Interallelic complementation in an inborn error of metabolism: genetic heterogeneity in argininosuccinate lyase deficiency

We used complementation analysis as a probe for the detection of genetic heterogeneity within a single locus affected in a human disease, argininosuccinate lyase (L-argininosuccinate arginine-lyase, EC 4.3.2.1) deficiency. Fibroblasts cultured from 28 unrelated patients were fused in all possible pairwise combinations, and the argininosuccinate lyase activity in heterokaryons was assayed by measuring the incorporation of 14C from L-[ureido-14C]citrulline into acid-precipitable material. Partial complementation was observed in fusions involving 20 of the 28 strains, with the lyase activity increasing from 2- to 10-fold. Thirteen of the mutants were identified by the complementation analysis as being phenotypically unique. Of the 20 complementing strains, 3 were remarkable because they participated in all but 2 of the 32 positive complementation tests; 2 others constituted a unique subgroup that produced the highest increases in argininosuccinate lyase activity of all fusions. The 8 strains that did not complement any others consisted of two types: 3 mutants with the highest residual argininosuccinate lyase activity of all strains and 5 mutants with low residual activity. All of the mutants mapped to a single major complementation group. The data could be summarized as a circular complementation map with an attached linear tail, the mutants being distributed among 12 subgroups in a complex pattern. We conclude that all of these mutants are affected at a single locus, that extensive genetic heterogeneity is present in the mutant population, and that the affected locus in argininosuccinate lyase deficiency is likely to be the structural gene coding for that enzyme.