Structure of the human argininosuccinate synthetase gene and an improved system for molecular diagnostics in patients with classical and mild citrullinemia

Abstract

Deficiency of argininosuccinate synthetase (ASS) causes citrullinemia, an autosomal recessive inherited defect of the urea cycle. Most patients described so far have presented with the classical form of the disease. There are also patients with a mild form of citrullinemia in whom the exact molecular basis and clinical relevance are uncertain. Mutations in the human ASS gene have not yet been described in mildly affected or asymptomatic patients with citrullinemia. The genomic sequence of the human ASS gene is not precisely known making mutation analysis difficult. Here, the entire genomic DNA sequence and mutations in the ASS gene of patients with the classical and mild form of the disease are described. The mutations c.1168G-->A (G390R) and IVS13+5 G-->A and the novel mutation c.323G-->T (R108L) have been found to be associated with classical citrullinemia, whereas the novel mutations c.535T-->G (W179R), and c.1085G-->T (G362V) have been detected on alleles of the mildly affected patients. Thus, mutations found in the human ASS gene of asymptomatic children with biochemical abnormalities and in some cases enzymatically proven citrullinemia have allowed us to classify these cases as ASS-deficient patients. The elucidation of the structure of the human ASS gene has made possible the use of intronic primers for molecular analysis of patients with mild disease and the classical form, and provides another option for prenatal diagnostics in affected families with the severe type.

Mutations and polymorphisms in the human argininosuccinate synthetase (ASS1) gene

“…A total of 17 mutations have been shown to cause neonatal onset of the disorder when found in a homozygous state (see Table 1) [Gao et al, 2003;Kakinoki et al, 1997;Kleijer et al, 2006;Kobayashi et al, 1990Kobayashi et al, , 1991Kobayashi et al, , 1994Kobayashi et al, , 1995Sander et al, 2003;Vilaseca et al, 2001]: among them are two nonsense mutations in addition to four splice-site defects, two deletions, and seven missense mutations. However, compound heterozygous carriers of some of these mutations (Ivs15-1G4C, R157C, V263M, E283K, G324S, R363W, and G390R) have been identified who, owing to the function of the second allele, present only mild clinical symptoms [Gao et al, 2003;Häberle et al, 2002;Hong et al, 2000;Sander et al, 2003]. A total of 30 mutations occur in compound heterozygotes with neonatal onset of citrullinemia [Gao et al, 2003;Häberle et al, 2002;Kakinoki et al, 1997;Kleijer et al, 2006;Kobayashi et al, 1990Kobayashi et al, , 1991Kobayashi et al, , 1994Li et al, 2001;Vilaseca et al, 2001].…”

“…However, compound heterozygous carriers of some of these mutations (Ivs15-1G4C, R157C, V263M, E283K, G324S, R363W, and G390R) have been identified who, owing to the function of the second allele, present only mild clinical symptoms [Gao et al, 2003;Häberle et al, 2002;Hong et al, 2000;Sander et al, 2003]. A total of 30 mutations occur in compound heterozygotes with neonatal onset of citrullinemia [Gao et al, 2003;Häberle et al, 2002;Kakinoki et al, 1997;Kleijer et al, 2006;Kobayashi et al, 1990Kobayashi et al, , 1991Kobayashi et al, , 1994Li et al, 2001;Vilaseca et al, 2001]. Again, some of these aberrations-such as G14S, G117D, and E191Q-were also revealed to account for mild or late onset forms of the disease.…”

“…A similar pattern is seen when the deleterious mutation is found on only one allele. The arginine replaced in R108L, a mutation described in heterozygous state in severely affected patients [Häberle et al, 2002;Vilaseca et al, 2001] is an exception because it is poorly conserved. The same applies to the replacement A192V, which has been identified on one allele of a Japanese patient with neonatal onset of citrullinemia [Kobayashi et al, 1994]; however, the fact that this amino acid position maps to the active center of the protein may account for the deleterious effect of the replacement.…”

“…Function impediment causes citrullinemia, a rare autosomal recessive disorder [Beaudet et al, 1986]. In accordance with the recommendations of Häberle et al [2002], the nomenclature of type I, II, or III citrullinemia should not be used any longer. Metabolic disturbances associated with impairment in the function of the ASS gene are named citrullinemia type I (MIM] 215700).…”

“…Diagnosis of ASS deficiency mainly relied on biochemical and enzymatic studies using liver tissue and fibroblasts [Beaudet et al, 1986;Kleijer et al, 1984b;Su et al, 1982] until elucidation of the genomic sequence of the ASS gene including the exonic and intronic regions permitted comprehensive molecular analyses [Häberle et al, 2002]. The ASS gene is located on the long arm of chromosome 9, within band 9q.34.11-9q34.12.…”