Loss of the polycystic kidney disease (PKD1) region of chromosome 16p13 in renal cyst cells supports a loss-of-function model for cyst pathogenesis

PKD1 intron 21: triplex DNA formation and effect on replication

Although autosomal dominant polycystic kidney disease is transmitted in an autosomal dominant fashion, there is evidence that the pathophysiology of cystogenesis involves a second hit somatic mutation superimposed upon the inherited germline mutation within the renal tubule cells. The polypurine.polypyrimidine (Pu.Py) tract of PKD1 intron 21 may play a role in promoting somatic mutations. To better characterize this tract and to evaluate its potential to participate in mutagenesis, we investigated the thermodynamics of intramolecular triplex formation by 15 Pu.Py mirror repeat tracts from PKD1 intron 21 by 2D gel electrophoresis. We demonstrate that intramolecular triplexes form with modest superhelical tensions for all the tracts examined. Primer extension studies demonstrated significant polymerase arrest within the Pu.Py tracts in one direction of replication only. We found correlation between polymerization arrest and both the potential length of the triplex and superhelical tension of intramolecular triplex formation. The presence of a Pu.Py tract also led to a replication blockade and double-strand breakage using an SV40 in vitro replication assay with HeLa cell extracts. During DNA replication, the G-rich template of the PKD1 Pu.Py tracts may form a triplex structure with the nascent strand, thereby blocking replication and potentially leading to recombination and mutation.

Altered distribution and co-localization of polycystin-2 with polycystin-1 in MDCK cells after wounding stress

Polycystin-1 and -2 are integral membrane glycoproteins defective in autosomal dominant polycystic kidney disease (ADPKD). Recent studies showed a coupled polycystin-1 and -2 action in cell signaling and channel activation suggesting an important biological role for the two proteins at the plasma membrane. To gain a better understanding about the (co)-distribution and dynamics of the polycystin-1 and -2 complex under stress conditions, we used a wound-healing model of Madine Darby canine kidney (MDCK) renal epithelial cells. In this model, cells near the wound edge undergo a process of reorganization to active migration, while cells further from the edge are unaffected and remain confluent. For the first time, endogenous polycystin-1 and -2 were found to partly co-localize in the plasma membrane of confluent monolayers, and both proteins co-localized in the primary cilium. Upon wound healing, the association of polycystin-2 to the membrane was greatly reduced at the wound edge and the submarginal cells. Polycystin-1 remained incorporated to the membrane at the edge of the cell sheet at all time points, although strongly reduced in lamellipodia-forming cells. Adherens junctions and desmosomes, and respective connected actin and keratin cytoskeleton were also disturbed in lamellipodia-forming cells. We propose that altered subcellular localization of polycystin-1 and -2 as a result of stress will affect signaling and other cellular processes mediated by these proteins.

Cyclic AMP activates B-Raf and ERK in cyst epithelial cells from autosomal-dominant polycystic kidneys

BACKGROUND

The proliferation of mural epithelial cells is a major cause of progressive cyst enlargement in autosomal-dominant polycystic kidney disease (ADPKD). Adenosine 3', 5' cyclic monophosphate (cAMP) stimulates the proliferation of cells from ADPKD cysts, but not cells from normal human kidney cortex (HKC), through the activation of protein kinase A (PKA), mitogen-activated protein kinase kinase (MEK), and extracellular signal-regulated kinase (ERK/MAPK). In the current study, we examined the signaling pathway between PKA and MEK in ADPKD and HKC cells.

METHODS

Primary cultures of human ADPKD and HKC cells were prepared from nephrectomy specimens. We determined the effects of cAMP and epidermal growth factor (EGF) on the activation of ERK, B-Raf and Raf-1 in ADPKD and HKC cells by immune kinase assay and Western blot.

RESULTS

8-Br-cAMP increased phosphorylated ERK (2.7- +/- 0.6-fold, N = 7), and B-Raf kinase activity (3.6- +/- 1.1-fold, N = 5) in cells from ADPKD kidneys; levels of phosphorylated Raf-1 were not changed. Inhibition of PKA by H89 strikingly decreased cAMP-stimulated phosphorylation of ERK and B-Raf, and MAPK inhibition by PD98059 blocked the effect of the nucleotide to activate ERK. By contrast, in HKC cells 8-Br-cAMP did not activate B-Raf and ERK. EGF stimulated the phosphorylation of ERK and Raf-1 in both ADPKD and HKC cells, but had no effect on B-Raf. 8-Br-cAMP and EGF conjointly increased ERK activation above that of either agonist alone in ADPKD cells, and this combined effect was abolished by PD98059, indicating that ERK was activated by EGF- and cAMP-responsive cascades that converge at MAPK.

CONCLUSION

cAMP activates ERK and increases proliferation of ADPKD epithelial cells, but not cells from normal human kidney cortex, through the sequential phosphorylation of PKA, B-Raf and MAPK in a pathway separate from, but complementary to, the classical receptor tyrosine kinase cascade. Consequently, cAMP and EGF have great potential to accelerate the progressive enlargement of renal cysts.

Molecular cytogenetic aberrations in autosomal dominant polycystic kidney disease tissue

Autosomal dominant polycystic kidney disease (ADPKD) is a genetically heterogeneous disorder characterized by focal cyst formation from any part of the nephron. The molecular bases include germinal mutation of either PKD1 or PKD2 genes, enhanced expression of several protooncogenes, alteration of the TGF-alpha/EGF/EGF receptor (EGFR) axis, and disturbed regulation of proliferative/apoptosis pathways. To identify new locations of ADPKD related oncogenes and/or tumor suppressor genes (TSG), comparative genomic hybridization (CGH) and loss of heterozygosity (LOH) analyses were performed for a series of individual cysts (n = 24) from eight polycystic kidneys. By CGH, imbalances were detected predominantly on chromosomes 1p, 9q, 16p, 19, and 22q in all tissues. DNA copy number gain was seen on chromosomes 3q and 4q in five samples. The CGH data were supplemented by LOH analysis using 83 polymorphic microsatellite markers distributed along chromosomes 1, 9, 16, 19, and 22. The highest frequency of LOH was found on the 1p35-36 and 16p13.3 segments in cysts from seven samples. Allelic losses on 9q were detected in six, whereas deletions at 19p13 and 22q11 bands were observed in three polycystic kidneys. These results indicate that the deleted chromosomal regions may contain genes important in ADPKD initiation and progression.

Renal activation of extracellular signal-regulated kinase in rats with autosomal-dominant polycystic kidney disease

BACKGROUND

Abnormal proliferation of renal tubule epithelial cells is a central factor in the biogenesis and sustained expansion of cysts in autosomal-dominant polycystic kidney disease (ADPKD). Recent evidence from in vitro studies of human cyst wall epithelial cells has implicated a role for the mitogen-activated protein (MAP) kinase pathway in this aberrant proliferation. To determine the extent to which this signaling pathway is involved in cyst pathogenesis in vivo, we measured the expression of select components of the MAP kinase cascade in Han:SPRD rats with ADPKD at an early stage of the disease.

METHODS

Kidneys of 8-week-old normal Han:SPRD rats (+/+) or rats heterozygous (Cy/+) for ADPKD were examined by Western blot analysis and immunohistochemistry to determine the expression of extracellular-regulated kinase (ERK), phosphorylated ERK (P-ERK), Raf-1 (MAPKKK), phosphorylated Raf-1 (P-Raf-1), B-Raf, Rap-1 and phosphorylated protein kinase A (P-PKA).

RESULTS

P-ERK was expressed to a greater extent in Cy/+ kidneys (3.74 +/- 1.07 fold) than in normal kidneys, whereas ERK abundance was not different. P-Raf-1 levels were higher in Cy/+ than in +/+ kidneys (1.53 +/- 0.08 fold) consistent with upstream stimulation of receptor tyrosine kinase. B-Raf and Raf-1 abundances were greater in Cy/+ than in +/+ (1.74 +/- 0.25 and 1.27 +/- 0.08 fold, respectively). In Cy/+, immunohistochemistry showed increased P-ERK and B-Raf expression in the abnormal mural epithelial cells within cysts. These findings, together with the detection of P-PKA and the small G protein, Rap-1, in cyst epithelial cells, implicate a potential role for cyclic adenosine monophosphate (AMP) in the activation of ERK in ADPKD cells.

CONCLUSIONS

We conclude that the MAP kinase pathway is activated to the level of ERK in the abnormal mural epithelial cells lining cysts in animals with a dominantly inherited type of polycystic kidney disease. We suggest that cAMP, acting through PKA, Rap-1 and B-Raf, may contribute to the activation of ERK in a way that complements receptor tyrosine kinase-mediated agonists in the promotion of cyst enlargement.

Loss of chromosome 16 from renal epithelial cells in humans

This work explores the notion that low-frequency, acquired aneuploidy may play a role in complex genetic traits such as essential hypertension. To this end, renal epithelial cells in urinary sediments and in renal cysts were examined by fluorescent in situ hybridization with DNA probes specific for the heterochromatic and centromere regions of chromosomes 16 and 1. Chromosome 16 was probed because it harbors variant genes causing monogenic hypertension. These genes have also been investigated for their role in essential hypertension. Chromosome 1 was also probed as an internal control. Higher proportions of renal epithelial cells in the urinary sediments showed monosomy of chromosome 16 than monosomy of chromosome 1 (P<0.001). We also observed in epithelial cells of renal cysts a preponderance of monosomy for chromosome 16 over monosomy for chromosome 1 (P<0.024). Low-frequency loss of heterozygosity that results from acquired monosomy of chromosome 16 and perhaps other chromosomes may contribute to expression of complex genetic traits such as essential hypertension, in which the diverse phenotypic manifestations are poorly understood.

The polycystic kidney disease-1 promoter is a target of the beta-catenin/T-cell factor pathway

Polycystic kidney disease (PKD) results from loss-of-function mutations in the PKD1 gene. There are also reports showing abnormally high levels of PKD1 expression in cystic epithelial cells. At present, nothing is known about the molecular mechanisms regulating the normal expression of the PKD1 gene or whether transcriptional disregulation of the PKD1 gene has a role in cyst formation. We have analyzed a 3.3-kb 5'-proximal portion of the human PKD1 gene. Sequence analysis revealed the presence of consensus sequences for numerous transactivating factors, including four T-cell factor (TCF) binding elements (TBEs). Transcriptional activity of the 3.3-kb fragment and a series of deletion constructs was assayed in HEK293T cells. A 2.0-kb proximal promoter region containing one of the four TBEs (TBE1) was inducible up to 6-fold by cotransfection with beta-catenin. beta-catenin-mediated induction was inhibited by dominant-negative TCF and by deletion of the TBE1 sequence. 15- or 109-bp sequences containing the TBE1 site, when cloned upstream of a minimal promoter, were shown to respond to beta-catenin induction. Gel shift assays confirmed that the TBE1 site is capable of forming complexes with TCF and beta-catenin. To determine whether expression of the endogenous PKD1 gene responds to beta-catenin, HT1080 cells were treated with LiCl, and HeLa cells were stably transfected with beta-catenin. In both cases, endogenous PKD1 mRNA levels were elevated in response to these treatments. Taken together, these studies define an active PKD1 promoter region and suggest that the PKD1 gene is a target of the beta-catenin/TCF pathway.

Expression of PKD1 and PKD2 transcripts and proteins in human embryo and during normal kidney development

Autosomal-dominant polycystic kidney disease, one of the most frequent human genetic disorders, is genetically heterogeneous. Most cases result from mutations of PKD1 or PKD2 encoding polycystin-1 or polycystin-2, respectively. Polycystin-1 is a large transmembrane protein containing several domains involved in cell-cell and/or cell-matrix interactions. Polycystin-2 is transmembrane glycoprotein sharing homology with some families of cation channels. Despite a large number of reports, the tissue distribution of these two proteins, especially of polycystin-1, is still debated. We investigated the expression pattern of PKD1 and PKD2 transcripts and proteins during human embryogenesis and kidney development, using Northern blot analysis, in situ hybridization, and immunohistochemical methods. For each gene, the expression pattern of transcripts and protein was concordant. In human 5- to 6-week-old embryos, both genes are widely expressed, mainly in neural tissue, cardiomyocytes, endodermal derivatives, and mesonephros. At this age, PKD2 but not PKD1 expression is observed in the ureteric bud and the uninduced metanephros. Thereafter, PKD2 is diffusely expressed at all stages of nephron development, whereas high PKD1 expression first appears in differentiated proximal tubules. Proximal tubule expression of both genes decreases from weeks 20 to 24 onwards. PKD1 transcripts, later restricted to distal tubules in fetal nephrogenesis, are no longer detected in adult kidneys, which nevertheless maintain a faint expression of polycystin-1, whereas persistent expression of PKD2 transcripts and protein is observed throughout nephrogenesis. Overall, contrary to previous observations, we found profound differences in the spatiotemporal expression of PKD1 and PKD2 during nephrogenesis, PKD2 being expressed earlier and more diffusely than PKD1. These data suggest that polycystins could interact with different partners, at least during kidney development.

Mutations of the human polycystic kidney disease 2 (PKD2) gene

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited nephropathy, usually of late onset (onset between third to seventh decade), primarily characterized by the formation of fluid-filled cysts in the kidneys. It is one of the most frequent inherited conditions affecting approximately 1:1,000 Caucasians. Two major genes have been identified and characterized in detail: PKD1 and PKD2, mapping on chromosomes 16p13.3 and 4q21-23, respectively. A third gene, PKD3, has been implicated in selected families. Polycystic kidney disease of types 1 or 2 follows a very similar course of symptoms, both being multisystem pleiotropic disorders of indistinguishable picture on clinical grounds. The only difference is that patients with PKD2 mutations run a milder course compared to PKD1 carriers, with an average 10-20 years later age of onset and lower probability to reach end-stage-renal failure. The proteins polycystin-1 and -2 are trans-membranous glycoproteins hypothesized to participate in a common signaling pathway, interacting with each other and with other proteins, and coordinately expressed in normal and cystic tissue. Renal cysts most probably arise after a second somatic event, which inactivates the inherited healthy allele of the same gene, or perhaps one of the alleles of the other gene counterpart, generating a trans-heterozygous state. This article reviews the reported mutations in PKD2. Mutations of all kinds have been reported over the entire sequence of the PKD2 gene, with no apparent significant clustering and with some evidence of genotype/phenotype correlation. Most families harbor their own private mutations but a few recurrent events have been reported in unrelated families.

The Jeremiah Metzger Lecture. Polycystic kidney disease: old disease in a new context

I want to thank the organizers for inviting me to present the Jeremiah Metzger Lecture at this, the 114th meeting of the ACCA. It is a high honor, indeed, to join a list of very distinguished predecessors. And for this opportunity to tell you about my passion in medicine and science, I am most grateful. Most of you in this room have passing knowledge of polycystic kidney disease, probably hearing about it in your medical school Pathology course where you were shown an especially grotesque, enormously enlarged kidney either encased in transparent plastic or submerged in a bucket of formaldehyde. In that minute or two when PKD was discussed in lecture, you may have been told that this is a rare, hereditary disorder that causes kidney failure and that nothing can be done to alter that course. Unless you chose to specialize in General Internal Medicine or Nephrology, you may not have encountered PKD again until today, despite the fact there are approximately 600,000 PKD patients in the USA and over 10,000,000 worldwide, and it accounts for approximately 5% of non-diabetic dialysis and renal transplant patients (Table 1). I might have overlooked PKD as well had it not been for a close friend that I grew up with who had inherited the disease from his mother. He was very open about the fact that he had cysts in his kidneys that caused bleeding into the urine from time to time, especially after a solid hit during a game of tackle football. We remained friends long after I left home for college and medical school. At an early stage of my research career in medicine, while wondering how nephron segments processed glomerular filtrate, I inadvertently discovered that renal tubules could secrete as well as reabsorb salt and water. This was quite an unexpected finding at the time (1). But it occurred to [table: see text] me that this might be a means to fill renal cysts with fluid and so I decided to learn more about the pathology and pathogenesis of PKD. This didn't take long, because there wasn't much literature on the subject. The clinical manifestations of PKD were described in the 19th century European medical literature and Sir William Osler had published on the topic in this country, but by and large only a few descriptions of small groups of patients were reported through the middle of the 20th century. In 1957, Dalgaard (2) reported in a classic doctoral thesis that the most common type of hereditary PKD is transmitted as an autosomal dominant trait (ADPKD) with complete penetrance. It is a bilateral renal condition, but cysts also occur in the liver (approximately 60%), pancreas (approximately 10%) and various other organs, and it is associated with cerebral aneurysms in approximately 5% of patients. A recessive form that affects infants and children primarily (ARPKD), is much rarer than ADPKD and commonly leads to death in infancy in association with massively enlarged kidneys (Table 2). I was also attracted to the study of PKD because the etiology was not in question: it had to be mutated DNA. Yet that fact proved to be a hindrance in attaining research support. As some of you will recall, not too long ago genetic diseases were viewed by kidney-oriented NIH review panels to be incurable. I was advised that a young scientist's time would be better spent determining how the kidneys excrete salt and water. Fortunately, the era of molecular genetics and biology was upon us, and we quickly learned that uncommon genetic disorders could lead to the discovery of novel molecules in metabolic and structural pathways. And that is just what happened in the PKD field. The autosomal dominant form of PKD led to the discovery of a unique family of highly complex proteins long before they would have been selected from a gene or proteomic micro-array by some desperate graduate student or fellow. The chromosomal location of the major ADPKD genotype, PKD1, was defined in 1985 (3), a date that marks the beginning of a remarkable period of discovery.

Mutational analysis within the 3' region of the PKD1 gene in Japanese families

Autosomal dominant polycystic kidney disease (ADPKD) is a widespread genetic disease that causes renal failure. One of the genes that is responsible for this disease, PKD1, has been identified and characterized. Many mutations of the PKD1 gene have been identified in the Caucasian population. We investigated the occurrence of mutations in this gene in the Japanese population. We analyzed each exon in the 3' single copy region of the gene between exons 35 and 46 in genomic DNA obtained from 69 patients, using a PCR-based direct sequencing method. Four missense mutations (T3509M, G3559R, R3718Q, R3752W), one deletion mutation (11307del61bp) and one polymorphism (L3753L) were identified, and their presence confirmed by allele-specific oligonucleotide (ASO) hybridization. These were novel mutations, except for R3752W, and three of them were identified in more than two families. Mutation analysis of the PKD1 gene in the Japanese population is being reported for the first time.

Dysplastic and polycystic kidneys: diagnosis, associations and management

Cystic and bright kidneys can pose a significant diagnostic dilemma when discovered as an incidental finding at the time of a routine fetal ultrasound scan. There are diverse aetiologies with equally variable implications for the prognosis in the affected fetus, and for future pregnancies. Accurate antenatal diagnosis in the absence of any positive family history is often not possible and a team approach to management (to include the fetal medicine specialist, paediatric nephrologist or urologist, geneticists and in some cases, pathologist) is essential. In this review we will attempt to describe the embryology and aetiology of these conditions and suggest an approach to management.

Autosomal dominant polycystic kidney disease: modification of disease progression

Autosomal dominant polycystic kidney disease is a common inherited disorder, which is characterised by the formation of fluid-filled cysts in both kidneys that leads to progressive renal failure. Mutations in two genes, PKD1 and PKD2, are associated with the disorder. We describe the various factors that cause variation in disease progression between patients. These include whether the patient has a germline mutation in the PKD1 or in the PKD2 gene, and the nature of the mutation. Detection of mutations in PKD1 is complicated, but the total number identified is rising and will enable genotype-to-phenotype studies. Another factor affecting disease progression is the occurrence of somatic mutations in PKD genes. Furthermore, modifying genes might directly affect the function of polycystins by affecting the rate of somatic mutations or the rate of protein interactions, or they might affect cystogenesis itself or clinical factors associated with disease progression. Finally, environmental factors that speed up or slow down progress towards chronic renal failure have been identified in rodents.

Novel PKD1 deletions and missense variants in a cohort of Hellenic polycystic kidney disease families

The autosomal dominant form of polycystic kidney disease is a very frequent genetically heterogeneous inherited condition affecting approximately 1 : 1000 individuals of the Caucasian population. The main symptom is the formation of fluid-filled cysts in the kidneys, which grow progressively in size and number with age, and leading to end-stage renal failure in approximately 50% of patients by age 60. About 85% of cases are caused by mutations in the PKD1 gene on chromosome 16p13.3, which encodes for polycystin-1, a membranous glycoprotein with 4302 amino acids and multiple domains. Mutation detection is still a challenge owing to various sequence characteristics that prevent easy PCR amplification and sequencing. Here we attempted a systematic screening of part of the duplicated region of the gene in a large cohort of 53 Hellenic families with the use of single-strand conformation polymorphism analysis of exons 16-34. Our analysis revealed eight most probably disease causing mutations, five deletions and three single amino acid substitutions, in the REJ domain of the protein. In one family, a 3-bp and an 8-bp deletion in exons 20 and 21 respectively, were co-inherited on the same PKD1 chromosome, causing disease in the mother and three sons. Interestingly we did not find any termination codon defects, so common in the unique part of the PKD1 gene. In the same cohort we identified 11 polymorphic sequence variants, four of which resulted in amino acid variations. This supports the notion that the PKD1 gene may be prone to mutagenesis, justifying the relatively high prevalence of polycystic kidney disease.

Polycystin-1 transforms the cAMP growth-responsive phenotype of M-1 cells

BACKGROUND

Polycystic kidney disease (PKD) is characterized by the abnormal proliferation of tubular epithelial cells. It was recently shown that the growth of PKD cyst-lining cells is stimulated by cyclic adenosine monophosphate (cAMP), whereas the growth of normal human kidney cortex cells is inhibited.

METHODS

We have examined the effects of overexpressing the C-terminal cytosolic tail of mouse polycystin-1, as a membrane-targeted fusion protein, on cAMP-responsive cell proliferation in stably transfected M-1 cortical collecting duct cells. Two cell lines that express high levels of the polycystin-1 fusion protein and two control cell lines that do not express the fusion protein were tested.

RESULTS

Growth of parental M-1 cells and the control cell lines was inhibited by 8-Br-cAMP and by a variety of cAMP agonists. In contrast, growth of the polycystin-1-expressing clones was stimulated by cAMP. Consistent with this, the protein kinase A (PKA) inhibitor H-89 caused either a positive or a negative growth effect depending on the primary response to cAMP. PD98059 blocked the cAMP stimulation of cell proliferation, indicating that the pathway is MEK1 dependent.

CONCLUSIONS

Expression of the polycystin-1 C-terminal tail disrupts normal cellular signaling and transforms the stably transfected M-1 cells to an abnormal PKD cell proliferation phenotype.

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.

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.

Nanobacteria: controversial pathogens in nephrolithiasis and polycystic kidney disease

Nanobacteria are unconventional agents 100-fold smaller than common bacteria that can replicate apatite-forming units. Nanobacteria are powerful mediators of biogenic apatite nucleation (crystal form of calcium phosphate) and crystal growth under conditions simulating blood and urine. Apatite is found in the central nidus of most kidney stones and in mineral plaques (Randall's plaques) in renal papilla. The direct injection of nanobacteria into rat kidneys resulted in stone formation in the nanobacteria-injected kidney during one month follow-up, but not in the control kidney injected with vehicle. After intravenous administration in rats and rabbits, nanobacteria are rapidly excreted from the blood into the urine, as a major elimination route, and damage renal collecting tubuli. Nanobacteria are cytotoxic to fibroblasts in vitro. Human kidney cyst fluids contain nanobacteria. Nanobacteria thus appear to be potential provocateurs and initiators of kidney stones, tubular damage, and kidney cyst formation. It is hypothesized that nanobacteria are the initial nidi on which kidney stone is built up, at a rate dependent on the supersaturation status of the urine. Those individuals having both nanobacteria and diminished defences against stone formation (i.e. genetic factors, diet and drinking habits) could be at high risk. Kidney cyst formation is hypothesized to involve nanobacteria-induced tubular damage and defective tissue regeneration yielding cyst formation, the extent of which is dependent on genetic vulnerability.

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.

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.

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.

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.

Development of autosomal recessive polycystic kidney disease in BALB/c-cpk/cpk mice

Autosomal recessive polycystic kidney disease (ARPKD) is a rare but devastating inherited disease in humans. Various strains of mice that are homozygous for the cpk gene display renal pathology similar to that seen in human ARPKD. The PKD progresses to renal insufficiency, azotemia, and ultimately a uremic death by approximately 3 wk of age. This study characterizes PKD in mice that are homozygous for the cpk gene on a BALB/c inbred mouse background. The BALB/c-cpk/cpk murine model displays renal as well as extrarenal pathology similar to that found in human ARPKD. The renal pathology includes the well-characterized early proximal tubule and, later, massive collecting duct cysts. The extrarenal defects in this murine model include common bile duct dilation, intrahepatic biliary duct cysts with periductal hyperplasia, and pancreatic dysplasia with cysts. Renal mRNA expression of c-myc, a proto-oncogene, and clusterin (SGP-2), a marker associated with immature collecting ducts, decreases during normal development but is upregulated in murine ARPKD. Expression of epidermal growth factor (EGF) mRNA is significantly diminished, whereas EGF receptor mRNA is upregulated in the BALB/c-cpk/cpk kidney compared with phenotypically normal littermates. To determine whether the altered EGF expression contributes to the development of PKD, neonatal mice were treated with exogenous EGF (1 microg/g body wt injected subcutaneously on postnatal days 3 through 9). EGF treatment reduced the relative kidney weight and common bile duct dilation and downregulated renal expression of clusterin and EGF receptor. However, exogenous EGF did not affect the degree of renal failure, the pancreatic pathology, or the misregulated renal expression of c-myc. In summary, the present study characterizes the renal and extrarenal pathology in the BALB/c-cpk/cpk murine model of ARPKD. Renal mRNA expression of EGF is diminished in this mouse model. EGF treatment did not prevent renal failure but ameliorated pathologic changes in the kidney and the biliary ducts of the BALB/c-cpk/cpk mouse.

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.

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.

Endotoxin and nanobacteria in polycystic kidney disease

BACKGROUND

Microbes have been suspected as provocateurs of polycystic kidney disease (PKD), but attempts to isolate viable organisms have failed. Bacterial endotoxin is the most often reported microbial product found in PKD fluids. We assessed potential microbial origins of endotoxin in cyst fluids from 13 PKD patients and urines of PKD and control individuals.

METHODS

Fluids were probed for endotoxin and nanobacteria, a new bacterium, by the differential Limulus Amebocyte Lysate assay (dLAL), genus-specific antilipopolysaccharide (LPS) antibodies, monoclonal antibodies to nanobacteria, and hyperimmune serum to Bartonella henselae (HS-Bh). Selected specimens were also assessed by transmission electron microscopy (TEM) and nanobacterial culture methods.

RESULTS

LPS or its antigenic metabolites were found in more than 75% of cyst fluids tested. Nanobacteria were cultured from 11 of 13 PKD kidneys, visualized in 8 of 8 kidneys by TEM, and immunodetected in all 13 PKD kidneys. By immunodetection, nanobacterial antigens were found in urine from 7 of 7 PKD males, 1 of 7 PKD females, 3 of 10 normal males, and 1 of 10 normal females. "Nanobacterium sanguineum" was dLAL positive and cross-reactive with antichlamydial LPS and HS-Bh. Some cyst fluids were also positive for LPS antigens from Escherichia coli, Bacteroides fragilis and/or Chlamydia, and HS-Bh, as were liver cyst fluids from one patient. Tetracycline and citrate inhibited nanobacterial growth in vitro.

CONCLUSION

Nanobacteria or its antigens were present in PKD kidney, liver, and urine. The identification of candidate microbial pathogens is the first step in ascertaining their contribution, if any, to human disease.

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.

Cardiac defects and renal failure in mice with targeted mutations in Pkd2

PKD2, mutations in which cause autosomal dominant polycystic kidney disease (ADPKD), encodes an integral membrane glycoprotein with similarity to calcium channel subunits. We induced two mutations in the mouse homologue Pkd2 (ref.4): an unstable allele (WS25; hereafter denoted Pkd2WS25) that can undergo homologous-recombination-based somatic rearrangement to form a null allele; and a true null mutation (WS183; hereafter denoted Pkd2-). We examined these mutations to understand the function of polycystin-2, the protein product of Pkd2, and to provide evidence that kidney and liver cyst formation associated with Pkd2 deficiency occurs by a two-hit mechanism. Pkd2-/- mice die in utero between embryonic day (E) 13.5 and parturition. They have structural defects in cardiac septation and cyst formation in maturing nephrons and pancreatic ducts. Pancreatic ductal cysts also occur in adult Pkd2WS25/- mice, suggesting that this clinical manifestation of ADPKD also occurs by a two-hit mechanism. As in human ADPKD, formation of kidney cysts in adult Pkd2WS25/- mice is associated with renal failure and early death (median survival, 65 weeks versus 94 weeks for controls). Adult Pkd2+/- mice have intermediate survival in the absence of cystic disease or renal failure, providing the first indication of a deleterious effect of haploinsufficiency at Pkd2on long-term survival. Our studies advance our understanding of the function of polycystin-2 in development and our mouse models recapitulate the complex human ADPKD phenotype.

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.

Genetic localization of interacting modifiers affecting severity in a murine model of polycystic kidney disease

Genetic analysis of mouse disease models provides a means to investigate how modifying loci cause variation in phenotypic expression. We have shown that polycystic kidney disease (PKD) progression in the juvenile cystic kidney (jck) mutation can be influenced by an epistatic interaction between alleles of different strain backgrounds and we localized one of these loci to chromosome 1. Using a chromosome 1 congenic strain, we improved the genetic analysis and mapped the interacting locus to proximal chromosome 4, with a highly significant lod association of 5.5. Re-analysis of the original F(2) cross reveals that in this cohort, while the lod association of the chromosome 4 locus alone is not significant, its effect is apparent when analyzed in combination with the chromosome 1 locus. This result suggests that correlation of paired genotype data with phenotype data will be an effective means to detect epistatic interactions contributing to complex traits, and that these associations can be tested using appropriate congenic lines.

Mutation detection of PKD1 identifies a novel mutation common to three families with aneurysms and/or very-early-onset disease

It is known that several of the most severe complications of autosomal-dominant polycystic kidney disease, such as intracranial aneurysms, cluster in families. There have been no studies reported to date, however, that have attempted to correlate severely affected pedigrees with a particular genotype. Until recently, in fact, mutation detection for most of the PKD1 gene was virtually impossible because of the presence of several highly homologous loci also located on chromosome 16. In this report we describe a cluster of 4 bp in exon 15 that are unique to PKD1. Forward and reverse PKD1-specific primers were designed in this location to amplify regions of the gene from exons 11-21 by use of long-range PCR. The two templates described were used to analyze 35 pedigrees selected for study because they included individuals with either intracranial aneurysms and/or very-early-onset disease. We identified eight novel truncating mutations, two missense mutations not found in a panel of controls, and several informative polymorphisms. Many of the polymorphisms were also present in the homologous loci, supporting the idea that they may serve as a reservoir for genetic variability in the PKD1 gene. Surprisingly, we found that three independently ascertained pedigrees had an identical 2-bp deletion in exon 15. This raises the possibility that particular genotypes may be associated with more-severe disease.

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

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

Identification and characterization of polycystin-2, the PKD2 gene product

PKD2, the second gene for the autosomal dominant polycystic kidney disease (ADPKD), encodes a protein, polycystin-2, with predicted structural similarity to cation channel subunits. However, the function of polycystin-2 remains unknown. We used polyclonal antisera specific for the intracellular NH(2) and COOH termini to identify polycystin-2 as an approximately 110-kDa integral membrane glycoprotein. Polycystin-2 from both native tissues and cells in culture is sensitive to Endo H suggesting the continued presence of high-mannose oligosaccharides typical of pre-middle Golgi proteins. Immunofluorescent cell staining of polycystin-2 shows a pattern consistent with localization in the endoplasmic reticulum. This finding is confirmed by co-localization with protein-disulfide isomerase as determined by double indirect immunofluorescence and co-distribution with calnexin in subcellular fractionation studies. Polycystin-2 translation products truncated at or after Gly(821) retain their exclusive endoplasmic reticulum localization while products truncated at or before Glu(787) additionally traffic to the plasma membrane. Truncation mutants that traffic to the plasma membrane acquire Endo H resistance and can be biotinylated on the cell surface in intact cells. The 34-amino acid region Glu(787)-Ser(820), containing two putative phosphorylation sites, is responsible for the exclusive endoplasmic reticulum localization of polycystin-2 and is the site of specific interaction with an as yet unidentified protein binding partner for polycystin-2. The localization of full-length polycystin-2 to intracellular membranes raises the possibility that the PKD2 gene product is a subunit of intracellular channel complexes.

Renal asymmetry in children with autosomal dominant polycystic kidney disease

Although for decades autosomal dominant polycystic kidney disease (ADPKD) was considered a disease of adults, our recent longitudinal studies on children from ADPKD families have shown that the disease is evident by ultrasound imaging in approximately 75% of children who are carriers of the ADPKD1 gene, the most common form of ADPKD. Here we report that, in contrast to adults, the disease appears to be unilateral initially in approximately 17% of children. Asymmetric enlargement of the kidneys is also frequently observed. This renal asymmetry can be extreme and lead to diagnostic confusion. We present 2 unusual cases of asymmetric renal involvement that we have observed during the last 10 years. The first is a 14-year-old boy who was scheduled for a nephrectomy to relieve pain and whose family requested a second opinion. The second is a 10-year-old girl who was diagnosed with ADPKD in utero by prenatal ultrasound. After birth, 1 kidney progressively developed cysts and enlarged, whereas the other had only a few tiny cysts and remained normal in size. A review of the literature shows that presentations like these often lead to a nephrectomy or surgical biopsy. A carefully obtained family history and examination of both parents with ultrasound can help to avoid unnecessary invasive procedures. If pain is a prominent symptom, it can be treated by cyst aspiration if there are only a few cysts or a single dominant cyst. The molecular mechanism for extremely asymmetric renal disease remains to be elucidated.

Polycystin-1 expression in PKD1, early-onset PKD1, and TSC2/PKD1 cystic tissue

BACKGROUND

The mutational mechanism responsible for cyst formation in polycystic kidney disease 1 gene (PKD1) remains controversial, with data indicating a two-hit mechanism, but also evidence of polycystin-1 expression in cystic tissue.

METHODS

To investigate this apparent paradox, we analyzed polycystin-1 expression in cystic renal or liver tissue from 10 patients with truncating PKD1 mutations (including one early-onset case) and 2 patients with severe disease associated with contiguous deletions of TSC2 and PKD1, using monoclonal antibodies (mAbs) to both extreme N-(7e12) and C-terminal (PKS-A) regions of the protein. Truncation of the C-terminal epitope from the putative mutant proteins in each case allowed exclusive assessment of the nontruncated protein with PKS-A.

RESULTS

In adult PKD1 tissue, the majority of cysts (approximately 80%) showed polycystin-1 expression, although staining was absent in a variable but significant minority (approximately 20%), in spite of the normal expression of marker proteins. Unlike adult PKD1, however, negative cysts were rarely found in infantile PKD1 or TSC2/PKD1 deletion cases.

CONCLUSIONS

If a two-hit mutational mechanism is operational, these results suggest that the majority of somatic mutations in adult PKD1 are likely to be missense changes. The low level of polycystin-1-negative cysts in the three "early-onset" cases, however, suggests that a somatic PKD1 mutation may not always be required for cyst formation.

Cellular localization and tissue distribution of polycystin-1

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the formation of fluid-filled cysts in both kidneys, in addition to a variety of extra-renal manifestations. The PKD1 gene product, polycystin-1, encodes a novel protein with a putative role in cell-cell/cell-matrix interactions. The present study we focused on the (sub)cellular localization of polycystin-1 in cultured cells, and on its tissue distribution in various organs. In Madin Darby canine kidney (MDCK) cells, several polyclonal antibodies showed intense staining at the sites of interaction between adjacent cells, which remained after Triton extraction. Weak cytoplasmic staining was observed. No signal was detected at the free borders of cell aggregates, supporting a role for polycystin-1 in cell-cell interactions. At the tissue level, polycystin-1 expression was observed in specific cell types in tissues with known manifestations of the disease, but also in tissues of organs which have not been reported to be affected in ADPKD. Expression was frequently seen in epithelia, but also in endocrine cells (pancreatic islets, parathyroid-producing cells, clusters in the adenohypophysis, clusters in the adrenal gland, and Leydig cells in the testis). In addition, expression was observed in myocardium and more weakly in myocytes of cardiac valves, of the cerebral arteries, and of skeletal muscles.

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.

Somatic PKD2 mutations in individual kidney and liver cysts support a "two-hit" model of cystogenesis in type 2 autosomal dominant polycystic kidney disease

An intriguing feature of autosomal dominant polycystic kidney disease (ADPKD) is the focal and sporadic formation of renal and extrarenal cysts. Recent documentation of somatic PKD1 mutations in cystic epithelia of patients with germ-line PKD1 mutations suggests a "two-hit" model for cystogenesis in type 1 ADPKD. This study tests whether the same mechanism for cystogenesis might also occur in type 2 ADPKD. Genomic DNA was obtained from 54 kidney and liver cysts from three patients with known germ-line PKD2 mutations, using procedures that minimize contamination of cells from noncystic tissue. Using intragenic and microsatellite markers, these cyst samples were screened for loss of heterozygosity. The same samples were also screened for somatic mutations in five of the 15 exons in PKD2 by single-stranded conformational polymorphism analysis. Loss of heterozygosity was found in five cysts, and unique intragenic mutations were found in seven other cysts. In 11 of these 12 cysts, it was also determined that the somatic mutation occurred nonrandomly in the copy of PKD2 inherited from the unaffected parent. These findings support the "two-hit" model as a unified mechanism for cystogenesis in ADPKD. In this model, the requirement of a somatic mutation as the rate-limiting step for individual cyst formation has potential therapeutic implications.

DNA structural transitions within the PKD1 gene

Autosomal dominant polycystic kidney disease (ADPKD) affects over 500 000 Americans. Eighty-five percent of these patients have mutations in the PKD1 gene. The focal nature of cyst formation has recently been attributed to innate instability in the PKD1 gene. Intron 21 of this gene contains the largest polypurine. polypyrimidine tract (2.5 kb) identified to date in the human genome. Polypurine.polypyrimidine mirror repeats form intramolecular triplexes, which may predispose the gene to mutagenesis. A recombinant plasmid containing the entire PKD1 intron 21 was analyzed by two-dimensional gel electrophoresis and it exhibited sharp structural transitions under conditions of negative supercoiling and acidic pH. The superhelical density at which the transition occurred was linearly related to pH, consistent with formation of protonated DNA structures. P1 nuclease mapping studies of a plasmid containing the entire intron 21 identified four single-stranded regions where structural transitions occurred at low superhelical densities. Two-dimensional gel electrophoresis and chemical modification studies of the plasmid containing a 46 bp mirror repeat from one of the four regions demonstrated the formation of an H-y3 triplex structure. In summary, these experiments demonstrate that a 2500 bp polypurine.polypyrimidine tract within the PKD1 gene is capable of forming multiple non-B-DNA structures.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Polycystin-2 expression is developmentally regulated

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

Identification of the major site of in vitro PKA phosphorylation in the polycystin-1 C-terminal cytosolic domain

Sequence analysis of the C-terminal cytosolic domain of human and mouse polycystin-1 has identified three RxS consensus protein kinase A (PKA) phosphorylation motifs. GST-fusion proteins containing the full-length and truncated C-terminal cytosolic domain of murine polycystin-1 were phosphorylated in vitro by the purified catalytic subunit of PKA. This identified a sequence of 25 amino acids, immediately downstream of a previously identified heterotrimeric G-protein activation sequence, as the major site of PKA phosphorylation. Phosphorylation of wild-type and alanine substituted synthetic peptides containing this motif demonstrated that alanine substitution of serine 4159 largely eliminated phosphorylation. Mutation of this residue in the fusion protein reduced phosphorylation by about 70%, whereas mutation of the other two conserved phosphorylation motifs had little effect. We conclude that serine 4159 is the major site of PKA phosphorylation in the C-terminal cytosolic domain of murine polycystin-1.

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

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

Renal cell carcinoma in children with diffuse cystic hyperplasia of the kidneys

We report the clinical, pathologic, and genetic features of renal malignancy in two children with diffuse cystic hyperplasia. Both presented with massive bilateral nephromegaly. Neither had a family history or clinical findings suggestive of tuberous sclerosis or von Hippel-Lindau disease. The kidneys of both children were extensively replaced by tubulocystic hyperplasia with large eosinophilic epithelial cells. The masses of hyperplastic tissue were nodular, compressing remnants of uninvolved renal parenchyma. Tubulopapillary carcinoma was present in both children, one of whom had bilateral multicentric carcinoma. No loss of heterozygosity was detected in the tumors at the TSC1, TSC2, or VHL gene regions, and no alterations in the VHL gene were detected using single-strand conformation polymorphism analysis. These cases of bilateral renal enlargement with diffuse cystic hyperplasia appear to represent a new clinical syndrome that may warrant bilateral nephrectomy because of the risk of malignancy.