Recent advances in understanding ion transport mechanisms in polycystic kidney disease

This review focuses on the most recent advances in the understanding of the electrolyte transport-related mechanisms important for the development of severe inherited renal disorders, autosomal dominant (AD) and recessive (AR) forms of polycystic kidney disease (PKD). We provide here a basic overview of the origins and clinical aspects of ARPKD and ADPKD and discuss the implications of electrolyte transport in cystogenesis. Special attention is devoted to intracellular calcium handling by the cystic cells, with a focus on polycystins and fibrocystin, as well as other calcium level regulators, such as transient receptor potential vanilloid type 4 (TRPV4) channels, ciliary machinery, and purinergic receptor remodeling. Sodium transport is reviewed with a focus on the epithelial sodium channel (ENaC), and the role of chloride-dependent fluid secretion in cystic fluid accumulation is discussed. In addition, we highlight the emerging promising concepts in the field, such as potassium transport, and suggest some new avenues for research related to electrolyte handling.

Clinical and genetic characteristics of autosomal recessive polycystic kidney disease in Oman

BACKGROUND

There is a high prevalence of rare genetic disorders in the Middle East, and their study provides unique clinical and genetic insights. Autosomal recessive polycystic kidney disease (ARPKD) is one of the leading causes of kidney and liver-associated morbidity and mortality in Oman. We describe the clinical and genetic profile of cohort of ARPKD patients.

METHODS

We studied patients with a clinical diagnosis of ARPKD (n = 40) and their relatives (parents (n = 24) and unaffected siblings (n = 10)) from 32 apparently unrelated families, who were referred to the National Genetic Centre in Oman between January 2015 and December 2018. Genetic analysis of PKHD1 if not previously known was performed using targeted exon PCR of known disease alleles and Sanger sequencing.

RESULTS

A clinical diagnosis of ARPKD was made prenatally in 8 patients, 21 were diagnosed during infancy (0-1 year), 9 during early childhood (2-8 years) and 2 at later ages (9-13 years). Clinical phenotypes included polycystic kidneys, hypertension, hepatic fibrosis and splenomegaly. Twenty-four patients had documented chronic kidney disease (median age 3 years). Twenty-four out of the 32 families had a family history suggesting an autosomal recessive pattern of inherited kidney disease, and there was known consanguinity in 21 families (66%). A molecular genetic diagnosis with biallelic PKHD1 mutations was known in 18 patients and newly identified in 20 other patients, totalling 38 patients from 30 different families. Two unrelated patients remained genetically unsolved. The different PKHD1 missense pathogenic variants were: c.107C > T, p.(Thr36Met); c.406A > G, p.(Thr136Ala); c.4870C > T, p.(Arg1624Trp) and c.9370C > T, p.(His3124Tyr) located in exons 3, 6, 32 and 58, respectively. The c.406A > G, p.(Thr136Ala) missense mutation was detected homozygously in one family and heterozygously with a c.107C > T, p.(Thr36Met) allele in 5 other families. Overall, the most commonly detected pathogenic allele was c.107C > T; (Thr36Met), which was seen in 24 families.

CONCLUSIONS

Molecular genetic screening of PKHD1 in clinically suspected ARPKD cases produced a high diagnostic rate. The limited number of PKHD1 missense variants identified in ARPKD cases suggests these may be common founder alleles in the Omani population. Cost effective targeted PCR analysis of these specific alleles can be a useful diagnostic tool for future cases of suspected ARPKD in Oman.

Salt-deficient diet exacerbates cystogenesis in ARPKD via epithelial sodium channel (ENaC)

Background

Autosomal Recessive Polycystic Kidney Disease (ARPKD) is marked by cyst formation in the renal tubules, primarily in the collecting duct (CD) system, ultimately leading to end-stage renal disease. Patients with PKD are generally advised to restrict their dietary sodium intake. This study was aimed at testing the outcomes of dietary salt manipulation in ARPKD.

Methods

PCK/CrljCrlPkhd1pck/CRL (PCK) rats, a model of ARPKD, were fed a normal (0.4% NaCl; NS), high salt (4% NaCl; HS), and sodium-deficient (0.01% NaCl; SD) diets for 8 weeks. Immunohistochemistry, GFR measurements, balance studies, and molecular biology approaches were applied to evaluate the outcomes of the protocol. Renin-angiotensin-aldosterone system (RAAS) levels were assessed using LC-MS/MS, and renal miRNA profiles were studied.

Findings

Both HS and SD diets resulted in an increase in cystogenesis. However, SD diet caused extensive growth of cysts in the renal cortical area, and hypertrophy of the tissue; RAAS components were enhanced in the SD group. We observed a reduction in epithelial Na⁺ channel (ENaC) expression in the SD group, accompanied with mRNA level increase. miRNA assay revealed that renal miR-9a-5p level was augmented in the SD group; we showed that this miRNA decreases ENaC channel number in CD cells.

Interpretation

Our data demonstrate a mechanism of ARPKD progression during salt restriction that involves activity of ENaC. We further show that miR-9a-5p potentially implicated in this mechanism and that miR-9a-5p downregulates ENaC in cultured CD cells. Our findings open new therapeutic possibilities and highlight the importance of understanding salt reabsorption in ARPKD.

Telmisartan ameliorates fibrocystic liver disease in an orthologous rat model of human autosomal recessive polycystic kidney disease

Human autosomal recessive polycystic kidney disease (ARPKD) produces kidneys which are massively enlarged due to multiple cysts, hypertension, and congenital hepatic fibrosis characterized by dilated bile ducts and portal hypertension. The PCK rat is an orthologous model of human ARPKD with numerous fluid-filled cysts caused by stimulated cellular proliferation in the renal tubules and hepatic bile duct epithelia, with interstitial fibrosis developed in the liver. We previously reported that a peroxisome proliferator activated receptor (PPAR)-γ full agonist ameliorated kidney and liver disease in PCK rats. Telmisartan is an angiotensin receptor blocker (ARB) used widely as an antihypertensive drug and shows partial PPAR-γ agonist activity. It also has nephroprotective activity in diabetes and renal injury and prevents the effects of drug-induced hepatotoxicity and hepatic fibrosis. In the present study, we determined whether telmisartan ameliorates progression of polycystic kidney and fibrocystic liver disease in PCK rats. Five male and 5 female PCK and normal control (+/+) rats were orally administered 3 mg/kg telmisartan or vehicle every day from 4 to 20 weeks of age. Treatment with telmisartan decreased blood pressure in both PCK and +/+ rats. Blood levels of aspartate amino transferase, alanine amino transferase and urea nitrogen were unaffected by telmisartan treatment. There was no effect on kidney disease progression, but liver weight relative to body weight, liver cystic area, hepatic fibrosis index, expression levels of Ki67 and TGF-β, and the number of Ki67- and TGF-β-positive interstitial cells in the liver were significantly decreased in telmisartan-treated PCK rats. Therefore, telmisartan ameliorates congenital hepatic fibrosis in ARPKD, possibly through the inhibition of signaling cascades responsible for cellular proliferation and interstitial fibrosis in PCK rats. The present results support the potential therapeutic use of ARBs for the treatment of fibrocystic liver disease in ARPKD patients.

Nephronophthisis-associated ciliopathies

Nephronophthisis (NPHP), an autosomal recessive cystic kidney disease, represents the most frequent genetic cause of end-stage kidney disease in the first three decades of life. Contrary to polycystic kidney disease, NPHP shows normal or diminished kidney size, cysts are concentrated at the corticomedullary junction, and tubulointerstitial fibrosis is dominant. NPHP can be associated with retinitis pigmentosa (Senior-Løken syndrome), liver fibrosis, and cerebellar vermis aplasia (Joubert syndrome) in approximately 10% of patients. Positional cloning of six novel genes (NPHP1 through 6) as mutated in NPHP and functional characterization of their encoded proteins have contributed to the concept of "ciliopathies." It has helped advance a new unifying theory of cystic kidney diseases. This theory states that the products of all genes that are mutated in cystic kidney diseases in humans, mice, or zebrafish are expressed in primary cilia or centrosomes of renal epithelial cells. Primary cilia are sensory organelles that connect mechanosensory, visual, osmotic, and other stimuli to mechanisms of cell-cycle control and epithelial cell polarity. The ciliary theory explains the multiple organ involvement in NPHP regarding retinitis pigmentosa, liver fibrosis, ataxia, situs inversus, and mental retardation. Mutations in NPHP genes cause defects in signaling mechanisms, including the noncanonical Wnt signaling pathway. The "ciliopathy" NPHP thereby is caused by defects in tissue differentiation and maintenance as a result of impaired processing of extracellular cues. Nephrocystins, the proteins that are encoded by NPHP genes, are highly conserved in evolution. Positional cloning of additional causative genes of NPHP will elucidate further signaling mechanisms that are involved, thereby establishing therapeutic approaches using animal models in mouse, zebrafish, and Caenorhabditis elegans.

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.

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.

Mechanisms of Disease: autosomal dominant and recessive polycystic kidney diseases

Autosomal dominant polycystic kidney disease and autosomal recessive polycystic kidney disease are the best known of a large family of inherited diseases characterized by the development of renal cysts of tubular epithelial cell origin. Autosomal dominant and recessive polycystic kidney diseases have overlapping but distinct pathogeneses. Identification of the causative mutated genes and elucidation of the function of their encoded proteins is shedding new light on the mechanisms that underlie tubular epithelial cell differentiation. This review summarizes recent literature on the role of primary cilia, intracellular calcium homeostasis, and signaling involving Wnt, cyclic AMP and Ras/MAPK, in the pathogenesis of polycystic kidney disease. Improved understanding of pathogenesis and the availability of animal models orthologous to the human diseases provide an excellent opportunity for the development of pathophysiology-based therapies. Some of these have proven effective in preclinical studies, and clinical trials have begun.

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.

The role of ultrasound in renal insufficiency: the essentials

Sonography is the best screening modality to evaluate patients presenting with renal insufficiency. Ultrasound findings can be normal in patients with renal disease, especially in prerenal azotemia and acute parenchymal renal disease. Echogenic kidneys indicate the presence of parenchymal renal disease; the kidneys may be of a normal size or enlarged. Small kidneys suggest advanced stage chronic kidney disease. Uncommonly, cystic disease of the kidney, especially adult type polycystic kidney disease may be the cause of the patient's renal insufficiency with bilaterally enlarged kidneys containing multiple cysts of various sizes. If hydronephrosis is present, the level and cause of the obstruction should be sought. When ultrasound cannot diagnose the level and cause of obstruction, other imaging modalities, including CT and MRI may be useful. When renovascular disease (arterial stenosis or venous thrombosis) is suspected, spectral and color Doppler can be useful in detecting abnormalities.

Mechanoregulation of intracellular Ca2+ concentration is attenuated in collecting duct of monocilium-impaired orpk mice

Autosomal recessive polycystic kidney disease (ARPKD) is characterized by the progressive dilatation of collecting ducts, the nephron segments responsible for the final renal regulation of sodium, potassium, acid-base, and water balance. Murine models of ARPKD possess mutations in genes encoding cilia-associated proteins, including Tg737 in orpk mice. New findings implicate defects in structure/function of primary cilia as central to the development of polycystic kidney disease. Our group (Liu W, Xu S, Woda C, Kim P, Weinbaum S, and Satlin LM, Am J Physiol Renal Physiol 285: F998-F1012, 2003) recently reported that increases in luminal flow rate in rabbit collecting ducts increase intracellular Ca(2+) concentration ([Ca(2+)](i)) in cells therein. We thus hypothesized that fluid shear acting on the apical membrane or hydrodynamic bending moments acting on the cilium increase renal epithelial [Ca(2+)](i). To further explore this, we tested whether flow-induced [Ca(2+)](i) transients in collecting ducts from mutant orpk mice, which possess structurally abnormal cilia, differ from those in controls. Isolated segments from 1- and 2-wk-old mice were microperfused in vitro and loaded with fura 2; [Ca(2+)](i) was measured by digital ratio fluorometry before and after the rate of luminal flow was increased. All collecting ducts responded to an increase in flow with an increase in [Ca(2+)](i), a response that appeared to be dependent on luminal Ca(2+) entry. However, the magnitude of the increase in [Ca(2+)](i) in 2- but not 1-wk-old mutant orpk animals was blunted. We speculate that this defect in mechano-induced Ca(2+) signaling in orpk mice leads to aberrant structure and function of the collecting duct in ARPKD.

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.

Impact of liver transplantation on renal function of patients with congenital hepatic fibrosis associated with autosomal recessive polycystic kidney disease

Congenital hepatic fibrosis (CHF) is an uncommon autosomal recessive malformation. It may be associated with extrahepatic manifestations such as polycystic kidney disease. The main consequence is portal hypertension and bleeding from varices. Despite liver transplantation as a therapeutic option for this patient, long-term impact of liver transplantation on renal functions of patients with autosomal recessive polycystic kidney disease with associated liver disease is not well known. In this study, we aimed to analyze the patient's renal function after liver transplantation by creatinine clearance, glomerular filtration rate, and renal resistive indexes. Between March 1997 and September 2002, three of 50 orthotopic liver transplantation (OLT) were performed because of CHF associated with ARPKD at Ege University Organ Transplantation and Research Center. Baseline immunosuppression consisted of prednisone and cyclosporine A (CSA). The mean follow-up of the patients was 2.1 yr. Blood urea and creatinine levels were decreased after operation in all patients and remained within the normal range at the sixth and 12th month, whereas the level of the third patient were increased at the 18th month. RRI values of patients were not found different at the sixth month whereas, RRI values of patients were decreased at the 12th month and remained unchanged at the 18th month of follow-up. During the study period hypertension developed in one patient at the 16th month and resolved with antihypertensive treatment and decreasing dosage of CSA. Kidney function has remained satisfactory in all of the patients despite the use of cyclosporine. OLT can provide good survival in patients with CHF associated with ARPKD.

EGF-related growth factors in the pathogenesis of murine ARPKD11See Editorial by Wilson, p. 2441

BACKGROUND

Epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha) and their receptor, EGFR, play key roles in polycystic kidney disease (PKD) pathogenesis. Renal expression of two related growth factors, amphiregulin and heparin-binding EGF, has not been examined previously in PKD. The aims of this study of murine autosomal-recessive polycystic kidney disease (ARPKD) were (1) to characterize amphiregulin and heparin-binding EGF expression in cystic versus normal kidneys and cells; and (2) to identify the functional effects of abnormal EGF-related growth factor expression.

METHODS

Amphiregulin and heparin-binding-EGF expression were examined by immunohistology and Western blot of kidneys and conditionally-immortalized collecting tubule cells obtained from cystic bpk mice (a murine model of ARPKD) and normal littermates. EGF, TGF-alpha, amphiregulin, and heparin-binding EGF in vitro effects on cystic and control collecting tubule cells were assessed by cell proliferation, cyst fluid mitogenicity, and EGFR activation.

RESULTS

By immunohistology, amphiregulin and heparin-binding EGF localized to apical and basolateral surfaces of proximal tubule cysts > normal proximal tubules. In cystic collecting tubules, heparin-binding EGF (but not amphiregulin) localized to both apical and basolateral surfaces; whereas in normal collecting tubules, amphiregulin and heparin-binding EGF localized to the basolateral surface only. Increased amphiregulin and heparin-binding EGF expression by Western blot was seen in cystic vs. normal kidneys and increased heparin-binding EGF (but not amphiregulin) expression was present in cystic collecting tubule cell lines vs. controls. EGF, TGF-alpha, amphiregulin, and heparin-binding EGF were all mitogenic to cystic > control collecting tubule cells. Immunoprecipitation of EGF and TGF-alpha reduced cyst fluid mitogenicity by almost 80%, whereas heparin-binding EGF and amphiregulin immunoprecipitations had minimal effects. Differential receptor activation was also seen: Heparin-binding EGF markedly activated EGFR (>EGF = TGF-alpha > amphiregulin), with a greater effect seen in cystic vs. control collecting tubule cells.

CONCLUSION

Multiple EGF-related growth factors are abnormally expressed in murine ARPKD and may have differential roles in disease pathogenesis. In particular, newly identified abnormalities in heparin-binding EGF expression in cystic kidneys and cells may have important implications for disease pathogenesis.

Decreased amiloride-sensitive Na+ absorption in collecting duct principal cells isolated from BPK ARPKD mice

The main feature of polycystic kidney diseases (PKD) is formation and progressive enlargement of renal cysts. Alterations in epithelial cell proliferation, extracellular matrix, and ion transport are thought to contribute to cyst enlargement and loss of renal function. Abnormal Cl- secretion is implicated in cyst enlargement in autosomal dominant PKD (ADPKD), but little is known about transport abnormalities in autosomal recessive PKD (ARPKD). We developed a method to isolate collecting duct (CD) principal cells (site of the lesion in ARPKD) from normal and ARPKD mice. A transgenic mouse (Hoxb7/GFP) in which enhanced green fluorescent protein (GFP) is expressed in CDs was bred with an ARPKD mouse (BPK), and GFP-positive cells from normal and cystic mice were selected by fluorescence-activated cell sorting. GFP-positive CD cells (>95 +/- 3%) obtained from either normal or cystic mice formed high-resistance, polarized epithelial monolayers. Expression patterns for marker proteins and the presence of a central cilium confirmed that the monolayers are composed of principal cells. Under basal conditions, the Cl- secretory responses elicited by elevation of cAMP or calcium were not significantly different between normal and cystic monolayers. In contrast, the amiloride-sensitive short-circuit current was significantly reduced in monolayers of cells isolated from cystic mice (12.9 +/- 1.6 microA/cm2; n = 10) compared with monolayers of cells isolated from normal mice (27.3 +/- 3.4 microA/cm2; n = 12). The results of these studies suggest that epithelial sodium channel-mediated sodium absorption is decreased in principal cells of ARPKD CD cysts and that the reduction in sodium absorption may contribute to the accumulation of luminal fluid.

Autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) is an important hereditary early childhood nephropathy. However, the clinical ARPKD spectrum is much more variable than is generally presumed. Presentation of ARPKD at a later age and survival into adulthood is well known. Diagnostic criteria, clinical course, differential diagnoses, genetics and molecular biology will be discussed along with the advantages and limitations of mutation detection in clinical practice.

Polycystic kidney disease--the ciliary connection

CONTEXT

"Cystic degeneration" of the kidneys was first described pathologically in 1841 and "polycystic kidneys" as a clinical syndrome in 1888. The heritable nature in some families was noted in 1899, and autosomal dominant and recessive patterns of inheritance of polycystic kidney disease (PKD) were later recognised. Autosomal dominant PKD is one of the most common human genetic diseases and results from mutations in PKD1 or PKD2. These genes encode two proteins, polycystin-1 and polycystin-2.

STARTING POINT

Primary cilia are cellular organelles previously thought by some to be vestigial. New findings from several species, including algae, nematodes, and mice, implicate defects in structure or function of primary cilia as a possible common mechanism central to the development of some forms of recessive PKD. Two recent reports propose a causal link between ciliary dysfunction and autosomal dominant PKD. B Yoder and colleagues (J Am Soc Nephrol 2002; 13:2508-16) show that polycystin-1 and polycystin-2 are localised to primary cilia in cultured renal epithelial cells. S Nauli and colleagues (Nat Genet 2003; 33:129-37) show that polycystin-1 and polycystin-2 function as flow-sensitive mechanosensors in the same signal-transduction pathway. WHERE NEXT? Cystic epithelial cells show many altered cellular properties, including changes in proliferation, apoptosis, adhesion, differentiation, polarity, extracellular matrix synthesis, and fluid transport. The next important steps in PKD research will be to define the physiological roles of primary renal cilia and how defects in ciliary structure and function lead to the development of a cystic phenotype in different forms of PKD.

Multiorgan mRNA misexpression in murine autosomal recessive polycystic kidney disease

BACKGROUND

BALB/c mice homozygous for the cpk mutation develop a form of polycystic kidney disease (PKD) with multiorgan pathology similar to human autosomal recessive PKD. Messenger RNA expression in multiple affected organs was analyzed to determine if common gene cascades were misexpressed in the cystic kidney and extrarenal sites of disease. In cystic kidneys, misexpressed mRNAs were found in one of four general groups: proliferation/cell growth, apoptosis, differentiation or extracellular matrix.

METHODS

RNA was isolated from kidney, liver and pancreas of cystic and normal BALB/c-cpk mice. Using Northern blot hybridization and ribonuclease protection assays (RPA), the expression of several genes thought to be associated with PKD, namely c-myc, epidermal growth factor receptor (EGF-R) and PKD-1, were evaluated. RPAs were used to assess mRNA expression of cyclins and members of the bax/bcl-2 family. In addition, kidney, liver and pancreas were immunostained for c-Myc and PCNA.

RESULTS

Cystic kidney, liver and pancreas all exhibited similar patterns of mRNA misexpression of c-myc, EGF-R and PKD-1. In addition, a number of cell proliferation and apoptosis-related mRNAs also were elevated in cystic kidney and pancreas. Renal epithelial cells expressing proliferation-associated proteins [c-Myc and proliferating cell nuclear antigen (PCNA)] were nearly absent in normal kidney; however, cells of cystic and non-cystic renal tubules plus liver and pancreatic cyst exhibited an increased number of nuclei labeled with antibodies to these proteins.

CONCLUSIONS

These data suggest that similar pathologic mechanisms (including the expression of c-myc, EGF-R, PKD-1, cyclin, and bax/bcl-2 family mRNAs) may be responsible for the development of cystic changes in kidney, liver and pancreas in murine autosomal recessive PKD. Treatments targeting these similarly misexpressed mRNAs may be efficacious in ameliorating the cystic pathology in the kidney as well as the other affected organs in ARPKD.

The Jeremiah Metzger Lecture. Polycystic kidney disease: old disease in a new context

I want to thank the organizers for inviting me to present the Jeremiah Metzger Lecture at this, the 114th meeting of the ACCA. It is a high honor, indeed, to join a list of very distinguished predecessors. And for this opportunity to tell you about my passion in medicine and science, I am most grateful. Most of you in this room have passing knowledge of polycystic kidney disease, probably hearing about it in your medical school Pathology course where you were shown an especially grotesque, enormously enlarged kidney either encased in transparent plastic or submerged in a bucket of formaldehyde. In that minute or two when PKD was discussed in lecture, you may have been told that this is a rare, hereditary disorder that causes kidney failure and that nothing can be done to alter that course. Unless you chose to specialize in General Internal Medicine or Nephrology, you may not have encountered PKD again until today, despite the fact there are approximately 600,000 PKD patients in the USA and over 10,000,000 worldwide, and it accounts for approximately 5% of non-diabetic dialysis and renal transplant patients (Table 1). I might have overlooked PKD as well had it not been for a close friend that I grew up with who had inherited the disease from his mother. He was very open about the fact that he had cysts in his kidneys that caused bleeding into the urine from time to time, especially after a solid hit during a game of tackle football. We remained friends long after I left home for college and medical school. At an early stage of my research career in medicine, while wondering how nephron segments processed glomerular filtrate, I inadvertently discovered that renal tubules could secrete as well as reabsorb salt and water. This was quite an unexpected finding at the time (1). But it occurred to [table: see text] me that this might be a means to fill renal cysts with fluid and so I decided to learn more about the pathology and pathogenesis of PKD. This didn't take long, because there wasn't much literature on the subject. The clinical manifestations of PKD were described in the 19th century European medical literature and Sir William Osler had published on the topic in this country, but by and large only a few descriptions of small groups of patients were reported through the middle of the 20th century. In 1957, Dalgaard (2) reported in a classic doctoral thesis that the most common type of hereditary PKD is transmitted as an autosomal dominant trait (ADPKD) with complete penetrance. It is a bilateral renal condition, but cysts also occur in the liver (approximately 60%), pancreas (approximately 10%) and various other organs, and it is associated with cerebral aneurysms in approximately 5% of patients. A recessive form that affects infants and children primarily (ARPKD), is much rarer than ADPKD and commonly leads to death in infancy in association with massively enlarged kidneys (Table 2). I was also attracted to the study of PKD because the etiology was not in question: it had to be mutated DNA. Yet that fact proved to be a hindrance in attaining research support. As some of you will recall, not too long ago genetic diseases were viewed by kidney-oriented NIH review panels to be incurable. I was advised that a young scientist's time would be better spent determining how the kidneys excrete salt and water. Fortunately, the era of molecular genetics and biology was upon us, and we quickly learned that uncommon genetic disorders could lead to the discovery of novel molecules in metabolic and structural pathways. And that is just what happened in the PKD field. The autosomal dominant form of PKD led to the discovery of a unique family of highly complex proteins long before they would have been selected from a gene or proteomic micro-array by some desperate graduate student or fellow. The chromosomal location of the major ADPKD genotype, PKD1, was defined in 1985 (3), a date that marks the beginning of a remarkable period of discovery.

Can progression of autosomal dominant or autosomal recessive polycystic kidney disease be prevented?

Data from animal and human studies suggest that the rate of progression of renal insufficiency can be retarded with careful control of blood pressure, institution of a low-protein diet, and the use of lipid-lowering agents. These therapeutic interventions become important when managing patients with renal insufficiency secondary to autosomal dominant polycystic kidney disease (PKD) and autosomal recessive polycystic kidney disease, in which end-stage renal disease is present in nearly 17,000 individuals per year. Several dietary and pharmacologic intervention strategies including blood pressure control, dietary modification, and the use of antioxidants as well as lipid-lowering agents have been studied in humans and animals with PKD in an effort to slow the rate of renal progression. This article reviews the current understanding of the effectiveness of these conventional therapies, as well as novel therapies that specifically target the mediators of cyst formation in PKD using tyrosine kinase inhibitors and gene therapy in an effort to identify potential strategies for retarding cyst formation and parenchymal injury in PKD. Current pharmacologic and dietary strategies fail to show any consistent benefits in preserving renal function and reducing renal injury in human PKD. The therapeutic potential for exciting new gene therapies and pharmacologic agents designed to target the pathophysiologic pathways involved in cyst formation are promising. Randomized, controlled trials in children and adults with early PKD are necessary to evaluate the effectiveness of these therapeutic interventions.

Autosomal recessive polycystic kidney disease in adulthood

BACKGROUND

Renal cysts arising from collecting ducts, congenital hepatic fibrosis, and recessive inheritance characterize autosomal recessive polycystic kidney disease (ARPKD). The disorder usually manifests in infancy, with a high mortality rate in the first year of life. For the patients who survive the neonatal period, the probability of being alive at 15 years of age ranges from 50 to 80%, with 56--67% of them not requiring renal replacement therapy at that stage. Some develop portal hypertension. Long-term outcome of adults escaping renal insufficiency above age 18 is largely unknown.

METHOD

In consecutive patients with ARPKD and autonomous renal function at age 18, clinical course of kidney and liver disease in adulthood and status at last follow-up were evaluated. Progression of renal insufficiency was assessed by the rate of decline of creatinine clearance, according to Schwartz's formula before age 18 and Cockcroft and Gault formula thereafter. Severity of liver involvement was estimated by imaging techniques, liver function tests, and endoscopy.

RESULTS

Sixteen patients from 15 families were included. ARPKD was diagnosed between 1 day and 13 years of age. From diagnosis, mean follow-up period lasted 24+/-9 years. Before age 18, nine patients (56%) were hypertensive, nine (56%) had renal failure, and four (25%) had portal hypertension. Beyond age 18, no additional patient became hypertensive, and another five developed progressive renal insufficiency; altogether, the mean yearly decline of creatinine clearance was 2.9+/-1.6 ml/min. Portal hypertension was recognized in two additional patients. Four patients experienced gastro-oesophageal bleeding, while recurrent cholangitis or cholangiocarcinoma developed in one case each. At the end of follow-up, 15/16 patients (94%) were alive at a mean age of 27 (18--55) years. Two patients had a normal renal function, 11 had chronic renal insufficiency, one was on regular dialysis, and two had functioning kidney grafts. Four patients had required a porto-systemic shunt.

CONCLUSIONS

A subset of ARPKD patients with autonomous renal function at age 18 experiences slowly progressive renal insufficiency. With prolonged renal survival, complications related to portal hypertension are not rare, requiring careful surveillance and appropriate management.

Role of CFTR in autosomal recessive polycystic kidney disease

An extensive body of in vitro data implicates epithelial chloride secretion, mediated through cystic fibrosis transmembrane conductance regulator (CFTR) protein, in generating or maintaining fluid filled cysts in MDCK cells and in human autosomal dominant polycystic kidney disease (ADPKD). In contrast, few studies have addressed the pathophysiology of fluid secretion in cyst formation and enlargement in autosomal recessive polycystic kidney disease (ARPKD). Murine models of targeted disruptions or deletions of specific genes have created opportunities to examine the role of individual gene products in normal development and/or disease pathophysiology. The creation of a murine model of CF, which lacks functional CFTR protein, provides the opportunity to determine whether CFTR activity is required for renal cyst formation in vivo. Therefore, this study sought to determine whether renal cyst formation could be prevented by genetic complementation of the BPK murine model of ARPKD with the CFTR knockout mouse. The results of this study reveal that in animals that are homozygous for the cystic gene (bpk), the lack of functional CFTR protein on the apical surface of cystic epithelium does not provide protection against cyst growth and subsequent decline in renal function. Double mutant mice (bpk -/-; cftr -/-) developed massively enlarged kidneys and died, on average, 7 d earlier than cystic, non-CF mice (bpk -/-; cftr +/+/-). This suggests fundamental differences in the mechanisms of transtubular fluid secretion in animal models of ARPKD compared with ADPKD.

Chlamydomonas IFT88 and its mouse homologue, polycystic kidney disease gene tg737, are required for assembly of cilia and flagella

Intraflagellar transport (IFT) is a rapid movement of multi-subunit protein particles along flagellar microtubules and is required for assembly and maintenance of eukaryotic flagella. We cloned and sequenced a Chlamydomonas cDNA encoding the IFT88 subunit of the IFT particle and identified a Chlamydomonas insertional mutant that is missing this gene. The phenotype of this mutant is normal except for the complete absence of flagella. IFT88 is homologous to mouse and human genes called Tg737. Mice with defects in Tg737 die shortly after birth from polycystic kidney disease. We show that the primary cilia in the kidney of Tg737 mutant mice are shorter than normal. This indicates that IFT is important for primary cilia assembly in mammals. It is likely that primary cilia have an important function in the kidney and that defects in their assembly can lead to polycystic kidney disease.

Developmental expression of urine concentration-associated genes and their altered expression in murine infantile-type polycystic kidney disease

Currently, there is little understanding of what factors regulate the development of urine concentrating capability in normal or polycystic kidney. The present study examined the developmental expression of genes associated with urine concentration in developing mice, including C57BL/6J-cpk/cpk mice with autosomal recessive-infantile (AR) polycystic kidney disease (PKD). Concentration of urine requires: 1) medullary collecting ducts (CD) located within a hypertonic interstitium, 2) CD cell expression of functional arginine vasopressin V2 receptors (AVP-V2R), and 3) the presence of appropriate CD water channels (aquaporins, AQP 2 and 3). An increase in urine osmolarity, normally seen between 1 and 3 weeks of age, was absent in cpk cystic mice. Aldose reductase mRNA expression (a gene upregulated by medullary hyperosmolarity) increased in normal mice, but remained low in the cystic kidney, suggesting the absence of a hypertonic medullary interstitium. AVP-V2R, AQP2, and AQP3 mRNA expression normally increase between 7 and 14 days. However, all were dramatically overexpressed even at 7 days of age in the cpk kidney in vivo, but decreased in vitro. Activation of the AVP-V2 receptor stimulates the production of cAMP, a substance known to promote cyst enlargement. To determine if CD cAMP, generated from increased AVP-V2Rs, was accelerating the PKD, cystic mice and their normal littermates were treated with OPC31260, a relatively specific AVP-V2R antagonist. OPC31260 treatment of cystic mice led to an amelioration of the cystic enlargement and azotemia. Treatment also decreased renal AQP2 mRNA but increased AVP-V2R and AQP3 mRNA expression in vivo. AVP upregulates the expression of AVP-V2R, AQP2, and AQP3 mRNAs in vitro. Renal EGF, known to inhibit AVP-V2R activity, downregulates AVP-V2R mRNA in vitro. Brief in vivo EGF treatment, known to decrease PKD in cpk mice, led to increased expression of AVP-V2R, AQP2, and AQP3 mRNAs at 2 weeks in both normal and cystic mice but no change was evident at 3 weeks of age. In conclusion, the development of urinary concentration ability correlates with the development of an increased medullary osmotic gradient which is diminished in murine ARPKD. However, CD genes associated with this process are overexpressed in vivo but underexpressed in vitro in the cystic kidney. The overexpression and/or overactivity of the AVP-V2R appears to contribute to the progression of PKD since an AVP-V2R antagonist inhibits cystic renal enlargement in the cpk mouse.

Analysis of chromosome 6p in Spanish families with recessive polycystic kidney disease

Several previous reports have suggested that autosomal recessive polycystic kidney disease (ARPKD) is caused by mutations in a single gene (the PKDH1 gene). Linkage analysis showed a positive linkage for polymorphic markers at the short arm of chromosome 6 (6p) in all families. PKHD1 has not been cloned. Recombinants in the critical region would permit the narrowing of the 6p interval containing the PKHD1 gene, thus facilitating the final identification (cloning) of this gene. Our study included 30 Spanish families. Each family consisted of both parents and at least two children, with at least one diagnosed with ARPKD by clinical and pathological parameters. DNA was obtained and 6p microsatellite markers were used to establish haplotypes for each family. A positive linkage to chromosome 6p was found for all families. In 2 cases, recombinants in the region containing the PKHD1 gene were found. These families will help narrow the size of the 6p region, facilitating the efforts to position and clone the PKHD1 gene. In conclusion, our analysis of Spanish ARPKD families confirms the lack of linkage heterogeneity. This suggests that mutations at a single locus on chromosome 6p21.1-p12 are responsible for the broad clinical spectrum of variable phenotypes.

New insights into the molecular pathophysiology of polycystic kidney disease

Polycystic kidney diseases are characterized by the progressive expansion of multiple cystic lesions, which compromise the function of normal parenchyma. Throughout the course of these diseases, renal tubular function and structure are altered, changing the tubular microenvironment and ultimately causing the formation and progressive expansion of cystic lesions. Renal tubules are predisposed to cystogenesis when a germ line mutation is inherited in either the human PKD1 or PKD2 genes in autosomal dominant polycystic kidney disease (ADPKD) or when a homozygous mutation in Tg737 is inherited in the orpk mouse model of autosomal recessive polycystic kidney disease (ARPKD). Recent information strongly suggests that the protein products of these disease genes may form a macromolecular signaling structure, the polycystin complex, which regulates fundamental aspects of renal epithelial development and cell biology. Here, we re-examine the cellular pathophysiology of renal cyst formation and enlargement in the context of our current understanding of the molecular genetics of ADPKD and ARPKD.

Multiple intracranial aneurysms in a patient with autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD) is usually characterized by early onset chronic renal failure due to innumerable dilated collecting ducts. Hepatic fibrosis is an obligate sign. Here, for the first time, we report a 31-year-old female with ARPKD who was diagnosed with symptomatic multiple intracranial aneurysms, a manifestation previously only known to be associated with autosomal dominant polycystic kidney disease (ADPKD).

The intrahepatic cholangiopathies

The intrahepatic biliary epithelial cells or cholangiocytes are the primary focus of injury in many congenital and acquired liver diseases of childhood. Although cholangiocytes account for only 3 to 5% of the liver cell population, injury and progressive loss of intrahepatic bile ducts can result in considerable morbidity and mortality. Table 1 provides an overview of the various disorders that affect the intrahepatic biliary tree. The more common disorders are discussed in detail below. One of the most important cholangiopathies, biliary atresia, characterized by complete destruction of the extrahepatic biliary tree, with variable involvement of the intrahepatic bile ducts, is discussed elsewhere in this series of articles. There has been considerable progress in our understanding of the embryology and physiology of the intrahepatic biliary system. These topics are also selectively reviewed, with an emphasis on advances that aid in the understanding of the pathophysiology of the disorders which affect the biliary tract in children.

Linkage of Gitelman syndrome to the thiazide-sensitive sodium-chloride cotransporter gene with identification of mutations in Dutch families

Gitelman syndrome is a mostly autosomal recessive disorder affecting the renal tubular function associated with hypokalemia and hypomagnesemia. Functional studies point to a defect in the distal renal tubule in the thiazide-sensitive, electroneutral sodium-chloride co-transporter (TSC). Based upon the localization of a 2.6 cDNA encoding the human TSC to chromosome 16q13, polymorphic markers spanning the region from 16p12 to 16q21 were tested for linkage to the Gitelman syndrome locus in three Dutch families with autosomal recessive inheritance of this disorder. Using two-point linkage analysis, a maximum LOD score (Zmax of 4.49 (at theta = 0.00) was found for the marker D16S408. One crucial recombination event places the Gitelman syndrome locus distal to D16S419 at 16q12-13. Subsequently we have tested our group of Gitelman patients for mutations in the human TSC gene. Two mutations were identified in three Gitelman families. Our study confirms that the human TSC gene is involved in Gitelman syndrome. Patients from three Gitelman families reveal two identical human TSC mutations, suggesting these families share a common ancestor.

Aggressive respiratory support and unilateral nephrectomy for infants with severe perinatal autosomal recessive polycystic kidney disease

Newborn infants with severe autosomal recessive polycystic kidney disease often receive minimal intervention because poor respiratory and renal outcomes are anticipated. We describe two patients whose respiratory failure was successfully treated with aggressive intervention. Massive kidneys restricted gastrointestinal capacity and limited feedings. Uninephrectomy allowed adequate enteral feedings and preserved sufficient renal function for homeostasis and growth.

Apoptosis and loss of renal tissue in polycystic kidney diseases

BACKGROUND

Polycystic kidney disease is characterized by the enlargement of renal cysts, interstitial fibrosis, and gradual loss of normal renal tissue in association with progressive deterioration of renal function. The process causing the progressive loss of renal tissue is unknown, but it could be the result of a form of programmed cell death known as apoptosis.

METHODS

We assayed apoptotic DNA fragmentation in normal and polycystic kidneys biochemically by gel electrophoresis and histochemically by in situ end-labeling. A DNA-specific dye, Hoechst 33258, was used to detect morphologic apoptosis in renal samples from patients with normal kidneys, polycystic kidney disease, and other kidney diseases.

RESULTS

Apoptotic DNA fragmentation was detected in polycystic kidneys from 5 patients without renal failure and 11 patients with renal failure but not in kidneys from 12 patients with no renal disease. In situ end-labeling revealed apoptotic cells in glomeruli, in cyst walls, and in both cystic and noncystic tubules of the polycystic kidneys. No tubular apoptosis was detected in renal-biopsy specimens from five patients with IgA nephropathy, three patients with nephrosclerosis, two patients with focal glomerulosclerosis, one patient with diabetic nephropathy, six patients with acute tubular necrosis, or four patients with acute and four patients with chronic renal-transplant rejection. The capacity of polycystic kidney cells to undergo apoptosis was retained in vitro in the absence of uremia, ischemia, and other confounding pathologic conditions.

CONCLUSIONS

Apoptotic loss of renal tissue may be associated with the progressive deterioration of renal function that occurs in patients with polycystic kidney disease.

Autosomal recessive polycystic kidney disease: long-term sonographic findings in patients surviving the neonatal period

OBJECTIVE

We studied the sonographic findings and the changes in renal function seen on long-term follow-up of children who had the initial diagnosis of autosomal recessive polycystic kidney disease made in the neonatal period.

MATERIALS AND METHODS

The case records and sonograms of 14 children with biopsy evidence of autosomal recessive polycystic kidney disease were evaluated. Nine children who survived the neonatal period were followed up for a mean of 13 years (range, 5-19 years) after diagnosis and form the basis of this study. Serial changes in renal size, echogenicity, and function were assessed sonographically. The imaging findings were compared with those described in published reports.

RESULTS

The sonographic findings showed that five of the nine children had a decrease in renal size, and three had stable renal size over a minimum follow-up period of 5 years. Only one of the nine survivors showed progressive increase in renal size. All had increased cortical echogenicity and large kidneys. Three patients showed a subjective change in renal echogenicity over time. A change in the echogenic pattern to one that resembles autosomal dominant polycystic kidney disease was noted with no evidence of increase in size of the kidneys. None of the surviving children had renal stones or massively enlarged kidneys. The renal function of seven of the nine survivors has remained stable with creatinine clearance nearly normal (> 60 ml/min/1.73 m2), and there was no correlation between renal size and renal function.

CONCLUSION

In patients with autosomal recessive polycystic kidney disease who survive the neonatal period, kidney size as seen on sonograms does not continue to increase despite the patients' linear growth and maintained normal renal function. Rather, a decrease in kidney size and change in echogenicity occurs, producing a pattern that is similar to that seen on sonograms of patients with autosomal dominant polycystic kidney disease but without the marked increase in kidney size that occurs in that entity. This changing cystic pattern on follow-up sonograms may be the reason that previous descriptions of the sonographic findings in cases of autosomal recessive polycystic kidney disease have varied and why a decrease in size may not herald deteriorating renal function.

Course of autosomal recessive polycystic kidney disease (ARPKD) in siblings: a clinical comparison of 20 sibships

Forty-two children out of 20 sibships with autosomal recessive polycystic kidney disease were observed pro- and retrospectively over a mean period of 3.7 years in a long-term study on cystic kidney diseases in children. The intra- and interfamilial variability in terms of age at diagnosis, administration of antihypertensive therapy, liver affection, and renal function were evaluated. According to the 1971 subclassification of Blyth & Ockenden, defining different grades of severity, 12 patients were assigned to the perinatal, nine to the neonatal, 13 to the infantile, and eight to the juvenile subtype of autosomal recessive polycystic kidney disease. In 11 of the 20 families different subtypes were observed among affected siblings. In seven families, affected sibs belonged to adjacent subtypes, while major intrafamilial differences were observed in only four families. The defined subtypes, therefore, cannot be regarded as appropriate in distinguishing genetic groups of autosomal recessive polycystic kidney disease. With respect to the severity of autosomal recessive polycystic kidney disease, there is a wide spectrum of phenotypic manifestations, ranging from stillbirths to mildly affected of phenotypic manifestations, ranging from stillbirths to mildly affected adults, while intrafamilial variability of the clinical picture is generally small with multiple allelism as the most likely genetic explanation. Age at death, however, showed gross variation in eight sibships. Differences in the clinical course between several siblings cannot be explained by a sex influence in autosomal recessive polycystic kidney disease.

Autosomal recessive polycystic kidney disease in 15 Arab children

The study includes 15 children (8 males, 7 females) with autosomal recessive polycystic kidney disease (ARPKD) whose ages at diagnosis ranged from 2 days to 7 years (median 10 months). Eleven (73.3%) patients were hypertensive on admission and 1 developed hypertension 4 months later; 5 patients became normotensive after receiving treatment for 18-36 months (mean 23.2 months). Patients were followed for a period of 1-48 months (mean 20.9 months). Glomerular filtration rate remained normal in 7 patients, improved in 4 and deteriorated in 1. Two patients died soon after diagnosis and 1 was lost to follow-up and is assumed dead. Of the 4 patients less than 6 months old at the time of diagnosis, only 1 is alive compared with 10 of 11 presenting after 6 months of age. The cumulative chance of survival from the time of diagnosis was 85% at 3 months and 77% at 6 months. The study highlights the reversible nature of hypertension in ARPKD. Survival is better in patients older than 6 months at the time of diagnosis and those surviving 6 months follow-up.