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

Prevalence, risk factors and disease knowledge of polycystic kidney disease in Pakistan

Polycystic kidneys disease refers to cyst(s) formation in kidneys with severe consequences of end stage renal disease thus have higher mortality. It is a common genetic disease occurring either as autosomal dominant polycystic kidney (ADPKD) or autosomal recessive polycystic kidney disease (ARPKD) with prevalence rates of 1/1000 and 1/40,000 respectively. Dominant forms presenting in later (>30) while recessive in earlier ages (infancy) and affecting both sexes and almost all race. The patient experiences many renal as well as extra-renal manifestations with marked hypertension and cyst formation in other organs predominantly in liver. Due to genetic basis, positive family history is considered as major risk factor. Ultrasonography remains the main stay of diagnosis along with family history, by indicating increased renal size and architectural modifications. Initially disease remains asymptomatic, later on symptomatic treatment is suggested with surgical interventions like cyst decortications or drainage. Dialysis proved to be beneficial in end stage renal disease. However renal transplantation is the treatment of choice.

Metabolomics Approaches for the Diagnosis and Understanding of Kidney Diseases

Diseases of the kidney are difficult to diagnose and treat. This review summarises the definition, cause, epidemiology and treatment of some of these diseases including chronic kidney disease, diabetic nephropathy, acute kidney injury, kidney cancer, kidney transplantation and polycystic kidney diseases. Numerous studies have adopted a metabolomics approach to uncover new small molecule biomarkers of kidney diseases to improve specificity and sensitivity of diagnosis and to uncover biochemical mechanisms that may elucidate the cause and progression of these diseases. This work includes a description of mass spectrometry-based metabolomics approaches, including some of the currently available tools, and emphasises findings from metabolomics studies of kidney diseases. We have included a varied selection of studies (disease, model, sample number, analytical platform) and focused on metabolites which were commonly reported as discriminating features between kidney disease and a control. These metabolites are likely to be robust indicators of kidney disease processes, and therefore potential biomarkers, warranting further investigation.

Generation of special autosomal dominant polycystic kidney disease iPSCs with the capability of functional kidney-like cell differentiation

BACKGROUND

Human induced pluripotent stem cells (iPSCs) have been verified as a powerful cell model for the study of pathogenesis in hereditary disease. Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations of PKD or non-PKD genes. The pathogenesis of ADPKD remains unexplored because of the lack of a true human cell model.

METHODS

Six ADPKD patients and four healthy individuals were recruited as donors of somatic cells from a Chinese ADPKD family without mutations of the PKD genes but carrying SAMSN1 gene deletion. The ADPKD-iPSCs were generated from somatic cells and were induced into kidney-like cells (KLCs) by a novel three-step method involving cytokines and renal epithelium growth medium. Furthermore, we analyzed functional properties of these KLCs by water transportation and albumin absorption assays.

RESULTS

We successfully generated iPSCs from ADPKD patients and differentiated them into KLCs that showed morphological and functional characteristics of human kidney cells. Further, we also found that ADPKD-iPSC-KLCs had a significantly higher rate of apoptosis and a significantly lower capacity for water transportation and albumin absorption compared to healthy sibling-derived differentiated KLCs. Furthermore, knockdown of SAMSN1 in control iPSCs may attenuate differentiation and/or function of KLCs.

CONCLUSIONS

These data show that we have created the first iPSCs established from ADPKD patients without mutations in the PKD genes, and suggest that the deletion mutation of SAMSN1 might be involved in the differentiation and/or function of KLCs. ADPKD-iPSC-KLCs can be used as a versatile model system for the study of kidney disease.

PKD2 mutation in an Iranian autosomal dominant polycystic kidney disease family with misleading linkage analysis data

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic renal disorder caused by mutation in 2 genes PKD1 and PKD2. Thus far, no mutation is identified in approximately 10% of ADPKD families, which can suggest further locus heterogeneity. Owing to the complexity of direct mutation detection, linkage analysis can initially identify the responsible gene in appropriate affected families. Here, we evaluated an Iranian ADPKD family apparently unlinked to both PKD1 and PKD2 genes. This is one of the pioneer studies in genetic analysis of ADPKD in Iranian population.

METHODS

Linkage reanalysis was performed by regenotyping of flanking microsatellite markers in 8 individuals of the ADPKD family. Direct mutation analysis was performed by Sanger sequencing.

RESULTS

Mutation analysis revealed a pathogenic mutation (c.1094+1G>A) in the PKD2 gene in the proband. Analyzing 2 healthy and 4 clinically affected members confirmed the correct segregation of the mutation within the family and also ruled out the disease in 1 suspected individual. Misinterpretation of the linkage data was due to the occurrence of 1 crossing over between the PKD2 intragenic and the nearest downstream marker (D4S2929). Homozygosity of upstream markers caused the recombination indistinguishable.

CONCLUSION

Although analysis of additive informative polymorphic markers can overcome the misleading haplotype data, it is limited because of the lack of other highly polymorphic microsatellite markers closer to the gene. Direct mutation screening can identify the causative mutation in the apparently unlinked pedigree; moreover, it is the only approach to achieve the confirmed diagnosis in individuals with equivocal imaging results.

Genetics and pathogenesis of autosomal dominant polycystic kidney disease: 20 years on

Autosomal dominant polycystic kidney disease (ADPKD), the most common inherited kidney disorder, is characterized by the progressive development and expansion of bilateral fluid-filled cysts derived from the renal tubule epithelial cells. Although typically leading to end-stage renal disease in late middle age, ADPKD represents a continuum, from neonates with hugely enlarged cystic kidneys to cases with adequate kidney function into old age. Since the identification of the first causative gene (i.e., PKD1, encoding polycystin 1) 20 years ago, genetic studies have uncovered a large part of the key factors that underlie the phenotype variability. Here, we provide a comprehensive review of these significant advances as well as those related to disease pathogenesis models, including mutation analysis of PKD1 and PKD2 (encoding polycystin 2), current mutation detection rate, allelic heterogeneity, genotype and phenotype relationships (in terms of three different inheritance patterns: classical autosomal dominant inheritance, complex inheritance, and somatic and germline mosaicism), modifier genes, the role of second somatic mutation hit in renal cystogenesis, and findings from mouse models of polycystic kidney disease. Based upon a combined consideration of the current knowledge, we attempted to propose a unifying framework for explaining the phenotype variability in ADPKD.

Characterization of the SAM domain of the PKD-related protein ANKS6 and its interaction with ANKS3

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic disorder leading to end-stage renal failure in humans. In the PKD/Mhm(cy/+) rat model of ADPKD, the point mutation R823W in the sterile alpha motif (SAM) domain of the protein ANKS6 is responsible for disease. SAM domains are known protein-protein interaction domains, capable of binding each other to form polymers and heterodimers. Despite its physiological importance, little is known about the function of ANKS6 and how the R823W point mutation leads to PKD. Recent work has revealed that ANKS6 interacts with a related protein called ANKS3. Both ANKS6 and ANKS3 have a similar domain structure, with ankyrin repeats at the N-terminus and a SAM domain at the C-terminus.

RESULTS

The SAM domain of ANKS3 is identified as a direct binding partner of the ANKS6 SAM domain. We find that ANKS3-SAM polymerizes and ANKS6-SAM can bind to one end of the polymer. We present crystal structures of both the ANKS3-SAM polymer and the ANKS3-SAM/ANKS6-SAM complex, revealing the molecular details of their association. We also learn how the R823W mutation disrupts ANKS6 function by dramatically destabilizing the SAM domain such that the interaction with ANKS3-SAM is lost.

CONCLUSIONS

ANKS3 is a direct interacting partner of ANKS6. By structurally and biochemically characterizing the interaction between the ANKS3 and ANKS6 SAM domains, our work provides a basis for future investigation of how the interaction between these proteins mediates kidney function.

Polycystic liver disease: an overview of pathogenesis, clinical manifestations and management

Polycystic liver disease (PLD) is the result of embryonic ductal plate malformation of the intrahepatic biliary tree. The phenotype consists of numerous cysts spread throughout the liver parenchyma. Cystic bile duct malformations originating from the peripheral biliary tree are called Von Meyenburg complexes (VMC). In these patients embryonic remnants develop into small hepatic cysts and usually remain silent during life. Symptomatic PLD occurs mainly in the context of isolated polycystic liver disease (PCLD) and autosomal dominant polycystic kidney disease (ADPKD). In advanced stages, PCLD and ADPKD patients have massively enlarged livers which cause a spectrum of clinical features and complications. Major complaints include abdominal pain, abdominal distension and atypical symptoms because of voluminous cysts resulting in compression of adjacent tissue or failure of the affected organ. Renal failure due to polycystic kidneys and non-renal extra-hepatic features are common in ADPKD in contrast to VMC and PCLD. In general, liver function remains prolonged preserved in PLD. Ultrasonography is the first instrument to assess liver phenotype. Indeed, PCLD and ADPKD diagnostic criteria rely on detection of hepatorenal cystogenesis, and secondly a positive family history compatible with an autosomal dominant inheritance pattern. Ambiguous imaging or screening may be assisted by genetic counseling and molecular diagnostics. Screening mutations of the genes causing PCLD (PRKCSH and SEC63) or ADPKD (PKD1 and PKD2) confirm the clinical diagnosis. Genetic studies showed that accumulation of somatic hits in cyst epithelium determine the rate-limiting step for cyst formation. Management of adult PLD is based on liver phenotype, severity of clinical features and quality of life. Conservative treatment is recommended for the majority of PLD patients. The primary aim is to halt cyst growth to allow abdominal decompression and ameliorate symptoms. Invasive procedures are required in a selective patient group with advanced PCLD, ADPKD or liver failure. Pharmacological therapy by somatostatin analogues lead to beneficial outcome of PLD in terms of symptom relief and liver volume reduction.

Evidence of a third ADPKD locus is not supported by re-analysis of designated PKD3 families

Mutations to PKD1 and PKD2 are associated with autosomal dominant polycystic kidney disease (ADPKD). The absence of apparent PKD1/PKD2 linkage in five published European or North American families with ADPKD suggested a third locus, designated PKD3. Here we re-evaluated these families by updating clinical information, re-sampling where possible, and mutation screening for PKD1/PKD2. In the French-Canadian family we identified PKD1: p.D3782_V3783insD, with misdiagnoses in two individuals and sample contamination explaining the lack of linkage. In the Portuguese family, PKD1: p.G3818A segregated with the disease in 10 individuals in three generations with likely misdiagnosis in one individual, sample contamination, and use of distant microsatellite markers explaining the linkage discrepancy. The mutation, PKD2: c.213delC, was found in the Bulgarian family, with linkage failure attributed to false positive diagnoses in two individuals. An affected son but not the mother, in the Italian family had the nonsense mutation, PKD1: p.R4228X, which appeared de novo in the son; with simple cysts probably explaining the mother’s phenotype. No likely mutation was found in the Spanish family, but the phenotype was atypical with kidney atrophy in one case. Thus, re-analysis does not support the existence of a PKD3 in ADPKD. False positive diagnoses by ultrasound in all resolved families shows the value of mutation screening, but not linkage, to understand families with discrepant data.

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.

Genotype-phenotype correlation in children with autosomal dominant polycystic kidney disease

Adults with autosomal dominant polycystic kidney disease (ADPKD) and PKD1 mutations have a more severe disease than do patients with PKD2 mutations. The aim of this study was to compare phenotypes between children with mutations in the PKD1/PKD2 genes. Fifty PKD1 children and ten PKD2 children were investigated. Their mean age was similar (8.6 +/- 5.4 years and 8.9 +/- 5.6 years). Renal ultrasound was performed, and office blood pressure (BP), ambulatory BP, creatinine clearance and proteinuria were measured. The PKD1 children had, in comparison with those with PKD2, significantly greater total of renal cysts (13.3 +/- 12.5 vs 3.0 +/- 2.1, P = 0.004), larger kidneys [right/left kidney length 0.89 +/- 1.22 standard deviation score (SDS) vs 0.17 +/- 1.03 SDS, P = 0.045, and 1.19 +/- 1.42 SDS vs 0.12 +/- 1.09 SDS, P = 0.014, successively] and higher ambulatory day-time and night-time systolic BP (day-time/night-time BP index 0.93 +/- 0.10 vs 0.86 +/- 0.05, P = 0.021 and 0.94 +/- 0.07 vs 0.89 +/- 0.04, P = 0.037, successively). There were no significant differences in office BP, creatinine clearance or proteinuria. Prenatal renal cysts (14%), hypertension defined by ambulatory BP (27%) and enlarged kidneys (32%) were observed only in the PKD1 children. This is the first study on genotype-phenotype correlation in children with ADPKD. PKD1 children have more and larger renal cysts, larger kidneys and higher ambulatory BP than do PKD2 children. Renal cysts and enlarged kidneys detected prenatally are highly specific for children with PKD1.

Presence of de novo mutations in autosomal dominant polycystic kidney disease patients without family history

BACKGROUND

At the University of Colorado Health Sciences Center, on detailed questioning, approximately 10% of patients with autosomal dominant polycystic kidney disease (ADPKD) gave no family history of ADPKD. There are several explanations for this observation, including occurrence of a de novo pathogenic sequence variant or extreme phenotypic variability. To confirm de novo sequence variants, we have undertaken clinical and genetic screening of affected offspring and their parents.

STUDY DESIGN

Case series.

SETTING & PARTICIPANTS

24 patients with a well-documented ADPKD phenotype and no family history of polycystic kidney disease (PKD) and both parents of each patient.

OUTCOME

Presence or absence of PKD1 or PKD2 pathogenic sequence variants in parents of affected offspring.

MEASUREMENTS

Abdominal ultrasound of affected offspring and their parents for ADPKD diagnosis. Parentage testing by genotyping. Complete screening of PKD1 and PKD2 genes by using genomic DNA from affected offspring; analysis of genomic DNA from both parents to confirm the absence or presence of all DNA variants found.

RESULTS

A positive diagnosis of ADPKD by means of ultrasound or genetic screening was made in 1 parent of 4 patients (17%). No PKD1 or PKD2 pathogenic sequence variants were identified in 10 patients (42%), whereas possible pathological DNA variants were identified in 4 patients (17%) and 1 of their respective parents. Parentage was confirmed in the remaining 6 patients (25%), and de novo sequence variants were documented.

LIMITATIONS

Size of patient group. No direct examination of RNA.

CONCLUSION

Causes other than de novo pathogenic sequence variants may explain the negative family history of ADPKD in certain families.

Diagnosis of autosomal dominant polycystic kidney disease

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disease and accounts for 5 - 10% of end stage renal disease. Mutations of two genes, PKD1 and PKD2, account for ∼ 85 and ∼ 15% of cases, respectively.

OBJECTIVE

This paper reviews the clinical features of ADPKD, highlights the current roles for image- and molecular-based diagnostics, and the potential for new innovations to improve the clinical diagnostics for ADPKD.

METHODS

This paper reviews the literature on the clinical features, differential diagnosis, and image- and molecular-based diagnostics for ADPKD.

RESULTS/CONCLUSION

At present, presymptomatic diagnosis of ADPKD in subjects born with 50% risk is typically performed by renal ultrasonography. Renal MRI, with improved sensitivity for detecting smaller cysts, is a promising modality. There is also a clear role for molecular diagnostics, especially in patients with equivocal imaging results, in those with a negative family history and in younger at-risk subjects with a negative ultrasound study being evaluated as a living-related kidney donor. Also, several classes of promising disease-modifying drugs are being tested in clinical trials and, if proved effective, some of them will be used in early disease. Therefore, it is likely that there will be an increased demand for accurate and early diagnosis of ADPKD in the not so distant future.

Genomic organization and mutation screening of the human ortholog of Pkdr1 associated with polycystic kidney disease in the rat

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common inherited disorders in humans. Although disease-causing mutations have been found in two genes, PKD1 and PKD2, a small number of ADPKD families exist that are unlinked to either of these genes, suggesting involvement of a third, as yet unidentified PKD3 gene. Susceptibility to renal cyst formation in the (cy/+) rat is caused by a missense mutation in Pkdr1 encoding the novel protein SamCystin. To initiate studies of the human orthologous gene, we determined the location and the organization of human PKDR1. We genotyped microsatellite markers flanking the human ortholog in PKD families that either are unlinked to known PKD genes, or in which mutations have not yet been identified and carried out mutation analysis in PKD patients. We identified eight novel single nucleotide polymorphisms, including three leading to amino acid changes. These variants are unlikely to account for PKD in these patients, yet the screening of other affected populations may provide information about the involvement of PKDR1 as a modifier gene in cystic kidney disease.

Diagnostic Approach in Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common Mendelian disorder of the kidney and affects all racial groups worldwide. It is characterized by focal development of renal and extrarenal cysts in an age-dependent manner. Typically, only a few renal cysts are detected in most affected individuals before 30 yr of age. However, by the fifth decade of life, hundreds to thousands of renal cysts will be found in the majority of patients. ADPKD is genetically heterogeneous. Mutations of two genes, PKD1 and PKD2, account for approximately 85 and 15% of cases, respectively. Although the clinical manifestations of these two genotypes overlap completely, patients with PKD1 have much more severe renal disease compared with those with PKD2, as evidenced by their ESRD occurring approximately 15 yr earlier. Renal ultrasonography commonly is used for the assessment of ADPKD, and age-dependent ultrasound diagnostic criteria with high sensitivity and specificity have been established for individuals who are born with 50% risk for PKD1. Although these diagnostic criteria are used widely for genetic counseling and for the evaluation of at-risk individuals as living-related kidney donors to their affected relatives, their application to individuals who are at risk for PKD2 or have undefined genotype needs to be refined further. Molecular genetic testing is available for ADPKD and may be useful for evaluation of at-risk individuals with equivocal imaging results, younger at-risk individuals as a living-related kidney donor, and individuals with atypical or de novo renal cystic disease.

Autosomal dominant polycystic liver disease in a family without polycystic kidney disease associated with a novel missense protein kinase C substrate 80K-H mutation

Polycystic liver disease (PLD) is characterized by the presence of multiple bile duct-derived epithelial cysts scattered in the liver parenchyma. PLD can manifest itself in patients with severe autosomal dominant polycystic kidney disease (ADPKD). Isolated autosomal dominant polycystic liver disease (ADPLD) is genetically distinct from PLD associated with ADPKD, although it may have similar pathogenesis and clinical manifestations. Recently, mutations in two causative genes for ADPLD, independently from ADPKD, have been identified. We report here a family (a mother and her daughter) with a severe form of ADPLD not associated with ADPKD produced by a novel missense protein kinase C substrate 80K-H (PRKCSH) mutation (R281W). This mutation causes a severe phenotype, since the two affected subjects manifested signs of portal hypertension. Doppler sonography, computed tomography (CT) and magnetic resonance (MR) imaging are effective in documenting the underlying lesions in a non-invasive way.

Missense mutation in sterile alpha motif of novel protein SamCystin is associated with polycystic kidney disease in (cy/+) rat

Autosomal dominant polycystic kidney disease (PKD) is the most common genetic disease that leads to kidney failure in humans. In addition to the known causative genes PKD1 and PKD2, there are mutations that result in cystic changes in the kidney, such as nephronophthisis, autosomal recessive polycystic kidney disease, or medullary cystic kidney disease. Recent efforts to improve the understanding of renal cystogenesis have been greatly enhanced by studies in rodent models of PKD. Genetic studies in the (cy/+) rat showed that PKD spontaneously develops as a consequence of a mutation in a gene different from the rat orthologs of PKD1 and PKD2 or other genes that are known to be involved in human cystic kidney diseases. This article reports the positional cloning and mutation analysis of the rat PKD gene, which revealed a C to T transition that replaces an arginine by a tryptophan at amino acid 823 in the protein sequence. It was determined that Pkdr1 is specifically expressed in renal proximal tubules and encodes a novel protein, SamCystin, that contains ankyrin repeats and a sterile alpha motif. The characterization of this protein, which does not share structural homologies with known polycystins, may give new insights into the pathophysiology of renal cyst development in patients.

Depletion of PKD1 by an antisense oligodeoxynucleotide induces premature G1/S-phase transition

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the growth of epithelial cells and the influx of cyst fluid. The 14-kb mRNA of the polycystic kidney disease gene, PKD1, encodes the polycystin-1 protein, whose function remains unknown. In this study, we observed that polycystin-1 localized in epithelial cell-cell contacts of 293 cells. We found, by bromodeoxyuridine (BrdU) incorporation experiments and Western blot analysis of S-phase-specific cyclins, that the depletion of polycystin-1 led to an increased cell proliferation rate and caused a premature G1/S-phase transition. In addition, we showed that the depletion of polycystin-1 reduced the amount of p53 in 293 cells irradiated by UV light, suggesting that polycystin-1 acts as a regulator of G1 checkpoint, which controls entry into the S phase and prevents the replication of damaged DNA. Our results might provide an insight into the formation and progression of ADPKD cysts.

Calcium signaling and polycystin-2

Polycystic kidney disease (PKD) is caused by mutations in two genes, PKD1 and PKD2, which encode for the proteins, polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Although disease-associated mutations have been identified in these two proteins, the sequence of molecular events leading up to clinical symptoms is still unknown. PC1 resides in the plasma membrane and it is thought to function in cell-cell and cell-matrix interactions, whereas PC2 is a calcium (Ca2+) permeable cation channel concentrated in the endoplasmic reticulum. Both proteins localize to the primary cilia where they function as a mechanosensitive receptor complex allowing the entry of Ca2+ into the cell. The downstream signaling pathway involves activation of intracellular Ca2+ release channels, especially the ryanodine receptor (RyR), but subsequent steps are still to be identified. Elucidation of the signaling pathway involved in normal PC1/PC2 function, the functional consequences of PC1/PC2 mutation, and the role of Ca2+ signaling will all help to unravel the molecular mechanisms of cystogenesis in PKD.

Recent advances in understanding the pathogenesis of polycystic kidney disease: therapeutic implications

Hereditary polycystic kidney disease (PKD) is a common cause of renal failure. Increasing knowledge is available regarding mechanisms of cyst development and progression, and renal functional deterioration in PKD. On the basis of this information and theories regarding the pathophysiology of these processes, studies to alter progression and potentially treat PKD have been reported. Cyst development and progression requires epithelial cell proliferation, transepithelial fluid secretion and extracellular matrix remodelling. Several interventions designed to inhibit cell proliferation or alter fluid secretion modify the progression of PKD in selected animal models. Renal functional deterioration appears to involve interstitial inflammation and fibrosis, and tubular apoptosis. Glucocorticoids with anti-inflammatory and antifibrotic properties slow the progression of cystic disease and renal functional deterioration in animal models of PKD. Other interventions, such as dietary modification and angiotensin antagonism, shown to be of benefit in non-PKD models of slowly progressive renal disease, are also of benefit in animal models of PKD. Caution should be used in extrapolating interventional studies in one animal model to another model and certainly to human disease, since examples exist in which treatments in one model of PKD have different effects in another model. Nonetheless, early attempts to determine whether potential treatments are tolerated and of potential benefit in patients with PKD are beginning to appear. Ultimately, treatment of PKD may involve efforts to identify patients at greatest risk for disease progression, thus allowing targeted therapy, use of surrogate markers for disease progression to assist assessment of therapeutic efficacy, and combination therapy to retard disease progression and renal functional deterioration in this common hereditary cause of chronic renal failure.

Polycystic disease of the liver

Autosomal dominant polycystic disease is genetically heterogeneous with mutations in two distinct genes predisposing to the combination of renal and liver cysts (AD-PKD1 and AD-PKD2) and mutations in a third gene yielding isolated liver cysts (the polycystic liver disease gene). Transcription and translation of the PKD1 gene produces polycystin-1, an integral membrane protein that may serve as an extracellular receptor. Mutations occur throughout the PKD1 gene, but more severe disease is associated with N-terminal mutations. The PKD2 gene product, polycystin-2, is an integral membrane protein with molecular characteristics of a calcium-permeant cation channel. Mutations occur throughout the PKD2 gene, and severity of disease may vary with site of mutation in PKD2 and the functional consequence on the resultant polycystin-2 protein. Polycystic liver disease is genetically linked to protein kinase C substrate 80K-H (PRKCSH). The PRKCSH gene encodes hepatocystin, a protein that moderates glycosylation and fibroblast growth factor receptor signaling. More prominent in women, hepatic cysts emerge after the onset of puberty and dramatically increase in number and size through the child-bearing years of early and middle adult life. Although liver failure or complications of advanced liver disease are rare, some patients develop massive hepatic cystic disease and become clinically symptomatic. There is no effective medical therapy. Interventional and surgical options include cyst aspiration and sclerosis, open or laparoscopic cyst fenestration, hepatic resection, and liver transplantation.

Polycystic disease of the liver

Autosomal dominant polycystic disease is genetically heterogeneous with mutations in two distinct genes predisposing to the combination of renal and liver cysts (AD-PKD1 and AD-PKD2) and mutations in a third gene yielding isolated liver cysts (the polycystic liver disease gene). Transcription and translation of the PKD1 gene produces polycystin-1, an integral membrane protein that may serve as an extracellular receptor. Mutations occur throughout the PKD1 gene, but more severe disease is associated with N-terminal mutations. The PKD2 gene product, polycystin-2, is an integral membrane protein with molecular characteristics of a calcium-permeant cation channel. Mutations occur throughout the PKD2 gene, and severity of disease may vary with site of mutation in PKD2 and the functional consequence on the resultant polycystin-2 protein. Polycystic liver disease is genetically linked to protein kinase C substrate 80K-H (PRKCSH). The PRKCSH gene encodes hepatocystin, a protein that moderates glycosylation and fibroblast growth factor receptor signaling. More prominent in women, hepatic cysts emerge after the onset of puberty and dramatically increase in number and size through the child-bearing years of early and middle adult life. Although liver failure or complications of advanced liver disease are rare, some patients develop massive hepatic cystic disease and become clinically symptomatic. There is no effective medical therapy. Interventional and surgical options include cyst aspiration and sclerosis, open or laparoscopic cyst fenestration, hepatic resection, and liver transplantation.

The role of parental hypertension in the frequency and age of diagnosis of hypertension in offspring with autosomal-dominant polycystic kidney disease

BACKGROUND

Hypertension in autosomal-dominant polycystic kidney disease (ADPKD) patients is associated with more rapid progression of renal disease and a high incidence of left ventricular hypertrophy (LVH). The present study was undertaken to examine the role of parental hypertension in the occurrence of hypertension in 475 ADPKD offspring.

METHODS

Adult subjects participating in an ongoing study of the natural history of ADPKD were included in the analysis if they were diagnosed with ADPKD, had a known affected parent, and knew the hypertensive status of both parents.

RESULTS

When the affected parent was hypertensive, the ADPKD male (82% versus 62%, P < 0.05) and female (61% versus 37%, P < 0.005) offspring had a significantly higher frequency of hypertension than when the ADPKD-affected parent was normotensive. The median age of diagnosis of hypertension was also significantly earlier in both male (33 years versus 40 years, P < 0.05) and female (38 years versus 50 years, P < 0.05) ADPKD patients when their affected parents were hypertensive as compared with normotensive. These effects of hypertension in the affected parent on hypertension in the ADPKD offspring were independent of age, renal volume, and renal function in the offspring. Hypertension in unaffected parents also increased the frequency of hypertension in the ADPKD female (69% versus 53%, P < 0.01), but not male (89% versus 77%, NS) subjects.

CONCLUSION

The results indicate that parental hypertension influences the frequency of hypertension in ADPKD patients.

Linkage analysis of candidate loci for end-stage renal disease due to diabetic nephropathy

Diabetic nephropathy (DN), a major cause of ESRD, is undoubtedly multifactorial and is caused by environmental and genetic factors. To identify a genetic basis for DN susceptibility, we are collecting multiplex DN families in the Caucasian (CA) and African-American (AA) populations for whole genome scanning and candidate gene analysis. A candidate gene search of diabetic sibs discordantly affected, concordantly affected and concordantly unaffected for DN was performed with microsatellite markers in genomic regions suspected to harbor nephropathy susceptibility loci. Regions examined were at human chromosome 10p,10q (orthologous to the rat renal susceptibility Rf-1 locus), and at NPHS1 (nephrin), CD2AP, Wilms tumor (WT1), and NPHS2 (podocin) loci. Linkage analyses were conducted using model-free methods (SIBPAL, S.A.G.E.) for AA, CA, and the combined sample. Allele frequencies and the identity by descent sharing were estimated separately for AA and CA, and race was included as a covariate in the final linkage analysis. To date, we have collected 212 sib pairs from 46 CA and 50 AA families. The average age of diabetes onset was 46.8 yr versus 36.2 yr for CA and 39.5 yr versus 40.2 yr for AA, in males versus females respectively. Genotyping data were available for 106 sib pairs (43 CA, 63 AA) from 27 CA (44% male probands) and 38 AA families (43% male probands). Average AA and CA sibship size was 2.73. Singlepoint and multipoint linkage analyses indicate that marker D10S1654 on chromosome 10p is potentially linked to DN (CA only multipoint P = 4 x 10(-3)). Interestingly, the majority of the linkage evidence derives from the CA sib pairs. We are now adding sib pairs and increasing marker density on chromosome 10. We have excluded linkage with candidate regions for nephrin, CD2AP, WT1, and podocin in this sample. In conjunction with previous reports, our data support evidence for a DN susceptibility locus on chromosome 10.

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.

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.

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.

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.

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.

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

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

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

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