Dents disease, due to mutations in the chloride/proton antiporter, CLC-5, represents one form of familial hypophosphataemic rickets. Dents disease patients also have: low-molecular-weight-proteinuria; hypercalciuria with nephrolithiasis and renal failure; and urinary loss of parathyroid hormone and vitamin D-binding protein, due to defective receptor-mediated endocytosis within the renal proximal tubule. However, there is variability in these clinical phenotypes such that only 25% of patients have rickets, while ~40% have phosphaturia, and we have previously demonstrated that this is also affected in the cellular phenotypes of human conditionally-immortalised proximal tubular epithelial cell-lines (ciPTECs) established from the urines of patients harbouring CLC-5 mutations. To further elucidate the mechanisms underlying the differences in the cell phenotypes and their structural-functional relationships we studied two ciPTECs harbouring CLC-5 mutations in different intracellular domains: an in-frame insertion (30:insHis) in the N-terminus that lies in a highly charged region; and a nonsense mutation (Arg637Stop) in the C-terminus, that causes loss of a region of CLC-5 known to interact with endocytic accessory proteins. Heterologous expression and whole-cell patch-clamp recordings within HEK293 cells revealed CLC-5 mutations to cause a reduction in chloride currents, to ~70% of wild-type for 30:insHis-CLC-5 (n=10), and ~30% of wild-type for Arg637Stop-CLC-5 (n=6) (P<0.05 in both). Confocal microscopy of ciPTECs demonstrated that 30:insHis-CLC-5 had reduced cell surface expression, while the Arg637Stop-CLC-5 was predominantly intracellular. Thus, the Arg637Stop-CLC-5 mutation may disturb interactions with endocytic proteins, thereby disrupting the trafficking of CLC-5 to the cell surface, and reducing the overall chloride conductance of the cell; whilst the 30:insHis-CLC-5 mutation which alters the amino acid charge balance of the N-terminus, likely affects chloride conductance, thereby impairing endosomal acidification and receptor-mediated endocytosis. These studies further demonstrate that different intracellular mechanisms give rise to Dents disease and these may account for the differences in patient phenotypes.
Declaration of funding: Yes.
Details: This work was supported by the Medical Research Council (Grants G9825289 and G1000467), Kidney Research UK, EuFP6-EuReGene, (05085) and the Fund for Scientific Research Flanders (Fundamental Clinical Investigatorship, 1801110N).