Genetic heterogeneity in autosomal dominant pseudohypoaldosteronism type I: exclusion of claudin-8 as a candidate gene.

BACKGROUND/AIMS: Pseudohypoaldosteronism type I (PHAI) is an inherited disorder characterized by renal salt wasting, hyperkalemic metabolic acidosis, and hyperaldosteronism. Its known causes are mutations in the mineralocorticoid receptor and the epithelial sodium channel (ENaC), but there are reports of genetic heterogeneity. Claudin-8 is a tight junction protein that acts as a paracellular cation barrier in the distal nephron. The aim of this study was to test the hypothesis that mutations in claudin-8, which would be expected to induce a distal tubule cation leak, can be a cause of PHAI. METHODS: We identified 10 patients with autosomal dominant PHAI in whom mutations in the mineralocorticoid receptor and ENaC had been excluded. The claudin-8 gene and upstream region was sequenced in all patients. RESULTS: No disease-associated claudin-8 mutations were identified. A novel polymorphic allele in the 3'-untranslated region was identified in 2 patients, but was also found in 15% of individuals in a panel of normal controls. CONCLUSION: We present further evidence for locus heterogeneity in PHAI. Mutations in claudin-8 are unlikely to be a cause of PHAI. Further studies of other claudins in this disease are warranted.

Expression of claudin-7 and -8 along the mouse nephron.

Claudins are integral membrane proteins of the tight junction that determine the magnitude and selectivity of paracellular permeability in epithelial tissues. The mammalian renal tubule exhibits considerable heterogeneity in the permeability properties of its different segments. To determine the nephron segment localization of claudin-7 and -8, immunofluorescence staining of mouse kidney sections was performed using isoform-specific antibodies. Claudin-8 was found to be expressed primarily at the tight junction along the entire aldosterone-sensitive distal nephron and in the late segments of the thin descending limbs of long-looped nephrons. This pattern of expression is consistent with the putative role of claudin-8 as a paracellular cation barrier. By contrast, claudin-7 was found in the same nephron segments as claudin-8, but it was expressed primarily at the basolateral membrane.