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Among the renal cystic diseases that result in end-stage renal disease, an important hereditary cause is medullary cystic kidney disease, which affects adults in an autosomal dominant pattern. It is characterized by progressive renal failure, tubulointerstitial fibrosis, and formation of small cysts in the renal medulla and corticomedullary junction. While the appearance of medullary/corticomedullary cysts may not be pathognomonic for medullary cystic kidney disease, encountering a patient with renal failure and medullary/corticomedullary cysts should prompt further investigation, given the implication of having the disease. Genetic testing can be used to identify potential renal donors as well as identify affected individuals in order to control risk factors for chronic renal disease that may mitigate the progression of the disease process. The treatment of choice is renal transplantation.

Background: Medullary cystic kidney disease (MCKD) is clinically indistinguishable from several other autosomal-dominant renal diseases; thus, molecular genetic testing is needed to establish a definitive diagnosis. A specific type of single cytosine insertion in the variable number tandem repeat (VNTR) of the mucin 1 (MUC1) gene is the only known cause of MCKD1; however, genetic analysis of this mutation is difficult and not yet offered routinely. To identify the causative mutation/s and establish a definitive diagnosis in a Chinese family with chronic kidney disease, clinical assessments and genetic analysis were performed, including using a modified genotyping method to identify the MUC1-VNTR single cytosine insertion. Methods: Clinical data from three patients in a Chinese family with chronic kidney disease were collected and evaluated. Linkage analysis was used to map the causative locus. Mutation analysis of uromodulin (UMOD) gene was performed using polymerase chain reaction (PCR) and direct sequencing. For MUC1 genotyping, the mutant repeat units were enriched by MwoI restriction, and then were amplified and introduced into pMD-18T vectors. The 192 clones per transformant were picked up and tested by colony PCR and second round of MwoI digestion. Finally, Sanger sequencing was used to confirm the MUC1 mutation. Results: Clinical findings and laboratory results were consistent with a tubulointerstitial lesion. Linkage analysis indicated that the family was compatible with the MCKD1 locus. No mutations were found in UMOD gene. Using the modified MUC1 genotyping method, we detected the MUC1-VNTR single cytosine insertion events in three patients of the family; and mutation-containing clones were 12/192, 14/192, and 5/96, respectively, in the three patients. Conclusions: Clinical and genetic findings could support the MCKD1 diagnosis. The modified strategy has been demonstrated to be a practical way to detect MUC1 mutation. Key words: Autosomal Dominant Tubulointerstitial Kidney Diseases; Genotyping; Medullary Cystic Kidney Disease; MUC1 Gene; Variable Number Tandem Repeat

Purpose of review Recent studies in the kidney have revealed that the well characterized tumor antigen mucin 1 (MUC1/Muc1) also has numerous functions in the normal and injured kidney. Recent findings Mucin 1 is a transmembrane mucin with a robust glycan-dependent apical targeting signal and efficient recycling from endosomes. It was recently reported that the TRPV5 calcium channel is stabilized on the cell surface by galectin-dependent cross-linking to mucin 1, providing a novel mechanism for regulation of ion channels and normal electrolyte balance.Our recent studies in mice show that Muc 1 is induced after ischemia, stabilizing hypoxia-inducible factor 1 (HIF-1)α and β-catenin levels, and transactivating the HIF-1 and β-catenin protective pathways. However, prolonged induction of either pathway in the injured kidney can proceed from apparent full recovery to chronic kidney disease. A very recent report indicates that aberrant activation of mucin 1 signaling after ischemic injury in mice and humans is associated with development of chronic kidney disease and fibrosis. A frameshift mutation in MUC1 was recently identified as the genetic lesion causing medullary cystic kidney disease type 1, now appropriately renamed MUC1 Kidney Disease. Summary Studies of mucin 1 in the kidney now reveal significant functions for the extracellular mucin-like domain and signaling through the cytoplasmic tail.

BACKGROUND: UMOD mutations cause familial juvenile hyperuricemic nephropathy (FJHN) and medullary cystic kidney disease (MCKD), although these phenotypes are nonspecific. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: We reviewed cases of UMOD mutations diagnosed in the genetic laboratories of Necker Hospital (Paris, France) and of Universite Catholique de Louvain (Brussels, Belgium). We also analyzed patients with MCKD/FJHN but no UMOD mutation. To determine thresholds for hyperuricemia and uric-acid excretion fraction (UAEF) according to GFR, these parameters were analyzed in 1097 patients with various renal diseases and renal function levels. RESULTS: Thirty-seven distinct UMOD mutations were found in 109 patients from 45 families, all in exon 4 or 5 except for three novel mutations in exon 8. Median renal survival was 54 years. The type of mutation had a modest effect on renal survival, and intrafamilial variability was high. Detailed data available in 70 patients showed renal cysts in 24 (34.3%) of nonspecific localization in most patients. Uricemia was >75th percentile in 31 (71.4%) of 42 patients not under dialysis or allopurinol therapy. UAEF (n = 27) was

Familial juvenile hyperuricemic nephropathy (FJHN [MIM 162000]) is an autosomal-dominant disorder characterized by abnormal tubular handling of urate and late development of chronic interstitial nephritis leading to progressive renal failure. A locus for FJHN was previously identified on chromosome 16p12 close to the MCKD2 locus, which is responsible for a variety of autosomal-dominant medullary cystic kidney disease (MCKD2). UMOD, the gene encoding the Tamm-Horsfall/uromodulin protein, maps within the FJHN/MCKD2 critical region. Mutations in UMOD were recently reported in nine families with FJHN/MCKD2 disease. A mutation in UMOD has been identified in 11 FJHN families (10 missense and one in-frame deletion)-10 of which are novel-clustering in the highly conserved exon 4. The consequences of UMOD mutations on uromodulin expression were investigated in urine samples and renal biopsies from nine patients in four families. There was a markedly increased expression of uromodulin in a cluster of tubule profiles, suggesting an accumulation of the protein in tubular cells. Consistent with this observation, urinary excretion of wild-type uromodulin was significantly decreased. The latter findings were not observed in patients with FJHN without UMOD mutations. In conclusion, this study points to a mutation clustering in exon 4 of UMOD as a major genetic defect in FJHN. Mutations in UMOD may critically affect the function of uromodulin, resulting in abnormal accumulation within tubular cells and reduced urinary excretion.

Nephronophthisis (NPH) and medullary cystic kidney disease (MCKD) constitute a group of renal cystic diseases that share the macroscopic feature of cyst development at the corticomedullary border of the kidneys. The disease variants also have in common a characteristic renal histologic triad of tubular basement membrane disintegration, tubular atrophy with cyst development, and interstitial cell infiltration with fibrosis. NPH and, in most instances, MCKD lead to chronic renal failure with an onset in the first two decades of life for recessive NPH and onset in adult life for autosomal dominant MCKD. There is extensive genetic heterogeneity with at least three different loci for NPH (NPHP1, NPHP2, and NPHP3) and two different loci for MCKD (MCKD1 and MCKD2). Juvenile nephronophthisis, in addition, can be associated with extrarenal organ involvement. As a first step toward understanding the pathogenesis of this disease group, the gene (NPH1) for juvenile nephronophthisis (NPH1) has been identified by positional cloning. Its gene product, nephrocystin, is a novel protein of unknown function that contains a src-homology 3 domain. It is hypothesized that the pathogenesis of NPH might be related to signaling processes at focal adhesions (the contact points between cells and extracellular matrix) and/or adherens junctions (the contact points between cells). This hypothesis is based on the fact that most src-homology 3-containing proteins are part of focal adhesion signaling complexes, on animal models that exhibit an NPH-like phenotype, and on the recent finding that nephrocystin binds to the protein p130(cas), a major mediator of focal adhesion signaling.

Nephronophthisis (NPHP), medullary cystic kidney disease (MCKD), and familial juvenile hyperuricemic nephropathy (FJHN) are three renal disorders with overlapping clinical features. Recent molecular genetic studies have begun to clarify the similarities and differences between these conditions.