Medullary cystic kidney disease type 1: mutational analysis in 37 genes based on haplotype sharing.

Medullary cystic kidney disease type 1 (MCKD1) is an autosomal dominant, tubulo-interstitial nephropathy that causes renal salt wasting and end-stage renal failure in the fourth to seventh decade of life. MCKD1 was localized to chromosome 1q21. We demonstrated haplotype sharing and confirmed the telomeric border by a recombination of D1S2624 in a Belgian kindred. Since the causative gene has been elusive, high resolution haplotype analysis was performed in 16 kindreds. Clinical data and blood samples of 257 individuals (including 75 affected individuals) from 26 different kindreds were collected. Within the defined critical region mutational analysis of 37 genes (374 exons) in 23 MCKD1 patients was performed. In addition, for nine kindreds RT-PCR analysis for the sequenced genes was done to screen for mutations activating cryptic splice sites. We found consistency with the haplotype sharing hypothesis in an additional nine kindreds, detecting three different haplotype subsets shared within a region of 1.19 Mb. Mutational analysis of all 37 positional candidate genes revealed sequence variations in 3 different genes, AK000210, CCT3, and SCAMP3, that were segregating in each affected kindred and were not found in 96 healthy individuals, indicating, that a single responsible gene causing MCKD1 remains elusive. This may point to involvement of different genes within the MCKD1 critical region.

Mapping a new suggestive gene locus for autosomal dominant nephrolithiasis to chromosome 9q33.2-q34.2 by total genome search for linkage.

BACKGROUND: Nephrolithiasis is a complex, multifactorial disease resulting from genetic and environmental interaction. The pathogenesis of nephrolithiasis is far from being understood. So far, no gene locus for autosomal dominant nephrolithiasis only has been described. We here identified a new suggestive gene locus for autosomal dominant nephrolithiasis by a genome-wide search for linkage in a Spanish kindred with nephrolithiasis. METHODS: Clinical data, blood and urine samples of 18 individuals from a Spanish kindred with nephrolithiasis were collected. We performed a genome-wide search for linkage using 380 polymorphic microsatellite markers. RESULTS: Nephrolithiasis segregated in this Spanish kindred in a pattern compatible with autosomal dominant inheritance. The total genome search yielded the highest two-point LOD score of Z(max) = 1.99 (theta = 0) for marker D9S159 on chromosome 9q33.2-q34.2. Multipoint analysis of 24 polymorphic markers used for further fine mapping resulted in a LOD score of Z(max) = 2.7 (theta = 0) for markers D9S1881-D9S164, thereby identifying a new gene locus for autosomal dominant nephrolithiasis (NPL1). Two recombination events define D9S1850 as the centromeric flanking marker and D9S1818 as the telomeric flanking marker, restricting the NPL1 locus to a 14 Mb interval. CONCLUSION: We here identified a new suggestive gene locus (NPL1) for autosomal dominant nephrolithiasis. It is localized on chromosome 9q33.2-q34.2. The identification of the responsible gene will provide new insights into the molecular basis of nephrolithiasis.

Medullary cystic kidney disease type 1 in a large Native-American kindred.

BACKGROUND: Autosomal dominant medullary cystic kidney disease type 1 (MCKD1; Mendelian Inheritance in Man 174000) is a hereditary tubulointerstitial renal disease. For MCKD1, a locus on chromosome 1q21 is published. Although there are characteristic biopsy and imaging findings for MCKD, clinical diagnosis of this disorder is still very difficult because unique phenotypic features are not always present in individual cases. METHODS: In a large Native-American kindred with apparent autosomal dominant nephropathy, clinical findings in more than 50 individuals were collected and evaluated. Haplotype analysis for 34 individuals was performed. RESULTS: We report the difficulties establishing the diagnosis of MCKD in a large Native-American kindred solely by means of clinical criteria. This kindred shows a wider range of age of disease onset than previously reported. Gout and hypertension were common, but no patient reported symptoms of salt wasting. By means of haplotype analysis linkage was shown to the MCKD1 locus (logarithm of the odds score, 3.34). CONCLUSION: Establishing a diagnosis of MCKD solely on clinical findings is difficult because signs and symptoms may be subtle, renal cysts may be absent in more than 50% of affected individuals, and renal histological abnormalities are nonspecific. In patients presenting with renal insufficiency from apparent interstitial disease, a thorough family history and genetic linkage studies are required to establish a diagnosis of MCKD. We suspect MCKD is underdiagnosed and the true incidence of MCKD1 in the general population may be underestimated. No further living-related transplantation should be performed until genetic testing can exclude potentially affected donors.

Prevalence of WT1 mutations in a large cohort of patients with steroid-resistant and steroid-sensitive nephrotic syndrome.

BACKGROUND: Nephrotic syndrome (NS) represents the association of proteinuria, hypoalbuminemia, edema, and hyperlipidemia. Steroid-resistant nephrotic syndrome (SRNS) is defined by primary resistance to standard steroid therapy. It remains one of the most intractable causes for end-stage renal disease (ESRD) in the first two decades of life. Sporadic mutations in the Wilms' tumor suppressor gene WT1 have been found to be present in patients with SRNS in association with Wilms' tumor (WT) and urinary or genital malformations, as well as in patients with isolated SRNS. METHODS: To further evaluate the incidence of WT1 mutations in patients with NS we performed mutational analysis in 115 sporadic cases of SRNS and in 110 sporadic cases of steroid-sensitive nephrotic syndrome (SSNS) as a control group. Sixty out of 115 (52%) patients with sporadic SRNS were male, 55/115 (48%) were female. Sex genotype was verified by haplotype analysis. Mutational analysis was performed by direct sequencing and by denaturing high-performance liquid chromatography (DHPLC). RESULTS: Mutations in WT1 were found in 3/60 (5%) male (sex genotype) cases and 5/55 (9%) female (sex genotype) cases of sporadic SRNS, and 0/110 (0%) sporadic cases of SSNS. One out of five female patients with mutations in WT1 developed a WT, 2/3 male patients presented with the association of urinary and genital malformations, 1/3 male patients presented with sexual reversal (female phenotype) and bilateral gonadoblastoma, and 4/5 female patients presented with isolated SRNS. CONCLUSION: According to the data acquired in this study, patients presenting with a female phenotype and SRNS and male patients presenting with genital abnormalities should especially be screened to take advantage of the important genetic information on potential Wilms' tumor risk and differential therapy. This will also help to provide more data on the phenotype/genotype correlation in this patient population.

Telomeric refinement of the MCKD1 locus on chromosome 1q21.

BACKGROUND: Autosomal-dominant medullary cystic kidney disease type 1 (MCKD1) is a tubulointerstitial nephropathy that causes renal salt wasting and end-stage renal failure in the sixth decade of life. The chromosomal locus for MCKD1 was localized to chromosome 1q21 in a Cyprotic kindred. In this report we describe further refinement of the critical genetic region by a recombination in a Belgian kindred. METHODS: Clinical data and blood samples of 33 individuals from a large Belgian kindred were collected and high-resolution haplotype analysis was performed. RESULTS: In the Belgian kindred linkage to the MCKD1 locus on chromosome 1q21 was found with a logarithm of odds (LOD) score significant for linkage. A recombination in individual III:7 for marker D1S2624 refines the critical genetic region to 2.1 Mb. In this kindred a wide variety of clinical symptoms and age of onset of renal failure was detected. CONCLUSION: We confirm the MCKD1 locus on chromosome 1q21 and show further refinement of the MCKD1 locus to 2.1 Mb. This allowed us to exclude another 17 genes as positional candidate genes.

Patients with mutations in NPHS2 (podocin) do not respond to standard steroid treatment of nephrotic syndrome.

Nephrotic syndrome (NS) represents the association of proteinuria, hypoalbuminemia, edema, and hyperlipidemia. Steroid-resistant NS (SRNS) is defined by primary resistance to standard steroid therapy. It remains one of the most intractable causes of ESRD in the first two decades of life. Mutations in the NPHS2 gene represent a frequent cause of SRNS, occurring in approximately 20 to 30% of sporadic cases of SRNS. On the basis of a very small number of patients, it was suspected that children with homozygous or compound heterozygous mutations in NPHS2 might exhibit primary steroid resistance and a decreased risk of FSGS recurrence after kidney transplantation. To test this hypothesis, NPHS2 mutational analysis was performed with direct sequencing for 190 patients with SRNS from 165 different families and, as a control sample, 124 patients with steroid-sensitive NS from 120 families. Homozygous or compound heterozygous mutations in NPHS2 were detected for 43 of 165 SRNS families (26%). Conversely, no homozygous or compound heterozygous mutations in NPHS2 were observed for the 120 steroid-sensitive NS families. Recurrence of FSGS in a renal transplant was noted for seven of 20 patients with SRNS (35%) without NPHS2 mutations, whereas it occurred for only two of 24 patients with SRNS (8%) with homozygous or compound heterozygous mutations in NPHS2. None of 29 patients with homozygous or compound heterozygous mutations in NPHS2 who were treated with cyclosporine A or cyclophosphamide demonstrated complete remission of NS. It was concluded that patients with SRNS with homozygous or compound heterozygous mutations in NPHS2 do not respond to standard steroid treatment and have a reduced risk for recurrence of FSGS in a renal transplant. Because these findings might affect the treatment plan for childhood SRNS, it might be advisable to perform mutational analysis of NPHS2, if the patient consents, in parallel with the start of the first course of standard steroid therapy.

Mutations of the Uromodulin gene in MCKD type 2 patients cluster in exon 4, which encodes three EGF-like domains.

BACKGROUND: Autosomal-dominant medullary cystic kidney disease type 2 (MCKD2) is a tubulointerstitial nephropathy that causes renal salt wasting, hyperuricemia, gout, and end-stage renal failure in the fifth decade of life. The chromosomal locus for MCKD2 was localized on chromosome 16p12. Within this chromosomal region, Uromodulin (UMOD) was located as a candidate gene. UMOD encodes the Tamm-Horsfall protein. By sequence analysis, one group formerly excluded UMOD as the disease-causing gene. In contrast, recently, another group described mutations in the UMOD gene as responsible for MCKD2 and familial juvenile hyperuricemic nephropathy (FJHN). METHODS: Haplotype analysis for linkage to MCKD2 was performed in 25 MCKD families. In the kindreds showing linkage to the MCKD2 locus on chromosome 16p12, mutational analysis of the UMOD gene was performed by exon polymerase chain reaction (PCR) and direct sequencing. RESULTS: In 19 families, haplotype analysis was compatible with linkage to the MCKD2 locus. All these kindreds were examined for mutations in the UMOD gene. In three different families, three novel heterozygous mutations in the UMOD gene were found and segregated with the phenotype in affected individuals. Mutations were found only in exon 4. CONCLUSION: We confirm the UMOD gene as the disease-causing gene for MCKD2. All three novel mutations were found in the fourth exon of UMOD, in which all mutations except one (this is located in the neighboring exon 5) published so far are located. These data point to a specific role of exon 4 encoded sequence of UMOD in the generation of the MCKD2 renal phenotype.