A novel mutation, outside of the candidate region for diagnosis, in the inverted formin 2 gene can cause focal segmental glomerulosclerosis.

Focal and segmental glomerulosclerosis (FSGS) is a histological pattern that has several etiologies, including genetics. The autosomal dominant form of FSGS is a heterogenic disease caused by mutations within three known genes: α-actinin 4 (ACTN4), canonical transient receptor potential 6 (TRPC6), and the inverted formin 2 (INF2) gene. More recently, INF2 mutations have also been attributed to Charcot-Marie-Tooth neuropathy associated with FSGS. Here we performed direct sequencing, histological characterization, and functional studies in a cohort of families with autosomal dominant FSGS. We detected a novel mutation in exon 6 of the INF2 gene outside of the exon 2-4 candidate region used for rapid diagnosis of autosomal dominant FSGS. This new mutation is predicted to alter a highly conserved amino-acid residue within the 17th α-helix of the diaphanous inhibitory domain of the protein. A long-term follow-up of this family indicated that all patients were diagnosed in adulthood, as opposed to early childhood, and progression to end-stage renal disease was at different times without clinical or electrodiagnostic evidence of neuropathy. Thus, this novel mutation in INF2 linked to nonsyndromic FSGS indicates the necessity for full gene sequencing if no mutation is found in the current rapid-screen region of the gene.

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.

Evaluating the clinical utility of a molecular genetic test for polycystic kidney disease.

Autosomal dominant polycystic kidney disease (ADPKD) is estimated to affect 1/600-1/1000 individuals worldwide. The disease is characterized by age dependent renal cyst formation that results in kidney failure during adulthood. Although ultrasound imaging may be an adequate diagnostic tool in at risk individuals older than 30, this modality may not be sufficiently sensitive in younger individuals or for those from PKD2 families who have milder disease. DNA based assays may be indicated in certain clinical situations where imaging cannot provide a definitive clinical diagnosis. The goal of this study was to evaluate the utility of direct DNA analysis in a test sample of 82 individuals who were judged to have polycystic kidney disease by standard clinical criteria. The samples were analyzed using a commercially available assay that employs sequencing of both genes responsible for the disorder. Definite disease causing mutations were identified in 34 (approximately 42%) study participants. An additional 30 (approximately 37%) subjects had either in frame insertions/deletions, non-canonical splice site alterations or a combination of missense changes that were also judged likely to be pathogenic. We noted striking sequence variability in the PKD1 gene, with a mean of 13.1 variants per participant (range 0-60). Our results and analysis highlight the complexity of assessing the pathogenicity of missense variants particularly when individuals have multiple amino acid substitutions. We conclude that a significant fraction of ADPKD mutations are caused by amino acid substitutions that need to be interpreted carefully when utilized in clinical decision-making.

A comparative study of three kidney biomarker tests in autosomal-dominant polycystic kidney disease.

BACKGROUND: The relationship between the progress of tubular damage and renal insufficiency in autosomal-dominant polycystic kidney disease (ADPKD) is a subject of doubtless interest, and is the object of this present work. METHODS: A total of 92 adult ADPKD patients of both genders were studied, none of which presented end-stage renal disease (ESRD), and classified according to an ultrasound score based on kidney size and number of cysts. Urinary albumin and beta-N-acetylhexosaminidase (Hex) and its isoenzymes were determined, together with serum glutathione peroxidase, cystatin C, creatinine, and urea. RESULTS: A frequent elevation of the urinary Hex was found and an alteration of its isoenzymatic profile, with 31% of the normotensive patients with normoalbuminuria already presenting an increased proportion of Hex B isoenzyme. Keeping age constant, a partial significant correlation was found between the ultrasound score and the proportion of Hex B (r = 0.352, P < 0.05), but not with albuminuria or cystatin C. In 42 patients the different biochemical variables were again determined after 1 year, finding that in the 13 normotensive patients with normoalbuminuria there had been a significant decrease in the concentration of cystatin C (P < 0.05), and a significant increase in the urinary excretion of albumin and Hex B isoenzyme (P < 0.05). By the other hand, in the other 29 patients with micro- or macroalbuminuria and hypertension, no significant differences were found. CONCLUSION: The results point toward an important participation of tubular damage in the pathogenesis of this disease. It may also be suggested that in normotensive and normoalbuminuric ADPKD patients, a gradual increase of glomerular filtration would be produced. After the start of hypertension and microalbuminuria, the glomerular filtration rate (GFR) would decrease progressively, although more slowly.

A novel pattern of mutation in uromodulin disorders: autosomal dominant medullary cystic kidney disease type 2, familial juvenile hyperuricemic nephropathy, and autosomal dominant glomerulocystic kidney disease.

BACKGROUND: Autosomal dominant medullary cystic kidney disease type 2 (MCKD2), familial juvenile hyperuricemic nephropathy (FJHN), and autosomal dominant glomerulocystic kidney disease (GCKD) constitute a hereditary renal disease group that may lead to end-stage renal failure caused by mutations of the UMOD gene and its product, uromodulin or Tamm-Horsfall protein. Of 34 different UMOD mutations described to date, 28 were located in exon 4. Based on such mutation clustering, some investigators have proposed that the sequencing of UMOD exon 4 might become a preliminary diagnostic test for patients with this phenotype. METHODS: We performed linkage analysis and sequencing of the entire codifying region of the UMOD gene in 4 Spanish families with MCKD/FJHN/GCKD. RESULTS: All families were shown to present mutations in the UMOD gene. In 3 families, the detected mutations were located in exon 5. Although 1 novel mutation (Gln316Pro) was observed in 2 of these families, a previously reported mutation (Cys300Gly) was found in the other kindred. The Cys300Gly mutation was found in the family presenting with a GCKD phenotype. CONCLUSION: Our data show a novel mutation pattern in UMOD , suggesting that exon 5 mutations can be more frequent in some populations. Our results support that every exon of the UMOD gene must be included in molecular testing and provide additional evidence for the existence of a fourth calcium-binding epidermal growth factor-like domain in the structure of Tamm-Horsfall protein. A second family reported to date is described, confirming that the GCKD phenotype may be caused by a UMOD mutation.

Comparison between siblings and twins supports a role for modifier genes in ADPKD.

BACKGROUND: Autosomal-dominant polycystic kidney disease (ADPKD) is characterized by intrafamilial variability in renal disease progression, which could result from a combination of environmental and genetic factors. Although a role for modifier genes has been evidenced in mouse models, direct evidence in ADPKD patients is lacking. The analysis of variability in affected siblings and monozygotic (MZ) twins would help evaluate the relative contribution of environment and genetic factors on renal disease progression in ADPKD. METHODS: The difference in the age at end-stage renal disease (ESRD) and the intraclass correlation coefficient (ICC) were quantified in a large series of ADPKD siblings from western Europe and compared with the values obtained in a series of MZ ADPKD twins from the same geographic area. RESULTS: Fifty-six sibships (including 129 patients) and nine pairs of MZ twins were included. The difference in the age at ESRD was significantly higher in siblings (6.9 +/- 6.0 years, range 2 months to 23 years) than in MZ twins (2.1 +/- 1.9 years, range 1 month to 6 years; P = 0.02). Furthermore, the intraclass correlation coefficient was significantly lower in siblings than in MZ twins (0.49 vs. 0.92, respectively; P = 0.003). The intrafamilial difference in the age at ESRD was not influenced by gender. CONCLUSION: These data substantiate the existence of a large intrafamilial variability in renal disease progression in ADPKD siblings. The fact that the variability in siblings is in a significant excess of that found in MZ twins strongly suggests that modifier genes account for a significant part of this variability.

Homozygosity for uromodulin disorders: FJHN and MCKD-type 2.

BACKGROUND: Autosomal-dominant medullary cystic kidney disease type 2 (MCKD2) and familial juvenile hyperuricemic nephropathy (FJHN) are heritable renal diseases with autosomal-dominant transmission and shared features, including polyuria, progressive renal failure, and abnormal urate handling, which leads to hyperuricemia and gout. Mutations of the UMOD gene, disrupting the tertiary structure of uromodulin, cause MCKD2 and FJHN. METHODS: Haplotype analysis of a large Spanish family with MCKD was carried out to determinate genetic linkage to MCKD2 locus. Mutation detection was performed by direct sequencing of the UMOD gene. The level of Tamm-Horsfall protein in the urine was measured by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot analysis. RESULTS: Linkage to MCKD2 locus was demonstrated (LOD score: 4.13), and a known pathogenic uromodulin mutation was found in exon 4, corresponding to Cys255Tyr, disrupting the light chain binding domain of the protein. In this consanguineous family there were three patients homozygous for the C255Y mutation, and multiple heterozygous cases, allowing the MCKD phenotypes associated with one or two mutant alleles to be compared. The homozygous individuals survived to adulthood, although presenting an earlier onset of hyperuricemia and faster progression to end-stage renal disease than heterozygous individuals. Western analysis revealed lower levels of urine THP in one heterozygous patient compared with a normal control patient, both with normal renal function. CONCLUSION: The study shows that individuals with two UMOD mutations are viable, but they do have more severe disease on average than heterozygotes. This family sheds light on the possible disease mechanism in this disorder.