Lysosome-associated membrane protein 2 deficiency (glycogen storage disease IIb, Danon disease)
- Basil T Darras, MD
Basil T Darras, MD
- Professor of Neurology
- Harvard Medical School
- William J Craigen, MD, PhD
William J Craigen, MD, PhD
- Professor of Molecular and Human Genetics
- Baylor College of Medicine
Glycogen is the stored form of glucose and serves as a buffer for glucose needs. It is composed of long polymers of a 1-4 linked glucose, interrupted by a 1-6 linked branch point every 4 to 10 residues. Glycogen is formed in periods of dietary carbohydrate loading and broken down when glucose demand is high or dietary availability is low (figure 1).
There are a number of inborn errors of glycogen metabolism that result from mutations in genes for virtually all of the proteins involved in glycogen synthesis, degradation, or regulation. Those disorders that result in abnormal storage of glycogen are known as glycogen storage diseases (GSDs). They have largely been categorized by number according to the chronology of recognition of the responsible enzyme defect (table 1). The age of onset varies from in utero to adulthood.
Glycogen is most abundant in liver and muscle, which are most affected by these disorders. The physiologic importance of a given enzyme in liver and muscle determines the clinical manifestations of the disease.
●The main role of glycogen in the liver is to store glucose for release to tissues that are unable to synthesize significant amounts during fasting. The major manifestations of disorders of glycogen metabolism affecting the liver are hypoglycemia and hepatomegaly. (See "Physiologic response to hypoglycemia in normal subjects and patients with diabetes mellitus".)
●Glycogen serves as the primary source of energy for high-intensity muscle activity by providing substrates for the generation of adenosine triphosphate (ATP). The major manifestations of disorders of glycogen metabolism affecting muscle are muscle cramps, exercise intolerance and easy fatigability, and progressive weakness.
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