Haploinsufficiency of Pkd2 is associated with increased tubular cell proliferation and interstitial fibrosis in two murine Pkd2 models

Background. Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited human kidney disease and is caused by germline mutations in PKD1 (85%) or PKD2 (15%) It has been estimated that around 1% of tubular cells give rise to cysts, and cell hyperproliferation has been noted to be a cardinal feature of cystic epithelium. Nevertheless, it is uncertain whether the increase in proliferative index observed is an early or late feature of the cystic ADPKD kidney. Methods. Two Pkd2 mouse mutants (WS25 and WS183) have been recently generated as orthologous models of PKD2. To determine the effect of Pkd2 dosage on cell proliferation, cyst formation and renal fibrosis, we studied renal tissue from Pkd2^WS25/WS25 and Pkd2^+/- mice by histological analysis. We also examined the proliferative index in archival nephrectomy tissue obtained from patients with ADPKD and normal controls. Results. The proliferative index of non-cystic tubules in Pkd2 mutant mice as assessed by proliferating cell nuclear antigen and Ki67-positive nuclei was between 1-2%, values 5-10 times higher than control tissue. Similarly, the proliferative index of non-cystic tubules in human ADPKD kidneys was 40 times higher than corresponding controls. In Pkd2 mutant mice, significant correlations were found between the fibrosis score and the mean cyst area as well as with the proliferative index. Of significance, proliferating tubular cells were uniformly positive for polycystin-2 expression in Pkd2^+/-kidney. Conclusion. These results suggest that an increase in cell proliferation is an early event preceding cyst formation and can result from haploinsufficiency at Pkd2. The possible pathogenic link between tubular cell proliferation, interstitial fibrosis and cyst formation is discussed.