Mutations in the glycogen branching enzyme (GBE) gene cause adult polyglucosan body disease. The GBE gene provides instructions for making the glycogen branching enzyme. This enzyme is involved in the production of a complex sugar called glycogen, which is a major source of stored energy in the body. Most GBE gene mutations result in a shortage of the glycogen branching enzyme, leading to the overproduction of glycogen molecules.
These abnormal glycogen molecules, called polyglucosan bodies, accumulate within cells; especially nerve cells (neurons). Essentially with APBD, the polyglucosan bodies block the pathways for brain messaging, causing the functional problems.

Source: NLM