Glycogen storage diseases: An update

Abstract

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.

Keywords: Glycogen storage disease; Hypoglycemia; Liver; Muscle.

Figure 1

Simplified pathway of glycogen synthesis and degradation in hepatocytes. Glucose and glycogen convert into one another via synthesis or degradation (glycogenolysis) through various steps. The liver plays a central role in maintaining normoglycemia. During the fasting state, the liver maintains glucose homeostasis via a metabolic shift from synthesizing glycogen to endogenous glucose production by glycogenolysis and gluconeogenesis. Specific enzyme or transporter defects in these pathways are associated with clinical and biochemical manifestations including hepatomegaly, hypoglycemia, hyperlipidemia, hypertriglyceridemia, hyperlactatemia, and hyperuricemia. GSD: Glycogen storage disease; UDP-Glucose: Uridine diphosphate glucose; Glucose-1-P: Glucose 1-phosphate; Glucose-6-P: Glucose-6-phosphate; Acetyl-CoA: Acetyl coenzyme A; TCA: Tricarboxylic acid.