Echocardiographic manifestations of Glycogen Storage Disease III: increase in wall thickness and left ventricular mass over time

Gene therapy for glycogen storage diseases

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

Glycogen storage disease with massive left ventricular hypertrophy and increased native T1: a case report

Background

Glycogen storage disease (GSD) type Ⅲa is a rare autosomal recessive disorder resulting in the accumulation of abnormally structured glycogen in the liver, skeletal muscle, and cardiac muscle. Cardiovascular magnetic resonance (CMR) tissue characteristics in GSD have rarely been reported.

Case summary

We report a 24-year-old male patient suffering from recurrent palpitation and atypical chest pain for 5 years with suspected hypertrophic cardiomyopathy. Laboratory tests revealed an elevated creatine kinase, and physical exam revealed hepatosplenomegaly. Cardiovascular magnetic resonance demonstrated asymmetrical massive left ventricular hypertrophy with a maximal thickness of 34.6 mm in the septum. In the regions with focal late gadolinium enhancement (LGE) in the anterior septum, both native T1 and extracellular volume (ECV) are elevated. However, in the LGE-negative regions of the myocardium, native T1 was elevated without elevation in ECV (septum, 22.7%; free wall, 20.9%). Whole exome sequencing revealed a novel pathogenic homozygous nonsense variant of the AGL gene (c.4284 T > G, p. Tyr1428*), confirming the diagnosis of the patients as GSD type Ⅲa.

Discussion

This case showed increased diffuse native T1 but not ECV on CMR in LGE-negative myocardium in GSD, which indicates that the T1 value is increased with an accumulation of glycogen in the myocardium, but the ECV space was not expanded in this process. Genetic testing should be obtained in severe LV hypertrophy when multi-organ involvement is present, and myocardial tissue characterization is discrepant between T1 elevation and normal ECV to consider glycogen storage disorder.

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.

Echocardiographic Assessment of Patients with Glycogen Storage Disease in a Single Center

Glycogen storage disease (GSD) is a hereditary metabolic disorder caused by enzyme deficiency resulting in glycogen accumulation in the liver, muscle, heart, or kidney. GSD types II, III, IV, and IX are associated with cardiac involvement. However, cardiac manifestation in other GSD types is unclear. This study aimed to describe whether energy deprivation and the toxic effects of accumulated glycogen affect the heart of patients with GSD. We evaluated the left ventricle (LV) wall mass, LV systolic and diastolic function and myocardial strain with conventional echocardiography and two-dimensional speckle-tracking echocardiography (2D STE) in 62 patients with GSD type I, III, VI and IX who visited the Wonju Severance Hospital in 2021. Among the GSD patients, the echocardiographic parameters of 55 pediatrics were converted into z-scores and analyzed. Of the patients, 43 (62.3%), 7 (11.3%) and 12 (19.4%) patients were diagnosed with GSD type I, type III, and type IX, respectively. The median age was 9 years (range, 1-36 years), with 55 children under 18 years old and seven adults over 18 years. For the 55 pediatric patients, the echocardiographic parameters were converted into a z-score and analyzed. Multiple linear regression analysis showed that the BMI z-score (p = 0.022) and CK (p = 0.020) predicted increased LV mass z-score, regardless of GSD type. There was no difference in the diastolic and systolic functions according to myocardial thickness; however, 2D STE showed a negative correlation with the LV mass (r = -0.28, p = 0.041). Given that patients with GSD tend to be overweight, serial evaluation with echocardiography might be required for all types of GSD.

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.

Starch based hydrogel NPs loaded by anthocyanins might treat glycogen storage at cardiomyopathy in animal fibrotic model

Many reports have been published recently confirmed the limitation of cargo molecules delivered into the heart. This failure is mostly associated with lymphatic or vascular channels washing or to the immune system recognition. Delivery of anthocyanins by encapsulation may augment it retention in the heart at early time points as the capsules are too large to wash out by lymphatic or venous channels and the physical structure of the capsule may shield the anthocyanins from immunoglobulins and cellular components of the immune system. In the current study, the cardiac dysfunction was induced by using carbon tetrachloride and then animal were treated orally by using anthocyanins incorporated into hydrogel NPs twice time /week for 4 weeks. The results showed anthocyanin loaded hydrogel NPs has ability to re-maintain the glycogen content in the liver and heart tissues of fibrotic group (13 ± 1.4 and 5 ± 0.7 μmol glucose/g tissue). Additionally, MDA and hydroxyproline were significantly reduced. PAS stain showed depletion of glycogen granules from heart tissue. It is concluded that starch based hydrogel loaded by anthocyanins can improve histological cardiac functions after their injury .

Hypertrophic Cardiomyopathy in Children: Pathophysiology, Diagnosis, and Treatment of Non-sarcomeric Causes

Hypertrophic cardiomyopathy (HCM) is a myocardial disease characterized by left ventricular hypertrophy not solely explained by abnormal loading conditions. Despite its rare prevalence in pediatric age, HCM carries a relevant risk of mortality and morbidity in both infants and children. Pediatric HCM is a large heterogeneous group of disorders. Other than mutations in sarcomeric genes, which represent the most important cause of HCM in adults, childhood HCM includes a high prevalence of non-sarcomeric causes, including inherited errors of metabolism (i.e., glycogen storage diseases, lysosomal storage diseases, and fatty acid oxidation disorders), malformation syndromes, neuromuscular diseases, and mitochondrial disease, which globally represent up to 35% of children with HCM. The age of presentation and the underlying etiology significantly impact the prognosis of children with HCM. Moreover, in recent years, different targeted approaches for non-sarcomeric etiologies of HCM have emerged. Therefore, the etiological diagnosis is a fundamental step in designing specific management and therapy in these subjects. The present review aims to provide an overview of the non-sarcomeric causes of HCM in children, focusing on the pathophysiology, clinical features, diagnosis, and treatment of these rare disorders.

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.

A Novel Gene Therapy Approach for GSD III Using an AAV Vector Encoding a Bacterial Glycogen Debranching Enzyme

Glycogen storage disease type III (GSD III) is an inherited disorder caused by a deficiency of glycogen debranching enzyme (GDE), which results in the accumulation of abnormal glycogen (limit dextrin) in the cytoplasm of liver, heart, and skeletal muscle cells. Currently, there is no curative treatment for this disease. Gene therapy with adeno-associated virus (AAV) provides an optimal treatment approach for monogenic diseases like GSD III. However, the 4.6 kb human GDE cDNA is too large to be packaged into a single AAV vector due to its small carrying capacity. To overcome this limitation, we tested a new gene therapy approach in GSD IIIa mice using an AAV vector ubiquitously expressing a smaller bacterial GDE, Pullulanase, whose cDNA is 2.2 kb. Intravenous injection of the AAV vector (AAV9-CB-Pull) into 2-week-old GSD IIIa mice blocked glycogen accumulation in both cardiac and skeletal muscles, but not in the liver, accompanied by the improvement of muscle functions. Subsequent treatment with a liver-restricted AAV vector (AAV8-LSP-Pull) reduced liver glycogen content by 75% and reversed hepatic fibrosis while maintaining the effect of AAV9-CB-Pull treatment on heart and skeletal muscle. Our results suggest that AAV-mediated gene therapy with Pullulanase is a possible treatment for GSD III.

A lower energetic, protein and uncooked cornstarch intake is associated with a more severe outcome in glycogen storage disease type III: an observational study of 50 patients

Background Glycogen storage disease type III (GSDIII), due to a deficiency of glycogen debrancher enzyme (GDE), is particularly frequent in Tunisia. Phenotypic particularities of Tunisian patients remain unknown. Our aim was to study complications of GSDIII in a Tunisian population and to explore factors interfering with its course. Methods A retrospective longitudinal study was conducted over 30 years (1986-2016) in the referral metabolic center in Tunisia. Results Fifty GSDIII patients (26 boys), followed for an average 6.75 years, were enrolled. At the last evaluation, the median age was 9.87 years and 24% of patients reached adulthood. Short stature persisted in eight patients and obesity in 19 patients. Lower frequency of hypertriglyceridemia (HTG) was associated with older patients (p<0.0001), higher protein diet (p=0.068) and lower caloric intake (p=0.025). Hepatic complications were rare. Cardiac involvement (CI) was frequent (91%) and occurred early at a median age of 2.6 years. Severe cardiomyopathy (50%) was related to lower doses of uncooked cornstarch (p=0.02). Neuromuscular involvement (NMI) was constant, leading to a functional discomfort in 64% of cases and was disabling in 34% of cases. Severe forms were related to lower caloric (p=0.005) and protein intake (p<0.015). Conclusions A low caloric, protein and uncooked cornstarch intake is associated with a more severe outcome in GSDIII Tunisian patients. Neuromuscular and CIs were particularly precocious and severe, even in childhood. Genetic and epigenetic factors deserve to be explored.

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.

•

GSDIII is a rare incurable disease due to lacking of glycogen debrancher enzyme.

•

Essential features are liver, heart and skeletal muscle impairment.

•

Two diets differing in protein and sugar amount were tested in Agl-mouse model.

•

Glucose-free/high-protein diet decreased glycogen storage and hepatomegaly.

•

Improved muscle performance and better metabolic compensation were achieved.

The effect of geographical indices on left ventricular structure in healthy Han Chinese population

The left ventricular posterior wall thickness (LVPWT) and interventricular septum thickness (IVST) are generally regarded as the functional parts of the left ventricular (LV) structure. This paper aims to examine the effects of geographical indices on healthy Han adults' LV structural indices and to offer a scientific basis for developing a unified standard for the reference values of adults' LV structural indices in China. Fifteen terrain, climate, and soil indices were examined as geographical explanatory variables. Statistical analysis was performed using correlation analysis. Moreover, a back propagation neural network (BPNN) and a support vector regression (SVR) were applied to developing models to predict the values of two indices. After the prediction models were built, distribution maps were produced. The results show that LV structural indices are characteristically associated with latitude, longitude, altitude, average temperature, average wind velocity, topsoil sand fraction, topsoil silt fraction, topsoil organic carbon, and topsoil sodicity. The model test analyses show the BPNN model possesses better simulative and predictive ability in comparison with the SVR model. The distribution maps of the LV structural indices show that, in China, the values are higher in the west and lower in the east. These results demonstrate that the reference values of the adults' LV structural indices will be different affected by different geographical environment. The reference values of LV structural indices in one region can be calculated by setting up a BPNN, which showed better applicability in this study. The distribution of the reference values of the LV structural indices can be seen clearly on the geographical distribution map.

Glycogen storage disease type III: diagnosis, genotype, management, clinical course and outcome

Glycogen storage disease type III (GSDIII) is a rare disorder of glycogenolysis due to AGL gene mutations, causing glycogen debranching enzyme deficiency and storage of limited dextrin. Patients with GSDIIIa show involvement of liver and cardiac/skeletal muscle, whereas GSDIIIb patients display only liver symptoms and signs. The International Study on Glycogen Storage Disease (ISGSDIII) is a descriptive retrospective, international, multi-centre cohort study of diagnosis, genotype, management, clinical course and outcome of 175 patients from 147 families (86 % GSDIIIa; 14 % GSDIIIb), with follow-up into adulthood in 91 patients. In total 58 AGL mutations (non-missense mutations were overrepresented and 21 novel mutations were observed) were identified in 76 families. GSDIII patients first presented before the age of 1.5 years, hepatomegaly was the most common presenting clinical sign. Dietary management was very diverse and included frequent meals, uncooked cornstarch and continuous gastric drip feeding. Chronic complications involved the liver (hepatic cirrhosis, adenoma(s), and/or hepatocellular carcinoma in 11 %), heart (cardiac involvement and cardiomyopathy, in 58 % and 15 %, respectively, generally presenting in early childhood), and muscle (pain in 34 %). Type 2 diabetes mellitus was diagnosed in eight out of 91 adult patients (9 %). In adult patients no significant correlation was detected between (non-) missense AGL genotypes and hepatic, cardiac or muscular complications. This study demonstrates heterogeneity in a large cohort of ageing GSDIII patients. An international GSD patient registry is warranted to prospectively define the clinical course, heterogeneity and the effect of different dietary interventions in patients with GSDIII.

Cross-sectional retrospective study of muscle function in patients with glycogen storage disease type III

Glycogen storage disease type III is an inherited metabolic disorder characterized by liver and muscle impairment. This study aimed to identify promising muscle function measures for future studies on natural disease progression and therapeutic trials. The age-effect on the manual muscle testing (MMT), the hand-held dynamometry (HHD), the motor function measure (MFM) and the Purdue pegboard test was evaluated by regression analysis in a cross-sectional retrospective single site study. In patients aged between 13 and 56 years old, the Purdue pegboard test and dynamometry of key pinch and knee extension strength were age-sensitive with annual losses of 1.49, 1.10 and 0.70% of the predicted values (%pred), respectively. The MFM score and handgrip strength were also age-sensitive but only in patients older than 29 and 37 years old with annual losses of 1.42 and 1.84%pred, respectively. Muscle strength assessed by MMT and elbow extension measured by HHD demonstrated an annual loss of less than 0.50%pred and are thus unlikely to be promising outcome measures for future clinical trials. In conclusion, our results identified age-sensitive outcomes from retrospective data and may serve for future longitudinal studies in which an estimation of the minimal number of subjects is provided.

Muscle Ultrasound in Patients with Glycogen Storage Disease Types I and III

In glycogen storage diseases (GSDs), improved longevity has resulted in the need for neuromuscular surveillance. In 12 children and 14 adults with the "hepatic" (GSD-I) and "myopathic" (GSD-III) phenotypes, we cross-sectionally assessed muscle ultrasound density (MUD) and muscle force. Children with both "hepatic" and "myopathic" GSD phenotypes had elevated MUD values (MUD Z-scores: GSD-I > 2.5 SD vs. GSD-III > 1 SD, p < 0.05) and muscle weakness (GSD-I muscle force; p < 0.05) of myopathic distribution. In "hepatic" GSD-I adults, MUD stabilized (GSD-I adults vs. GSD-I children, not significant), concurring with moderate muscle weakness (GSD-I adults vs. healthy matched pairs, p < 0.05). In "myopathic" GSD-III adults, MUD increased with age (MUD-GSD III vs. age: r = 0.71-0.83, GSD-III adults > GSD-III children, p < 0.05), concurring with pronounced muscle weakness (GSD-III adults vs. GSD-I adults, p < 0.05) of myopathic distribution. Children with "hepatic" and "myopathic" GSD phenotypes were both found to have myopathy. Myopathy stabilizes in "hepatic" GSD-I adults, whereas it progresses in "myopathic" GSD-III adults. Muscle ultrasonography provides an excellent, non-invasive tool for neuromuscular surveillance per GSD phenotype.

Skeletal and cardiac muscle involvement in children with glycogen storage disease type III

Glycogen storage disease type III (GSD III) may present with hepatic disease or may involve both skeletal and cardiac muscles as well. To assess the prevalence of neuromuscular and cardiac involvement in a group of children with GSD III, 28 children with GSD III, diagnosed by enzymatic assay, were enrolled in the study after an informed consent was obtained from their parents/guardians and after the study protocol was approved by our institutional ethical committee. Their mean age was 6.6 + 3.1 years. All cases were assessed neurologically by clinical examination, electromyography (EMG), and nerve conduction velocity. The heart was examined clinically by electrocardiogram and echocardiography. Seventeen patients (61 %) had myopathic changes by EMG, three of them had associated neuropathic changes. Creatine phosphokinase (CPK) was elevated in all myopathic cases except one. Children with myopathic changes were significantly older (p = 0.02), and CPK was significantly higher (p < 0.0001). Nine cases had left ventricular (LV) hypertrophy, seven of them had myopathic changes by EMG.

CONCLUSION

Myopathic changes are not uncommon in children with GSD III. Myopathic changes tend to occur in older age and are associated with higher CPK level. Cardiac muscle involvement is less common in this age group and may, on occasion, occur alone without skeletal muscle involvement. Despite mild degrees of affection in this age group, it is recommended to perform prospective annual screening using EMG and echocardiography in order to augment dietary therapy regimen to prevent progression to life threatening complications.

Dietary management in glycogen storage disease type III: what is the evidence?

In childhood, GSD type III causes relatively severe fasting intolerance, classically associated with ketotic hypoglycaemia. During follow up, history of (documented) hypoglycaemia, clinical parameters (growth, liver size, motor development, neuromuscular parameters), laboratory parameters (glucose, lactate, ALAT, cholesterol, triglycerides, creatine kinase and ketones) and cardiac parameters all need to be integrated in order to titrate dietary management, for which age-dependent requirements need to be taken into account. Evidence from case studies and small cohort studies in both children and adults with GSD III demonstrate that prevention of hypoglycaemia and maintenance of euglycemia is not sufficient to prevent complications. Moreover, over-treatment with carbohydrates may even be harmful. The ageing cohort of GSD III patients, including the non-traditional clinical presentations in adulthood, raises ‬‬‬new questions.

Glycogen storage disease type III: A novel Agl knockout mouse model

Glycogen storage disease type III is an autosomal recessive disease characterized by a deficiency in the glycogen debranching enzyme, encoded by AGL. Essential features of this disease are hepatomegaly, hypoglycemia, hyperlipidemia, and growth retardation. Progressive skeletal myopathy, neuropathy, and/or cardiomyopathy become prominent in adults. Currently, there is no available cure. We generated an Agl knockout mouse model by deletion of the carboxy terminus of the protein, including the carboxy end of the glucosidase domain and the glycogen-binding domain. Agl knockout mice presented serious hepatomegaly, but we did not observe signs of cirrhosis or adenomas. In affected tissues, glycogen storage was higher than in wild-type mice, even in the central nervous system which has never been tested in GSDIII patients. The biochemical findings were in accordance with histological data, which clearly documented tissue impairment due to glycogen accumulation. Indeed, electron microscopy revealed the disruption of contractile units due to glycogen infiltrations. Furthermore, adult Agl knockout animals appeared less prompt to move, and they exhibited kyphosis. Three-mo-old Agl knockout mice could not run, and adult mice showed exercise intolerance. In addition, older affected animals exhibited an accelerated respiratory rate even at basal conditions. This observation was correlated with severe glycogen accumulation in the diaphragm. Diffuse glycogen deposition was observed in the tongues of affected mice. Our results demonstrate that this Agl knockout mouse is a reliable model for human glycogenosis type III, as it recapitulates the essential phenotypic features of the disease.

Neuromuscular disorders of glycogen metabolism

Disorders of glycogen metabolism are inborn errors of energy homeostasis affecting primarily skeletal muscle, heart, liver, and, less frequently, the central nervous system. These rare diseases are quite variable in age of onset, symptoms, morbidity, and mortality. This review provides an update on disorders of glycogen metabolism affecting skeletal muscle exclusively or predominantly. From a pathogenetic perspective, we classify these diseases as primary, if the defective enzyme is directly involved in glycogen/glucose metabolism, or secondary, if the genetic mutation affects proteins which indirectly regulate glycogen or glucose processing. In addition to summarizing the most recent clinical reports in this field, we briefly describe animal models of human glycogen disorders. These experimental models are greatly improving the understanding of the pathogenetic mechanisms underlying the muscle degenerative process associated to these diseases and provide in vivo platforms to test new therapeutic strategies.

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.

Evaluation of cardiac manifestations in pediatric liver transplant candidates

Knowledge concerning the involvement of the cardiovascular system in children awaiting liver transplant is limited. Therefore, no guidelines have been established on evaluating this group of patients for cardiac disease. This review examines the diverse cardiovascular manifestations of liver disease in children. We also discuss the available testing and its applicability in screening for cardiac disease in this vulnerable population.

Cardiac Pathology in Glycogen Storage Disease Type III

PURPOSE

To investigate the distribution and clinical impact of glycogen accumulation on heart structure and function in individuals with GSD III.

METHODS

We examined cardiac tissue and the clinical records of three individuals with GSD IIIa who died or underwent cardiac transplantation. Of the two patients that died, one was from infection and the other was from sudden cardiac death. The third patient required cardiac transplantation for end-stage heart failure with severe hypertrophic cardiomyopathy.

RESULTS

Macro- and microscopic examination revealed cardiac fibrosis (n = 1), moderate to severe vacuolation of cardiac myocytes (n = 3), mild to severe glycogen accumulation in the atrioventricular (AV) node (n = 3), and glycogen accumulation in smooth muscle cells of intramyocardial arteries associated with smooth muscle hyperplasia and profoundly thickened vascular walls (n = 1).

CONCLUSION

Our findings document diffuse though variable involvement of cardiac structures in GSD III patients. Furthermore, our results also show a potential for serious arrhythmia and symptomatic heart failure in some GSD III patients, and this should be considered when managing this patient population.