Functional polycystin-1 dosage governs autosomal dominant polycystic kidney disease severity

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations to PKD1 or PKD2, triggering progressive cystogenesis and typically leading to end-stage renal disease in midlife. The phenotypic spectrum, however, ranges from in utero onset to adequate renal function at old age. Recent patient data suggest that the disease is dosage dependent, where incompletely penetrant alleles influence disease severity. Here, we have developed a knockin mouse model matching a likely disease variant, PKD1 p.R3277C (RC), and have proved that its functionally hypomorphic nature modifies the ADPKD phenotype. While Pkd1^+/null mice are normal, Pkd1^RC/null mice have rapidly progressive disease, and Pkd1^RC/RC animals develop gradual cystogenesis. These models effectively mimic the pathophysiological features of in utero–onset and typical ADPKD, respectively, correlating the level of functional Pkd1 product with disease severity, highlighting the dosage dependence of cystogenesis. Additionally, molecular analyses identified p.R3277C as a temperature-sensitive folding/trafficking mutant, and length defects in collecting duct primary cilia, the organelle central to PKD pathogenesis, were clearly detected for the first time to our knowledge in PKD1. Altogether, this study highlights the role that in trans variants at the disease locus can play in phenotypic modification of dominant diseases and provides a truly orthologous PKD1 model, optimal for therapeutic testing.     

Mechanism-based therapeutics for autosomal dominant polycystic kidney disease: recent progress and future prospects

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease, accounting for up to 10% of patients on renal replacement therapy. There are presently no proven treatments for ADPKD and an effective disease-modifying drug would have significant implications for patients and their families. Since the identification of PKD1 and PKD2, there has been an explosion in knowledge identifying new disease mechanisms and testing new drugs. Currently, the three major treatment strategies are to: (1) reduce cAMP levels; (2) inhibit cell proliferation, and (3) reduce fluid secretion. Several compounds shown to be effective in preclinical models have already undergone clinical trials and more are planned. In addition, a whole raft of other compounds have been developed from preclinical studies. The purpose of this paper is to evaluate the results of recent published trials, review current trials and highlight the most promising compounds in the pipeline. There appears to be no shortage of potential candidates, but several key issues need to be addressed to facilitate clinical translation.     

Therapies to slow polycystic kidney disease

Advances in the understanding of cystogenesis and availability of animal models orthologous to human autosomal dominant polycystic kidney disease (ADPKD) and recessive polycystic kidney disease (ARPKD) will likely facilitate the development of treatments for these diseases. Proteins mutated in ADPKD and ARPKD, as well as in several animal models, are localized to renal primary cilia. These are thought to have a sensory function and contribute to the regulation of the intracellular calcium ([Ca2+]i). It seems likely that the maintenance of a differentiated renal epithelial phenotype, characterized by controlled fluid secretion and cell proliferation, requires precise functional coordination of cAMP and Ras/Raf/MEK/ERK signaling by [Ca2+]i. [Ca2+]i alterations, linked to genetic defects causing polycystic kidney disease, may hinder negative feedback mechanisms that control cAMP and Ras/Raf/MEK/ERK signaling, and result in increased fluid secretion and cell proliferation. cAMP levels, Raf kinase activities and ERK phosphorylation are increased in polycystic kidneys. There is also evidence of abnormal cross-talk between cAMP and MAPK pathways, that can be reproduced in wild-type cells by altering [Ca2+]i. While cAMP inhibits Ras-Raf-1-stimulated phosphorylation of ERK in normal kidney cells, it markedly increases B-Raf kinase activity and ERK phosphorylation in polycystic kidney cells. Treatment strategies should probably be aimed at increasing [Ca2+]i, inhibiting Ras/Raf/MEK/ERK signaling or lowering cAMP in the distal nephron and collecting duct. Vasopressin is the major adenylyl cyclase agonist in the collecting duct principal cells via a V2 receptor. OPC31260, a V2 receptor antagonist, lowers renal cAMP and markedly inhibits cystogenesis in four animal models of polycystic kidney disease, three of which are orthologous to human diseases (PCK rat, ARPKD; pcy mouse, adolescent nephronophthisis; Pkd2WS25/- mouse, ADPKD). The renal selectivity and safety profile of this class of drugs make it an excellent candidate for clinical trials.     

Failure to ubiquitinate c-Met leads to hyperactivation of mTOR signaling in a mouse model of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common inherited disorder that is caused by mutations at two loci, polycystin 1 (PKD1) and polycystin 2 (PKD2). It is characterized by the formation of multiple cysts in the kidneys that can lead to chronic renal failure. Previous studies have suggested a role for hyperactivation of mammalian target of rapamycin (mTOR) in cystogenesis, but the etiology of mTOR hyperactivation has not been fully elucidated. In this report we have shown that mTOR is hyperactivated in Pkd1-null mouse cells due to failure of the HGF receptor c-Met to be properly ubiquitinated and subsequently degraded after stimulation by HGF. In Pkd1-null cells, Casitas B-lineage lymphoma (c-Cbl), an E3-ubiquitin ligase for c-Met, was sequestered in the Golgi apparatus with α₃β₁ integrin, resulting in the inability to ubiquitinate c-Met. Treatment of mouse Pkd1-null cystic kidneys in organ culture with a c-Met pharmacological inhibitor resulted in inhibition of mTOR activity and blocked cystogenesis in this mouse model of ADPKD. We therefore suggest that blockade of c-Met is a potential novel therapeutic approach to the treatment of ADPKD.     

Practical genetics for autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common mendelian disorder of the kidney and accounts for ~5% of end-stage renal disease in North America. It is characterized by focal development of renal cysts which increase in number and size with age. Mutations of PKD1 and PKD2 account for most cases. Although the clinical manifestations of both gene types overlap completely, PKD1 is associated with more severe disease than PKD2, with larger kidneys and earlier onset of end-stage renal disease. Furthermore, marked within-family renal disease variability is well documented in ADPKD and suggests a strong modifier effect from as yet unknown genetic and environmental factors. In turn, the significant inter- and intra-familial renal disease variability poses a challenge for diagnosis and genetic counseling. In general, renal ultrasonography is commonly used for the diagnosis, and age-dependent criteria have been defined for subjects at risk of PKD1. However, the utility of the PKD1 ultrasound criteria in the clinical setting is unclear since their performance characteristics have not been defined for the milder PKD2 and the gene type for most test subjects is unknown. Recently, highly predictive ultrasound diagnostic criteria have been derived for at-risk subjects of unknown gene type. Additionally, both DNA linkage and gene-based direct sequencing are available for the diagnosis of ADPKD, especially in subjects with equivocal imaging results, a negative or indeterminate family history, or in younger at-risk individuals being evaluated as potential living related kidney donor. This review will highlight the utility and limitations of clinical predictors of gene types, imaging- and molecular-based diagnostic tests, and present an integrated approach for evaluating individuals suspected to have ADPKD.     

The pathogenesis of autosomal dominant polycystic kidney disease

In individuals with autosomal dominant polycystic kidney disease (ADPKD), renal function deteriorates as the kidneys become replaced by multitudes of fluid-filled cysts. Although the PKD genes were identified a decade ago, the pathway(s) leading from mutation to disease remain the subject of intense investigation. As a result of this work, it has become apparent that the polycystins are multifunctional proteins that, in the broadest sense, appear to be involved in the transduction of a number of environmental cues into appropriate cellular responses. It is likely that the central pathogenetic pathway for cystogenesis stems from de-differentiation of tubular epithelial cells. Available evidence indicates that loss of polycystin activity leads to subtle derangements of cell calcium regulation through several possible pathways. Abnormal cell calcium homeostasis might then lead to altered differentiation in affected cells. The study of the polycystins has revealed some entirely novel insights into fundamental cell biology but these have not yet been satisfactorily integrated into a verified pathogenetic pathway for the development of ADPKD.     

Autosomal dominant polycystic kidney disease: recent advances in pathogenesis and treatment

Autosomal dominant polycystic kidney disease (ADPKD) is a common genetic disorder affecting 1 in 1,000 people in the general population and accounts for up to 10% of all patients on renal replacement therapy. Numerous fluid-filled epithelial cysts arise from different nephron segments as spherical dilatations or small out-pouchings, enlarge progressively and eventually become disconnected from the rest of the renal tubule. The development of cysts is accompanied by destruction of the renal parenchyma, interstitial fibrosis, cellular infiltration and loss of functional nephrons. ADPKD is not only a kidney disease but also a systemic disorder associated with intracranial arterial aneurysms, cardiac valvular defects, colonic diverticulosis and cyst formation in other organs such as the liver, spleen and pancreas. The identification of PKD1 and PKD2 together with the drive to elucidate the functions of their encoded proteins, polycystin-1 (PC1) and polycystin-2 (PC2), has led to an explosion of clinical and scientific interest in this common disorder. The aim of this review is to highlight recent advances in our understanding of ADPKD pathogenesis which are leading to exciting new treatment strategies.     

Molecular genetics of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common Mendelian disorder, occurring in approximately 1 in 1000 births and accounting for 8% to 10% of cases of end-stage renal disease (ESRD). Mutations of 2 genes, PKD1 and PKD2, account for the disease in approximately 80% to 85% and 10% to 15% of families respectively. The gene products (polycystin 1 and 2) of PKD1 and PKD2 are plasma membrane proteins and components of a novel signalling pathway that regulates epithelial cell growth and differentiation. Significant inter- and intrafamilial renal disease variability in ADPKD has been well documented and is influenced by both germline and somatic genetic events. Specifically, genetic locus heterogeneity and 2 rare Mendelian syndromes have been shown to strongly influence the variability of interfamilial renal disease, and as-yet-unknown genetic and environmental factors likely modify both inter- and intrafamilial renal disease severity. Furthermore, individual cyst formation in ADPKD represents an aberration of monoclonal growth triggered by somatic PKD1 or PKD2 mutations within individual epithelial cells. Current studies are in progress to identify major genetic and environmental modifiers of renal disease variability. A thorough knowledge of these determinants will allow better patient risk assessment and development of mechanism-based therapy in ADPKD.     

Genetic mechanisms and signaling pathways in autosomal dominant polycystic kidney disease

Recent advances in defining the genetic mechanisms of disease causation and modification in autosomal dominant polycystic kidney disease (ADPKD) have helped to explain some extreme disease manifestations and other phenotypic variability. Studies of the ADPKD proteins, polycystin-1 and -2, and the development and characterization of animal models that better mimic the human disease, have also helped us to understand pathogenesis and facilitated treatment evaluation. In addition, an improved understanding of aberrant downstream pathways in ADPKD, such as proliferation/secretion-related signaling, energy metabolism, and activated macrophages, in which cAMP and calcium changes may play a role, is leading to the identification of therapeutic targets. Finally, results from recent and ongoing preclinical and clinical trials are greatly improving the prospects for available, effective ADPKD treatments.     

Mutation analysis of autosomal dominant polycystic kidney disease genes in Han Chinese

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in two genes, PKD1 and PKD2. The complexity of these genes, particularly PKD1, has complicated genetic screening, though recent advances have provided new opportunities for amplifying these genes. In the Han Chinese population, no complete mutational analysis has previously been conducted across the entire span of PKD1 and PKD2. Here, we used single-strand conformation polymorphism (SSCP) analysis to screen the entire coding sequence of PKD1 and PKD2 in 85 healthy controls and 72 Han Chinese from 24 ADPKD pedigrees. In addition to 11 normal variants, we identified 17 mutations (12 in PKD1 and 5 in PKD2), 15 of which were novel ones (11 for PKD1 and 4 for PKD2). We did not identify any seeming mutational hot spots in PKD1 and PKD2. Notably, we found several disease-associated C-T or G-A mutations that led to charge or hydrophobicity changes in the corresponding amino acids. This suggests that the mutations cause conformational alterations in the PKD1 and PKD2 protein products that may impact the normal protein functions. Our study is the first report of screenable mutations in the full-length PKD1 and PKD2 genes of the Han Chinese, and also offers a benchmark for comparisons between Caucasian and Han ADPKD pedigrees and patients.     

The molecular genetic analysis of polycystic kidney disease]

Autosomal dominant polycystic kidney disease (ADPKD) is one of common single gene disorders. The development of molecular genetic techniques has shown that mutant PKD1 gene assigned to ADPKD was closely linked to alpha-globin on the short arm of chromosome 16. This location was established when genetic linkage was found between ADPKD and a highly polymorphic region at the 3' end of the alpha-globin cluster (3' HVR). The discover of genetic linkage markers such as 3' HVR probe has provided a diagnostic test in presymptomatic stage. We performed this diagnostic test using DNA probes in 3 patients with ADPKD of one Japanese family. They also showed PKD1 gene linkage as previously described by Reeders et al. Linkage analysis of the PKD1 gene might be available to diagnostic test of ADPKD. DNA diagnosis of ADPKD however has to be performed carefully because of an ethical standpoint.     

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.     

Pkd1 regulates immortalized proliferation of renal tubular epithelial cells through p53 induction and JNK activation

Autosomal dominant polycystic kidney disease (ADPKD) is the most common human monogenic genetic disorder and is characterized by progressive bilateral renal cysts and the development of renal insufficiency. The cystogenesis of ADPKD is believed to be a monoclonal proliferation of PKD-deficient (PKD(-/-)) renal tubular epithelial cells. To define the function of Pkd1, we generated chimeric mice by aggregation of Pkd1(-/-) ES cells and Pkd1(+/+) morulae from ROSA26 mice. As occurs in humans with ADPKD, these mice developed cysts in the kidney, liver, and pancreas. Surprisingly, the cyst epithelia of the kidney were composed of both Pkd1(-/-) and Pkd1(+/+) renal tubular epithelial cells in the early stages of cystogenesis. Pkd1(-/-) cyst epithelial cells changed in shape from cuboidal to flat and replaced Pkd1(+/+) cyst epithelial cells lost by JNK-mediated apoptosis in intermediate stages. In late-stage cysts, Pkd1(-/-) cells continued immortalized proliferation with downregulation of p53. These results provide a novel understanding of the cystogenesis of ADPKD patients. Furthermore, immortalized proliferation without induction of p53 was frequently observed in 3T3-type culture of mouse embryonic fibroblasts from Pkd1(-/-) mice. Thus, Pkd1 plays a role in preventing immortalized proliferation of renal tubular epithelial cells through the induction of p53 and activation of JNK.     

Macrophage migration inhibitory factor promotes cyst growth in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by renal cyst formation, inflammation, and fibrosis. Macrophages infiltrate cystic kidneys, but the role of these and other inflammatory factors in disease progression are poorly understood. Here, we identified macrophage migration inhibitory factor (MIF) as an important regulator of cyst growth in ADPKD. MIF was upregulated in cyst-lining epithelial cells in polycystin-1–deficient murine kidneys and accumulated in cyst fluid of human ADPKD kidneys. MIF promoted cystic epithelial cell proliferation by activating ERK, mTOR, and Rb/E2F pathways and by increasing glucose uptake and ATP production, which inhibited AMP-activated protein kinase signaling. MIF also regulated cystic renal epithelial cell apoptosis through p53-dependent signaling. In polycystin-1–deficient mice, MIF was required for recruitment and retention of renal macrophages, which promoted cyst expansion, and Mif deletion or pharmacologic inhibition delayed cyst growth in multiple murine ADPKD models. MIF-dependent macrophage recruitment was associated with upregulation of monocyte chemotactic protein 1 (MCP-1) and inflammatory cytokine TNF-α. TNF-α induced MIF expression, and MIF subsequently exacerbated TNF-α expression in renal epithelial cells, suggesting a positive feedback loop between TNF-α and MIF during cyst development. Our study indicates MIF is a central and upstream regulator of ADPKD pathogenesis and provides a rationale for further exploration of MIF as a therapeutic target for ADPKD.     

Recent advances in the molecular diagnosis of polycystic kidney disease

Introduction: Polycystic kidney disease (PKD) is clinically and genetically heterogeneous and constitutes the most common heritable kidney disease. Most patients are affected by the autosomal dominant form (ADPKD) which generally is an adult-onset multisystem disorder. By contrast, the rarer recessive form ARPKD usually already manifests perinatally or in childhood. In some patients, however, ADPKD and ARPKD can phenotypically overlap with early manifestation in ADPKD and only late onset in ARPKD. Progressive fibrocystic renal changes are often accompanied by severe hepatobiliary changes or other extrarenal abnormalities.

Areas covered: A reduced dosage of disease proteins disturbs cell homeostasis and explains a more severe clinical course in some PKD patients. Cystic kidney disease is also a common feature of other ciliopathies and genetic syndromes. Genetic diagnosis may guide clinical management and helps to avoid invasive measures and to detect renal and extrarenal comorbidities early in the clinical course.

Expert Commentary: The broad phenotypic and genetic heterogeneity of cystic and polycystic kidney diseases make NGS a particularly powerful approach. Interpretation of data becomes the challenge and bench and bedside benefit from digitized multidisciplinary interrelationships.     

A common polymorphism in exon 46 of the human autosomal dominant polycystic kidney disease 1 gene (PKD1)

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common single gene diseases in humans. We have identified a synonymous T to C transition polymorphism in exon 46 of the PKD1 gene (12838T→C, Pro4209Pro). The polymorphism was present with similar frequencies in ADPKD patients and unaffected individuals. The heterozygosity, determined in 89 Italian individuals, was 0·347. The frequency of the rarer allele was 0·222. This polymorphism is easy to determine as it abolishes a naturally occurring DdeI restriction site. The availability of an additional intragenic marker in the PKD1 gene will improve the accuracy of linkage studies in ADPKD families.     

A role for microRNA in cystic liver and kidney diseases

The polycystic liver and kidney diseases are a family of disorders with heterogeneous etiologies. Proposed mechanisms of disease include ciliary dysfunction, excess cell proliferation, and altered cell-cell or cell-matrix interactions. In this issue of the JCI, Lee and colleagues provide data to support a novel mechanism for cystogenesis involving microRNA (miRNA) (see the related article beginning on page 3714). They demonstrate that levels of the miRNA miR15a are decreased in livers of patients with autosomal recessive and autosomal dominant polycystic kidney disease (ARPKD and ADPKD, respectively) and congenital hepatic fibrosis as well as in the PKC rat model of ARPKD. This results in increased expression of the cell-cycle regulator Cdc25A, which is a direct target of miR15a, and increased cellular proliferation and cystogenesis in vitro. These findings suggest that other miRNAs may also participate in the molecular pathogenesis of cystic liver and kidney diseases.     

Molecular diagnosis of autosomal dominant polycystic kidney disease

Introduction: Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease that accounts for 5–10% of end-stage renal disease in developed countries. Mutations in PKD1 and PKD2 account for a majority of cases. Mutation screening of PKD1 is technically challenging largely due to the complexity resulting from duplication of its first 33 exons in six highly homologous pseudogenes (i.e. PKD1P1-P6). Protocol using locus-specific long-range and nested PCR has enabled comprehensive PKD1 mutation screening but is labor-intensive and costly. Here, the authors review how recent advances in Next Generation Sequencing are poised to transform and extend molecular diagnosis of ADPKD.

Areas covered: Key original research articles and reviews of the topic published in English identified through PubMed from 1957–2017.

Expert commentary: The authors review current and evolving approaches using targeted resequencing or whole genome sequencing for screening typical as well as challenging cases (e.g. cases with no detectable PKD1 and PKD2 mutations which may be due to somatic mosaicism or other cystic disease; and complex genetics such as bilineal disease).     

常染色体显性多囊肾病的预后评估及治疗

常染色体显性多囊肾病(autosomal dominant polycystic kidney disease, ADPKD)患病率为1‰~2‰, 属于罕见病, 临床主要表现为双侧肾囊肿且逐渐发展, 肾脏体积进行性增大, 肾功能逐步降低。PKD1基因突变约占81%, PKD2基因突变约占10.5%~22%。血管加压素(arginine vasopressin, AVP)和环磷酸腺苷(cyclic adenosine monophosphate, cAMP)信号通路在ADPKD囊肿发展过程中发挥重要作用。近年来发表的梅奥风险评估模型和PROPKD(predicting renal outcome in polycystic kidney disease)评分是ADPKD较好的预后评估模型, 已成为临床医生决策的重要依据。通过拮抗AVP受体, 抑制cAMP通路的托伐普坦已成为ADPKD首个特异治疗药物, 可有效抑制总肾脏体积的增长和保护肾功能。药物的长期安全性仍需进一步研究。