Genotyp-/Phänotyp-Analyse und klinische Charakterisierung von 25 Familien mit familiärer Hypomagnesiämie mit Hyperkalziurie und Nephrokalzinose

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal recessive tubular disorder that is frequently associated with progressive renal failure. The primary defect is related to impaired tubular reabsorption of magnesium and calcium in the thick ascending limb of Henle’s loop (TAL) in the nephron. Mutations in CLDN16 (former PCLN1), which encodes the renal tight junction protein claudin-16, were identified as the underlying genetic defects. Comprehensive clinical data and the results of CLDN16 mutation analysis of 25 FHHNC families with 33 affected individuals are presented. The onset of disease occurs mainly in early childhood. In our cohort patients presented mainly with urinary tract infections, polyuria and hematuria at a median age of 3.5 years. In contrast to other tubulopathies FHHNC is generally complicated by chronic renal failure in early childhood or adolescence. At the time of diagnosis the GFR was already decreased to <60 ml/min per 1.73 m2 for eleven patients. Twelve patients exhibited progression to end-stage renal disease at a median age of 14.5 years. Treatment with magnesium salts and thiazides seemed to have no effect on the progression of renal failure. Genotype analysis revealed CLDN16 mutations in all except three mutant alleles (94%). Fifteen different mutations were observed, including eight novel mutations. The accumulation of mutations affecting the first extracellular loop was striking, with 48% of all mutant alleles exhibiting a Leu151Phe exchange. Haplotype analysis strongly suggested a founder effect among patients with FHHNC who originated from Germany or eastern European countries. Former publications assumed that the human CLDN16 gene encodes a protein of 305 amino acids. Gene sequence analysis suggested two possible start codons at methionine 1 and methionine 71. Sequence alignments of human claudin-16 and other species and a common frameshift polymorphism at amino acid residue 55 support the hypothesis that methionine 71 is the original translational start site. The correlation of genotype and phenotype revealed no obvious genotype-phenotype relationship in our cohort. However, with respect to the progression of renal failure, we observed a rather close intrafamilial phenotypic concordance in the majority of mutliplex families supporting the hypothesis that different genotypes of CLDN16 play an important role in the progression of renal failure. Recently expression studies of a large cohort including our 33 patients revealed that some claudin-16 mutations led to partial loss of claudin-16 function and others to complete loss of function. Patients who had mutations resulting in complete loss of function of both alleles were significantly younger at the onset of symptoms and had a more rapid decline in renal function. In 13 von 23 families hypercalciuria and/or nephrolithiasis were observed in otherwise unaffected family members, indicating a possible role of heterozygous CLDN16 mutations in yielding hypercalciuric stone-forming conditions. CLDN16 is a possible candidate gene for familial hypercalciuria and familial disorders of nephrolithiasis.