THE MOLECULAR-BASIS OF CYSTINURIA - THE ROLE OF THE RBAT GENE

The cDNAs of mammalian amino acid transporters already identified coul d be grouped into four families. One of these protein families is comp osed of the protein rBAT and the heavy chain of the cell surface antig en 4F2 (4F2hc). The cRNAs of rBAT and 4F2hc induce amino acid transpor t activity via systems b(0,+) -like and y(+)L -like in Xenopus oocytes respectively. Surprisingly, neither rBAT nor 4F2hc is very hydrophobi c, and they seem to be unable to form a pore in the plasma membrane. T his prompted the hypothesis that rBAT and 4F2hc are subunits or modula tors of the corresponding amino acid transporters. The association of rBAT with a light subunit of similar to 40kDa has been suggested, and such an association has been demonstrated for 4F2hc. The b(0,+)-like s ystem expressed in oocytes by rBAT cRNA transports L-cystine, L-dibasi c and L-neutral amino acids with high-affinity. This transport system shows exchange of amino acids through the plasma membrane of Xenopus o ocytes, suggesting a tertiary active transport mechanism. The rBAT gen e is mainly expressed in the outer stripe of the outer medulla of the kidney and in the mucosa of the small intestine. The protein localizes to the microvilli of the proximal straight tubules (S3 segment) of th e nephron and the mucosa of the small intestine. All this suggested th e participation of rBAT in a high-affinity reabsorption system of cyst ine and dibasic amino acids in kidney and intestine, and indicated rBA T (named SLC3A1 in Gene Data Bank) as a good candidate gene for cystin uria. This is an inherited aminoaciduria due to defective renal and in testinal reabsorption of cystine and dibasic amino acids. The poor sol ubility of cystine causes the formation of renal cystine calculi. Muta tional analysis of the rBAT gene of patients with cystinuria is reveal ing a growing number (similar to 20) of cystinuria-specific mutations, including missense, nonsense, deletions and insertions. Mutations M46 7T (substitution of methionine 467 residue for threonine) and R270X (s top codon at arginine residue 270) represent approximately half of the cystinuric chromosomes where mutations have been found. Mutation M467 T reduces transport activity of rBAT in oocytes. All this demonstrates that mutations in the rBAT gene cause cystinuria. Three types of cyst inuria (types, I, II and III) have been described on the basis of the genetic, biochemical and clinical manifestations of the disease. Type I cystinuria has a complete recessive inheritance; type I heterozygote s are totally silent. In contrast, type II and III heterozygotes shaw, respectively, high or moderate hyperaminoaciduria of cystine and diba sic amino acids. Type III homozygotes show moderate, if any, alteratio n of intestinal absorption of cystine and dibasic amino acids type II homozygotes clearly show defective intestinal absorption of these amin o acids. To date, all the rBAT cystinuria-specific mutations we have f ound are associated with type I cystinuria (similar to 70% of the chro mosomes studied) but not to types II or III. This strongly suggests ge netic heterogeneity for cystinuria. Genetic linkage analysis with mark ers of the genomic region of rBAT in chromosome 2 (G band 2p16.3) and intragenic markers of rBAT have demonstrated genetic heterogeneity for cystinuria; the rBAT gene is linked to type I cystinuria, but not to type III. Biochemical, genetic and clinical studies are needed to iden tify the additional cystinuria genes; a low-affinity cystine reabsorti on system and the putative light subunit of rBAT are additional candid ate genes for cystinuria.