Presentation Authors: Yan Liu*, Ellen Shapiro, Herbert Lepor, Xue-Ru Wu, New York, NY
Introduction: Mice lacking one or both alleles of Tamm-Horsfall protein (THP) gene, thus excreting low or no urinary THP, are prone to developing spontaneous calcium phosphate (apatite) crystals in the interstitium of renal papillae, particularly in aged mice. We tested whether increasing urine concentration of phosphorus or oxalate would accelerate renal calcinosis or convert crystals into stones by developing double knockout (KO) mice bearing two genetic alterations: (i) THP deficiency plus hyperphosphaturia, or (ii) THP deficiency plus hyperoxaluria.
Methods: To produce hyperphosphaturia, THP KO mice were cross-bred with Slc34a (Npt2) KO mice in which the lack of sodium/phosphate cotransporter in the proximal tubules leads to hyperphosphaturia. To produce hyperoxaluria, THP KO mice were cross-bred with Slc26a1 (Sat1) KO mice in which the lack of sulfate anion transporter-1 in the intestine leads to hyperoxaluria. Both crosses of mice were further bred to homozygous status, after which single and double KO mice were compared for renal calcification by von Kossa staining. Urine phosphate and oxalate levels were determined using assay kits (Sigma) and normalized to creatinine.
Results: Single KO mice lacking THP, Npt2 or Sat1 all spontaneously developed intra-renal calcinosis, whereas their wild-type littermates were devoid of intra-renal crystal. Of the single KO mice, THP KO mice had the greatest amount of crystals localized primarily interstitially in the papillae. Npt2 KO mice had moderate amount of crystals that were both intra-tubular and interstitial localized throughout the nephron. Sat1 KO mice had small amount of crystals localized interstitially throughout the nephron. THP/Npt2 double KO mice had significantly more crystals and wider distribution including plugging crystals in the ducts of Bellini and renal pelvis than the single KO mice. THP/Sat1 double KO mice had similar amounts of crystals to the single KO mice but slightly wider distribution in the nephron. Urine phosphate level in THP/Npt2 double KO mice was significantly higher than the single KO mice. Urine oxalate level in THP/Sat1 double KO mice was significantly higher than THP KO mice, but did not differ from Sat1 KO mice.
Conclusions: These results indicate that, in the absence of THP, hyperphosphaturia significantly shortens the latency and increases the severity of renal calcinosis, while hyperoxaluria only has a moderate effect. Renal calcium oxalate stone formation atop interstitial apatite crystals may require other important factors yet to be identified.
Source of Funding: NIH