PKHD1, the polycystic kidney and hepatic disease 1 gene, encodes a novel large protein containing multiple immunoglobulin-like plexin-transcription-factor domains and parallel beta-helix 1 repeats

Genetic interaction studies link autosomal dominant and recessive polycystic kidney disease in a common pathway

Polycystic kidney disease (PKD) describes a heterogeneous collection of disorders that differ significantly with respect to their etiology and clinical presentation. They share, however, abnormal tubular morphology as a common feature, leading to the hypothesis that their respective gene products may function cooperatively in a common pathway to maintain tubular integrity. To study the pathobiology of one major form of human PKD, we generated a mouse line with a floxed allele of Pkhd1, the orthologue of the gene mutated in human autosomal recessive PKD. Cre-mediated excision of exons 3-4 results in a probable hypomorphic allele. Pkhd1(del3-4/del3-4) developed a range of phenotypes that recapitulate key features of the human disease. Like in humans, abnormalities of the biliary tract were an invariant finding. Most mice 6 months or older also developed renal cysts. Subsets of animals presented with either perinatal respiratory failure or exhibited growth retardation that was not due to the renal disease. We then tested for genetic interaction between Pkhd1 and Pkd1, the mouse orthologue of the gene most commonly linked to human autosomal dominant PKD. Pkd1(+/-); Pkhd1(del3-4/del3-4) mice had markedly more severe disease than Pkd1(+/+); Pkhd1(del3-4/del3-4) littermates. These studies are the first to show genetic interaction between the major loci responsible for human renal cystic disease in a common PKD pathway.

A mouse model of autosomal recessive polycystic kidney disease with biliary duct and proximal tubule dilatation

Autosomal recessive polycystic kidney disease (ARPKD) is caused by mutations in the polycystic kidney and hepatic disease (PKHD1) gene encoding the protein fibrocystin/polyductin. The aim of our study was to produce a mouse model of ARPKD in which there was no functional fibrocystin/polyductin to study the pathophysiology of cystic and fibrocystic disease in renal and non-renal tissues. Exon 2 of the gene was deleted and replaced with a neomycin resistance cassette flanked by loxP sites, which could be subsequently removed by Cre-lox recombinase. Homozygous Pkhd1(del2/del2) mice were viable, fertile and exhibited hepatic, pancreatic, and renal abnormalities. The biliary phenotype displayed progressive bile duct dilatation, resulting in grossly cystic and fibrotic livers in all animals. The primary cilia in the bile ducts of these mutant mice had structural abnormalities and were significantly shorter than those of wild-type (WT) animals. The Pkhd1(del2/del2) mice often developed pancreatic cysts and some exhibited gross pancreatic enlargement. In the kidneys of affected female mice, there was tubular dilatation of the S3 segment of the proximal tubule (PT) starting at about 9 months of age, whereas male mice had normal kidneys up to 18 months of age. Inbreeding the mutation onto BALBc/J or C57BL/6J background mice resulted in females developing PT dilatation by 3 months of age. These inbred mice will be useful resources for studying the mechanisms underlying the pathogenesis of ARPKD.

Genotype-phenotype correlations in autosomal dominant and autosomal recessive polycystic kidney disease

The phenotypes that are associated with the common forms of polycystic kidney disease (PKD)--autosomal dominant (ADPKD) and autosomal recessive (ARPKD)--are highly variable in penetrance. This is in terms of severity of renal disease, which can range from neonatal death to adequate function into old age, characteristics of the liver disease, and other extrarenal manifestations in ADPKD. Influences of the germline mutation are at the genic and allelic levels, but intrafamilial variability indicates that genetic background and environmental factors are also key. In ADPKD, the gene involved, PKD1 or PKD2, is a major factor, with ESRD occurring 20 yr later in PKD2. Mutation position may also be significant, especially in terms of the likelihood of vascular events, with 5' mutations most detrimental. Variance component analysis in ADPKD populations indicates that genetic modifiers are important, but few such factors (beyond co-inheritance of a TSC2 mutation) have been identified. Hormonal influences, especially associated with more severe liver disease in female individuals, indicate a role for nongenetic factors. In ARPKD, the combination of mutations is critical to the phenotypic outcome. Patients with two truncating mutations have a lethal phenotype, whereas the presence of at least one missense change can be compatible with life, indicating that many missense changes are hypomorphic alleles that generate partially functional protein. Clues from animal models and other forms of PKD highlight potential modifiers. The information that is now available on both genes is of considerable prognostic value with the prospects from the ongoing genetic revolution that additional risk factors will be revealed.

Octreotide inhibits hepatic cystogenesis in a rodent model of polycystic liver disease by reducing cholangiocyte adenosine 3',5'-cyclic monophosphate

BACKGROUND AND AIMS

In polycystic liver diseases (PCLDs), increased cholangiocyte proliferation and fluid secretion are key features and cholangiocyte adenosine 3',5'-cyclic monophosphate (cAMP) is an important regulator of these processes. Thus, we assessed cAMP levels and evaluated octreotide (an analogue of somatostatin known to inhibit cAMP) in hepatic cyst growth using an in vitro model of cystogenesis and an in vivo animal model of autosomal recessive polycystic kidney disease (ARPKD), one of the PCLDs.

METHODS

Expression of somatostatin receptors (SSTRs) was assessed by reverse-transcription polymerase chain reaction and confocal microscopy in cholangiocytes from normal and polycystic kidney (PCK) rats, the ARPKD model of autosomal recessive polycystic kidney disease. Effects of octreotide on cAMP levels and cyst expansion were studied in vitro using PCK bile ducts grown in 3-dimensional culture. The effects of octreotide on hepatic and renal cystogenesis were investigated in PCK rats in vivo.

RESULTS

In cholangiocytes and serum of PCK rats, cAMP concentrations were approximately 2 times higher than in normal rats. SSTR subtypes that bind octreotide (ie, SSTR2, SSTR3, and SSTR5) were expressed in both normal and PCK cholangiocytes. In vitro, octreotide inhibited cAMP levels by 35% and reduced cyst growth by 44%. In vivo, octreotide lowered cAMP content in cholangiocytes and serum by 32%-39% and inhibited hepatic disease progression, leading to 22%-60% reductions in liver weight, cyst volume, hepatic fibrosis, and mitotic indices. Similar effects were observed in kidneys of PCK rats.

CONCLUSIONS

This preclinical study provides a strong rationale for assessing the potential value of octreotide in the treatment of PCLDs.

Fibrocystin/polyductin, found in the same protein complex with polycystin-2, regulates calcium responses in kidney epithelia

Recent evidence suggests that fibrocystin/polyductin (FPC), polycystin-1 (PC1), and polycystin-2 (PC2) are all localized at the plasma membrane and the primary cilium, where PC1 and PC2 contribute to fluid flow sensation and may function in the same mechanotransduction pathways. To further define the exact subcellular localization of FPC, the protein product encoded by the PKHD1 gene responsible for autosomal recessive polycystic kidney disease (PKD) in humans, and whether FPC has direct and/or indirect cross talk with PC2, which, in turn, is pivotal for the pathogenesis of autosomal dominant PKD, we performed double immunostaining and coimmunoprecipitation as well as a microfluorimetry study of kidney tubular epithelial cells. FPC and PC2 are found to completely or partially colocalize at the plasma membrane and the primary cilium and can be reciprocally coimmunoprecipitated. Although incomplete removal of FPC by small interfering RNA and antibody 803 to intracellular epitopes of FPC did not abolish flow-induced intracellular calcium responses, antibody 804 to extracellular epitopes of FPC blocked cellular calcium responses to flow stimulation. These findings suggest that FPC and polycystins share, at least in part, a common mechanotransduction pathway.

Galectin-3 associates with the primary cilium and modulates cyst growth in congenital polycystic kidney disease

Several lines of evidence implicate the beta-galactoside-binding lectin galectin-3 in development and pathological processes in renal collecting ducts: galectin-3 is expressed in the ureteric bud/collecting duct lineage during nephrogenesis, modulates collecting duct growth/differentiation in vitro, and is expressed in human autosomal recessive polycystic kidney disease in cyst epithelia, almost all of which arise from collecting ducts. Moreover, exogenous galectin-3 restricts growth of cysts generated by Madin-Darby canine kidney collecting duct-derived cells in three-dimensional culture in collagen. Using the cpk mouse model of recessively inherited polycystic kidney disease, we observed widespread galectin-3 mRNA and protein in cyst epithelia. Exogenous galectin-3 reduced cyst formation in suspension culture, and mice-null mutant for galectin-3 had more extensive renal cysts in vivo. Galectin-3 was also detected for the first time in the centrosome/primary cilium, which has been implicated in diverse polycystic kidney disease. Cilia structure/number appeared normal in galectin-3-null mutants. Finally, paclitaxel, a therapy that retards polycystic kidney disease in cpk mice, increased extracellular galectin-3, in which the lectin could potentially interact with cilia. These data raise the possibility that galectin-3 may act as a natural brake on cystogenesis in cpk mice, perhaps via ciliary roles.

Polycystic kidney disease: genes, proteins, animal models, disease mechanisms and therapeutic opportunities

An increased understanding of the genetic, molecular and cellular mechanisms responsible for the development of polycystic kidney disease has laid out the foundation for the development of rational therapies. Many animal models where these therapies can be tested are currently available. This review summarizes the rationale for these treatments, the results of preclinical trials and the prospects for clinical trials, some already in early phases of implementation.

Inhibition of intrahepatic bile duct dilation of the polycystic kidney rat with a novel tyrosine kinase inhibitor gefitinib

The polycystic kidney (PCK) rat represents a liver and kidney cyst pathology corresponding to Caroli's disease with congenital hepatic fibrosis and autosomal recessive polycystic kidney disease. We previously reported that an epidermal growth factor receptor tyrosine kinase inhibitor, gefitinib (Iressa), significantly inhibited the abnormal growth of biliary epithelial cells of PCK rats in vitro. This study investigated the effects of gefitinib on cyst pathogenesis of the PCK rat both in vitro and in vivo. A three-dimensional culture model of biliary epithelial cells in the collagen gel matrix was used for in vitro analysis. For in vivo experiments, PCK and control rats were treated with gefitinib between 3 and 10 weeks of age. In vitro, gefitinib had strong inhibitory effects on biliary cyst formation of PCK rats. In vivo, treatment with gefitinib significantly inhibited the cystic dilatation of the intrahepatic bile ducts of PCK rats, which was accompanied by improvement of liver fibrosis. By contrast, no beneficial effects were observed on renal cyst development because of the treatment. These results suggest that signaling pathways mediated by epidermal growth factor receptor are involved in biliary dysgenesis of the PCK rat, with the mechanisms of cyst progression being different between the liver and kidney.

Long-lasting arrest of murine polycystic kidney disease with CDK inhibitor roscovitine

Polycystic kidney diseases (PKDs) are primarily characterized by the growth of fluid-filled cysts in renal tubules leading to end-stage renal disease. Mutations in the PKD1 or PKD2 genes lead to autosomal dominant PKD (ADPKD), a slowly developing adult form. Autosomal recessive polycystic kidney disease results from mutations in the PKHD1 gene, affects newborn infants and progresses very rapidly. No effective treatment is currently available for PKD. A previously unrecognized site of subcellular localization was recently discovered for all proteins known to be disrupted in PKD: primary cilia. Because ciliary functions seem to be involved in cell cycle regulation, disruption of proteins associated with cilia or centrioles may directly affect the cell cycle and proliferation, resulting in cystic disease. We therefore reasoned that the dysregulated cell cycle may be the most proximal cause of cystogenesis, and that intervention targeted at this point could provide significant therapeutic benefit for PKD. Here we show that treatment with the cyclin-dependent kinase (CDK) inhibitor (R)-roscovitine does indeed yield effective arrest of cystic disease in jck and cpk mouse models of PKD. Continuous daily administration of the drug is not required to achieve efficacy; pulse treatment provides a robust, long-lasting effect, indicating potential clinical benefits for a lifelong therapy. Molecular studies of the mechanism of action reveal effective cell-cycle arrest, transcriptional inhibition and attenuation of apoptosis. We found that roscovitine is active against cysts originating from different parts of the nephron, a desirable feature for the treatment of ADPKD, in which cysts form in multiple nephron segments. Our results indicate that inhibition of CDK is a new and effective approach to the treatment of PKD.

Diagnosis, pathogenesis, and treatment prospects in cystic kidney disease

Cystic kidney diseases (CKDs) are a clinically and genetically heterogeneous group of disorders characterized by progressive fibrocystic renal and hepatobiliary changes. Recent findings have proven the cystogenic process to be compatible with cellular dedifferentiation, i. e. increased apoptosis and proliferation rates, altered protein sorting and secretory characteristics, as well as disorganization of the extracellular matrix. Compelling evidence suggests that cilia play a central pathogenic role and most cystic kidney disorders converge into a common pathogenic pathway. Recently, several promising trials have further extended our understanding of the pathophysiology of CKD and may have the potential for rational personalized therapies in future years. This review aims to summarize the current state of knowledge of the structure and function of proteins underlying polycystic kidney disease, to explore the clinical consequences of changes in respective genes, and to discuss potential therapeutic approaches.

Renal cystic diseases: diverse phenotypes converge on the cilium/centrosome complex

Inherited renal cystic diseases constitute an important set of single-gene disorders that frequently progress to end stage renal disease (ESRD). Transmitted as autosomal dominant, autosomal recessive, or X-linked traits, renal cystic diseases are phenotypically diverse with respect to age at onset, rate of disease progression, and associated extra-renal manifestations. These disorders involve defects in a set of gene products commonly referred to as cystoproteins that, while functionally distinct, appear to co-localize, at least in part, with the cilia/centrosome complex. Therefore, investigations are increasingly focused on the role of this complex in the pathogenesis of renal cystic disease. Sorting out the functional relationship between these cystoproteins and the cilia/centrosome complex will undoubtedly provide a better understanding of renal cystic disease pathogenesis and, potentially, identify new targets for therapeutic intervention.

Renal cystic disease: new insights for the clinician

This article cannot comprehensively cover the enormous strides made in defining the molecular and cellular basis of renal cystic diseases over the last decade. Therefore, it provides a brief overview and categorization of inherited, developmental, and acquired renal cystic diseases, providing a relevant, up-to-date bibliography as well as a useful list of informative Internet Web sites. Its major focus is the translational biology of polycystic kidney disease. It demonstrates how emerging molecular and cellular knowledge of the pathophysiology of particular diseases such as autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ADPKD) can translate into innovative therapeutic insights.

Kinesin-2 mediates physical and functional interactions between polycystin-2 and fibrocystin

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1, encoding polycystin-1 (PC1), or PKD2 (polycystin-2, PC2). Autosomal recessive PKD (ARPKD) is caused by mutations in PKHD1, encoding fibrocystin/polyductin (FPC). No molecular link between ADPKD and ARPKD has been determined. Here, we demonstrated, by yeast two-hybrid and biochemical assays, that KIF3B, a motor subunit of kinesin-2, associates with PC2 and FPC. Co-immunoprecipitation experiments using Madin-Darby canine kidney (MDCK) and inner medullary collecting duct (IMCD) cells and human kidney revealed that PC2 and KIF3B, FPC and KIF3B and, furthermore, PC2 and FPC are endogenously in the same complex(es), though no direct association between the PC2 and FPC intracellular termini was detected. In vitro binding and Far Western blot experiments demonstrated that PC2 and FPC are in the same complex only if KIF3B is present, presumably by forming a PC2-KIF3B-FPC complex. This was supported by our observation that altering KIF3B level in IMCD cells by over-expression or siRNA significantly affected complexing between PC2 and FPC. Immunofluorescence experiments showed that PC2, FPC and KIF3B partially co-localized in primary cilia of over-confluent and perinuclear regions of sub-confluent cells. Furthermore, KIF3B mediated functional modulation of purified PC2 channels by FPC in a planer lipid bilayer electrophysiology system. The FPC C-terminus substantially stimulated PC2 channel activity in the presence of KIF3B, whereas FPC or KIF3B alone had no effect. Taken together, we discovered that kinesin-2 is a linker between PC2 and FPC and mediates the regulation of PC2 channel function by FPC. Our study may be important for elucidating common molecular pathways for PKD of different genotypes.

Proteolytic cleavage and nuclear translocation of fibrocystin is regulated by intracellular Ca2+ and activation of protein kinase C

Fibrocystin, a type I membrane protein of unknown function, is the protein affected in the autosomal recessive form of polycystic kidney disease. Here we show that fibrocystin undergoes regulated proteolysis. Several proteolytic cleavages occur within the predicted ectodomain, whereas at least one cleavage occurs within the cytoplasmic portion. The latter generates a C-terminal intracellular fragment that harbors the nuclear localization signal KRKVSRLAVTGERTATPAPKIPRIT and translocates to the nucleus. Proteolytic cleavage of fibrocystin occurs constitutively in long term cultures of polarized inner medullary collecting duct cells (mIMCD-3). Activation of protein kinase C and release of intracellular Ca2+ are required for proteolysis under these conditions. In short term cultures of human embryonic kidney 293 cells (HEK-293), proteolytic cleavage of fibrocystin can be elicited by stimulation of intracellular Ca2+ release or activation of protein kinase C. These results identify a novel Ca2+-dependent pathway that signals from fibrocystin located in the cell membrane to the nucleus.

Development and characterization of a cholangiocyte cell line from the PCK rat, an animal model of Autosomal Recessive Polycystic Kidney Disease

In the PCK rat, a rodent model of Autosomal Recessive Polycystic Kidney Disease (ARPKD), a spontaneous splicing mutation of Pkhd1 initiates hepatic cyst development. Cystic cholangiocytes possess short and malformed cilia that do not express fibrocystin, the Pkhd1 protein. During the disease course, cysts continue to grow; however, the mechanisms underlying cyst progression are unclear due in part to the lack of suitable cell lines to study cystogenesis. Here, we describe the development of a PCK-derived cholangiocyte cell line (PCK-CCL). Normal rat cholangiocytes (NRCs) were used as a control. The PCK-CCL maintained a cholangiocyte phenotype as assessed by the expression of the CK-19, CK-7 and GGT. PCK-CCL grown on collagen formed a polarized monolayer with well-developed junctional complexes, and distinct apical and basolateral membranes. Compared to NRCs, cilia in the PCK-CCL were short and malformed and did not express fibrocystin. The PCK-CCL exhibited a higher rate of proliferation (P<0.05) with a doubling time approximately half that of NRCs. By RT-PCR analysis of exons 33-37, an approximately 800 bp product of Pkhd1 was amplified in NRCs. In contrast and as expected, in the PCK-CCL, the Pkhd1 amplicon was smaller ( approximately 630 bp) reflecting the IVS35-2A --> T mutation. PCK-CCL and NRCs seeded in 3-D cultures formed cystic structures; however, the PCK cysts expanded progressively up to day 21 while cysts formed by NRCs remained the same size after day 9. In summary, we have developed a cholangiocyte cell line from the PCK rat that retains properties of the cholangiocytes lining hepatic cysts in vivo. The cells have been grown continuously for approximately 18 month and 45 passages without crisis or senescence. The morphology and growth characteristics of the PCK-CCL are consistent with those seen in vivo in the PCK rat, suggesting that this cell line will be useful in dissecting the mechanisms of hepatic cyst formation.

[Polycystic kidney diseases: molecular genetics and counselling]

Autosomal dominant polycystic kidney disease (ADPKD) affects 1 newborn in 400 to 1000 making it the most common inherited form of genetic kidney disease and an important cause of medical morbidity and account for about 10% of end-stage renal disease. Autosomal recessive polycystic kidney disease (ARPKD) is a rare (1/20,000 to 1/40,000) inherited disease in children characterized by the association of dilation of collecting ducts and biliary dysgenesis. The clinical spectrum is variable but it represents an important cause of renal and liver-related morbidity and mortality in neonates and infancy. Symptoms of autosomal recessive PKD can begin before birth. ARPKD is genetically different from ADPKD. Parents who do not have the disease can have a child with the disease if both parents carry the abnormal gene and both pass the gene to their baby. Recently important advances in understanding the molecular basis of ADPKD (i.e. ADPKD1 and ADPKD2) and autosomal recessive PKD (i.e. PKHD1) have been done and are reported here. Genetic counselling is particularly advised in early onset disease families. It permits to determine the type of transmission, to describe the course and the major complications of the disease and to explain currents therapeutics possibilities.

Functional analysis of PKHD1 splicing in autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD) is caused by mutations in the PKHD1 (polycystic kidney and hepatic disease 1) gene on chromosome 6p12. The longest continuous open reading frame comprises 66 exons encoding a novel 4,074 aa multidomain integral membrane protein (polyductin/fibrocystin) of unknown function. Various alternatively spliced transcripts may additionally result in different isoproteins. Overall, the large size of PKHD1, its complex pattern of splicing, multiple allelism and lack of knowledge of the encoded protein's/proteins' functions pose significant challenges to DNA-based diagnostic testing. Nucleotide substitutions, particularly if residing in regulatory elements or introns outside the splice consensus sites, are often difficult to assess without further functional analyses and cannot be unambiguously classified as disease-associated. Investigations on the transcript level, however, are hampered as PKHD1 is not widely expressed in blood lymphocytes. We thus determined the functional significance of the novel splice site mutation c.53-3C>A in intron 2 by RNA analyses by minigene-construction. The mutant allele was shown to cause skipping of exon 3. Thus, given the minigene results together with 400 control chromosomes negative for this change, segregation of the mutation with the phenotype, and a significant lowering of the strength of the splice site by bioinformatics, the mutant allele is most likely pathogenic. To the best of our knowledge, this is the first study that defines the consequences of a PKHD1 splice mutation and underlines the relevance of functional analyses in determining the pathogenicity of changes of unknown significance.

Autosomal recessive polycystic kidney disease and congenital hepatic fibrosis: summary statement of a first National Institutes of Health/Office of Rare Diseases conference

Researchers and clinicians with expertise in autosomal recessive polycystic kidney disease and congenital hepatic fibrosis (ARPKD/CHF) and related fields met on May 5-6, 2005, on the National Institutes of Health (NIH) campus for a 1.5-day symposium sponsored by the NIH Office of Rare Diseases, the National Human Genome Research Institute (NHGRI), and in part by the ARPKD/CHF Alliance. The meeting addressed the present status and the future of ARPKD/CHF research.

Four mutations in Epidermodysplasia verruciformis 1 (EVER1) gene are not contributors to susceptibility in RRP

OBJECTIVE

Epidermodysplasia verruciformis is a skin disease characterized by abnormal susceptibility to human papilloma viruses. Recently four mutations in the Epidermodysplasia verruciformis 1 gene (EVER1, also known as TMC6) have been associated with the disease. Because of the phenotypic similarity between Epidermodysplasia verruciformis and recurrent respiratory papillomatosis, we decided to investigate whether any of these mutations accounts for the susceptibility to human papilloma viruses in subjects with recurrent respiratory papillomatosis (RRP).

METHODS

Allele-specific PCR and restriction fragment length polymorphisms (RFLPs) were employed for genotyping a cohort of 101 patients with recurrent respiratory papillomatosis.

RESULTS

None of these four mutations were found in the studied subjects.

CONCLUSION

The absence of these mutations in RRP patients might indicate that EVER 1 alleles are not associated with susceptibility to RRP, or that other, as yet unidentified, mutations in the Epidermodysplasia verruciformis 1 gene, might account for the susceptibility to RRP.

Molecular and cellular pathophysiology of autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) belongs to a group of congenital hepatorenal fibrocystic syndromes characterized by dual renal and hepatic involvement of variable severity. Despite the wide clinical spectrum of ARPKD (MIM 263200), genetic linkage studies indicate that mutations at a single locus, PKHD1 (polycystic kidney and hepatic disease 1), located on human chromosome region 6p21.1-p12, are responsible for all phenotypes of ARPKD. Identification of cystic disease genes and their encoded proteins has provided investigators with critical tools to begin to unravel the molecular and cellular mechanisms of PKD. PKD cystic epithelia share common phenotypic abnormalities despite the different genetic mutations that underlie the disease. Recent studies have shown that many cyst-causing proteins are expressed in multimeric complexes at distinct subcellular locations within epithelia. This co-expression of cystoproteins suggests that cyst formation, regardless of the underlying disease gene, results from perturbations in convergent and/or integrated signal transduction pathways. To date, no specific therapies are in clinical use for ameliorating cyst growth in ARPKD. However, studies noted in this review suggest that therapeutic targeting of the cAMP and epidermal growth factor receptor (EGFR)-axis abnormalities in cystic epithelia may translate into effective therapies for ARPKD and, by analogy, autosomal dominant polycystic kidney disease (ADPKD). A particularly promising approach appears to be the targeting of downstream intermediates of both the cAMP and EGFR axis. This review focuses on ARPKD and presents a concise summary of the current understanding of the molecular genetics and cellular pathophysiology of this disease. It also highlights phenotypic and mechanistic similarities between ARPKD and ADPKD.

Is congenital hepatic fibrosis a pure liver disease?

OBJECTIVES

An association between congenital hepatic fibrosis (CHF) and several different conditions is being increasingly recognized. We aimed to investigate, prospectively, these associated disorders and the clinical consequences for patients with CHF.

MATERIALS AND METHODS

CHF was diagnosed using liver biopsy, abdominal ultrasound (US), Doppler US, upper endoscopy, and abdominal computed tomography (CT) in 19 patients (13 women, 6 men). CT portography and splenoportography with digital subtraction angiography were performed if indicated. Endoscopic retrograde cholangiopancreatography (ERCP) was performed to investigate the extent of portal vein involvement of the common bile duct if it existed, to remove a stone located in the common bile duct when documented, and to confirm the diagnosis of Caroli's syndrome. Cranial MRI was done when clinical findings suggested brain involvement.

RESULTS

The mean age of the patients was 29.47+/-12.06, ranging from 13 to 57. CHF-associated diseases were Caroli's syndrome, polycystic kidney disease, cavernous transformation of the portal vein, Joubert's syndrome, von Meyenburg complex, polydactyly, medullary sponge kidney, and pancreatic duct atrophy. In two cases, cholangiocarcinoma had developed. There was only one case with pure CHF. Portosystemic shunt, TIPS, or splenectomy were performed in some cases to control bleeding from esophageal varices. Papillotomy and stone extraction from the common bile duct were performed in four patients with Caroli's syndrome complicated by cholangitis. Three patients died of complications of CHF. Two patients with Caroli's syndrome underwent liver transplantation.

CONCLUSION

In this prospective study, it seems that CHF is not a pure liver disease but rather a multiorgan disorder involving the brain, portal vein, kidneys, and bile ducts. In most cases, the clinical picture includes other organ involvement, rather than purely the liver parenchyma.

Cilia and centrosomes: a unifying pathogenic concept for cystic kidney disease?

Cystic kidney diseases are among the most frequent lethal genetic diseases. Positional cloning of novel cystic kidney disease genes revealed that their products (cystoproteins) are expressed in sensory organelles called primary cilia, in basal bodies or in centrosomes. Primary cilia link mechanosensory, visual, osmotic, gustatory and other stimuli to mechanisms of cell-cycle control and epithelial cell polarity. The ciliary expression of cystoproteins explains why many other organs might be also affected in patients with cystic kidney disease. Protein-protein interactions among cystoproteins, and their strong evolutionary conservation, provide a basis for a multidisciplinary approach to unravelling the novel signalling mechanisms that are involved in this disease group.

Algorithm for efficient PKHD1 mutation screening in autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) is an important cause of childhood renal- and liver-related morbidity and mortality with variable disease expression. While most cases manifest peri-/neonatally with a high mortality rate in the first month of life, others survive to adulthood. ARPKD is caused by mutations in the Polycystic Kidney and Hepatic Disease 1 (PKHD1) gene on chromosome 6p12. PKHD1 is an exceptionally large gene (470 kb) with a longest open reading frame transcript of 67 exons predicted to encode a 4,074-amino acid (aa) (447 kDa) multidomain integral membrane protein (fibrocystin/polyductin) of unknown function. Recent DHPLC-based mutational studies have reported detection rates of about 80% and a minimum of one PKHD1 mutation in more than 95% of families. Thus far, a total of 263 different PKHD1 mutations (639 mutated alleles) are included in the locus-specific database (www.humgen.rwth-aachen.de). Except for a few population-specific founder alleles and the common c.107C>T (p.Thr36Met) missense change, PKHD1 is characterized by significant allelic diversity, making mutation screening time-consuming and labor-intensive. Mutations are distributed throughout the gene's coding sequence; however, they are not equally scattered. Thus, we aimed to set up an algorithm for efficient molecular genetic diagnostics in ARPKD. A total of 80% of known PKHD1 mutations can be identified if a subset of 27 out of 77 DHPLC fragments is screened. The current study provides an essential platform for PKHD1 mutation screening in a routine setting that will largely alleviate molecular genetic diagnostics in patients suspected to have ARPKD.

Successful transplantation in a child with rapid progression of autosomal recessive polycystic kidney disease associated with a novel mutation

Autosomal recessive polycystic kidney disease (ARPKD) is the most common pediatric renal cystic disease with liver involvement. The vast majority of patients with ARPKD carry mutations in the recently characterized PKHD1 gene on chromosome 6p12. A Turkish female demonstrated rapid growth of both kidneys after delivery. Accelerated growth of both kidneys and increasing respiratory distress necessitated right-sided nephrectomy at the age of three months. Because of persistent dyspnea and ongoing growth of the remaining kidney, the second kidney also had to be removed one month later. Biopsies taken from the kidney and the liver confirmed the diagnosis of ARPKD histologically. Renal ultrasound of the patient's consanguineous parents and her older brother showed normal results. PKHD1 mutation analysis yielded a novel homozygous missense mutation (c.1116C >G, F372L) in exon 14, coding for an Ig-like domain (TIG), possibly involved in the increased growth of the kidneys. Peritoneal dialysis was performed for 12 months. The patient had successful transplantation at the age of 15 months and is doing well with actual immunosuppression with cyclosporine, mycophenolate mofetil, and prednisolone. In conclusion, the present case clearly demonstrates the favorable outcome of a child with severe ARPKD after bilateral nephrectomy, pre-emptive dialysis, and successful transplantation.

Renal cystic disease: the role of the primary cilium/centrosome complex in pathogenesis

Cystic diseases of the kidney are among the most frequent inherited disorders and constitute a significant cause of end-stage renal disease. Identification of the genes involved in these disorders and their protein products has provided key insights into the cellular processes that underlie cyst development and mediate disease progression. Of particular note, the proteins implicated in these disorders localize to the cilia/centrosome complex and have focused recent research on the role of this complex in the pathogenesis of renal cystic disease. Unraveling the spatial and functional relationship between these cystoproteins and the cilia/centrosome complex will undoubtedly provide a better understanding of the pathogenesis of cystic diseases and potentially offer windows for therapeutic intervention.

Renal cystic diseases: a review

This review aims to assist in the categorization of inherited, developmental, and acquired cystic disease of the kidney as well as to provide a pertinent, up-to-date bibliography. The conditions included are autosomal-dominant polycystic kidney disease, autosomal-recessive polycystic kidney disease, unilateral renal cystic disease (localized cystic disease), renal simple cysts, multicystic dysplastic kidney, pluricystic kidney of the multiple malformation syndromes, juvenile nephronophthisis and medullary cystic disease, medullary sponge kidney, primary glomerulocystic kidney disease, and glomerulocystic kidney associated with several systemic disorders mainly of genetic or chromosomal etiology, cystic kidney in tuberous sclerosis, and in von Hippel-Lindau syndrome, cystic nephroma, cystic variant of congenital mesoblastic nephroma, mixed epithelial stromal tumor of the kidney, renal lymphangioma, pyelocalyceal cyst, peripylic cyst and perinephric pseudocyst, acquired renal cystic disease of long-term dialysis, and cystic renal cell carcinoma and sarcoma. Whereas the gross and histologic appearance of some of these conditions may be diagnostic, clinical and sometimes molecular studies may be necessary to define other types.

Clinical and molecular characterization defines a broadened spectrum of autosomal recessive polycystic kidney disease (ARPKD)

The autosomal recessive form of polycystic kidney disease (ARPKD) is generally considered an infantile disorder with the typical presentation of greatly enlarged echogenic kidneys detected in utero or within the neonatal period, often resulting in neonatal demise. However, there is an increasing realization that survivors often thrive into adulthood with complications of the ductal plate malformation, manifesting as congenital hepatic fibrosis and Caroli disease, becoming prominent. Previous natural history studies have concentrated almost exclusively on the infantile presenting group. However, developments in understanding the genetic basis of ARPKD, through identification of the disease gene, PKHD1, have allowed exploration of the etiology in patients with ARPKD-like disease or congenital hepatic fibrosis presenting later in childhood or as adults. In the current study we retrospectively reviewed the clinical records, and where possible performed PKHD1 mutation screening, in patients diagnosed with ARPKD or congenital hepatic fibrosis at the Mayo Clinic, Rochester, MN, from 1961 to 2004. Of a total of 133 cases reviewed, 65 were considered to meet the diagnostic criteria with an average duration of follow-up of 8.6 +/- 6.4 years. Fifty-five cases had ARPKD and 10 had isolated congenital hepatic fibrosis with no or minimal renal involvement. The patients were analyzed as 3 groups categorized by the age at diagnosis; <1 years (n = 22), 1-20 years (n = 23), and >20 years (n = 20). The presenting feature in the neonates was typically associated with renal enlargement, but in the older groups, more often involved manifestations of liver disease, including hepatosplenomegaly, hypersplenism, variceal bleeding, and cholangitis. During follow-up, 22 patients had renal insufficiency and 8 developed end-stage renal disease (ESRD), most from the neonatal group. Liver disease was evident on follow-up in all diagnostic groups but particularly prevalent in those diagnosed later in life. A total of 12 patients died, 6 in the neonatal period, but 86% of patients were alive at 40 years of age. The likelihood of being alive without ESRD differed significantly between the diagnostic groups with 36%, 80%, and 88% survival in the 3 diagnostic groups, respectively, 20 years after the diagnosis. Considerable evidence of intrafamilial phenotype variability was observed. Mutation analysis was performed in 31 families and at least 1 mutation was detected in 25 (81%), with 76% of mutant alleles detected in those cases. Consistent with the relatively mild disease manifestations in this population, the majority of changes were missense (79%) and no case had 2 truncating changes. Mutations were detected in all diagnostic groups, indicating that congenital hepatic fibrosis with minimal kidney involvement can result from PKHD1 mutation. The finding of 6 cases with no detected mutations may represent missed mutations or possible evidence of genetic heterogeneity. The current study indicates a broadened spectrum for the ARPKD phenotype and that later presenting cases with predominant liver disease should be considered part of ARPKD.

[Cystic liver diseases. Genetics and cell biology]

In 50% of cases, polycystic liver disease is associated with autosomal dominant polycystic kidney disease, which is caused by mutations in the PKD1 and PKD2 genes that encode polycystin-1 and -2, respectively. These proteins form a polycystin-1/2 complex on the plasma membrane, including that localized on the surface of primary cilia, where they act as mechanosensors. Polycystin-1 acts as a (mechano)receptor of environmental signals, and polycystin-2 as a calcium channel mediating intracellular transduction. Isolated autosomal dominant polycystic liver disease is caused by mutations in PRKCSH that encodes hepatocystin, a protein of the endoplasmic reticulum, which may participate in the N-glycosylation and maturation of proteins addressed to the cell surface. Congenital hepatic fibrosis whether it is accompanied by bile duct dilatations (Caroli's syndrome) or not, may be associated with autosomal recessive polycystic kidney disease, which is caused by mutations in PKHD1 that encodes fibrocystin, a protein of primary cilia. Genetic defects in fibrocystin cause ciliary dysfunction, presently considered as a major pathogenic event in cystogenesis. Excessive cell proliferation, a hallmark of cystic biliary epithelium, occurs in combination with deregulation of the epidermal growth factor (EGF) and probably also estrogen receptors. EGF receptor antagonists inhibit kidney and liver cyst development in animal models, and are currently under investigation in phase I and II clinical trials in patients with autosomal dominant polycystic kidney disease.

Analysis of missense variants in the PKHD1-gene in patients with autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) is a severe form of polycystic kidney disease characterized by enlarged kidneys and congenital hepatic fibrosis. Given the poor prognosis for the majority of children with the severe perinatal ARPKD phenotype, there is a regular request for prenatal testing. ARPKD is caused by mutations in the polycystic kidney and hepatic disease 1 (PKHD1) gene, which consists of 86 exons that are variably assembled into a number of alternatively spliced transcripts. The longest transcript, comprising 67 exons, encodes the protein fibrocystin/polyductin. We have set up mutation analysis by direct sequencing of these 67 exons. In 39 mainly Dutch families we identified: 11 nonsense mutations, 15 deletions/insertions, 5 splice site mutations, and 39 missense mutations. To classify missense variants we combined evolutionary conservation, using the human, chimpanzee, dog, mouse, chicken and frog Pkhd1 sequences, with the Grantham score for chemical differences. Thirty-three missense mutations were considered pathogenic and seven were classified as rare, probably pathogenic variants. In addition to sequence analysis, multiplex ligation-dependent probe amplification (MLPA) was used to identify multiple exon deletions. However, no large deletions in the PKHD1 gene were identified. In 31 index patients two mutations were found, in 6 patients one mutation was found, leading to a mutation detection rate of 87%. The analysis of amino acid conservation as well as applying the Grantham score for chemical differences allowed us to determine the pathogeneity for nearly all new missense mutations and thus proved to be useful tools to classify missense variants.

Fibrocystin interacts with CAML, a protein involved in Ca2+ signaling

The predicted structure of the autosomal recessive polycystic kidney disease protein, fibrocystin, suggests that it may function as a receptor, but its function remains unknown. To understand its function, we searched for proteins that interact with the intracellular C-terminus of fibrocystin using the yeast two-hybrid system. From the screening, we found calcium modulating cyclophilin ligand (CAML), a protein involved in Ca(2+) signaling. Immunofluorescent analysis showed that both proteins are co-localized in the apical membrane, primary cilia, and the basal body of cells derived from the distal nephron Epitope-tagged expression constructs of both proteins were co-immunoprecipitated from COS7 cells. The intracellular C-terminus of fibrocystin interacts with CAML, a protein with an intracellular distribution that is similar to that of PKD2. Fibrocystin may participate in regulation of intracellular Ca(2+) in the distal nephron in a manner similar to PKD1 and PKD2 that are involved in autosomal dominant polycystic kidney disease.

Liver disease in autosomal recessive polycystic kidney disease

Hepatic complications occur in a significant proportion of children with autosomal recessive polycystic kidney disease (ARPKD). PKHD1/fibrocystin, the defective gene in ARPKD, is expressed in the cilia of bile duct epithelium and leads to abnormalities in the rubric of the ductal plate malformation. Portal hypertension and biliary disease are the major liver problems seen in ARPKD. Complete blood counting, physical examination, ultrasonography and magnetic resonance (MR) cholangiography are indicated as screening procedures for hepatic disease in ARPKD. Medical and surgical interventions are potentially indicated for children with portal hypertension and/or biliary disease. A high index of suspicion for the diagnosis of cholangitis needs to be maintained in children with biliary disease. The implications of hepatic disease need to be considered in the decision-making regarding renal transplantation in ARPKD.

Congenital hepatic fibrosis: a very uncommon cause of pancytopenia in children

The disease presentation of autosomal recessive polycystic kidney disease (OMIM #263200, ARPKD) is highly variable and includes polycystic kidneys, pulmonary hypoplasia, and congenital hepatic fibrosis. The authors report an unusual case of ARPKD presenting with hepatosplenomegaly and cytopenia mimicking acute leukemia.

Haplotype analysis improves molecular diagnostics of autosomal recessive polycystic kidney disease

BACKGROUND

Autosomal recessive polycystic kidney disease (ARPKD) is characterized by wide phenotypic variability, ranging from in utero detection with enlarged, echogenic kidneys to an adult presentation with congenital hepatic fibrosis. The ARPKD gene, PKHD1 , covers about 470 kb of DNA (67 exons), and mutation studies have found marked allelic heterogeneity with a high level of novel missense changes and neutral polymorphisms. To improve the prospects for molecular diagnostics and to study the origin of some relatively common mutations, the authors have developed a strategy for improved ARPKD haplotyping.

METHODS

A protocol of multiplex PCR and fluorescence genotyping in a single capillary has been developed to assay 7 highly informative simple sequence repeat (SSR) markers that are intragenic or closely flanking PKHD1.

RESULTS

Examples in which haplotype analysis, used in combination with mutation screening, improved the utility of molecular diagnostics, especially in families in which just a single PKHD1 mutation has been identified, are illustrated. The new markers also allow screening for larger DNA deletions, detecting unknown consanguinity and exploring the disease mechanism. Analysis of 8 recurring mutations has shown likely common haplotypes for each, and the divergence from the ancestral haplotype, by recombination, can be used to trace the history of the mutation. The common mutation, T36M, was found to have a single European origin, about 1,225 years ago.

CONCLUSION

Improved haplotype analysis of ARPKD complements mutation-based diagnostics and helps trace the history of common PKHD1 mutations.

Inhibition of Pkhd1 impairs tubulomorphogenesis of cultured IMCD cells

Fibrocystin/polyductin (FPC), the gene product of PKHD1, is responsible for autosomal recessive polycystic kidney disease (ARPKD). This disease is characterized by symmetrically large kidneys with ectasia of collecting ducts. In the kidney, FPC predominantly localizes to the apical domain of tubule cells, where it associates with the basal bodies/primary cilia; however, the functional role of this protein is still unknown. In this study, we established stable IMCD (mouse inner medullary collecting duct) cell lines, in which FPC was silenced by short hairpin RNA inhibition (shRNA). We showed that inhibition of FPC disrupted tubulomorphogenesis of IMCD cells grown in three-dimensional cultures. Pkhd1-silenced cells developed abnormalities in cell-cell contact, actin cytoskeleton organization, cell-ECM interactions, cell proliferation, and apoptosis, which may be mediated by dysregulation of extracellular-regulated kinase (ERK) and focal adhesion kinase (FAK) signaling. These alterations in cell function in vitro may explain the characteristics of ARPKD phenotypes in vivo.

Feline polycystic kidney disease is linked to the PKD1 region

Autosomal dominant polycystic kidney disease (ADPKD) is a commonly inherited disorder (1/1000) in humans characterized by fluid-filled cysts in the kidneys. Defects in the PKD genes, PKD1 and PKD2, cause 85% and 15% of human ADPKD cases, respectively. Mutations in the PKHD1 gene cause autosomal recessive PKD (ARPKD). Mutations in several genes, including Nek8, cause PKD in mice. Although PKD affects 38% of Persian cats worldwide, making it the most prominent inherited feline disease, a causative gene has not been identified. Feline PKD is an autosomal dominant disease with clinical presentations similar to human ADPKD. Forty-three microsatellites were chosen from the feline genetic maps based on known homology with human chromosomal regions containing the PKD1, PKD2, PKHD1, and Nek8 genes. Linkage analysis using seven Persian cat pedigrees segregating for PKD has shown significant linkage and no recombinants (Z=5.83, theta=0) between the PKD disease phenotype and marker FCA476, which is within 10 cR of the feline PKD1 gene on Chromosome E3. This suggests that the PKD1 gene or another gene within this region may cause feline PKD. Further investigation into the cause of PKD will be valuable for feline health and provide insights into human ADPKD.

A mechanistic approach to inherited polycystic kidney disease

There are approximately six and a half million people, of the estimated world population of six billion, with inherited polycystic kidney disease. Polycystic kidney diseases have a broad spectrum of associated findings that distinguish and define them as specific disease states. The dysregulation of renal tubular epithelial cell biology, including cell polarity, cell signaling, proliferation and apoptosis, basement membrane and matrix abnormalities, and fluid transport, has been postulated to contribute to cystogenesis. Evidence is currently accumulating that supports an association of the primary cilium and basal body, as well as the focal adhesion assembly, with polycystic kidney diseases. Renal cystogenesis may be the result of a disruption of a critical feedback loop that regulates tissue morphology based on the epithelial cell environment.

A mouse model for cystic biliary dysgenesis in autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) is an important cause of liver- and renal-related morbidity and mortality in childhood. Recently, PKHD1, the gene encoding the transmembrane protein polyductin, was shown to be mutated in ARPKD patients. We here describe the first mouse strain, generated by targeted mutation of Pkhd1. Due to exon skipping, Pkhd1ex40 mice express a modified Pkhd1 transcript and develop severe malformations of intrahepatic bile ducts. Cholangiocytes maintain a proliferative phenotype and continuously synthesize TGF-beta1. Subsequently, mesenchymal cells within the hepatic portal tracts continue to synthesize collagen, resulting in progressive portal fibrosis and portal hypertension. Fibrosis did not involve the hepatic lobules, and we did not observe any pathological changes in morphology or function of hepatocytes. Surprisingly and in contrast to human ARPKD individuals, Pkhd1ex40 mice develop morphologically and functionally normal kidneys. In conclusion,our data indicate that subsequent to formation of the embryonic ductal plate, dysgenesis of terminally differentiated bile ducts occurs in response to the Pkhd1ex40 mutation. The role of polyductin in liver and kidney may be functionally divergent, because protein domains essential for bile duct development do not affect nephrogenesis in our mouse model.

Cilium-generated signaling and cilia-related disorders

Biologists have long known that humans experience their environment through cilia. Light, odorant, and sound perception depend on these microtubule-filled, complex organelles present on cells in primary sensory tissues. Recently, discoveries on the mechanism of assembly of cilia (flagella) in the lowly, biflagellated, eucaryotic green alga Chlamydomonas have triggered a renaissance of interest in the organelles along with a recognition of their key sensory roles in nonsensory tissues. Chlamydomonas researchers uncovered an entirely new set of cellular machinery essential for transporting the protein components of cilia and flagella in all ciliated/flagellated eukaryotic cells between their site of synthesis in the cell body and their site of assembly at the tip of the flagellum (intraflagellar transport: IFT). Prompted by the surprising observations that disruption of IFT genes in mice led to polycystic kidney disease (PKD) and that PKD proteins are present on the sensory cilia of Caenorhabditis elegans, researchers have made a direct connection between PKD and cilia. At least five (and possibly all) of the seven identified human genes disrupted in PKD and a related disorder nephronophthisis encode proteins expressed in the primary cilia that project into the lumen from the epithelial cells that line renal tubules. Moreover, the renal cilia are flow sensors and at least two of the PKD genes encode ciliary transmembrane proteins essential for mechanosensation. Although their roles have not yet been as clearly identified, cilia also are at the center of a rare human disorder, Bardet-Biedl syndrome (BBS), in which patients exhibit phenotypes of common human diseases, including obesity and increased incidence of hypertension and diabetes. Five of the eight known BBS genes encode basal body or cilia proteins in mice or humans, and homologues of two of the remaining genes are present in basal bodies/cilia of model organisms. Here we briefly describe the biology of cilia and flagella, we outline how studies on model organisms have led to our current understanding of the roles of these organelles and their proteins in health and disease, and we highlight the notion that the primary cilia present on cells throughout the body, even those on brain neurons, may be essential for as yet undiscovered cilium-generated signaling functions.

Activation of the MEK5/ERK5 cascade is responsible for biliary dysgenesis in a rat model of Caroli's disease

Polycystic kidney (PCK) rats exhibit a multiorgan cyst pathology similar to human autosomal recessive polycystic kidney disease, and are proposed as an animal model of Caroli's disease with congenital hepatic fibrosis (CHF). This study investigated the expression and function of selected components of the mitogen activated protein kinase (MAPK) pathway in cultured intrahepatic biliary epithelial cells (BECs) of PCK rats. Compared to the proliferative activity of cultured BECs of control rats, those of the PCK rats were hyperresponsive to epidermal growth factor (EGF). The increase in BEC proliferation was accompanied by overexpression of MAPK/extracellular signal-regulated protein kinase (ERK) kinase 5 (MEK5), and subsequent phosphorylation of ERK5 in vitro. The increased proliferative activity was significantly inhibited by the transfection of short interfering RNA against MEK5 mRNA. An EGF receptor tyrosine kinase inhibitor, gefitinib ("Iressa", ZD1839), also significantly inhibited the abnormal growth of cultured BECs of PCK rats. By contrast, treatment with PD98059 and U0126, inhibitors for MEK1/2, was less effective. These results suggest that the activation of the MEK5-ERK5 cascade plays a pivotal role in the biliary dysgenesis of PCK rats, and also provide insights into the pathogenesis of Caroli's disease with CHF. As the MEK5-ERK5 interaction is highly specific, it may represent a potential target of therapy.

Role of the Hepatocyte Nuclear Factor-1β (HNF-1β) C-terminal Domain in Pkhd1 (ARPKD) Gene Transcription and Renal Cystogenesis

Hepatocyte nuclear factor-1beta (HNF-1beta) is a homeodomain-containing transcription factor that regulates tissue-specific gene expression in the kidney and other epithelial organs. Mutations of HNF-1beta produce congenital cystic abnormalities of the kidney, and previous studies showed that HNF-1beta regulates the expression of the autosomal recessive polycystic kidney disease (ARPKD) gene, Pkhd1. Here we show that the C-terminal region of HNF-1beta contains an activation domain that is functional when fused to a heterologous DNA-binding domain. An HNF-1beta deletion mutant lacking the C-terminal domain interacts with wild-type HNF-1beta, binds DNA, and functions as a dominant-negative inhibitor of a chromosomally integrated Pkhd1 promoter. The activation of the Pkhd1 promoter by wild-type HNF-1beta is stimulated by sodium butyrate or coactivators CREB (cAMP-response element)-binding protein (CBP) and P/CAF. The interaction with CBP and P/CAF requires the C-terminal domain. Expression of an HNF-1beta C-terminal deletion mutant in transgenic mice produces renal cysts, increased cell proliferation, and dilatation of the ureter similar to mice with kidney-specific inactivation of HNF-1beta. Pkhd1 expression is inhibited in cystic collecting ducts but not in non-cystic proximal tubules, despite transgene expression in this nephron segment. We conclude that the C-terminal domain of HNF-1beta is required for the activation of the Pkhd1 promoter. Deletion mutants lacking the C-terminal domain function as dominant-negative mutants, possibly by preventing the recruitment of histone acetylases to the promoter. Cyst formation correlates with inhibition of Pkhd1 expression, which argues that mutations of HNF-1beta produce kidney cysts by down-regulating the ARPKD gene, Pkhd1. Expression of HNF-1alpha in proximal tubules may protect against cystogenesis.

Clinical consequences of PKHD1 mutations in 164 patients with autosomal-recessive polycystic kidney disease (ARPKD)

BACKGROUND

ARPKD is associated with mutations in the PKHD1 gene on chromosome 6p12. Most cases manifest peri-/neonatally with a high mortality rate in the first month of life while the clinical spectrum of surviving patients is much more variable than generally perceived.

METHODS

We examined the clinical course of 164 neonatal survivors (126 unrelated families) over a mean observation period of 6 years (range 0 to 35 years). PKHD1 mutation screening was done by denaturing high-performance liquid chromatography (DHPLC) for the 66 exons encoding the 4074 aa fibrocystin/polyductin protein.

RESULTS AND CONCLUSION

This is the first study that reports the long-term outcome of ARPKD patients with defined PKHD1 mutations. The 1- and 10-year survival rates were 85% and 82%, respectively. Chronic renal failure was first detected at a mean age of 4 years. Actuarial renal survival rates [end point defined as start of dialysis/renal transplantation (RTX) or by death due to end-stage renal disease (ESRD)] were 86% at 5 years, 71% at 10 years, and 42% at 20 years. All but six patients (92%) had a kidney length above or on the 97th centile for age. About 75% of the study population developed systemic hypertension. Sequelae of congenital hepatic fibrosis and portal hypertension developed in 44% of patients and were related with age. Positive correlations could further be demonstrated between renal and hepatobiliary-related morbidity suggesting uniform disease progression rather than organ-specific patterns. PKHD1 mutation analysis revealed 193 mutations (70 novel ones; 77% nonconservative missense mutations). No patient carried two truncating mutations corroborating that one missense mutation is indispensable for survival of newborns. We attempted to set up genotype-phenotype correlations and to categorize missense mutations. In 96% of families we identified at least one mutated PKHD1 allele (overall detection rate 76.6%) indicating that PKHD1 mutation screening is a powerful diagnostic tool in patients suspected with ARPKD.

New options for prenatal diagnosis in autosomal recessive polycystic kidney disease by mutation analysis of the PKHD1 gene

Due to the poor prognosis of severe autosomal recessive polycystic kidney disease (ARPKD), there is a strong demand for prenatal diagnosis (PD). Reliable PD testing is possible by molecular genetic analysis only. Although haplotype-based analysis is feasible in most cases, it is associated with a risk of misdiagnosis in families without pathoanatomically proven diagnosis. Linkage analysis is impossible in families where DNA of the index patient is not available. Direct mutation analysis of the recently identified polycystic kidney and hepatic disease 1 gene opens new options in families to whom a reliable PD cannot be offered on the basis of linkage analysis. We for the first time report two cases with PD based on mutation detection, illustrating the new options for PD in ARPKD.

Kinesin family member 12 is a candidate polycystic kidney disease modifier in the cpk mouse

The cpk mouse is the most extensively characterized model of autosomal recessive polycystic kidney disease (ARPKD). The major ARPKD-related renal and biliary phenotypes are modulated in F2 mutants by genetic background, suggesting that quantitative trait loci (QTL) modulate disease severity. In 461 F2 cpk mice, kidney length, weight, and volume were scored as quantitative traits (QT), and a semiquantitative method to assess biliary duct number, area (BDA), portal vein area, and total area of each portal field, as well as the severity of cholangitis, was developed. QTL mapping was performed with Pseudomarker v1.02. Candidate genes were identified within the QTL intervals on the basis of expression profiling, reverse transcriptase-PCR, haplotypes, and sequence analysis. The renal QT were normally distributed in the F2 cohort and strongly correlated (P < 0.001). Among the biliary QT, only BDA correlated with the renal QT (P < 0.01). Genome-wide scan identified a major effect QTL on chromosome (Chr) 4 for the renal traits, adjusted BDA, and cholangitis with logarithm of odds scores of 18, 8, and 5, respectively. Regression modeling refined the Chr 4 main effect into an approximately 50-cM region with three distinct QTL peaks at 16, 34, and 54 cM. Kif12, a gene encoding a novel kinesin, mapped beneath the 34 cM QTL peak and has expression level variants and strain-specific sequences that were associated with renal disease severity in affected mice. Therefore, the positional candidate gene, Kif12, fulfills the major criteria for QTL gene discovery established by the Complex Trait Consortium, and, thus, it is proposed that Kif12 is a cpk modifier gene.

Liver development update: new embryo models, cell lineage control, and morphogenesis

The three phases of liver development that are the focus of this review are: the specification of hepatoblasts within the endoderm, the lineage split of hepatoblasts into hepatocytes and biliary cells, and the interaction of these cells with different mesodermal cell derivatives during liver morphogenesis. Advances in these areas include new genes and experimental models for studying liver development, the role of HNF6 and HNF1beta transcription factors and notch signaling in the hepatocyte-biliary cell lineage decision, the identification of genomic targets for HNF4, and HNF4's role in controlling hepatic epithelial structure and the sinusoidal organization of the liver.

Differential expression of Cux-1 and p21 in polycystic kidneys from Pkd1 null and cpk mice

BACKGROUND

Cux-1 is a murine homeodomain protein that functions as a cell cycle-dependent transcriptional repressor in proliferating cells. Targets of Cux-1 repression include the cyclin kinase inhibitors p21 and p27. In the kidney, Cux-1 is spatially and temporally regulated, and ectopic expression of Cux-1 in transgenic mice results in renal hyperplasia. Previously, we observed that Cux-1 is deregulated in cystic kidneys from cpk mice. Recent studies have suggested a role for the cyclin kinase inhibitor p21 in the development of polycystic kidney disease (PKD) in mice lacking PKD1.

METHODS

Since p21 is a target of transcriptional repression by Cux-1, we compared the expression of Cux-1 and p21 in kidneys from Pkd1 null and cpk mice by immunohistochemistry and Western blotting. We also evaluated apoptosis and the expression of the cyclin kinase inhibitor p27 in Pkd1 null and cpk mice by terminal deoxynucleotidal transferase (TdT)-mediated deoxyuridine triphosphate (dUTP) nick-end labeling (TUNEL) staining, immunohistochemistry, and Western blotting.

RESULTS

In both early and late embryonic kidneys from Pkd1 null mice, Cux-1 was highly and ectopically expressed in normal-appearing tubule epithelium, interstitial cells, and in the epithelial cells lining the cysts, where it colocalized with proliferating cell nuclear antigen (PCNA). Increased Cux-1 expression in Pkd1 null kidneys was also associated with a decrease in p27 expression at late stages of cystogenesis. In cpk kidneys, Cux-1 was not up-regulated until late stages of cyst development. Moreover, in contrast to Pkd1 null kidneys, p21 and p27 were highly expressed in cpk kidneys. In late stages of cystogenesis, Cux-1 and p21 colocalized in cyst lining cells, which also showed a high incidence of apoptosis.

CONCLUSION

These results suggest that cyst development in Pkd1 null mice and cpk mice proceeds through different mechanisms. In Pkd1 null mice, ectopic expression of Cux-1 is associated with increased cell proliferation. In contrast, in cpk mice, ectopic expression of Cux-1 is associated with apoptosis.

Germline hepatocyte nuclear factor 1alpha and 1beta mutations in renal cell carcinomas

Mutations in one copy of the hepatocyte nuclear factors (HNF) 1alpha and 1beta homeodomain containing transcription factors predispose the carrier to maturity-onset diabetes of the young (MODY) types 3 and 5, respectively. Moreover, previous identification of biallelic inactivation of HNF1alpha in hepatocellular adenoma identified its tumor suppressor function in hepatocarcinogenesis. The seminal observation of an ovarian carcinoma in a MODY5 patient who subsequently developed a chromophobe renal cell carcinoma, prompted us to screen for HNF1beta and HNF1alpha inactivation in a series of 20 ovarian and 35 renal neoplasms. Biallelic HNF1beta inactivation was found in two of 12 chromophobe renal carcinomas by association of a germline mutation and a somatic gene deletion. In these cases, the expression of PKHD1 (polycystic kidney and hepatic disease 1) and UMOD (Uromodulin), two genes regulated by HNF1beta, was turned off. Interestingly, in two of 13 clear cell renal carcinomas, we found a monoallelic germline mutation of HNF1alpha with no associated suppression of target mRNA expression. In normal and tumor renal tissues, we showed the existence of a network of transcription factors differentially regulated in tumor subtypes. We identified two related clusters of co-regulated genes associating HNF1beta, PKHD1 and UMOD in the first group and HNF1alpha, HNF4alpha, FABP1 and UGT2B7 in the second group. Finally, these results suggest that germline mutations of HNF1beta and HNF1alpha may predispose to renal tumors. Furthermore, we suggest that HNF1beta functions as a tumor suppressor gene in chromophobe renal cell carcinogenesis through a PKHD1 expression control.

Polycystic liver and kidney diseases

There have been remarkable advances in research on polycystic liver and kidney diseases recently, covering cloning of new genes, refining disease classifications, and advances in understanding more about the molecular pathology of these diseases. Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary disease affecting kidneys. It affects 1/400 to 1/1000 live births and accounts for 5% of the end stage renal disease in the United States and Europe, and is caused by gene defects in the PKD1 or PKD2 genes. Compared to ADPKD, polycystic liver disease (PCLD) is a milder disease and does not lower life expectancy. Both diseases are usually adult-onset diseases. Defects in genes, which code the hepatocystin and SEC63 proteins, have just recently been found to cause PCLD. It now seems that ADPKD is caused by malfunction of the primary cilia, a cell organ sensing fluid movement, and that PCLD is a sequel from defects in protein processing. Autosomal recessive polycystic kidney disease (ARPKD) belongs to a group of congenital hepatorenal fibrocystic syndromes. All ARPKD patients have a gene defect in a gene called PKHD1, the protein product of which localizes to primary cilia. We summarize the present clinical and molecular knowledge of these diseases in this review.

Biliary dysgenesis in the PCK rat, an orthologous model of autosomal recessive polycystic kidney disease

Hepatic polycystic disease occurs alone or in combination with polycystic kidney disease (PKD). In autosomal recessive PKD (ARPKD), liver lesions are the major cause of morbidity and mortality in older patients. ARPKD is caused by a mutation to PKHD1 and the PCK rat is an orthologous model of disease. Recently, we showed that fibrocystin, Pkhd1 protein, is localized to primary cilia in rat cholangiocytes and that disruption of its ciliary expression results in biliary cystogenesis. This study describes biliary phenotype in the PCK rat using micro-computed tomography scanning and three-dimensional reconstruction, and light, scanning, and transmission microscopy. Our results show that the biliary tree undergoes extensive remodeling resulting in bile duct dilatation, focal budding, and formation of cysts that are initially connected to bile ducts, but throughout time separate from them. Progressive liver enlargement results from massive cyst formation while liver parenchymal volume remains unchanged. Cilia in cystic cells are abnormal consistent with the notion that the primary defect in ARPKD resulting in cystogenesis may be linked to ciliary dysfunction. Our results suggest that the PCK rat is a useful model for studies of biliary cystogenesis and treatment options of inherited cystic liver disease.