Refinement of the critical region for MCKD1 by detection of transcontinental haplotype sharing

Refinement of the critical region for MCKD1 by detection of transcontinental haplotype sharing.BackgroundAutosomal-dominant medullary cystic kidney disease type 1 (MCKD1) [OMIM 174000] is a hereditary nephropathy that leads to renal salt wasting and end-stage renal failure at a median age of 62 years. In a Welsh MCKD1 kindred we have recently demonstrated linkage to the MCKD1 locus on chromosome 1q23.1 and refined the critical MCKD1 region to <3.3Mb.MethodsIn order to refine the candidate gene region for MCKD1, high-resolution haplotype analysis in three large kindreds with MCKD1 was performed.ResultsWe report here on high-resolution haplotype analysis in this Welsh kindred, as well as in the Arizona kindred, which was used for the first definition of MCKD as a disease entity, and in a kindred from the Dutch/German border. We detected extensive haplotype sharing among all affected individuals of all three kindreds. Scrutinization of the genealogy of the Arizona kindred revealed an origin from Germany in the 17th century, thereby providing historical data for haplotype sharing by descent at the MCKD1 locus.ConclusionUnder the hypothesis of haplotype sharing by descent, we refined the critical genetic interval to <650kb, thus enabling candidate gene analysis

Mucin-1 Increases Renal TRPV5 Activity In Vitro

Hypercalciuria is a major risk factor for nephrolithiasis. We previously reported that Uromodulin (UMOD) protects against nephrolithiasis by upregulating the renal calcium channel TRPV5. This channel is crucial for calcium reabsorption in the distal convoluted tubule (DCT). Recently, mutations in the gene encoding Mucin-1 (MUC1) were found to cause autosomal dominant tubulointerstitial kidney disease, the same disease caused by UMOD mutations. Because of the similarities between UMOD and MUC1 regarding associated disease phenotype, protein structure, and function as a cellular barrier, we examined whether urinary MUC1 also enhances TRPV5 channel activity and protects against nephrolithiasis. We established a semiquantitative assay for detecting MUC1 in human urine and found that, compared with controls (n=12), patients (n=12) with hypercalciuric nephrolithiasis had significantly decreased levels of urinary MUC1. Immunofluorescence showed MUC1 in the thick ascending limb, DCT, and collecting duct. Applying whole–cell patch-clamp recording of HEK cells, we found that wild-type but not disease mutant MUC1 increased TRPV5 activity by impairing dynamin-2– and caveolin-1–mediated endocytosis of TRPV5. Coimmunoprecipitation confirmed a physical interaction between TRPV5 and MUC1. However, MUC1 did not increase the activity of N-glycan–deficient TRPV5. MUC1 is characterized by variable number tandem repeats (VNTRs) that bind the lectin galectin-3; galectin-3 siRNA but not galectin-1 siRNA prevented MUC1-induced upregulation of TRPV5 activity. Additionally, MUC1 lacking VNTRs did not increase TRPV5 activity. Our results suggest that MUC1 forms a lattice with the N-glycan of TRPV5 via galectin-3, which impairs TRPV5 endocytosis and increases urinary calcium reabsorption

Individuals with mutations in XPNPEP3, which encodes a mitochondrial protein, develop a nephronophthisis-like nephropathy

The autosomal recessive kidney disease nephronophthisis (NPHP) constitutes the most frequent genetic cause of terminal renal failure in the first 3 decades of life. Ten causative genes (NPHP1–NPHP9 and NPHP11), whose products localize to the primary cilia-centrosome complex, support the unifying concept that cystic kidney diseases are “ciliopathies”. Using genome-wide homozygosity mapping, we report here what we believe to be a new locus (NPHP-like 1 [NPHPL1]) for an NPHP-like nephropathy. In 2 families with an NPHP-like phenotype, we detected homozygous frameshift and splice-site mutations, respectively, in the X-prolyl aminopeptidase 3 (XPNPEP3) gene. In contrast to all known NPHP proteins, XPNPEP3 localizes to mitochondria of renal cells. However, in vivo analyses also revealed a likely cilia-related function; suppression of zebrafish xpnpep3 phenocopied the developmental phenotypes of ciliopathy morphants, and this effect was rescued by human XPNPEP3 that was devoid of a mitochondrial localization signal. Consistent with a role for XPNPEP3 in ciliary function, several ciliary cystogenic proteins were found to be XPNPEP3 substrates, for which resistance to N-terminal proline cleavage resulted in attenuated protein function in vivo in zebrafish. Our data highlight an emerging link between mitochondria and ciliary dysfunction, and suggest that further understanding the enzymatic activity and substrates of XPNPEP3 will illuminate novel cystogenic pathways