Epithelial Ca2+ channel (ECAC1) in autosomal dominant idiopathic hypercalciuria

Background. The epithelial Ca2+ channel (ECaC) exhibits the defining properties for being the gatekeeper in 1,25-dihydroxyvitamin D-3-regulated Ca2+ (re)absorption. Its recently cloned human orthologue (ECaCl) could, therefore, represent a crucial molecule in human disorders related to Ca2+-wasting such as idiopathic hypercalciuria (IH). Methods. Fifty-seven members of nine families with IH were investigated. Phenotyping was performed by measurements of urinary Ca2+ excretion, while other underlying disorders were appropriately excluded. Initially, the recently suggested locus for kidney stone-associated hypercalciuria on chromosome 1q23.3-q24 was investigated. Next, direct mutation analysis of all 15 exons of the ECACl gene and 2.9 kb upstream from the start codon was performed. hECaCl, heterologously expressed in human embryonic kidney 293 cells, was characterized by patch-clamp analysis. Results. The mode of inheritance in the studied pedigrees is consistent with an autosomal dominant trait. Haplotype analysis did not implicate a role of the locus on chromosome 1. The coding sequence of the ECACl gene was not different between the affected and the non-affected family members. In the 5'-flanking region, three single nucleotide polymorphisms were encountered, but these polymorphisms were observed regardless of the affection status of the screened family members. Patch-clamp analysis of hECaCl was performed as the putative pore region contains four non-conserved amino acid substitutions compared with the other species. This analysis revealed the distinctive properties of ECaC, including a high Ca2+ selectivity, inward rectification, and Ca2+-dependent inactivation. Conclusion. These results do not support a primary role for hECaCl in IH in nine affected families. Because of the heterogeneity of the disease, however, the involvement of ECaCl in other subtypes of IH cannot be excluded and needs further investigation. The electrophysiological properties of hECaCl further substantiate its prime role in Ca2+ (re)absorption.