Phenotype and genotype heterogeneity in autosomal dominant polycystic kidney disease

Aberrant splicing in the PKD2 gene as a cause of polycystic kidney disease

It is estimated that approximately 15% of families with autosomal dominant polycystic kidney disease (ADPKD) have mutations in PKD2. Identification of these mutations is central to identifying functionally important regions of gene and to understanding the mechanisms underlying the pathogenesis of the disorder. The current study describes mutations in six type 2 ADPKD families. Two single base substitution mutations discovered in the ORF in exon 14 constitute the most COOH-terminal pathogenic variants described to date. One of these mutations is a nonsense change and the other encodes an apparent missense variant. Reverse transcription-PCR from patient lymphoblast RNA showed that, in addition, both mutations resulted in out-of-frame splice variants by activating cryptic splice sites via different mechanisms. The apparent missense variant produced such a strong splicing signal that the processed transcript from the mutant chromosome did not contain any of the normally spliced, missense product. A third mutation, a nonconservative missense change effecting a negatively charged residue in the third transmembrane span, is likely pathogenic and defines a highly conserved residue consistent with a potential channel subunit function for polycystin-2. The remaining three mutations included two frame shifts resulting from deletion of one or two bases in exons 6 and 10, respectively, and a nonsense mutation due to a single base substitution in exon 4. The study also defined a novel intragenic polymorphism in exon 1 that will be useful in analyzing "second hits" in PKD2. Finally, the study demonstrates that there are reduced levels of normal polycystin-2 protein in lymphoblast lines from PKD2-affected individuals and that truncated mutant polycystin-2 cannot be detected in patient lymphoblasts, suggesting that the latter may be unstable in at least some tissues. The mutations described will serve as critical reagents for future functional studies in PKD2.

Autosomal dominant polycystic kidney disease types 1 and 2: assessment of US sensitivity for diagnosis

PURPOSE

To estimate the sensitivity and specificity of ultrasonography (US) in the diagnosis of autosomal dominant polycystic kidney disease (ADPKD) types 1 and 2, as compared with those of genetic linkage analysis.

MATERIALS AND METHODS

A renal US and DNA analysis for ADPKD was performed in 319 patients who were at risk, 161 of whom were younger than 30 years, from 54 families with ADPKD. The sensitivity of US for diagnosis was estimated by comparing the US results with genotypes inferred from linkage studies.

RESULTS

The sensitivity of US in individuals younger than 30 years who were at risk was 95% for ADPKD type 1 but only 67% for ADPKD type 2. The sensitivity of US for either ADPKD type 1 or ADPKD type 2 in individuals aged 30 years or older who were at risk was 100%. The overall sensitivity in individuals younger than 30 years was 93%. For both ADPKD types 1 and 2 in all patients, US demonstrated a sensitivity of 97%, a specificity of 100%, and an accuracy of 98%.

CONCLUSION

US is the first-line imaging technique that should be used in the diagnosis of ADPKD. The sensitivity in individuals aged 30 years or older is 100%, but if there is a clinical suspicion of ADPKD type 2 in individuals younger than 30 years, linkage analysis should also be considered.

A loss-of-function model for cystogenesis in human autosomal dominant polycystic kidney disease type 2

Autosomal dominant polycystic kidney disease (ADPKD) is genetically heterogeneous, with at least three chromosomal loci (PKD1, PKD2, and PKD3) that account for the disease. Mutations in the PKD2 gene, on the long arm of chromosome 4, are expected to be responsible for approximately 15% of cases of ADPKD. Although ADPKD is a systemic disease, it shows a focal expression, because <1% of nephrons become cystic. A feasible explanation for the focal nature of events in PKD1, proposed on the basis of the two-hit theory, suggests that cystogenesis results from the inactivation of the normal copy of the PKD1 gene by a second somatic mutation. The aim of this study is to demonstrate that somatic mutations are present in renal cysts from a PKD2 kidney. We have studied 30 renal cysts from a patient with PKD2 in which the germline mutation was shown to be a deletion that encompassed most of the disease gene. Loss-of-heterozygosity (LOH) studies showed loss of the wild-type allele in 10% of cysts. Screening of six exons of the gene by SSCP detected eight different somatic mutations, all of them expected to produce truncated proteins. Overall, >/=37% of the cysts studied presented somatic mutations. No LOH for the PKD1 gene or locus D3S1478 were observed in those cysts, which demonstrates that somatic alterations are specific. We have identified second-hit mutations in human PKD2 cysts, which suggests that this mechanism could be a crucial event in the development of cystogenesis in human ADPKD-type 2.

Familial phenotype differences in PKD11

Familial phenotype differences in PKD1.

BACKGROUND

Mutations within the PKD1 gene are responsible for the most common and most severe form of autosomal dominant polycystic kidney disease (ADPKD). Although it is known that there is a wide range of disease severity within PKD1 families, it is uncertain whether differences in clinical severity also occur among PKD1 families.

METHODS

Ten large South Wales ADPKD families with at least 12 affected members were included in the study. From affected members, clinical information was obtained, including survival data and the presence of ADPKD-associated complications. Family members who were at risk of having inherited ADPKD but were proven to be non-affected were included as controls. Linkage and haplotype analysis were performed with highly polymorphic microsatellite markers closely linked to the PKD1 gene. Survival data were analyzed by the Kaplan-Meier method and the log rank test. Logistic regression analysis was used to test for differences in complication rates between families.

RESULTS

Haplotype analysis revealed that each family had PKD1-linked disease with a unique disease-associated haplotype. Interfamily differences were observed in overall survival (P = 0.0004), renal survival (P = 0.0001), hypertension prevalence (P = 0.013), and hernia (P = 0.048). Individuals with hypertension had significantly worse overall (P = 0.0085) and renal (P = 0.03) survival compared with those without hypertension. No statistically significant differences in the prevalence of hypertension and hernia were observed among controls.

CONCLUSION

We conclude that phenotype differences exist between PKD1 families, which, on the basis of having unique disease-associated haplotypes, are likely to be associated with a heterogeneous range of underlying PKD1 mutations.

Seven novel mutations of the PKD2 gene in families with autosomal dominant polycystic kidney disease

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is genetically heterogeneous, with at least three chromosomal loci accounting for the disease. Mutations in the PKD2 gene on the long arm of chromosome 4 are expected to be responsible for approximately 15% of cases of ADPKD.

METHODS

We report a systematic screening for mutations covering the 15 exons of the PKD2 gene in eight unrelated families with ADPKD type 2, using the heteroduplex technique.

RESULTS

Seven novel mutations were identified and characterized that, together with the previously described changes, amount to a detection rate of 85% in the population studied. The newly described mutations are two nonsense mutations, a 1 bp deletion, a 1 bp insertion, a mutation that involves both a substitution and a deletion (2511AG-->C), a complex mutation in exon 6 consisting of a simultaneous 7 bp inversion and a 4 bp deletion, and the last one is a G-->C transversion that may be a missense mutation. Most of these mutations are expected to lead to the formation of shorter truncated proteins lacking the carboxyl terminus of PKD2. We have also characterized a frequent polymorphism, Arg-Pro, at codon 28 in this gene. The clinical features of these PKD2 patients are similar to the previously described, with the mean age of end-stage renal disease being 75.5 years (SE +/- 3.8 years).

CONCLUSIONS

Our results confirm that many different mutations are likely to be responsible for the disease and that most pathogenic defects probably are point or small changes in the coding region of the gene.

Polycystin-2 expression is developmentally regulated

PKD2 encodes a protein of unknown function that is mutated in 15% of autosomal dominant polycystic kidney disease (ADPKD) families. We used polyclonal antisera against PKD2 to examine the pattern of Pkd2 expression in staged mouse embryos. Staining for Pkd2 was documented as early as the 6th embryonic day (day E6) in the embryonic ectoderm and endoderm. Low-intensity staining is seen in metanephric ureteric bud at day E12.5. By day E15.5, the adult pattern of expression is established with low level staining in proximal tubules and high level, basolateral staining in distal tubules. Pkd2 expression is first detected in the medullary collecting ducts at postnatal day 14. Outside of the kidney, Pkd2 expression is widely distributed in utero and more restricted postnatally. The greatest intensity of staining is seen in the fetal but not adult adrenal cortex and in red blood cell precursors. Expression also is seen in multiple endocrine organs, in cardiac, skeletal, and smooth muscle, and in multiple mesenchymal tissues. The diffuse distribution and early expression of Pkd2 suggest a fundamental developmental role. The persistent strong expression in adult kidney is consistent with a more organ-specific function in the maintenance of the mature metanephric tubule.

Coordinate expression of the autosomal dominant polycystic kidney disease proteins, polycystin-2 and polycystin-1, in normal and cystic tissue

A second gene for autosomal dominant polycystic kidney disease (ADPKD), PKD2, has been recently identified. Using antisera raised to the human PKD2 protein, polycystin-2, we describe for the first time its distribution in human fetal tissues, as well as its expression in adult kidney and polycystic PKD2 tissues. Its expression pattern is correlated with that of the PKD1 protein, polycystin-1. In normal kidney, expression of polycystin-2 strikingly parallels that of polycystin-1, with prominent expression by maturing proximal and distal tubules during development, but with a more pronounced distal pattern in adult life. In nonrenal tissues expression of both polycystin molecules is identical and especially notable in the developing epithelial structures of the pancreas, liver, lung, bowel, brain, reproductive organs, placenta, and thymus. Of interest, nonepithelial cell types such as vascular smooth muscle, skeletal muscle, myocardial cells, and neurons also express both proteins. In PKD2 cystic kidney and liver, we find polycystin-2 expression in the majority of cysts, although a significant minority are negative, a pattern mirrored by the PKD1 protein. The continued expression of polycystin-2 in PKD2 cysts is similar to that seen by polycystin-1 in PKD1 cysts, but contrasts with the reported absence of polycystin-2 expression in the renal cysts of Pkd2+/- mice. These results suggest that if a two-hit mechanism is required for cyst formation in PKD2 there is a high rate of somatic missense mutation. The coordinate presence or loss of both polycystin molecules in the same cysts supports previous experimental evidence that heterotypic interactions may stabilize these proteins.

Mutational analysis within the 3' region of the PKD1 gene

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common genetic diseases in humans, affecting 1 out of 1000 individuals. At least three different genes are involved in this disease. The search for mutations in PKD1 is complicated because most of the transcript is encoded by a genomic region reiterated more proximally on chromosome 16, and no prevalent mutation has been reported.

METHODS

We have screened DNA from exon 43 through exon 46 and intron 40 of the PKD1 sequence by single-stranded conformational polymorphism (SSCP) analysis in 175 ADPKD patients.

RESULTS

We have found 25 differences with respect to the reported PKD1 DNA sequence, seven of which are mutations (Q4041X, Q4124X, IVS44-1G-->C, IVS45-1G-->A, 12801del28, R4275W, and Q4224P). We found different phenotypical expressions of the same mutation in the families studied. We have detected several common polymorphisms, and some of them cosegregate, suggesting a common origin of these alleles in PKD1.

CONCLUSIONS

The detection of only seven mutations in 175 unrelated ADPKD patients for this region of the PKD1 analyzed suggests that mutations could be widespread throughout all of the gene and that a prevalent mutation is not expected to occur. The identified PKD1 missense mutations may help to refine critical regions of the protein. Until a quicker and more sensitive method for the detection of mutations becomes available, linkage studies will continue to be the basis for the molecular diagnosis of ADPKD families.

Comparison of phenotypes of polycystic kidney disease types 1 and 2. European PKD1-PKD2 Study Group

BACKGROUND

Although autosomal dominant polycystic kidney disease type 2 (PKD2) is known to have a milder clinical phenotype than PKD1, neither disorder has been compared with an unaffected control population in terms of survival. We report the findings of a multicentre survey that aimed to define more precisely the survival and clinical expression of PKD1 and PKD2.

METHODS

Clinical data from 333 people with PKD1 (31 families) were compared with data from 291 people with PKD2 (31 families) and 398 geographically matched controls. Survival analysis was used to compare age-at-event data. Differences in the prevalence of complications were assessed by logistic regression.

FINDINGS

Median age at death or onset of end-stage renal disease was 53.0 years (95% CI 51.2-54.8) in individuals with PKD1, 69.1 years (66.9-71.3) in those with PKD2, and 78.0 years (73.8-82.2) in controls. Women with PKD2 had a significantly longer median survival than men (71.0 [67.4-74.8] vs 67.3 [64.9-69.7] years), but no sex influence was apparent in PKD1. Age at presentation with kidney failure was later in PKD2 than in PKD1 (median age 74.0 [67.2-80.8] vs 54.3 [52.7-55.9] years). PKD2 patients were less likely to have hypertension (odds ratio 0.25 [95% CI 0.15-0.42]), a history of urinary-tract infection (0.50 [0.31-0.83]), or haematuria (0.59 [0.35-0.98]).

INTERPRETATION

Although PKD2 is clinically milder than PKD1, it has a deleterious impact on overall life expectancy and cannot be regarded as a benign disorder.

The use of ultrasonography and linkage studies for early diagnosis of autosomal dominant polycystic kidney disease (ADPKD)

To define possibly affected members of 69 families and to identify the factors influencing the progression of autosomal dominant polycystic kidney disease (ADPKD), 276 subjects at risk of having inherited the mutant gene underwent ultrasonographic scanning (US), using an ultrasound real-time scanner. At a mean age of 26 +/- 12 years (range 4-71), 85/276 individuals (31%) presented ultrasound evidence of the disease (at least two cysts in one kidney and one cyst in the other) (US: positive), while only 19/85 (22%) had one or more manifestations of ADPKD prior to diagnosis. The prevalence of the disease in subjects at risk aged < 30 years was 53/154 (34%), while hepatic cysts were also detected in 7/85 ADPKD probands (8%) (five females) at a mean age of 40 +/- 6 years (range 30-45) and their frequency correlated with the number of pregnancies. History was proved to be important in suspecting the disease since symptoms were more common in US positive as compared to negative subjects (22% vs 6%, p < 0.001). On the other hand, physical examination and routine laboratory data at presentation revealed abnormal signs mainly in US positive individuals aged 30-39 years. Forty ADPKD families met the criterion for genetic study (at least two members affected) but in three of them (7.5%), no linkage to DNA-markers for the short arm of chromosome 16 was detected ("unlinked" or ADPKD2). DNA-analysis in the rest 37 "linked" (ADPKD1) families identified the gene-carrier state in 18/123 (15%) US negative subjects at risk, at a mean age of 13 +/- 7 years (range 3-25). There were significantly more US positive subjects aged > or = 30 years in ADPKD2 as compared to ADPKD1 families (83% vs 35%, p < 0.05) suggesting that the progression of the disease is slower in the former families. During a 5-year follow-up, 6/18 gene-carriers (33%) had already developed distinct renal cysts on US, at a mean age of 20 +/- 9 years (range 8-29). On the contrary, none of the ADPKD1 non-carriers and the US negative ADPKD2 subjects had shown any ultrasound findings of cystic renal disease at that period.

Neurology and the kidney

Renal failure is relatively common, but except in association with spina bifida or paraplegia it is unlikely to occur as a result of disease of the CNS. Renal failure, however, commonly affects the nervous system. The effects of kidney failure on the nervous system are more pronounced when failure is acute. In addition to the important problems related to renal failure there are both acquired and genetically determined diseases which may affect the kidney and the brain. Those acquired diseases include the vasculitides, the paraproteinaemias, and various granulomatous conditions (considered in other chapters of Neurology and Medicine). In two of the most commonly encountered genetically determined diseases, Von Hippel-Lindau disease and polycystic kidney disease, location of pathogenic mutations will provide improved screening programmes and, possibly, allow therapeutic intervention. Uraemia may affect both the central and peripheral nervous systems. Whereas the clinical features of uraemia are well documented, the pathophysiology is less well understood and probably multifactorial. Uraemic encephalopathy, which classically fluctuates, is associated with problems in cognition and memory and may progress to delirium, convulsions, and coma. The encephalopathy may initially worsen with periods of dialysis and almost certainly relates to altered metabolic states in association with ionic changes and possibly impaired synaptic function. Renal failure may affect the peripheral nervous system, resulting in a neuropathy which shows a predilection for large diameter axons. This may be reversed by dialysis and transplantation. The myopathy seen in renal failure, often associated with bone pain and tenderness, is similar to that encountered in primary hyperparathyroidism and osteomalacia. Dialysis itself is associated with neurological syndromes including the dysequilibrium syndrome, subdural haematoma, and Wernicke's encephalopathy. Dialysis dementia, which was prevalent during the 1970s, has reduced in frequency with the use of aluminium free dialysate. With the introduction of transplantation and the concomitant use of powerful immunosuppressive drugs, the pattern of neurological problems encountered in renal replacement therapy has shifted. Five per cent of patients develop nerve injuries during renal transplantation, and up to 40% of patients experience neurological side effects from cyclosporine. Furthermore, CNS infections, often fungal in type, have been reported in up to 45% of transplant patients coming to postmortem. The nature of the involvement of neurologists with their nephrology colleagues is therefore evolving.

A novel frameshift mutation induced by an adenosine insertion in the polycystic kidney disease 2 (PKD2) gene

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common Mendelian disorders and is genetically heterogeneous. Linkage studies have shown that the majority (approximately 85%) of ADPKD cases are due to mutations in PKD1 on chromosome 16p13.3, while mutations in PKD2 on chromosome 4q21-q23 are thought to account for most of the remaining cases. In this report, we describe the mutation in a large four-generation ADPKD family (TOR-PKD77) which we had mapped to the PKD2 locus by linkage analysis. In this family, we screened for mutations by directly sequencing two nested RT-PCR fragments (PKD2N1 and PKD2N2) that cover approximately 90% of the PKD2 open reading frame. In the affected members, we identified a novel single adenosine insertion (2160InsA) in the PKD2N2 fragment. This mutation occurred in the polyadenosine tract (nt2152-2159) of exon 11 and is predicted to result in a frameshift with premature translation termination of the PKD2 product, polycystin 22, immediately after codon 723. The truncated polycystin 2 is predicted to lack the calcium-binding EF-hand domain and two cytoplasmic domains required for the homodimerization of polycystin 2 with itself and for the heterodimerization of polycystin 2 with polycystin 1.

Somatic inactivation of Pkd2 results in polycystic kidney disease

Germline mutations in PKD2 cause autosomal dominant polycystic kidney disease. We have introduced a mutant exon 1 in tandem with the wild-type exon 1 at the mouse Pkd2 locus. This is an unstable allele that undergoes somatic inactivation by intragenic homologous recombination to produce a true null allele. Mice heterozygous and homozygous for this mutation, as well as Pkd+/- mice, develop polycystic kidney and liver lesions that are indistinguishable from the human phenotype. In all cases, renal cysts arise from renal tubular cells that lose the capacity to produce Pkd2 protein. Somatic loss of Pkd2 expression is both necessary and sufficient for renal cyst formation in ADPKD, suggesting that PKD2 occurs by a cellular recessive mechanism.

A spectrum of mutations in the second gene for autosomal dominant polycystic kidney disease (PKD2)

Recently the second gene for autosomal dominant polycystic kidney disease (ADPKD), located on chromosome 4q21-q22, has been cloned and characterized. The gene encodes an integral membrane protein, polycystin-2, that shows amino acid similarity to the PKD1 gene product and to the family of voltage-activated calcium (and sodium) channels. We have systematically screened the gene for mutations by single-strand conformation-polymorphism analysis in 35 families with the second type of ADPKD and have identified 20 mutations. So far, most mutations found seem to be unique and occur throughout the gene, without any evidence of clustering. In addition to small deletions, insertions, and substitutions leading to premature translation stops, one amino acid substitution and five possible splice-site mutations have been found. These findings suggest that the first step toward cyst formation in PKD2 patients is the loss of one functional copy of polycystin-2.

Association of the angiotensin I converting enzyme gene deletion polymorphism with early onset of ESRF in PKD1 adult polycystic kidney disease

To determine the effect of the ACE gene insertion/deletion (I/D) polymorphism, angiotensinogen gene M235T polymorphism and the angiotensin 1 receptor gene A1166C polymorphism on the age of onset of end-stage renal failure (ESRF) in PKD1 adult autosomal-dominant polycystic kidney disease (ADPKD), 189 individuals from 46 families with PKD1 were genotyped for each polymorphism. Of the 189 patients 52 (28%) reached ESRF at an average age of 48 +/- 1 year. In patients genotyped for the ACE gene insertion/deletion polymorphism the frequencies of the DD, ID and II genotypes were similar to those expected from Hardy Weinberg equilibrium. In patients with ESRF there was an excess of patients homozygous for the deletion allele (DD: 48% chi2 = 9.97 (1df) P = 0.002). Cumulative renal survival was significantly reduced among those with DD genotype compared to ID and II genotypes. The estimated mean renal survival (95% confidence intervals) were: DD, 52 years [48, 57]; II, 59 years [54, 63]; ID, 64 years [56, 72]; chi2 = 6.13 (1df) P = 0.013, DD versus ID/II. The mean age of renal failure was significantly younger in the DD genotype compared to ID and II genotypes (DD, ID, and II: 44 +/- 2, 49 +/- 2 and 54 +/- 3 years, respectively; P < 0.05 DD vs. ID, P < 0.05 DD vs. II). Ten of the eleven patients who reached ESRF before the age of 40 were homozygous for the deletion allele. The relative risk for ESRF below the age 40 for DD genotype was 17. For all ages there was an overall increased risk of 1.4 for ESRF with the DD genotype. There was no interaction between age of onset of ESRF and either the angiotensinogen M235T allele or angiotensin 1 receptor A1166C polymorphism. This study strongly suggests that PKD 1 patients homozygous for the deletion allele of the ACE gene are at increased risk of developing ESRF at a early age.

The pathogenesis of hypertension in autosomal dominant polycystic kidney disease

BACKGROUND

Hypertension is a common and serious complication of autosomal dominant polycystic kidney disease (ADPKD), often occurring early in the disease before the renal function starts to decrease. The pathogenesis of this early hypertension is controversial.

OBJECTIVE

To review studies on the pathogenesis of early and late hypertension in ADPKD.

STUDY SELECTION

Studies on ADPKD and hypertension were retrieved from Medline from the last 20 years, with an emphasis on the last 10 years. These studies, together with selected published abstracts from recent hypertension and nephrology meetings, were reviewed critically.

RESULTS

Cyst growth, renal handling of sodium, activation of the renin-angiotensin-aldosterone system, volume expansion, an elevated plasma volume, and increased plasma atrial natriuretic peptide and plasma endothelin levels have all been found to be associated with hypertension in ADPKD. In some studies an inappropriate activity of the renin-angiotensin-aldosterone system that could be related to cyst growth and intrarenal ischemia was found. An increase in renal vascular resistance has been demonstrated and might be caused by intrarenal release of angiotensin II. Interestingly, the protective effect of angiotensin converting enzyme inhibitors on the renal function could not be demonstrated in ADPKD patients with a moderately decreased renal function. The importance, if any, of endothelial vasodilatory factors is not known. Sympathetic nervous activity seems to be increased in ADPKD, but the importance of this for the blood pressure level is not known.

CONCLUSION

The pathogenesis of hypertension in ADPKD is complex and likely to be dependent on the interaction of hemodynamic, endocrine and neurogenic factors.

Autosomal dominant polycystic kidney disease with anticipation and Caroli's disease associated with a PKD1 mutation. Rapid communication

Autosomal dominant polycystic kidney disease (ADPKD) is the most common renal hereditary disorder. Clinical expression of ADPKD shows interfamilial and intrafamilial variability. We screened for mutations the 3' region of the PKD1 gene, from exon 43 to exon 46, in a family showing anticipation and Caroli's disease and have found a 28 base pairs deletion in exon 46 (12801del28) and a new DNA variant in exon 43 (12184 C to G conserving Ala 3991) segregating with the disease. The mutation should result in a protein 44 amino acids longer then the wild-type PKD1. This PKD1 mutation manifests as typical adult-onset disease in the father, but in the proband, a 26-year-old man, ADPKD was diagnosed as a newborn and was associated with Caroli's disease at the age of 18 years. A renal biopsy performed in childhood disclosed a predominance of glomerular cysts. Mutation 12801del28 is the first molecular defect associated with Caroli's disease and the PKD1 phenotype. The finding of the same mutation in two different members of the same family with different expression of the disease indicates that the phenotypic variation in ADPKD must be due to modifying factors that may radically affect the course of the disease.

Identification of mutations in the duplicated region of the polycystic kidney disease 1 gene (PKD1) by a novel approach

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.

The molecular basis of focal cyst formation in human autosomal dominant polycystic kidney disease type I

Autosomal dominant polycystic kidney disease (ADPKD) is a common disease and an important cause of renal failure. It is characterized by considerable intrafamilial phenotypic variation and focal cyst formation. To elucidate the molecular basis for these observations, we have developed a novel method for isolating renal cystic epithelia from single cysts and have used it to show that individual renal cysts in ADPKD are monoclonal. Loss of heterozygosity was discovered within a subset of cysts for two closely linked polymorphic markers located within the PKD1 gene. Genetic analysis revealed that it was the normal haplotype that was lost. This study provides a molecular explanation for the focal nature of cyst formation and a probable mechanism whereby mutations cause disease. The high rate at which "second hits" must occur to account for the large number of cysts observed suggests that unique structural features of the PKD1 gene may be responsible for its mutability.

An Italian family with autosomal dominant polycystic kidney disease unlinked to either the PKD1 or PKD2 gene

We describe a family with autosomal dominant polycystic kidney disease in which molecular typing with closely linked markers for the PKD1 and PKD2 genes indicated absence of linkage. Thus, a third still unknown locus appears likely to be involved in disease development. This is the fourth "PKD3-linked" family described to date and the first from Italy.

Autosomal dominant polycystic kidney disease (ADPKD) in an Italian family carrying a novel nonsense mutation and two missense changes in exons 44 and 45 of the PKD1 Gene

Sixty-seven Italian patients with autosomal dominant polycystic kidney disease (ADPKD) were screened for mutations in the 3' unique region of the PKD1 gene, using heteroduplex DNA analysis. Novel aberrant bands were detected in 3 patients from the same family. DNA sequencing showed a C to T transition in exon 44 (C12269T), resulting in a premature stop codon (R4020X), predicted to impair the synthesis of the putative intracytoplasmic C-terminus tail of the PKD1 protein, polycystin. The mutation also generates a novel DdeI restriction site, and the abnormal restriction pattern was observed both on genomic DNA and on cDNA from the affected relatives, indicating that this is indeed the pathogenetic molecular lesion. Reverse transcriptase-polymerase chain reaction (RT-PCR) performed on lymphocyte mRNA showed that the mutant transcript is normally present and stable. No aberrantly spliced mRNAs were detected. Interestingly, the mutant PKD1 chromosome in this family also bears two missense mutations downstream (A12341G and C12384T), not found in the other ADPKD families studied.

Presymptomatic molecular diagnosis of autosomal dominant polycystic kidney disease using PKD1- and PKD2-linked markers in Cypriot families

Autosomal dominant polycystic kidney disease (ADPKD), is a heterogeneous disorder, primarily characterized by the formation of cysts in the kidneys, and the late development in life of progressive chronic kidney failure. Three genes are implicated in causing ADPKD. One on chromosome 16, PKD1, accounts for 85-90% of all cases, and the PKD2 gene on chromosome 4 accounts for the remainder. A very rare third locus is still of unknown location. We used PKD1- and PKD2-linked polymorphic markers to make the diagnosis of ADPKD in young presymptomatic members in affected families. We showed that in young members of families where clinical diagnosis cannot be definitively established, molecular linkage analysis can assist clinicians in the diagnosis. In one family a 24-year old had one cyst on the right kidney; however, molecular analysis showed clearly that he had inherited the normal haplotype. In another family, in one part of the pedigree there was co-inheritance of the disease with a PKD1-linked haplotype which originated in a non-affected 78-year-old father. Analysis with PKD2-linked markers excluded this locus. The data can be explained in one of two ways. Either this family phenotype is linked to a third locus, or the proband was the first affected person, most probably because of a novel mutation in one of her father's chromosomes. In conclusion, the combined use of markers around the PKD1 and the PKD2 locus provides more definitive answers in cases where presymptomatic diagnosis is requested by concerned families.

PKD2, a gene for polycystic kidney disease that encodes an integral membrane protein

A second gene for autosomal dominant polycystic kidney disease was identified by positional cloning. Nonsense mutations in this gene (PKD2) segregated with the disease in three PKD2 families. The predicted 968-amino acid sequence of the PKD2 gene product has six transmembrane spans with intracellular amino- and carboxyl-termini. The PKD2 protein has amino acid similarity with PKD1, the Caenorhabditis elegans homolog of PKD1, and the family of voltage-activated calcium (and sodium) channels, and it contains a potential calcium-binding domain.

Renal-hepatic-pancreatic dysplasia: an autosomal recessive malformation

We report two brothers with a cystic malformation of the kidneys, liver, and pancreas. In both cases the malformation was fatal and the children died shortly after birth. The pathological findings, consisting of multicystic dysplastic kidneys, dilated and dysgenetic bile ducts, dilated pancreatic ducts, and polysplenia, correspond to those reported by Ivemark as renal-hepatic-pancreatic dysplasia. Many polymalformation syndromes include cystic affectation of these three organs, so this syndrome could be an isolated entity or a final common pathway of response of these organs to a variety of developmental disturbances, which could also include splenic abnormalities. We propose an autosomal recessive pattern of inheritance for renal-hepatic-pancreatic dysplasia.

Advances in polycystic kidney disease

Until recently, the nature of the molecules involved in inherited cystic disease of the kidney remained unknown. These diseases are characterized by the development of multiple abnormal fluid-filled sacs or dilations in the kidney parenchyma, often leading to significant renal failure. The recent characterization of the PKD1 gene product and of other genes involved in murine polycystic models underscores the complexity of the pathways that lead to renal cystic disease.

Genetic mapping of the polycystic kidney gene, pcy, on mouse chromosome 9

The murine polycystic kidney disease gene, pcy, is an autosomal recessive trait located on chromosome 9. To determine the genetic locus of pcy, 222 intraspecific backcross mice were obtained by mating C57BL/6FG-pcy and Mus molossinus. Restriction fragment length polymorphism analysis of 70 of the 222 backcross progeny showed that pcy, dilute coat color (d), and cholecystokinin (Cck) were located in the order d--pcy--Cck from the centromere. Simple sequence repeat length polymorphism analysis of DNA of all 222 backcross mice was carried out using four markers which were located near the central regions of d and Cck. One and eight recombinations were detected between D9Mit24 and pcy and between D9Mit16 and pcy, respectively. However, no recombinant was observed among pcy, D9Mit14, and D9Mit148. These findings strongly suggest that D9Mit14 and D9Mit148 are located near the pcy gene and are good markers for chromosomal walking to this gene.

Autosomal dominant polycystic kidney disease: evidence for the existence of a third locus in a Portuguese family

Autosomal dominant polycystic kidney disease is characterized by clinical and genetic heterogeneity. Two loci implicated in the disease have previously been mapped (PKD1 on chromosome 16 and PKD2 on chromosome 4). By two point and multipoint linkage analysis, negative lod scores have been found for both chromosome 16 and chromosome 4 markers in a large Portuguese family, indicating that a third PKD locus is involved in the development of the disease.

The polycystic kidney disease 1 (PKD1) gene encodes a novel protein with multiple cell recognition domains

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.

Genetic analysis of 20 families with autosomal dominant adult polycystic kidney disease from South West Thames Region

Twenty families with autosomal dominant polycystic kidney disease from S. W. Thames Region were analysed using markers for chromosome 16p13.3, the site of the common mutation (PKD1). Six families gave a negative lod-score for 3'HVR, the most informative distal marker. This could be explained in four cases by recombination events. Of the two families where this was not an explanation, one, of Italian origin, was unequivocally unlinked for all markers, and the other was more likely to be non-PKD1 than linked to 16p13.3. The Italian family was ascertained through the Blood Pressure Unit, and the other via the Genetic Clinic. No members of either family had ever attended a renal clinic. The remaining 18 families either came via renal clinics, or had at least one member attending such a centre.

A family with autosomal dominant polycystic kidney disease linked to 4q21-23

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A century of mortality in five large families with polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) characteristically leads to end-stage renal failure in the fifth or sixth decade of life, which in the absence of therapeutic measures will lead to premature death. To determine excess mortality relative to the general population and chromosome 16-linked ADPKD patients, we studied 348 individuals who belonged to five large ADPKD families and who had at least a 50% probability of carrying the gene; the study data derive from a time span of approximately one century. Assessment of the diagnosis of ADPKD in the present generation was based on the characteristic roentgenographic appearance of polycystic kidneys and was confirmed by DNA analysis with flanking polymorphic markers around the polycystic gene. In the previous generation, we used Mendelian reasoning after pedigree analysis to identify persons with a 50% or 100% probability of carrying the polycystic gene. During the study period (1889 to 1992), 83 deaths occurred in 10,279 person-years. Mortality was increased 1.6-fold (95% confidence interval, 1.3 to 2.0) relative to the general population and was independent of the gender of the affected family member as well as the gender of the transmitting parent. The increased mortality was strongest in the 50 to 59 year age group (relative mortality, 3.2; 95% confidence interval, 2.0 to 4.8), but decreased after the 1970s, probably as a result of improvements in supportive care and, eventually, renal replacement therapy. In conclusion, the total life-span in ADPKD patients is improving, but remains low in comparison to the general population, and the gender of the transmitting parent or of the affected individual does not influence relative mortality.

Haplotype analysis in autosomal dominant polycystic kidney disease

Haplotype analysis was performed in 35 autosomal dominant polycystic kidney disease (ADPKD) families typed with 13 markers close to the PKD1 locus. The identification of recombinants close to the PKD1 gene on chromosome 16p indicates that PKD1 lies between CMM65 distally and 26-6 proximally. In addition, three unlinked (PKD2) families and two families with potential new mutation were identified.

Evidence for a third genetic locus for autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a genetically heterogeneous disease with loci on chromosomes 16p and 4q. It has a moderately high spontaneous mutation rate, although the relative frequency of such mutations at each gene locus is unknown. In studying genetic heterogeneity in the French-Canadian population, we identified a family in which a classical clinical presentation of ADPKD resulted from a mutation at a locus genetically distinct from either of the previously described loci for this disease. This suggests the existence of a third genetic locus for ADPKD.

Gene linkage analysis and DNA based detection of autosomal dominant polycystic kidney disease (ADPKD) in a newborn infant. Case report

Bilateral polycystic kidneys were detected by ultrasound at 23 weeks gestation in a male fetus. Bilateral renal cysts were subsequently also found in the asymptomatic propositus' mother and grandmother, suggesting the diagnosis of autosomal dominant polycystic kidney disease (ADPKD). The renal ultrasonograms showed cortical cysts with normal or decreased-sized kidneys. Renal function was normal. Seven available members of the family were genotyped for flanking DNA markers tightly linked to the PKD1 gene on chromosome 16p, and for a polymorphism close to a second putative disease gene (PKD2) on chromosome 2. The genetic linkage approach allowed us to detect with a high degree of accuracy the ADPKD1 at risk chromosome in the three patients, as well as in a 28-year-old unaffected female. This report illustrates the feasibility and the usefulness of recent molecular genetic strategies for diagnostic purposes in ADPKD, especially when clinical and radiological data are atypical. Furthermore, it also confirms that early or very early onset forms of the disease are not uncommon, and should be considered in the differential diagnosis of childhood cystic disease.

End-stage renal failure appears earlier in men than in women with polycystic kidney disease

Age-specific and cumulative incidence rates calculated using data from the Australia and New Zealand Dialysis and Transplant Registry indicate that more men than women with polycystic kidney disease enter end-stage renal failure programs between the ages of 25 and 44 years and 65 to 74 years. As this autosomal dominant inherited condition is equally likely to occur in the two sexes, gender-related factors appear to modulate the susceptibility to renal failure in persons with polycystic kidney disease.

Autosomal dominant polycystic kidney disease in the fetus

We report on 3 cases with a fetal presentation of autosomal dominant polycystic kidney disease (ADPKD), which illustrate the variable expression of ADPKD during fetal life. Fetus 1 was diagnosed at 20 weeks of gestation by ultrasonography; a molecular prenatal diagnosis was performed at 10 weeks on fetus 2, a sib of fetus 1; and ADPKD was an incidental finding in fetus 3 who was aborted at 16 weeks for anencephaly. All pregnancies were terminated and pathologic studies of the fetal kidneys were performed. From these cases and a review of the literature, we draw the following conclusions: (1) so far, all fetal ADPKD kidneys that have been histologically studied have shown cystic dilatations; 28/32 of these fetuses had ultrasonographic manifestations of the disease and/or had sibs with an early-onset form of it; (2) these cysts can be found in newly formed nephrons (fetus 2), predominantly in the more mature nephrons of the deep cortex (fetus 1) or more sparsely distributed in the cortex (fetus 3); these different patterns may reflect different rates of progression of the disease; (3) in contrast to the histologic findings in adult kidneys, glomeruli seem to be predominantly affected in fetal ADPKD; (4) severe fetal expression of ADPKD seems to cluster in some families; and (5) so far, all DNA analyses performed in families with subjects presenting during the fetal or neonatal period have been consistent with linkage to the PKD1 locus.

Evaluation of ultrasonographic diagnostic criteria for autosomal dominant polycystic kidney disease 1

Although ultrasound is commonly used for screening subjects at risk of polycystic kidney disease 1 (PKD1), there has been no evaluation of ultrasonographic diagnostic criteria. We used DNA linkage among subjects from 128 sibships within 18 PKD1 families as the basis for an assessment of ultrasound sensitivity. Positive and negative predictive values were calculated to allow assessment of different diagnostic cut-off points in previously undiagnosed cases. Currently used criteria (bilateral cysts with at least two in one kidney) provided good sensitivity (88.5% at age 15-29 years and 100% at 30 years and above) but performance could be improved by less stringent criteria in subjects aged 15-29 years and more stringent criteria in older family members, in whom simple renal cysts are frequent. The presence of at least two renal cysts (unilateral or bilateral) in individuals at risk and younger than 30 years may be regarded as sufficient to establish a diagnosis; among those aged 30-59 years, the presence of at least two cysts in each kidney may be required, and among those aged 60 years and above, at least four cysts in each kidney should be required.

Autosomal dominant polycystic kidney disease: localization of the second gene to chromosome 4q13-q23

At least two loci are known to exist for autosomal dominant polycystic kidney disease (ADPKD). One was localized to 16p, but the second less common locus has remained unlinked. Over 100 microsatellite markers, distributed across all chromosomes, have been typed on informative family members from the large Sicilian kindred in which the genetic heterogeneity was first discovered. Both the affected and the unaffected status of every family member used in the study were confirmed by renal ultrasonography. This search has resulted in the successful localization of a second ADPKD gene to chromosome 4q. It was found to be flanked by the markers D4S231 and D4S414, defining a segment that spans about 9 cM. The new locus has been designated PKD4. This second localization will allow researchers to target another ADPKD gene for isolation in an effort to understand the pathogenesis of this common disorder. Furthermore, when flanking markers for the second ADPKD gene are used in conjunction with flanking markers for PKD1, the accuracy of the diagnosis of the subtype of ADPKD present in any particular family will be enhanced. This will improve the accuracy of linkage-based presymptomatic diagnoses by reducing the error due to genetic heterogeneity.

Chromosome 4 localization of a second gene for autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a genetically heterogeneous disorder. A gene defect located on the short arm of chromosome 16 is responsible for the disease in roughly 86% of affected European families. Using highly polymorphic microsatellite DNA markers, we have assigned a second gene for ADPKD to chromosome 4. In eight families with clear evidence against linkage to chromosome 16 markers, linkage analysis with the markers D4S231 and D4S423, demonstrated a multipoint lod score of 22.42.

Estimating locus heterogeneity in autosomal dominant polycystic kidney disease (ADPKD) in the Spanish population

Although most mutations causing ADPKD in European populations have been mapped to the PKD1 locus on chromosome 16, some of them appear to be unlinked to this locus. To evaluate the incidence of unlinked mutations in Spain we have typed 31 Spanish families from different geographical sites for six closely linked DNA polymorphic marker loci flanking PKD1 detected by probes D16S85, D16S21, D16S259, D16S125, D16S246, and D16S80. Multilocus linkage analysis indicated that in 26 families the disease resulted from PKD1 mutations, whereas in three families it resulted from mutations in a locus other than PKD1. The two other families were not informative. Using the HOMOG test, the incidence of the PKD1 linked mutations in Spain is 85%. Multipoint linkage analysis in the 26 PKD1 families showed that the disease locus lies in the interval between D16S259(pGGG1) and D16S125(26.6).

A family with autosomal dominant polycystic kidney disease not linked to chromosome 16p13.3

A family of Sicilian origin with autosomal dominant polycystic kidney disease (APKD) has been shown to be unlinked to chromosome 16 markers. LOD scores for the polymorphic markers 3'HVR and SM7 flanking the PKD 1 locus, were -1.4 and -2.33 respectively, and theta max was 0.5 for each marker. The clinical phenotype of this family is consistent with that of the other non-linked families with APKD reported in the literature, all outside the United Kingdom, which have a milder progression than those linked to 16p13.3. Assuming that a clinic population represents the most severe forms of a disease and non PKD-1 is a less aggressive phenotype, the degree of genetic heterogeneity for APKD in the population may well be much greater than at present suggested.