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

Polycystin 1 is required for the structural integrity of blood vessels

Autosomal dominant polycystic kidney disease (ADPKD), often caused by mutations in the PKD1 gene, is associated with life-threatening vascular abnormalities that are commonly attributed to the frequent occurrence of hypertension. A previously reported targeted mutation of the mouse homologue of PKD1 was not associated with vascular fragility, leading to the suggestion that the vascular lesion may be of a secondary nature. Here we demonstrate a primary role of PKD1 mutations in vascular fragility. Mouse embryos homozygous for the mutant allele (Pkd1(L)) exhibit s.c. edema, vascular leaks, and rupture of blood vessels, culminating in embryonic lethality at embryonic day 15.5. Kidney and pancreatic ductal cysts are present. The Pkd1-encoded protein, mouse polycystin 1, was detected in normal endothelium and the surrounding vascular smooth muscle cells. These data reveal a requisite role for polycystin 1 in maintaining the structural integrity of the vasculature as well as epithelium and suggest that the nature of the PKD1 mutation contributes to the phenotypic variance in ADPKD.

Autosomal dominant polycystic kidney disease-type 2. Ultrasound, genetic and clinical correlations

BACKGROUND

Ultrasound, genetic and clinical correlations are available for ADPKD-1, but lacking for ADPKD-2. The present study was carried out to address: (i) the age-related diagnostic usefulness of ultrasound compared with genetic linkage studies; (ii) the age-related incidence and prevalence of relevant symptoms and complications; and (iii) the age and causes of death in patients with ADPKD-2.

METHODS

Two hundred and eleven alive subjects, from three ADPKD-2 families at 50% risk, were evaluated by physical examination, consultation of hospital records, biochemical parameters, ultrasound and with genetic linkage and DNA mutation analyses. Nineteen deceased and affected family members were also included in the study.

RESULTS

Of the 211 alive members, DNA linkage studies and direct mutation analyses showed that 106 were affected and 105 were not. Ultrasound indicated 94 affected, 108 not affected and nine equivocal results in nine children under the age of 15. For all ages, the false-positive diagnostic rate for ultrasound was 7.5% and the false-negative rate was 12.9%. The difference between ultrasound and DNA findings was most evident in children aged 5-14 years where the ultrasound was correct in only 50% and wrong or inconclusive in the remaining 50%. The mean age of the 106 alive, ADPKD-2 genetically affected patients was 37.9 years (range: 6-66 years). Among them, 23.5% had experienced episodes of renal pain, 22.6% were treated for hypertension, 22.6% had experienced at least one urinary tract infection, 19.8% had nephrolithiasis, 11.3% had at least one episode of haematuria, 9.4% had asymptomatic liver cysts, 7.5% had developed chronic renal failure and 0.9% had reached end-stage renal failure. Of the 19 deceased members, nine died before reaching end-stage renal failure at a mean age of 58.7 years (range: 40-68 years), mainly due to vascular complications, while the remaining 10 died on haemodialysis at a mean age of 71.4 years (range: 66-82 years).

CONCLUSIONS

DNA analysis is the gold standard for the diagnosis of ADPKD-2, especially in young people. Ultrasound diagnosis is highly dependent on age. Under the age of 14, ultrasound is not recommended as a routine diagnostic procedure, but ultrasound becomes 100% reliable in excluding ADPKD-2 in family members at 50% risk, over the age of 30. ADPKD-2 represents a mild variant of polycystic kidney disease with a low prevalence of symptoms and a late onset of end-stage renal failure.

Novel mutations of the PKD1 gene in Korean patients with autosomal dominant polycystic kidney disease

The gene for the most common form of autosomal dominant polycystic kidney disease (ADPKD), PKD1, has recently been characterized and shown to encode an integral membrane protein, polycystin-1, which is involved in cell-cell and cell-matrix interactions. Until now, approximately 30 mutations of the 3' single copy region of the PKD1 gene have been reported in European and American populations. However, there is no report of mutations in Asian populations. Using the polymerase chain reaction and single-strand conformation polymorphism (SSCP) analysis, 91 Korean patients with ADPKD were screened for mutation in the 3' single copy region of the PKD1 gene. As a result, we have identified and characterized six mutations: three frameshift mutations (11548del8bp, 11674insG and 12722delT), a nonsense mutation (Q4010X), and two missense mutations (R3752W and D3814N). Five mutations except for Q4010X are reported here for the first time. Our findings also indicate that many different mutations are likely to be responsible for ADPKD in the Korean population. The detection of additional disease-causing PKD1 mutations will help in identifying the location of the important functional regions of polycystin-1 and help us to better understand the pathophysiology of ADPKD.

Mutations in a NIMA-related kinase gene, Nek1, cause pleiotropic effects including a progressive polycystic kidney disease in mice

We previously have described a mouse model for polycystic kidney disease (PKD) caused by either of two mutations, kat or kat(2J), that map to the same locus on chromosome 8. The homozygous mutant animals have a latent onset, slowly progressing form of PKD with renal pathology similar to the human autosomal-dominant PKD. In addition, the mutant animals show pleiotropic effects that include facial dysmorphism, dwarfing, male sterility, anemia, and cystic choroid plexus. We previously fine-mapped the kat(2J) mutation to a genetic distance of 0.28 +/- 0.12 centimorgan between D8Mit128 and D8Mit129. To identify the underlying molecular defect in this locus, we constructed an integrated genetic and physical map of the critical region surrounding the kat(2J) mutation. Cloning and expression analysis of the transcribed sequences from this region identified Nek1, a NIMA (never in mitosis A)-related kinase as a candidate gene. Further analysis of the Nek1 gene from both kat/kat and kat(2J)/kat(2J) mutant animals identified a partial internal deletion and a single-base insertion as the molecular basis for these mutations. The complex pleiotropic phenotypes seen in the homozygous mutant animals suggest that the NEK1 protein participates in different signaling pathways to regulate diverse cellular processes. Our findings identify a previously unsuspected role for Nek1 in the kidney and open a new avenue for studying cystogenesis and identifying possible modes of therapy.

Molecular genetics and mechanism of autosomal dominant polycystic kidney disease

Considerable progress toward understanding pathogenesis of autosomal dominant polycystic disease (ADPKD) has been made during the past 15 years. ADPKD is a heterogeneous human disease resulting from mutations in either of two genes, PKD1 and PKD2. The similarity in the clinical presentation and evidence of direct interaction between the COOH termini of polycystin-1 and polycystin-2, the respective gene products, suggest that both proteins act in the same molecular pathway. The fact that most mutations from ADPKD patients result in truncated polycystins as well as evidence of a loss of heterozygosity mechanism in individual PKD cysts indicate that the loss of the function of either PKD1 or PKD2 is the most likely pathogenic mechanism for ADPKD. A novel mouse model, WS25, has been generated with a targeted mutation at Pkd2 locus in which a mutant exon 1 created by inserting a neo(r) cassette exists in tandem with the wild-type exon 1. This causes an unstable allele that undergoes secondary recombination to produce a true null allele at Pkd2 locus. Therefore, the model Pkd2(WS25/-), which carries the WS25 unstable allele and a true null allele, produces somatic second hits during mouse development or adult life and establishes an extremely faithful model of human ADPKD.

Treatment of polycystic kidney disease with a novel tyrosine kinase inhibitor

Treatment of polycystic kidney disease with a novel tyrosine kinase inhibitor.

BACKGROUND

We have previously demonstrated an essential role for increased epidermal growth factor receptor (EGFR) activity in mediating renal cyst formation and biliary epithelial hyperplasia in murine models of autosomal recessive polycystic kidney disease (ARPKD). This study was designed to determine whether or not treatment with a newly developed inhibitor of EGFR tyrosine kinase activity (EKI-785) would reduce renal and biliary abnormalities in murine ARPKD.

METHODS

Balb/c-bpk/bpk (BPK) litters were treated with EKI-785, an EGFR-specific tyrosine kinase inhibitor. Animals were treated by intraperitoneal injection beginning at postnatal day 7 and were treated until postnatal day 24 or 48. EKI-785's effectiveness was measured by a reduction in the renal cystic index, an increased life span, and maintenance of normal renal function.

RESULTS

Treatment of BPK mice with EKI-785 resulted in a marked reduction of collecting tubule (CT) cystic lesions, improved renal function, decreased biliary epithelial abnormalities, and an increased life span. Untreated cystic animals died of renal failure at postnatal day 24 (P-24) with a CT cystic index of 4.8, a maximal urine osmolarity of 361 mOsm, and moderate to severe biliary abnormalities. Cystic animals treated with EKI-785 to postnatal day 48 (P-48) were alive and well with normal renal function, a reduced CT cystic index of 2.0 (P < 0.02), a threefold increased in maximum urinary concentrating ability (P < 0.01), and a significant decrease in biliary epithelial proliferation/fibrosis (P < 0.01).

CONCLUSION

This study demonstrates that EKI-785 has therapeutic effectiveness in improving histopathologic abnormalities and decreasing mortality in murine ARPKD.

The rat pkd2 protein assumes distinct subcellular distributions in different organs

Mutations in the PKD2 gene account for approximately 15% of all cases of autosomal-dominant polycystic kidney disease. In the present study the cellular distribution of the Pkd2 protein was investigated by immunohistochemistry in different rat organs. Although the Pkd2 protein showed a widespread expression, a strikingly different distribution of the protein was observed between individual organs. Whereas in renal distal tubules and in striated ducts of salivary glands a basal-to-basolateral distribution of Pkd2 was found, a punctate cytoplasmic location was detected in the adrenal gland, ovary, cornea, and smooth muscle cells of blood vessels. Interestingly, in the adrenal gland and ovary, the rat Pkd2 protein was more heavily N-glycosylated than in the kidney and salivary gland. These results suggest that Pkd2 accomplishes its functions by interacting with proteins located in different cellular compartments. The extrarenal expression pattern of the Pkd2 protein hints at other candidate sites of disease manifestations in patients carrying PKD2 mutations.

Polycystin-1, the PKD1 gene product, is in a complex containing E-cadherin and the catenins

Autosomal dominant polycystic kidney disease (ADPKD) is a common human genetic disease characterized by cyst formation in kidney tubules and other ductular epithelia. Cells lining the cysts have abnormalities in cell proliferation and cell polarity. The majority of ADPKD cases are caused by mutations in the PKD1 gene, which codes for polycystin-1, a large integral membrane protein of unknown function that is expressed on the plasma membrane of renal tubular epithelial cells in fetal kidneys. Because signaling from cell-cell and cell-matrix adhesion complexes regulates cell proliferation and polarity, we speculated that polycystin-1 might interact with these complexes. We show here that polycystin-1 colocalized with the cell adhesion molecules E-cadherin and alpha-, beta-, and gamma-catenin. Polycystin-1 coprecipitated with these proteins and comigrated with them on sucrose density gradients, but it did not colocalize, coprecipitate, or comigrate with focal adhesion kinase, a component of the focal adhesion. We conclude that polycystin-1 is in a complex containing E-cadherin and alpha-, beta-, and gamma-catenin. These observations raise the question of whether the defects in cell proliferation and cell polarity observed in ADPKD are mediated by E-cadherin or the catenins.

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.

Influence of the ACE gene polymorphism in the progression of renal failure in autosomal dominant polycystic kidney disease

The recent description of a polymorphism in the gene for angiotensin-converting enzyme (ACE), with the D allele associated with greater plasma levels of ACE, allows us to perform studies of the relationship between this polymorphism and chronic renal diseases in which the renin-angiotensin system could be implicated. We examined 155 patients with autosomal dominant polycystic kidney disease (ADPKD) with linkage to the PKD1 locus. The ACE insertion/deletion (I/D) polymorphism was amplified with the previously published flanking primers, and the polymerase chain reaction product was separated, sized on a 2% agarose gel, and visualized by ultraviolet transillumination. The ACE genotype distributions were 11.6%, 63.8%, and 24.5% for II, ID, and DD, respectively. There were no significant differences among the three genotypes with respect to mean age, sex distribution, and prevalence of hypertension. The ACE genotype distribution in patients with end-stage renal failure at the time of data compilation was similar to that of the entire study population. In the subgroup of patients who received renal replacement therapy before the age of 50 years, we found a significant association between DD genotype and onset of end-stage renal disease (ESRD) before the age of 50 years compared with II and ID (P = 0.017). We calculated the estimated median renal survival time as 51 years for the II genotype, 53 years for the ID genotype, and 48 years for the DD genotype. There were statistically significant differences between DD and ID patients (P = 0.025). In conclusion, we found DD genotype implies a worse renal prognosis based on both the significantly lower median renal survival time and significantly greater percentage of patients who reach ESRD before the age of 50 years, without implying a greater prevalence of hypertension.

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.

Frequency of ovarian cysts in patients with autosomal dominant polycystic kidney disease

Extrarenal cysts occur in patients with autosomal dominant polycystic kidney disease (ADPKD) most frequently in the liver. Ovarian cysts have been reported in women with ADPKD, but their frequency has not been determined. Therefore, we analyzed the historical data in our database of 337 women with ADPKD and 199 of their unaffected female family members (NADPKD). In addition, we prospectively studied 25 nonpregnant, premenopausal women with ADPKD and 25 nonpregnant, premenopausal, age-matched control women recruited from the general population to assess the occurrence of ovarian cysts. No women in either the control or ADPKD groups were receiving exogenous estrogen or progesterone. All women underwent sonographic examination using a 5- or 7.5-MHz vaginal probe. A normal ovarian follicle was defined as a fluid-filled structure less than 2 cm in average diameter, and an ovarian cyst as one of 2 cm or greater. From the historical data, 28% of the women with ADPKD gave a history of ovarian cysts compared with 18% of the NADPKD women (P < 0.05). In the prospective study, the mean age of the women with ADPKD was not different from that of the control women (40.9 +/- 1.2 v 39.3 +/- 1.2 years; P = not significant [NS]). There was no difference in frequency of normal follicles found in women with ADPKD or controls (80% v 96%; P = NS), nor was there a difference in the frequency of ovarian cysts found in women with ADPKD or controls (12% v 12%; P = NS). There was no difference in the calculated ovarian volumes between the women with ADPKD and controls (9.9 +/- 2. 5 v 7.2 +/- 1.2 cm3). Among the women with ADPKD, there was no correlation between mean ovarian volume and mean renal volume, nor was there a significant relationship between the occurrence of hepatic cystic disease and ovarian cysts. Therefore, a prospective imaging study suggests that ovarian cysts have no increased frequency in women with ADPKD compared with women in the general population.

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.

Genetic modifiers of polycystic kidney disease in intersubspecific KAT2J mutants

Polycystic kidney disease (PKD) is a genetically heterogeneous disorder. In addition to the many PKD-causative loci mapped in mouse and human, a number of reports indicate that modifier loci greatly influence the course of disease progression. Recently we reported a new mouse mutation, kat2J, on chromosome (Chr) 8 that causes late-onset PKD and anemia. During the mapping studies it was noted that the severity of PKD in the mutant (C57BL/6J-kat2J/+ x CAST/Ei)F2 generation was more variable than that in the parental C57BL/6J strain. This suggested that genetic background or modifier genes alter the clinical manifestations and progression of PKD. Genome scans using molecular markers revealed three loci that affect the severity of PKD. The CAST-derived modifier on Chr 1 affects both kidney weight and hematocrit. The CAST-derived modifier on Chr 19 affects kidney weight, and the C57BL/6J-derived modifier on Chr 2 affects hematocrit. Additional modifier loci are noted that interact with and modulate the effects of these three loci. The mapping of these modifier genes and their eventual identification will help to uncover factors that can delay disease progression. These, in turn, could be used to design suitable modes of therapy for various forms of human PKD.

Cellular activation triggered by the autosomal dominant polycystic kidney disease gene product PKD2

Autosomal dominant polycystic kidney disease (ADPKD) is caused by germ line mutations in at least three ADPKD genes. Two recently isolated ADPKD genes, PKD1 and PKD2, encode integral membrane proteins of unknown function. We found that PKD2 upregulated AP-1-dependent transcription in human embryonic kidney 293T cells. The PKD2-mediated AP-1 activity was dependent upon activation of the mitogen-activated protein kinases p38 and JNK1 and protein kinase C (PKC) epsilon, a calcium-independent PKC isozyme. Staurosporine, but not the calcium chelator BAPTA [1,2-bis(o-aminophenoxy)ethane-N,N,N', N'-tetraacetate], inhibited PKD2-mediated signaling, consistent with the involvement of a calcium-independent PKC isozyme. Coexpression of PKD2 with the interacting C terminus of PKD1 dramatically augmented PKD2-mediated AP-1 activation. The synergistic signaling between PKD1 and PKD2 involved the activation of two distinct PKC isozymes, PKC alpha and PKC epsilon, respectively. Our findings are consistent with others that support a functional connection between PKD1 and PKD2 involving multiple signaling pathways that converge to induce AP-1 activity, a transcription factor that regulates different cellular programs such as proliferation, differentiation, and apoptosis. Activation of these signaling cascades may promote the full maturation of developing tubular epithelial cells, while inactivation of these signaling cascades may impair terminal differentiation and facilitate the development of renal tubular cysts.

Murine Pkd1 is a developmentally regulated gene from morula to adulthood: role in tissue condensation and patterning

PKD1 is the most common genetically mutated gene involved in autosomal dominant polycystic kidney disease (ADPKD). Our previous studies have shown that the pathogenesis of human and murine polycystic kidney disease (PKD) involves failure to switch out of a renal developmental program, suggesting a role for PKD1 in development. To investigate this hypothesis, we have cloned a portion of the murine Pkd1 gene and characterized the fetal to adult tissue expression pattern of Pkd1. We chose to clone the transmembrane region of Pkd1, a region prone to mutations in ADPKD. The transmembrane coding region (2.6 kb) has 80.3% nucleotide homology with human PKD1 and 85.3% amino acid similarity. The cloned murine Pkd1 fragment closely resembles that of human PKD1 with respect to both genomic size and exon/intron position. We have demonstrated that this Pkd1 region is not conserved in lower organisms and is mammalian specific. A detailed expression analysis of Pkd1 revealed expression as early as the morula stage and in ES cells with differential expression levels in various tissues/organs throughout development. Highest expression levels were observed in the early condensing mesenchyme of primitive mesoderm and ectoderm. Pkd1 was also expressed at high levels in developing neural tube, neural crest derivatives, prechondrogenic tissue, metanephros, bladder, salivary glands, lung, and blood vessels with lower expression levels in other organs and tissues. Specific spatial and temporal patterns of Pkd1 expression were demonstrated in individual organs, such as lung, kidney, brain, indicating it is highly developmentally regulated. Particularly high levels persisted in mature derivatives of neural tube, neural crest, chondrogenic tissue, metanephros, and lung. In summary, our data suggest that Pkd1 has at least two cellular functions, one a basic function involved in early tissue condensation processes, and the other a mammalian-specific function, that evolved with tissue patterning and tubulogenesis in metanephric and pulmonary development.

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.

Identification of PKD2L, a human PKD2-related gene: tissue-specific expression and mapping to chromosome 10q25

Mutations in PKD2 cause autosomal dominant kidney disease (ADPKD). Polycystin-2, the PKD2 gene product, is an integral membrane glycoprotein of unknown function. We have identified PKD2L, another member of the PKD2 gene family. PKD2L is expressed in adult heart and skeletal muscle, brain, spleen, testis, and retina, and alternative transcripts of 2.4, 2.7, and 3.0 kb are seen. PKD2L shows 56% identity and 76% similarity with polycystin-2 over a 581-amino-acid span; however, the COOH-terminal 65 residues of PKD2L are unrelated to PKD2. PKD2L is localized to chromosome 10q25 and is excluded as a candidate gene for autosomal recessive polycystic kidney disease, autosomal dominant polycystic liver disease, and the third form of ADPKD. Given the high degree of homology between PKD2L and PKD2, it is likely that the respective functions of these proteins are also closely related.

Identification of PKDL, a novel polycystic kidney disease 2-like gene whose murine homologue is deleted in mice with kidney and retinal defects

Polycystin-1 and polycystin-2 are the products of PKD1 and PKD2, genes that are mutated in most cases of autosomal dominant polycystic kidney disease. Polycystin-2 shares approximately 46% homology with pore-forming domains of a number of cation channels. It has been suggested that polycystin-2 may function as a subunit of an ion channel whose activity is regulated by polycystin-1. Here we report the identification of a human gene, PKDL, which encodes a new member of the polycystin protein family designated polycystin-L. Polycystin-L has 50% amino acid sequence identity and 71% homology to polycystin-2 and has striking sequence and structural resemblance to the pore-forming alpha1 subunits of Ca2+ channels, suggesting that polycystin-L may function as a subunit of an ion channel. The full-length transcript of PKDL is expressed at high levels in fetal tissues, including kidney and liver, and down-regulated in adult tissues. PKDL was assigned to 10q24 by fluorescence in situ hybridization and is linked to D10S603 by radiation hybrid mapping. There is no evidence of linkage to PKDL in six ADPKD families that are unlinked to PKD1 or PKD2. The mouse homologue of PKDL is deleted in Krd mice, a deletion mutant with defects in the kidney and eye. We propose that PKDL is an excellent candidate for as yet unmapped cystic diseases in man and animals.

Pathological consequences of sequence duplications in the human genome

As large-scale sequencing accumulates momentum, an increasing number of instances are being revealed in which genes or other relatively rare sequences are duplicated, either in tandem or at nearby locations. Such duplications are a source of considerable polymorphism in populations, and also increase the evolutionary possibilities for the coregulation of juxtaposed sequences. As a further consequence, they promote inversions and deletions that are responsible for significant inherited pathology. Here we review known examples of genomic duplications present on the human X chromosome and autosomes.

Epithelial transport in polycystic kidney disease

In autosomal dominant polycystic kidney disease (ADPKD), the genetic defect results in the slow growth of a multitude of epithelial cysts within the renal parenchyma. Cysts originate within the glomeruli and all tubular structures, and their growth is the result of proliferation of incompletely differentiated epithelial cells and the accumulation of fluid within the cysts. The majority of cysts disconnect from tubular structures as they grow but still accumulate fluid within the lumen. The fluid accumulation is the result of secretion of fluid driven by active transepithelial Cl- secretion. Proliferation of the cells and fluid secretion are activated by agonists of the cAMP signaling pathway. The transport mechanisms involved include the cystic fibrosis transmembrane conductance regulator (CFTR) present in the apical membrane of the cystic cells and a bumetanide-sensitive transporter located in the basolateral membrane. A lipid factor, called cyst activating factor, has been found in the cystic fluid. Cyst activating factor stimulates cAMP production, proliferation, and fluid secretion by cultured renal epithelial cells and also is a chemotactic agent. Cysts also appear in the intrahepatic biliary tree in ADPKD. Normal ductal cells secrete Cl- and HCO3-. The cystic ductal cell also secretes Cl-, but HCO3- secretion is diminished, probably as the result of a lower population of Cl-/HCO3- exchangers in the apical membrane as compared with the normal cells. Some segments of the normal renal tubule are also capable of utilizing CFTR to secrete Cl-, particularly the inner medullary collecting duct. The ability of Madin-Darby canine kidney cells and normal human kidney cortex cells to form cysts in culture and to secrete fluid and the functional similarities between these incompletely differentiated, proliferative cells and developing cells in the intestinal crypt and in the fetal lung have led us to suggest that Cl- and fluid secretion may be a common property of at least some renal epithelial cells in an intermediate stage of development. The genetic defect in ADPKD may not directly affect membrane transport mechanisms but rather may arrest the development of certain renal epithelial cells in an incompletely differentiated, proliferative stage.

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.

Pathogenesis of autosomal dominant polycystic kidney disease: role of apoptosis

Autosomal dominant polycystic kidney disease (ADPKD) is a common monogenic disorder with progressive distension of multiple tubular segments, and is manifested by fluid accumulation, growth of epithelial cells, and remodeling of the extracellular matrix, ultimately resulting in renal insufficiency in one half of affected individuals. The process causing the progressive loss of renal tissue is unclear. Recent studies demonstrate that apoptosis is one of the major histopathologic features of ADPKD and may be causally related to the progressive deterioration of the renal function in this population. Further studies are required to elucidate the mechanisms by which some cysts upregulate the process of programmed cell death in the kidney.

The development of the kidney

This chapter describes the earlier stages of development of the vertebrate metanephric kidney. It focuses on the mouse and descriptive morphology is used for considering both molecular mechanisms, underpinning kidney morphogenesis and differentiation, and the ways in which these processes can go awry and lead to congenital kidney disorders—particularly in humans. The mature kidney is a fairly complex organ attached to an arterial input vessel and two output vessels, the vein and the ureter. Inside, the artery and vein are connected by a complex network of capillaries that invade a large number of glomeruli, the proximal entrance to nephrons, which are filtration units that link to an arborized collecting-duct system that drains into the ureter. The ability of the kidney and isolated metanephrogenic mesenchyme, to develop in culture means that the developing tissues can be subjected to a wide variety of experimental procedures designed to investigate their molecular and cellular properties and to test hypotheses about developmental mechanisms.

Prenatal diagnosis of autosomal dominant polycystic kidney disease (PKD1) presenting in utero and prognosis for very early onset disease

We describe four prenatal diagnoses in a family with autosomal dominant polycystic kidney disease. Two pregnancies were terminated following the detection of enlarged echogenic fetal kidneys with cysts. Histopathological examination confirmed the diagnosis of polycystic kidney disease. Linkage to PKD1 was obtained by the analysis of DNA from relatives in three generations and from paraffin blocks and formalin fixed fetal tissues. Prenatal DNA analysis in subsequent pregnancies identified one unaffected fetus and one fetus carrying the high risk PKD1 allelle. Information on survival and subsequent outcome of PKD cases presenting in utero was requested by this family before prenatal testing was performed. Of 83 reported cases of ADPKD presenting in utero (excluding termination of pregnancy) or in the first few months of life, 43% died before 1 year. Longitudinal follow up of 24 children in two studies showed that 67% of survivors developed hypertension, of whom three had end stage renal failure at a mean age of 3 years.

Polycystin expression during embryonic development of human kidney in adult tissues and ADPKD tissue

Normal renal tissue, ranging from 8 weeks' gestation to full term to adult, was probed with polyclonal antibodies raised to peptide epitopes within the translated PKD1 gene sequence. Three antibodies were studied, all of which gave similar results. Renal tissue from patients with autosomal dominant polycystic kidney disease (ADPKD) and samples from normal adult liver, heart, brain, skeletal muscle and lymph node were also studied. Tissue staining demonstrated that the pattern of polycystin expression changed with gestational age in normal kidney. Whereas the precursors to the renal excretory unit were stained at 12 weeks, and the proximal and distal convoluted tubules stained to differing degrees throughout development, the glomeruli were poorly stained until full term and also in the adult. Extrarenal tissue stained in both adult and juvenile samples, with the exception of lymph node, which remained unstained. The intensity of polycystin staining increased in ADPKD renal tissue. The widespread distribution of polycystin was consistent with the systemic nature of ADPKD and the role of epithelial cells in the disease.

Genetic heterogeneity of autosomal dominant polycystic kidney disease in Argentina

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited disorder with genetic heterogeneity. Up to three loci are involved in this disease, PKD1 on chromosome 16p13.3, PKD2 on 4q21, and a third locus of unknown location. Here we report the existence of locus heterogeneity for this disease in the Argentinian population by performing linkage analysis on 12 families of Caucasian origin. Eleven families showed linkage to PKD 1 and one family showed linkage to PKD2. Two recombinants in the latter family placed the locus PKD2 proximal to D4S1563, in agreement with data recently published on the cloning of this gene. Analysis of clinical data suggests a milder ADPKD phenotype for the PKD2 family.

Molecular cloning, cDNA sequence analysis, and chromosomal localization of mouse Pkd2

The gene responsible for the second form of autosomal dominant polycystic kidney disease, PKD2, has recently been identified. We now describe the cloning, genomic localization, cDNA sequence, and expression analysis of its murine homologue, Pkd2. The cloned cDNA sequence is 5134 bp long and is predicted to encode a 966-amino-acid integral membrane protein with six membrane-spanning domains and intracellular NH2 and COOH termini. Pkd2 is highly conserved with 91% identity and 98% similarity to polycystin-2 at the amino acid level. Pkd2 mRNA is widely expressed in mouse tissues. Pkd2 maps to mouse Chromosome 5 and is excluded as a candidate gene for previously mapped mouse mutations resulting in a polycystic kidney phenotype.

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.

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.

Characterization of the exon structure of the polycystic kidney disease 2 gene (PKD2)

PKD2, the gene defective in the second form of autosomal dominant polycystic kidney disease (ADPKD), has been identified by positional cloning and found to encode an integral membrane protein with similarity to the gene for the more common form of ADPKD and to calcium channels. We have determined the exon-intron structure of the PKD2 gene. PKD2 is encoded in at least 15 exons with the translation start site in exon 1. All the splice acceptor and donor sites conform to the AG/GT rule. We have designed a series of intronic oligonucleotide primers for amplifying the entire coding sequence from genomic DNA in segments well suited to mutation analysis using conventional screening strategies such as SSCA or heteroduplex analysis.

Insulin resistance in adult polycystic kidney disease

Adult polycystic kidney disease (APKD) is a common hereditary disease with renal and extra-renal manifestations. There are at least three genes responsible for this disease. The polycystic kidney disease 1 (PKD1) gene product is a membrane protein involved in cell-cell and cell-matrix interactions and has a widespread tissue distribution. Abnormal membrane fluidity in erythrocytes from APKD patients is due to altered membrane proteins. Membrane fluidity of mononuclear cells is related to whole body insulin sensitivity. Insulin sensitivity might therefore be disturbed in APKD if the erythrocyte membrane abnormality is also present in other cells. Therefore, we investigated insulin sensitivity in 15 APKD patients and 20 normal subjects matched for age and sex. Insulin sensitivity was assessed by a short insulin tolerance test to derive the first-order rate constant for the disappearance of glucose (Kitt) and mononuclear leukocyte membrane fluidity was measured by fluorescence anisotropy. The Kitt value (% mmol.liter-1.min-1) was lower in APKD patients than in normal subjects [median (range) 2.2 (1.5 to 6.3) vs. 4.1 (2.0 to 5.4). P < 0.001]. Fasting plasma insulin concentrations were negatively correlated with the Kitt values (r = -0.66, P < 0.001). Core region anisotropy was significantly lower (higher fluidity) in leukocytes from APKD patients [mean (SEM) 0.164 (0.003) vs. 0.174 (0.001), P < 0.001]. Insulin sensitivity was positively correlated with the fluorescence anisotropy of the core region of leukocyte membranes (r = 0.81, P = 0.0001). In conclusion, APKD patients were insulin resistant and some patients were hyperinsulinemic, which may indicate increased cardiovascular risk. The cellular basis of the insulin resistance may be directly related to the proteins causing the disease or to the general change in membrane properties.

A family with a milder form of adult dominant polycystic kidney disease not linked to the PKD1 (16p) or PKD2 (4q) genes

Autosomal dominant polycystic kidney disease (ADPKD) is a genetically heterogeneous disease. Most families show positive linkage to polymorphic markers around the PKD1 (16p13.3) or PKD2 (4q21-23) loci. The PKD1 and PKD2 genes have been cloned and mutations defined in a number of patients. Several clinical studies have described a milder phenotype for PKD2 patients. More recently, evidence for a third genetic locus has been found in one Portuguese, one French-Canadian, and one Italian family. We identified a Spanish family with negative linkage to the PKD1 and the PKD2 loci. This family showed a very mild clinical phenotype compared to the other forms of ADPKD, including the non-PKD1/non-PKD2 families previously described.

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.

A translation frameshift mutation induced by a cytosine insertion in the polycystic kidney disease 2 gene (PDK2)

Mutations in the PKD2 gene on the long arm of chromosome 4 are responsible for approximately 15% of cases of polycystic kidney disease. Perhaps the only difference from the more common ADPKD1 cases is the rate of progression of cystic changes, and the age of onset, which is 10-15 years later for the ADPKD2 form. In Cyprus there are at least three large families, documented by molecular linkage analysis, that map to the PKD2 locus. For two of them the defects were recently shown to be nonsense mutations at positions arginine 742 and glutamine 405. In this report, we describe the mutation in the third family, CY1602. For this, the entire coding sequence was systematically screened by single strand conformation analysis and heteroduplex formation. A novel mutation was identified in exon 2 where a new cytosine residue was inserted immediately after codon 231 (231insC). It causes a translation frameshift and is expected to lead to the introduction of 37 novel amino acids before the translation reaches a new STOP codon. It is the most amino terminal mutation reported to date, and based on the protein's modeled structure, is predicted to be within the first transmembrane domain. It is the fourth PKD2 mutation reported thus far, and the first which is not a nonsense mutation.

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.

PKD1 interacts with PKD2 through a probable coiled-coil domain

Autosomal dominant polycystic kidney disease (ADPKD) describes a group of at least three genetically distinct disorders with almost identical clinical features that collectively affects 1:1,000 of the population. Affected individuals typically develop large cystic kidneys and approximately one half develop end-stage renal disease by their seventh decade. It has been suggested that the diseases result from defects in interactive factors involved in a common pathway. The recent discovery of the genes for the two most common forms of ADPKD has provided an opportunity to test this hypothesis. We describe a previously unrecognized coiled-coil domain within the C terminus of the PKD1 gene product, polycystin, and demonstrate that it binds specifically to the C terminus of PKD2. Homotypic interactions involving the C terminus of each are also demonstrated. We show that naturally occurring pathogenic mutations of PKD1 and PKD2 disrupt their associations. We have characterized the structural basis of their heterotypic interactions by deletional and site-specific mutagenesis. Our data suggest that PKD1 and PKD2 associate physically in vivo and may be partners of a common signalling cascade involved in tubular morphogenesis.

Genetics of intracranial aneurysms

The etiology and pathogenesis of intracranial aneurysms are clearly multifactorial, with genetic factors playing an increasingly recognized role. Intracranial aneurysms have been associated with numerous heritable connective tissue disorders, which account for at least 5% of cases. Of these disorders, the most important are Ehlers-Danlos syndrome Type IV, Marfan's syndrome, neurofibromatosis Type 1, and autosomal dominant polycystic kidney disease; the association with intracranial aneurysms, however, has been firmly established only for polycystic kidney disease. Familial intracranial aneurysms are not rare but account for 7 to 20% of patients with aneurysmal subarachnoid hemorrhage and are generally not associated with any of the known heritable connective tissue disorders. First-degree relatives of patients with aneurysmal subarachnoid hemorrhage are at an approximately fourfold increased risk of suffering ruptured intracranial aneurysms, compared to the general population. Various possible modes of inheritance have been identified in families with intracranial aneurysms, suggesting genetic heterogeneity. Although the benefits have never been quantified, screening for asymptomatic intracranial aneurysms should be considered in families with two or more affected members. The yield of such a screening program may approximate 10%. Although it is unlikely that there is a single gene with major effect, much effort is currently being directed at locating intracranial aneurysm genes.