Molecular basis of renal cyst formation--one hit or two?

Somatic mutations acquired during life: state of the art and implications for the kidney

As a consequence of continuous interaction with mutagens, the genome sequence accumulates changes, which are referred to as somatic mutations. Somatic variants acquired by normal cells during a lifetime are difficult to detect with common sequencing methods. This review provides a basic description of currently available technologies for somatic mutation detection and summarizes the studies that have explored somatic mutation in the kidneys. Given the role of somatic mutations in the formation of kidney cysts, genomic analyses can be used to investigate mechanisms that influence disease progression in inherited cystic kidney disorders. Moreover, genomic analyses are an important method to explore the evolution from a normal cell to cancer, providing insights into mechanisms of tumor initiation. Somatic mutation data can be used to discover endogenous and exogenous mutagens that harness the kidneys, including tobacco and aristolochic acid. In addition, genomic analyses have highlighted a link between kidney damage and mutation. This information is going to be key for understanding lifestyle factors that influence kidney cancer risk, overall impacting clinical decisions and public health strategies.

Generation of heterozygous PKD1 mutant pigs exhibiting early-onset renal cyst formation

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease, manifesting as the progressive development of fluid-filled renal cysts. In approximately half of all patients with ADPKD, end-stage renal disease results in decreased renal function. In this study, we used CRISPR-Cas9 and somatic cell cloning to produce pigs with the unique mutation c.152_153insG (PKD1insG/+). Pathological analysis of founder cloned animals and progeny revealed that PKD1insG/+ pigs developed many pathological conditions similar to those of patients with heterozygous mutations in PKD1. Pathological similarities included the formation of macroscopic renal cysts at the neonatal stage, number and cystogenic dynamics of the renal cysts formed, interstitial fibrosis of the renal tissue, and presence of a premature asymptomatic stage. Our findings demonstrate that PKD1insG/+ pigs recapitulate the characteristic symptoms of ADPKD.

Insights into Autosomal Dominant Polycystic Kidney Disease from Genetic Studies

Autosomal dominant polycystic kidney disease is the most common monogenic cause of ESKD. Genetic studies from patients and animal models have informed disease pathobiology and strongly support a "threshold model" in which cyst formation is triggered by reduced functional polycystin dosage below a critical threshold within individual tubular epithelial cells due to (1) germline and somatic PKD1 and/or PKD2 mutations, (2) mutations of genes (e.g., SEC63, SEC61B, GANAB, PRKCSH, DNAJB11, ALG8, and ALG9) in the endoplasmic reticulum protein biosynthetic pathway, or (3) somatic mosaicism. Genetic testing has the potential to provide diagnostic and prognostic information in cystic kidney disease. However, mutation screening of PKD1 is challenging due to its large size and complexity, making it both costly and labor intensive. Moreover, conventional Sanger sequencing-based genetic testing is currently limited in elucidating the causes of atypical polycystic kidney disease, such as within-family disease discordance, atypical kidney imaging patterns, and discordant disease severity between total kidney volume and rate of eGFR decline. In addition, environmental factors, genetic modifiers, and somatic mosaicism also contribute to disease variability, further limiting prognostication by mutation class in individual patients. Recent innovations in next-generation sequencing are poised to transform and extend molecular diagnostics at reasonable costs. By comprehensive screening of multiple cystic disease and modifier genes, targeted gene panel, whole-exome, or whole-genome sequencing is expected to improve both diagnostic and prognostic accuracy to advance personalized medicine in autosomal dominant polycystic kidney disease.

Genetics and mechanisms of hepatic cystogenesis

Polycystic liver disease (PLD) is a heterogeneous genetic condition. PKD1 and PKD2 germline mutations are found in patients with autosomal dominant polycystic kidney disease (ADPKD). Autosomal dominant polycystic liver disease (ADPLD) is associated with germline mutations in PRKCSH, SEC63, LRP5, and recently ALG8 and SEC61. GANAB mutations are found in both patient groups. Loss of heterozygosity of PLD-genes in cyst epithelium contributes to the development of hepatic cysts. A genetic interaction network is implied in hepatic cystogenesis that connects the endoplasmic glycoprotein control mechanisms and polycystin expression and localization. Wnt signalling could be the major downstream signalling pathway that results in hepatic cyst growth. PLD in ADPLD and ADPKD probably results from changes in one common final pathway that initiates cyst growth. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

A polycystin-centric view of cyst formation and disease: the polycystins revisited

It is 20 years since the identification of PKD1, the major gene mutated in autosomal dominant polycystic kidney disease (ADPKD), followed closely by the cloning of PKD2. These major breakthroughs have led in turn to a period of intense investigation into the function of the two proteins encoded, polycystin-1 and polycystin-2, and how defects in either protein lead to cyst formation and nonrenal phenotypes. In this review, we summarize the major findings in this area and present a current model of how the polycystin proteins function in health and disease.

Translational research in ADPKD: lessons from animal models

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 or PKD2, which encode polycystin-1 and polycystin-2, respectively. Rodent models are available to study the pathogenesis of polycystic kidney disease (PKD) and for preclinical testing of potential therapies-either genetically engineered models carrying mutations in Pkd1 or Pkd2 or models of renal cystic disease that do not have mutations in these genes. The models are characterized by age at onset of disease, rate of disease progression, the affected nephron segment, the number of affected nephrons, synchronized or unsynchronized cyst formation and the extent of fibrosis and inflammation. Mouse models have provided valuable mechanistic insights into the pathogenesis of PKD; for example, mutated Pkd1 or Pkd2 cause renal cysts but additional factors are also required, and the rate of cyst formation is increased in the presence of renal injury. Animal studies have also revealed complex genetic and functional interactions among various genes and proteins associated with PKD. Here, we provide an update on the preclinical models commonly used to study the molecular pathogenesis of ADPKD and test potential therapeutic strategies. Progress made in understanding the pathophysiology of human ADPKD through these animal models is also discussed.

Cyst expansion and regression in a mouse model of polycystic kidney disease

Autosomal-dominant polycystic kidney disease is characterized by progressive cyst formation and fibrosis in the kidneys. Here we describe an orthologous Pkd1(nl,nl) mouse model, with reduced expression of the normal Pkd1 transcript, on a fixed genetic background of equal parts C57Bl/6 and 129Ola/Hsd mice (B6Ola-Pkd1(nl,nl)). In these mice, the first cysts develop from mature proximal tubules around birth. Subsequently, larger cysts become visible at day 7, followed by distal tubule and collecting duct cyst formation, and progressive cystic enlargement to develop into large cystic kidneys within 4 weeks. Interestingly, cyst expansion was followed by renal volume regression due to cyst collapse. This was accompanied by focal formation of fibrotic areas, an increased expression of genes involved in matrix remodeling and subsequently an increase in infiltrating immune cells. After an initial increase in blood urea within the first 4 weeks, renal function remained stable over time and the mice were able to survive up to a year. Also, in kidneys of ADPKD patients collapsed cysts were observed, in addition to massive fibrosis and immune infiltrates. Thus, B6Ola-Pkd1(nl,nl) mice show regression of cysts and renal volume that is not accompanied by a reduction in blood urea levels.

Genomic instability in patients with autosomal-dominant polycystic kidney disease

OBJECTIVE

Autosomal-dominant polycystic kidney disease (ADPKD) is a systemic disorder affecting multiple organs that results in renal and extrarenal cysts. Patients with ADPKD may have genomic instability, making them more vulnerable to developing cancer. This study aimed to investigate latent genomic instability in patients with ADPKD, using single-cell gel electrophoresis (comet assay).

METHODS

The susceptibility of peripheral blood lymphocytes to DNA damage induced by X-ray treatment (0.5 Gy) was tested in 20 patients with ADPKD using single-cell gel electrophoresis. The percentage of DNA in the comet tail (TDNA%) before and after irradiation was compared between patients with ADPKD and 20 sex- and age-matched healthy control subjects.

RESULTS

Renal and extrarenal cysts were observed in patients with ADPKD. A significantly higher mean TDNA% was determined in patients with ADPKD compared with control subjects (8.85% versus 7.50%). After in vitro irradiation, DNA damage was significantly increased in all participants, but the increase was significantly greater in patients with ADPKD compared with control subjects.

CONCLUSION

These data suggest that patients with ADPKD have genomic instability, which may trigger renal and extrarenal cyst formation.

Lysophosphatidic acid is a modulator of cyst growth in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the slow growth of multiple fluid-filled cysts predominately in the kidney tubules and liver bile ducts. Elucidation of mechanisms that control cyst growth will provide the basis for rational therapeutic intervention. We used electrophysiological methods to identify lysophosphatidic acid (LPA) as a component of cyst fluid and serum that stimulates secretory Cl- transport in the epithelial cell type that lines renal cysts. LPA effects are manifested through receptors located on the basolateral membrane of the epithelial cells resulting in stimulation of channel activity in the apical membrane. Concentrations of LPA measured in human ADPKD cyst fluid and in normal serum are sufficient to maximally stimulate ion transport. Thus, cyst fluid seepage and/or leakage of vascular LPA into the interstitial space are capable of stimulating epithelial cell secretion resulting in cyst enlargement. These observations are particularly relevant to the rapid decline in renal function in late-stage disease and to the "third hit" hypothesis that renal injury exacerbates cyst growth.

Polycystic kidney disease: the complexity of planar cell polarity and signaling during tissue regeneration and cyst formation

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is an inherited systemic disease with intrarenal cystogenesis as its primary characteristic. A variety of mouse models provided information on the requirement of loss of balanced polycystin levels for initiation of cyst formation, the role of proliferation in cystogenesis and the signaling pathways involved in cyst growth and expansion. Here we will review the involvement of different signaling pathways during renal development, renal epithelial regeneration and cyst formation in ADPKD, focusing on planar cell polarity (PCP) and oriented cell division (OCD). This will be discussed in context of the hypothesis that aberrant PCP signaling causes cyst formation. In addition, the role of the Hippo pathway, which was recently found to be involved in cyst growth and tissue regeneration, and well-known for regulating organ size control, will be reviewed. The fact that Hippo signaling is linked to PCP signaling makes the Hippo pathway a novel cascade in cystogenesis. The newly gained understanding of the complex signaling network involved in cystogenesis and disease progression, not only necessitates refining of the current hypothesis regarding initiation of cystogenesis, but also has implications for therapeutic intervention strategies. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Diagnosis and management of childhood polycystic kidney disease

A number of syndromic disorders have renal cysts as a component of their phenotypes. These disorders can generally be distinguished from autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ARPKD) by imaging studies of their characteristic, predominantly non-renal associated abnormalities. Therefore, a major distinction in the differential diagnosis of enlarge echogenic kidneys is delineating ARPKD from ADPKD. ADPKD and ARPKD can be diagnosed by imaging the kidney with ultrasound, computed tomography, or magnetic resonance imaging (MRI), although ultrasound is still the method of choice for diagnosis in utero and in young children due to ease of use, cost, and safety. Differences in ultrasound characteristics, the presence or absence of associated extrarenal abnormalities, and the screening of the parents >40 years of age usually allow the clinician to make an accurate diagnosis. Early diagnosis of ADPKD and ARPKD affords the opportunity for maximal anticipatory care (i.e. blood pressure control) and in the not-too-distant future, the opportunity to benefit from new therapies currently being developed. If results are equivocal, genetic testing is available for both ARPKD and ADPKD. Specialized centers are now offering preimplantation genetic diagnosis and in vitro fertilization for parents who have previously had a child with ARPKD. For ADPKD patients, a number of therapeutic interventions are currently in clinical trial and may soon be available.

Unravelling the genetic basis of renal diseases; from single gene to multifactorial disorders

Chronic kidney disease is common with up to 5% of the adult population reported to have an estimated glomerular filtration rate of < 60 ml/min/1.73 m(2). A large number of pathogenic mutations have been identified that are responsible for 'single gene' renal disorders, such as autosomal dominant polycystic kidney disease and X-linked Alport syndrome. These single gene disorders account for < 15% of the burden of end-stage renal disease that requires dialysis or kidney transplantation. It has proved more difficult to identify the genetic susceptibility underlying common, complex, multifactorial kidney conditions, such as diabetic nephropathy and hypertensive nephrosclerosis. This review describes success to date and explores strategies currently employed in defining the genetic basis for a number of renal disorders. The complementary use of linkage studies, candidate gene and genome-wide association analyses are described and a collation of renal genetic resources highlighted.

Pkd2 dosage influences cellular repair responses following ischemia-reperfusion injury

Autosomal dominant polycystic kidney disease (ADPKD) results from mutations in either PKD1 or PKD2 and accounts for 10% of all patients on renal replacement therapy. The kidney disease phenotype is primarily characterized by cyst formation, but there are also prominent interstitial changes (inflammation, apoptosis, proliferation, and fibrosis). Using a model of unilateral ischemia-reperfusion injury, we tested the hypothesis that Pkd2 heterozygous kidneys are more sensitive to injury and that this could lead to interstitial inflammation and fibrosis. Baseline tubular proliferation in heterozygous kidneys was twofold higher than in wild-type kidneys. The magnitude and duration of tubular and interstitial proliferative responses was consistently greater in injured heterozygous compared with wild-type kidneys at all time points. Conversely, tubular p21 expression in heterozygotes was lower at baseline and following injury at all time points. Significantly more neutrophils and macrophages were detected in injured Pkd2 heterozygous kidneys at 2 days, correlating with increased expression of the cytokines interleukin (IL)-1beta and keratinocyte-derived chemokine and resulting in interstitial fibrosis at 28 days. We conclude that Pkd2 dosage influences both susceptibility and nature of the repair responses following injury. Polycystin-2 is therefore likely to play multiple roles in regulating tubular cell viability, repair, and remodeling in the mature kidney.

Treatment of autosomal dominant polycystic kidney disease (ADPKD): the new horizon for children with ADPKD

Polycystic kidney disease (PKD) is the most common inherited renal disorder. Patients with PKD remain clinically asymptomatic for decades, while significant anatomic and physiologic systemic changes take place. Sequencing of the responsible genes and identification of their protein products have significantly expanded our understanding of the pathophysiology of PKD. The molecular basis for cystogenesis is being unraveled, leading to new targets for therapy and giving hope to millions of people suffering from PKD. This has direct implications for children with PKD with regard to screening for the disease and identification of high-risk individuals. In this article we provide a review of the clinical manifestations in children with autosomal dominant polycystic kidney disease (ADPKD), the genetic and molecular basis for the disease, and a concise review of potential therapies being evaluated.

Hyperproliferation of PKD1 cystic cells is induced by insulin-like growth factor-1 activation of the Ras/Raf signalling system

Autosomal dominant polycystic kidney disease (ADPKD) largely results from mutations in the PKD1 gene leading to hyperproliferation of renal tubular epithelial cells and consequent cyst formation. Rodent models of PKD suggest that the multifunctional hormone insulin-like growth factor-1 (IGF-1) could play a pathogenic role in renal cyst formation. In order to test this possibility, conditionally immortalized renal epithelial cells were prepared from normal individuals and from ADPKD patients with known germline mutations in PKD1. All patient cell lines had a decreased or absence of polycystin-1 but not polycystin-2. These cells had an increased sensitivity to IGF-1 and to cyclic AMP, which required phosphatidylinositol-3 (PI3)-kinase and the mitogen-activated protein kinase, extracellular signal-regulated protein kinase (ERK) for enhanced growth. Inhibition of Ras or Raf abolished the stimulated cell proliferation. Our results suggest that haploinsufficiency of polycystin-1 lowers the activation threshold of the Ras/Raf signalling system leading to growth factor-induced hyperproliferation. Inhibition of Ras or Raf activity may be a therapeutic option for decreasing tubular cell proliferation in ADPKD.

Cerebral cavernous malformations: clinical insights from genetic studies

✓ Familial disease is responsible for one third to one half of cerebral cavernous malformation (CCM) cases presenting to clinical attention. Much has been learned in the past decade about the genetics of these cases, which are all inherited in an autosomal dominant pattern, at three known chromosome loci. Unique features of inherited CCMs in Hispanic-Americans of Mexican descent have been described. The respective genes for each locus have been identified and preliminary observations on disease pathways and mechanisms are coming to light, including possible explanations for selectivity of neural milieu and relationships to endothelial layer abnormalities. Mechanisms of lesion genesis in cases of genetic predisposition are being investigated, with evidence to support a two-hit model emerging from somatic mutation screening of the lesions themselves and from lesion formation in transgenic murine models of the disease. Other information on potential inflammatory factors has emerged from differential gene expression studies. Unique phenotypic features of solitary versus familial cases have emerged: different associations with venous developmental anomaly and the exceptionally high penetrance rates that are found in inherited cases when high-sensitivity screening is performed with gradient echo magnetic resonance imaging. This information has changed the landscape of screening and counseling for patients and their families, and promises to lead to the development of new tools for predicting, explaining, and modifying disease behavior.

Haploinsufficiency of Pkd2 is associated with increased tubular cell proliferation and interstitial fibrosis in two murine Pkd2 models

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited human kidney disease and is caused by germline mutations in PKD1 (85%) or PKD2 (15%). It has been estimated that around 1% of tubular cells give rise to cysts, and cell hyperproliferation has been noted to be a cardinal feature of cystic epithelium. Nevertheless, it is uncertain whether the increase in proliferative index observed is an early or late feature of the cystic ADPKD kidney.

METHODS

Two Pkd2 mouse mutants (WS25 and WS183) have been recently generated as orthologous models of PKD2. To determine the effect of Pkd2 dosage on cell proliferation, cyst formation and renal fibrosis, we studied renal tissue from Pkd2(WS25/WS25) and Pkd2(+/-) mice by histological analysis. We also examined the proliferative index in archival nephrectomy tissue obtained from patients with ADPKD and normal controls.

RESULTS

The proliferative index of non-cystic tubules in Pkd2 mutant mice as assessed by proliferating cell nuclear antigen and Ki67-positive nuclei was between 1-2%, values 5-10 times higher than control tissue. Similarly, the proliferative index of non-cystic tubules in human ADPKD kidneys was 40 times higher than corresponding controls. In Pkd2 mutant mice, significant correlations were found between the fibrosis score and the mean cyst area as well as with the proliferative index. Of significance, proliferating tubular cells were uniformly positive for polycystin-2 expression in Pkd2(+/-) kidney.

CONCLUSION

These results suggest that an increase in cell proliferation is an early event preceding cyst formation and can result from haploinsufficiency at Pkd2. The possible pathogenic link between tubular cell proliferation, interstitial fibrosis and cyst formation is discussed.

Molecular pathogenesis of ADPKD: the polycystin complex gets complex

Autosomal-dominant polycystic kidney disease (ADPKD) is one of the most common human monogenic diseases with an incidence of 1:400 to 1:1000. It is characterized by the progressive development and enlargement of focal cysts in both kidneys, typically resulting in end-stage renal disease (ESRD) by the fifth decade. The cystogenic process is highly complex with a cellular phenotype consistent with "dedifferentiation" (i.e., a high proliferative rate, increased apoptosis, altered protein sorting, changed secretory characteristics, and disorganization of the extracellular matrix). Although cystic renal disease is the major cause of morbidity, the occurrence of nonrenal cysts, most notably in the liver (occasionally resulting in clinically significant polycystic liver disease) and the increased prevalence of other abnormalities including intracranial aneurysms, indicate that ADPKD is a systemic disorder. Following the identification of the first ADPKD gene, PKD1, 10 years ago and PKD2 2 years later, considerable progress has been made in defining the etiology and understanding the pathogenesis of this disorder, knowledge that is now leading to the development of several promising new therapies. The purpose of this review is to summarize our current state of knowledge as to the structure and function of the PKD1 and PKD2 proteins, polycystin-1 and -2, respectively, and explore how mutation at these loci results in the spectrum of changes seen in ADPKD.

Functional analysis of PKD1 transgenic lines reveals a direct role for polycystin-1 in mediating cell-cell adhesion

The PKD1 protein, polycystin-1, is a large transmembrane protein of uncertain function and topology. To study the putative functions of polycystin-1, conditionally immortalized kidney cells transgenic for PKD1 were generated and an interaction between transgenic polycystin-1 and endogenous polycystin-2 has been recently demonstrated in these cells. This study provides the first functional evidence that transgenic polycystin-1 directly mediates cell-cell adhesion. In non-permeabilized cells, polycystin-1 localized to the lateral cell borders with N-terminal antibodies but not with a C-terminal antibody; there was a clear difference in surface intensity between transgenic and non-transgenic cells. Compared with non-transgenic cells, transgenic cells showed a dramatic increase in resistance to the disruptive effect of a polycystin-1 antibody raised to the PKD domains of polycystin-1 (IgPKD) in both cell adhesion and cell aggregation assays. The differential effect on cell adhesion between transgenic and non-transgenic cells could be reproduced using recombinant fusion proteins encoding non-overlapping regions of the IgPKD domains. In contrast, antibodies raised to other extracellular domains of polycystin-1 had no effect on cell adhesion. Finally, the specificity of this finding was confirmed by the lack of effect of IgPKD antibody on cell adhesion in a PKD1 cystic cell line deficient in polycystin-1. These results demonstrate that one of the primary functions of polycystin-1 is to mediate cell-cell adhesion in renal epithelial cells, probably via homophilic or heterophilic interactions of the PKD domains. Disruption of cell-cell adhesion during tubular morphogenesis may be an early initiating event for cyst formation in ADPKD.

Identification, characterization, and localization of a novel kidney polycystin-1-polycystin-2 complex

The functions of the two proteins defective in autosomal dominant polycystic kidney disease, polycystin-1 and polycystin-2, have not been fully clarified, but it has been hypothesized that they may heterodimerize to form a "polycystin complex" involved in cell adhesion. In this paper, we demonstrate for the first time the existence of a native polycystin complex in mouse kidney tubular cells transgenic for PKD1, non-transgenic kidney cells, and normal adult human kidney. Polycystin-1 is heavily N-glycosylated, and several glycosylated forms of polycystin-1 differing in their sensitivity to endoglycosidase H (Endo H) were found; in contrast, native polycystin-2 was fully Endo H-sensitive. Using highly specific antibodies to both proteins, we show that polycystin-2 associates selectively with two species of full-length polycystin-1, one Endo H-sensitive and the other Endo H-resistant; importantly, the latter could be further enriched in plasma membrane fractions and co-immunoprecipitated with polycystin-2. Finally, a subpopulation of this complex co-localized to the lateral cell borders of PKD1 transgenic kidney cells. These results demonstrate that polycystin-1 and polycystin-2 interact in vivo to form a stable heterodimeric complex and suggest that disruption of this complex is likely to be of primary relevance to the pathogenesis of cyst formation in autosomal dominant polycystic kidney disease.

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.

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.

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.

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.

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.

Somatic mutation in individual liver cysts supports a two-hit model of cystogenesis in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD), Type I is a common genetic disorder and an important cause of renal failure. The disease is characterized by progressive cyst formation in a variety of organs including the kidney, liver and pancreas. We have previously shown that in the case of PKD1, renal cyst development is likely to require somatic inactivation of the normal allele coupled to a germline PKD1 mutation. In this report, we have used unique reagents to show that intragenic, somatic mutations are common in hepatic cysts. All pathogenic mutations were shown to have altered the previously normal copy of the gene. These data extend the "two-hit" model of cystogenesis to include a second focal manifestation of the disease.

Somatic inactivation of Pkd2 results in polycystic kidney disease

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