title: N-carbamylglutamate enhancement of ureagenesis leads to discovery of a novel deleterious mutation in a newly defined enhancer of the NAGS gene and to effective therapy.
abstract: N-acetylglutamate synthase (NAGS) catalyzes the conversion of glutamate and acetyl-CoA to NAG, the essential allosteric activator of carbamyl phosphate synthetase I, the first urea cycle enzyme in mammals. A 17-year-old female with recurrent hyperammonemia attacks, the cause of which remained undiagnosed for 8 years in spite of multiple molecular and biochemical investigations, showed markedly enhanced ureagenesis (measured by isotope incorporation) in response to N-carbamylglutamate (NCG). This led to sequencing of the regulatory regions of the NAGS gene and identification of a deleterious single-base substitution in the upstream enhancer. The homozygous mutation (c.-3064C>A), affecting a highly conserved nucleotide within the hepatic nuclear factor 1 (HNF-1) binding site, was not found in single nucleotide polymorphism databases and in a screen of 1,086 alleles from a diverse population. Functional assays demonstrated that this mutation decreases transcription and binding of HNF-1 to the NAGS gene, while a consensus HNF-1 binding sequence enhances binding to HNF-1 and increases transcription. Oral daily NCG therapy restored ureagenesis in this patient, normalizing her biochemical markers, and allowing discontinuation of alternate pathway therapy and normalization of her diet with no recurrence of hyperammonemia. Inc.
authors.affiliation: Research Center for Genetic Medicine, Children's National Medical Center, The George Washington University, Washington, DC, USA.
authors.forename: Sandra K
authors.initials: SK
authors.lastname: Heibel
authors.forename: Nicholas
authors.initials: N
authors.lastname: Ah Mew
authors.forename: Ljubica
authors.initials: L
authors.lastname: Caldovic
authors.forename: Yevgeny
authors.initials: Y
authors.lastname: Daikhin
authors.forename: Marc
authors.initials: M
authors.lastname: Yudkoff
authors.forename: Mendel
authors.initials: M
authors.lastname: Tuchman
mesh_terms: D000293:Adolescent; D000483:Alleles; D051046:Amino-Acid N-Acetyltransferase / Q000235:genetics*; D001483:Base Sequence; D001665:Binding Sites; D045744:Cell Line, Tumor; D002648:Child; D004742:Enhancer Elements, Genetic; D005260:Female; D005787:Gene Frequency; D005971:Glutamates / Q000378:metabolism / Q000627:therapeutic use*; D056945:Hep G2 Cells; D051537:Hepatocyte Nuclear Factor 1 / Q000378:metabolism; D006801:Humans; D059372:Nucleotide Motifs; D020641:Polymorphism, Single Nucleotide; D016415:Sequence Alignment; D017384:Sequence Deletion; D016896:Treatment Outcome; D056806:Urea Cycle Disorders, Inborn / Q000188:drug therapy* / Q000235:genetics* / Q000378:metabolism
chemical_list: D005971:Glutamates; C006895:N-carbamylglutamate; D051537:Hepatocyte Nuclear Factor 1; D051046:Amino-Acid N-Acetyltransferase