Glycogen is the stored form of glucose and serves as a tissue reserve for the body’s glucose needs. It is composed of polymers of a 1-4 linked glucose, interrupted by a 1-6 linked branch point every 4-10 residues. Glycogen is formed in periods of dietary carbohydrate surplus and is broken down during starvation or periods of high glucose demand. Several inborn errors of glycogen metabolism have been described, and they result from mutations in genes that code for proteins involved in various steps of glycogen synthesis, degradation, or regulation. These disorders result in abnormal storage of glycogen, and hence the phrase glycogen storage diseases (GSDs).
Glucose-6-phosphatase (Glc-6-Pase) deficiency, also termed GSD type I or von Gierke disease, is a rare form of GSD. The hydrolysis and transport of glucose 6-phosphate requires a hydrolase and microsomal transporters, pyrophosphate and glucose. Type Ia results from a deficiency in the glucose 6-phosphate hydrolase activity, and makes up more than 80% of cases. Types Ib (glucose-6-phosphate transporter deficiency), Ic, and Id are allelic defects in the translocase associated with glucose-6-phosphatase.