Types of Glycogen Storage Diseases (GSDs)

There are many types of Glycogen Storage Diseases. This is because there are different enzymes involved in various stages of glycogen breakdown and production. Indeed, there are over 9 types. Each disorder has a different enzyme malfunction.

• Type Ia (von Gierke's disease), type Ib.
• Type II (Pompe's disease).
• Type III (Forbes-Cori disease).
• Type IV (Andersen's disease).
• Type V (McArdle's disease).
• Type VI (Hers' disease).
• Type VII (Tarui's disease).
• Type IX (liver glycogen phosphorylase kinase deficiency).
• Type 0 (Lewis' disease).

We will be focusing only on Type I. Type I GSD accounts for 90% of all GSD cases. 1 in 100,000 live births suffer from Type I. Researchers have identified two types of Type I GSD. These two types differ in their signs and symptoms and also their genetic causes.

Type Ia, or von Gierke's disease, is caused by the glucose-6-phosphatase deficiency in the liver, kidney, and small intestine. Glucose-6-phosphatase is a type of enzyme. Without glucose-6-phosphatase, glycogen cannot be broken down into glucose and the synthesis of glucose is disrupted. As a result, the liver is clogged with excess glycogen.

Type Ib is caused by glucose-6-phosphatase translocase T1 deficiency. Glucose-6-phosphate translocase is also a type of enzyme. Without glucose-6-phosphate translocase, glucose-6-phosphate cannot be transported back to the cell, to meet glucose-6-phosphatase, and no glucose will be formed. Thus glycogen cannot be broken down into glucose. 

Later, other translocases were discovered, adding 2 more subtypes of Type I GSD: Type Ic and Type Id. They are caused by defects in the translocase system.