Functional analysis of novel mutations in y(+)LAT-1 amino acid transportergene causing lysinuric protein intolerance (LPI)

Lysinuric protein intolerance (LPI; MIM 222700) is an autosomal recessive d isorder characterized by defective transport of the cationic amino acids ly sine, arginine and ornithine at the basolateral membrane of the polar epith elial cells in the intestine and renal tubules, and by hyperammonemia after high-protein meals, LPI is caused by mutations in the SLC7A7 (solute carri er family 7, member 7) gene encoding y(+)LAT-1 (y(+)L amino acid transporte r-1), which co-induces together with 4F2 heavy chain (4F2hc) system y(+)L i n Xenopus oocytes, All Finnish LPI patients share the same founder mutation 1181-2A-->T (LPIFin) not found in LPI patients elsewhere. Mutation screeni ng of 20 non-Finnish LPI patients revealed 10 novel mutations: four deletio ns, two missense mutations, two nonsense mutations, a splice site mutation and a tandem duplication. Five LPI mutations (L334R, G54V, 1291delCTTT, 154 8delC and LPIFin) were studied functionally, Ail mutant proteins failed to co-induce amino acid transport activity when expressed with 4F2hc in Xenopu s oocytes, Immunostaining experiments revealed that frameshift mutants 1291 delCTTT, 1548delC and LPIFin remained intracellular on expression with 4F2h c. In contrast, the missense mutants L334R and G54V reached the oocyte plas ma membrane when coexpressed with 4F2hc, demonstrating that they are transp ort-inactivating mutations. This finding, together with the strong degree o f conservation among all members of this family of amino acid transporters, indicates that residues L334 and G54 play a crucial role in the function o f the y+LAT-1 transporter.