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textabstractAutosomal dominant polycystic kidney disease (ADPKD) is a common genetic disorder that frequently results in renal fallure due to progressive cyst development. The major locus, PKD1, maps to 16p13.3. We identified a chromosome translocation associated with ADPKD that disrupts a gene (PBP) encoding a 14 kb transcript in the PKD1 candidate region. Further mutations of the PBP gene were found in PKD1 patients, two deletions (one a de novo event) and a splicing defect, confirming that PBP is the PKD1 gene. This gene is located adjacent to the TSC2 locus in a genomic region that is reiterated more proximally on 16p. The duplicate area encodes three transcripts substantially homologous to the PKD1 transcript. Partial sequence analysis of the PKD1 transcript shows that it encodes a novel protein whose function is at present unknown.

SummaryMutation screening of the major autosomal dominant polycystic kidney disease gene (PKD1) has been complicated by the large transcript size (>14 kb) and by reiteration of the genomic area encoding 75% of the protein on the same chromosome (the HG loci). The sequence similarity between the PKD1 and HG regions has precluded specific analysis of the duplicated region of PKD1, and consequently all previously described mutations map to the unique 3′ region of PKD1. We have now developed a novel anchored reverse-transcription-PCR (RT-PCR) approach to specifically amplify duplicated regions of PKD1, employing one primer situated within the single-copy region and one within the reiterated area. This strategy has been incorporated in a mutation screen of 100 patients for more than half of the PKD1 exons (exons 22-46; 37% of the coding region), including 11 (exons 22-32) within the duplicated gene region, by use of the protein-truncation test (PTT). Sixty of these patients also were screened for missense changes, by use of the nonisotopic RNase cleavage assay (NIRCA), in exons 23-36. Eleven mutations have been identified, six within the duplicated region, and these consist of three stop mutations, three frameshifting deletions of a single nucleotide, two splicing defects, and three possible missense changes. Each mutation was detected in just one family (although one has been described elsewhere); no mutation hot spot was identified. The nature and distribution of mutations, plus the lack of a clear phenotype/genotype correlation, suggest that they may inactivate the molecule. RT-PCR/PTT proved to be a rapid and efficient method to detect PKD1 mutations (differentiating pathogenic changes from polymorphisms), and we recommend this procedure as a first-pass mutation screen in this disorder.

Autosomal dominant polycystic kidney disease (ADPKD) is a common genetic disease which frequently results in renal failure. The major ADPKD gene, polycystic kidney disease 1 (PKD1), has recently been identified. In an attempt to understand better the aetiology of this disorder we have searched for mutations in the PKD1 gene. Analysis of three regions in the 3' part of the gene has revealed two mutations that occur by a novel mechanism. Both mutations are deletions (of 18 or 20 bp) within the same 75 bp intron and although these deletions do not disrupt the splice donor or acceptor sites at the boundary of the intron, they nevertheless result in aberrant splicing. Two different transcripts are produced in each case; one includes the deleted intron while the other has a 66 bp deletion due to activation of a cryptic 5' splice site. No normal product is generated from the deleted gene. Aberrant splicing probably occurs because the deleted intron is too small for spliceosome assembly using the authentic splice sites; this mechanism has previously only been described from in vitro studies of vertebrate genes. A 9 bp direct repeat has been identified within the intron, which probably facilitated deletion by promoting misalignment of sequence. The possible phenotypic implications of producing more than one aberrant PKD1 transcript in these cases are discussed.

Autosomal dominant polycystic kidney disease (ADPKD) is the most common single gene disorder resulting in renal failure. It is generally an adult onset disease, but rarely, cases of severe childhood polycystic disease arise in ADPKD families. The clear clinical anticipation in these pedigrees has led to the suggestion that the mutation may be an unstable trinucleotide repeat. We have now identified a nonsense mutation, Tyr3818Stop, in one such family (P117) within the major ADPKD gene, polycystic kidney disease 1 (PKD1). The mutation is shown to be a de novo change in the father, and of grandpaternal origin. PKD1 manifests as typical adult onset disease in the father, but is seen as severe disease, detected as enlarged polycystic kidneys in utero, in one of a pair of dizygotic twins; the other twin has the mutation but no evidence of cysts, consistent with an adult onset disease course. The finding of the same stable mutation associated with very different disease severity in this family indicates that phenotypic variation in PKD1 is not due to a dynamic mutation. It seems most likely that a small number of modifying factors may radically affect the course of disease in PKD1; identification of such factors will have important prognostic implications in this disorder.

Characterization of the polycystic kidney disease 1 (PKD1) gene has been complicated by genomic rearrangements on chromosome 16. We have used an exon linking strategy, taking RNA from a cell line containing PKD1 but not the duplicate loci, to clone a cDNA contig of the entire transcript. The transcript consists of 14,148 bp (including a correction to the previously described C terminus), distributed among 46 exons spanning 52 kb. The predicted PKD1 protein, polycystin, is a glycoprotein with multiple transmembrane domains and a cytoplasmic C-tail. The N–terminal extracellular region of over 2,500 aa contains leucine–rich repeats, a C–type lectin, 16 immunoglobulin–like repeats and four type III fibronectin–related domains. Our results indicate that polycystin is an integral membrane protein involved in cell–cell/matrix interactions.

Major genes which cause tuberous sclerosis (TSC) and autosomal dominant polycystic kidney disease (ADPKD), known as TSC2 and PKD1 respectively, lie immediately adjacent to each other on chromosome 16p. Renal cysts are often found in TSC, but a specific renal phenotype, distinguished by the severity and infantile presentation of the cystic changes, is seen in a small proportion of cases. We have identified large deletions disrupting TSC2 and PKD1 in each of six such cases studied. Analysis of the deletions indicates that they inactivate PKD1, in contrast to the mutations reported in ADPKD patients, where in each case abnormal transcripts have been detected.