G>A, p.Asp215Asn mutation is related with type IIIa, as this mutation was found homozygously in two type IIIa patients. In addition to five novel mutations, we present new genotype-phenotype relationships for c.2039G>A, p.Trp680X; c.753_756delCAGA, p.Asp251fs; and the intron 32 c.4260-12A>G splice site mutation. The p.Trp680X mutation was found homozygously in four patients, presenting a mild IIIa phenotype with mild skeletal myopathy, elevated CK values, and no cardiomyopathy. The p.Asp251fs mutation was found homozygously in one patient presenting with a severe IIIa phenotype, with skeletal myopathy, and severe symptomatic cardiomyopathy. The c.4260-12A>G mutation was found heterozygously, together with the p.Arg343Trp mutation in a severe IIIb patient who developed liver cirrhosis and hepatocellular carcinoma, necessitating an orthotopic liver transplantation.

Molecular and biochemical characterization of Tunisian patients with glycogen storage disease type III

Glycogen storage disease type III (GSD III) is an autosomal recessive inborn error of metabolism caused by mutations in the glycogen debranching enzyme amylo-1,6-glucosidase gene, which is located on chromosome 1p21.2. GSD III is characterized by the storage of structurally abnormal glycogen, termed limit dextrin, in both skeletal and cardiac muscle and/or liver, with great variability in resultant organ dysfunction. The spectrum of AGL gene mutations in GSD III patients depends on ethnic group. The most prevalent mutations have been reported in the North African Jewish population and in an isolate such as the Faroe Islands. Here, we present the molecular and biochemical analyses of 22 Tunisian GSD III patients. Molecular analysis revealed three novel mutations: nonsense (Tyr1148X) and two deletions (3033_3036del AATT and 3216_3217del GA) and five known mutations: three nonsense (R864X, W1327X and W255X), a missense (R524H) and an acceptor splice-site mutation (IVS32-12A>G). Each mutation is associated to a specific haplotype. This is the first report of screening for mutations of AGL gene in the Tunisian population.

Inborn errors of energy metabolism associated with myopathies

Inherited neuromuscular disorders affect approximately one in 3,500 children. Structural muscular defects are most common; however functional impairment of skeletal and cardiac muscle in both children and adults may be caused by inborn errors of energy metabolism as well. Patients suffering from metabolic myopathies due to compromised energy metabolism may present with exercise intolerance, muscle pain, reversible or progressive muscle weakness, and myoglobinuria. In this review, the physiology of energy metabolism in muscle is described, followed by the presentation of distinct disorders affecting skeletal and cardiac muscle: glycogen storage diseases types III, V, VII, fatty acid oxidation defects, and respiratory chain defects (i.e., mitochondriopathies). The diagnostic work-up and therapeutic options in these disorders are discussed.

Egyptian glycogen storage disease type III - identification of six novel AGL mutations, including a large 1.5 kb deletion and a missense mutation p.L620P with subtype IIId

BACKGROUND

Glycogen storage disease type III (GSD III) is caused by mutations in AGL which encodes for a single protein with two enzyme activities: oligo-1, 4-1, 4-glucantransferase (transferase) and amylo-1, 6-glucosidase. Activity of both enzymes is lost in most patients with GSD III, but in the very rare subtype IIId, transferase activity is deficient. Since the spectrum of AGL mutations is dependent on the ethnic group, we investigated the clinical and molecular characteristics in Egyptian patients with GSD III.

METHODS

Clinical features were examined in five Egyptian patients. AGL was sequenced and AGL haplotypes were determined.

RESULTS

Six novel AGL mutations were identified: a large deletion (c.3481-3588+1417del1525 bp), two insertions (c.1389insG and c.2368insA), two small deletions (c.2223-2224delGT and c.4041delT), and a missense mutation (p.L620P). p.L620P was found in a patient with IIId. Each mutation was located on a different AGL haplotype.

CONCLUSIONS

Our results suggest that there is allelic and phenotypic heterogeneity of GSD III in Egypt. This is the second description of a large deletion in AGL. p.L620P is the second mutation found in GSD IIId.

Distinct mutations in the glycogen debranching enzyme found in glycogen storage disease type III lead to impairment in diverse cellular functions

Glycogen storage disease type III (GSDIII) is a metabolic disorder characterized by a deficiency in the glycogen debranching enzyme, amylo-1,6-glucosidase,4-alpha-glucanotransferase (AGL). Patients with GSDIII commonly exhibit hypoglycemia, along with variable organ dysfunction of the liver, muscle or heart tissues. The AGL protein binds to glycogen through its C-terminal region, and possesses two separate domains for the transferase and glucosidase activities. Most causative mutations are nonsense, and how they affect the enzyme is not well understood. Here we investigated four rare missense mutations to determine the molecular basis of how they affect AGL function leading to GSDIII. The L620P mutant primarily abolishes transferase activity while the R1147G variant impairs glucosidase function. Interestingly, mutations in the carbohydrate-binding domain (CBD; G1448R and Y1445ins) are more severe in nature, leading to significant loss of all enzymatic activities and carbohydrate binding ability, as well as enhancing targeting for proteasomal degradation. This region (Y1445-G1448R) displays virtual identity across human and bacterial species, suggesting an important role that has been conserved throughout evolution. Our results clearly indicate that inactivation of either enzymatic activity is sufficient to cause GSDIII disease and suggest that the CBD of AGL plays a major role to coordinate its functions and regulation by the ubiquitin-proteasome system.

Glycogen storage diseases: New perspectives

Glycogen storage diseases (GSD) are inherited metabolic disorders of glycogen metabolism. Different hormones, including insulin, glucagon, and cortisol regulate the relationship of glycolysis, gluconeogenesis and glycogen synthesis. The overall GSD incidence is estimated 1 case per 20000-43000 live births. There are over 12 types and they are classified based on the enzyme deficiency and the affected tissue. Disorders of glycogen degradation may affect primarily the liver, the muscle, or both. Type Ia involves the liver, kidney and intestine (and Ib also leukocytes), and the clinical manifestations are hepatomegaly, failure to thrive, hypoglycemia, hyperlactatemia, hyperuricemia and hyperlipidemia. Type IIIa involves both the liver and muscle, and IIIb solely the liver. The liver symptoms generally improve with age. Type IV usually presents in the first year of life, with hepatomegaly and growth retardation. The disease in general is progressive to cirrhosis. Type VI and IX are a heterogeneous group of diseases caused by a deficiency of the liver phosphorylase and phosphorylase kinase system. There is no hyperuricemia or hyperlactatemia. Type XI is characterized by hepatic glycogenosis and renal Fanconi syndrome. Type II is a prototype of inborn lysosomal storage diseases and involves many organs but primarily the muscle. Types V and VII involve only the muscle.

Glucogenosis tipo III asociada a carcinoma hepatocelular

Type III glycogen storage disease is a hereditary disorder with autosomal recessive transmission. It is characterized by accumulation of abnormal glycogen in the liver and, in 80% of patients, in muscle. The liver can also show fibrosis and sometimes cirrhosis. Until 2000, 9 cases of cirrhosis had been published, 3 of which showed associated hepatocarcinoma. We present the case of a 31-year-old woman, diagnosed in childhood with type III glycogen storage disease, who 30 years after onset developed a hepatocellular carcinoma with portal thrombosis in the context of advanced cirrhosis. This is the first case to be reported in the Spanish literature of type III glycogen storage disease associated with hepatocellular carcinoma.

DNA-based subtyping of glycogen storage disease type III: mutation and haplotype analysis of the AGL gene in Chinese

Glycogen storage disease type III (GSD III) is an inborn error of glycogen metabolism caused by a deficiency of glycogen debranching enzyme (AGL). Here, we investigate two unrelated Hong Kong Chinese GSD III patients and identify a novel 5-base pair deletional mutation, 2715_2719delTCAGAin exon 22, in one patient and a nonsense mutation, 1222C>T (R408X) in exon 11, in another patient. Since GSD IIIb is only caused by mutation in exon 3 of the AGL gene, we diagnose our patients to have GSD IIIa, which is consistent with the clinical diagnosis. Until now, R408X has only been reported in Faroe Islands GSDIII patients and was thought to demonstrate a founder effect. In this study, haplotyping of the disease-bearing chromosomes in the AGL locus by 19 intragenic single nucleotide polymorphisms shows that R408X is linked with IVS16+8T and IVS23-21T in our patient while R408X is linked with IVS16+8C and IVS23-21A in the Faroe Islands. The different haplotypes of R408X in Chinese and Faroese indicated that R408X is a recurrent mutation.

Clinical and genetic variability of glycogen storage disease type IIIa: seven novel AGL gene mutations in the Mediterranean area

Deficiency of amylo-1,6-glucosidase, 4-alpha-glucanotransferase enzyme (AGL or glycogen debrancher enzyme) is responsible for glycogen storage disease type III, a rare autosomal recessive disorder of glycogen metabolism. The AGL gene is located on chromosome 1p21, and contains 35 exons translated in a monomeric protein product. The disease has recognized clinical and biochemical heterogeneity, reflecting the genotype-phenotype heterogeneity among different subjects. The clinical manifestations of GSD III are represented by hepatomegaly, hypoglycemia, hyperlipidemia, short stature and, in a number of subjects, cardiomyopathy and myopathy. In this article, we discuss the genotypic-phenotypic heterogeneity of GSD III by the molecular characterization of mutations responsible for the disease on a collection of 18 independent alleles from the Mediterranean area. We identified by heteroduplex band shift, DNA direct sequencing, and restriction analysis, seven novel mutations (four nonsense point-mutations: R34X, S530X, R1218X, W1398X; two microinsertions: 1072insT and 4724insAA; and one bp deletion: 676DeltaG), together with two new cases carrying a IVS21 + 1 G --> A splicing site mutation previously described in Italian patients. Altogether, 15 alleles were characterized. The correlation between type of mutation and clinical severity was studied in six patients in whom both mutated alleles were detected. Our data confirm the extreme genetic heterogeneity of this disease, thus precluding a strategy of mutation finding based on screening of recurrent common mutations.

Novel exon 11 skipping mutation in a patient with glycogen storage disease type IIId

We report the molecular genetic abnormalities of a patient with GSD IIId presenting with progressive myopathy and cardiopathy leading to a fatal outcome. We identified two independent deletions including a 4 bp deletion (117-1120) and a 98 bp deletion (1135-1232) in cDNA. Sequencing of the genomic DNA of the corresponding region revealed a 4 bp deletion in exon 10; however, the other 98 bp deletion corresponding to exon 11, which was deleted in cDNA, was present in genomic DNA. We therefore concluded that skipping of exon 11 occurred in the cDNA of the patient. Intron/exon boundary analysis of the skipped exon 11 revealed no mutation in the consensus splice-site sequence. If normal splicing had occurred, a stop codon would have appeared within exon II due to frameshift mutation. The mechanism of exon skipping observed in our patient is as yet unknown, and it is still not clear whether intraexonal mutation of the preceding exon can influence splice-site selection. It is possible that a unique exon skipping occurred, preventing the appearance of a stop codon in our patient.

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.

Hepatocellular carcinoma complicating liver cirrhosis in type IIIa glycogen storage disease

Type III glycogen storage disease (GSD III) is an autosomal recessive disorder characterized by the accumulation of abnormal glycogen in the liver and, in most patients, in the muscle. Although liver fibrosis is a well-known consequence of GSD III, until now only eight cases of liver cirrhosis and two cases of hepatocellular carcinoma have been described in patients affected by this disease. In this case report, the authors describe the clinical history of a patient affected by GSD III who developed severe liver disease during her adult life, progressing from fibrosis to cirrhosis and finally to hepatocellular carcinoma. Until now, the hepatic involvement in GSD III has been considered by most authors as mild and almost always self-limiting. This report, together with the previously published cases, clearly indicates that severe and progressive liver disease may complicate this metabolic disorder. These observations advise a careful hepatologic follow-up of patients affected by GSD III.

Novel mutations in two Japanese cases of glycogen storage disease type IIIa and a review of the literature of the molecular basis of glycogen storage disease type III

We report two novel mutations in two Japanese patients with glycogen storage disease type IIIa (GSD IIIa). In addition, we review the literature on mutations in GSD III to understand better the molecular basis of GSD III. In our first case, the homozygous A-to-C mutation at the acceptor site of intron 5 (IVS5-2A > C) was identified. This leads to the skipping of exon 6 and the predicted mutant protein was found to be 68 amino acids shorter than normal. This is the first report of skipping exon 6, which encodes one of the putative active sites, resulting in a profoundly deleterious effect on debrancher activity. In our second case, the homozygous deletion of an A at position 4234 (4234delA) was identified; this induces a frameshift resulting in the appearance of a stop codon at amino acid position 1276 (1276X). In patients with GSD IIIa, several mutations of the debrancher gene located in the C-terminal region containing putative glycogen binding domains have been identified as well as 4234delA in our second case. On the other hand, specific localization of the mutations within exon 3 was proposed in patients with GSD IIIb.

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.

Genotype-phenotype correlation in two frequent mutations and mutation update in type III glycogen storage disease

Deficiency of glycogen debranching enzyme (AGL) activity causes glycogen storage disease type III (GSD-III). Generalized loss of AGL activity results in GSD-IIIa, and muscle-specific retention of AGL activity results in GSD-IIIb. To date, no common mutation has been described among GSD-III patients, except for three alleles; two linked specifically with GSD-IIIb, and the third found only in North African Jews with GSD-IIIa. Here we report two frequent mutations, each of which was found in the homozygous state in multiple patients, and each of which was associated with a subset of clinical phenotype in those patients with that mutation. A novel point mutation of a single T deletion at cDNA position 3964 (3964delT) was first detected in an African American patient, who has a severe phenotype and early onset of clinical symptoms. The second mutation was an A to G transition at position -12 upstream of the 3' splice site of intron 32 (IVS32-12A > G). This lesion, previously implicated as a IIIb mutation in a Japanese patient, was identified in a confirmed GSD-IIIa Caucasian patient presenting with mild clinical symptoms. These two mutations together account for more than 12% of the molecular defects in the GSD-III patients tested. Our molecular and clinical data suggest a genotype-phenotype correlation for each of these mutations. Furthermore, this current study, coupled with our previous reports, describes the molecular tools necessary for the development of a DNA-based diagnostic test for GSD-III.

Glycogen storage diseases. Phenotypic, genetic, and biochemical characteristics, and therapy

The glycogen storage diseases are caused by inherited deficiencies of enzymes that regulate the synthesis or degradation of glycogen. In the past decade, considerable progress has been made in identifying the precise genetic abnormalities that cause the specific impairments of enzyme function. Likewise, improved understanding of the pathophysiologic derangements resulting from individual enzyme defects has led to the development of effective nutritional therapies for each of these disorders. Meticulous adherence to dietary therapy prevents hypoglycemia, ameliorates the biochemical abnormalities, decreases the size of the liver, and results in normal or nearly normal physical growth and development. Nevertheless, serious long-term complications, including nephropathy that can cause renal failure and hepatic adenomata that can become malignant, are a major concern in GSD-I. In GSD-III, the risk for hypoglycemia diminishes with age, and the liver decreases in size during puberty. Cirrhosis develops in some adult patients, and progressive myopathy and cardiomyopathy occur in patients with absent GDE activity in muscle. It remains unclear whether these complications of glycogen storage disease can be prevented by dietary therapy. Glycogen storage diseases caused by lack of phosphorylase activity are milder disorders with a good prognosis. The liver decreases in size, and biochemical abnormalities disappear by puberty.

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.

Neonatal metabolic myopathies

The primary presentations of neuromuscular disease in the newborn period are hypotonia and weakness. Although metabolic myopathies are inherited disorders that present from birth and may present with subtle to marked neonatal hypotonia, a number of these defects are diagnosed classically in childhood, adolescence, or adulthood. Disorders of glycogen, lipid, or mitochondrial metabolism may cause three main clinical syndromes in muscle, namely, (1) progressive weakness with hypotonia (e.g., acid maltase, debrancher enzyme, and brancher enzyme deficiencies among the glycogenoses; carnitine uptake and carnitine acylcarnitine translocase defects among the fatty acid oxidation (FAO) defects; and cytochrome oxidase deficiency among the mitochondrial disorders) or (2) acute, recurrent, reversible muscle dysfunction with exercise intolerance and acute muscle breakdown or myoglobinuria (with or without cramps), e.g., phosphorylase, phosphofructokinase, and phosphoglycerate kinase among the glycogenoses and carnitine palmitoyltransferase II deficiency among the disorders of FAO or (3) both (e.g., long-chain or very long-chain acyl coenzyme A (CoA) dehydrogenase, short-chain L-3-hydroxyacyl-CoA dehydrogenase, and trifunctional protein deficiencies among the FAO defects). Episodes of exercise-induced myoglobinuria tend to present in later childhood or adolescence; however, myoglobinuria in the first year of life may occur in FAO disorders during catabolic crises precipitated by fasting or infection. The following is a survey of genetic disorders of glycogen and lipid metabolism resulting in myopathy, focusing primarily on those defects, to date, that have presented in the neonatal or early infancy period. Disorders of mitochondrial metabolism are discussed in another chapter.

Case report: rupture of a gastric varix in liver cirrhosis associated with glycogen storage disease type III

Glycogen storage disease type III, or Cori's disease, is caused by a deficiency of amylo-1,6-glucosidase (debranching enzyme), which leads to the storage of an abnormal glycogen in the liver and in skeletal and heart muscle. Glycogen storage disease type III is usually characterized by hepatic symptoms, growth failure and myopathy. Even though liver cirrhosis is reported, portal hypertension is a rare complication of this disease. We describe the case of a glycogen storage disease type III patient who was diagnosed at 3 years of age and developed complications (liver cirrhosis and rupture of a gastric varix) at 31 years of age. We discuss the histological progression to cirrhosis of the liver and describe the liver enzyme profile at 3 and 31 years of age.

Isolation and nucleotide sequence of human liver glycogen debranching enzyme mRNA: identification of multiple tissue-specific isoforms

Glycogen storage disease type III (GSD-III) is caused by a deficiency of glycogen debranching enzyme (AGL) activity. Patients are found to have deficient AGL activity in both muscle and liver, and also enzyme deficiency in the liver, but not in muscle. To determine the molecular basis of enzymatic variability in GSD-III and to elucidate the mechanism for control of tissue-specific expression of AGL, we previously cloned and sequenced the human muscle AGL cDNA. Here we report the isolation and nucleotide sequence of liver AGL cDNA and the tissue distribution of the isoform mRNAs. The predominant form of human liver AGL cDNA (isoform 1) contained 400 bp of 5' untranslated region, 4596 bp of coding region, and 2371 bp of 3' untranslated region. The liver AGL mRNA sequence was identical to the previously published muscle sequence (isoform 5) for most of the length, except for the 5' end, in which the liver sequence diverged completely from the muscle sequence. The divergence began with the transcription start point and extended 82 nucleotides downstream from the translation initiation codon. Six isoforms of AGL mRNA were identified and sequenced from liver and muscle. These isoforms differed only at the 5' end. Tissue distribution studies showed that liver, kidney and lymphoblastoid cells expressed predominantly isoform 1; whereas muscle and heart expressed not only isoform 1, but also muscle-specific isoform mRNAs (isoforms 2, 3 and 4). Defining tissue-specific AGL isoform mRNAs is an important step toward understanding the molecular basis of enzymatic variability in GSD-III.

Comparison of the functional significance of left ventricular hypertrophy in hypertrophic cardiomyopathy and glycogenosis type III

A comparison of blood pressure response with exercise stress, thallium scintigraphy, and 24-hour electrocardiographic monitoring between 5 patients with left ventricular hypertrophy associated with glycogen storage disease type III and 10 matched patients with hypertrophic cardiomyopathy revealed normal results in the former group. These data highlight the importance of the etiology of left ventricular hypertrophy before the application of risk stratification.

Polymorphic markers of the glycogen debranching enzyme gene allowing linkage analysis in families with glycogen storage disease type III

Glycogen storage disease type III (GSD-III), an autosomal recessive disease, is caused by deficient glycogen debranching enzyme (GDE) activity. We identified three polymorphic markers in the GDE gene using single strand conformation polymorphism (SSCP) analysis and DNA sequencing. They were -10G/A in the 5' non-translated region of exon 3,2001 + 8C/T in intron 16, and 3199C/T (P1067S) in exon 25. Two polymorphic markers (-10G/A and 2001 + 8C/T) were highly informative in both controls and GSD-III patients with heterozygosity values of 0.50 and 0.46, respectively. The third marker (3199C/T) had a heterozygosity value of 0.26. Restriction analysis of the PCR amplified genomic DNA products in two GSD-III families showed for the first time the potential use of these markers for carrier detection and prenatal diagnosis in this disease.

A nonsense mutation due to a single base insertion in the 3'-coding region of glycogen debranching enzyme gene associated with a severe phenotype in a patient with glycogen storage disease type IIIa

Glycogen storage disease type III (GSD-III) is an autosomal recessive disease resulting from deficient glycogen debranching enzyme (GDE) activity. A child with GDE deficient in both liver and muscle (GSD-IIIa) had recurrent hypoglycemia, seizures, severe cardiomegaly, and hepatomegaly and died at 4 years of age. Analysis of the GDE gene in this child by single-strand conformation polymorphism, followed by direct DNA sequencing and restriction analysis, revealed an insertion of a nucleotide A into position 4529 of the GDE cDNA (4529insA). This insertion resulted in substitution of a tyrosine to a stop codon at amino acid 1510 (Y1510X). The 4529insA mutation appeared to be homozygous in this patient and was not found in 20 unrelated controls or 18 other GSD-III patients (14 GSD-IIIa and 4 GSD-IIIb). This is the first identification of a disease mutation in this gene, and the data suggest that homozygous 4529insA may be associated with a severe phenotype in GSD-IIIa.

Mutations in exon 3 of the glycogen debranching enzyme gene are associated with glycogen storage disease type III that is differentially expressed in liver and muscle

Glycogen storage disease type HI (GSD-III), an autosomal recessive disease, is caused by deficient glycogen debranching enzyme (GDE) activity. Most GSD-III patients are GDE deficient in both liver and muscle (type IIIa), and some GSD-III patients have GDE absent in liver but retained in muscle (type IIIb). The molecular basis for this enzymatic variability is largely unknown. In the present study, the analysis of the GDE gene in three GSD-IIIb patients by single-strand conformation polymorphism (SSCP), DNA sequencing, restriction analysis, and family studies, revealed each of them as being a compound heterozygote for two different mutations. The first mutant alleles in all three patients involved mutations in exon 3 at amino acid codon 6 of the GDE protein. Two had an AG deletion at nucleotides 17 and 18 of the GDE cDNA (17delAG) which resulted in change of subsequent amino acid sequence and a truncated protein (25X); the other had a C to T transition at nucleotide 16 of the cDNA which changed a Glutamine codon to a stop codon (Q6X). The 17delAG mutation was also found in 8 of the 10 additional GSD-IIIb patients. The Q6X mutation was found in one of the remaining two GSD-IIIb patients. These two mutations were not found in any of the 31 GSD-IIIa patients, 2 GSD-IIId patients, nor 28 unrelated normal controls. The second mutant alleles in each of the three GSD-IIIb patients were R864X, R1228X, and W68OX. The R864X and R1228X were not unique for GSD-IIIb as they were also found in GSD-IIIa patients (frequency of 10.3% and 5.2% in Caucasian patients, respectively). Our data demonstrated that both IIIa and IIIb had mutations in the same GDE gene and established for the first time the molecular basis of GSD-III that differentially expressed in liver and muscle. The striking and specific association of exon 3 mutations with GSD-IIIb may provide insight into mechanisms controlling tissue-specific expression of the GDE gene. The identification of exon 3 mutations has clinical significance as well because it distinguished GSD-IIIb from IIIa hence permitting diagnosis from a blood sample rather than a more invasive muscle biopsy.

Metabolic myopathies

Disorders of glycogen, lipid or mitochondrial metabolism may cause two main clinical syndromes, namely (1) progressive weakness (eg, acid maltase, debrancher enzyme, and brancher enzyme deficiencies among the glycogenoses; long- and very-long-chain acyl-CoA dehydrogenase (LCAD, VLCAD), and trifunctional enzyme deficiencies among the fatty acid oxidation (FAO) defects; and mitochondrial enzyme deficiencies) or (2) acute, recurrent, reversible muscle dysfunction with exercise intolerance and acute muscle breakdown or myoglobinuria (with or without cramps) (eg, phosphorylase (PPL), phosphorylase b kinase (PBK), phosphofructokinase (PFK), phosphoglycerate kinase (PGK), phosphoglycerate mutase (PGAM), and lactate dehydrogenase (LDH) among the glycogenoses and carnitine palmitoyltransferase II (CPT II) deficiency among the disorders of FAO or (3) both (eg, PPL, PBK, PFK among the glycogenoses; LCAD, VLCAD, short-chain L-3-hydroxyacyl-CoA dehydrogenase (SCHAD), and trifunctional enzyme deficiencies among the FAO defects; and multiple mitochondrial DNA (mtDNA) deletions). Myoadenylate deaminase deficiency, a purine nucleotide cycle defect, is somewhat controversial and is characterized by exercise-related cramps leading rarely to myoglobinuria.

A man with type III glycogenosis associated with cirrhosis and portal hypertension

Type III glycogenosis, an inherited disorder of glycogen metabolism that results from reduced or absent activity of the enzyme amylo-1,6-glycosidase (debranching enzyme), has not been frequently associated with cirrhosis and portal hypertension in adults. An adult Caucasian man with well-document type IIIa glycogenosis, who presented with a variceal hemorrhage secondary to hepatic cirrhosis, is described here. No other cause of cirrhosis was found.

Hypoglycemia in infants

Because the infant's brain is to a large extent dependent on glucose utilization, hypoglycemia of infants can have grave effects on brain function, and it is important to diagnose it and, when possible, treat it promptly. Causes of hypoglycemia in infants are (a) excess insulin secretion, (b) factitious hyperinsulinemia, (c) GH or ACTH deficiency, (d) primary glucocorticoid deficiency, (e) defects of the enzymes involved in hepatic glucose production, or (f) defects in hepatic fatty acid oxidation.

Glycogen debranching enzyme deficiency: long-term study of serum enzyme activities and clinical features

In glycogen storage disease type III (glycogen debranching enzyme (DE) deficiency), the activities of serum alanine aminotransferase, aspartate aminotransferase and lactate dehydrogenase may be strikingly elevated during childhood but are low during adult life. To determine the pattern of the elevated serum enzyme activities in relationship to diet, the biochemical subtype and clinical symptoms, 13 patients with DE deficiency were studied. Activities of serum aspartate and alanine transaminases, lactate dehydrogenase, and alkaline phosphatase were markedly elevated during infancy. Continued elevation of enzyme activities during childhood appeared to be related to DE deficiency in liver, but unrelated to DE deficiency in muscle. Activity elevations correlated inconsistently with diet and poorly with childhood growth rate or the presence of hypoglycaemia. The serum enzyme activities declined around puberty concomitantly with a decrease in liver size. Although periportal fibrosis and micronodular cirrhosis indicated the presence of hepatocellular damage during childhood, the decline in serum enzyme activities with age and the absence of overt hepatic dysfunction suggest that the fibrotic process may not always progress.

[Muscular glycogenoses]

Muscular glycogenosis is a disease resulting from genetic abnormalities altering an enzyme which is involved in glycogen metabolism. In addition to disorders of glycogenolysis and glycolysis, there are other pathological processes such as acid maltase (alpha-glucosidase) deficiency and diseases associated with abnormal glycogen structure. Glycolysis is the only metabolic pathway that can produce ATP in the absence of oxygen. It is then easy to understand that any disturbance in this energy pathway can result in dysfunction of the muscle machine and in a number of symptoms which are common to these abnormalities. An overall review of the various diseases know to exist on the glycogenolytic and glycolytic pathway will enable the reader to acquire a better knowledge of their particular features.

Biochemical diagnosis of hepatic glycogen storage diseases: 20 years French experience

French experience of 242 cases of liver glycogenoses is reported. Screening tests based on serum biochemical data and glucagon tolerance tests are briefly reviewed. The diagnosis of types I glycogen storage disease (GSD) was ascertained in 73 patients' liver biopsies by measurement of glycogen content and by studying the glucose-6-phosphatase system. Liver biopsies were also required at the beginning for the diagnosis of other hepatic GSDs; later on, the possibilities of diagnosis using peripheral blood cells were investigated. Eighty-four cases of type III GSD were confirmed by measurement of debranching enzyme activity and glycogen content using either liver biopsies (78 cases) and/or erythrocytes (37 cases); enzyme determination was also performed in leukocytes and/or fibroblasts for 18 patients. Twenty-four cases of type VI GSD underwent liver biopsies, and the diagnosis could be confirmed using mononuclear or polymorphonuclear cells for 11 of these patients. Sixty-one patients were identified as type IX GSD; phosphorylase kinase deficiency was demonstrated in erythrocytes for all patients, and a liver biopsy was analyzed for 26 of these cases. From this experience, the possibilities of diagnosis of liver GSD using peripheral blood cells are emphasized.

Type III glycogen storage disease: an adult case with mild disease but complete absence of debrancher protein

A 54-yr-old woman who presented with chest pain and elevated serum creatine kinase levels was found to have type III glycogen storage disease. Except for a history of hepatomegaly in childhood, she was healthy and lived a normal life. There was no hypoglycemia, seizure disorder or growth retardation. Muscle weakness was not apparent until the sixth decade. Despite the mild clinical course, debranching enzyme activity was not detectable by biochemical assay, and immunoblot analysis using a polyclonal antibody showed a complete absence of debrancher protein. Thus, mild clinical manifestations in this patient could not be explained by the residual debrancher enzyme and/or activity.