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.

Effects of acute nutritional ketosis during exercise in adults with glycogen storage disease type IIIa are phenotype-specific: An investigator-initiated, randomized, crossover study.

Glycogen storage disease type IIIa (GSDIIIa) is an inborn error of carbohydrate metabolism caused by a debranching enzyme deficiency. A subgroup of GSDIIIa patients develops severe myopathy. The purpose of this study was to investigate whether acute nutritional ketosis (ANK) in response to ketone-ester (KE) ingestion is effective to deliver oxidative substrate to exercising muscle in GSDIIIa patients. This was an investigator-initiated, researcher-blinded, randomized, crossover study in six adult GSDIIIa patients. Prior to exercise subjects ingested a carbohydrate drink (~66 g, CHO) or a ketone-ester (395 mg/kg, KE) + carbohydrate drink (30 g, KE + CHO). Subjects performed 15-minute cycling exercise on an upright ergometer followed by 10-minute supine cycling in a magnetic resonance (MR) scanner at two submaximal workloads (30% and 60% of individual maximum, respectively). Blood metabolites, indirect calorimetry data, and in vivo 31 P-MR spectra from quadriceps muscle were collected during exercise. KE + CHO induced ANK in all six subjects with median peak ßHB concentration of 2.6 mmol/L (range: 1.6-3.1). Subjects remained normoglycemic in both study arms, but delta glucose concentration was 2-fold lower in the KE + CHO arm. The respiratory exchange ratio did not increase in the KE + CHO arm when workload was doubled in subjects with overt myopathy. In vivo 31 P MR spectra showed a favorable change in quadriceps energetic state during exercise in the KE + CHO arm compared to CHO in subjects with overt myopathy. Effects of ANK during exercise are phenotype-specific in adult GSDIIIa patients. ANK presents a promising therapy in GSDIIIa patients with a severe myopathic phenotype. TRIAL REGISTRATION NUMBER: ClinicalTrials.gov identifier: NCT03011203.

Liver fibrosis during clinical ascertainment of glycogen storage disease type III: a need for improved and systematic monitoring.

PURPOSE: In glycogen storage disease type III (GSD III), liver aminotransferases tend to normalize with age giving an impression that hepatic manifestations improve with age. However, despite dietary treatment, long-term liver complications emerge. We present a GSD III liver natural history study in children to better understand changes in hepatic parameters with age. METHODS: We reviewed clinical, biochemical, histological, and radiological data in pediatric patients with GSD III, and performed a literature review of GSD III hepatic findings. RESULTS: Twenty-six patients (median age 12.5 years, range 2-22) with GSD IIIa (n = 23) and IIIb (n = 3) were enrolled in the study. Six of seven pediatric patients showed severe fibrosis on liver biopsy (median [range] age: 1.25 [0.75-7] years). Markers of liver injury (aminotransferases), dysfunction (cholesterol, triglycerides), and glycogen storage (glucose tetrasaccharide, Glc4) were elevated at an early age, and decreased significantly thereafter (p < 0.001). Creatine phosphokinase was also elevated with no significant correlation with age (p = 0.4). CONCLUSION: Liver fibrosis can occur at an early age, and may explain the decrease in aminotransferases and Glc4 with age. Our data outlines the need for systematic follow-up and specific biochemical and radiological tools to monitor the silent course of the liver disease process.

Rescue of GSDIII Phenotype with Gene Transfer Requires Liver- and Muscle-Targeted GDE Expression.

Glycogen storage disease type III (GSDIII) is an autosomal recessive disorder caused by a deficiency of glycogen-debranching enzyme (GDE), which results in profound liver metabolism impairment and muscle weakness. To date, no cure is available for GSDIII and current treatments are mostly based on diet. Here we describe the development of a mouse model of GSDIII, which faithfully recapitulates the main features of the human condition. We used this model to develop and test novel therapies based on adeno-associated virus (AAV) vector-mediated gene transfer. First, we showed that overexpression of the lysosomal enzyme alpha-acid glucosidase (GAA) with an AAV vector led to a decrease in liver glycogen content but failed to reverse the disease phenotype. Using dual overlapping AAV vectors expressing the GDE transgene in muscle, we showed functional rescue with no impact on glucose metabolism. Liver expression of GDE, conversely, had a direct impact on blood glucose levels. These results provide proof of concept of correction of GSDIII with AAV vectors, and they indicate that restoration of the enzyme deficiency in muscle and liver is necessary to address both the metabolic and neuromuscular manifestations of the disease.

Genetic analysis and clinical assessment of four patients with Glycogen Storage Disease Type IIIa in China.

BACKGROUND: Glycogen Storage Disease Type III (GSD III) is a rare autosomal recessive metabolic disorder caused by AGL gene mutation. There is significant heterogeneity between the clinical manifestations and the gene mutation of AGL among different ethnic groups. However, GSD III is rarely reported in Chinese population. CASE PRESENTATION: In this study, we aimed to study the genetic and clinical characteristics of four patients with GSD IIIa from China, especially the neurological manifestations. Meanwhile, we conducted a literature review of GSD IIIa cases reported in Chinese population to investigate the relationship between genotype and phenotype. CONCLUSIONS: Three different AGL gene mutations were identified in our patients: c.206dupA, c.1735 + 1G > T and c.2590 C>T. Moreover, progressive myopathy accompanied by elevated creatine kinase level was the main manifestation of our patients in adolescents. Our results showed that AGL c.206dupA was a novel mutation and caused severe clinical manifestations. AGL c.1735 + 1G > T might be a recurrent mutation in the Chinese population. Genetic analysis of AGL gene mutation combined with muscle magnetic resonance imaging (MRI) might provide greater benefit to the patient in diagnosing GSD IIIa, rather than an invasive diagnostic procedure of biopsy.

[Characteristics of lipid metabolism and the cardiovascular system in glycogenosis types I and III]. / Kharakteristika lipidnogo obmena i sostoianie serdechno-sosudistoi sistemy pri glikogenozakh I i III tipov.

Glycogen storage disease (GSD) is an inherited metabolic disorder characterized by early childhood lipid metabolic disturbances with potentially proatherogenic effects. The review outlines the characteristics of impaired lipid composition and other changes in the cardiovascular system in GSD types I and III. It analyzes the factors enabling and inhibiting the development of atherosclerosis in patients with GSD. The review describes the paradox of vascular resistance to the development of early atherosclerosis despite the proatherogenic composition of lipids in the patients of this group.

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.

[Mitochondrial dysfunction in children with hepatic forms of glycogen storage disease].

AIM: The purpose of the study was to assess mitochondrial dysfunction severity in patients with hepatic forms of glycogen storage disease (GSD). PATIENTS AND METHODS: We examined 53 children with GSD in the dynamics. Distribution of children by disease types was: 1st group--children with GSD type I, 2nd group--children with GSD type III, 3rd group--children with GSD type VI and IX; comparison group consisted of 34 healthy children. Intracellular dehydrogenases activity: succinate dehydrogenase (SDH), glycerol-3-phosphate-dehydrogenase (GPDH). nicotinamideadenin-H-dehydrogenase (NADH-D) and lactatdehydrogenase (LDH) was measured using the quantitative cytochemical method in the peripheral lymphocytes. RESULTS: It was revealed decrease of SDH- (p < 0.001) and GPDH-activities (p < 0.001), along with increase of the NADH-D activity (p < 0.05) in all patients with GSD, (SDH/ NADH-D) index was decreased (p < 0.001). LDH activity was increased in groups 1 (p < 0.05) and 3 (p < 0.01), compared with comparison group. The most pronounced intracellular enzymes activity deviations were observed in children with GSD type I, that correspond to more severe clinical form of GSD. It was found strong correlation between intracellular enzymes activity and both hepatomegaly level (R = 0.867) and metabolic acidosis severity (R = 0.987). CONCLUSION: Our investigation revealed features of mitochondrial dysfunction in children with GSD, depending on the GSD type. Activities of lymphocytes enzymes correlates with the main disease severity parameters and can be used as an additional diagnostic criteria in children with hepatic form of GSD.

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.

Molecular and biochemical characterization of a novel intronic single point mutation in a Tunisian family with glycogen storage disease type III.

Genetic deficiency of the glycogen debranching enzyme causes glycogen storage disease type III, an autosomal recessive inherited disorder. The gene encoding this enzyme is designated as AGL gene. The disease is characterized by fasting hypoglycemia, hepatomegaly, growth retardation, progressive myopathy and cardiomyopathy. In the present study, we present clinical features and molecular characterization of two consanguineous Tunisian siblings suffering from Glycogen storage disease type III. The full coding exons of the AGL gene and their corresponding exon-intron boundaries were amplified for the patients and their parents. Gene sequencing identified a novel single point mutation at the conserved polypyrimidine tract of intron 21 in a homozygous state (IVS21-8A>G). This variant cosegregated with the disease and was absent in 102 control chromosomes. In silico analysis using online resources showed a decreased score of the acceptor splice site of intron 21. RT-PCR analysis of the AGL splicing pattern revealed a 7 bp sequence insertion between exon 21 and exon 22 due to the creation of a new 3' splice site. The predicted mutant enzyme was truncated by the loss of 637 carboxyl-terminal amino acids as a result of premature termination. This novel mutation is the first mutation identified in the region of Bizerte and the tenth AGL mutation identified in Tunisia. Screening for this mutation can improve the genetic counseling and prenatal diagnosis of GSD III.

Generation of three induced pluripotent stem cell lines from patients with glycogen storage disease type III.

Glycogen storage disease type III (GSDIII) is an autosomal recessive disorder characterized by a deficiency of glycogen debranching enzyme (GDE) leading to cytosolic glycogen accumulation and inducing liver and muscle pathology. Skin fibroblasts from three GSDIII patients were reprogrammed into induced pluripotent stem cells (iPSCs) using non-integrated Sendai virus. All of the three lines exhibited normal morphology, expression of pluripotent markers, stable karyotype, potential of trilineage differentiation and absence of GDE expression, making them valuable tools for modeling GSDIII disease in vitro, studying pathological mechanisms and investigating potential treatments.

Bulbar muscle weakness and fatty lingual infiltration in glycogen storage disorder type IIIa.

Glycogen storage disorder type III (GSD III) is a rare autosomal recessive disorder resulting from a deficiency of glycogen debranching enzyme, critical in cytosolic glycogen degradation. GSD IIIa, the most common form of GSD III, primarily affects the liver, cardiac muscle, and skeletal muscle. Although skeletal muscle weakness occurs commonly in GSD IIIa, bulbar muscle involvement has not been previously reported. Here we present three GSD IIIa patients with clinical evidence of bulbar weakness based on instrumental assessment of lingual strength. Dysarthria and/or dysphagia, generally mild in severity, were evident in all three individuals. One patient also underwent correlative magnetic resonance imaging (MRI) which was remarkable for fatty infiltration at the base of the intrinsic tongue musculature, as well as abnormal expansion of the fibro-fatty lingual septum. Additionally, we provide supportive evidence of diffuse glycogen infiltration of the tongue at necropsy in a naturally occurring canine model of GSD IIIa. While further investigation in a larger group of patients with GSD III is needed to determine the incidence of bulbar muscle involvement in this condition and whether it occurs in GSD IIIb, clinical surveillance of lingual strength is recommended.

Liver transplantation in glycogen storage disease: a single-center experience.

BACKGROUND: Glycogen storage diseases (GSDs) are inherited glycogen metabolic disorders which have various subtypes. GSDs of type I, III, IV, VI, and IX show liver involvement and are considered as hepatic types of GSDs. Thus, liver transplantation (LT) has been proposed as a final therapy for these types of GSD. LT corrects the primary hepatic enzyme defect; however, the long-term outcomes of LT in these patients have not been extensively evaluated so far. There are few reports in the English literature about the outcome of GSD patients after LT. There has been no report from Iran. The present retrospective study aimed to evaluate the long-term outcomes of eight patients with GSD types I, III, and IV who underwent LT in the affiliated hospitals of Shiraz University of Medical Sciences, from March 2013 to June 2021. During this period, there were no patients with GSD VI and IX identified in this center. RESULTS: The median time of diagnosis of the GSDs and at transplant was 1 year and 11 years, respectively. All eight transplanted patients were alive at the time of follow-up in this study. None of them required a re-transplant. All of the patients showed normalized liver enzymes after LT with no sign of hypoglycemia. CONCLUSIONS: LT is an achievable treatment for end-stage hepatic involvement of GSDs with a cure for metabolic deficiency. Our experience in these eight patients shows a favorable outcome with no mortality and no major complication.

Usher syndrome type 2A complicated with glycogen storage disease type 3 due to paternal uniparental isodisomy of chromosome 1 in a sporadic patient.

BACKGROUND: The condition of uniparental disomy (UPD) occurs when an individual inherits two copies of a chromosome, or part of a chromosome, from one parent. Most cases of uniparental heterodisomy (UPhD) do not cause diseases, whereas cases of uniparental isodisomy (UPiD), while rare, may be pathogenic. Theoretically, UPiD may cause rare genetic diseases in a homozygous recessive manner. METHODS: A 4-year-old girl presented with congenital hearing loss, developmental delay, hepatomegaly, and other clinical features. She and her parents were genetically tested using trio whole exome sequencing (Trio-WES) and copy number variation sequencing (CNV-seq). In addition, we built a structural model to further examine the pathogenicity of the UPiD variants. RESULTS: Trio-WES identified a paternal UPiD in chromosome 1, and two homozygous pathogenic variants AGL c.4284T>G/p.Tyr1428* and USH2A c.6528T>A/p.Tyr2176* in the UPiD region. We further analyzed the pathogenicity of these two variations. The patient was diagnosed with Usher syndrome type 2A (USH2A) and glycogen storage disease type III (GSD3). CONCLUSIONS: Our study reports a rare case of a patient carrying two pathogenic variants of different genes caused by paternal UPiD, supporting the potential application of Trio-WES in detecting and facilitating the diagnosis of UPD.

Glycogen storage disease type III with hypoketosis.

A rare case of glycogen storage disease type III with unusually absent ketone body production during hypoglycemia is presented. A 10-month-old boy presented with asymptomatic hepatomegaly. GOT/GPT 2555/1160 IU/L, CK 302 IU/L, triglycerides 1223 mg/dL, cholesterol 702 mg/dL and uric acid 7.9 mg/dL. After a 9-hour fast, glucose was 27 mg/dL and adequate lipolysis without ketogenesis was observed (total/free carnitine 34.5/20 micromol/L, free fatty acids 1620 micromol/L and beta-hydroxybutyrate 172 micromol/L). Result of MCT (medium-chain triglycerides) load test: basal hydroxybutyrate 29 micromol/L rose to 5748 micromol/L. Treatment with a fat-restricted diet supplemented with formula containing MCT was initiated and the patient presented a satisfactory initial evolution. Three months later, CK were 3000 IU/L. Muscle biopsy was diagnostic of glycogenosis. Enzymatic activity in skin fibroblasts was 0% for amylo-1,6-glucosidase. The diagnosis of glycogenosis type III was established. Echocardiography performed at that time showed non-obstructive ventricular hypertrophy. Until now hypoketosis during hypoglycemia has only been described in glycogenosis type I.

Reduction in bone mineral density in glycogenosis type III may be due to a mixed muscle and bone deficit.

UNLABELLED: Glycogen storage disease type III (GSD III; OMIM 232400) is an autosomal recessive deficiency of the glycogen debrancher enzyme, amylo-1,6-glucosidase (EC 3.2.1.33). Patients with other hepatic glycogenoses are known to have reduced bone mineral content (BMC) and to be at consequent risk of fractures. They have key metabolic differences from GSD III patients, however. This study examines bone density and metabolism in 15 GSD III patients (6 female) from childhood to adulthood (aged 10-34 years). The results demonstrate that patients with GSD III have low bone mass at all skeletal sites compared with healthy individuals of the same age and sex, with a significant proportion (40-64%) having BMD > 2 standard deviations below the mean for whole body and lumbar spine. The deficiency seems to be attributable to a mixed muscle andbone deficit. Lower bone mass was found at all sites for GSD IIIa patients (combined liver and muscle defect) compared with GSD IIIb patients (liver only defect). CONCLUSION: Patients with GSD III have significantly abnormal bone mass, placing them at increased risk of potential fracture. The underlying mechanism is probably multifactorial with contributions from abnormal muscle physiology, abnormal metabolic milieu and altered nutrition affecting micronutrient intake. Therapies need to address all these factors to be successful.

Glucose-free/high-protein diet improves hepatomegaly and exercise intolerance in glycogen storage disease type III mice.

Glycogen disease type III (GSDIII), a rare incurable autosomal recessive disorder due to glycogen debranching enzyme deficiency, presents with liver, heart and skeletal muscle impairment, hepatomegaly and ketotic hypoglycemia. Muscle weakness usually worsens to fixed myopathy and cardiac involvement may present in about half of the patients during disease. Management relies on careful follow-up of symptoms and diet. No common agreement was reached on sugar restriction and treatment in adulthood. We administered two dietary regimens differing in their protein and carbohydrate content, high-protein (HPD) and high-protein/glucose-free (GFD), to our mouse model of GSDIII, starting at one month of age. Mice were monitored, either by histological, biochemical and molecular analysis and motor functional tests, until 10 months of age. GFD ameliorated muscle performance up to 10 months of age, while HPD showed little improvement only in young mice. In GFD mice, a decreased muscle glycogen content and fiber vacuolization was observed, even in aged animals indicating a protective role of proteins against skeletal muscle degeneration, at least in some districts. Hepatomegaly was reduced by about 20%. Moreover, the long-term administration of GFD did not worsen serum parameters even after eight months of high-protein diet. A decreased phosphofructokinase and pyruvate kinase activities and an increased expression of Krebs cycle and gluconeogenesis genes were seen in the liver of GFD fed mice. Our data show that the concurrent use of proteins and a strictly controlled glucose supply could reduce muscle wasting, and indicate a better metabolic control in mice with a glucose-free/high-protein diet.

Reduced bone mineral density in glycogen storage disease type III: evidence for a possible connection between metabolic imbalance and bone homeostasis.

INTRODUCTION: Glycogen storage disease type III (GSDIII) is an inborn error of carbohydrate metabolism caused by deficient activity of glycogen debranching enzyme (GDE). It is characterized by liver, cardiac muscle and skeletal muscle involvement. The presence of systemic complications such as growth retardation, ovarian polycystosis, diabetes mellitus and osteopenia/osteoporosis has been reported. The pathogenesis of osteopenia/osteoporosis is still unclear. OBJECTIVES: The aim of the current study was to evaluate the bone mineral density (BMD) in GSDIII patients and the role of metabolic and endocrine factors and physical activity on bone status. METHODS: Nine GSDIII patients were enrolled (age 2-20years) and compared to eighteen age and sex matched controls. BMD was evaluated by Dual-emission-X-ray absorptiometry (DXA) and Quantitative ultrasound (QUS). Clinical and biochemical parameters of endocrine system function and bone metabolism were analyzed. Serum levels of the metabolic control markers were evaluated. Physical activity was evaluated by administering the International Physical Activity Questionnaire (IPAQ). RESULTS: GSDIII patients showed reduced BMD detected at both DXA and QUS, decreased serum levels of IGF-1, free IGF-1, insulin, calcitonin, osteocalcin (OC) and increased serum levels of C-terminal cross-linking telopeptide of type I collagen (CTX). IGF-1 serum levels inversely correlated with AST and ALT serum levels. DXA Z-score inversely correlated with cholesterol and triglycerides serum levels and directly correlated with IGF-1/IGFBP3 molar ratio. No difference in physical activity was observed between GSDIII patients and controls. DISCUSSION: Our data confirm the presence of reduced BMD in GSDIII. On the basis of the results, we hypothesized that metabolic imbalance could be the key factor leading to osteopenia, acting through different mechanisms: chronic hyperlipidemia, reduced IGF-1, Insulin and OC serum levels. Thus, the mechanism of osteopenia/osteoporosis in GSDIII is probably multifactorial and we speculate on the factors involved in its pathogenesis.

Natural Progression of Canine Glycogen Storage Disease Type IIIa.

Glycogen storage disease type IIIa (GSD IIIa) is caused by a deficiency of glycogen debranching enzyme activity. Hepatomegaly, muscle degeneration, and hypoglycemia occur in human patients at an early age. Long-term complications include liver cirrhosis, hepatic adenomas, and generalized myopathy. A naturally occurring canine model of GSD IIIa that mimics the human disease has been described, with progressive liver disease and skeletal muscle damage likely due to excess glycogen deposition. In the current study, long-term follow-up of previously described GSD IIIa dogs until 32 mo of age (n = 4) and of family-owned GSD IIIa dogs until 11 to 12 y of age (n = 2) revealed that elevated concentrations of liver and muscle enzyme (AST, ALT, ALP, and creatine phosphokinase) decreased over time, consistent with hepatic cirrhosis and muscle fibrosis. Glycogen deposition in many skeletal muscles; the tongue, diaphragm, and heart; and the phrenic and sciatic nerves occurred also. Furthermore, the urinary biomarker Glc4, which has been described in many types of GSD, was first elevated and then decreased later in life. This urinary biomarker demonstrated a similar trend as AST and ALT in GSD IIIa dogs, indicating that Glc4 might be a less invasive biomarker of hepatocellular disease. Finally, the current study further demonstrates that the canine GSD IIIa model adheres to the clinical course in human patients with this disorder and is an appropriate model for developing novel therapies.