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

N-acetylglutamate synthase deficiency (NAGSD, MIM #237310) 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 sequence variants in the NAGS regulatory regions. All three patients had hyperammonemia that resolved upon treatment with NCG. Sequence variants NM_153006.2:c.-3065A>C and NM_153006.2:c-3098C>T reside in the NAGS enhancer, within known HNF1 and predicted glucocorticoid receptor binding sites, respectively. Sequence variants NM_153006.2:c.426+326G>A and NM_153006.2:c.427-218A>C reside in the first intron of NAGS and define a novel NAGS regulatory element that binds retinoic X receptor α. 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 non-coding regions of NAGS and other urea cycle genes can reveal molecular causes of disease and identify novel regulators of ureagenesis.

Gene delivery corrects N-acetylglutamate synthase deficiency and enables insights in the physiological impact of L-arginine activation of N-acetylglutamate synthase

The urea cycle protects the central nervous system from ammonia toxicity by converting ammonia to urea. N-acetylglutamate synthase (NAGS) catalyzes formation of N-acetylglutamate, an essential allosteric activator of carbamylphosphate synthetase 1. Enzymatic activity of mammalian NAGS doubles in the presence of L-arginine, but the physiological significance of NAGS activation by L-arginine has been unknown. The NAGS knockout (Nags−/−) mouse is an animal model of inducible hyperammonemia, which develops hyperammonemia without N-carbamylglutamate and L-citrulline supplementation (NCG + Cit). We used adeno associated virus (AAV) based gene transfer to correct NAGS deficiency in the Nags−/− mice, established the dose of the vector needed to rescue Nags−/− mice from hyperammonemia and measured expression levels of Nags mRNA and NAGS protein in the livers of rescued animals. This methodology was used to investigate the effect of L-arginine on ureagenesis in vivo by treating Nags−/− mice with AAV vectors encoding either wild-type or E354A mutant mouse NAGS (mNAGS), which is not activated by L-arginine. The Nags−/− mice expressing E354A mNAGS were viable but had elevated plasma ammonia concentration despite similar levels of the E354A and wild-type mNAGS proteins. The corresponding mutation in human NAGS (NP_694551.1:p.E360D) that abolishes binding and activation by L-arginine was identified in a patient with NAGS deficiency. Our results show that NAGS deficiency can be rescued by gene therapy, and suggest that L-arginine binding to the NAGS enzyme is essential for normal ureagenesis.

Impaired ureagenesis due to arginine-insensitive N-acetylglutamate synthase

The urea cycle protects the central nervous system from ammonia toxicity by converting ammonia to non-toxic urea. N-acetylglutamate synthase (NAGS) is an enzyme that catalyzes the formation of N-acetylglutamate (NAG), an allosteric activator of carbamylphosphate synthetase 1 (CPS1), the rate limiting enzyme of the urea cycle. Enzymatic activity of mammalian NAGS doubles in the presence of L-arginine but the physiological significance of NAGS activation by L-arginine is unknown. Previously, we have described the creation of a NAGS knockout (Nags−/−) mouse, which develops hyperammonemia without N-carbamylglutamate and L-citrulline supplementation (NCG+Cit). In order to investigate the effect of L-arginine on ureagenesis in vivo, we used adeno associated virus (AAV) mediated gene transfer to deliver either wild-type or E354A mutant mouse NAGS (mNAGS), which is not activated by L-arginine, to Nags−/− mice. The ability of the E354A mNAGS mutant protein to rescue Nags−/− mice was determined by measuring their activity on the voluntary wheel following NCG+Cit withdrawal. The Nags−/− mice that received E354A mNAGS remained apparently healthy and active but had elevated plasma ammonia concentration despite similar expression levels of the E354A mNAGS and control wild-type NAGS proteins. The corresponding mutation in human NAGS (NP 694551.1:p.E360D) that abolishes binding and activation by L-arginine was also identified in a patient with hyperammonemia due to NAGS deficiency. Taken together, our results suggest that L-arginine binding to the NAGS enzyme is essential for normal ureagenesis.

Identification, cloning and expression of the mouse N-acetylglutamate synthase gene

In ureotelic animals, N-acetylglutamate (NAG) is an essential allosteric activator of carbamylphosphate synthetase I (CPSI), the first enzyme in the urea cycle. NAG synthase (NAGS; EC 2.3.1.1) catalyses the formation of NAG from glutamate and acetyl-CoA in liver and intestinal mitochondria. This enzyme is supposed to regulate ureagenesis by producing variable amounts of NAG, thus modulating CPSI activity. Moreover, inherited deficiencies in NAGS have been associated with hyperammonaemia, probably due to the loss of CPSI activity. Although the existence of the NAGS protein in mammals has been known for decades, the gene has remained elusive. We identified the mouse (Mus musculus) and human NAGS genes using their similarity to the respective Neurospora crassa gene. NAGS was cloned from a mouse liver cDNA library and was found to encode a 2.3kb message, highly expressed in liver and small intestine with lower expression levels in kidney, spleen and testis. The deduced amino acid sequence contains a putative mitochondrial targeting signal at the N-terminus. The cDNA sequence complements an argA (NAGS)-deficient Escherichia coli strain, reversing its arginine auxotrophy. His-tagged versions of the pre-protein and two putative mature proteins were each overexpressed in E. coli, and purified to apparent homogeneity by using a nickel-affinity column. The pre-protein and the two putative mature proteins catalysed the NAGS reaction but one of the putative mature enzymes had significantly higher activity than the pre-protein. The addition of l-arginine increased the catalytic activity of the purified recombinant NAGS enzymes by approx. 2–6-fold.

Inefficient growth arrest in response to dNTP starvation stimulates gene amplification through bridge-breakage-fusion cycles.

Cells often acquire resistance to the antiproliferative agents methotrexate (MTX) or N-phosphonacetyl-L-aspartate (PALA) through amplification of genes encoding the target enzymes dihydrofolate reductase or carbamylphosphate synthetase/aspartate transcarbamylase/dihydroorotase (CAD), respectively. We showed previously that Syrian hamster BHK cells resistant to selective concentrations of PALA (approximately 3 x ID50) arise at a rate of approximately 10(-4) per cell per generation and contain amplifications of the CAD gene as ladder-like structures on one of the two B9 chromosomes, where CAD is normally located. We now find that BHK cells resistant to high concentrations of PALA (approximately 15 x ID50) appear only after prior exposure to selective concentrations of PALA for approximately 72 h. Furthermore, in contrast to untreated cells, BHK cells pretreated with selective concentrations of MTX give colonies in high concentrations of PALA, and cells pretreated with selective concentrations of PALA give colonies in high concentrations of MTX or 5-fluorouracil. As judged by measuring numbers of cells and metaphase cell pairs, BHK cells do not arrest completely when starved for pyrimidine nucleotides by treatment with selective concentrations of PALA for up to 72 h. We propose that DNA damage, caused when cells fail to stop DNA synthesis promptly under conditions of dNTP starvation, stimulates amplification throughout the genome by mechanisms--such as bridge-breakage-fusion cycles--that are triggered by broken DNA. Amplified CAD genes were analyzed by fluorescence in situ hybridization both in cells where amplification was induced by PALA pretreatment and in cells in which the amplification occurred spontaneously, before selection with PALA. The ladder-like structures that result from bridge-breakage-fusion cycles were observed in both cases.