Genomic organization and mutation screening of the human ortholog of Pkdr1 associated with polycystic kidney disease in the rat.

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common inherited disorders in humans. Although disease-causing mutations have been found in two genes, PKD1 and PKD2, a small number of ADPKD families exist that are unlinked to either of these genes, suggesting involvement of a third, as yet unidentified PKD3 gene. Susceptibility to renal cyst formation in the (cy/+) rat is caused by a missense mutation in Pkdr1 encoding the novel protein SamCystin. To initiate studies of the human orthologous gene, we determined the location and the organization of human PKDR1. We genotyped microsatellite markers flanking the human ortholog in PKD families that either are unlinked to known PKD genes, or in which mutations have not yet been identified and carried out mutation analysis in PKD patients. We identified eight novel single nucleotide polymorphisms, including three leading to amino acid changes. These variants are unlikely to account for PKD in these patients, yet the screening of other affected populations may provide information about the involvement of PKDR1 as a modifier gene in cystic kidney disease.

Missense mutation in sterile alpha motif of novel protein SamCystin is associated with polycystic kidney disease in (cy/+) rat.

Autosomal dominant polycystic kidney disease (PKD) is the most common genetic disease that leads to kidney failure in humans. In addition to the known causative genes PKD1 and PKD2, there are mutations that result in cystic changes in the kidney, such as nephronophthisis, autosomal recessive polycystic kidney disease, or medullary cystic kidney disease. Recent efforts to improve the understanding of renal cystogenesis have been greatly enhanced by studies in rodent models of PKD. Genetic studies in the (cy/+) rat showed that PKD spontaneously develops as a consequence of a mutation in a gene different from the rat orthologs of PKD1 and PKD2 or other genes that are known to be involved in human cystic kidney diseases. This article reports the positional cloning and mutation analysis of the rat PKD gene, which revealed a C to T transition that replaces an arginine by a tryptophan at amino acid 823 in the protein sequence. It was determined that Pkdr1 is specifically expressed in renal proximal tubules and encodes a novel protein, SamCystin, that contains ankyrin repeats and a sterile alpha motif. The characterization of this protein, which does not share structural homologies with known polycystins, may give new insights into the pathophysiology of renal cyst development in patients.

Characterization of a major modifier locus for polycystic kidney disease (Modpkdr1) in the Han:SPRD(cy/+) rat in a region conserved with a mouse modifier locus for Alport syndrome.

The genetic analysis of rodent disease models provides a powerful tool to investigate how modifier loci cause variation in the phenotypic expression of a disease. In order to test the existence of modifier loci influencing polycystic kidney disease (PKD) phenotypes, we derived a backcross between PKD susceptible Han:SPRD(cy/+) and control Brown Norway (BN) rats, and performed a whole-genome scan in 182 PKD affected hybrids showing different grades of disease severity. The genetic dissection of PKD in the cross allowed us to detect a modifier locus, Modpkdr1, on rat chromosome 8 that controls PKD severity, kidney mass and plasma urea concentration. Results from database searches and computational analyses demonstrated that the Modpkdr1 locus shows strong evidence of synteny conservation with human and mouse chromosomal regions controlling kidney diseases, including disease progression of Alport syndrome. Comparative genome mapping also provided an inventory of potential candidate genes for modifier(s) of PKD. Analyses of the coding regions for four strong candidates (Ctsh, Bcl2a1, Trpc1 and Slc21a2) in (cy/+), BN and Lewis rat strains did not reveal sequence variants that could be associated with PKD. The characterization of Modpkdr1 may provide new insights into modulating mechanisms involved in the pathogenesis of PKD that could delay disease progression in humans. It may also have strong implications in the identification of pathophysiological factors common to different renal disorders.