cDNA sequence, interspecies comparison, and gene mapping analysis of argininosuccinate lyase

Unstable argininosuccinate lyase in variant forms of the urea cycle disorder argininosuccinic aciduria

Loss of function of the urea cycle enzyme argininosuccinate lyase (ASL) is caused by mutations in the ASL gene leading to ASL deficiency (ASLD). ASLD has a broad clinical spectrum ranging from life-threatening severe neonatal to asymptomatic forms. Different levels of residual ASL activity probably contribute to the phenotypic variability but reliable expression systems allowing clinically useful conclusions are not yet available. In order to define the molecular characteristics underlying the phenotypic variability, we investigated all ASL mutations that were hitherto identified in patients with late onset or mild clinical and biochemical courses by ASL expression in human embryonic kidney 293 T cells. We found residual activities >3% of ASL wild type (WT) in nine of 11 ASL mutations. Six ASL mutations (p.Arg95Cys, p.Ile100Thr, p.Val178Met, p.Glu189Gly, p.Val335Leu, and p.Arg379Cys) with residual activities ≥16% of ASL WT showed no significant or less than twofold reduced Km values, but displayed thermal instability. Computational structural analysis supported the biochemical findings by revealing multiple effects including protein instability, disruption of ionic interactions and hydrogen bonds between residues in the monomeric form of the protein, and disruption of contacts between adjacent monomeric units in the ASL tetramer. These findings suggest that the clinical and biochemical course in variant forms of ASLD is associated with relevant residual levels of ASL activity as well as instability of mutant ASL proteins. Since about 30% of known ASLD genotypes are affected by mutations studied here, ASLD should be considered as a candidate for chaperone treatment to improve mutant protein stability.

Understanding the role of argininosuccinate lyase transcript variants in the clinical and biochemical variability of the urea cycle disorder argininosuccinic aciduria

Argininosuccinic aciduria (ASA) is an autosomal recessive urea cycle disorder caused by deficiency of argininosuccinate lyase (ASL) with a wide clinical spectrum from asymptomatic to severe hyperammonemic neonatal onset life-threatening courses. We investigated the role of ASL transcript variants in the clinical and biochemical variability of ASA. Recombinant proteins for ASL wild type, mutant p.E189G, and the frequently occurring transcript variants with exon 2 or 7 deletions were (co-)expressed in human embryonic kidney 293T cells. We found that exon 2-deleted ASL forms a stable truncated protein with no relevant activity but a dose-dependent dominant negative effect on enzymatic activity after co-expression with wild type or mutant ASL, whereas exon 7-deleted ASL is unstable but seems to have, nevertheless, a dominant negative effect on mutant ASL. These findings were supported by structural modeling predictions for ASL heterotetramer/homotetramer formation. Illustrating the physiological relevance, the predominant occurrence of exon 7-deleted ASL was found in two patients who were both heterozygous for the ASL mutant p.E189G. Our results suggest that ASL transcripts can contribute to the highly variable phenotype in ASA patients if expressed at high levels. Especially, the exon 2-deleted ASL variant may form a heterotetramer with wild type or mutant ASL, causing markedly reduced ASL activity.

Identification of a common novel mutation in Saudi patients with argininosuccinic aciduria

We have identified a common novel mutation (Q354X) in the argininosuccinate lyase (ASL) gene in Saudi patients with argininosuccinic aciduria (ASAuria; McKusick 207900). The two index patients were siblings, had a neonatal onset of the disease and were diagnosed based on the clinical presentation and confirmed by analysis of their dried blood spots (DBS) by tandem mass spectrometry (MS/MS). The ASL gene was then analysed by direct sequencing. A further 28 patients with a confirmed diagnosis of ASAuria based on MS/MS of their DBS were tested by sequencing for the presence of the Q354X mutation. This mutation was found in 14 out of the 28 patients (50%) tested. Our work indicates that the Q354X allele is common, may account for 50% of the abnormal ASL genes in the Saudi population, and is likely to be associated with the neonatal form of the disease. We recommend that all patients diagnosed with ASAuria in Saudi Arabia or of Arab origin be tested for this mutation and for Q116X, which has been described previously. In addition, further analysis is needed to identify other underlying disease mutations for ASAuria in the Saudi population.

Clearance of amino acids by hemodialysis in argininosuccinate synthetase deficiency

We determined the dialytic clearance of amino acids involved in ammoniagenesis and nitrogen excretion in a neonate with argininosuccinate synthetase deficiency who underwent acute hemodialysis. Plasma ammonia and plasma and dialysate amino acid concentrations were obtained at baseline, 30-minute intervals during hemodialysis, and 30 minutes after the completion of hemodialysis. Plasma ammonia concentrations declined by 56% during the 90-minute hemodialysis treatment, whereas arginine, citrulline, glutamine, and glycine concentrations decreased by 65%, 55%, 40%, and 34%, respectively. Mean dialytic clearances for arginine, citrulline, glutamine, and glycine were 24, 282, 263, and 189 mL/min per 1.73 m(2), respectively. The high dialytic clearance of citrulline suggests a novel mechanism of hemodialysis removal of nitrogen. Dialytic clearances of glutamine and glycine may prevent further ammoniagenesis in hyperammonemic patients. However, our data suggest that hemodialysis affects the precursors of alternative pathway removal of ammonia. Further study is needed to optimize the intradialytic and interdialytic dosing of substrates.

Generation and comparative analysis of approximately 3.3 Mb of mouse genomic sequence orthologous to the region of human chromosome 7q11.23 implicated in Williams syndrome

Williams syndrome is a complex developmental disorder that results from the heterozygous deletion of a approximately 1.6-Mb segment of human chromosome 7q11.23. These deletions are mediated by large (approximately 300 kb) duplicated blocks of DNA of near-identical sequence. Previously, we showed that the orthologous region of the mouse genome is devoid of such duplicated segments. Here, we extend our studies to include the generation of approximately 3.3 Mb of genomic sequence from the mouse Williams syndrome region, of which just over 1.4 Mb is finished to high accuracy. Comparative analyses of the mouse and human sequences within and immediately flanking the interval commonly deleted in Williams syndrome have facilitated the identification of nine previously unreported genes, provided detailed sequence-based information regarding 30 genes residing in the region, and revealed a number of potentially interesting conserved noncoding sequences. Finally, to facilitate comparative sequence analysis, we implemented several enhancements to the program, including the addition of links from annotated features within a generated percent-identity plot to specific records in public databases. Taken together, the results reported here provide an important comparative sequence resource that should catalyze additional studies of Williams syndrome, including those that aim to characterize genes within the commonly deleted interval and to develop mouse models of the disorder.

Detection of neonatal argininosuccinate lyase deficiency by serum tandem mass spectrometry

Argininosuccinate lyase (ASL) deficiency (McKusick 207900) is an inborn error of the urea cycle. The leading symptom is progressive hyperammonaemia, which is a life-threatening condition, particularly in patients with a neonatal onset. Early diagnosis and treatment of the hyperammonaemia are necessary to improve survival and the long-term outcome of ASL-deficient patients. Currently, the diagnosis of ASL deficiency is based on the measurement of urea cycle intermediates and amino acids by automated quantitative ion exchange chromatography in plasma and urine. Here, we report a newborn presenting with coma and severe hyperammonaemia. ASL deficiency was suspected on the basis of an adapted tandem mass spectrometric (MS-MS) procedure which allows determination of argininosuccinate in addition to the amino acids in serum samples. MS-MS measurements revealed a characteristic increase of argininosuccinate, a moderate increase of citrulline, and lowered levels of arginine and ornithine in the serum of the patient. The diagnosis was confirmed by the detection of a novel homozygous frameshift mutation in exon 14 of the argininosuccinate lyase gene. We propose MS-MS as a diagnostic tool suitable for the rapid detection of specific alterations in the amino acid spectra caused by ASL deficiency.

Intragenic complementation at the human argininosuccinate lyase locus. Identification of the major complementing alleles

To determine the molecular and biochemical basis of intragenic complementation observed at the human argininosuccinate lyase (ASL) locus, we identified the ASL alleles in ASL-deficient cell strains with two unique complementation phenotypes: (i) frequent complementers, strains that participated in the majority of complementation events, and (ii) high activity complementers, strains in which complementation was associated with a relatively high level of restoration of ASL activity. Four mutations (Q286R, D87G, A398D, and a deletion of exon 13) were identified in the four strains examined. One of the two frequent complementers was homozygous, and the other heterozygous, for the Q286R allele. Similarly, one of the two high activity complementers was homozygous, and the other heterozygous, for the D87G allele. When the Q286R and D87G mutations were introduced by site-directed mutagenesis into wild-type ASL cDNA, each conferred loss of ASL activity in COS cell transfection assays. To test directly the hypothesis that intragenic complementation occurs at the ASL locus, one of the major complementation events observed previously, between strains carrying the Q286R and D87G alleles, was reconstructed in COS cell transfection assays. A partial restoration of ASL activity, comparable with the increase seen in the fibroblast complementation analysis, was observed on joint cotransfection of these two alleles. The results provide molecular confirmation of the major features of the ASL mutant complementation map, identify the Q286R and D87D alleles as the frequent and high activity complementing alleles, respectively, and provide direct proof of intragenic complementation at the ASL locus.

Transcriptional regulation of genes for ornithine cycle enzymes

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Differential expression of the two delta-crystallin genes in lens and non-lens tissues: shift favoring delta 2 expression from embryonic to adult chickens

Chicken argininosuccinate lyase (ASL)/delta-crystallin, a lens enzyme-crystallin, is encoded in two linked genes (delta 1 and delta 2); only the delta 2 polypeptide contains ASL activity. Here we have quantified delta 1- and delta 2-crystallin mRNA in the lens, cornea, neural retina, heart, and brain at different stages of embryonic development and in 1-wk-old and 1-yr-old chickens by the polymerase chain reaction using internal delta 1 and delta 2 RNA standards. The delta 1/delta 2 mRNA ratio differed for every tissue and was regulated during development. In the embryo there was more delta 1 than delta 2 mRNA in the lens (50-100 times), cornea (3-4 times), and neural retina (2-20 times), about equal amounts of delta 1 and delta 2 mRNA in the heart, and more delta 2 mRNA in the brain (15 times). delta 1-Crystallin mRNA differentially decreased in every tissue after hatching; by contrast, the delta 2 mRNA remained about the same except for the lens, where it decreased 50-fold between 1 wk and 1 yr after hatching. In the 1-yr-old chicken, the delta 2/delta 1 mRNA ratios were 7 in the lens, 175 in the cornea, 22 in the neural retina, 107 in the heart, and 136 in the brain, indicating that delta 2-crystallin is strongly favored in all adult tissues of the chicken. The excess of delta 1 to delta 2 mRNA in the embryonic lens, cornea, and neural retina is intriguing, and suggests some connection with developing transparent eye tissues. Finally, we raise the possibility that expression of both delta-crystallin genes may create tetrameric ASL isoenzymes (perhaps with different specific activities). The unexpected predominance of delta 2 mRNA in the 1-yr-old lens suggests that both the enzymatic and refractive functions of ASL/delta-crystallin are operative and spatially separated, with the enzymatic role present in the cortical fibers and the refractive role in the center of the lens.

Molecular analysis of human argininosuccinate lyase: mutant characterization and alternative splicing of the coding region

Argininosuccinic acid lyase (ASAL) deficiency is a clinically heterogeneous autosomal recessive urea cycle disorder. We previously established by complementation analysis that 28 ASAL-deficient patients have heterogeneous mutations in a single gene. To prove that the ASAL structural gene is the affected locus, we sequenced polymerase chain reaction-amplified ASAL cDNA of a representative mutant from the single complementation group. Fibroblast strain 944 (approximately 1% of residual ASAL activity), from a late-onset patient who was the product of a consanguineous mating, had only a single base-pair change in the coding region, a C-283----T transition at a CpG dinucleotide in exon 3. This substitution converts Arg-95 to Cys (R95C), occurs in a stretch of 13 residues that is identical in yeast and human ASAL, and was present in both of the patient's alleles but not in 14 other mutant or 10 normal alleles. Expression in COS cells demonstrated that the R95C mutation produces normal amounts of ASAL mRNA but little protein and less than 1% ASAL activity. We observed that amplified cDNA from mutant 944 and normal cells (liver, keratinocytes, lymphoblasts, and fibroblasts) contained, in addition to the expected 5' 513-base-pair band, a prominent 318-base-pair ASAL band formed by the splicing of exon 2 from the transcript. The short transcript maintains the ASAL reading frame but removes Lys-51, a residue that may be essential for catalysis, since it binds the argininosuccinate substrate. We conclude (i) that the identification of the R95C mutation in strain 944 demonstrates that virtually all ASAL deficiency results from defects in the ASAL structural gene and (ii) that minor alternative splicing of the coding region occurs at the ASAL locus.