No abstract

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.

Mutation screening of the major autosomal dominant polycystic kidney disease (ADPKD) locus, PKD1, has proved difficult because of the large transcript and complex reiterated gene region. We have developed methods, employing long polymerase chain reaction (PCR) and specific reverse transcription-PCR, to amplify all of the PKD1 coding area. The gene was screened for mutations in 131 unrelated patients with ADPKD, using the protein-truncation test and direct sequencing. Mutations were identified in 57 families, and, including 24 previously characterized changes from this cohort, a detection rate of 52.3% was achieved in 155 families. Mutations were found in all areas of the gene, from exons 1 to 46, with no clear hotspot identified. There was no significant difference in mutation frequency between the single-copy and duplicated areas, but mutations were more than twice as frequent in the 3' half of the gene, compared with the 5' half. The majority of changes were predicted to truncate the protein through nonsense mutations (32%), insertions or deletions (29.6%), or splicing changes (6.2%), although the figures were biased by the methods employed, and, in sequenced areas, approximately 50% of all mutations were missense or in-frame. Studies elsewhere have suggested that gene conversion may be a significant cause of mutation at PKD1, but only 3 of 69 different mutations matched PKD1-like HG sequence. A relatively high rate of new PKD1 mutation was calculated, 1.8x10-5 mutations per generation, consistent with the many different mutations identified (69 in 81 pedigrees) and suggesting significant selection against mutant alleles. The mutation detection rate, in this study, of>50% is comparable to that achieved for other large multiexon genes and shows the feasibility of genetic diagnosis in this disorder.

Mutation 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 firstpass mutation screen in this disorder.