How many forms of glycogen storage disease type I?

Glucose-6-phosphatase is a multicomponent enzymatic system of the endoplasm ic reticulum, which catalyses the terminal steps of gluconeogenesis and gly cogenolysis by converting glucose-6-phosphate to glucose and inorganic phos phate. Glycogen storage diseases type I (GSD I) are a group of metabolic di sorders arising from a defect in a component of this enzymatic system, i.e. the glucose-6-phosphate hydrolase (GSD Ia), the glucose-6-phosphate transl ocase (GSD Ib) and possibly also the translocases for inorganic phosphate ( GSD Ic) or glucose (GSD Id). The genes encoding the glucose-6-phosphate hyd rolase and the glucose-6-phosphate translocase have both been cloned and as signed to human chromosomes 17q21 and 11q33, respectively. Investigation of patients with GSD I shows that those with GSD Ia are mutated in the glucos e-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 the refore GSD Ib patients, in agreement with the fact that they all have neutr openia or neutrophil dysfunction. This suggests that the biochemical assays used to differentiate GSD Ic and GSD Id from GSD Ib are not reliable. Conclusion In practice therefore appears to be only two types of GSD I (Ta and Ib), which can be differentiated by (1) measurement of glucose-6-phosph atase activity in fresh and detergent-treated homogenates and (2) by mutati on search in the genes encoding the glucose-6-phosphate hydrolase and the g lucose-6-phosphate translocase.