Glycogen storage disease type I and G6Pase-β deficiency: etiology and therapy

Glycogen storage disease type I (GSD-I) consists of two subtypes: GSD-Ia, a deficiency in glucose-6-phosphatase-alpha (G6Pase-alpha) and GSD-Ib, which is characterized by an absence of a glucose-6-phosphate (G6P) transporter (G6PT). A third disorder, G6Pase-beta deficiency, shares similarities with this group of diseases. G6Pase-alpha and G6Pase-beta are G6P hydrolases in the membrane of the endoplasmic reticulum, which depend on G6PT to transport G6P from the cytoplasm into the lumen. A functional complex of G6PT and G6Pase-alpha maintains interprandial glucose homeostasis, whereas G6PT and G6Pase-beta act in conjunction to maintain neutrophil function and homeostasis. Patients with GSD-Ia and those with GSD-Ib exhibit a common metabolic phenotype of disturbed glucose homeostasis that is not evident in patients with G6Pase-beta deficiency. Patients with a deficiency in G6PT and those lacking G6Pase-beta display a common myeloid phenotype that is not shared by patients with GSD-Ia. Previous studies have shown that neutrophils express the complex of G6PT and G6Pase-beta to produce endogenous glucose. Inactivation of either G6PT or G6Pase-beta increases neutrophil apoptosis, which underlies, at least in part, neutrophil loss (neutropenia) and dysfunction in GSD-Ib and G6Pase-beta deficiency. Dietary and/or granulocyte colony-stimulating factor therapies are available; however, many aspects of the diseases are still poorly understood. This Review will address the etiology of GSD-Ia, GSD-Ib and G6Pase-beta deficiency and highlight advances in diagnosis and new treatment approaches, including gene therapy.