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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

Authors

  • Sandra K. Heibel,

    1. Research Center for Genetic Medicine, Children's National Medical Center, The George Washington University, Washington, DC
    2. Molecular and Cellular Biology Program, University of Maryland, College Park, Maryland
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    • These authors contributed equally.

  • Nicholas Ah Mew,

    1. Research Center for Genetic Medicine, Children's National Medical Center, The George Washington University, Washington, DC
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    • These authors contributed equally.

  • Ljubica Caldovic,

    1. Research Center for Genetic Medicine, Children's National Medical Center, The George Washington University, Washington, DC
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  • Yevgeny Daikhin,

    1. Department of Pediatrics, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
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  • Marc Yudkoff,

    1. Department of Pediatrics, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
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  • Mendel Tuchman

    Corresponding author
    1. Research Center for Genetic Medicine, Children's National Medical Center, The George Washington University, Washington, DC
    • Children's National Medical Center, 111 Michigan Ave, NW, Washington, DC 20010.
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  • Communicated by Mark Paalman

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. Hum Mutat 32:1153–1160, 2011. ©2011 Wiley-Liss, Inc.

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