PKD1 interacts with PKD2 through a probable coiled-coil domain

Physiology, phylogeny, and functions of the TRP superfamily of cation channels

The transient receptor potential (TRP) protein superfamily consists of a diverse group of Ca(2+) permeable nonselective cation channels that bear structural similarities to Drosophila TRP. TRP-related proteins play important roles in nonexcitable cells, as demonstrated by the recent finding that a mammalian TRPC protein is expressed in endothelial cells and functions in vasorelaxation. However, an emerging theme is that many TRP-related proteins are expressed predominantly in the nervous system and function in sensory physiology. The TRP superfamily can be divided into six subfamilies, the first of which is composed of the "classical TRPs" (TRPC subfamily). These proteins all share the common features of three to four ankryin repeats, >/=30% amino acid homology over >/=750 amino acids, and a gating mechanism that operates through phospholipase C. Some classical TRPs may be store-operated channels (SOCs), which are activated by release of Ca(2+) from internal stores. The mammalian TRPC proteins are also expressed in the central nervous system, and several are highly enriched in the brain. One TRPC protein has been implicated in the pheromone response. The archetypal TRP, Drosophila TRP, is predominantly expressed in the visual system and is required for phototransduction. Many members of a second subfamily (TRPV) function in sensory physiology. These include VR1 and OSM-9, which respond to heat, osmolarity, odorants, and mechanical stimuli. A third subfamily, TRPN, includes proteins with many ankyrin repeats, one of which, NOMPC, participates in mechanotransduction. Among the members of a fourth subfamily, TRPM, is a putative tumor suppressor termed melastatin, and a bifunctional protein, TRP-PLIK, consisting of a TRPM channel fused to a protein kinase. PKD2 and mucolipidin are the founding members of the TRPP and TRPML subfamilies, respectively. Mutations in PKD2 are responsible for polycystic kidney disease, and mutations in mucolipidin result in a severe neurodegenerative disorder. Recent studies suggest that alterations in the activities of SOC and TRP channels may be at the heart of several additional neurodegenerative diseases. Thus, TRP channels may prove to be important new targets for drug discovery.

Polycystic kidney disease: from the bedside to the gene and back

Collated in this highly personal commentary are the most important research findings of the past 10 years that deal primarily with the renal manifestations of inherited polycystic kidney diseases. Progress in understanding these complex disorders has followed two major concurrent and convergent lines of investigation: genes and genetic mechanisms, and pathogenesis and progression. The field has moved from descriptive pathobiology to the elucidation of molecular mechanisms consequent to genetic and epigenetic events. Doubtless, the favorite works of some who have labored diligently in this field have not been fully exalted, and for this I apologize. Were I the editor, this entire celebratory volume would be used to extol the thrilling growth of knowledge during the tenure of this polycystic kidney disease watcher.

Homologues to the first gene for autosomal dominant polycystic kidney disease are pseudogenes

PKD1 is the first gene identified to be causative for the condition of autosomal dominant polycystic kidney disease. There are several genes homologous to PKD1 that are located proximal to the master gene on the same chromosome. Two of these genes have been recently covered in a large sequencing work on chromosome 16, and their structure has been broadly analyzed. However, the major question whether homologous genes (HG) code for functionally active polypeptides has not been resolved so far. The current study identifies and partially characterizes four more homologues of PKD1, different from the previously published sequence, two of which were found by screening of a BAC library and the other two contained in available databases. Analysis of HG transcripts shows that they are not translated in the model cell line T98G. Taken together, these findings suggest that homologues to PKD1 form a family of pseudogenes.

Pkd1 unusual DNA conformations are recognized by nucleotide excision repair

The 2.5-kilobase pair poly(purine.pyrimidine) (poly(R.Y)) tract present in intron 21 of the polycystic kidney disease 1 (PKD1) gene has been proposed to contribute to the high mutation frequency of the gene. To evaluate this hypothesis, we investigated the growth rates of 11 Escherichia coli strains, with mutations in the nucleotide excision repair, SOS, and topoisomerase I and/or gyrase genes, harboring plasmids containing the full-length tract, six 5'-truncations of the tract, and a control plasmid (pSPL3). The full-length poly(R.Y) tract induced dramatic losses of cell viability during the first few hours of growth and lengthened the doubling times of the populations in strains with an inducible SOS response. The extent of cell loss was correlated with the length of the poly(R.Y) tract and the levels of negative supercoiling as modulated by the genotype of the strains or drugs that specifically inhibited DNA gyrase or bound to DNA directly, thereby affecting conformations at specific loci. We conclude that the unusual DNA conformations formed by the PKD1 poly(R.Y) tract under the influence of negative supercoiling induced the SOS response pathway, and they were recognized as lesions by the nucleotide excision repair system and were cleaved, causing delays in cell division and loss of the plasmid. These data support a role for this sequence in the mutation of the PKD1 gene by stimulating repair and/or recombination functions.

Human disease: calcium signaling in polycystic kidney disease

Polycystic kidney disease results from loss of function of either of two novel proteins, polycystin-1 or polycystin-2. Recent studies show that intracellular calcium signaling is important in kidney development, and define defects in this signaling pathway as the basis of cyst formation in polycystic kidney disease.

Molecular genetics and pathogenesis of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common and systemic disease characterized by formation of focal cysts. Of the three potential causes of cysts, downstream obstruction, compositional changes in extracellular matrix, and proliferation of partially dedifferentiated cells, evidence strongly supports the latter as the primary abnormality. In the vast majority of cases, the disease is caused by mutations in PKD1 or PKD2, and appears to be recessive at the cellular level. Somatic second hits in the normal allele of cells containing the germ line mutation initiate or accelerate formation of cysts. The intrinsically high frequency of somatic second hits in epithelia appears to be sufficient to explain the frequent occurrence of somatic second hits in the disease-causing genes. PKD1 and PKD2 encode a putative adhesive/ion channel regulatory protein and an ion channel, respectively. The two proteins interact directly in vitro. Their cellular and subcellular localization suggest that they may also function independently in a common signaling pathway that may involve the membrane skeleton and that links cell-cell and cell-matrix adhesion to the development of cell polarity.

Transport function of the naturally occurring pathogenic polycystin-2 mutant, R742X

Most patients with autosomal dominant polycystic kidney disease (ADPKD) harbor mutations truncating polycystin-1 (PC1) or polycystin-2 (PC2), products of the PKD1 and PKD2 genes, respectively. A third member of the polycystin family, polycystin-L (PCL), was recently shown to function as a Ca(2+)-modulated nonselective cation channel. More recently, PC2 was also shown to be a nonselective cation channel with comparable properties to PCL, though the membrane targeting of PC2 likely varies with cell types. Here we show that PC2 expressed heterologously in Xenopus oocytes is targeted to intracellular compartments. By contrast, a truncated form of mouse PC2 corresponding to a naturally occurring human mutation R742X is targeted predominantly to the plasma membrane where it mediates K(+), Na(+), and Ca(2+) currents. Unlike PCL, the truncated form does not display Ca(2+)-activated transport activities, possibly due to loss of an EF-hand at the C-terminus. We propose that PC2 forms ion channels utilizing structural components which are preserved in the R742X form of the protein. Implications for epithelial cell signaling are discussed.

Novel mutations in the duplicated region of the polycystic kidney disease 1 (PKD1) gene provides supporting evidence for gene conversion

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common human single-gene disorders, and is the most common inherited form of cystic kidney disease. It is estimated that approximately 85% of ADPKD is due to mutations in the PKD1 gene, which is located on chromosome 16p13.3. Mutation analysis in this gene is difficult, because more than two-thirds of reiterated several times at 16p13.1. In this study, mutation screening in 90 ADPKD patients was carried out on exons in the duplicated region of the PKD1 gene (23-34), using genomic long-range PCR followed by nested PCR and single-strand conformation polymorphism (SSCP), and finally cycle sequencing. Two nonconservative missense mutations were detected in exons 25 and 31, and two conservative mutations were found in exons 24 and 29. A novel splicing mutation, which is expected to cause skipping of exon 30, was detected in one case. Moreover, six intronic variants, three silent variants, and one polymorphic variant were detected in this study. Comparison between some of these changes and published sequences from the homologous genes on 16p13.1, revealed supporting evidence for the gene conversion theory as a mechanism responsible for some of the mutations in the PKD1 gene. Factors likely to facilitate gene conversion in this region of the PKD1 gene are discussed.

A "two-hit" model of cystogenesis in autosomal dominant polycystic kidney disease?

An intriguing feature of autosomal dominant polycystic kidney disease (ADPKD) is the focal and sporadic nature of individual cyst formation. Typically, only a few renal cysts are detectable in an affected individual during the first two decades of life. By the fifth decade, however, hundreds to thousands of renal cysts can be found in most patients. Additionally, significant intra-familial variability of ADPKD has been well documented. Taken together, these findings suggest that factor(s) in addition to the germline mutation of a polycystic kidney disease gene might be required for individual cyst formation. Indeed, recent studies have provided compelling evidence in support of a "two-hit" model of cystogenesis in ADPKD. In this model, inactivation of both copies of a polycystic kidney disease gene by germline and somatic mutations within an epithelial cell provides growth advantages for it to proliferate clonally into a cyst. This article highlights key findings of these recent studies and discusses the controversies and implications of the "two-hit" model in ADPKD.

Polycystin: new aspects of structure, function, and regulation

Polycystin-1 is a modular membrane protein with a long extracellular N-terminal portion that bears several ligand-binding domains, 11 transmembrane domains, and a > or =200 amino acid intracellular C-terminal portion with several phosphorylation signaling sites. Polycystin-1 is highly expressed in the basal membranes of ureteric bud epithelia during early development of the metanephric kidney, and disruption of the PKD1 gene in mice leads to cystic kidneys and embryonic or perinatal death. It is proposed that polycystin-1 functions as a matrix receptor to link the extracellular matrix to the actin cytoskeleton via focal adhesion proteins. Co-localization, co-sedimentation, and co-immunoprecipitation studies show that polycystin-1 forms multiprotein complexes with alpha2beta1-integrin, talin, vinculin, paxillin, p130cas, focal adhesion kinase, and c-src in normal human fetal collecting tubules and sub-confluent epithelial cultures. In normal adult kidneys and confluent epithelial cultures, polycystin-1 is downregulated and forms complexes with the cell-cell adherens junction proteins E-cadherin and beta-, gamma-, and alpha-catenin. Polycystin-1 activation at the cell membrane leads to intracellular signaling via phosphorylation through the c-Jun terminal kinase and wnt pathways leading to activation of AP-1 and TCF/LEF-dependent genes, respectively. The C-terminal of polcystin-1 has been shown to be phosphorylated by c-src at Y4237, by protein kinase A at S4252, and by focal adhesion kinase and protein kinase X at yet-to-be identified residues. Inhibition of tyrosine phosphorylation or increased cellular calcium increases polycystin-1 focal adhesion complexes versus polycystin-1 adherens junction complexes, whereas disruption of the actin cytoskeleton dissociates all polycystin-1 complexes. Genetic evidence suggests that PKD1, PKD2, NPHP1, and tensin are in the same pathway.

Polycystin-2 is a novel cation channel implicated in defective intracellular Ca(2+) homeostasis in polycystic kidney disease

Mutations in polycystins-1 and -2 (PC1 and PC2) cause autosomal dominant polycystic kidney disease (ADPKD), which is characterized by progressive development of epithelial renal cysts, ultimately leading to renal failure. The functions of these polycystins remain elusive. Here we show that PC2 is a Ca(2+)-permeable cation channel with properties distinct from any known intracellular channels. Its kinetic behavior is characterized by frequent transitions between closed and open states over a wide voltage range. The activity of the PC2 channel is transiently increased by elevating cytosolic Ca(2+). Given the predominant endoplasmic reticulum (ER) location of PC2 and its unresponsiveness to the known modulators of mediating Ca(2+) release from the ER, inositol-trisphosphate (IP(3)) and ryanodine, these results suggest that PC2 represents a novel type of channel with properties distinct from those of the other Ca(2+)-release channels. Our data also show that the PC2 channel can be translocated to the plasma membranes by defined chemical chaperones and proteasome modulators, suggesting that in vivo, it may also function in the plasma membrane under specific conditions. The sensitivity of the PC2 channel to changes of intracellular Ca(2+) concentration is deficient in a mutant found in ADPKD patients. The dysfunction of such mutants may result in defective coupling of PC2 to intracellular Ca(2+) homeostasis associated with the pathogenesis of ADPKD.

A putative prokaryote voltage-gated Ca(2+) channel with only one 6TM motif per subunit

Until now, voltage-gated Ca(2+) channel proteins have been found only in eukaryotes. Here we report that a gene recently discovered in the eubacterium Bacillus halodurans codes for a protein closely related to eukaryotic Ca(2+) channels, but that has only one 6-transmembrane-segement (6TM) motif, instead of four, in its pore-forming subunit. This is supported by the comparison of consensus sequences, which, along with the patterns of residue conservation, indicates a similar structure in the membrane to voltage-gated K(+) channels. From this we hypothesize that Ca(2+) channels originally evolved in bacteria, and that the specific eubacteria protein highlighted here is an ideal candidate for structure determination efforts.

Identification of the gene for oral-facial-digital type I syndrome

Oral-facial-digital type 1 syndrome (OFD1 [MIM 311200]) is transmitted as an X-linked dominant condition with lethality in males and is characterized by malformations of the face, oral cavity, and digits, and by a highly variable expressivity even within the same family. Malformation of the brain and polycystic kidneys are commonly associated with this disorder. The locus for OFD1 was mapped by linkage analysis to a 12-Mb interval, flanked by markers DXS85 and DXS7105 in the Xp22 region. To identify the gene responsible for this syndrome, we analyzed several transcripts mapping to the region and found mutations in OFD1 (formerly named "Cxorf5/71-7a"), encoding a protein containing coiled-coil alpha-helical domains. Seven patients with OFD1, including three with familial and four with sporadic cases, were analyzed. Analysis of the familial cases revealed a missense mutation, a 19-bp deletion, and a single base-pair deletion leading to a frameshift. In the sporadic cases, we found a missense (de novo), a nonsense, a splice, and a frameshift mutation. RNA in situ studies on mouse embryo tissue sections show that Ofd1 is developmentally regulated and is expressed in all tissues affected in OFD1 syndrome. The involvement of OFD1 in oral-facial-digital type I syndrome demonstrates an important role of this gene in human development.

Lack of association of ACE/angiotensinogen genotype with renal function in autosomal dominant polycystic kidney disease

ACE polymorphisms have recently been shown to associate with worse renal and or cardiovascular outcome, with the D allele widely reported as a risk factor for cardiovascular disease. In autosomal dominant polycystic kidney disease (ADPKD), there are conflicting reports of an association between ACE polymorphisms and disease phenotype. There are no previous reports of any association between angiotensinogen polymorphisms and clinical phenotype in ADPKD. We examined the ACE I/D and angiotensinogen M235T polymorphisms in 176 patients with ADPKD. Patients are categorized into three groups according to the reason for initial investigation. Clinical history and examination findings were recorded at the time of first referral. A cohort of 17 patients had progressive renal impairment observed after 3 or more years of follow-up. Reciprocal creatinine against time was plotted in this group. From the patient population of 176, a total of 33 patients reached end-stage renal failure (ESRF) or a serum creatinine greater than 500 microm/liter. ACE genotype and M235T polymorphism frequencies were compared across groups. Serum creatinine and presence of hypertension and onset of ESRF were taken as outcome variables; age and source of referral were taken as confounding variables. There was no association of any genotype or allele with either creatinine, inverse creatinine, hypertension, or age at end-stage renal failure. These findings do not support the proposition that ACE genotype or angiotensinogen polymorphisms are associated with a worse prognosis in patients with ADPKD.

The cytoplasmic C-terminal fragment of polycystin-1 regulates a Ca2+-permeable cation channel

The cytoplasmic C-terminal portion of the polycystin-1 polypeptide (PKD1(1-226)) regulates several important cell signaling pathways, and its deletion suffices to cause autosomal dominant polycystic kidney disease. However, a functional link between PKD1 and the ion transport processes required to drive renal cyst enlargement has remained elusive. We report here that expression at the Xenopus oocyte surface of a transmembrane fusion protein encoding the C-terminal portion of the PKD1 cytoplasmic tail, PKD1(115-226), but not the N-terminal portion, induced a large, Ca(2+)-permeable cation current, which shifted oocyte reversal potential (E(rev)) by +33 mV. Whole cell currents were sensitive to inhibition by La(3+), Gd(3+), and Zn(2+), and partially inhibited by SKF96365 and amiloride. Currents were not activated by bath hypertonicity, but were inhibited by acid pH. Outside-out patches pulled from PKD1(115-226)-expressing oocytes exhibited a 5.1-fold increased NP(o) of endogenous 20-picosiemens cation channels of linear conductance. PKD1(115-226)-injected oocytes also exhibited elevated NP(o) of unitary calcium currents in outside-out and cell-attached patches, and elevated calcium permeability documented by fluorescence ratio and (45)Ca(2+) flux experiments. Both Ca(2+) conductance and influx were inhibited by La(3+). Mutation of candidate phosphorylation sites within PKD1(115-226) abolished the cation current. We conclude that the C-terminal cytoplasmic tail of PKD1 up-regulates inward current that includes a major contribution from Ca(2+)-permeable nonspecific cation channels. Dysregulation of these or similar channels in autosomal dominant polycystic kidney disease may contribute to cyst formation or expansion.

Bilineal disease and trans-heterozygotes in autosomal dominant polycystic kidney disease

In searching for a putative third gene for autosomal dominant polycystic kidney disease (ADPKD), we studied the genetic inheritance of a large family (NFL10) previously excluded from linkage to both the PKD1 locus and the PKD2 locus. We screened 48 members of the NFL10 pedigree, by ultrasonography, and genotyped them, with informative markers, at both the PKD1 locus and the PKD2 locus. Twenty-eight of 48 individuals assessed were affected with ADPKD. Inspection of the haplotypes of these individuals suggested the possibility of bilineal disease from independently segregating PKD1 and PKD2 mutations. Using single-stranded conformational analysis, we screened for and found a PKD2 mutation (i.e., 2152delA; L736X) in 12 affected pedigree members. Additionally, when the disease status of these individuals was coded as "unknown" in linkage analysis, we also found, with markers at the PKD1 locus, significant LOD scores (i.e., >3.0). These findings strongly support the presence of a PKD1 mutation in 15 other affected pedigree members, who lack the PKD2 mutation. Two additional affected individuals had trans-heterozygous mutations involving both genes, and they had renal disease that was more severe than that in affected individuals who had either mutation alone. This is the first documentation of bilineal disease in ADPKD. In humans, trans-heterozygous mutations involving both PKD1 and PKD2 are not necessarily embryonically lethal. However, the disease associated with the presence of both mutations appears to be more severe than the disease associated with either mutation alone. The presence of bilineal disease as a confounder needs to be considered seriously in the search for the elusive PKD3 locus.

Polycystin-2, the protein mutated in autosomal dominant polycystic kidney disease (ADPKD), is a Ca2+-permeable nonselective cation channel

Defects in polycystin-2, a ubiquitous transmembrane glycoprotein of unknown function, is a major cause of autosomal dominant polycystic kidney disease (ADPKD), whose manifestation entails the development of fluid-filled cysts in target organs. Here, we demonstrate that polycystin-2 is present in term human syncytiotrophoblast, where it behaves as a nonselective cation channel. Lipid bilayer reconstitution of polycystin-2-positive human syncytiotrophoblast apical membranes displayed a nonselective cation channel with multiple subconductance states, and a high perm-selectivity to Ca2+. This channel was inhibited by anti-polycystin-2 antibody, Ca2+, La3+, Gd3+, and the diuretic amiloride. Channel function by polycystin-2 was confirmed by patch-clamping experiments of polycystin-2 heterologously infected Sf9 insect cells. Further, purified insect cell-derived recombinant polycystin-2 and in vitro translated human polycystin-2 had similar ion channel activity. The polycystin-2 channel may be associated with fluid accumulation and/or ion transport regulation in target epithelia, including placenta. Dysregulation of this channel provides a mechanism for the onset and progression of ADPKD.

Current advances in molecular genetics of autosomal-dominant polycystic kidney disease

Autosomal-dominant polycystic kidney disease results from at least two causal genes, PKD1 and PKD2. The identical clinical phenotype in human patients and targeted Pkd1 and Pkd2 mutant mouse models provides evidence that both gene products act in the same pathogenic pathway. The discovery of direct PKD1 and PKD2 interactions implies that both gene products, polycystin-1 and polycystin-2, play a functional role in the same molecular complex. The spectrum of germ-line mutations in both genes and the somatic mutations identified from individual PKD1 or PKD2 cysts indicate that loss of function of either PKD1 or PKD2 is the mechanism of cystogenesis in autosomal-dominant polycystic kidney disease. A novel mouse model, Pkd2WS25/-, has proved that loss of heterozygosity is the molecular mechanism of autosomal-dominant polycystic kidney disease. Recently, studies on the expression patterns of PKD1 and PKD2 in humans or mice indicate that polycystin 1 and polycystin 2 seem to have their own respective functional roles, even though most of the functions of these polycystins are parallel during human and mouse development. Pkd2-deficient mice have cardiac septum defects, but Pkd1 knockout mice do not have this phenotype. On the other hand, Pkd2 has a very low level of expression in the central nervous system when compared with Pkd1. In addition, the level of expression of Pkd1 is increased during mesenchymal condensation, whereas Pkd2 expression is unchanged. Preliminary data have shown that the PKD1/PKD2 compound trans-heterozygous has a more severe cystic phenotype in the kidney than that of an age-matched heterozygous type 1 or type 2 of autosomal-dominant polycystic kidney disease alone. This finding suggests that PKD1 may be a modifier of disease severity for PKD2, and vice versa. The characteristics of the contiguous PKD1/TSC2 syndrome phenotypes and the data from Krd mice imply that TSC2 and PAX2 may also serve as potential modifiers for the disease severity of autosomal-dominant polycystic kidney disease.

Vascular expression of polycystin-2

The expression of polycystin-1 in the vascular smooth muscle cells (VSMC) of elastic and large distributive arteries suggests that some vascular manifestations of autosomal-dominant polycystic kidney disease (ADPKD) result directly from the genetic defect. Intracranial aneurysms have been reported in PKD2, as well as in PKD1 families. To determine whether the vascular expression of polycystin-2 is similar to that of polycystin-1, the expression of PKD2 mRNA and protein in cultured pig aortic VSMC was studied and immunofluorescence and immunohistochemistry were used to study the localization of polycystin-2 in cultured pig aortic VSMC, pig ascending thoracic aorta, and normal elastic and intracranial arteries and intracranial aneurysms obtained at autopsy from patients without or with ADPKD. Tissues derived from Pkd2 wild-type and Pkd2 null mice were used to confirm the specificity of the immunostaining for polycystin-2. Northern blots of VSMC revealed the expected 5.3-kb band. Western blotting detected a 110-kb band in a 100,000 x g fraction of VSMC homogenates. Cultured VSMC as well as VSMC between the elastic lamellae of pig thoracic aorta were positive for polycystin-2 by immunofluorescence. The staining pattern was cytoplasmic. Treatment of the cells before fixation with Taxol, colchicine, or cytochalasin-D altered the pattern of staining in a way suggesting alignment with the cytoskeleton. The immunohistochemical staining for polycystin-2 was abolished by extraction with 0.5% Triton X-100, indicating that polycystin-2 is not associated with the cytoskeleton. Weak immunoreactivity for polycystin-2, which was markedly enhanced by protease digestion, was detected in formaldehyde-fixed normal human elastic and intracranial arteries. Immunostaining of variable intensity for polycystin-2, which was not consistently enhanced by protease digestion, was seen in the spindle-shaped cells of the wall of the intracranial aneurysms. The similar expression of polycystin-1 and polycystin-2 in the vascular smooth muscle is consistent with the proposed interaction of these proteins in a single pathway. These observations suggest a direct pathogenic role for PKD1 and PKD2 mutations in the vascular complications of ADPKD.

Co-assembly of polycystin-1 and -2 produces unique cation-permeable currents

The human kidney is composed of roughly 1.2-million renal tubules that must maintain their tubular structure to function properly. In autosomal dominant polycystic kidney disease (ADPKD) cysts develop from renal tubules and enlarge independently, in a process that ultimately causes renal failure in 50% of affected individuals. Mutations in either PKD1 or PKD2 are associated with ADPKD but the function of these genes is unknown. PKD1 is thought to encode a membrane protein, polycystin-1, involved in cell-cell or cell-matrix interactions, whereas the PKD2 gene product, polycystin-2, is thought to be a channel protein. Here we show that polycystin-1 and -2 interact to produce new calcium-permeable non-selective cation currents. Neither polycystin-1 nor -2 alone is capable of producing currents. Moreover, disease-associated mutant forms of either polycystin protein that are incapable of heterodimerization do not result in new channel activity. We also show that polycystin-2 is localized in the cell in the absence of polycystin-1, but is translocated to the plasma membrane in its presence. Thus, polycystin-1 and -2 co-assemble at the plasma membrane to produce a new channel and to regulate renal tubular morphology and function.

Modification of the composition of polycystin-1 multiprotein complexes by calcium and tyrosine phosphorylation

Mutations in the PKD1 gene are responsible for >85% of autosomal dominant polycystic kidney disease (ADPKD). The protein product of PKD1, polycystin-1, is a large, modular membrane protein, with putative ligand-binding motifs in the extracelluar N-terminal portion, 9-11 transmembrane domains and an intracellular C-terminal portion with phosphorylation sites. A role for polycystin-1 as a cell surface receptor involved in cell-matrix and cell-cell interactions has been proposed. In this study, we have analyzed polycystin-1 and associated protein distribution in normal human epithelial cells and examined the role of cell-matrix versus cell-cell interactions in regulation of the assembly of polycystin-1 multiprotein complexes. Immunocytochemistry, sucrose density gradient sedimentation, co-immunoprecipitation analyses and in vitro binding assays have shown that polycystin-1 associates with the focal adhesion proteins talin, vinculin, p130Cas, FAK, alpha-actinin, paxillin and pp60c-src in subconfluent normal human fetal collecting tubule (HFCT) epithelia when cell-matrix interactions predominate. Polycystin-1 also forms higher S value complexes with the cell-cell adherens junction proteins E-cadherin, beta- and gamma-catenins in confluent cultures when cell-cell interactions are predominant. Polycystin-1 multiprotein complexes can be disrupted by cytochalasin D but not by colchicine, suggesting involvement of the actin cytoskeleton. Although inhibition of tyrosine phosphorylation by tyrphostin inhibits polycystin-1-FAK interactions, E-cadherin interactions are enhanced. High calcium treatment also increases polycystin-1-E-cadherin interactions.

Polycystin-1, the gene product of PKD1, induces resistance to apoptosis and spontaneous tubulogenesis in MDCK cells

The major form of autosomal dominant polycystic kidney disease (ADPKD) results from mutation of a gene (PKD1) of unknown function that is essential for the later stages of renal tubular differentiation. In this report, we describe a novel cell culture system for studying how PKD1 regulates this process. We show that expression of human PKD1 in MDCK cells slows their growth and protects them from programmed cell death. MDCK cells expressing PKD1 also spontaneously form branching tubules while control cells form simple cysts. Increased cell proliferation and apoptosis have been implicated in the pathogenesis of cystic diseases. Our study suggests that PKD1 may function to regulate both pathways, allowing cells to enter a differentiation pathway that results in tubule formation.

Chlamydomonas IFT88 and its mouse homologue, polycystic kidney disease gene tg737, are required for assembly of cilia and flagella

Intraflagellar transport (IFT) is a rapid movement of multi-subunit protein particles along flagellar microtubules and is required for assembly and maintenance of eukaryotic flagella. We cloned and sequenced a Chlamydomonas cDNA encoding the IFT88 subunit of the IFT particle and identified a Chlamydomonas insertional mutant that is missing this gene. The phenotype of this mutant is normal except for the complete absence of flagella. IFT88 is homologous to mouse and human genes called Tg737. Mice with defects in Tg737 die shortly after birth from polycystic kidney disease. We show that the primary cilia in the kidney of Tg737 mutant mice are shorter than normal. This indicates that IFT is important for primary cilia assembly in mammals. It is likely that primary cilia have an important function in the kidney and that defects in their assembly can lead to polycystic kidney disease.

In vivo interaction of the adapter protein CD2-associated protein with the type 2 polycystic kidney disease protein, polycystin-2

We identified a developmentally regulated gene from mouse kidney whose expression is up-regulated in metanephrogenic mesenchyme cells when they are induced to differentiate to epithelial cells during kidney organogenesis. The deduced 70.5-kDa protein, originally named METS-1 (mesenchyme-to-epithelium transition protein with SH3 domains), has since been cloned as a CD2-associated protein (CD2AP). CD2AP is strongly expressed in glomerular podocytes, and the absence of CD2AP in mice results in congenital nephrotic syndrome. We have found that METS-1/CD2AP (hereafter referred to as CD2AP) is expressed at lower levels in renal tubular epithelial cells in the adult kidney, particularly in distal nephron segments. Independent yeast two-hybrid screens using the COOH-terminal region of either CD2AP or polycystin-2 as bait identified the COOH termini of polycystin-2 and CD2AP, respectively, as strong interacting partners. This interaction was confirmed in cultured cells by co-immunoprecipitation of endogenous polycystin-2 with endogenous CD2AP and vice versa. CD2AP shows a diffuse reticular cytoplasmic and perinuclear pattern of distribution, similar to polycystin-2, in cultured cells, and the two proteins co-localize by indirect double immunofluorescence microscopy. CD2AP is an adapter molecule that associates with a variety of membrane proteins to organize the cytoskeleton around a polarized site. Such a function fits well with that hypothesized for the polycystin proteins in renal tubular epithelial cells, and the present findings suggest that CD2AP has a role in polycystin-2 function.

Thirteen novel mutations of the replicated region of PKD1 in an Asian population

BACKGROUND

Mutations of PKD1 are thought to account for approximately 85% of all mutations in autosomal dominant polycystic kidney disease (ADPKD). The search for PKD1 mutations has been hindered by both its large size and complicated genomic structure. To date, few mutations that affect the replicated segment of PKD1 have been described, and virtually all have been reported in Caucasian patients.

METHODS

In the present study, we have used a long-range polymerase chain reaction (PCR)-based strategy previously developed by our laboratory to analyze exons in the replicated region of PKD1 in a population of 41 unrelated Thai and 6 unrelated Korean families with ADPKD. We have amplified approximately 3.5 and approximately 5 kb PKD1 gene-specific fragments (5'MR and 5'LR) containing exons 13 to 15 and 15 to 21 and performed single-stand conformation analysis (SSCA) on nested PCR products.

RESULTS

Nine novel pathogenic mutations were detected, including six nonsense and three frameshift mutations. One of the deletions was shown to be a de novo mutation. Four potentially pathogenic variants, including one 3 bp insertion and three missense mutations, were also discovered. Two of the nonconservative amino acid substitutions were predicted to disrupt the three-dimensional structure of the PKD repeats. In addition, six polymorphisms, including two missense and four silent nucleotide substitutions, were identified. Approximately 25% of both the pathogenic and normal variants were found to be present in at least one of the homologous loci.

CONCLUSION

To our knowledge, this is the first report of mutation analysis of the replicated region of PKD1 in a non-Caucasian population. The methods used in this study are widely applicable and can be used to characterize PKD1 in a number of ethnic groups using DNA samples prepared using standard techniques. Our data suggest that gene conversion may play a significant role in producing variability of the PKD1 sequence in this population. The identification of additional mutations will help guide the study of polycystin-1 and better help us to understand the pathophysiology of this common disease.

Increased prevalence of polycystic kidney disease type 2 among elderly polycystic patients

Autosomal dominant polycystic kidney disease (ADPKD) is genetically heterogeneous, with at least three chromosomal loci (PKD1, PKD2, and PKD3) accounting for the disease. Mutations in the PKD2 gene, on the long arm of chromosome 4, are estimated to be responsible for 15% of the cases of ADPKD, based on linkage studies. PKD2 is a milder form of the disease, with a mean age of end-stage renal disease (ESRD) approximately 20 years later than PKD1. The object of this study is to determine the proportion of elderly patients with ADPKD with ESRD who harbor mutations in the PKD2 gene. We analyzed all exons and intron-exon boundaries of the PKD2 gene by single-strand conformation polymorphism analysis and silver staining technique in 46 patients with ADPKD who reached ESRD after the age of 63 years or were not yet undergoing renal replacement therapy (RRT) by that age. We performed exactly the same studies in a control group of 40 patients with ADPKD with unknown gene status aged younger than 63 years. In 22 patients, a mutation in the PKD2 gene was defined: 18 of 46 patients from the elderly group and 4 of 40 patients from the control group. We identified 14 different mutations: 4 nonsense mutations, 1 missense mutation, 5 small deletions, 2 insertions, 1 deletion of the whole PKD2 gene, and 1 splicing mutation. Five of these mutations previously were described by our group. Three of the mutations reported in the present study are recurrent. The prevalence of PKD2 disease among elderly patients with ADPKD undergoing RRT is 39.1%, almost three times the prevalence of the disease in the general ADPKD population.

Screening the 3' region of the polycystic kidney disease 1 (PKD1) gene in 41 Bulgarian and Australian kindreds reveals a prevalence of protein truncating mutations

Screening for disease-causing mutations in the unique region of the polycystic kidney disease 1 (PKD1) gene was performed in 41 unrelated individuals with autosomal dominant polycystic kidney disease. Exons 34-41 and 43-46 were assayed using PCR amplification and SSCP analysis followed by direct sequencing of amplicons presenting variant SSCP patterns. We have identified seven disease-causing mutations of which five are novel [c.10634-10656del; c.11587delG; IVS37-10C>A; c.11669-11674del; c.13069-13070ins39] and two have been reported previously [Q4010X; Q4041X]. Defects in this part of the gene thus account for 17% of our group of patients. Five of the seven sequence alterations detected are protein-truncating which is in agreement with mutation screening data for this part of the gene by other groups. The two other mutations are in-frame deletions or insertions which could destroy important functional properties of polycystin 1. These findings suggest that the first step toward cyst formation in PKD1 patients is the loss of one functional copy of polycystin 1, which indirectly supports the "two-hit" model of cystogenesis where a second somatic mutation inactivating the normal allele is necessary to occur for development of the disease condition.

Interaction between LIS1 and doublecortin, two lissencephaly gene products

Mutations in either LIS1 or DCX are the most common cause for type I lissencephaly. Here we report that LIS1 and DCX interact physically both in vitro and in vivo. Epitope-tagged DCX transiently expressed in COS cells can be co-immunoprecipitated with endogenous LIS1. Furthermore, endogenous DCX could be co-immunoprecipitated with endogenous LIS1 in embryonic brain extracts, demonstrating an in vivo association. The two protein products also co-localize in transfected cells and in primary neuronal cells. In addition, we demonstrate homodimerization of DCX in vitro. Using fragments of both LIS1 and DCX, the domains of interaction were mapped. LIS1 and DCX interact with tubulin and microtubules. Our results suggest that addition of DCX and LIS1 to tubulin enhances polymerization in an additive fashion. In in vitro competition assays, when LIS1 is added first, DCX competes with LIS1 in its binding to microtubules, but when DCX is added prior to the addition of LIS1 it enhances the binding of LIS1 to microtubules. We conclude that LIS1 and DCX cross-talk is important to microtubule function in the developing cerebral cortex.

Molecular genetics of nephronophthisis and medullary cystic kidney disease

Nephronophthisis (NPH) and medullary cystic kidney disease (MCKD) constitute a group of renal cystic diseases that share the macroscopic feature of cyst development at the corticomedullary border of the kidneys. The disease variants also have in common a characteristic renal histologic triad of tubular basement membrane disintegration, tubular atrophy with cyst development, and interstitial cell infiltration with fibrosis. NPH and, in most instances, MCKD lead to chronic renal failure with an onset in the first two decades of life for recessive NPH and onset in adult life for autosomal dominant MCKD. There is extensive genetic heterogeneity with at least three different loci for NPH (NPHP1, NPHP2, and NPHP3) and two different loci for MCKD (MCKD1 and MCKD2). Juvenile nephronophthisis, in addition, can be associated with extrarenal organ involvement. As a first step toward understanding the pathogenesis of this disease group, the gene (NPH1) for juvenile nephronophthisis (NPH1) has been identified by positional cloning. Its gene product, nephrocystin, is a novel protein of unknown function that contains a src-homology 3 domain. It is hypothesized that the pathogenesis of NPH might be related to signaling processes at focal adhesions (the contact points between cells and extracellular matrix) and/or adherens junctions (the contact points between cells). This hypothesis is based on the fact that most src-homology 3-containing proteins are part of focal adhesion signaling complexes, on animal models that exhibit an NPH-like phenotype, and on the recent finding that nephrocystin binds to the protein p130(cas), a major mediator of focal adhesion signaling.

ADPKD: a human disease altering Golgi function and basolateral exocytosis in renal epithelia

Epithelial cells explanted from autosomal dominant polycystic kidney disease (ADPKD) tissue exhibit impaired exocytosis, specifically between the Golgi and basolateral membrane (Charron A, Nakamura B, Bacallo R, Wandinger-Ness A. Compromised cytoarchitecture and polarized trafficking in autosomal dominant polycystic kidney disease cells. J Cell Biol 2000; 148: 111-124.). Here the defect is shown to result in the accumulation of the basolateral transport marker vesicular stomatitis virus (VSV) G protein in the Golgi complex. Golgi complex morphology is consequently altered in the disease cells, evident in the noticeable fenestration and dilation of the cisternae. Further detailed microscopic evaluation of normal kidney and ADPKD cells revealed that ineffective basolateral exocytosis correlated with modulations in the localization of select post-Golgi transport effectors. The cytosolic coat proteins p200/myosin II and caveolin exhibited enhanced association with the cytoskeleton or the Golgi of the disease cells, respectively. Most cytoskeletal components with known roles in vesicle translocation or formation were normally arrayed with the exception of Golgi beta-spectrin, which was less prevalent on vesicles. The rab8 GTPase, important for basolateral vesicle targeting, was redistributed from the perinuclear Golgi region to disperse vesicles in ADPKD cells. At the basolateral membrane of ADPKD cells, there was a notable loss of the exocyst components sec6/sec8 and an unidentified syntaxin. It is postulated that dysregulated basolateral transport effector function precipitates the disruption of basolateral exocytosis and dilation of the ADPKD cell Golgi as basolateral cargo accumulates within the cisternae.

The pathogenesis of autosomal dominant polycystic kidney disease: an update

The identification of PKD1 and PKD2, the two major genes responsible for autosomal dominant polycystic kidney disease, are the seminal discoveries upon which much of the current investigation into the pathogenesis of this common heritable disease is based. A major mechanistic insight was achieved with the discovery that autosomal dominant polycystic kidney disease occurs by a two-hit mechanism requiring somatic inactivation of the normal allele in individual polarized epithelial cells. Most recent advances are focused on the function of the respective protein products, polycystin-1 and polycystin-2. Indirect evidence supports an interaction between polycystin-1 and -2, albeit it is unlikely that they work in concert in all tissues and at all times. They associate in yeast two hybrid and cotransfection assays and there is a striking similarity in the renal and pancreatic cystic phenotypes of Pkd2-/- and Pkd1del34/del34 mice. Also, the respective homologues of both proteins are expressed in the same sensory neuronal cells in the nematode and the human disease phenotypes remain completely overlapping with the major difference being in relative severity. Mounting evidence supports the hypothesis that polycystin-1 is a cell surface receptor. A close homologue in the sea urchin sperm mediates the acrosome reaction in response to contact with egg-jelly, the nematode homologue functions in mechano- or chemosensation, and the solution structure of the repeated extracellular polycystic kidney disease domains reveals a beta-sandwich fold commonly found in surface receptor molecules. Indirect evidence also supports the initial hypothesis that polycystin-2 is a calcium channel subunit. Several closely related homologues retain the calcium channel signature motif but differ in their predicted interaction domains, and one of these homologues has been shown to be a calcium regulated cation channel. Several important distinctions in polcystin-1 and -2 function have also been discovered. Polycystin-2 has a role in cardiac development that polcystin-1 does not. High level polycystin-2 expression in renal epithelial cells coincides with maturation and elongation of tubules and, unlike polycystin-1, persists into adulthood. In cells in tissue culture, polycystin-2 is expressed exclusively in the endoplasmic reticulum whilst the cellular expression of polycystin-1 remains unknown. Overall, the difficult task of understanding the autosomal dominant polycystic disease process is proceeding apace.

Quantitative trait loci modulate renal cystic disease severity in the mouse bpk model

Numerous mouse models of polycystic kidney disease (PKD) have been described in which the mutant phenotypes closely resemble human PKD with regard to morphology, cyst localization, and disease progression. As in human PKD, genetic background affects the disease phenotype in mouse PKD models. Using experimental crosses, these modifying effects can be dissected into discrete genetic factors referred to as quantitative trait loci. The locus for the mouse bpk model was recently mapped to chromosome (Chr) 10. In the course of these studies, marked variability was observed in the renal cystic disease expressed in F2 bpk/bpk homozygotes of a (BALB/c-+/bpk x CAST/Ei)F1 intercross. The current study was undertaken to further characterize the renal cystic disease as quantitative trait in this F2 cohort and to map the genetic modifiers that modulate this phenotype. Whole-genome scans revealed a CAST-derived locus on distal Chr 6, near D6Mit14, that affects renal cystic disease severity. Additional analyses identified loci on Chr 1, Chr 2, and Chr 4, as well as a possible interaction between the Chr 6 locus and a locus on distal Chr 1, near D1Mit17. Interestingly, the gene encoding RGS7, a regulator of G protein signaling that binds to polycystin-1, was mapped to the same Chr 1 interval. It is concluded that the severity of the bpk renal cystic disease phenotype is modulated by multiple loci and possibly by epistatic interaction among them. It is hypothesized that the gene encoding the polycystin-binding partner RGS7 is a candidate for the Chr 1 genetic modifier.

Mutations of PKD1 in ADPKD2 cysts suggest a pathogenic effect of trans-heterozygous mutations

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 and PKD2. The products of these genes associate to form heteromeric complexes. Several models have been proposed to explain the mechanism of cyst formation. Here we find somatic mutations of PKD2 in 71% of ADPKD2 cysts analysed. Clonal somatic mutations of PKD1 were identified in a subset of cysts that lacked PKD2 mutations.

Distinct and common developmental expression patterns of the murine Pkd2 and Pkd1 genes

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most commonly inherited renal diseases. At least two genes, PKD2 and PKD1 are implicated in the development of this disease. Our pathogenetic studies showed that the human and murine polycystic kidney disease (PKD) involves failure to switch out of a renal developmental program. We have thus undertaken a detailed comparative expression analysis of Pkd2 and Pkd1 from the morula stage to adulthood. Pkd2 expression was detected as early as the morula and blastocyst stages as observed for Pkd1. Strong Pkd2 expression, similar to Pkd1, was displayed in all mesenchymal and cartilaginous tissues during mouse development. However major differences in Pkd2 expression in comparison to Pkd1 were identified. First, in contrast to Pkd1, the neural crest cell-derived tissues displayed a low to undetectable Pkd2 expression at all ages. Second, no increase in Pkd2 expression was detected during mesenchymal condensation. Third, high Pkd2 expression in the kidneys was localized mainly to the tubular epithelium of the cortical region from murine development to adulthood.

Cellular and subcellular distribution of polycystin-2, the protein product of the PKD2 gene

Mutations in the PKD1 and PKD2 genes account for 85 and 15% of cases of autosomal dominant polycystic kidney disease, respectively. Polycystin-2, the product of the PKD2 gene, is predicted to be an integral membrane protein with homology to a family of voltage-activated Ca(2+) channels. In vitro studies suggest that it may interact with polycystin-1, the PKD1 gene product, via coiled-coil domains present in their C-terminal domains. In this study, the cellular and subcellular distribution of polycystin-2 is defined and compared with polycystin-1. A panel of rabbit polyclonal antisera was raised against polycystin-2 and shown to recognize a single band consistent with polycystin-2 in multiple tissues and cell lines by immunoprecipitation and Western blotting. Immunostaining of human and murine renal tissues demonstrated widespread and developmentally regulated expression of polycytin-2, with highest levels in the kidney in the thick ascending limbs of the loop of Henle and the distal convoluted tubule. In contrast, polycystin-1 expression, while localizing to the same tubular segments, was highest in the collecting ducts. Immunohistochemical staining and immunofluorescence microscopy localized polycystin-2 to the basolateral plasma membrane of kidney tubular epithelial cells compared with the junctional localization of polycystin-1. Differences in the developmental, cellular, and subcellular expression of polycystin-1 and polycystin-2 suggest that they may be able to function independently of each other in addition to a potential in vivo interaction via their C-termini.

The polycystic kidney disease protein PKD2 interacts with Hax-1, a protein associated with the actin cytoskeleton

Despite the recent positional cloning of the PKD1 and PKD2 genes, which are mutated in the great majority of patients with autosomal-dominant polycystic kidney disease (PKD), the pathogenic mechanism for cyst formation is still unclear. The finding, that the PKD1 and PKD2 proteins interact with each other through their COOH termini, suggests that both proteins are part of the same protein complex or signal transduction pathway. Using a yeast two-hybrid screen with the PKD2 protein, we isolated the PKD2-interacting protein Hax-1. The specificity of the interaction was demonstrated by the fact that PKD2L, a protein closely related to PKD2, failed to interact with Hax-1. Immunofluorescence experiments showed that in most cells PKD2 and Hax-1 colocalized in the cell body, but in some cells PKD2 and Hax-1 also were sorted into cellular processes and lamellipodia. Furthermore we demonstrated an association between Hax-1 and the F-actin-binding protein cortactin, which suggests a link between PKD2 and the actin cytoskeleton. We speculate that PKD2 is involved in the formation of cell-matrix contacts, which are dysfunctional without a wild-type PKD2 protein, thus leading to cystic enlargement of tubular structures in the kidney, liver, and pancreas.

Compromised cytoarchitecture and polarized trafficking in autosomal dominant polycystic kidney disease cells

Cystogenesis associated with autosomal dominant polycystic kidney disease (ADPKD) is characterized by perturbations in the polarized phenotype and function of cyst-lining epithelial cells. The polycystins, the protein products of the genes mutated in the majority of ADPKD cases, have been described recently, but the pathological mechanism by which causal mutations result in the mislocalization of cell membrane proteins has remained unclear. This report documents the dissociation from the ADPKD cell basolateral membrane of three molecules essential for spatial organization and exocytosis. The adherens junction protein E-cadherin, the subcellular disposition of which governs intercellular and intracellular architecture, was discovered sequestered in an internal ADPKD cell compartment. At the same time, sec6 and sec8, components of a complex critical for basolateral cargo delivery normally arrayed at the apico-lateral apex, were depleted from the ADPKD cell plasma membrane. An analysis of membrane transport revealed that basolateral trafficking of proteins and lipids was impaired as a result of delayed cargo exit from the ADPKD cell Golgi apparatus. Apical transport proceeded normally. Taken together with recent documentation of an association between polycystin-1 and E-cadherin (Huan and van Adelsberg 1999), the data suggest that causal mutations disrupt E-cadherin-dependent cytoarchitecture, adversely affecting protein assemblies crucial for basolateral trafficking.

Location of mutations within the PKD2 gene influences clinical outcome

BACKGROUND

Since the cloning of the gene for autosomal dominant polycystic kidney disease type 2 (PKD2), approximately 40 different mutations of that gene have been reported to be associated with the disease. The relationship between the PKD2 genotype and phenotype, however, remains unclear.

METHODS

Detailed clinical information was collected for PKD2 families in which the underlying mutation had been identified. Logistic regression analysis was employed to assess the influence of age and sex on hypertension, hematuria, renal calculi, and urinary tract infections, and a clinical phenotype score was computed. Patients were then grouped according to the relative location of their mutation within the cDNA sequence, and differences in the mean phenotypic score between groups were tested for statistical significance by means of a multiple pairwise t-test.

RESULTS

While phenotypic scores for each mutational group revealed a considerable degree of intragroup variability, the variability in phenotypic scores was significantly higher between mutational groups than within groups. A group-wise comparison of the mean phenotypic scores confirmed the observation of significant nonlinear variation in disease severity, with high- and low-scoring mutational groups interspersed along the gene sequence.

CONCLUSION

The identification of groups of mutations in the PKD2 gene, which differ significantly with respect to clinical outcome, is to our knowledge the first description of a genotype/phenotype correlation in autosomal dominant polycystic kidney disease. It also provides evidence against complete loss of function of the mutant PKD2 gene product.

Co-occurrence of autosomal dominant polycystic kidney disease and Marfan syndrome in a kindred

Several reports exist of the co-occurrence of autosomal dominant polycystic kidney disease (ADPKD) and Marfan syndrome, including a report of ADPKD and "overlap" connective tissue disorder in a family with linkage to the PKD1 locus. We report the results of clinical and linkage investigations of an ADPKD family in whom several affected subjects also had aortic vascular complications as well as features of Marfan syndrome. Detailed clinical assessment and linkage analysis were performed with polymorphic microsatellite markers closely linked to the PKD1 and FBN1 loci. Survival data were compared with 10 geographically matched PKD1 families. Although several subjects had features of both ADPKD and Marfan syndrome, detailed clinical examination of the extended family indicated that the two conditions had converged within the kindred. For those with ADPKD, linkage was established to the PKD1 locus (lod score, 6.04). Among those with features of Marfan syndrome, linkage was confirmed to the FBN1 locus (lod score, 1.87). Five of six subjects with both ADPKD and the high-risk FBN1 haplotype had associated vascular complications. In contrast, among the remaining nine individuals with PKD1 alone, seven had aortic assessments, and none were found to have aortic complications. Our experience suggests that when prominent features of connective tissue disease or vascular complications are found in ADPKD patients, alternative additional diagnoses should be considered, including the possibility of a coinherited FBN1 mutation responsible for Marfan syndrome or, alternatively, an associated milder FBN1 phenotype in the absence of sufficient other clinical features to allow Marfan syndrome to be diagnosed.

A novel frameshift mutation (2436insT) produces an immediate stop codon in the autosomal dominant polycystic kidney disease 2 (PKD2) gene

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is a genetically heterogeneous disorder that can be caused by mutations in at least three different genes. Several mutations have been identified in PKD1 and PKD2 genes. Most of the mutations found in PKD2 gene are predicted to cause premature termination of the protein.

METHODS

We analysed an Argentinian family characterized previously as PKD2. The PKD2 gene was amplified from genomic DNA using 17 primer pairs and the products were analysed by heteroduplex analysis. PCR products that showed a variation by heteroduplex analysis were sequenced directly. The mutation was confirmed by sequencing relatives. The segregation of the mutation in this family was verified by restriction endonuclease digestion of PCR products obtained from genomic DNA of all family members. Results and conclusions. Here, we report a novel mutation present in an Argentinian family characterized as PKD2 by linkage analysis. The mutation, shared by all affected members of the family, is a thymidine insertion at position 2436 of the gene, which results in a translation frameshift and creates an immediate stop codon. This mutation is expected to lead to a truncated protein that lacks the interacting domain with the PKD1 gene product. The thymidine insertion abolished a Ddel restriction site, allowing a rapid test for detection of PKD2 carriers in the family.

Reduced Pax2 gene dosage increases apoptosis and slows the progression of renal cystic disease

The murine cpk mouse develops a rapid-onset polycystic kidney disease (PKD) with many similarities to human PKD. During kidney development, the transcription factor Pax2 is required for the specification and differentiation of the renal epithelium. In humans, Pax2 is also expressed in juvenile cystic kidneys where it correlates with cell proliferation. In this report, Pax2 expression is demonstrated in the cystic epithelium of the mouse cpk kidneys. To assess the role of Pax2 during the development of polycystic kidney disease, the progression of renal cysts was examined in cpk mutants carrying one or two alleles of Pax2. Reduced Pax2 gene dosage resulted in a significant inhibition of renal cyst growth while maintaining more normal renal structures. The inhibition of cyst growth was not due to reduced proliferation of the cystic epithelium, rather to increased cell death in the Pax2 heterozygotes. Increased apoptosis with reduced Pax2 gene dosage was also observed in normal developing kidneys. Thus, increased cell death is an integral part of the Pax2 heterozygous phenotype and may be the underlying cause of Pax gene haploinsufficiency. That the cystic epithelium requires Pax2 for continued expansion underscores the embryonic nature of the renal cystic cells and may provide new insights toward growth suppression strategies.

Identification and characterization of a novel polycystin family member, polycystin-L2, in mouse and human: sequence, expression, alternative splicing, and chromosomal localization

Polycystins-1, -2, -L, and -REJ are the four known members of the polycystin family of proteins. In this study, we describe a fifth member of the family, polycystin-L2, encoded by PKD2L2 in human and Pkd2l2 in mouse. Full-length cDNA sequences for both mouse and human polycystin-L2 were obtained from testis cDNA. Sequence analysis predicts that the mouse and human polycystin-L2 proteins consist of 621 and 624 amino acid residues, respectively. Polycystin-L2 has significant homology with polycystins-L and -2, with similarities of 58 and 59%, respectively. Both human and murine polycystin-L2 proteins are predicted to have seven putative transmembrane (TM) domains, and, by comparison with transient receptor potential channels, the six carboxyl-terminal TM domains are likely to constitute an ion channel subunit. Northern blot analysis indicated that mouse Pkd2l2 has an abundant approximately 2.5-kb transcript in testis and an approximately 2.2-kb transcript in heart. RT-PCR analysis showed that the full-length transcript is expressed in human brain, kidney, testis, and HepG2 cells, and there are three alternatively spliced variants that were differentially expressed. PKD2L2 consists of 17 exons spanning approximately 50 kb of genomic DNA. PKD2L2 was mapped to human chromosome 5q31 and Pkd2l2 to mouse chromosome 18 in band C.

Apoptosis in polycystic kidney disease: involvement of caspases

Polycystic kidney disease (PKD) is characterized by the development of large renal cysts and progressive loss of renal function. Although the cause of the development of renal cysts is unknown, recent evidence suggests that excessive apoptosis occurs in PKD. With the use of terminal deoxynucleotidyl transferase dUTP nick-end labeling staining, we have confirmed the presence of apoptotic bodies in cystic kidneys of congenital polycystic kidney (cpk) disease mice carrying a homozygous mutation at 3 wk of age. Apoptosis was localized primarily to the interstitium with little evidence of cell death in cyst epithelium or noncystic tubules. In addition, we observed that the expression of various caspases, bax and bcl-2, was upregulated in cystic kidneys. With the use of various substrates in enzyme activity assays, we have demonstrated a greater than sevenfold increase in caspase 4 activity and a sixfold increase in caspase 3 activity. These data suggest that there is a caspase-dependent apoptosis pathway associated with PKD and support the hypothesis that apoptotic cell death contributes to cyst formation in PKD.

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.

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.

Nephrocystin: gene expression and sequence conservation between human, mouse, and Caenorhabditis elegans

Juvenile nephronophthisis, an autosomal recessive cystic kidney disease, is the primary genetic cause for chronic renal failure in children. The gene (NPHP1) for nephronophthisis type 1 has recently been identified. Its gene product, nephrocystin, is a novel protein of unknown function, which contains a src-homology 3 domain. To study tissue expression and analyze amino acid sequence conservation of nephrocystin, the full-length murine Nphp1 cDNA sequence was obtained and Northern and in situ hybridization analyses were performed for extensive expression studies. The results demonstrate widespread but relatively weak NPHP1 expression in the human adult. In the adult mouse there is strong expression in testis. This expression occurs specifically in cell stages of the first meiotic division and thereafter. In situ hybridization to whole mouse embryos demonstrated widespread and uniform expression at all developmental stages. Amino acid sequence conservation studies in human, mouse, and Caenorhabditis elegans show that in nephrocystin the src-homology 3 domain is embedded in a novel context of other putative domains of protein-protein interaction, such as coiled-coil and E-rich domains. It is concluded that for multiple putative protein-protein interaction domains of nephrocystin, sequence conservation dates back at least to Caenorhabditis elegans. The previously described discrepancy between widespread tissue expression and the restriction of symptoms to the kidney has now been confirmed by an in-depth expression study.