Molecular characterization of congenital hyperinsulinism/hyperammonemia caused by glutamate dehydrogenase gene defects

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Congenital hyperinsulinism and hyperammonemia (CHH) is caused by dysregulation of glutamate dehydrogenase (GDH). We characterized the GDH gene in two Japanese patients with CHH. Patient 1 showed late-onset and mild hypoglycemic episodes and mild hyperammonemia, compared with patient 2. In GDH activity of lymphoblasts, patient 1 showed 2-fold higher basal GDH activity than control subjects and mild insensitivity for GTP inhibition. Patient 2 showed severe insensitivity for GTP inhibition, and similar allosteric stimulation by ADP in the controls. Genetic studies identified heterozygous and de novo L413V and G446D mutations in patients 1 and 2, respectively. COS cell expression study confirmed that both mutations were disease-causing gene. The insensitivity for GTP inhibition in L413V and G446D was emphasized in COS cell expression system as a result of the dosage effect of mutant GDH gene. L413V showed less impairment of GDH than G446D based on biochemical and genetic results, which was consistent with the clinical phenotype. Based on the structure of bovine GDH, G446D was located in GTP binding site of pivot helix and its surroundings, while L413V was located in α-helix of antenna-like structure. These different locations of mutations gave different effects on GDH enzyme. The antenna-like structure plays an important role in GDH activity.