Novel NCC mutants and functional analysis in a new cohort of patients with Gitelman syndrome

Item does not contain fulltextGitelman syndrome (GS) is an autosomal recessive disorder characterized by hypokalemic metabolic alkalosis in conjunction with significant hypomagnesemia and hypocalciuria. The GS phenotype is caused by mutations in the solute carrier family 12, member 3 (SLC12A3) gene that encodes the thiazide-sensitive NaCl cotransporter (NCC). We analyzed DNA samples of 163 patients with a clinical suspicion of GS by direct sequencing of all 26 exons of the SLC12A3 gene. In total, 114 different mutations were identified, 31 of which have not been reported before. These novel variants include 3 deletions, 18 missense, 6 splice site and 4 nonsense mutations. We selected seven missense mutations to investigate their effect on NCC activity and plasma membrane localization by using the Xenopus laevis oocyte expression system. The Thr392Ile mutant did not display transport activity (probably class 2 mutation), while the Asn442Ser and Gln1030Arg NCC mutants showed decreased plasma membrane localization and consequently function, likely due to impaired trafficking (class 3 mutation). Even though the NaCl uptake was hampered for NCC mutants Glu121Asp, Pro751Leu, Ser475Cys and Tyr489His, the transporters reached the plasma membrane (class 4 mutation), suggesting an effect on NCC regulation or ion affinity. The present study shows the identification of 38 novel mutations in the SLC12A3 gene and provides insight into the mechanisms that regulate NCC

Gitelman's Syndrome: characterization of a novel c.1181G>A point mutation and functional classification of the known mutations.

We have investigated the mechanisms by which a novel missense point mutation (c.1181G>A) found in two sisters causes Gitelman’s syndrome by impairing the sodium chloride co-transporter (NCC, encoded by SLC12A3 gene) function. The cDNA and in vitro transcribed mRNA of either wild-type or mutated SLC12A3 were transfected into HEK293 cells and injected into Xenopus laevis oocytes, respectively. The expression, maturation, trafficking, and function of the mutated and wild-type NCC were assessed by Western blotting, immunohistochemistry and 22Na+ uptake studies. By immunoblotting of lysates from HEK293 cells and oocytes expressing wild-type NCC, two NCC-related bands of approximately 130 kDa and 115 kDa, corresponding to fully and core-glycosylated NCC, respectively, were identified. In contrast, the mutant NCC only showed a single band of approximately 115 kDa, indicating impaired maturation of the protein. Moreover, oocytes injected with wild-type NCC showed thiazide-sensitive 22Na+ uptake, which was absent in those injected with the mutant NCC. The novel mutation was discussed in the context of the functionally characterized NCC mutations causing Gitelman’s syndrome, which fit into five classes. In conclusion, the functional characterization of this novel Gly394Asp NCC and its localization on the NCC structure, alongside that of previously known mutations causing Gitelman’s syndrome, may provide novel information on the function of the different domains of the human NCC