Mice homozygous for the disrupted renal type 11a sodium/phosphate (Na/Pi) cotransporter gene, Npt2, (Npt2 KO) exhibit renal Pi wasting and hypercalciuria, predisposing factors for renal stone formation. We observed that Npt2 KO mice, but not wild-type littermates form renal stones. The renal stones were evident in newborn, weanling and adult mice and composed of calcium (Ca) and Pi. The presence of renal calcification correlated with the absence of Npt2 gene expression and the presence of genes responsible for the synthesis (1alpha-hydroxylase) and catabolism (24-hydroxylase) of 1,25-dihydroxyvitamin D, whose elevated levels contribute to the hypercalciuria and renal calcification in Npt2 KO mice. The renal calcification was associated with increased osteopontin (OPN) mRNA expression and colocalized with OPN, the latter associates with renal stones in vivo and inhibits Ca mineralization in vitro). These data demonstrate that hyperphosphaturia and hypercalciuria, secondary to Npt2 gene disruption, are sufficient for the development of renal calcification.

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