Extrarenal manifestations of autosomal dominant polycystic kidney disease

Cystic kidney diseases and planar cell polarity signaling

Renal cystic diseases are a major clinical concern as they are the most common genetic cause of end-stage renal disease. While many of the genes causing cystic disease have been identified in recent years, knowing the molecular nature of the mutations has not clarified the mechanisms underlying cyst formation. Recent research in model organisms has suggested that cyst formation may be because of defective planar cell polarity (PCP) and/or ciliary defects. In this review, we first outline the clinical features of renal cystic diseases and then discuss current research linking our understanding of cystic kidney disease to PCP and cilia.

Novel method for genomic analysis of PKD1 and PKD2 mutations in autosomal dominant polycystic kidney disease

Genetic testing of PKD1 and PKD2 is useful for diagnosis and prognosis of autosomal dominant polycystic kidney disease (ADPKD), particularly in asymptomatic individuals or those without a family history. PKD1 testing is complicated by the large transcript size, complexity of the gene region, and the extent of gene variations. A molecular assay was developed using Transgenomic's SURVEYOR Nuclease and WAVE Nucleic Acid High Sensitivity Fragment Analysis System to screen for PKD1 and PKD2 variants, followed by sequencing of variant gene segments, thereby reducing the sequencing reactions by 80%. This method was compared to complete DNA sequencing performed by a reference laboratory for 25 ADPKD patients from 22 families. The pathogenic potential of gene variations of unknown significance was examined by evolutionary comparison, effects of amino acid substitutions on protein structure, and effects of splice-site alterations. A total of 90 variations were identified, including all 82 reported by the reference laboratory (100% sensitivity). A total of 76 variations (84.4%) were in PKD1 and 14 (15.6%) in PKD2. Definite pathogenic mutations (seven nonsense, four truncation, and three splicing defects) were detected in 64% (14/22) of families. The remaining 76 variants included 26 missense, 33 silent, and 17 intronic changes. Two heterozygous nonsense mutations were incorrectly determined by the reference laboratory as homozygous. "Probably pathogenic" mutations were identified in an additional five families (overall detection rate 86%). In conclusion, the SURVEYOR nuclease method was comparable to direct sequencing for detecting ADPKD mutations, achieving high sensitivity with lower cost, providing an important tool for genetic analysis of complex genes.

Spontaneous coronary artery dissection in a woman with polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD), characterized by renal cyst formation, is known to cause such vascular abnormalities as arterial dilatation and dissection. However, spontaneous coronary artery dissection (SCAD) is observed only rarely in patients with ADPKD. We report a patient with ADPKD who developed SCAD and presented with acute myocardial infarction. Her coronary angiography showed a long spiral dissection of the left anterior descending coronary artery. She underwent successful coronary angioplasty with insertion of 3 drug-eluting stents. To the best of our knowledge, this is the first reported case of percutaneous coronary intervention for coronary dissection in a patient with ADPKD. The pathophysiological characteristics of vascular complications in patients with ADPKD are discussed. Polycystins are strongly expressed in human adult vascular smooth muscle cells, and the vascular abnormalities in patients with ADPKD may be related to altered expression of polycystins. Because early recoginition and prompt efforts at mechanical reperfusion, if indicated, are crucial for successful management of SCAD, it would be worthwhile to consider SCAD in the differential diagnoses of acute coronary syndrome in patients with ADPKD.

TRP channels and kidney disease: lessons from polycystic kidney disease

Important insights in to the function of members of the TRP (transient receptor potential) channel superfamily have been gained from the identification of disease-related mutations. In particular the identification of mutations in the PKD2 gene in autosomal dominant polycystic kidney disease has revealed a link between TRP channel function, mechanosensation and the role of the primary cilium in renal cyst formation. The PKD2 gene encodes TRPP2 (transient receptor potential polycystin 2) that has significant homology to voltage-activated calcium and sodium TRP channels. It interacts with polycystin-1 to form a large membrane-associated complex that is localized to the renal primary cilium. Functional characterization of this polycystin complex reveals that it can respond to mechanical stimuli such as flow, resulting in influx of extracellular calcium and release of calcium from intracellular stores. TRPP2 is expressed in the endoplasmic reticulum/sarcoplasmic reticulum where it also regulates intracellular calcium signalling. Therefore TRPP2 modulates many cellular processes via intracellular calcium-dependent signalling pathways.

Ets factors regulate the polycystic kidney disease-1 promoter

The Ets family of transcription factors consists of a group of highly conserved sequence-specific DNA binding proteins that functionally cooperate with other transcription factors to regulate a number of diverse cellular processes including proliferation, differentiation, and apoptosis. We have analyzed a 3.3kb 5'-upstream region of the human PKD1 promoter, using transient transfection in HEK293T cells and Drosophila SL2 cells, to demonstrate that the PKD1 promoter is a target of Ets family transcription factors. Our studies showed that PKD1 promoter-luciferase reporter gene expression is downregulated by cotransfected Fli-1 and is upregulated by cotransfected Ets-1. Using deletion constructs, we demonstrated that the sequences responding to Fli-1 and Ets-1 lie within the -200 to +33bp proximal promoter. This region was found to contain two putative Ets response elements (EREs): an upstream (Ets-A) sequence 5'-CGGAA-3' (-181 to -185) and a downstream (Ets-B) sequence 5'-CGGAT-3' (-129 to -133). Site-directed mutagenesis indicated that both EREs are functional. A Fli-1 DNA binding domain mutant construct (W321R), which is incapable of binding DNA, was unable to inhibit basal promoter activity. In contrast, a Fli-1 DNA binding domain truncation mutant construct, which only contains the DNA binding domain and lacks the transactivation domain, was able to inhibit. These results suggest that the effect of Fli-1 is through direct binding to these EREs. Direct binding of Fli-1 and Ets-1 to the Ets-A and Ets-B sites was supported by electrophoretic mobility shift assays. Lastly, competition between Fli-1 and Ets-1 for the two EREs was demonstrated by showing that increasing amounts of Ets-1 could overcome Fli-1 repression of promoter activity. Taken together, these experiments define the proximal PKD1 promoter region as a potential target of Ets family transcription factors.

ADPKD: molecular characterization and quest for treatment

Autosomal-dominant polycystic kidney disease (ADPKD) is a common hereditary disease that features multiple cystogenesis in various organs and vascular defects. The genes responsible for ADPKD, PKD1, and PKD2 have been identified, and the pathological processes of the disease are becoming clearer. This review focuses on recent findings about the molecular and cellular biology of ADPKD, and especially on PKD1. PKD1 and its product, polycystin-1, play pivotal roles in cellular differentiation because they regulate the cell cycle and because polycystin-1 is a component of adherens junctions. A possible link between polycystin-1 and PPARgamma is discussed. The extraordinarily fast research progress in this area in the last decade has now reached a stage where the development of a remedy for ADPKD might become possible in the near future.

Aberrant expression of SPARC and its impact on proliferation and apoptosis in ADPKD cyst-lining epithelia

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) results from a combination of environmental and genetic factors. Secreted protein acidic and rich in cysteine (SPARC) can be expressed by many different cell types and is associated with development, remodelling, cell turnover and tissue repair. The analysis of SPARC would help evaluate the effect of the unique matricellular glycoprotein on renal disease progression in ADPKD.

METHODS

The concentration of SPARC was measured with an enzyme-linked immunosorbent assay (ELISA); distribution and expression levels were measured with in situ hybridization, immunohistochemistry, reverse transcription-polymerase chain reaction (RT-PCR) and western blot assays. Apoptosis was assessed by morphological observation and fluorescence-activated cell sorting (FACS) apoptosis index (AI) analysis. Cell cycle phase was examined by FACS analysis. Cell proliferation was studied using bromodeoxyuridine (BrdU) incorporation ELISA.

RESULTS

The SPARC level in the renal cyst fluid of patients with ADPKD was greater than that in patients with simple renal cyst (SRC), and also greater than that found in the plasma and urine of patients with either ADPKD or SRC and normal subjects. SPARC mRNA and protein levels in polycystic renal tissue were greater than that in normal renal tissue. Additionally, SPARC could inhibit cyst-lining epithelial cell proliferation, bring about cell cycle arrest in the G0/G1 phase and induce apoptosis in vitro. SPARC treatment resulted in decreased mRNA levels of PCNA (proliferating cell nuclear antigen), MCM2 (minichromosome maintenance protein 2), ClnD1 and Bcl-2, but an increased mRNA level of p21(Waf1) in cyst-lining epithelial cells.

CONCLUSION

Our findings suggest that the increased SPARC expression in ADPKD renal tissue may provide negative feedback in ADPKD patients.

Magnetic Resonance Imaging Evaluation of Hepatic Cysts in Early Autosomal-Dominant Polycystic Kidney Disease: The Consortium for Radiologic Imaging Studies of Polycystic Kidney Disease Cohort

The objective of this study was to investigate the prevalence of hepatic cysts by age and gender in patients with early autosomal-dominant polycystic kidney disease (ADPKD) and to determine whether hepatic cyst volume is related to renal and renal cyst volumes by using magnetic resonance imaging (MRI). A total of 230 patients with ADPKD (94 men and 136 women) who were aged 15 to 46 yr and had relatively preserved renal function were studied. MRI images of the kidney and liver were obtained to measure renal, renal cyst, and hepatic cyst volumes. These volume measurements and hepatic cyst prevalence were compared in all patients and in subgroups on the basis of gender and age (15 to 24, 25 to 34, and 35 to 46 yr). The overall prevalence of hepatic cysts was 83%; the prevalence was 58, 85, and 94% in the sequential age groups and 85% in women and 79% in men. The prevalence was related directly to renal volume (chi2 = 4.30, P = 0.04) and to renal cyst volume (chi2 = 5.59, P = 0.02). The total hepatic cyst volume was significantly greater in women than in men (a logarithmic transformation mean of 5.27 versus 1.94 ml; P = 0.003). The average hepatic cyst volume was 0.25, 5.75, and 22.78 ml in sequential age groups. Hepatic cysts are evident in 94% of patients who are older than 35 yr and in 55% of individuals who are younger than 25 yr. Hepatic cysts are more prevalent and larger in total cyst volume in women than in men. Hepatic cyst prevalence and aggregate total hepatic cyst volume increased with age.

Psoriatic arthritis associated with adult polycystic kidney disease, seminal vesicle, and epididymal cysts

Patients with seminal vesicle and epididymal cysts are mostly asymptomatic. To date, only one patient presenting with bloody ejaculate and acute scrotum has been reported. Different extrarenal manifestations and the association of adult polycystic kidney disease (APKD) with some connective tissue diseases are known. We report on a 60-year-old male patient with bloody ejaculate and acute scrotum who had been diagnosed as having APKD 1 year earlier and whose past medical history revealed inflammatory low back pain, psoriasis, and the diagnosis of psoriatic arthritis. Cultures of urine and ejaculate were sterile, and the patient's renal functions were normal. Ultrasound showed epididymal and seminal vesicle cysts in addition to hepatic and renal cysts. Our case is the first in which psoriatic arthritis accompanied APKD, seminal vesicle cysts, and epididymal cysts. We also review other APKD cases that have accompanied seminal vesicle cysts.

Polycystins 1 and 2 mediate mechanosensation in the primary cilium of kidney cells

Several proteins implicated in the pathogenesis of polycystic kidney disease (PKD) localize to cilia. Furthermore, cilia are malformed in mice with PKD with mutations in TgN737Rpw (encoding polaris). It is not known, however, whether ciliary dysfunction occurs or is relevant to cyst formation in PKD. Here, we show that polycystin-1 (PC1) and polycystin-2 (PC2), proteins respectively encoded by Pkd1 and Pkd2, mouse orthologs of genes mutated in human autosomal dominant PKD, co-distribute in the primary cilia of kidney epithelium. Cells isolated from transgenic mice that lack functional PC1 formed cilia but did not increase Ca(2+) influx in response to physiological fluid flow. Blocking antibodies directed against PC2 similarly abolished the flow response in wild-type cells as did inhibitors of the ryanodine receptor, whereas inhibitors of G-proteins, phospholipase C and InsP(3) receptors had no effect. These data suggest that PC1 and PC2 contribute to fluid-flow sensation by the primary cilium in renal epithelium and that they both function in the same mechanotransduction pathway. Loss or dysfunction of PC1 or PC2 may therefore lead to PKD owing to the inability of cells to sense mechanical cues that normally regulate tissue morphogenesis.

Cardiovascular, skeletal, and renal defects in mice with a targeted disruption of the Pkd1 gene

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by cyst formation in the kidney, liver, and pancreas and is associated often with cardiovascular abnormalities such as hypertension, mitral valve prolapse, and intracranial aneurysms. It is caused by mutations in PKD1 or PKD2, encoding polycystin-1 and -2, which together form a cell surface nonselective cation ion channel. Pkd2-/- mice have cysts in the kidney and pancreas and defects in cardiac septation, whereas Pkd1(del34) -/- and Pkd1(L) -/- mice have cysts but no cardiac abnormalities, although vascular fragility was reported in the latter. Here we describe mice carrying a targeted mutation in Pkd1 (Pkd1(del17-21betageo)), which defines its expression pattern by using a lacZ reporter gene and may identify novel functions for polycystin-1. Although Pkd1(del17-21betageo) +/- adult mice develop renal and hepatic cysts, Pkd1(del17-21betageo) -/- embryos die at embryonic days 13.5-14.5 from a primary cardiovascular defect that includes double outflow right ventricle, disorganized myocardium, and abnormal atrio-ventricular septation. Skeletal development is also severely compromised. These abnormalities correlate with the major sites of Pkd1 expression. During nephrogenesis, Pkd1 is expressed in maturing tubular epithelial cells from embryonic day 15.5. This expression coincides with the onset of cyst formation in Pkd1(del34) -/-, Pkd1(L) -/-, and Pkd2-/- mice, supporting the hypothesis that polycystin-1 and polycystin-2 interact in vivo and that their failure to do so leads to abnormalities in tubule morphology and function.

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.

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.