Analysis of naturally occurring and site-directed mutations in the argininosuccinate lyase gene

Argininosuccinic aciduria is an inborn error of metabolism due to the genetic deficiency of argininosuccinate lyase. In order to determine the molecular basis for the disease, RNA isolated from cultured skin fibroblasts derived from four unrelated patients was reverse-transcribed and amplified using the polymerase chain reaction and the products were cloned and sequenced. Three single base missense mutations were identified: Arg111----Trp, Gln286----Arg, and Arg193----Gln. One single base amber mutation was identified at Gln454. One mutation involved a 13-base pair deletion within exon 13, and it was noted that the majority of the mature RNA derived from this allele was deleted for the entire exon rather than containing the exon with the 13 bases deleted. A final mutation was observed in which exon 2 was deleted from the mature RNA. The molecular basis for this deletion was not determined. Of the eight potential mutations present in the four cell lines studied, six mutations were identified and further data indicate that the remaining two unidentified mutations were different from those identified. Two site-directed mutations were created in the cDNA, Lys51----Asn and His89----Gln, and these were expressed in yeast. The Lys51 mutation caused an approximate 2-fold reduction in activity and the His89 mutation resulted in an approximate 10-fold reduction in activity. The combination of determination of naturally occurring mutations and the study of the effect of site-directed mutations on the activity of argininosuccinate lyase provide insight into the amino acid residues critical to the function of the enzyme.