Mutation analysis in patients with N-acetylglutamate synthase deficiency

Abstract

N-acetylglutamate synthase (NAGS) is the key enzyme for the regulation of the hepatic urea cycle and is also highly expressed in kidney and gut. The reaction product, N-acetylglutamate, is an allosteric activator of carbamylphosphate synthetase 1 in the liver, catalyzing the initial step of ammonia detoxification. NAGS deficiency is a rare inborn error of metabolism inherited as an autosomal recessive trait leading to hyperammonemia. Using homology search based on genetic information of ascomycetes, we identified the human gene for NAGS on chromosome 17q21.31. There is a distinct pattern of organospecific expression of transcripts in liver, small intestine, and kidney similar to the other mitochondrially located enzymes of the urea cycle. The encoded 534 amino acid polypeptide has a consensus sequence for a 49 amino acid mitochondrial leader peptide. We identified private mutations of the NAGS gene in patients with severe early onset of clinical symptoms (IVS3-2A>T, c.1306_1307insT, c.971G>A/W324X, c.1289T>C/L430P, c.1299G>C/E433S, c.1450T>C/W484R), as well as in a case with late onset (c.835G>A/A279P). Four out of seven mutations were detected on exon 6. This is the first report of mutation analysis in a series of families affected with deficiency of NAGS. Molecular analysis of patients and reliable antenatal diagnostics for affected families are now feasible.

Mutations and polymorphisms in the human N-acetylglutamate synthase (NAGS) gene

“…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[Caldovic et al, , 2002bHaberle et al, 2003b]. NAGS is expressed in the liver and intestine and possibly to a lesser degree in other tissues [Caldovic et al, 2002b].…”

“…Since exon 1 encodes the mitochondrial targeting sequence and variable segment, which are poorly conserved, amino acid substitutions in this exon may be less deleterious than in other parts of the enzyme [Caldovic et al, 2002a]. Four null mutations (c.545delT, c.1026delG, c.1036dupC, and W324X) out of 21 were associated with acute neonatal hyperammonemia in homozygous patients [Caldovic et al, 2003;Elpeleg et al, 2002;Haberle et al, 2003b]. An additional two null mutations (IVS3-2A4T and c.1307dupT) resulted in acute neonatal disease in a patient that was a compound heterozygote [Haberle et al, 2003a].…”

Mammalian N-acetylglutamate synthase

“…In the current alignment, 197 amino acids are identical across vertebral species. Deleterious mutations of NAGS have been identified in patients corresponding to these regions of strong conservation [13,31,33].…”

“…Moreover, a specific treatment for NAGS deficiency using Ncarbamylglutamate, a NAG structural analog, to activate CPSI is available and approved for use in Europe (Carbaglu®, Orphan Europe). Several reports have shown that carbamylglutamate is apparently effective in the treatment of NAGS deficiency [31,[33][34][35][36][37][38]. With the availability of molecular diagnosis, early confirmation of NAGS deficiency becomes particularly compelling, as treatment with N-carbamylglutamate may be effective in preventing hyperammonemia and brain damage.…”