Favourable long-term outcome after immediate treatment of neonatal hyperammonemia due to N-acetylglutamate synthase deficiency

N-Acetylglutamate synthase (NAGS) deficiency is a rare urea cycle disorder, which may present in the neonatal period with severe hyperammonemia and marked neurological impairment. We report on a Turkish family with a patient who died due to hyperammonemia in the neonatal period. Reduced activity of NAGS and carbamyl phosphate synthetase were found at autopsy. A second child who developed hyperammonemia on the second day of life was immediately treated with arginine hydrochloride, sodium benzoate and protein restriction. After NAGS deficiency was suspected by enzyme analysis, sodium benzoate was replaced by N-carbamylglutamate (NCG). A third child who developed slight hyperammonemia on the third day of life was treated with NCG before enzyme analysis confirmed reduced NAGS activity. Neither of the patients developed hyperammonemia in the following years. After the human NAGS gene was identified, mutation analysis revealed that the older sibling on NCG therapy was homozygous for a 971G > A (W324X) mutation. The parents and the younger sibling were heterozygous. Therapy was continued in the older sibling until now without any adverse effects and favourable neurodevelopment outcome. In the younger sibling, therapy was stopped without any deterioration of urea cycle function. NAGS deficiency can be successfully treated with NCG and arginine hydrochloride with favourable outcome. Molecular diagnostic rather than enzyme analysis should be used in patients with suspected NAGS deficiency.