Molecular variants of the thiazide-sensitive Na+-Cl- cotransporter in hypertensive families

Background The thiazide- sensitive Na+- ClS cotransporter ( TSC) is involved in the fine regulation of sodium excretion by the kidney, and an increase in its activity causes salt- sensitive hypertension and hypercalciuria. Methods To explore the possibility that activating mutations in the TSC gene may be involved in the pathogenesis of primary hypertension, we screened genomic DNA of 68 individuals from hypertensive families of patients with hypercalciuria for variations using single- strand conformation polymorphism and investigated the functional properties using the Xenopus laevis oocyte expression system. Results We identified 18 variants in the TSC and, of those, four were nonsynonymous. The A728T, R904Q and R919C variants were introduced into the human TSC cDNA and expressed in Xenopus oocytes to study their effect on Na+ transport. Although T728 and Q904 did not show any difference in Na+ uptake, the R919C mutant showed a 60% increase compared to wild- type human TSC and an exaggerated inhibitory action of hydrochlorothiazide. Immunocytochemical analysis revealed no difference in cell surface expression of R919C mutants compared to wild- type hTSC. There was no significant difference in the inhibitory effect of the carboxyl terminus of the serinethreonine kinase WNK4 on the R919C mutant and wild- type hTSC. Conclusions Our results show that the substitution of arginine for cysteine at position 919 of TSC increases Na+ transport function, and provide support for the hypothesis that mutations in renal tubular sodium transporters may contribute to the development of primary hypertension, a polygenic disorder, by increasing renal sodium reabsorption.