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Hyperinsulinism-hyperammonemia syndrome (HIHA) is a frequent form of diazoxide-sensitive diffuse hyperinsulinism (see this term), characterized by an excessive/ uncontrolled insulin secretion (inappropriate for the level of glycemia), asymptomatic hyperammonemia and recurrent episodes of profound hypoglycemia induced by fasting and protein rich meals, requiring rapid and intensive treatment to prevent neurological sequelae. Epilepsy and cognitive deficit that are unrelated to hypoglycemia may also occur.
ORPHA:35878Classification level: Disorder
Prevalence is estimated at 1 in 200,000.
Clinical features are similar to those described in congenital isolated hyperinsulinism (see this term), but the manifestations are milder with diagnosis delayed until late infancy. Hypoglycemia is triggered by fasting or a protein-rich meal (leucine sensitive hypoglycemia) and is easily controlled by diazoxide. Asymptomatic and persistent hyperammonemia (about 3 to 5 times the normal range) is observed. Children with HIHA frequently present seizures (most commonly atypical or absence) and learning difficulties. Neurologic abnormalities appear to be unrelated to hypoglycemia.
Glutamate dehydrogenase (GDH) is a mitochondrial matrix enzyme that catalyzes the oxidative deamination of glutamate, yielding alpha-ketoglutarate and an ammonia by-product. Production of alpha-ketoglutarate leads to an increase in intracellular ATP causing the closure of ATP-sensitive potassium channels (SUR1/Kir6.2). This in turn leads to an influx of calcium ions and the ensuing cellular depolarization triggers the secretion of insulin from pancreatic beta-cells. GDH is under a complex regulatory control of allosterically activating leucine and inhibiting GTP. Dominantly expressed, missense mutations of GLUD1 (10q23.3) that encodes GDH, increase enzyme activity by reducing its sensitivity to allosteric inhibition by guanosine triphosphate (GTP). The role of hyperammonemia is not known; it may primarily be the result of the renal expression of altered forms of GDH.
Most activating mutations of the gene GLUD1 are de novo, but the familial forms that have been reported are dominant. Incomplete variance and clinical variability are noted within the same families.
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