Mutation analysis in patients with N-acetylglutamate synthase deficiency

Use of pure recombinant human enzymes to assess the disease-causing potential of missense mutations in urea cycle disorders, applied to N-acetylglutamate synthase deficiency

N-acetylglutamate synthase (NAGS) makes acetylglutamate, the essential activator of the first, regulatory enzyme of the urea cycle, carbamoyl phosphate synthetase 1 (CPS1). NAGS deficiency (NAGSD) and CPS1 deficiency (CPS1D) present identical phenotypes. However, they must be distinguished, because NAGSD is cured by substitutive therapy with the N-acetyl-L-glutamate analogue N-carbamyl-L-glutamate, while curative therapy of CPS1D requires liver transplantation. Since their differentiation is done genetically, it is important to ascertain the disease-causing potential of CPS1 and NAGS genetic variants. With this goal, we previously carried out site-directed mutagenesis studies with pure recombinant human CPS1. We could not do the same with human NAGS (HuNAGS) because of enzyme instability, leading to our prior utilization of a bacterial NAGS as an imperfect surrogate of HuNAGS. We now use genuine HuNAGS, stabilized as a chimera of its conserved domain (cHuNAGS) with the maltose binding protein (MBP), and produced in Escherichia coli. MBP-cHuNAGS linker cleavage allowed assessment of the enzymatic properties and thermal stability of cHuNAGS, either wild-type or hosting each one of 23 nonsynonymous single-base changes found in NAGSD patients. For all but one change, disease causation was accounted by the enzymatic alterations identified, including, depending on the variant, loss of arginine activation, increased Km Glutamate, active site inactivation, decreased thermal stability, and protein misfolding. Our present approach outperforms experimental in vitro use of bacterial NAGS or in silico utilization of prediction servers (including AlphaMissense), illustrating with HuNAGS the value for UCDs of using recombinant enzymes for assessing disease-causation and molecular pathogenesis, and for therapeutic guidance.

Novel pathogenic variant (c.2947C > T) of the carbamoyl phosphate synthetase 1 gene in neonatal-onset deficiency

Background

Carbamoyl phosphate synthetase 1 deficiency (CPS1D) is a rare autosomal recessive urea cycle disorder characterized by hyperammonaemia. The biochemical measurement of the intermediate metabolites is helpful for CPS1D diagnosis; it however cannot distinguish CPS1D from N-acetylglutamate synthetase deficiency. Therefore, next-generation sequencing (NGS) is often essential for the accurate diagnosis of CPS1D.

Methods

NGS was performed to identify candidate gene variants of CPS1D in a Asian neonatal patient presented with poor feeding, reduced activity, tachypnea, lethargy, and convulsions. The potential pathogenicity of the identified variants was predicted by various types of bioinformatical analyses, including evolution conservation, domain and 3D structure simulations.

Results

Compound heterozygosity of CPS1D were identified. One was in exon 24 with a novel heterozygous missense variant c.2947C > T (p.P983S), and another was previously reported in exon 20 with c.2548C > T (p.R850C). Both variants were predicted to be deleterious. Conservation analysis and structural modeling showed that the two substituted amino acids were highly evolutionarily conserved, resulting in potential decreases of the binding pocket stability and the partial loss of enzyme activity.

Conclusion

In this study, two pathogenic missense variants were identified with NGS, expanding the variants pectrum of the CPS1 gene. The variants and related structural knowledge of CPS enzyme demonstrate the applicability for the accurate diagnosis of CPS1D.

Noncoding sequence variants define a novel regulatory element in the first intron of the N-acetylglutamate synthase gene

N-acetylglutamate synthase deficiency is an autosomal recessive urea cycle disorder caused either by decreased expression of the NAGS gene or defective NAGS enzyme resulting in decreased production of N-acetylglutamate (NAG), an allosteric activator of carbamylphosphate synthetase 1 (CPS1). NAGSD is the only urea cycle disorder that can be effectively treated with a single drug, N-carbamylglutamate (NCG), a stable NAG analog, which activates CPS1 to restore ureagenesis. We describe three patients with NAGSD due to four novel noncoding sequence variants in the NAGS regulatory regions. All three patients had hyperammonemia that resolved upon treatment with NCG. Sequence variants NM_153006.2:c.427-222G>A and NM_153006.2:c.427-218A>C reside in the 547 bp-long first intron of NAGS and define a novel NAGS regulatory element that binds retinoic X receptor α. Sequence variants NC_000017.10:g.42078967A>T (NM_153006.2:c.-3065A>T) and NC_000017.10:g.42078934C>T (NM_153006.2:c.-3098C>T) reside in the NAGS enhancer, within known HNF1 and predicted glucocorticoid receptor binding sites, respectively. Reporter gene assays in HepG2 and HuH-7 cells demonstrated that all four substitutions could result in reduced expression of NAGS. These findings show that analyzing noncoding regions of NAGS and other urea cycle genes can reveal molecular causes of disease and identify novel regulators of ureagenesis.

Clinical and structural insights into potential dominant negative triggers of proximal urea cycle disorders

Despite biochemical and genetic testing being the golden standards for identification of proximal urea cycle disorders (UCDs), genotype-phenotype correlations are often unclear. Co-occurring partial defects affecting more than one gene have not been demonstrated so far in proximal UCDs. Here, we analyzed the mutational spectrum of 557 suspected proximal UCD individuals. We probed oligomerizing forms of NAGS, CPS1 and OTC, and evaluated the surface exposure of residues mutated in heterozygously affected individuals. BN-PAGE and gel-filtration chromatography were employed to discover protein-protein interactions within recombinant enzymes. From a total of 281 confirmed patients, only 15 were identified as "heterozygous-only" candidates (i.e. single defective allele). Within these cases, the only missense variants to potentially qualify as dominant negative triggers were CPS1 p.Gly401Arg and NAGS p.Thr181Ala and p.Tyr512Cys, as assessed by residue oligomerization capacity and surface exposure. However, all three candidates seem to participate in critical intramolecular functions, thus, unlikely to facilitate protein-protein interactions. This interpretation is further supported by BN-PAGE and gel-filtration analyses revealing no multiprotein proximal urea cycle complex formation. Collectively, genetic analysis, structural considerations and in vitro experiments point against a prominent role of dominant negative effects in human proximal UCDs.

Presentation and management of N-acetylglutamate synthase deficiency: a review of the literature

Background

N-Acetylglutamate synthase (NAGS) deficiency is an extremely rare autosomal recessive metabolic disorder affecting the urea cycle, leading to episodes of hyperammonemia which can cause significant morbidity and mortality. Since its recognition in 1981, NAGS deficiency has been treated with carbamylglutamate with or without other measures (nutritional, ammonia scavengers, dialytic, etc.). We conducted a systematic literature review of NAGS deficiency to summarize current knowledge around presentation and management.

Methods

Case reports and case series were identified using the Medline database, as well as references from other articles and a general internet search. Clinical data related to presentation and management were abstracted by two reviewers.

Results

In total, 98 cases of NAGS deficiency from 79 families, in 48 articles or abstracts were identified. Of these, 1 was diagnosed prenatally, 57 were neonatal cases, 34 were post-neonatal, and 6 did not specify age at presentation or were asymptomatic at diagnosis. Twenty-one cases had relevant family history. We summarize triggers of hyperammonemic episodes, diagnosis, clinical signs and symptoms, and management strategies. DNA testing is the preferred method of diagnosis, although therapeutic trials to assess response of ammonia levels to carbamylglutamate may also be helpful. Management usually consists of treatment with carbamylglutamate, although the reported maintenance dose varied across case reports. Protein restriction was sometimes used in conjunction with carbamylglutamate. Supplementation with citrulline, arginine, and sodium benzoate also were reported.

Conclusions

Presentation of NAGS deficiency varies by age and symptoms. In addition, both diagnosis and management have evolved over time and vary across clinics. Prompt recognition and appropriate treatment of NAGS deficiency with carbamylglutamate may improve outcomes of affected individuals. Further research is needed to assess the roles of protein restriction and supplements in the treatment of NAGS deficiency, especially during times of illness or lack of access to carbamylglutamate.

N-Acetylglutamate Synthase Deficiency Due to a Recurrent Sequence Variant in the N-acetylglutamate Synthase Enhancer Region

N-acetylglutamate synthase deficiency (NAGSD, MIM #237310) is an autosomal recessive disorder of the urea cycle that results from absent or decreased production of N-acetylglutamate (NAG) due to either decreased NAGS gene expression or defective NAGS enzyme. NAG is essential for the activity of carbamylphosphate synthetase 1 (CPS1), the first and rate-limiting enzyme of the urea cycle. NAGSD is the only urea cycle disorder that can be treated with a single drug, N-carbamylglutamate (NCG), which can activate CPS1 and completely restore ureagenesis in patients with NAGSD. We describe a novel sequence variant NM_153006.2:c.-3026C > T in the NAGS enhancer that was found in three patients from two families with NAGSD; two patients had hyperammonemia that resolved upon treatment with NCG, while the third patient increased dietary protein intake after initiation of NCG therapy. Two patients were homozygous for the variant while the third patient had the c.-3026C > T variant and a partial uniparental disomy that encompassed the NAGS gene on chromosome 17. The c.-3026C > T sequence variant affects a base pair that is highly conserved in vertebrates; the variant is predicted to be deleterious by several bioinformatics tools. Functional assays in cultured HepG2 cells demonstrated that the c.-3026C > T substitution could result in reduced expression of the NAGS gene. These findings underscore the importance of analyzing NAGS gene regulatory regions when looking for molecular causes of NAGSD.

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

INTRODUCTION

N-carbamoyl-L-glutamic acid (NCG) is a synthetic analogue of N-acetyl glutamate (NAG) that works effectively as a cofactor for carbamoyl phosphate synthase 1 and enhances ureagenesis by activating the urea cycle. NCG (brand name, Carbaglu) was recently approved by the United States Food and Drug Administration (US FDA) for the management of NAGS deficiency and by the European Medicines Agency (EMA) for the treatment of NAGS deficiency as well as for the treatment of hyperammonenia in propionic, methylmalonic and isovaleric acidemias in Europe.

AREAS COVERED

The history, mechanism of action, and efficacy of this new drug are described. Moreover, clinical utility of NCG in a variety of inborn errors of metabolism with secondary NAGS deficiency is discussed.

EXPERT COMMENTARY

NCG has favorable pharmacological features including better bioavailability compared to NAG. The clinical use of NCG has proven to be so effective as to make dietary protein restriction unnecessary for patients with NAGS deficiency. It has been also demonstrated to be effective for hyperammonemia secondary to other types of inborn errors of metabolism. NCG may have additional therapeutic potential in conditions such as hepatic hyperammonemic encephalopathy secondary to chemotherapies or other liver pathology.

N-acetylglutamate synthase deficiency: Novel mutation associated with neonatal presentation and literature review of molecular and phenotypic spectra

The urea cycle is the main pathway for the disposal of excess nitrogen. Carbamoylphosphate synthetase 1 (CPS1), the first and rate-limiting enzyme of urea cycle, is activated by N-acetylglutamate (NAG), and thus N-acetylglutamate synthase (NAGS) is an essential part of the urea cycle. Although NAGS deficiency is the rarest urea cycle disorder, it is the only one that can be specifically and effectively treated by a drug, N-carbamylglutamate, a stable structural analogous of NAG that activates CPS1. Here we report an infant with NAGS deficiency who presented with neonatal hyperammonemia. She was found to have a novel homozygous splice-site mutation, c.1097-2A>T, in the NAGS gene. We describe the clinical course of this infant, who had rapid response to N-carbamylglutamate treatment. In addition, we reviewed the clinical and molecular spectra of previously reported individuals with NAGS deficiency, which presents in most cases with neonatal hyperammonemia, and in some cases the presentation is later, with a broad spectrum of ages and manifestations. With this broad later-onset phenotypic spectrum, maintaining a high index of suspicion is needed for the early diagnosis of this treatable disease.

Human Metabolic Enzymes Deficiency: A Genetic Mutation Based Approach

One of the extreme challenges in biology is to ameliorate the understanding of the mechanisms which emphasize metabolic enzyme deficiency (MED) and how these pretend to have influence on human health. However, it has been manifested that MED could be either inherited as inborn error of metabolism (IEM) or acquired, which carries a high risk of interrupted biochemical reactions. Enzyme deficiency results in accumulation of toxic compounds that may disrupt normal organ functions and cause failure in producing crucial biological compounds and other intermediates. The MED related disorders cover widespread clinical presentations and can involve almost any organ system. To sum up the causal factors of almost all the MED-associated disorders, we decided to embark on a less traveled but nonetheless relevant direction, by focusing our attention on associated gene family products, regulation of their expression, genetic mutation, and mutation types. In addition, the review also outlines the clinical presentations as well as diagnostic and therapeutic approaches.

The N-Acetylglutamate Synthase Family: Structures, Function and Mechanisms

N-acetylglutamate synthase (NAGS) catalyzes the production of N-acetylglutamate (NAG) from acetyl-CoA and l-glutamate. In microorganisms and plants, the enzyme functions in the arginine biosynthetic pathway, while in mammals, its major role is to produce the essential co-factor of carbamoyl phosphate synthetase 1 (CPS1) in the urea cycle. Recent work has shown that several different genes encode enzymes that can catalyze NAG formation. A bifunctional enzyme was identified in certain bacteria, which catalyzes both NAGS and N-acetylglutamate kinase (NAGK) activities, the first two steps of the arginine biosynthetic pathway. Interestingly, these bifunctional enzymes have higher sequence similarity to vertebrate NAGS than those of the classical (mono-functional) bacterial NAGS. Solving the structures for both classical bacterial NAGS and bifunctional vertebrate-like NAGS/K has advanced our insight into the regulation and catalytic mechanisms of NAGS, and the evolutionary relationship between the two NAGS groups.

Recurrent encephalopathy: NAGS (N-acetylglutamate synthase) deficiency in adults

N-acetyl-glutamate synthase (NAGS) deficiency is a rare autosomal recessive urea cycle disorder (UCD) that uncommonly presents in adulthood. Adult presentations of UCDs include; confusional episodes, neuropsychiatric symptoms and encephalopathy. To date, there have been no detailed neurological descriptions of an adult onset presentation of NAGS deficiency. In this review we examine the clinical presentation and management of UCDs with an emphasis on NAGS deficiency. An illustrative case is provided. Plasma ammonia levels should be measured in all adult patients with unexplained encephalopathy, as treatment can be potentially life-saving. Availability of N-carbamylglutamate (NCG; carglumic acid) has made protein restriction largely unnecessary in treatment regimens currently employed. Genetic counselling remains an essential component of management of NAGS.

Carglumic acid for the treatment of N-acetylglutamate synthase deficiency and acute hyperammonemia

Carglumic acid is a structural analog and the first registered synthetic form of the naturally occurring allosteric activator of the urea cycle, N-acetylglutamate (NAG), which is the product of the enzyme NAG synthase (NAGS). Because NAG is essential for the function of carbamoylphosphate synthetase 1 as the first step of the urea cycle, a decreased availability of NAG due to primary or secondary defects of NAGS will affect ammonia detoxification in the urea cycle. Carglumic acid (Carbaglu®, Orphan Europe SARL, Paris, France) is approved for the acute and long-term treatment of primary defects of NAGS in Europe and the USA. In addition, it is approved in Europe for the treatment of acute hyperammonemia in patients with specific organic acidurias that can lead to NAG deficiency secondary to inhibition of NAGS. This article reviews the use of carglumic acid for both approved indications and considers the potential of this compound for acute hyperammonemias in general.

Insight on an arginine synthesis metabolon from the tetrameric structure of yeast acetylglutamate kinase

N-acetyl-L-glutamate kinase (NAGK) catalyzes the second, generally controlling, step of arginine biosynthesis. In yeasts, NAGK exists either alone or forming a metabolon with N-acetyl-L-glutamate synthase (NAGS), which catalyzes the first step and exists only within the metabolon. Yeast NAGK (yNAGK) has, in addition to the amino acid kinase (AAK) domain found in other NAGKs, a ∼150-residue C-terminal domain of unclear significance belonging to the DUF619 domain family. We deleted this domain, proving that it stabilizes yNAGK, slows catalysis and modulates feed-back inhibition by arginine. We determined the crystal structures of both the DUF619 domain-lacking yNAGK, ligand-free as well as complexed with acetylglutamate or acetylglutamate and arginine, and of complete mature yNAGK. While all other known arginine-inhibitable NAGKs are doughnut-like hexameric trimers of dimers of AAK domains, yNAGK has as central structure a flat tetramer formed by two dimers of AAK domains. These dimers differ from canonical AAK dimers in the −110° rotation of one subunit with respect to the other. In the hexameric enzymes, an N-terminal extension, found in all arginine-inhibitable NAGKs, forms a protruding helix that interlaces the dimers. In yNAGK, however, it conforms a two-helix platform that mediates interdimeric interactions. Arginine appears to freeze an open inactive AAK domain conformation. In the complete yNAGK structure, two pairs of DUF619 domains flank the AAK domain tetramer, providing a mechanism for the DUF619 domain modulatory functions. The DUF619 domain exhibits the histone acetyltransferase fold, resembling the catalytic domain of bacterial NAGS. However, the putative acetyl CoA site is blocked, explaining the lack of NAGS activity of yNAGK. We conclude that the tetrameric architecture is an adaptation to metabolon formation and propose an organization for this metabolon, suggesting that yNAGK may be a good model also for yeast and human NAGSs.

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

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.

N-acetylglutamate synthase deficiency: an insight into the genetics, epidemiology, pathophysiology, and treatment

The conversion of ammonia into urea by the human liver requires the coordinated function of the 6 enzymes and 2 transporters of the urea cycle. The initial and rate-limiting enzyme of the urea cycle, carbamylphosphate synthetase 1 (CPS1), requires an allosteric activator, N-acetylglutamate (NAG). The formation of this unique cofactor from glutamate and acetyl Coenzyme-A is catalyzed by N-acetylglutamate synthase (NAGS). An absence of NAG as a consequence of NAGS deficiency may compromise flux through CPS1 and result in hyperammonemia. The NAGS gene encodes a 528-amino acid protein, consisting of a C-terminal catalytic domain, a variable segment, and an N-terminal mitochondrial targeting signal. Only 22 mutations in the NAGS gene have been reported to date, mostly in the catalytic domain. NAGS is primarily expressed in the liver and intestine. However, it is also surprisingly expressed in testis, stomach and spleen, and during early embryonic development at levels not concordant with the expression of other urea cycle enzymes, CPS1, or ornithine transcarbamylase. The purpose of NAGS expression in these tissues, and its significance to NAGS deficiency is as yet unknown. Inherited NAGS deficiency is the rarest of the urea cycle disorders, and we review the currently reported 34 cases. Treatment of NAGS deficiency with N-carbamyglutamate, a stable analog of NAG, can restore deficient urea cycle function and normalize blood ammonia in affected patients.

Role of carglumic acid in the treatment of acute hyperammonemia due to N-acetylglutamate synthase deficiency

N-acetylglutamate synthase (NAGS) deficiency is a rare inborn error of metabolism affecting ammonia detoxification in the urea cycle. The product of NAGS is N-acetylglutamate which is the absolutely required allosteric activator of the first urea cycle enzyme carbamoylphosphate synthetase 1. In defects of NAGS, the urea cycle function can be severely affected resulting in fatal hyperammonemia in neonatal patients or at any later stage in life. NAGS deficiency can be treated with a structural analog of N-acetylglutamate, N-carbamyl-L-glutamate, which is available for enteral use as a licensed drug. Since NAGS deficiency is an extremely rare disorder, reports on the use of N-carbamyl-L-glutamate are mainly based on single patients. According to these, the drug is very effective in treating acute hyperammonemia by avoiding the need for detoxification during the acute metabolic decompensation. Also during long-term treatment, N-carbamyl-L-glutamate is effective in maintaining normal plasma ammonia levels and avoiding the need for additional drug therapy or protein-restricted diet. Open questions remain which concern the optimal dosage in acute and long-term use of N-carbamyl-L-glutamate and potential additional disorders in which the drug might also be effective in treating acute hyperammonemia. This review focuses on the role of N-carbamyl-L-glutamate for the treatment of acute hyperammonemia due to primary NAGS deficiency but will briefly discuss the current knowledge on the role of N-carbamyl-L-glutamate for treatment of secondary NAGS deficiencies.

The nuclear receptor FXR regulates hepatic transport and metabolism of glutamine and glutamate

Hepatic transport and metabolism of glutamate and glutamine are regulated by intervention of several proteins. Glutamine is taken up by periportal hepatocytes and is the major source of ammonia for urea synthesis and glutamate for N-acetylglutamate (NAG) synthesis, which is catalyzed by the N-acetylglutamate synthase (NAGS). Glutamate is taken up by perivenous hepatocytes and is the main source for the synthesis of glutamine, catalyzed by glutamine synthase (GS). Accumulation of glutamate and ammonia is a common feature of chronic liver failure, but mechanism that leads to failure of the urea cycle in this setting is unknown. The Farnesoid X Receptor (FXR) is a bile acid sensor in hepatocytes. Here, we have investigated its role in the regulation of the metabolism of both glutamine and glutamate. In vitro studies in primary cultures of hepatocytes from wild type and FXR(-/-) mice and HepG2 cells, and in vivo studies, in FXR(-/-) mice as well as in a rodent model of hepatic liver failure induced by carbon tetrachloride (CCl(4)), demonstrate a role for FXR in regulating this metabolism. Further on, promoter analysis studies demonstrate that both human and mouse NAGS promoters contain a putative FXRE, an ER8 sequence. EMSA, ChIP and luciferase experiments carried out to investigate the functionality of this sequence demonstrate that FXR is essential to induce the expression of NAGS. In conclusion, FXR activation regulates glutamine and glutamate metabolism and FXR ligands might have utility in the treatment of hyperammonemia states.

New developments in the treatment of hyperammonemia: emerging use of carglumic acid

Hyperammonemia is a true neonatal emergency with high toxicity for the central nervous system and developmental delay. The causes of neonatal hyperammonemia are genetic defects of urea cycle enzymes, organic acidemias, lysinuric protein intolerance, hyperammonemia-hyperornithinemia- homocitrullinemia syndrome, transient hyperammonemia of the newborn, and congenital hyperinsulinism with hyperammonemia. In some of these conditions the high blood ammonia levels are due to the reduction of N-acetylglutamate, an essential cofactor necessary for the function of the urea cycle, or to the reduction of carbamoyl-phosphate synthase-I activity. In these cases, N-carbamylglutamate (carglumic acid) can be administered together with the conventional therapy. Carglumic acid is an analog of N-acetylglutamate that has a direct action on carbamoyl-phosphate synthase-I. Its effects are reactivation of the urea cycle and reduction of plasma ammonia levels. As a consequence it improves the traditional treatment, avoiding the need of hemodialysis and peritoneal dialysis. In this review we evaluate the possible field of application of carglumic acid and its effectiveness and safety.

Genetic, structural and biochemical basis of carbamoyl phosphate synthetase 1 deficiency

Carbamoyl phosphate synthetase 1 (CPS1) plays a paramount role in liver ureagenesis since it catalyzes the first and rate-limiting step of the urea cycle, the major pathway for nitrogen disposal in humans. CPS1 deficiency (CPS1D) is an autosomal recessive inborn error which leads to hyperammonemia due to mutations in the CPS1 gene, or is caused secondarily by lack of its allosteric activator NAG. Proteolytic, immunological and structural data indicate that human CPS1 resembles Escherichia coli CPS in structure, and a 3D model of CPS1 has been presented for elucidating the pathogenic role of missense mutations. Recent availability of CPS1 expression systems also can provide valuable tools for structure-function analysis and pathogenicity-testing of mutations in CPS1. In this paper, we provide a comprehensive compilation of clinical CPS1 mutations, and discuss how structural knowledge of CPS enzymes in combination with in vitro analyses can be a useful tool for diagnosis of CPS1D.

Favourable long-term outcome after immediate treatment of neonatal hyperammonemia due to N-acetylglutamate synthase deficiency

INTRODUCTION

N-Acetylglutamate synthase (NAGS) deficiency is a rare urea cycle disorder, which may present in the neonatal period with severe hyperammonemia and marked neurological impairment.

CASE REPORT

We report on a Turkish family with a patient who died due to hyperammonemia in the neonatal period. Reduced activity of NAGS and carbamyl phosphate synthetase were found at autopsy. A second child who developed hyperammonemia on the second day of life was immediately treated with arginine hydrochloride, sodium benzoate and protein restriction. After NAGS deficiency was suspected by enzyme analysis, sodium benzoate was replaced by N-carbamylglutamate (NCG). A third child who developed slight hyperammonemia on the third day of life was treated with NCG before enzyme analysis confirmed reduced NAGS activity. Neither of the patients developed hyperammonemia in the following years. After the human NAGS gene was identified, mutation analysis revealed that the older sibling on NCG therapy was homozygous for a 971G>A (W324X) mutation. The parents and the younger sibling were heterozygous. Therapy was continued in the older sibling until now without any adverse effects and favourable neurodevelopment outcome. In the younger sibling, therapy was stopped without any deterioration of urea cycle function.

CONCLUSION

NAGS deficiency can be successfully treated with NCG and arginine hydrochloride with favourable outcome. Molecular diagnostic rather than enzyme analysis should be used in patients with suspected NAGS deficiency.

N-acetylglutamate synthase: structure, function and defects

N-acetylglutamate (NAG) is a unique enzyme cofactor, essential for liver ureagenesis in mammals while it is the first committed substrate for de novo arginine biosynthesis in microorganisms and plants. The enzyme that produces NAG from glutamate and CoA, NAG synthase (NAGS), is allosterically inhibited by arginine in microorganisms and plants and activated in mammals. This transition of the allosteric effect occurred when tetrapods moved from sea to land. The first mammalian NAGS gene (from mouse) was cloned in 2002 and revealed significant differences from the NAGS ortholog in microorganisms. Almost all NAGS genes possess a C-terminus transferase domain in which the catalytic activity resides and an N-terminus kinase domain where arginine binds. The three-dimensional structure of NAGS shows two distinctly folded domains. The kinase domain binds arginine while the acetyltransferase domain contains the catalytic site. NAGS deficiency in humans leads to hyperammonemia and can be primary, due to mutations in the NAGS gene or secondary due to other mitochondrial aberrations that interfere with the normal function of the same enzyme. For either condition, N-carbamylglutamate (NCG), a stable functional analog of NAG, was found to either restore or improve the deficient urea-cycle function.

Effects of meals high in carbohydrate, protein, and fat on ghrelin and peptide YY secretion in prepubertal children

CONTEXT

Ghrelin and peptide YY (PYY) are two hormones produced by the gastrointestinal tract that have effects on appetite. However, little is known about their secretion in response to meals high in individual macronutrients in prepubertal children.

OBJECTIVE

We sought to understand how meals high in carbohydrate, protein, and fat affect serum concentrations of total ghrelin and total PYY, hypothesizing that these macronutrients would exert differential effects on their secretion.

DESIGN AND SETTING

This was a cross-sectional study at one tertiary care center.

SUBJECTS

Subjects were 7- to 11-yr-old healthy normal-weight (NW) and obese (OB) volunteers recruited from local advertisements.

INTERVENTIONS

After an overnight fast, the subjects were given a breakfast high in carbohydrate, protein, or fat at 0800 h. Blood samples for total ghrelin and total PYY were taken at baseline, 30 min, and hourly from 0900 to 1200 h.

MAIN OUTCOME MEASURE

We assessed postprandial ghrelin suppression and PYY elevation, as well as changes in reported hunger and satiety, after the three test meals.

RESULTS

After the high-protein meal, ghrelin declined gradually in both groups over the study period without subsequent increase, whereas ghrelin suppressed more rapidly to a nadir at 60 min after the high-carbohydrate meal in both NW and OB children, followed by rebound in ghrelin levels. Similarly, after the high-protein meal, PYY concentrations increased steadily over the course of the morning in both groups without decline, whereas PYY levels peaked 30 min after the high-carbohydrate meal in both NW and OB subjects with significant decline thereafter. Ghrelin and PYY responses to the high-fat meal were somewhat intermediate between that observed with high carbohydrate and high protein. The OB children reported higher hunger and lower satiety after the high-carbohydrate meal compared to the NW subjects, whereas appetite ratings were similar between the groups after the high-protein and high-fat meals. Additionally, within the OB group, area under the curve (AUC) analysis revealed significantly greater PYY response, as well as lower AUC hunger and higher AUC satiety, to the high-protein meal than the high-carbohydrate and high-fat meals.

CONCLUSIONS

The patterns of secretion of ghrelin and PYY in our study of prepubertal children suggest that they may play a role in the effectiveness of high-protein/low-carbohydrate diets in promoting weight loss.

Contrasting features of urea cycle disorders in human patients and knockout mouse models

The urea cycle exists for the removal of excess nitrogen from the body. Six separate enzymes comprise the urea cycle, and a deficiency in any one of them causes a urea cycle disorder (UCD) in humans. Arginase is the only urea cycle enzyme with an alternate isoform, though no known human disorder currently exists due to a deficiency in the second isoform. While all of the UCDs usually present with hyperammonemia in the first few days to months of life, most disorders are distinguished by a characteristic profile of plasma amino acid alterations that can be utilized for diagnosis. While enzyme assay is possible, an analysis of the underlying mutation is preferable for an accurate diagnosis. Mouse models for each of the urea cycle disorders exist (with the exception of NAGS deficiency), and for almost all of them, their clinical and biochemical phenotypes rather closely resemble the phenotypes seen in human patients. Consequently, all of the current mouse models are highly useful for future research into novel pharmacological and dietary treatments and gene therapy protocols for the management of urea cycle disorders.

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

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; 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.

Expression, crystallization and preliminary crystallographic studies of a novel bifunctional N-acetylglutamate synthase/kinase from Xanthomonas campestris homologous to vertebrate N-acetylglutamate synthase

A novel N-acetylglutamate synthase/kinase bifunctional enzyme of arginine biosynthesis that was homologous to vertebrate N-acetylglutamate synthases was identified in Xanthomonas campestris. The protein was overexpressed, purified and crystallized. The crystals belong to the hexagonal space group P6(2)22, with unit-cell parameters a = b = 134.60, c = 192.11 A, and diffract to about 3.0 A resolution. Selenomethionine-substituted recombinant protein was produced and selenomethionine substitution was verified by mass spectroscopy. Multiple anomalous dispersion (MAD) data were collected at three wavelengths at SER-CAT, Advanced Photon Source, Argonne National Laboratory. Structure determination is under way using the MAD phasing method.

Biochemical properties of recombinant human and mouse N-acetylglutamate synthase

N-Acetylglutamate synthase (NAGS, EC 2.3.1.1) is a mitochondrial enzyme that catalyzes the formation of N-acetylglutamate (NAG) from glutamate and acetylcoenzyme A. NAG is an obligatory activator of carbamylphosphate I (CPSI), the first and a rate limiting enzyme of ureagenesis. The enzymatic activity of NAGS increases in the presence of arginine. Since the level of NAGS activity depends on the concentrations of two amino acids, glutamate and arginine, and it supplies the essential cofactor for CPSI, NAGS may play an important role in the regulation of ureagenesis. The amino acid sequences of human and mouse NAGS consist of three regions with different degrees of conservation: the mitochondrial targeting signal (MTS), the variable domain, and the conserved domain. Removal of the MTS results in mature NAGS (NAGS-M) while removal of the MTS and the variable domain results in conserved NAGS (NAGS-C). The biochemical properties of purified recombinant human and mouse NAGS-M and NAGS-C were determined in this study with the goal of better understanding the role of the variable domain in NAGS function. The activity of all four proteins doubled in the presence of arginine, while the affinities for substrates changed less than two fold. The turnover numbers of NAGS-C are double those of NAGS-M proteins. Processing of NAGS-M to form NAGS-C results in an enzyme with higher catalytic activity and could play a role in the regulation of NAG production, CPSI function, and urea synthesis.

Neonatal hyperammonemia: the N-carbamoyl-L-glutamic acid test

In a prospective study, patients with a suspected urea cycle defect underwent oral N-carbamoyl-L-glutamic acid loading testing. In patients with subsequently confirmed N-acetylglutamate synthase deficiency, hyperammonemia normalized within 8 hours. This test may be useful in the early diagnosis of patients with suspected urea cycle disorders.

Identification of novel mutations of the human N-acetylglutamate synthase gene and their functional investigation by expression studies

The mitochondrial enzyme N-acetylglutamate synthase (NAGS) produces N-acetylglutamate serving as an allosteric activator of carbamylphosphate synthetase 1, the first enzyme of the urea cycle. Autosomal recessively inherited NAGS deficiency (NAGSD) leads to severe neonatal or late-onset hyperammonemia. To date few patients have been described and the gene involved was described only recently. In this study, another three families affected by NAGSD were analyzed for NAGS gene mutations resulting in the identification of three novel missense mutations (C200R [c.598T > C], S410P [c.1228T > C], A518T [c.1552G > A]). In order to investigate the effects of these three and two additional previously published missense mutations on enzyme activity, the mutated proteins were overexpressed in a bacterial expression system using the NAGS deficient E. coli strain NK5992. All mutated proteins showed a severe decrease in enzyme activity providing evidence for the disease-causing nature of the mutations. In addition, we expressed the full-length NAGS wild type protein including the mitochondrial leading sequence, the mature protein as well as a highly conserved core protein. NAGS activity was detected in all three recombinant proteins but varied regarding activity levels and response to stimulation by l-arginine. In conclusion, overexpression of wild type and mutated NAGS proteins in E. coli provides a suitable tool for functional analysis of NAGS deficiency.

Misleading diagnosis of partial N-acetylglutamate synthase deficiency based on enzyme measurement corrected by mutation analysis

N-acetylglutamate synthase (NAGS) deficiency is a rare urea cycle disorder. Most of the patients present in the early neonatal period with severe hyperammonaemia and marked neurological impairment. We report on a Turkish family with an index patient, who died due to hyperammonemia, and another three siblings, who received a prophylactic treatment consisting of arginine hydrochloride, sodium benzoate and phenylbutyrate directly after birth. Enzyme measurement in a liver biopsy suggested a diagnosis of partial NAGS deficiency in all three siblings. Thereafter, N-carbamylglutamate was added to the treatment. None of the patients developed hyperammonaemia. After the human NAGS gene was identified, mutation analysis revealed that the consanguineous parents and two siblings were heterozygous for a private mutation (W484R), whereas the wild-type gene was found in the eldest sibling. Therapy was stopped without any deterioration of urea cycle function.

CONCLUSION

Diagnosis of partial NAGS deficiency based on enzyme measurement may be misleading and should be completed by mutation analysis.

Late onset N-acetylglutamate synthase deficiency caused by hypomorphic alleles

N-acetylglutamate (NAG) is a unique cofactor that is essential for the conversion of ammonia to urea in the liver. N-acetylglutamate synthase (NAGS) catalyzes the formation of NAG. Deficiency of NAGS causes a block in ureagenesis resulting in hyperammonemia. Although a number of mutations have been identified in the NAGS gene, their effects on NAGS enzymatic activity have not been examined. We describe here three mutations in two families with NAGS deficiency. Studies of the purified recombinant mutant proteins revealed deleterious effects on NAGS affinity for substrates, and on the rate of catalysis. These studies provide a better understanding of the function of NAGS, and the mechanisms for deleterious effect of mutations causing inherited NAGS deficiency.

Clinical manifestations of inborn errors of the urea cycle and related metabolic disorders during childhood

Various disorders cause hyperammonemia during childhood. Among them are those caused by inherited defects in urea synthesis and related metabolic pathways. These disorders can be grouped into two types: disorders of the enzymes that comprise the urea cycle, and disorders of the transporters or metabolites of the amino acids related to the urea cycle. Principal clinical features of these disorders are caused by elevated levels of blood ammonium. Additional disease-specific symptoms are related to the particular metabolic defect. These specific clinical manifestations are often due to an excess or lack of specific amino acids. Treatment of urea cycle disorders and related metabolic diseases consists of nutritional restriction of proteins, administration of specific amino acids, and use of alternative pathways for discarding excess nitrogen. Although combinations of these treatments are extensively employed, the prognosis of severe cases remains unsatisfactory. Liver transplantation is one alternative for which a better prognosis is reported.

Mammalian N-acetylglutamate synthase

N-Acetylglutamate synthase (NAGS, E.C. 2.3.1.1) is a mitochondrial enzyme that catalyzes the formation of N-acetylglutamate (NAG), an essential allosteric activator of carbamylphosphate synthetase I (CPSI). The mouse and human NAGS genes have been identified based on similarity to regions of NAGS from Neurospora crassa and cloned from liver cDNA libraries. These genes were shown to complement an argA- (NAGS) deficient Escherichia coli strain, and enzymatic activity of the proteins was confirmed by a new stable isotope dilution assay. The deduced amino acid sequence of mammalian NAGS contains a putative mitochondrial-targeting signal at the N-terminus. The mouse NAGS preprotein was overexpressed in insect cells to determine post-translational modifications and two processed proteins with different N-terminal truncations have been identified. Sequence analysis using a hidden Markov model suggests that the vertebrate NAGS protein contains domains with a carbamate kinase fold and an acyl-CoA N-acyltransferase fold, and protein crystallization experiments are currently underway. Inherited NAGS deficiency results in hyperammonemia, presumably due to the loss of CPSI activity. We, and others, have recently identified mutations in families with neonatal and late-onset NAGS deficiency and the identification of the gene has now made carrier testing and prenatal diagnosis feasible. A structural analog of NAG, carbamylglutamate, has been shown to bind and activate CPSI, and several patients have been reported to respond favorably to this drug (Carbaglu).

Diagnosis of N-acetylglutamate synthase deficiency by use of cultured fibroblasts and avoidance of nonsense-mediated mRNA decay

Molecular diagnosis of N-acetylglutamate synthase deficiency (NAGSD) has become possible now that the corresponding gene has been identified. We describe the genetic analysis of a patient with NAGSD using low-level transcripts derived from cultured fibroblasts. One defective allele (c.1306-1307insT) was detected by PCR amplification. However, the transcript from a second mutation (IVS3 - 2A>T), causing aberrant splicing with the generation of a premature termination codon, was not detected until interference of nonsense-mediated mRNA decay was abrogated by the translation inhibitor cycloheximide. We demonstrate that low-level transcripts in cells that do not express significant enzyme activity are a valuable tool for molecular studies of genetic alterations, and suggest routine abrogation of nonsense-mediated mRNA decay using cycloheximide when transcript analysis is performed.