The clinical utility of renal concentrating capacity in polycystic kidney disease

A pilot clinical study to evaluate changes in urine osmolality and urine cAMP in response to acute and chronic water loading in autosomal dominant polycystic kidney disease

BACKGROUND AND OBJECTIVES

Autosomal dominant polycystic kidney disease (ADPKD) leads to kidney failure in half of those affected. Increased levels of adenosine 3':5'-cyclic monophosphate (cAMP) play a critical role in disease progression in animal models. Water loading, by suppressing arginine vasopressin (AVP)-stimulated cAMP production, is a proposed therapy for ADPKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The effects of acute and sustained water loading on levels of urine osmolality (Uosm) and cAMP in 13 subjects with ADPKD and 10 healthy controls were studied. Uosm and cAMP concentrations were measured before and after water loading.

RESULTS

Urine [cAMP] indexed to Uosm significantly decreased with acute water loading in both groups (58% in controls and 35% in ADPKD). Chronic water loading resulted in a nonsignificant 13% decrease in 24-hour urine cAMP excretion in ADPKD participants, despite an increase in 24-hour urine volume by 64% to 3.14 +/- 0.32 L and decrease in mean Uosm by 46%, to below that of plasma (270 +/- 21 mOsm/L).

CONCLUSIONS

Increased water intake of 3 L per day decreased Uosm in most ADPKD subjects. While urine [cAMP] accurately reflects changes in Uosm during acute water loading in ADPKD subjects, chronic water loading did not lower 24-hour urine cAMP excretion, although subjects with higher baseline [cAMP] (>2 nmol/mg Cr) responded best. Decreases in urine [cAMP] and osmolality are consistent with decreased AVP activity. These results support the need for a larger study to evaluate the effect of chronic water loading on ADPKD progression.

A case for water in the treatment of polycystic kidney disease

Autosomal dominant polycystic disease (ADPKD) is an inherited disorder characterized by the development within renal tubules of innumerable cysts that progressively expand to cause renal insufficiency. Tubule cell proliferation and transepithelial fluid secretion combine to enlarge renal cysts, and 3'-5'-cyclic adenosine monophosphate (cAMP) stimulates that growth. The antidiuretic hormone, arginine vasopressin (AVP), operates continuously in ADPKD patients to stimulate the formation of cAMP, thereby contributing to cyst and kidney enlargement and renal dysfunction. Studies in animal models of ADPKD provide convincing evidence that blocking the action of AVP dramatically ameliorates the disease process. In the current analysis, the authors reason that increasing the amount of solute-free water drunk evenly throughout the day in patients with ADPKD and normal renal function will decrease plasma AVP concentrations and mitigate the action of cAMP on the renal cysts. Potential pitfalls of increasing fluid intake in ADPKD patients are considered, and suggestions for how physicians may prudently implement this therapy are offered.

MRI-based kidney volume measurements in ADPKD: reliability and effect of gadolinium enhancement

BACKGROUND AND OBJECTIVES

To evaluate the inter- and intrareader reliability and the effect of gadolinium enhancement on kidney volume measurements obtained from pre- and postgadolinium T1 MR images in patients with autosomal dominant polycystic kidney disease (ADPKD).

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Twenty subjects were randomly selected with approximately equal frequency from three kidney-size groups. Pre- and postgadolinium 3D T1 (pre-T1, post-T1) MR images were obtained. The stereology method was applied to segment and measure kidney volumes. The measurement process was repeated at two-wk intervals by two radiologists. Reliability was assessed with correlation coefficients. Intra- and inter-reader bias and measure differences were assessed with paired T-tests. The size effect on the pre- and post-T1 measurements was evaluated with one-way ANOVA.

RESULTS

The intra- and inter-reader reliability was extremely high in all measurements. No systematic intrareader bias but a small inter-reader bias for the post-T1 measurements was observed. All kidney volumes measured on the pre- and post-T1 images were highly correlated with each other for both readers. The post-T1 volumes were significantly higher than pre-T1 volumes. While the post-pre volume differences were relatively constant across the three kidney-size groups, the post-pre percent volume differences were significantly smaller as the size of the kidney increased.

CONCLUSIONS

Kidney volume measurements can be made with minimum intra- and inter-reader variability on both pre- and post-T1 MR images. Kidney volumes measured on the pre-T1 were smaller than those on post-T1, and percent differences between pre-T1 and post-T1 kidney volumes decreased with increasing kidney size.

Evaluation of nephrolithiasis in autosomal dominant polycystic kidney disease patients

BACKGROUND AND OBJECTIVES

Nephrolithiasis (LIT) is more prevalent in patients with autosomal dominant polycystic kidney disease (ADPKD) than in the general population. Renal ultrasonography may underdetect renal stones because of difficulties imposed by parenchymal and/or cyst wall calcifications.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A total of 125 patients with ADPKD underwent ultrasonography and unenhanced computed tomography (CT) scan, routine blood chemistry, and spot and 24-h urine collections.

RESULTS

CT scan detected calculi in 32 patients, including 20 whose previous ultrasonography revealed no calculi. The percentage of hypocitraturia was high but not statistically different between patients with ADPKD+LIT or ADPKD. Hyperuricosuria and distal renal tubular acidosis were less prevalent but also did not differ between groups, whereas hyperoxaluria was significantly higher in the former. Hypercalciuria was not detected. Renal volume was significantly higher in patients with ADPKD+LIT versus ADPKD, and a stepwise multivariate logistic regression analysis showed that a renal volume >or=500 ml was a significant predictor of LIT in patients with ADPKD and normal renal function, after adjustments for age and hypertension.

CONCLUSIONS

CT scan was better than ultrasonography to detect LIT in patients with ADPKD. Larger kidneys from patients with ADPKD were more prone to develop stones, irrespective of the presence of metabolic disturbances.

Cystinuria in a patient with polycystic kidney disease

Cystinuria is a rare autosomal recessive metabolic disorder of renal and intestinal cystine transport. Cystine stones are found in only 1–2% of all stone formers. Patients with cystinuria are at high risk for nephrolithiasis and subsequent morbidity. Our patient is a 37-year-old male who presented for routine follow-up for polycystic kidney disease (PKD). He denied any history of passing nephroliths. He had no family history of PKD or personal history of kidney stones. Serum creatinine was 1.2 mg%. On routine urine microscopy, he was found to have multiple hexagonal cystine crystals. Urine pH was 7.5. Renal CT scan revealed enlarged polycystic kidneys and scattered bilateral intra-renal calculi. Urinary quantification of cystine was 1645 mg/day (normal excretion rate 30 mg/day). Patients with PKD are at increased risk for nephrolithiasis for a number of reasons including urinary acidification, concentrating defects and hypocitraturia. The molecular, cellular and genetic basis for cystinuria is distinctly different and presumably unrelated to the genetic defects in PKD. We suspect that the occurrence of these two unrelated genetic diseases in the same patient is a coincidental finding. Even after a thorough review of the published literature, we were unable to find a genetic relationship between cystinuria and cystic renal diseases. To our knowledge, this is the first report of a finding of cystinuria in an adult with PKD.

Jouberin localizes to collecting ducts and interacts with nephrocystin-1

Joubert syndrome and related disorders are autosomal recessive multisystem diseases characterized by cerebellar vermis aplasia/hypoplasia, retinal degeneration and cystic kidney disease. There are five known genes; mutations of which give rise to a spectrum of renal cystic diseases the most common of which is nephronophthisis, a disorder characterized by early loss of urinary concentrating ability, renal fibrosis, corticomedullary cyst formation and renal failure. Many of the proteins encoded by these genes interact with one another and are located at adherens junctions or the primary cilia and or basal bodies. Here we characterize Jouberin, a multi-domain protein encoded by the AHI1 gene. Immunohistochemistry with a novel antibody showed that endogenous Jouberin is expressed in brain, kidney and HEK293 cells. In the kidney, Jouberin co-localized with aquaporin-2 in the collecting ducts. We show that Jouberin interacts with nephrocystin-1 as determined by yeast-2-hybrid system and this was confirmed by exogenous and endogenous co-immunoprecipitation in HEK293 cells. Jouberin is expressed at cell-cell junctions, primary cilia and basal body of mIMCD3 cells while a Jouberin-GFP construct localized to centrosomes in subconfluent and dividing MDCK cells. Our results suggest that Jouberin is a protein whose expression pattern supports both the adherens junction and the ciliary hypotheses for abnormalities leading to nephronophthisis.

Treatment of autosomal dominant polycystic kidney disease (ADPKD): the new horizon for children with ADPKD

Polycystic kidney disease (PKD) is the most common inherited renal disorder. Patients with PKD remain clinically asymptomatic for decades, while significant anatomic and physiologic systemic changes take place. Sequencing of the responsible genes and identification of their protein products have significantly expanded our understanding of the pathophysiology of PKD. The molecular basis for cystogenesis is being unraveled, leading to new targets for therapy and giving hope to millions of people suffering from PKD. This has direct implications for children with PKD with regard to screening for the disease and identification of high-risk individuals. In this article we provide a review of the clinical manifestations in children with autosomal dominant polycystic kidney disease (ADPKD), the genetic and molecular basis for the disease, and a concise review of potential therapies being evaluated.

Urinary concentration defects and mechanisms underlying nephronophthisis

The cystic kidney disease nephronophthisis (NPHP) is the commonest genetic cause of end-stage renal failure in young people and children. Histologically the disease is characterized by interstitial fibrosis, tubular atrophy with corticomedullary cyst development and disruption of the tubular basement membrane. Affected children present with polydipsia and polyuria, secondary to a urinary concentration defect, before these structural changes develop. Recently, molecular genetic advances have identified several genes mutated in NPHP, providing novel insights into its pathophysiology for the first time in decades. Here we review the normal physiological mechanisms of urinary concentration and explain, in the context of recent discoveries, the possible mechanisms underlying urinary concentration defects in patients with NPHP. The pattern of a ciliary and adherens junction subcellular localization of nephrocystin proteins is discussed. Recent animal models of cystic kidney disease and treatment with vasopressin V2 receptor antagonists are reviewed and a hypothesis regarding urinary concentration defects in NPHP is proposed. Understanding the cellular mechanisms underlying NPHP and other cystic kidney diseases will provide the rationale for therapeutic interventions in this disease. Early urinary concentration defects provide both a clue to clinical diagnosis of NPHP and potential therapeutic targets for pharmacological treatment of this condition.

Potential pharmacological interventions in polycystic kidney disease

Polycystic kidney diseases (autosomal dominant and autosomal recessive) are progressive renal tubular cystic diseases, which are characterised by cyst expansion and loss of normal kidney structure and function. Autosomal dominant polycystic kidney disease (ADPKD) is the most common life- threatening, hereditary disease. ADPKD is more prevalent than Huntington's disease, haemophilia, sickle cell disease, cystic fibrosis, myotonic dystrophy and Down's syndrome combined. Early diagnosis and treatment of hypertension with inhibitors of the renin-angiotensin-aldosterone system (RAAS) and its potential protective effect on left ventricular hypertrophy has been one of the major therapeutic goals to decrease cardiac complications and contribute to improved prognosis of the disease. Advances in the understanding of the genetics, molecular biology and pathophysiology of the disease are likely to facilitate the improvement of treatments for these diseases. Developments in describing the role of intracellular calcium ([Ca(2+)](i)) and its correlation with cellular signalling systems, Ras/Raf/mitogen extracellular kinase (MEK)/extracellular signal-regulated protein kinase (ERK), and interaction of these pathways with cyclic adenosine monophosphate (cAMP) levels, provide new insights on treatment strategies. Blocking the vasopressin V(2) receptor, a major adenylyl cyclase agonist, demonstrated significant improvements in inhibiting cytogenesis in animal models. Because of activation of the mammalian target of rapamycin (mTOR) pathway, the use of sirolimus (rapamycin) an mTOR inhibitor, markedly reduced cyst formation and decreased polycystic kidney size in several animal models. Caspase inhibitors have been shown to decrease cytogenesis and renal failure in rats with cystic disease. Cystic fluid secretion results in cyst enlargement and somatostatin analogues have been shown to decrease renal cyst progression in patients with ADPKD. The safety and efficacy of these classes of drugs provide potential interventions for experimental and clinical trials.

PKD1 haploinsufficiency causes a syndrome of inappropriate antidiuresis in mice

Mutations in PKD1 are associated with autosomal dominant polycystic kidney disease. Studies in mouse models suggest that the vasopressin (AVP) V2 receptor (V2R) pathway is involved in renal cyst progression, but potential changes before cystogenesis are unknown. This study used a noncystic mouse model to investigate the effect of Pkd1 haploinsufficiency on water handling and AVP signaling in the collecting duct (CD). In comparison with wild-type littermates, Pkd1(+/-) mice showed inappropriate antidiuresis with higher urine osmolality and lower plasma osmolality at baseline, despite similar renal function and water intake. The Pkd1(+/-) mice had a decreased aquaretic response to both a water load and a selective V2R antagonist, despite similar V2R distribution and affinity. They showed an inappropriate expression of AVP in brain, irrespective of the hypo-osmolality. The cAMP levels in kidney and urine were unchanged, as were the mRNA levels of aquaporin-2 (AQP2), V2R, and cAMP-dependent mediators in kidney. However, the (Ser256) phosphorylated AQP2 was upregulated in Pkd1(+/-) kidneys, with AQP2 recruitment to the apical plasma membrane of CD principal cells. The basal intracellular Ca(2+) concentration was significantly lower in isolated Pkd1(+/-) CD, with downregulated phosphorylated extracellular signal-regulated kinase 1/2 and decreased RhoA activity. Thus, in absence of cystic changes, reduced Pkd1 gene dosage is associated with a syndrome of inappropriate antidiuresis (positive water balance) reflecting decreased intracellular Ca(2+) concentration, decreased activity of RhoA, recruitment of AQP2 in the CD, and inappropriate expression of AVP in the brain. These data give new insights in the potential roles of polycystin-1 in the AVP and Ca(2+) signaling and the trafficking of AQP2 in the CD.

Autosomal Dominant Polycystic Kidney Disease: Time for a Change?

Diagnosis and treatment of autosomal dominant polycystic kidney disease (ADPKD) is rapidly changing. Cellular pathways that involve the polycystins are being mapped and involve the primary cilium, intracellular calcium and cAMP regulation, and the mammalian target of rapamycin (mTOR) pathway. With the use of new imaging approaches, earlier diagnosis of hepatic cystic disease is possible, and measurement of kidney and cystic growth as well as kidney blood flow is possible over relatively short periods. PKD gene type, gender, proteinuria, and the presence of hypertension relate to the rate of kidney growth in ADPKD. On the basis of risk factors for progression to ESRD and the pathogenic roles that intracellular cAMP and mTOR play in cystogenesis, novel therapies are now being tested, including maximal inhibition of the renin-angiotensin system, inhibition of renal intracellular cAMP using vasopressin V2 receptor antagonists, and somatostatin analogues, as well as inhibitors of mTOR. This review addresses the current understanding of the pathogenesis and the natural history of ADPKD; accuracy and reliability of diagnostic approaches in utero, childhood, and adulthood; the value of reliable magnetic resonance imaging to measure disease progression early in the course of ADPKD; and novel therapeutic approaches that are being evaluated in ADPKD.

Increased water intake decreases progression of polycystic kidney disease in the PCK rat

Renal enlargement in polycystic kidney disease (PKD) is caused by the proliferation of mural epithelial cells and transepithelial fluid secretion into the cavities of innumerable cysts. Arginine vasopressin (AVP) stimulates the proliferation of human PKD cells in vitro via cAMP-dependent activation of the B-Raf/MEK (MAPK/ERK kinase/extracellular signal-regulated kinase (ERK) pathway. ERK activity is elevated in cells that line the cysts in animals with PKD, and AVP receptor antagonists reduce ERK activity and halt disease progression. For suppression of the effect of AVP physiologically, water intake was increased in PCK rats, a model of PKD, and the effect on renal morphology, cellular mechanism, and function was determined. The addition of 5% glucose in the drinking water increased fluid intake approximately 3.5-fold compared with rats that received tap water. In PCK rats, increased water intake for 10 wk reduced urinary AVP excretion (68.3%), and urine osmolality fell below 290 mOsmol/kg. High water intake was associated with reduced renal expression of AVP V2 receptors (41.0%), B-Raf (15.4%), phosphorylated ERK (38.1%), and proliferating cell nuclear antigen-positive renal cells (61.7%). High water intake reduced the kidney/body weight ratio 28.0% and improved renal function. Taken together, these data demonstrate that water intake that is sufficient to cause persistent water diuresis suppresses B-Raf/MEK/ERK activity and decreases cyst and renal volumes in PCK rats. It is suggested that limiting serum AVP levels by increased water intake may be beneficial to some patients with PKD.

Design and baseline characteristics of participants in the study of antihypertensive therapy in children and adolescents with autosomal dominant polycystic kidney disease (ADPKD)

In this manuscript, we describe our ongoing randomized clinical trial to assess the efficacy of blood pressure control with angiotensin converting enzyme (ACE) inhibition on renal cyst growth over a 5-year study period in children and young adults aged 4-21 years with autosomal dominant polycystic kidney disease (ADPKD). Baseline demographic and laboratory data for the study groups are reported. Results of this study could significantly impact the standard of care for management of ADPKD in this population.

Effective treatment of an orthologous model of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a leading cause of end-stage renal disease. The vasopressin V2 receptor (VPV2R) antagonist OPC31260 has been effective in two animal models of PKD with pathologies that are probably related. Here we show, in a mouse model of ADPKD (Pkd2(-/tm1Som)), a similar cellular phenotype and response to OPC31260 treatment, with reduction of renal cyclic AMP (cAMP) levels, prevention of renal enlargement, marked inhibition of cystogenesis and protection of renal function.

Inhibition of renal cystic disease development and progression by a vasopressin V2 receptor antagonist

The polycystic kidney diseases (PKDs) are a group of genetic disorders causing significant renal failure and death in children and adults. There are no effective treatments. Two childhood forms, autosomal recessive PKD (ARPKD) and nephronophthisis (NPH), are characterized by collecting-duct cysts. We used animal models orthologous to the human disorders to test whether a vasopressin V2 receptor (VPV2R) antagonist, OPC31260, would be effective against early or established disease. Adenosine-3',5'-cyclic monophosphate (cAMP) has a major role in cystogenesis, and the VPV2R is the major cAMP agonist in the collecting duct. OPC31260 administration lowered renal cAMP, inhibited disease development and either halted progression or caused regression of established disease. These results indicate that OPC31260 may be an effective treatment for these disorders and that clinical trials should be considered.

High urine volume and low urine osmolality are risk factors for faster progression of renal disease

BACKGROUND

Increased fluid intake slows renal disease progression in animal models. The relevance of these findings to human renal disease is not clear, although increased fluid intake often is recommended to patients with chronic renal insufficiency. This study tested the hypothesis that urine volume, urine osmolality (Uosm), or both are significantly associated with glomerular filtration rate (GFR) decline in patients with chronic renal insufficiency.

METHODS

This is a retrospective analysis of Modification of Diet in Renal Disease (MDRD) study A patients with (N = 139) and without polycystic kidney disease (PKD; N = 442). The key outcome measure was GFR slope in relation to mean 24-hour urine volume and Uosm during follow-up in study A (mean, 2.3 years).

RESULTS

The regression of GFR slope on mean follow-up 24-hour urine volume (adjusted for body surface area and MDRD diet and blood pressure group) showed that the greater the urine volume, the faster the GFR decline in patients both with and without PKD. For example, the difference in GFR slope for those with a mean follow-up 24-hour urine volume of 2.4 versus 1.4 L was -1.01 mL/min/y (confidence interval, -0.27 to -1.75) for patients without PKD and -1.20 mL/min/y (confidence interval, -0.06 to -2.34) for those with PKD. A similar but inverse relationship was shown between GFR decline and mean 24-hour Uosm in patients with (P = 0.01) and without PKD (P = 0.001). These associations remained significant after adjustment for 13 relevant baseline and follow-up covariates.

CONCLUSION

Sustained high urine volume and low Uosm are independent risk factors for faster GFR decline in patients with chronic renal insufficiency. Thus, high fluid intake does not appear to slow renal disease progression in humans. We suggest that until better evidence becomes available, patients with chronic renal insufficiency should generally let their thirst guide fluid intake. The advice to avoid "pushing fluids" might be particularly important for patients with PKD.

Factors affecting the fate of prolonged forgotten 'J' stents

PURPOSE

To compare the effects of age and presence or absence of renal impairment at the time of 'J' stent insertion on the subsequent fate of prolonged forgotten stents.

PATIENTS AND METHODS

A patient was described as having a prolonged forgotten 'J' stent, if the patient's 'J' stent was removed more than 12 months after it had been inserted. We compared the effect of age at insertion and presence or absence of renal impairment on the complications encountered in such patients. We describe briefly the management of the complications. We defined a patient as having moderate renal impairment if the patient has serum creatinine >200 < 500 micromol/l and is not on dialysis.

RESULTS

We treated 17 patients with forgotten 'J' stents between 1994 and 2000. Fifteen were adults, mean age 25 (range 18-72) years, and 2 were children 9 and 10 years respectively at the time the stents were inserted. The mean duration of stent retention was 24.30 (range 12-60) months. In 12 patients the stents were forgotten for between 12 and 18 months. In these, the stents had varying degrees of calcification but were easily removed intact endoscopically in 11 out of 12 cases. One 10-year-old boy in this group required open surgical removal of the stent. In one 35-year-old patient, the stent was forgotten for 36 months. It had fractured spontaneously in 7 places and required endoscopic and open removal of stent fragments. In 2 cases, a growing 9-year-old boy, and a 30-year-old man the stents were forgotten for 46 and 48 months respectively. After 46 months of retaining the stent, the stent spontaneously fractured in 11 places in the growing child, while in the adult it became heavily calcified and fractured during attempts to remove it endoscopically. Two adult patients with moderate renal failure at the time of stent insertion retained the stents for 40 and 60 months respectively. One of these 2 stents had a minor calcification at the tip of the stent in the renal pelvis. Both stents were removed intact endoscopically and showed no sign of fracture or calcification.

CONCLUSION

In a growing child a prolonged forgotten 'J' stent is very likely to undergo spontaneous fracture due to the stress exerted on it as a result of cranio-caudal growth of the child. In adults, prolonged forgotten stents become calcified, brittle and lose tensile strength after more than one year of placement and may fracture either spontaneously or during attempts to remove them endoscopically. In patients producing hypotonic urine such as patients with moderate renal failure, a prolonged forgotten stent may remain little affected by the passage of time.

Anatomic and metabolic risk factors for nephrolithiasis in patients with autosomal dominant polycystic kidney disease

The prevalence of nephrolithiasis is considerably greater in patients with autosomal dominant polycystic kidney disease (ADPKD) than in the general population. We evaluated anatomic and metabolic factors that may be associated with an increased prevalence of nephrolithiasis in patients with ADPKD. We compared anatomic parameters among ADPKD patients with or without nephrolithiasis as diagnosed by ultrasonography, whereas metabolic factors were determined by 24-hour urinary chemical analysis. Patients with ADPKD and nephrolithiasis had more renal cysts (P < 0.05) and a larger predominant renal cyst size (P < 0.005) than patients without nephrolithiasis. Concurrently, individual stone-forming kidneys had a greater cyst number (P < 0.05) and a significantly larger predominant cyst size (P < 0.01) compared with kidneys without stones. Patients with ADPKD and nephrolithiasis had a significantly lower creatinine clearance than those without nephrolithiasis (68.7 +/- 8.6 versus 94.8 +/- 5.4 mL/min, respectively; P < 0.05). Twenty-four-hour urinary analysis showed that patients with ADPKD and nephrolithiasis had significantly lower urinary volumes (P < 0. 05), and levels of urinary phosphate (P < 0.05), magnesium (P < 0. 005), and potassium (P < 0.05). Although not statistically significant, patients with ADPKD with stones tended to have lower levels of urinary citrate, and both groups showed a high percentage (range, 49% to 60%) of patients with hypocitraturia. Our data are consistent with the hypothesis that patients with ADPKD who develop nephrolithiasis do so because of increased intrarenal anatomic obstruction, as well as lower levels of such urinary inhibitors of stones as magnesium and citrate.

Volumetric measurement of renal cysts and parenchyma using MRI: phantoms and patients with polycystic kidney disease

We have developed an MR method to measure the volumes of renal cysts and parenchyma in patients with polycystic kidney disease. Phantoms were designed to simulate polycystic kidneys. Four patients were recruited. MR scans were performed on the phantoms and patients. A stereology technique was applied for image segmentation and volume measurement. Volumetric measurement of renal cysts and parenchyma was accurate in phantom studies and reliable in both phantom and patient studies in these limited examples.

Mutation detection of PKD1 identifies a novel mutation common to three families with aneurysms and/or very-early-onset disease

It is known that several of the most severe complications of autosomal-dominant polycystic kidney disease, such as intracranial aneurysms, cluster in families. There have been no studies reported to date, however, that have attempted to correlate severely affected pedigrees with a particular genotype. Until recently, in fact, mutation detection for most of the PKD1 gene was virtually impossible because of the presence of several highly homologous loci also located on chromosome 16. In this report we describe a cluster of 4 bp in exon 15 that are unique to PKD1. Forward and reverse PKD1-specific primers were designed in this location to amplify regions of the gene from exons 11-21 by use of long-range PCR. The two templates described were used to analyze 35 pedigrees selected for study because they included individuals with either intracranial aneurysms and/or very-early-onset disease. We identified eight novel truncating mutations, two missense mutations not found in a panel of controls, and several informative polymorphisms. Many of the polymorphisms were also present in the homologous loci, supporting the idea that they may serve as a reservoir for genetic variability in the PKD1 gene. Surprisingly, we found that three independently ascertained pedigrees had an identical 2-bp deletion in exon 15. This raises the possibility that particular genotypes may be associated with more-severe disease.

Epithelial transport in polycystic kidney disease

In autosomal dominant polycystic kidney disease (ADPKD), the genetic defect results in the slow growth of a multitude of epithelial cysts within the renal parenchyma. Cysts originate within the glomeruli and all tubular structures, and their growth is the result of proliferation of incompletely differentiated epithelial cells and the accumulation of fluid within the cysts. The majority of cysts disconnect from tubular structures as they grow but still accumulate fluid within the lumen. The fluid accumulation is the result of secretion of fluid driven by active transepithelial Cl- secretion. Proliferation of the cells and fluid secretion are activated by agonists of the cAMP signaling pathway. The transport mechanisms involved include the cystic fibrosis transmembrane conductance regulator (CFTR) present in the apical membrane of the cystic cells and a bumetanide-sensitive transporter located in the basolateral membrane. A lipid factor, called cyst activating factor, has been found in the cystic fluid. Cyst activating factor stimulates cAMP production, proliferation, and fluid secretion by cultured renal epithelial cells and also is a chemotactic agent. Cysts also appear in the intrahepatic biliary tree in ADPKD. Normal ductal cells secrete Cl- and HCO3-. The cystic ductal cell also secretes Cl-, but HCO3- secretion is diminished, probably as the result of a lower population of Cl-/HCO3- exchangers in the apical membrane as compared with the normal cells. Some segments of the normal renal tubule are also capable of utilizing CFTR to secrete Cl-, particularly the inner medullary collecting duct. The ability of Madin-Darby canine kidney cells and normal human kidney cortex cells to form cysts in culture and to secrete fluid and the functional similarities between these incompletely differentiated, proliferative cells and developing cells in the intestinal crypt and in the fetal lung have led us to suggest that Cl- and fluid secretion may be a common property of at least some renal epithelial cells in an intermediate stage of development. The genetic defect in ADPKD may not directly affect membrane transport mechanisms but rather may arrest the development of certain renal epithelial cells in an incompletely differentiated, proliferative stage.

Renal tubular dysfunction in patients with cystic disease of the kidneys

OBJECTIVE

To define the renal tubular functional abnormalities in patients with cystic disease of the kidneys.

METHODS

Patients with autosomal dominant polycystic kidney disease (ADPKD) (n = 4) and medullary sponge kidneys (MSK) (n = 3) with normal glomerular filtration rate (GFR), determined by inulin clearance, and effective renal plasma flow (ERPF), measured by p-aminohippurate clearance, underwent measurement of proximal and distal tubular functions. Proximal tubular functions were determined by the maximum reabsorption of glucose (TmGlucose) and the maximum secretion of p-aminohippurate (TmPAH). Distal tubular functions were measured by the maximum urinary concentrating and diluting mechanisms, and the urinary acidification response to acid load.

RESULTS

TmGlucose was low in both groups (209 +/- 25 mg/min/1.73 m2 in the ADPKD group and 110 +/- 28 mg/min/1.73 m2 in the MSK, compared with 375 +/- 40 mg/min/1.73 m2 in healthy controls; P < 0.05). Likewise, TmPAH was significantly diminished in patients with ADPKD (72 +/- 6 mg/min/1.73 m2) and MSK (63 +/- 5 mg/min/1.73 m2) when compared with healthy controls (89 +/- 4 mg/min/1.73 m2; P < 0.05). Urinary maximum concentration after fluid deprivation was impaired in both ADPKD and MSK patients, but the diluting mechanism was intact. Finally, the ability to excrete urinary ammonium and titratable acids following an oral acid load was inadequate in both the ADPKD and MSK groups.

CONCLUSIONS

Proximal and distal tubular functions are impaired in patients with ADPKD and MSK when GFR and ERPF are normal, indicating tubular disruption by the cysts and the alteration of the tubulo-interstitial vascular relationship.

Polycystic kidney disease: etiology, pathogenesis, and treatment

Once viewed as hopelessly incurable disorders and the dustbin for careers in academic medicine, the polycystic kidney diseases have emerged as prime targets of pathophysiologic study and palliative and definitive treatment in the era of molecular medicine. Polycystic kidney disease (PKD) may be hereditary or acquired. The major inherited types are autosomal dominant (AD) and autosomal recessive (AR). ADPKD is caused by at least two (and possibly three) genes located on separate chromosomes, while ADPKD-1 is due to a 14 kb transcript in a duplicated region on the short arm of chromosome 16 very near the alpha-globin gene cluster and the gene for one form of tuberous sclerosis. ADPKD-2 has been assigned to the long arm of chromosome 4. ARPKD is due to a mutated gene on both copies of the long arm of chromosome 6. Cysts originate in renal tubules. Proliferation of tubule epithelial cells modulated by endocrine, paracrine, and autocrine factors is a major element in the pathogenesis of renal cystic diseases. In addition, fluid that is abnormally accumulated within the cysts is derived from glomerular filtrate and, to a greater extent, by transepithelial fluid secretion. Abnormal synthesis and degradation of matrix components associated with interstitial inflammation are additional features in the pathogenesis of renal cystic diseases. The ADPKD genotypes are characterized by bilateral kidney cysts, hypertension, hematuria, renal infection, stones, and renal insufficiency. ADPKD is a systemic disorder; cysts appear with decreasing frequency in the kidneys, liver, pancreas, brain, spleen, ovaries, and testis. Cardiac valvular disorders, abdominal and inguinal hernias, and aneurysms of cerebral and coronary arteries and aorta are also associated with ADPKD. Treatment is supportive: dietary regulation of salt and protein intake, control of hypertension and renal stones, and dialysis and transplantation at the end stage. ARPKD is a relatively rare disease that causes clinical symptoms at birth, with significant mortality in the first month of life. The cysts develop primarily in the collecting ducts because of a failure in the maturation process. Early complications include Potter's syndrome; excessive size of the kidneys, causing respiratory dysfunction; hypertension; and renal insufficiency. Hepatic fibrosis is an associated extrarenal problem that results in significant morbidity in young children and adolescents. Treatment includes supportive care, dialysis, and renal transplantation. Acquired cysts (solitary/simple) are commonplace in older persons. Multiple cysts may be seen in association with potassium deficiency, congenital disorders, metabolic diseases, and toxic renal injury.(ABSTRACT TRUNCATED AT 400 WORDS)

Fibrosis causes progressive kidney failure

Fibrosis is usually regarded as an uninteresting end-point of injury. However, it influences function in heart, lung, and liver. Kidney fibrosis correlates well with overall renal function, but is only rarely recognized as affecting function itself. There is evidence that organ contraction from fibrotic scarring may affect kidney function, and could contribute to progressive renal failure.

Renal ammonia in autosomal dominant polycystic kidney disease

Recent studies have suggested that defective medullary trapping of ammonia underlies the acidosis associated with renal failure and sets in motion maladaptive compensatory mechanisms that contribute to the progression of renal disease. Since a renal concentrating defect is an early functional abnormality in autosomal dominant polycystic kidney disease (ADPKD), defective medullary trapping and urinary excretion of ammonia may also occur early and have important pathophysiological consequences. The urinary pH and excretions of ammonia, titratable acid, and bicarbonate, were measured during a 24-hour baseline period and following the administration of ammonium chloride (100 mg/kg body wt) in ADPKD patients with normal glomerular filtration rate and in age- and gender-matched healthy control subjects. The distal nephron hydrogen ion secretory capacity was assessed during a bicarbonate infusion. Ammonia, sodium, pH, C3dg, and C5b-9 were measured in cyst fluid samples. The excretion rates of ammonia during the 24-hour baseline period and following the administration of ammonium chloride were significantly lower, and the relationship of ammonia excretion to urinary pH was significantly shifted downward in ADPKD. No difference in the increment of urinary pCO2 (delta pCO2) or the peripheral blood-urine pCO2 gradient (U-B pCO2) between ADPKD patients and control subjects was detected during a sodium bicarbonate infusion. Calculated concentrations of free-base ammonia in cyst fluid samples exceeded those calculated from reported concentrations of ammonia in renal venous blood of normal subjects. C3dg and C5b-9 were detected in some cyst fluids. The urinary excretion of ammonia is reduced in ADPKD patients with normal glomerular filtration rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathogenesis of renal cyst expansion: opportunities for therapy

Renal epithelial cysts are caused by hereditary or acquired etiologies of undetermined molecular mechanisms. The cysts develop within renal tubules by pathogenetic processes that involve cellular proliferation, accumulation of tubule fluid within distended cavities, and remodelling of extracellular matrix. Evidence is accumulating to support the view that renal cysts are composed of moderately dedifferentiated, immature epithelial cells that proliferate abnormally and transport solute and fluid by secretion, in contrast to the usual absorptive movement of liquid in normal renal tubules. The formation and expansion of renal cysts occurs in conjunction with alterations in the extracellular matrix, including thickening of the tubule basement membrane, infiltration of the interstitium with mononuclear inflammatory cells, and interstitial fibrosis. The pathogenetic elements of cyst formation are understood well enough that it is reasonable, and of considerable importance, to examine in animal models treatment strategies that may be hypothesized to allay the progression of cystic disease to end-stage renal failure.

Renal stone disease in autosomal dominant polycystic kidney disease

Nephrolithiasis is an important manifestation of autosomal dominant polycystic kidney disease (ADPKD), which occurs in approximately 20% of patients. It should always enter the differential diagnosis of flank pain in patients with ADPKD. The diagnosis is hindered by the distorted anatomy of the polycystic kidneys and the frequent occurrence of parenchymal and cyst wall calcifications, and requires demonstration of the relationship to the collecting system by intravenous urography and/or computed tomography. Computed tomography is the most sensitive imaging technique for detection of stones or calcifications, whereas intravenous urography is the most sensitive for visualization of the intrarenal collecting system. Precaliceal tubular ectasia can be detected in 15% of patients with ADPKD and nephrolithiasis, but this association may not be specific to ADPKD. The composition of the stones is most frequently uric acid and/or calcium oxalate. Metabolic factors are important in their pathogenesis. Distal acidification defects may be important in a few patients, while an abnormal transport of ammonium, low urine pH, and hypocitruria are the most common abnormalities. The treatment of nephrolithiasis in patients with ADPKD is not different from that in patients without ADPKD. Extracorporeal shock wave lithotripsy and percutaneous nephrostolithotomy in patients with early disease and normal renal function are not contraindicated.

Clinical profiles of gross hematuria in autosomal dominant polycystic kidney disease

There is little information on the characteristics, management, or sequelae of gross hematuria in autosomal dominant polycystic kidney disease (ADPKD). Therefore, we obtained detailed information regarding gross hematuria in 191 adult ADPKD subjects. Forty-two percent (N = 81) experienced at least one episode of gross hematuria. The mean age of the initial episode was 30 +/- 1 years; only 10% of subjects reported the first episode before age 16. Twenty-three percent of those with gross hematuria had experienced more than six occurrences. Sixty-two percent of patients with bleeding indicated a presumptive precipitating event, most commonly urinary tract infection (42% overall, 61% of females v 17% of males, P less than 0.01), or sports or strenuous activity (20% of males v 11% of females, NS). In 56% of subjects, the episode persisted for 2 to 7 days. Hypertensive ADPKD subjects were more likely to have gross hematuria than normotensive subjects (48% v 30%, P less than 0.02) and those with gross hematuria had larger renal size (820 +/- 87 v 588 +/- 52 cm3, P less than 0.03). Moreover, those subjects with more episodes of gross hematuria had a higher serum creatinine concentration than those with fewer episodes (serum creatinine: 0 episodes, 120 +/- 10 v greater than 5 episodes, 190 +/- 30 mumol/L, P less than 0.04 [1.4 +/- 0.1 v 2.1 +/- 0.3 mg/dL]). This association suggests that, although self-limited, cumulative episodes of gross hematuria may have an unfavorable impact on long-term renal function.

Factors affecting the progression of renal disease in autosomal-dominant polycystic kidney disease

Autosomal-dominant polycystic kidney disease results in renal failure at a varying age from childhood to old age. We postulated that factors other than the culprit gene alone contribute to the course of progression of the renal failure. We studied 580 subjects with autosomal-dominant polycystic kidney disease and 194 unaffected family members. We calculated survival curves to end-stage renal failure or death and developed a linear model for testing the effects of single or multiple variables on the progression of renal failure as estimated from the reciprocal of serum creatinine. Fifty-two subjects died and 94 reached end-stage renal failure during the period of observation, yielding functional survivals of 71% at age 50 years, 53% at 58 years and 23% at 70 years. The following variables were independently associated with worse mean renal function at a given age (P value less than 0.01): the PKD1 gene, younger age at diagnosis, male gender, hypertension, increased left ventricular mass, hepatic cysts in women, three or more pregnancies, gross hematuria, urinary tract infections in men and renal size expressed as renal volume. The following were not associated significantly with the course of renal function: gender of affected parent, mitral valve prolapse, intracranial aneurysms, any pregnancy, hepatic cysts in men and urinary tract infections in women. The identification of unalterable maleficent factors such as the PKD1 gene and male gender permit more informed counseling while the identification of alterable factors such as hypertension, number of pregnancies and recurrent urinary tract infections provides the clinician with the opportunity to modify these factors and improve the management of patients with autosomal-dominant polycystic kidney disease.

Autosomal dominant polycystic kidney disease--more than a renal disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic disease, affecting a half million Americans. The clinical phenotype can result from at least two different gene defects. One gene that can cause ADPKD has been located on the short arm of chromosome 16. This discovery has made possible new methods for diagnosing the disorder in gene carriers prior to the development of renal cysts. Although renal cysts are clearly an important manifestation of the gene defect, other systemic manifestations are both common and clinically important. Cardiac valvular lesions, intracranial aneurysms, hepatic cysts, and diverticula are included in the array of systemic manifestations. Moreover, renal cysts are only one of a myriad of renal manifestations. Although ADPKD was long considered an adult cystic disease, it is also a common cause of childhood cystic disease and must be considered in the differential diagnosis in that setting.

The renin-angiotensin-aldosterone system and autosomal dominant polycystic kidney disease

BACKGROUND

A high incidence of hypertension (50 to 75 percent) occurs early in the course of autosomal dominant polycystic kidney disease. Cyst enlargement, causing bilateral renal ischemia and subsequent release of renin, is proposed as the cause of this form of hypertension.

METHODS

To investigate this hypothesis, we measured plasma renin activity and aldosterone concentrations during short-term and long-term converting-enzyme inhibition in 14 patients with hypertension due to polycystic kidney disease, 9 patients with essential hypertension, 11 normotensive patients with polycystic kidney disease, and 13 normal subjects. The groups were comparable with respect to age, sex, body-surface area, degree of hypertension, sodium excretion, and renal function.

RESULTS

During the short-term study, the mean (+/- SE) plasma renin activity was significantly higher in the hypertensive patients with polycystic kidney disease than in the patients with essential hypertension, in the supine (0.36 +/- 0.06 vs. 0.22 +/- 0.06 ng per liter.second, P = 0.05) and upright positions (1.03 +/- 0.14 vs. 0.61 +/- 0.08 ng per liter.second, P less than 0.03) and after converting-enzyme inhibition (1.97 +/- 0.28 vs. 0.67 +/- 0.17 ng per liter.second, P less than 0.0006). The mean arterial pressures measured in the supine and upright positions and the plasma aldosterone concentrations measured in the upright position were significantly higher in the normotensive patients with polycystic kidney disease than in the normal subjects. After six weeks of converting-enzyme inhibition, renal plasma flow increased (P less than 0.005), and both renal vascular resistance (P less than 0.007) and the filtration fraction (P less than 0.02) decreased significantly in the hypertensive patients with polycystic kidney disease but not in the patients with essential hypertension.

CONCLUSIONS

The renin-angiotensin-aldosterone system is stimulated significantly more in hypertensive patients with polycystic kidney disease than in comparable patients with essential hypertension. The increased renin release, perhaps due to renal ischemia caused by cyst expansion, probably contributes to the early development of hypertension in polycystic kidney disease.

Uric acid handling in autosomal dominant polycystic kidney disease with normal filtration rates

PURPOSE

Patients with autosomal dominant polycystic kidney disease (ADPKD) are alleged to have more frequent or more pronounced alterations of uric acid homeostasis than are seen in most other types of chronic renal diseases. We performed this study to examine the hypothesis that individuals with ADPKD have abnormal uric acid homeostasis that is manifest before the development of renal insufficiency.

PATIENTS AND METHODS

We studied 301 subjects, 163 with ADPKD and 138 relatives without ADPKD (NADPKD), by ultrasonography. The subjects were interviewed and examined. Venous blood and two 24-hour urine collections were obtained for uric acid and creatinine determinations.

RESULTS

Presence of hyperuricemia, serum uric acid levels, uric acid clearance, and fractional excretion of uric acid did not differ between ADPKD and NADPKD subjects with normal renal function (creatinine clearance greater than 80 mL/minute/1.73 m2). Clearance of uric acid decreased and fractional excretion increased in subjects with decreased renal function in both groups. Female gender enhanced renal excretion of uric acid in both groups and hypertension depressed it except in men with ADPKD, who had higher fractional excretions of uric acid than did hypertensive NADPKD men.

CONCLUSIONS

Uric acid homeostasis is preserved in individuals with ADPKD with normal renal function when compared to unaffected family members. Hyperuricemia and decreased renal excretion of uric acid develop as renal function worsens in ADPKD, similar to that in control subjects. The expected depressing effect of hypertension on renal handling of uric acid was not seen in men with ADPKD, speculatively due to an effect of atrial natriuretic factor.