Null mutations in the N-acetylglutamate synthase gene associated with acute neonatal disease and hyperammonemia
ABSTRACT N-acetylglutamate synthase (NAGS) is a mitochondrial enzyme that catalyzes the formation of N-acetylglutamate, an essential allosteric activator of carbamyl phosphate synthetase I, the first enzyme of the urea cycle. Liver NAGS deficiency has previously been found in a small number of patients with hyperammonemia. The mouse and human NAGS genes have recently been cloned and expressed in our laboratory. We searched for mutations in the NAGS gene of two families with presumed NAGS deficiency. The exons and exon/intron boundaries of the NAGS gene were sequenced from genomic DNA obtained from the parents of an infant from the Faroe Islands who died in the neonatal period and from two Hispanic sisters who presented with acute neonatal hyperammonemia. Both parents of the first patient were found to be heterozygous for a null mutation in exon 4 (TGG-->TAG, Trp324Ter). Both sisters from the second family were homozygous for a single base deletion in exon 4 (1025delG) causing a frameshift and premature termination of translation. The finding of deleterious mutations in the NAGS gene confirms the genetic origin of NAGS deficiency. This disorder can now be diagnosed by DNA testing allowing for carrier detection and prenatal diagnosis.
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ABSTRACT: Hyperammonemia related to urea cycle disorders is a rare cause of potentially fatal encephalopathy that is encountered in intensive care units (ICUs). Left undiagnosed, this condition may manifest irreversible neuronal damage. However, timely diagnosis and treatment initiation can be facilitated simply by increased awareness of the ICU staff. Here, we describe a patient with acute severe pancreatitis who developed hyperammonemia and encephalopathy without liver disease. Urea cycle disorder was suspected and hemodialysis was initiated. Following reduction of ammonia levels, subsequent treatment included protein restriction and administration of arginine and sodium benzoate. The patient was discharged to home after 47 days with plasma ammonia within normal range and without neurological symptoms. In clinical care settings, patients with neurological symptoms unexplained by the present illness should be assessed for serum ammonia levels to disclose any urea cycle disorders to initiate timely treatment and improve outcome.Case Reports in Medicine 08/2013; 2013:903546. DOI:10.1155/2013/903546
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ABSTRACT: N-acetylglutamate synthase (NAGS) cat-alyzes the conversion of glutamate and acetyl-CoA to NAG, the essential allosteric activator of carbamyl phos-phate synthetase I, the first urea cycle enzyme in mam-mals. A 17-year-old female with recurrent hyperammone-mia attacks, the cause of which remained undiagnosed for 8 years in spite of multiple molecular and biochemi-cal 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), af-fecting 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 as-says demonstrated that this mutation decreases transcrip-tion 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 ther-apy restored ureagenesis in this patient, normalizing her biochemical markers, and allowing discontinuation of al-ternate pathway therapy and normalization of her diet with no recurrence of hyperammonemia. Hum Mutat 32:1153–1160, 2011. C 2011 Wiley-Liss, Inc.Human Mutation 01/2011; 32(10). · 5.05 Impact Factor
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ABSTRACT: Communicated by David Rosenblatt N-acetylglutamate synthase (NAGS) deficiency, an autosomal recessive disorder, is the last urea cycle disorder for which molecular testing became available. This is the first comprehensive report of 21 mutations that cause NAGS deficiency and of commonly found polymorphisms in the NAGS gene. Five mutations are reported here for the first time. A total of 10 disease-causing mutations are associated with acute neonatal hyperammonemia; the remaining mutations were found in patients with late onset disease. Residual enzymatic activities are included in this report and the deleterious effects of eight mutations were confirmed by expression studies. Mutations in the NAGS gene are distributed throughout its reading frame. No mutations have been found in exon 1, which encodes for the putative mitochondrial targeting signal and variable segment of NAGS. Three polymorphisms have been found. Early, accurate, and specific diagnosis of NAGS deficiency is critical since this condition can be successfully treated with N-carbamylglutamate (NCG, Carbaglu s ; Orphan Europe). Treatment with NCG should be initiated as soon as a patient is suspected of having NAGS deficiency. Molecular testing represents the most reliable method of diagnosis. Hum Mutat 28(8), 754–759, 2007. Published 2007 Wiley-Liss, Inc. y KEY WORDS: NAGS; urea cycle; hyperammonemia; inherited metabolic disease; inborn errors of metabolism N-acetylglutamate synthase (NAGS; MIM] 608300; EC 126.96.36.199) is located in the mitochondrial matrix of liver and intestinal cells, and catalyzes formation of N-acetylglutamate (NAG) from glutamate and acetyl coenzyme A (AcCoA) [Bachmann et al., 1982b; Shigesada and Tatibana, 1978; Sonoda and Tatibana, 1983]. NAG is an essential allosteric activator of carbamylphosphate synthetase I (CPSI), the first and rate limiting enzyme of the urea cycle [Hall et al., 1958; Waterlow, 1999]. NAGS deficiency is an autosomal recessive disorder that appeared to be the least prevalent of all urea cycle disorders [Brusilow and Horwich, 2001]. However, the apparent low prevalence may have been biased because the liver-enzyme assay requires an open biopsy, is technically difficult, and is not completely reliable [Colombo et al., 1982; Heckmann et al., 2005], and molecular diagnosis of NAGS deficiency has only recently been possible [Caldovic et al., 2003; Elpeleg et al., 2002; Haberle et al., 2003b]. NAGS was the last urea cycle gene to be cloned [Caldovic et al., 2002b]. The gene is located on the long arm of chromosome 17 within band 17q21.31 [Caldovic et al., 2002a]. The gene spans 4.5 kb, has an open reading frame of 1,605 nucleotides, and contains seven exons and six introns [Caldovic et al., 2002a, 2002b; Haberle et al., 2003b]. NAGS is expressed in the liver and intestine and possibly to a lesser degree in other tissues [Caldovic et al., 2002b]. The mRNA encodes a precursor protein containing 534 amino acids with a predicted molecular weight of 58.1 kDa. At the N-terminus of human NAGS preprotein is a predicted mitochondrial targeting signal that is 50 amino acids long. It is presumably cleaved upon NAGS import into the mitochondria to produce a 486–amino acid–long mature protein with a predicted molecular weight of 53.2 kDa. The functional enzyme is likely to be a homodimer, based on its similarity to bacterial NAG kinases [Shi et al., 2006]. The phenotype of NAGS deficiency ranges from acute neonatal hyperammonemic coma [Caldovic et al., 2003; Elpeleg et al., 2002] to adult onset hyperammonemia that can result in coma and death in the most severe cases [Caldovic et al., 2005]. Heterozygous individuals appear to be asymptomatic [Caldovic et al., 2005]. Affected newborns with complete deficiency of the NAGS enzyme present with acute neonatal hyperammonemia, usually within the first 72 hr of life, and their presentation is indistinguishable clinically and biochemically from CPSI deficiency [Bachmann et al., 1988; Brusilow and Horwich, 2001]. Neonatal presentation correlates with the presence of either two NAGS null alleles [Caldovic et al., 2003; Elpeleg et al., 2002; Haberle et al., 2003a] or two missense mutations that completely abolish NAGS enzymatic activity [Schmidt et al., 2005]. Patients with partialHuman Mutation 01/2007; 28:754-759. · 5.05 Impact Factor