Genotype-penotype correlations provide new insights in the pathogenesis of Gitelman's syndrome

Abstract

Gitelman's syndrome (GS) is a relatively frequent salt-losing tubulopathy caused by mutations in the SLC12A3 gene encoding the sodium-chloride cotransporter NCC. Although is often described as a mild variant of the salt-losing tubular disorders, clinical studies have revealed that GS patients show a considerable phenotypic variability. A limited number of patients may show an early onset, severe neuromuscular manifestations, growth retardation, chondrocalcinosis, and ventricular arrhythmia. Even if genetic background and environmental effects may influence disease severity, one may hypothesize that the extent to which a given SLC12A3 mutation modifies the NCC cotransporter activity is crucial. Thus far, the molecular basis of the phenotype variability in GS remains unknown. Functional analysis has only been performed for a limited number of mutations, without phenotypical counterpart, and the potential correlations between the position/nature of mutation in SLC12A3, the mutation-determined transcriptional profile and the clinical manifestations have not been addressed. In this work we focused on the molecular genetics of GS, taking advantage of a large series of patients with GS. A total of 26 different mutations in the SLC12A3 gene (8 novel) were identified, including 22 missenses, 3 splice site and 1 duplication. We documented significant phenotype heterogeneity among 25 patients harbouring SLC12A3 mutations, including a subgroup of 9 patients with early-onset and severe manifestations. We further characterized biochemically and structurally a panel of missense mutants in Xenopus laevis oocytes and delineated their association with phenotypic information. These data provide the first evidence suggesting that the potential severity of GS is related to a combination of the nature and functional consequence of the SLC12A3 mutations with male gender. They also extend the range of pathogenic mechanisms that underline GS by identifying NCC mutations resulting in a defective intrinsic transport activity, a mechanism that has not been shown before to underline GS. This project integrates for the first time genetic, molecular, functional and clinical data to provide new clues into the molecular mechanisms and phenotype variability of the disease. These studies provided us the tools for the molecular diagnosis of GS, improved our understanding on the factors potentially involved in the phenotypic heterogeneity observed in the patients, and expanded our knowledge of the pathogenic mechanisms involved in ion transport operating in the distal tubule of the nephronThèse de doctorat en sciences biomédicales (orientation : génétique-physiopathologie) (SBIM 3)--UCL, 200