Autosomal Dominant Hypercalciuria in a Mouse Model Due to a Mutation of the Epithelial Calcium Channel, TRPV5
Loh NY., Bentley L., Dimke H., Verkaart S., Tammaro P., Gorvin CM., Stechman MJ., Ahmad BN., Hannan FM., Piret SE., Evans H., Bellantuono I., Hough TA., Fraser WD., Hoenderop JGJ., Ashcroft FM., Brown SDM., Bindels RJM., Cox RD., Thakker RV.
Hypercalciuria is a major cause of nephrolithiasis, and is a common and complex disorder involving genetic and environmental factors. Identification of genetic factors for monogenic forms of hypercalciuria is hampered by the limited availability of large families, and to facilitate such studies, we screened for hypercalciuria in mice from an N-ethyl-N-nitrosourea mutagenesis programme. We identified a mouse with autosomal dominant hypercalciuria (HCALC1). Linkage studies mapped the Hcalc1 locus to a 11.94 Mb region on chromosome 6 containing the transient receptor potential cation channel, subfamily V, members 5 (Trpv5) and 6 (Trpv6) genes. DNA sequence analysis of coding regions, intron-exon boundaries and promoters of Trpv5 and Trpv6 identified a novel T to C transition in codon 682 of TRPV5, mutating a conserved serine to a proline (S682P). Compared to wild-type littermates, heterozygous (Trpv5682P/+) and homozygous (Trpv5682P/682P) mutant mice had hypercalciuria, polyuria, hyperphosphaturia and a more acidic urine, and ~10% of males developed tubulointerstitial nephritis. Trpv5682P/682P mice also had normal plasma parathyroid hormone but increased 1,25-dihydroxyvitamin D3 concentrations without increased bone resorption, consistent with a renal defect for the hypercalciuria. Expression of the S682P mutation in human embryonic kidney cells revealed that TRPV5-S682P-expressing cells had a lower baseline intracellular calcium concentration than wild-type TRPV5-expressing cells, suggesting an altered calcium permeability. Immunohistological studies revealed a selective decrease in TRPV5-expression from the renal distal convoluted tubules of Trpv5682P/+ and Trpv5682P/682P mice consistent with a trafficking defect. In addition, Trpv5682P/682P mice had a reduction in renal expression of the intracellular calcium-binding protein, calbindin-D28K, consistent with a specific defect in TRPV5-mediated renal calcium reabsorption. Thus, our findings indicate that the TRPV5 S682P mutant is functionally significant and study of HCALC1, a novel model for autosomal dominant hypercalciuria, may help further our understanding of renal calcium reabsorption and hypercalciuria. © 2013 Loh et al.