Mutationsanalyse des thiazidsensitiven Natrium-Chlorid-Kotransporters im distalen Tubuluskonvolut der Niere

Gitelmans syndrome (GS), an autosomal recessive salt-losing tubulopathy, is characterized by the combination of hypokalemic alkalosis, hypomagnesemia and hypocalciuria. The clinical onset covers a broad spectrum of symptoms – of our study group more than two thirds of all patients presented with symptoms or electrolyte abnormalities before or at beginning of adolescence. Due to unspecific symptoms such as salt craving, weakness and fatigue, the diagnosis of GS is largely based on detection of the typical electrolyte constellation. Specific neuromuscular presentations such as carpopedal spasms, tetany and paresthesia are rarely seen [63] and, moreover, severe manifestations such as chondrocalcinosis, rhabdomyolysis, seizures and fetal ventricular arrhythmia have been reported only in a very limited number of cases [80]. GS is associated with inactivating mutations in the SLC12A3 gene, which encodes for the thiazid-sensitive sodium-chloride-cotransporter (NCCT) that is expressed in the apical membrane of the cells lining the distal convoluted tubule (DCT) [85]. More than 100 different mutations of the SLC12A3 gene have been identified so far. However, the classical view of GS as a genetically homogeneous disorder linked to SLC12A3 has been disturbed by the identification of mutations in the CLCNKB gene, which may also be coincident with electrolyte abnormalities characteristic for GS [36]. Nevertheless, CLCNKB mutations more commonly lead to Bartter syndrome (cBS), another salt-losing tubulopathy which manifests in the first year of life with polyuria and failure to thrive [63, 64, 84]. CLCNKB codes for the basolateraly expressed renal chloride channel (ClC-Kb). Both, NCCT and CIC-Kb, are expressed oppositionally in the DCT cell and are crucial for the transcellular chloride reabsorption [2]. In the present study, 28 patients from 22 different families presenting with a hypokalemic saltlosing tubulopathy (19 GS-, 4 cBS-, 5 cBS/GS phenotype) were screened for mutations in the SLC12A3 gene. The occurrence of CLCNKB mutations in these patients could be excluded. Interestingly, all 8 informative families showed cosegregation with the SLC12A3 locus, but SLC12A3 mutations could be detected in only 6 families. In summary, 20 patients harboured mutations in the SLC12A3 gene (71% detection rate; 11 homozygous, 2 compound heterozygous, 7 heterozygous) and 13 different mutations could be identified (9 missense, 4 nonsense mutations: 2 deletions and 2 donor splice site mutations). Intriguingly, of all 13 mutations detected in our patient cohort, five haven´t been described before and can be considered as novel SLC12A3 mutations for GS. The majority of these mutations are placed in the intracellular C-terminal-domain, an important regulatory element for transporter activity [46]. Moreover, all NCCT mutations affect evolutionary highly conserved aminoacid residues within the cation-chloride-cotransporter family. Although functional analysis of the detected mutations are not covered in this study, it is tempting to speculate that these mutations drastically impair NCCT function. 11 of 19 patients with the typical GS-phenotype presented according to expectation mutations in the SLC12A3 gene. Surprisingly, all 4 patients with the clinical phenotype of cBS also harboured SLC12A3 mutations. Interestingly, in a case of dizygous twins with two identical homozygous SLC12A3 mutations, one presented with the classical GS features and the other one with the clinical picture of cBS. This example highlights the variable expression of SLC12A3 mutations. In this study the phenotype of SLC12A3 mutations was associated with a broader clinical spectrum than expected. In addition, neither nature nor position of the detected mutations had an impact on the severity of the phenotype. In summary, these new insights demonstrate that the historical differentiation between GS and cBS is overage. From a clinical and pathophysiological point of view it appears to be more practicable to consider both, GS and cBS, as one common disease of the renal distal tubule - the DCT disease.