Glycogen Storage Diseases

Glycogen storage diseases are inherited disorders in which an enzyme defect in glycogen synthesis or breakdown leads to abnormal quantity or structure of glycogen in tissues, chiefly liver and muscle. They are classically numbered by type (for example type I / von Gierke, type II / Pompe, type V / McArdle) according to the enzyme involved, and they divide broadly into hepatic forms that disturb blood-glucose homeostasis and muscle forms that impair energy supply to working muscle.

Definition

A glycogen storage disease is an inherited disorder caused by deficiency of an enzyme of glycogen synthesis, degradation, or glycolytic regulation, resulting in excess or structurally abnormal glycogen accumulation, most often in liver, skeletal muscle, or both.

Scope

The entry covers the shared concept of a block in glycogen metabolism, the distinction between hepatic and muscle phenotypes, the principal enzyme defects, and the biochemical reasoning used to localise them. Individual disease subtypes are described only to illustrate the categories. It is a reference overview and not a guide to managing any specific glycogenosis.

Key concepts

• Glycogen synthesis and glycogenolysis
• Glucose-6-phosphatase deficiency (type I, von Gierke)
• Acid alpha-glucosidase deficiency (type II, Pompe)
• Myophosphorylase deficiency (type V, McArdle)
• Hepatic versus muscle phenotypes
• Fasting hypoglycaemia and hepatomegaly
• Exercise intolerance and the second-wind phenomenon
• Lysosomal versus cytosolic glycogen accumulation

Mechanisms

Glycogen is the branched storage polymer of glucose; its synthesis and breakdown are controlled by a chain of enzymes, and a defect at any step produces a distinct disorder. In hepatic forms such as type I, deficiency of glucose-6-phosphatase prevents the liver from releasing free glucose during fasting, causing hepatomegaly and fasting hypoglycaemia with lactic acidosis; Cori and Cori's demonstration of absent hepatic glucose-6-phosphatase activity first tied a clinical glycogenosis to a specific enzyme defect. In muscle forms such as type V, deficiency of myophosphorylase blocks glycogen mobilisation during exercise, producing exercise intolerance and the characteristic second-wind phenomenon. Type II (Pompe) is distinctive in that glycogen accumulates within lysosomes because of deficient acid alpha-glucosidase, making it both a glycogenosis and a lysosomal storage disorder.

Clinical relevance

Glycogen storage diseases connect a single enzyme defect to recognisable patterns - fasting hypoglycaemia with hepatomegaly, or exertional muscle symptoms - and so illustrate how pathway logic guides classification. Practice guidelines such as those of the American College of Medical Genetics and Genomics organise how type I is diagnosed and monitored. This entry summarises that landscape for reference and does not provide individualised diagnostic or treatment recommendations.

Epidemiology

Glycogen storage diseases are individually rare; type I and type II are among the more frequently described forms, and several are now detectable through enzyme assay, biochemical testing, or, for Pompe disease, newborn screening in some programmes. Precise incidence varies by population and ascertainment.

History

The glycogen storage diseases were among the first metabolic disorders to be defined enzymatically. Von Gierke described the hepatorenal glycogenosis that bears his name in the 1920s, and Gerty and Carl Cori's 1952 identification of absent hepatic glucose-6-phosphatase activity provided one of the earliest direct links between a clinical disorder and a specific missing enzyme, anchoring the type I classification. Subsequent decades added the muscle phosphorylase defect of McArdle disease and the lysosomal defect of Pompe disease, and modern guidelines now codify diagnosis and follow-up.

Key figures

• Gerty Cori
• Carl Cori
• Edgar von Gierke
• Joannes Pompe
• Brian McArdle
• Priya Kishnani

Seminal works

• cori-cori-1952
• kishnani-2014
• labrune-2002

Frequently asked questions

Why do some glycogen storage diseases cause hypoglycaemia and others cause muscle symptoms?

Hepatic forms (such as type I) impair the liver's ability to release glucose during fasting, leading to low blood sugar and an enlarged liver, whereas muscle forms (such as type V) block glycogen breakdown in muscle, leading to exercise intolerance rather than hypoglycaemia.

Why is Pompe disease grouped with both glycogen storage and lysosomal disorders?

In Pompe disease the missing enzyme, acid alpha-glucosidase, normally works inside lysosomes, so glycogen accumulates within these organelles. This makes it both a glycogenosis and a lysosomal storage disease.