GBE1 and disorder of glycogen metabolism: Adult polyglucosan body disease (APBD) is a rare autosomal recessive glycogenosis caused by bi-allelic variants in GBE1. Impaired glycogen-branching enzyme (GBE) activity and upregulation of glycogen synthase cause the accumulation of polyglucosan bodies with detrimental effects on neurons and glia in the central and peripheral nervous system.