We describe a Japanese case of neonatal hyperinsulinism due to a de novo mutation (Gly446Asp) in glutamate dehydrogenase gene (GLUD1). A boy suffered from hypoglycemic coma with relative hyperinsulinemia on day 1 after birth, and received subtotal pancreatectomy. Examination of the resected pancreas revealed a diffuse increase in endocrine cells, consistent with 'nesidioblastosis'. He is now 15 years old and has exhibited mild but persistent hyperammonemia, which is a very unique feature of the disorder caused by GLUD1 activating mutations. He has also been suffering from seizures and mental retardation. Thus, GLUD1 mutations can be a cause of congenital hyperinsulinism in Japanese.
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