Characterization and mRNA expression analysis of a novel ARG1 splicing mutation causing hyperargininemia.

OBJECTIVES: Biallelic mutations in the ARG1 gene result in an uncommon autosomal recessive inborn defect of the urea cycle known as hyperargininemia (OMIM #207800). ARG1 splicing mutations are not reported often, and they are probably related to a more severe phenotype than missense mutations. In this article, we describe the results of molecular studies in a young hyperargininemia patient carrying a novel splicing mutation in ARG1. DESIGN AND METHODS: Molecular analyses included PCR amplification and direct nucleotide sequencing of the ARG1 gene. RT-PCR analysis was performed to investigate the effect of the mutation in mRNA splicing and in the expression of ARG1 isoforms. RESULTS: Mutational analysis identified a novel homozygous ARG1 IVS4-1G>C point mutation in the patient's DNA. Blood leukocyte mRNA was analyzed to demonstrate the splicing defect caused by this mutation. Sequencing of ARG1 RT-PCR products allowed the characterization of a mutated transcript retaining 51-bp from intron 4. In addition, two new, alternatively spliced ARG1 transcripts lacking either exon 4 or exons 4 and 5 were identified in mRNA from the patient and from controls. CONCLUSIONS: Our results expand the mutational spectrum in hyperargininemia patients and indicate that the novel splicing mutation results in an aberrant transcript retaining intronic sequences. Two novel alternatively spliced ARG1 transcripts were also recognized.

Analysis of novel ARG1 mutations causing hyperargininemia and correlation with arginase I activity in erythrocytes.

Hyperargininemia (HA) is an autosomal recessive disease that typically has a clinical presentation that is distinct from other urea cycle disorders. It is caused by the deficient activity of the enzyme arginase I, encoded by the gene ARG1. We screened for ARG1 mutations and measured erythrocyte enzyme activity in a series of 16 Brazilian HA patients. Novel mutations, in addition to previously described missense mutations, were analysed for their effect on the structure, stability and/or function of arginase I (ARG1) using bioinformatics tools. Three previously reported mutations were found (p.R21X; p.I11T and p.W122X), and five novel mutations were identified (p.G27D; p.G74V; p.T134I; p.R308Q; p.I174fs179). The p.T134I mutation was the most frequent in the Brazilian population. Patients carrying the p.R308Q mutation had higher residual ARG1 decreased activity, but presented no distinguishable phenotype compared to the other patients. Bioinformatics analyses revealed that missense mutations (1) affect the ARG1 active site, (2) interfere with the stability of the ARG1 folded conformation or (3) alter the quaternary structure of the ARG1. Our study reinforced the role of Arg308 residue for assembly of the ARG1 homotrimer. The panel of heterogeneous ARG1 mutations that cause HA was expanded, nevertheless a clear genotype-phenotype correlation was not observed in our series.