MOLECULAR-GENETICS OF CYSTINURIA - MUTATION ANALYSIS OF SLC3A1 AND EVIDENCE FOR ANOTHER GENE IN THE TYPE-I (SILENT) PHENOTYPE

Background. Cystinuria is a hereditary disorder that affects luminal t ransport of cystine and dibasic amino acids in kidney and small intest ine. Three subtypes have been defined on the basis of urinary excretio n of cystine in obligate heterozygotes. Mutations in the SLC3A1 gene h ave been associated with the Type I phenotype. Methods. We investigate d 20 cystinuria patients from Quebec (8 Type I/I, 9 Type III/I and 3 T ype II/N) for mutations in SLC3A1. DNA was studied by Southern blottin g and by the single strand conformation polymorphism (SSCP) protocol t o identify mutations. Expression of mutations in Xenopus oocytes was p erformed to confirm the effect of missense mutations on cystine uptake . Results. Six novel mutations (2 large deletions, a 2 bp deletion and 3 single bp substitutions) were identified on the Type I allele. Four missense mutations (T216M, S217R, R270L and I618M) were expressed in vitro; the first three changes significantly decreased uptake. Conclus ions. Combined with our previous work, we have identified 15/16 mutati ons in SLC3A1 on Type I alleles in the eight Type I/I patients, but on ly one SLC3A1 mutation on the nine Type I alleles of the Type I/III pa tients. Therefore, we propose that the Type I phenotype could be cause d by mutations in other, as yet unidentified cystinuria genes.