Understanding N-Acetyl-L-Glutamate Synthase Deficiency: Mutational Spectrum, Impact of Clinical Mutations on Enzyme Functionality, and Structural Considerations

Abstract

N-acetyl-L-glutamate synthase (NAGS) deficiency (NAGSD), the rarest urea cycle defect, is clinically indistinguishable from carbamoyl phosphate synthetase 1 deficiency, rendering the identification of NAGS gene mutations key for differentiation, which is crucial, as only NAGSD has substitutive therapy. Over the last 13 years, we have identified 43 patients from 33 families with NAGS mutations, of which 14 were novel. Overall, 36 NAGS mutations have been found so far in 56 patients from 42 families, of which 76% are homozygous for the mutant allele. 61% of mutations are missense changes. Lack or decrease of NAGS protein is predicted for ∼1/3 of mutations. Missense mutations frequency is inhomogeneous along NAGS: null for exon 1, but six in exon 6, which reflects the paramount substrate binding/catalytic role of the C-terminal domain (GNAT domain). Correspondingly, phenotypes associated with missense mutations mapping in the GNAT domain are more severe than phenotypes of amino acid kinase domain-mapping missense mutations. Enzyme activity and stability assays with 12 mutations introduced into pure recombinant Pseudomonas aeruginosa NAGS, together with in silico structural analysis, support the pathogenic role of most NAGSD-associated mutations found. The disease-causing mechanisms appear to be, from higher to lower frequency, decreased solubility/stability, aberrant kinetics/catalysis, and altered arginine modulation.

Normal Neurological Development During Infancy Despite Massive Hyperammonemia in Early Treated NAGS Deficiency

“…This was later confirmed by DNA-analysis, which identified a known homozygous nonsense mutation in exon 4 of the NAGS gene, c.971G>A (p.Trp324*) (University Children's Hospital, Z€ urich, Switzerland) (Sancho-Vaello et al 2016). The parents were confirmed to be heterozygous for the same mutation.…”

“…Prognosis is considered very poor with ammonia levels above 1,000 mmol/l, although the impact of the absolute value also depends on the duration of hyperammonemia (H€ aberle et al 2012;Picca et al 2001). With timely treatment several patients with NAGS deficiency have had a normal outcome (Sancho-Vaello et al 2016).…”

Efficacy of N-carbamoyl-L-glutamic acid for the treatment of inherited metabolic disorders

“…Subsequently, another NAGS deficiency case was identified in a neonate with a similar lethal presentation [14]. Since then, more than 50 patients have been reported worldwide [15,16]. Of interest, clinical symptoms of NAGS deficiency are diverse ranging from irritability, vomiting, and lethargy to behavioral abnormalities.…”

“…In general, patients with the severe form of the disorder become symptomatic during the neonatal period with hyperammonemic encephalopathy, while milder cases present in adulthood with neuropsychiatric or behavioral symptoms [15]. A recent cohort and biochemical study identified that mutations in acetyltransferase (GNAT) domain are more frequent than in the other kinetic amino acid kinase domain [16]. Additionally, mutations affecting GNAT domain are more deleterious and cause severe neonatal onset form [16].…”

“…A recent cohort and biochemical study identified that mutations in acetyltransferase (GNAT) domain are more frequent than in the other kinetic amino acid kinase domain [16]. Additionally, mutations affecting GNAT domain are more deleterious and cause severe neonatal onset form [16]. Because of its rarity, the prevalence of this disorder is unclear but the distribution of cases is panethnic [15,17].…”