How many forms of glycogen storage disease type I?

Glucose-6-phosphatase is a multicomponent enzymatic system of the endoplasmic reticulum, which catalyses the terminal steps of gluconeogenesis and glycogenolysis by converting glucose-6-phosphate to glucose and inorganic phosphate. Glycogen storage diseases type I (GSD I) are a group of metabolic disorders arising from a defect in a component of this enzymatic system, i.e. the glucose-6-phosphate hydrolase (GSD Ia), the glucose-6-phosphate translocase (GSD Ib) and possibly also the translocases for inorganic phosphate (GSD Ic) or glucose (GSD Id). The genes encoding the glucose-6-phosphate hydrolase and the glucose-6-phosphate translocase have both been cloned and assigned to human chromosomes 17q21 and 11q23, respectively. Investigation of patients with GSD I shows that those with GSD Ia are mutated in the glucose-6-phosphate hydrolase gene, whereas those diagnosed as GSD Ib, GSD Ic or GSD Id are mutated in the glucose-6-phosphate translocase gene, and are therefore GSD Ib patients, in agreement with the fact that they all have neutropenia or neutrophil dysfunction. This suggests that the biochemical assays used to differentiate GSD Ic and GSD Id from GSD Ib are not reliable.

In practice therefore appears to be only two types of GSD I (Ia and Ib), which can be differentiated by (1) measurement of glucose-6-phosphatase activity in fresh and detergent-treated homogenates and (2) by mutation search in the genes encoding the glucose-6-phosphate hydrolase and the glucose-6-phosphate translocase.