The association of nephrolithiasis and autosomal dominant polycystic kidney disease

Acute Kidney Injury Due to Nephrolithiasis in Autosomal Dominant Polycystic Kidney Disease Treated With Tolvaptan: A Case Report

A 61-year-old asymptomatic female with autosomal dominant polycystic kidney disease (ADPKD) on tolvaptan therapy was hospitalized for acute kidney injury (AKI). Nephrolithiasis had already been diagnosed; however, the patient had not undergone any interventions. She also presented with hyponatremia possibly caused by overhydration. Because the estimated glomerular filtration rate (eGFR) decline was significantly higher than the predicted rate, we considered a possible case of postrenal AKI and examined computed tomography (CT), which revealed left hydronephrosis with a 9.4-mm ureteric stone at the level of L3/L4. We restricted fluid intake, which resulted in an increase in sodium levels. She was treated with transurethral lithotripsy (TUL) twice, which successfully improved her kidney function. Although the serum sodium levels increase because of aquaresis in almost all patients treated with tolvaptan, our case was unique in that the patient presented with hyponatremia. We should pay more attention to the periodical follow-up of nephrolithiasis in addition to the increase in total kidney volume and decide the appropriate time to treat nephrolithiasis depending on the case. We should also keep in mind that ADPKD patients have a high frequency of nephrolithiasis and, even if asymptomatic, investigate urinary tract obstruction and hydronephrosis in case of AKI.

How Does ADPKD Severity Differ Between Family Members?

Thousands of pathogenic variants in more than 100 genes can cause kidney cysts with substantial variability in phenotype and risk of subsequent kidney failure. Despite an established genotype-phenotype correlation in cystic kidney diseases, incomplete penetrance and variable disease expressivity are present as is the case in all monogenic diseases. In family members with autosomal dominant polycystic kidney disease (ADPKD), the same causal variant is responsible in all affected family members; however, there can still be striking discordance in phenotype severity. This narrative review explores contributors to within-family discordance in ADPKD severity. Cases of biallelic and digenic inheritance, where 2 rare pathogenic variants in cystogenic genes are coexistent in one family, account for a small proportion of within-family discordance. Genetic background, including cis and trans factors and the polygenic propensity for comorbid disease, also plays a role but has not yet been exhaustively quantified. Environmental exposures, including diet; smoking; alcohol, salt, and protein intake, and comorbid diseases, including obesity, diabetes, hypertension, kidney stones, dyslipidemia, and additional coexistent kidney diseases all contribute to ADPKD phenotypic variability among family members. Given that many of the factors contributing to phenotype variability are preventable, modifiable, or treatable, health care providers and patients need to be aware of these factors and address them in the treatment of ADPKD.

Investigational agents for autosomal dominant polycystic kidney disease: preclinical and early phase study insights

INTRODUCTION

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is the most common inherited kidney condition caused by a single-gene mutation. It leads patients to kidney failure in more than 50% of cases by the age of 60, and, given the dominant inheritance, this disease is present in the family history in more than 90% of cases.

AREAS COVERED

This review aims to analyze the set of preclinical and early-phase studies to provide a general view of the current progress on ADPKD therapeutic options. Articles from PubMed and the current status of the trials listed in clinicaltrials.gov were examined for the review.

EXPERT OPINION

Many potential therapeutic targets are currently under study for the treatment of ADPKD. A few drugs have reached the clinical phase, while many are currently still in the preclinical phase. Organoids could be a novel approach to the study of drugs in this phase. Other than pharmacological options, very important developing approaches are represented by gene therapy and the use of MiRNA inhibitors.

Assessment of metabolic risk factors for nephrolithiasis in patients with autosomal dominant polycystic kidney disease: a cross-sectional study

BACKGROUND

Nephrolithiasis is more common in autosomal dominant polycystic kidney disease (ADPKD) than in the normal population. We aimed to investigate the anatomical and metabolic factors that may be associated with nephrolithiasis in patients with ADPKD METHODS: In this cross-sectional study, a total of 180 participants were included. Eighty-five patients with ADPKD [42 patients with nephrolithiasis (PKD N +) and 43 without nephrolithiasis (PKD N -)] were recruited. Forty-seven nephrolithiasis patients without ADPKD (N) and 48 healthy controls (HC) were selected as control groups. 24-h urine collections were measured in all participants. 24-h urine citrate, calcium, urate, oxalate, magnesium and sodium, serum electrolytes, and eGFRs were compared.

RESULTS

Total kidney volumes were not different between patients with PKD N + and PKD N -. Hypocitraturia was common in all patients with ADPKD (69.4%), and it was not different between PKD N + (76.2%) and PKD N- (62.8%). However, hypocitraturia was statistically higher in PKD N + and PKD N - than in N (38.3%) and HC (12.5%) (p<0.05). 24-h urine calcium, urate, and oxalate levels were similar between PKD N + and PKD N - CONCLUSIONS: Hypocitraturia was found to be significantly higher in patients with ADPKD than in healthy adults and other kidney stone patients.

Supine PCNL in autosomal dominant polycystic kidney disease

AIMS AND OBJECTIVES

Standard percutaneous nephrolithotomy (PCNL) has routinely been performed in prone position in autosomal dominant polycystic kidney disease (ADPKD) with nephrolithiasis. The objectives of our present study is to ensure optimum access to the renal collecting system, reducing operative time, and anesthetic morbidity during supine PCNL in ADPKD with nephrolithiasis.

METHODS

Seven patients were selected randomly. There were no preference for age, gender, size, location and laterality of stone, or BMI . All the patients fit into the AUA guideline criteria for management by percutaneous nephrolithotomy. Preoperative, perioperative, and follow up data were collected prospectively.

RESULTS

Seven patients underwent supine PCNL in approximately 2.5 year in modified supine position. There was no intraoperative, post operative, or on follow up complications in any patient. In all the patients stones were cleared completely in single sitting.

CONCLUSIONS

Supine PCNL in ADPKD with nephrolithiasis is an alternative with similar outcomes to the standard prone PCNL. It provides an additional benefit of performing the procedure in a single position, which is known to reduce total operating time, less anesthesia related complications, less neuromusculoskeletal injury, and reduce physical strain on operating.

Increased Body Fat and Organic Acid Anions Production Are Associated with Larger Kidney Size in ADPKD

Background and Objectives: A high body mass index (BMI) is associated with the progression of autosomal dominant polycystic kidney disease (ADPKD). However, body fat (BF), which is another adiposity marker, has not yet been studied. Excessive weight may promote elevation in the endogenous synthesis of organic acid (OA) anions. Accordingly, we aimed to investigate the possible association of the aforementioned markers with kidney volume and renal function in patients with ADPKD. Materials and Methods: We conducted a retrospective cohort study of adult ADPKD outpatients involving clinical, serum, and urinary laboratorial data and body composition assessments retrieved from their medical records. BF was estimated by skinfold thickness (mm) on the non-dominant arm and was considered as normal or high for each sex. Total kidney volume (TKV) and height-adjusted volume (htTKV) were measured by magnetic resonance imaging. The annual estimated glomerular filtration rate (eGFR) slope was analyzed during a median follow-up time of 6 (5.0–7.0) years to calculate rapid progression (decline in renal function ≥2.5 mL/min/year over 5 years). Results: A total of 104 patients were included (41.9 ± 11.9 years old, 38.5% men), with 62.5% of the patients classified as high BF. The High BF group presented higher levels of OA, glycosylated hemoglobin (HbA1c), C-reactive protein (CRP), 24 h urinary sodium (UNa), and htTKV, and lower eGFR than those with a normal BF. In the multivariate linear regression, the associated variables with TKV were high BF, OA and BMI (std. β 0.47, p < 0.05; std. β 0.36, p = 0.001; std. β 0.25, p = 0.01, respectively). In the binary logistic regression, when adjusted for potential confounders, UNa was the only parameter associated with an increased risk of eGFR decline ≥2.5 mL/min/year (OR 1.02, 95% CI 1.01–1.03, p = 0.02). Conclusions: Increased body fat and endogenous production of organic acid anions are associated with larger kidney size in ADPKD but not with a decline in renal function.

Hypocitraturia is present when renal function is impaired in diverse nephropathies and is not related with serum bicarbonate levels

BACKGROUND

In autosomal dominant polycystic kidney disease (ADPKD) it is frequently found a reduction in urinary citrate of unknown origin. It has been suggested that it could be a marker of acid retention in chronic kidney disease. Our aim was to compare urinary citrate in ADPKD with other nephropathies and to show its relation with serum bicarbonate.

METHODS

We determined urinary citrate in patients with several nephropathies and varied renal function. We included 291 patients, 119 with glomerular diseases, 116 with ADPKD, 21 with other nephropathies, and 35 patients with normal renal function.

RESULTS

Urinary citrate was higher in women and in patients with normal renal function. ADPKD patients showed similar values of urinary citrate to patients with glomerular diseases and with other nephropathies. We observed a progressive reduction in urinary citrate with renal impairment, in a comparable way among patients with ADPKD and glomerular diseases. We did not observe a relationship with serum bicarbonate. Serum uric acid levels were significantly higher in patients with glomerular diseases than in ADPKD patients, even after correction with the degree of renal function.

CONCLUSIONS

Hypocitraturia is not specific of ADPKD but it is also present in all tested nephropathies and is related with renal impairment and not with serum bicarbonate. It could be interesting to study urinary citrate as a marker of renal function and as a prognostic factor.

Expanding the variability of the ADPKD-GANAB clinical phenotype in a family of Italian ancestry

BACKGROUND

Causative mutations in the GANAB gene have been described in only 14 families, 9 diagnosed with late-onset Autosomal Dominant Polycystic Kidney Disease (ADPKD) and 5 with Autosomal Dominant Polycystic Liver Disease (ADPLD).

CASE

Diagnosis of ADPKD was made in a 45-year old man during screening for hernia repair. CT scan showed enlarged cystic kidneys, nephrolithiasis and normal-sized liver with multiple cysts. Hematuria, hypertension and aortic root dilatation were also documented. Renal function was normal. Molecular analysis of PKD genes disclosed a heterozygous p.R839W GANAB variant inherited from the mother. Both his elderly parents presented normal-sized bilateral cystic kidneys but normal renal function. The GANAB-ADPKD mother had no liver cysts. The father was screened for PKD-related genes and no variant was found.

GENETIC ANALYSIS

We describe a new family with late-onset ADPKD due to the p.R839W GANAB variant, previously reported in a severe ADPLD patient, requiring liver transplantation.

DISCUSSION

Since ADPKD-GANAB is an ultrarare, recently described disease, reporting further patients may help unraveling gene-related phenotype. In our patients the p.R839W GANAB variant was not related to severe ADPLD, as previously reported, but with mild ADPKD and a plethora of renal and extrarenal manifestations, usually described in PKD1/PKD2 patients. The evidence that the GANAB variant may cause both ADPKD and ADPLD of variable severity supports that renal and hepatic cystogenesis are the result of a common defective polycystin-1 pathway.

Urinary citrate as a marker of renal function in patients with autosomal dominant polycystic kidney disease

INTRODUCTION

Autosomal dominant polycystic kidney disease (ADPKD) is frequent to find low urinary citrate levels. Recently, it has been suggested that urinary citrate could be a marker of covert metabolic acidosis in chronic kidney disease.

OBJECTIVE

Our aim was to analyze relationship between urinary citrate levels, renal function, and serum bicarbonate in ADPKD patients.

METHODS

We determined citrate in 24-h collected urine from ADPKD patients and correlated with glomerular filtration rate (CKD-EPI equation) and serum bicarbonate concentration.

RESULTS

We included 120 patients, 60% men, eGFR was 71 ± 32 mL/min/1.73 m². Urinary citrate/creatinine ratio was 195 ± 152 mg/gCr (range 1.2-689) with levels significantly higher in females. Urinary citrate lower than 300 mg/gCr was present in 75% of patients and when considering chronic kidney stages (CKD), we observed reduced levels in 48.8% in CKD1 stage, in 79.4% in CKD2 stage, in 96.2% in CKD3 stage, and in 94.7% of patients in CKD4 stage. Urinary citrate was correlated with serum creatinine (r = - 0.61, p < 0.001) and eGFR (r = 0.55, p < 0.001) in both gender. We did not find any correlation with serum bicarbonate. Using a general linear modeling analysis, we found as predictors of urinary citrate/creatinine ratio to glomerular filtration rate, gender, and age. Lower levels of urinary citrate were accompanied by a decline in urinary osmolality and in renal excretion of calcium and uric acid. In a subgroup of patients, we measured total kidney volume and we found an inverse correlation with urinary citrate levels that disappeared when it was corrected with glomerular filtration rate.

CONCLUSIONS

Urinary citrate is very frequently reduced in ADPKD patients being present from very early CKD stages. Their levels in urine are inversely correlated with glomerular filtration rate and it is not related with serum bicarbonate concentration. We think that it would be interesting to study urinary citrate as a marker of chronic kidney disease in ADPKD patients.

A comprehensive PGT-M strategy for ADPKD patients with de novo PKD1 mutations using affected embryo or gametes as proband

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease characterized by the development of renal cysts and progression to renal failure. Preimplantation genetic testing-monogenic disease (PGT-M) is an alternative option to obtain healthy babies. However, de novo PKD1 mutation of one of the spouses or the absence of a positive family history poses a serious challenge to PGT-M. Here, we described a comprehensive strategy which includes preimplantation genetic testing for aneuploidies (PGT-A) study and monogenic diagnosis study for ADPKD patients bearing de novo mutations. The innovation of our strategy is to use the gamete (polar body or single sperm) as proband for single-nucleotide polymorphism (SNP) linkage analysis to detect an embryo's carrier status. Nine ADPKD couples with either de novo mutation or without a positive family history were recruited and a total of 34 embryos from 13 PGT-M cycles were examined. Within these nine couples, two successfully delivered healthy babies had their genetic status confirmed by amniocentesis. This study provides a creative approach for embryo diagnosis of patients with de novo mutations or patients who lack essential family members for linkage analysis.

Percutaneous Nephrolithotomy Under X-Ray-Free Technique in Upper Urinary Stone Patients with Autosomal Dominant Polycystic Kidney Disease: Experience from a Large-Volume Stone Management Center

Objectives: To present our large single-center experience in the management of autosomal dominant polycystic kidney disease (ADPKD) with total ultrasound (US)-guided percutaneous nephrolithotomy (PNL) and to evaluate the role of PNL under US in these patients. Patients and Methods: We retrospectively reviewed the charts of patients with ADPKD who underwent PNL from August 2011 to December 2019. A total of 56 patients were included in this study; all procedures were completed by the total US-guided technique. Demographic characteristics, operative parameters, and postoperative data were collected and analyzed. Results: Effective renal access was achieved in all patients. The mean stone size was 3.1 cm (range 1.7-6.5 cm). The initial stone-free rate was 70.6% (36/51); five patients underwent second-look PNL to remove residual stones. The other patients underwent oral medication therapy. No severe intraoperative complications occurred; one patient received selective embolization for bleeding on the contralateral side and one patient needed percutaneous drainage for perinephric abscess. Clavien I or II complications were seen in nine patients. Renal function was improved or stable in most patients; the condition of only one patient deteriorated after surgery. Conclusion: Total US-guided PNL is a safe and efficient treatment for kidney stones in patients with ADPKD; perioperative renal function was not adversely affected and complications were acceptable compared with patients in the general population.

Urinary Lithogenic Risk Profile in ADPKD Patients Treated with Tolvaptan

BACKGROUND AND OBJECTIVES

Nephrolithiasis is a common health problem in autosomal dominant polycystic kidney disease (ADPKD) and significantly contributes to patient morbidity. Recently, Tolvaptan has been introduced for the treatment of ADPKD, but whether it is associated with alterations of the urinary lithogenic risk profile remains unknown.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We conducted an analysis of participants enrolled in the Bern ADPKD registry, a prospective observational cohort study. Twenty-four-hour urine analyses were performed at baseline and then at yearly follow-ups. Relative supersaturation ratios for calcium oxalate, brushite, and uric acid were calculated with the program EQUIL2. Unadjusted and multivariable mixed-effects linear regression models, adjusted for age, sex, body mass index, eGFR, net acid excretion, and height-adjusted total kidney volume, were used to assess the association of Tolvaptan with urinary parameters relevant for kidney stone formation. The maximum individual follow-up time was 3 years, median follow-up time 1.9 years, and cumulative follow-up time 169 years.

RESULTS

In total, 125 participants (38 with and 87 without Tolvaptan treatment) were included in the analysis. In multivariable analysis, Tolvaptan treatment was associated [adjusted estimate of the difference between Tolvaptan and no Tolvaptan; 95% confidence interval (CI)] with lower urine relative supersaturation ratios for calcium oxalate (-0.56; 95% CI, -0.82 to -0.3; P<0.001), brushite (-0.33; 95% CI, -0.54 to -0.11; P=0.004), and uric acid (-0.62; 95% CI, -0.88 to -0.37; P<0.001), and with higher urine citrate in mmol/mmol creatinine per day (0.25; 95% CI, 0.05 to 0.46; P=0.02) and calcium in mmol/mmol creatinine per day (0.31; 95% CI, 0.09 to 0.53; P=0.006) excretion. In addition, Tolvaptan treatment was associated with lower net acid excretion in mEq/mmol creatinine per day (-0.54; 95% CI, -0.90 to -0.17; P=0.004) and higher net gastrointestinal alkali absorption in mEq/mmol creatinine per day (0.57; 95% CI, 0.26 to 0.88; P<0.001).

CONCLUSIONS

Tolvaptan treatment is associated with a significantly improved urinary lithogenic risk profile in patients with ADPKD.

Stone Prevalence in Autosomal Dominant Polycystic Kidney Disease: A Systematic Review and Meta-Analysis

Background:

It is uncertain how often patients with autosomal dominant polycystic kidney disease (ADPKD) develop kidney stones.

Objective:

To review English-language studies reporting the incidence and prevalence of stones and stone interventions in adults with ADPKD.

Design:

Systematic review and meta-analysis.

Setting:

Any country of origin.

Patients:

Adult patients with ADPKD.

Measurements:

Incidence or prevalence of kidney stones and stone interventions.

Methods:

We reviewed 1812 citations from bibliographic databases, abstracted data from 49 eligible studies, and assessed methodological quality in duplicate. In some studies, the proportion of adults with ADPKD with the outcome were compared to adults without ADPKD; for these studies, prevalence risk ratios were calculated and pooled using a random effects model.

Results:

We identified 49 articles that met our review criteria. The methodological quality of many studies was limited (scores ranging from 2 to 14 out of 22, with a higher score indicating higher quality). No study clearly reported stone incidence, and in the cross-sectional studies, the definition of stones was often unclear. The prevalence of stones ranged from 3% to 59%, and a prevalence of stone interventions ranged from 1% to 8%; the average patient age at the time of assessment ranged from 26 to 61 years across the studies. Two studies reported a nonstatistically significant higher stone prevalence in patients with ADPKD compared to unaffected family members. Compared to unaffected family members, patients with ADPKD had a higher prevalence of kidney stones (6 cross-sectional studies; unadjusted prevalence ratio: 1.8; 95% confidence interval: 1.3 to 2.6; P = .0007; test for heterogeneity: I² = 0%, P = .8).

Limitations:

Studies were limited to articles published in English.

Conclusions:

The prevalence of kidney stones and stone interventions in adults with ADPKD remains uncertain. Future studies of higher methodological quality are needed to better characterize the incidence and prevalence of kidney stones in patients with ADPKD.

Trial registration:

We did not register the protocol for this systematic review.

Insights into cellular and molecular basis for urinary tract infection in autosomal-dominant polycystic kidney disease

Urinary tract infection (UTI) is a broad term referring to an infection of the kidneys, ureters, bladder, and/or urethra. Because of its prevalence, frequent recurrence, and rising resistance to antibiotics, UTI has become a challenge in clinical practice. Autosomal-dominant polycystic kidney disease (ADPKD) is the most common monogenic disorder of the kidney and is characterized by the growth of fluid-filled cysts in both kidneys. Progressive cystic enlargement, inflammation, and interstitial fibrosis result in nephron loss with subsequent decline in kidney function. ADPKD patients frequently develop UTI; however, the cellular and molecular mechanisms responsible for the high UTI incidence in ADPKD patients remain virtually unaddressed. Emerging evidence suggests that α-intercalated cells (α-ICs) of the collecting ducts function in the innate immune defense against UTI. α-ICs inhibit bacterial growth by acidifying urine and secreting neutrophil gelatinase-associated lipocalin (NGAL) that chelates siderophore-containing iron. It is necessary to determine, therefore, if ADPKD patients with recurrent UTI have a reduced number and/or impaired function of α-ICs. Identification of the underlying cellular and molecular mechanisms may lead to the development of novel strategies to reduce UTI in ADPKD.

Inherited renal cystic diseases

A number of inherited renal diseases present with renal cysts and often lead to end-stage renal disease. With recent advances in genetics, increasing number of genes and mutations have been associated with cystic renal diseases. Although genetic testing can provide a definite diagnosis, it is often reserved for equivocal cases or for ongoing investigational research. Therefore, imaging findings are essential in the routine diagnosis, follow-up, and detection of complications in patients with inherited cystic renal diseases. In this article, the most recent classification, genetic analysis, clinical presentations, and imaging findings of inherited cystic renal diseases will be discussed.

Association between Nephrolithiasis, Hypertension and Obesity in Polycystic Kidney Disease

AIM:

We aim to define the correlations between nephrolithiasis, hypertension, age and obesity in patients with autosomal dominant polycystic kidney disease (ADPKD) in Albania.

MATERIAL AND METHODS:

We included 100 patients with autosomal dominant polycystic kidney from 2011 to 2014. The patients underwent X-ray and renal ultrasonography. We performed the metabolic evaluation of blood and urine.

RESULTS:

The patients with renal stones had a higher level of mean systolic and diastolic blood pressure compared with patients without stones (155 ± 12 mmHg vs. 145 ± 8 mmHg, and 105 ± 0.9 mmHg vs. 92 ± 1.28 mmHg, respectively). Patients with renal stones were older (47 ± 15 vs. 38 ± 5 years), had a higher prevalence of obesity [body mass index (BMI): 28 ± 2.4 vs. 25.7 ± 0.6], had higher levels of total cholesterol level (220 ± 5 mg/dl vs. 203 ± 4 mg/dl) as well as triglyceride levels (160 ± 9 mg/dl vs. 126 ± 4 mg/dl), compared with no renal stone individuals.

CONCLUSION:

ADPKD patients with renal stones in our study had a higher mean level of systolic and diastolic blood pressure, BMI and cholesterol and triglycerides levels compared with individuals without renal stones.

Anatomy and Physiology of the Urinary Tract: Relation to Host Defense and Microbial Infection

The urinary tract exits to a body surface area that is densely populated by a wide range of microbes. Yet, under most normal circumstances, it is typically considered sterile, i.e., devoid of microbes, a stark contrast to the gastrointestinal and upper respiratory tracts where many commensal and pathogenic microbes call home. Not surprisingly, infection of the urinary tract over a healthy person's lifetime is relatively infrequent, occurring once or twice or not at all for most people. For those who do experience an initial infection, the great majority (70% to 80%) thankfully do not go on to suffer from multiple episodes. This is a far cry from the upper respiratory tract infections, which can afflict an otherwise healthy individual countless times. The fact that urinary tract infections are hard to elicit in experimental animals except with inoculum 3-5 orders of magnitude greater than the colony counts that define an acute urinary infection in humans (105 cfu/ml), also speaks to the robustness of the urinary tract defense. How can the urinary tract be so effective in fending off harmful microbes despite its orifice in a close vicinity to that of the microbe-laden gastrointestinal tract? While a complete picture is still evolving, the general consensus is that the anatomical and physiological integrity of the urinary tract is of paramount importance in maintaining a healthy urinary tract. When this integrity is breached, however, the urinary tract can be at a heightened risk or even recurrent episodes of microbial infections. In fact, recurrent urinary tract infections are a significant cause of morbidity and time lost from work and a major challenge to manage clinically. Additionally, infections of the upper urinary tract often require hospitalization and prolonged antibiotic therapy. In this chapter, we provide an overview of the basic anatomy and physiology of the urinary tract with an emphasis on their specific roles in host defense. We also highlight the important structural and functional abnormalities that predispose the urinary tract to microbial infections.

[Pathophysiology, epidemiology, clinical presentation, diagnosis and treatment options for autosomal dominant polycystic kidney disease]

Autosomal dominant polycystic kidney disease (ADPKD) is the leading genetic cause of end-stage renal disease (ESRD) worldwide. Its prevalence is evaluated according to studies and population between 1/1000 and 1/4000 live births and it accounts for 6 to 8% of incident ESRD patients in developed countries. ADPKD is characterized by numerous cysts in both kidneys and various extrarenal manifestations that are detailed in this review. Clinico-radiological and genetic diagnosis are also discussed. Mutations in the PKD1 and PKD2 codifying for polycystin-1 (PC-1) and polycystin-2 (PC-2) are responsible for the 85 and 15% of ADPKD cases, respectively. In primary cilia of normal kidney epithelial cells, PC-1 and PC-2 interact forming a complex involved in flow- and cilia-dependant signalling pathways where intracellular calcium and cAMP play a central role. Alteration of these multiple signal transduction pathways leads to cystogenesis accompanied by dysregulated planar cell polarity, excessive cell proliferation and fluid secretion, and pathogenic interactions of epithelial cells with an abnormal extracellular matrix. The mass effect of expanding cyst is responsible for the decline in glomerular filtration rate that occurs late in the course of the disease. For many decades, the treatment for ADPKD aims to lessen the condition's symptoms, limit kidney damage, and prevent complications. Recently, the development of promising specific treatment raises the hope to slow the growth of cysts and delay the disease. Treatment strategies targeting cAMP signalling such as vasopressin receptor antagonists or somatostatin analogs have been tested successfully in clinical trials with relative safety. Newer treatments supported by preclinical trials will become available in the next future. Recognizing early markers of renal progression (clinical, imaging, and genetic markers) to identify high-risk patients and multidrug approaches with synergistic effects may provide new opportunities for the treatment of ADPKD.

A stepwise approach for effective management of chronic pain in autosomal-dominant polycystic kidney disease

Chronic pain, defined as pain existing for >4-6 weeks, affects >60% of patients with autosomal-dominant polycystic disease (ADPKD). It can have various causes, indirectly or directly related to the increase in kidney and liver volume in these patients. Chronic pain in ADPKD patients is often severe, impacting physical activity and social relationships, and frequently difficult to manage. This review provides an overview of pathophysiological mechanisms that can lead to pain and discusses the sensory innervation of the kidneys and the upper abdominal organs, including the liver. In addition, the results of a systematic literature search of ADPKD-specific treatment options are presented. Based on pathophysiological knowledge and evidence derived from the literature an argumentative stepwise approach for effective management of chronic pain in ADPKD is proposed.

Determination of urinary lithogenic parameters in murine models orthologous to autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD), a genetic disease caused by mutations in PKD1 or PKD2 genes, is associated with a high prevalence of nephrolithiasis. The underlying mechanisms may encompass structural abnormalities resulting from cyst growth, urinary metabolic abnormalities or both. An increased frequency of hypocitraturia has been described in ADPKD even in the absence of nephrolithiasis, suggesting that metabolic alterations may be associated with ADPKD per se. We aimed to investigate whether non-cystic Pkd1-haploinsufficient (Pkd1(+/-)) and/or nestin-Cre Pkd1-targeted cystic (Pkd1(cond/cond):Nestin(cre)) mouse models develop urinary metabolic abnormalities potentially related to nephrolithiasis in ADPKD. 24-h urine samples were collected during three non-consecutive days from 10-12 and 18-20 week-old animals. At 10-12 weeks of age, urinary oxalate, calcium, magnesium, citrate and uric acid did not differ between test and their respective control groups. At 18-20 weeks, Pkd1(+/-) showed slightly but significantly higher urinary uric acid vs. controls while cystic animals did not. The absence of hypocitraturia, hyperoxaluria and hyperuricosuria in the cystic model at both ages and the finding of hyperuricosuria in the 18-20 week-old animals suggest that anatomic cystic distortions per se do not generate the metabolic disturbances described in human ADPKD-related nephrolithiasis, while Pkd1 haploinsufficiency may contribute to this phenotype in this animal model.

Medullary sponge kidney: state of the art

Medullary sponge kidney (MSK) is a kidney malformation that generally manifests with nephrocalcinosis and recurrent renal stones; other signs may be renal acidification and concentration defects, and pre-calyceal duct ectasias. MSK is generally considered a sporadic disorder, but an apparently autosomal dominant inheritance has also been observed. As MSK reveals abnormalities in both the lower and the upper nephron and is often associated with urinary tract developmental anomalies, its pathogenesis should probably be sought in one of the numerous steps characterizing renal morphogenesis. Given the key role of the GDNF-RET interaction in kidney and urinary tract development and nephrogenesis, anomalies in these molecules are reasonable candidates for explaining a disorder such as MSK. As a matter of fact, we detected two, hitherto unknown, rare variants of the GDNF gene in MSK patients. We surmise that a defective distal acidification has a central role in MSK and is followed by a chain of events including defective bone mineralization, hypercalciuria, hypocitraturia and stone formation.

Kidney transplantation with and without simultaneous bilateral native nephrectomy in patients with polycystic kidney disease: a comparative retrospective study

BACKGROUND

Patients with autosomal dominant polycystic kidney disease (ADPKD) often need to undergo native nephrectomy and are candidates for kidney transplantation. The necessity and timing of nephrectomy are controversial. Some authors recommend simultaneous bilateral native nephrectomy (SBN) as the preferred option in living-donor kidney transplantation (LDKT). These recommendations are based on small study populations. We therefore set out to study outcomes of LDKT with SBN, compared with LDKT alone in a larger single-center cohort.

METHODS

A consecutive series of 159 patients with ADPKD undergoing LDKT were included in the study. Of the 159 patients, 2 were excluded because of missing data, 79 underwent LDKT alone (group A), and 78 underwent LDKT with SBN (group B). Demographic data and intraoperative and postoperative data were collected from patient charts and the national kidney registry.

RESULTS

There were no differences regarding background data. Group B experienced significantly longer operating times (183.7 vs. 319.3 min, P<0.001), a greater need for blood transfusions (0.1 vs. 1.6 units, P<0.001) and plasma products (35.1 vs. 438.3 mL, P<0.001), and longer hospital stays (11.8 vs. 15.4 days, P<0.001). It also experienced more intraoperative events and postoperative complications but fewer reoperations/reinterventions. There were no differences in patient and graft survival rates.

CONCLUSIONS

SBN in patients undergoing LDKT for ADPKD does not have a significant negative impact on patient and graft survival rates. It obviates a separate surgical procedure but requires longer hospital stay. It may be associated with more postoperative complications and risk of graft loss. These considerations should be communicated to the recipient and the donor.

Management of nephrolithiasis in autosomal dominant polycystic kidney disease - A single center experience

PURPOSE

To evaluate available options for the management of nephrolithiasis in patients with autosomal dominant polycystic kidney disease (ADPKD).

MATERIALS AND METHODS

Case files of all the patients with ADPKD treated in our hospital in the last 18 years were evaluated. Their demographic details, clinical presentations, investigations, treatments, and outcomes were critically analyzed.

RESULTS

There were a total of 19 patients (23 renal units) with nephrolithiasis among 452 consecutive cases of ADPKD. Male-to-female ratio was 3.75:1. The mean age of the patients was 43.3 years (range 23 to 60 years). The most common presentations were pain and hematuria (27.7% each). Mean serum creatinine was 7.2 mg/ dl (range 0.8-18.1 mg/dl) at presentation. The mean stone size was 115 mm(2) (range 36 to 980 mm(2)). The majority of the stones were calyceal (n = 10). Ten renal units (nine patients) required intervention, while the rest were treated conservatively. Treatment offered included open nephrectomy for non-functioning infected kidney (n = 1), extracorporeal shock wave lithotripsy (ESWL, n = 3), ureterorenoscopy (URS, n = 3), and percutaneous nephrolithotomy (PCNL, n = 3). All patients undergoing URS and PCNL had complete clearance, while those undergoing ESWL had a residual stone. Two failed ESWL patients required an auxiliary procedure (retrograde intrarenal surgery, RIRS) and the other was kept under observation. Mean follow-up after treatment was 4.2 years (one month to six years). None of the patients had major complications.

CONCLUSION

Careful selection of the endourological procedure can give good results in patients of ADPKD with nephrolithiasis.

Pelvi-ureteric junction obstruction in autosomal-dominant polycystic kidney disease: an association yet to be reported

Autosomal-dominant polycystic kidney disease (ADPKD) is the most common inherited renal cystic disease. It is characterised by the development of renal parenchymal cysts and a variety of other extrarenal manifestations. Pelvi-ureteric junction (PUJ) obstruction has not been described in association with ADPKD in the literature. We present a case of a 23-year-old man presenting with bilateral flank pain. On evaluation he was diagnosed to have ADPKD with bilateral renal calculi and left-sided PUJ obstruction. He underwent successful right percutaneous nephrolithotomy and left laparoscopic dismembered pyeloplasty with simultaneous stone removal.

An 11-year-old child with autosomal dominant polycystic kidney disease who presented with nephrolithiasis

Patients with autosomal dominant polycystic kidney disease become symptomatic and are diagnosed usually at adulthood. The rate of nephrolithiasis in these patients is 5-10 times the rate in the general population, and both anatomic and metabolic abnormalities play role in the formation of renal stones. However, nephrolithiasis is rare in childhood age group. In this paper, an 11-year-old child with autosomal dominant polycystic kidney disease presenting with nephrolithiasis is discussed.

Hepatorenal findings in obligate heterozygotes for autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD), characterized by progressive cystic degeneration of the kidneys and congenital hepatic fibrosis (CHF), is the most common childhood onset ciliopathy, with an estimated frequency of 1 in 20,000 births. It is caused by mutations in PKHD1. The carrier frequency for ARPKD in the general population is estimated at 1 in 70. Given the recessive inheritance pattern, individuals who are heterozygous for PKHD1 mutations are not expected to have clinical findings. We performed ultrasound (USG) evaluations on 110 parents from 64 independent ARPKD families and identified increased medullary echogenicity in 6 (5.5%) and multiple small liver cysts in 10 parents (9%). All ARPKD parents with these abnormal imaging findings were asymptomatic; kidney and liver function tests were unremarkable. Complete sequencing of PKHD1 in the 16 ARPKD parents with abnormal imaging confirmed the mutation transmitted to the proband, but did not reveal any other pathogenic variants. Our data suggest that carrier status for ARPKD is a predisposition to polycystic liver disease and renal involvement associated with increased medullary echogenicity on USG. Whether some of these individuals become symptomatic as they age remains to be determined.

Nephrolithiasis in medullary sponge kidney: evaluation of clinical and metabolic features

OBJECTIVE

Medullary sponge kidney (MSK) is a disorder characterized by tubular dilation of renal collecting ducts and cystic dilation of medullary pyramids that has been associated with stone disease. The significance of nephrolithiasis and the mechanisms by which it occurs are incompletely understood. We describe clinical and metabolic features of nephrolithiasis in a cohort of patients with MSK.

METHODS

Records were reviewed of 56 patients, all with radiographic diagnosis of medullary sponge kidney and data collected pertaining to presentation, stone events and recurrences, stone composition, and metabolic profile to perform a descriptive study with median 3.7 years follow-up.

RESULTS

Nephrolithiasis was confirmed radiographically in 39/56 patients (69.6%). No patient without evidence of nephrolithiasis developed a stone event, whereas 13/39 (33%) of those with nephrolithiasis developed a recurrent stone event. Stones were composed of calcium oxalate monohydrate, calcium oxalate dihydrate, calcium phosphate apatite, and uric acid. Metabolic profile was obtained for 26 of 39 (67%) stone-forming patients demonstrating abnormalities in 22/26 (84.6%). These included hypercalciuria, 58% (15/26); low urine volume, 35% (9/26); hyperuricosuria, 27% (7/26); hypocitraturia, 19% (5/26); elevated urine sodium, 15% (4/26); and hyperoxaluria, 12% (3/26).

CONCLUSION

Many patients with MSK have no evidence of nephrolithiasis. Among those who do, recurrence is common, and metabolic profile and composition are varied as in the general stone-forming population.

Primary nonfunction of renal allograft secondary to acute oxalate nephropathy

Primary nonfunction (PNF) accounts for 0.6 to 8% of renal allograft failure, and the focus on causes of PNF has changed from rejection to other causes. Calcium oxalate (CaOx) deposition is common in early allograft biopsies, and it contributes in moderate intensity to higher incidence of acute tubular necrosis and poor graft survival. A-49-year old male with ESRD secondary to polycystic kidney disease underwent extended criteria donor kidney transplantation. Posttransplant, patient developed delayed graft function (DGF), and the biopsy showed moderately intense CaOx deposition that persisted on subsequent biopsies for 16 weeks, eventually resulting in PNF. The serum oxalate level was 3 times more than normal at 85 μmol/L (normal <27 μmol/L). Allograft nephrectomy showed massive aggregates of CaOx crystal deposition in renal collecting system. In conclusion, acute oxalate nephropathy should be considered in the differential diagnosis of DGF since optimal management could change the outcome of the allograft.

Evaluation and management of pain in autosomal dominant polycystic kidney disease

Transient episodes of pain are common in autosomal dominant polycystic kidney disease (ADPKD). A small fraction of patients have disabling chronic pain. In this review, we discuss the etiologies of pain in ADPKD; review how ADPKD patients should be assessed; and discuss medical, surgical, and other management options.

Induction of apoptosis with cisplatin enhances calcium oxalate crystal adherence to inner medullary collecting duct cells

Attachment of stone crystals to tubular epithelium may initiate kidney stone formation. We previously reported that apical nucleolin related protein (NRP) expression during mitosis enhance attachment of Ca oxalate monohydrate crystals (COM). Some forms of injury may also increase affinity for crystals. We examined changes in subcellular localization of NRP during the course of cisplatin-induced apoptosis in cultured inner medullary collecting duct cells. Caspase-3 activation and chromatin condensation followed by nuclear fragmentation occurred after 20 h exposure to cisplatin, indicating the development of apoptosis. Cells were fixed without permeabilization and stained for surface NRP. Cells with condensed chromatin showed little or no cytoplasmic or apical NRP. Those at an early stage of nuclear fragmentation had cytoplasmic but not apical NRP and cells with advanced nuclear fragmentation were positively stained for apical NRP. Membrane proteins isolated by apical biotinylation and precipitated with avidin were analyzed by Western blot. Apical NRP was markedly increased after cisplatin compared to control, while expression of the apical marker, GP-135, and other putative attachment protein were unchanged. Hyaluronic acid was decreased. Cultures with apoptotic cells demonstrated increased adherence of COM that was inhibited by the polyanion (poly)aspartic acid. We conclude that pre-existing apoptotic injury may promote calcium oxalate crystals attachment to renal tubular epithelium via apical NRP expression.

Percutaneous nephrolithotomy for large or multiple upper tract calculi and autosomal dominant polycystic kidney disease

PURPOSE

Percutaneous nephrolithotomy is standard therapy for upper tract calculi larger than 2 cm. However, the role of percutaneous nephrolithotomy in patients with autosomal dominant polycystic kidney disease has not been well evaluated. We report our experience with percutaneous nephrolithotomy in patients with autosomal dominant polycystic kidney disease.

MATERIALS AND METHODS

We retrospectively reviewed the charts of all patients with autosomal dominant polycystic kidney disease and subsequent renal calculi managed by percutaneous nephrolithotomy from October 1981 to the present.

RESULTS

We identified 9 patients. Percutaneous nephrolithotomy was performed in 11 kidneys. Flank pain was the presenting symptom in 6 patients. Average stone burden was 2.5 cm (range 1.6 to 3.6). Two access tracts were necessary in 5 kidneys. No intraoperative complications occurred. In 2 kidneys a second stage endoscopic procedure with ultrasonic lithotripsy was required to achieve stone-free status. Nephrostogram 24 hours after the final procedure showed no residual stone fragments in 9 of 11 kidneys (82%). The remaining 2 patients underwent percutaneous basket extraction to render them stone-free. There were no postoperative complications or recurrent stones. No patient required blood transfusion. Mean followup was 2.7 years (range 0.3 to 4). Mean calculated creatinine clearance was stable at 85.6 (range 45.9 to 126.6) and 89.5 mg/dl per minute (range 39.6 to 126.6) preoperatively and at last followup, respectively (p = 0.783).

CONCLUSIONS

Autosomal dominant polycystic kidney disease increased operative complexity, the need for multiple percutaneous access tracts and the likelihood of repeat endoscopy. Despite the altered anatomy percutaneous nephrolithotomy was a safe, efficacious approach for autosomal dominant polycystic kidney disease. At last followup there was no stone recurrence and renal function was stable.

Pkd1 transgenic mice: adult model of polycystic kidney disease with extrarenal and renal phenotypes

While high levels of Pkd1 expression are detected in tissues of patients with autosomal dominant polycystic kidney disease (ADPKD), it is unclear whether enhanced expression could be a pathogenetic mechanism for this systemic disorder. Three transgenic mouse lines were generated from a Pkd1-BAC modified by introducing a silent tag via homologous recombination to target a sustained wild-type genomic Pkd1 expression within the native tissue and temporal regulation. These mice specifically overexpressed the Pkd1 transgene in extrarenal and renal tissues from approximately 2- to 15-fold over Pkd1 endogenous levels in a copy-dependent manner. All transgenic mice reproducibly developed tubular and glomerular cysts leading to renal insufficiency. Interestingly, Pkd1(TAG) mice also exhibited renal fibrosis and calcium deposits in papilla reminiscent of nephrolithiasis as frequently observed in ADPKD. Similar to human ADPKD, these mice consistently displayed hepatic fibrosis and approximately 15% intrahepatic cysts of the bile ducts affecting females preferentially. Moreover, a significant proportion of mice developed cardiac anomalies with severe left-ventricular hypertrophy, marked aortic arch distention and/or valvular stenosis and calcification that had profound functional impact. Of significance, Pkd1(TAG) mice displayed occasional cerebral lesions with evidence of ruptured and unruptured cerebral aneurysms. This Pkd1(TAG) mouse model demonstrates that overexpression of wild-type Pkd1 can trigger the typical adult renal and extrarenal phenotypes resembling human ADPKD.

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.

Developments in the management of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent life- threatening, hereditary disease. ADPKD is more common than sickle cell anemia, cystic fibrosis, muscular dystrophy, hemophilia, Down's syndrome, and Huntington's disease combined. ADPKD is a multisystemic disorder characterized by the progressive development of renal cysts and marked renal enlargement. Structural and functional renal deterioration occurs in ADPKD patients and is the fourth leading cause of end-stage renal disease (ESRD) in adults. Aside from the renal manifestations, extrarenal structural abnormalities, such as liver cysts, cardiovascular abnormalities, and intracranial aneurysms may lead to morbidity and mortality. Recent studies have identified prognostic factors for progressive renal impairment including gender, race, age, proteinuria, hematuria, hypertension and increased left ventricular mass index (LVMI). Early diagnosis and better understanding of the pathophysiology of the disease provides the opportunity to aggressivly treat hypertension with renin-angiotensin-aldosterone system inhibitors and thereby potentially reduce LVMI, prevent cardiovascular morbidity and mortality and slow progression of the renal disease.

Autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease is the most prevalent, potentially lethal, monogenic disorder. It is associated with large interfamilial and intrafamilial variability, which can be explained to a large extent by its genetic heterogeneity and modifier genes. An increased understanding of the disorder's underlying genetic, molecular, and cellular mechanisms and a better appreciation of its progression and systemic manifestations have laid out the foundation for the development of clinical trials and potentially effective treatments.

Renal and extrarenal manifestations of autosomal dominant polycystic kidney disease

The objective of the present study was to determine the frequency of the most common clinical features in patients with autosomal dominant polycystic kidney disease in a sample of the Brazilian population. The medical records of 92 patients with autosomal dominant polycystic kidney disease attended during the period from 1985 to 2003 were reviewed. The following data were recorded: age at diagnosis, gender, associated clinical manifestations, occurrence of stroke, age at loss of renal function (beginning of dialysis), and presence of a family history. The involvement of abdominal viscera was investigated by ultrasonography. Intracranial alterations were prospectively investigated by magnetic resonance angiography in 42 asymptomatic patients, and complemented with digital subtraction arteriography when indicated. Mean age at diagnosis was 35.1 +/- 14.9 years, and mean serum creatinine at referral was 2.4 +/- 2.8 mg/dL. The most frequent clinical manifestations during the disease were arterial hypertension (63.3%), lumbar pain (55.4%), an abdominal mass (47.8%), and urinary infection (35.8%). Loss of renal function occurred in 27 patients (mean age: 45.4 +/- 9.5 years). The liver was the second organ most frequently affected (39.1%). Stroke occurred in 7.6% of the patients. Asymptomatic intracranial aneurysm was detected in 3 patients and arachnoid cysts in 3 other patients. In conclusion, the most common clinical features were lumbar pain, arterial hypertension, abdominal mass, and urinary infection, and the most serious complications were chronic renal failure and stroke. Both intracranial aneurysms and arachnoid cysts occurred in asymptomatic patients at a frequency of 7.14%.

Medullary sponge kidney (Lenarduzzi–Cacchi–Ricci disease): A Padua Medical School discovery in the 1930s

The introduction of radiological contrast media and intravenous (i.v.) urography in clinical diagnostics in the 1930s enabled the discovery of several diseases, including the medullary sponge kidney (MSK). MSK is a renal malformation characterized by cystic anomalies of precalyceal ducts, which is frequently associated with nephrocalcinosis and renal stones. Although it was first recognized by G Lenarduzzi in 1939, its thorough description was the result of the ante litteram multidisciplinary cooperation between a radiologist (Lenarduzzi), a urologist (Cacchi), and a pathologist (Ricci), all at the Padua University Hospital. These authors 'established' the paradigm for its diagnosis that is still used today. I.v. urography is the gold standard for the diagnosis of MSK, but as the technique is used less and less, there is a concrete possibility of this renal condition being forgotten in the future. Although the pathogenesis of MSK has yet to be elucidated, its association with different malformative conditions supports the idea that it is a developmental disorder. Recent findings suggest that MSK may be the consequence of a disruption of the ureteral-bud/metanephric-blastema interface.