Effect of a high-citrate beverage on urine chemistry in patients with calcium kidney stones.

A well-accepted strategy to prevent kidney stones is to increase urine volume by increasing oral intake of fluids, especially water, to lower supersaturation of the relevant, relatively insoluble salts, and thereby lower the risk of precipitation. Randomized controlled trials have shown that this strategy works. It is inexpensive, safe, and intuitively attractive to patients. However, although any beverage can increase urine volume, and citrus juices can increase urine citrate content and pH, no beverage other than water has been clearly shown by randomized controlled trial to prevent kidney stones. We designed an innovative, palatable, low-calorie, high alkali citrate beverage to prevent kidney stones, called Moonstone. One packet of Moonstone powder, mixed in 500 ml of water, contains 24.5 meq of alkali citrate. We administered one packet twice a day to ten calcium stone formers. Moonstone resulted in an increase in mean 24-h urine citrate and urine pH, and a decrease in supersaturation of calcium oxalate in calcium stone formers compared to an equal volume of water. These changes, comparable to those seen in a prior study of a similar amount of (potassium-magnesium) citrate, will likely be associated with a clinically meaningful reduction in kidney stone burden in patients with calcium stones. The effect to increase urine pH would also be expected to benefit patients with uric acid and cystine stones, groups that we hope to study in a subsequent study. The study preparation was well tolerated and was selected as a preferred preventative strategy by about half the participants. Moonstone is an alternative, over-the-counter therapy for kidney stone prevention.

Nedosiran in primary hyperoxaluria subtype 3: results from a phase I, single-dose study (PHYOX4).

Nedosiran is an N-acetyl-D-galactosamine (GalNAc)-conjugated RNA interference agent targeting hepatic lactate dehydrogenase (encoded by the LDHA gene), the putative enzyme mediating the final step of oxalate production in all three genetic subtypes of primary hyperoxaluria (PH). This phase I study assessed the safety, pharmacokinetics (PK), and pharmacodynamics (PD) of subcutaneous nedosiran in patients with PH subtype 3 (PH3) and an estimated glomerular filtration rate  ≥ 30 mL/min/1.73 m2. Single-dose nedosiran 3 mg/kg or placebo was administered in a randomized (2:1), double-blinded manner. Safety/tolerability, 24-h urinary oxalate (Uox) concentrations, and plasma nedosiran concentrations were assessed. The main PD endpoint was the proportion of participants achieving a > 30% decrease from baseline in 24-h Uox at two consecutive visits. Six participants enrolled in and completed the study (nedosiran, n = 4; placebo, n = 2). Nedosiran was well-tolerated and lacked safety concerns. Although the PD response was not met, 24-h Uox excretion declined 24.5% in the nedosiran group and increased 10.5% in the placebo group at Day 85. Three of four nedosiran recipients had a > 30% reduction in 24-h Uox excretion during at least one visit, and one attained near-normal (i.e., ≥ 0.46 to < 0.60 mmol/24 h; ≥ 1.0 to < 1.3 × upper limit of the normal reference range) 24-h Uox excretion from Day 29 to Day 85. Nedosiran displayed predictable plasma PK. The acceptable safety and trend toward Uox-lowering after single-dose nedosiran treatment enables further clinical development of nedosiran in patients with PH3 who currently have no viable therapeutic options. A plain language summary is available in the supplementary information.

A Summary of Current Guidelines and Future Directions for Medical Management and Monitoring of Patients with Cystinuria.

Cystinuria is the most common genetic cause of recurrent kidney stones. As the result of a genetic defect in proximal tubular reabsorption of filtered cystine, increased urine levels of the poorly soluble amino acid result in recurrent cystine nephrolithiasis. Recurrent cystine stones not only adversely affect the quality of patients suffering from cystinuria but also may result in chronic kidney disease (CKD) from recurrent renal injury. Thus, the mainstay of medical management revolves around prevention of stones. Recently published consensus statements on guidelines for managing cystinuria were released from both the United States and Europe. The purpose of this review is to summarize guidelines for medical management of patients with cystinuria, to provide new insight into the utility and clinical significance of cystine capacity-an assay for monitoring cystinuria, and to discuss future directions for research on treatment of cystinuria. We discuss future directions, including the potential use of cystine mimetics, gene therapy, V2-receptor blockers, and SGLT2 inhibitors, topics which have not appeared in more recent reviews. It is notable that in the absence of randomized, controlled trials, the recommendations cited here and in the guidelines are based on our best understanding of the disorder's pathophysiology, observational studies, and clinical experience.

Accurate 24-h urine cystine quantification for patients on cystine-binding thiol drugs.

Cystinuria is a rare disorder resulting in development of recurrent kidney stones, adversely affecting patient quality of life. The goal of cystinuria management is to reduce stone formation by increasing cystine solubility in urine, which includes lowering the urinary cystine level below its solubility limit. Treatment usually involves alkalinization of the urine and often requires initiating pharmacotherapy with a cystine-binding thiol drug (CBTD) such as tiopronin; however, proper dose adjustment requires accurate measurement of urinary cystine. The goal of this study was to validate a novel high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) method for quantification of cystine in the urine of patients with cystinuria receiving a CBTD. Urine samples were collected over 24 h from 24 patients and separated into 2 aliquots. Chromatographic separation of samples was conducted and separation of cystine from the cysteine-tiopronin drug complex was complete in < 3 min. The method was validated for accuracy, precision, linearity, limit of detection (LOD), and limit of quantification (LOQ). Mean accuracy range was 97.7-102.3%; intermediate precision was high with relative percent difference values calculated at 1.2-9.3%; the calibration curve resulted in a linear response throughout the concentration range (R2 = 0.998); and the LOD and LOQ were 0.002 and 0.005 mg/mL, respectively. Mean (range) cystine concentrations measured were 111.10 (51.31-179.46) and 242.21 (61.14-741.80) g/L in Aliquots A and B, respectively. The HPLC-MS/MS method presented here indicates that urine cystine can be reliably quantified in patients receiving a CBTD.

Comparison of Empiric Preventative Pharmacologic Therapies on Stone Recurrence Among Patients with Kidney Stone Disease.

OBJECTIVE: To compare the frequency of stone-related events among patients receiving thiazides, alkali citrate, and allopurinol without prior 24 h urine testing.  It is unknown whether 1 preventative pharmacological therapy (PPT) medication class is more beneficial for reducing kidney stone recurrence when prescribed empirically. MATERIALS AND METHODS: Using medical claims data from working-age adults with kidney stone disease diagnoses (2008-2018), we identified those prescribed thiazides, alkali citrate, or allopurinol. We excluded those who received 24 h urine testing prior to initiating PPT and those with less than 3 years of follow-up. We fit multivariable regression models to estimate the association between the occurrence of a stone-related event (emergency department visit, hospitalization, or surgery for stones) and PPT medication class. RESULTS: Our cohort consisted of 1834 (60%), 654 (21%), and 558 (18%) patients empirically prescribed thiazides, alkali citrate, or allopurinol, respectively. After controlling for patient factors including medication adherence and concomitant conditions that increase recurrence risk, the adjusted rate of any stone event was lowest for the thiazide group (14.8%) compared to alkali citrate (20.4%) or allopurinol (20.4%) (each P < .001). Thiazides, compared to allopurinol, were associated with 32% lower odds of a subsequent stone event by 3 years (OR 0.68, 95% CI 0.53-0.88). No such association was observed when comparing alkali citrate to allopurinol (OR 1.00, 95% CI 0.75-1.34). CONCLUSION: Empiric PPT with thiazides is associated with significantly lower odds of subsequent stone-related events. When 24 h urine testing is unavailable, thiazides may be preferred over alkali citrate or allopurinol for empiric PPT.

Update on Uric Acid and the Kidney.

PURPOSE OF REVIEW: In this review, we report on new findings regarding associations of uric acid with kidney health. We discuss kidney stones, effects of uric acid in chronic kidney disease (CKD), and management of gout in CKD. Recent studies on neuroprotective effects of raising uric acid provide interesting data regarding nephrolithiasis. RECENT FINDINGS: Elevated urate levels have been implicated in the progression of chronic kidney disease (CKD), but the results from PERL and CKD-FIX studies did not demonstrate that allopurinol slowed CKD progression. The SURE-PD3 sought to determine if increasing uric acid would slow the progression of Parkinson's disease. Results ultimately did not support this hypothesis, but high urinary uric acid levels caused uric acid stones, not calcium stones. Low urinary pH remains the key to the formation of uric acid stones. Thiazolidinediones improve insulin resistance, which is associated with an increase in urine pH. The most recent research has not supported the hypothesis that lowering serum uric acid levels will slow the progression of CKD or provide neuroprotection in Parkinson's disease. It is still unclear as to why uric acid stone formers have a high net acid excretion. The STOP-GOUT trial demonstrates that there was a lack of significant adverse events with higher urate-lowering dosages of allopurinol and febuxostat, despite patients' kidney function. This may push other studies to administer higher dosages per ACR guidelines. Future studies could then demonstrate decreased progression of CKD.

Extracorporeal Treatment for Methotrexate Poisoning: Systematic Review and Recommendations from the EXTRIP Workgroup.

Methotrexate is used in the treatment of many malignancies, rheumatological diseases, and inflammatory bowel disease. Toxicity from use is associated with severe morbidity and mortality. Rescue treatments include intravenous hydration, folinic acid, and, in some centers, glucarpidase. We conducted systematic reviews of the literature following published EXtracorporeal TReatments In Poisoning (EXTRIP) methods to determine the utility of extracorporeal treatments in the management of methotrexate toxicity. The quality of the evidence and the strength of recommendations (either "strong" or "weak/conditional") were graded according to the GRADE approach. A formal voting process using a modified Delphi method assessed the level of agreement between panelists on the final recommendations. A total of 92 articles met inclusion criteria. Toxicokinetic data were available on 90 patients (89 with impaired kidney function). Methotrexate was considered to be moderately dialyzable by intermittent hemodialysis. Data were available for clinical analysis on 109 patients (high-dose methotrexate [>0.5 g/m2]: 91 patients; low-dose [≤0.5 g/m2]: 18). Overall mortality in these publications was 19.5% and 26.7% in those with high-dose and low-dose methotrexate-related toxicity, respectively. Although one observational study reported lower mortality in patients treated with glucarpidase compared with those treated with hemodialysis, there were important limitations in the study. For patients with severe methotrexate toxicity receiving standard care, the EXTRIP workgroup: (1) suggested against extracorporeal treatments when glucarpidase is not administered; (2) recommended against extracorporeal treatments when glucarpidase is administered; and (3) recommended against extracorporeal treatments instead of administering glucarpidase. The quality of evidence for these recommendations was very low. Rationales for these recommendations included: (1) extracorporeal treatments mainly remove drugs in the intravascular compartment, whereas methotrexate rapidly distributes into cells; (2) extracorporeal treatments remove folinic acid; (3) in rare cases where fast removal of methotrexate is required, glucarpidase will outperform any extracorporeal treatment; and (4) extracorporeal treatments do not appear to reduce the incidence and magnitude of methotrexate toxicity.

Comparison of Selective vs Empiric Pharmacologic Preventive Therapy of Kidney Stone Recurrence With High-Risk Features.

OBJECTIVE: To compare the frequency of stone-related events among subgroups of high-risk patients with and without 24-hour urine testing before preventive pharmacological therapy (PPT) prescription. While recent studies show, on average, no benefit to a selective approach to PPT for urinary stone disease (USD), there could be heterogeneity in treatment effect across patient subgroups. MATERIALS AND METHODS: Using medical claims data from working-age adults and their dependents with USD (2008-2019), we identified those with a prescription fill for a PPT agent (thiazide diuretic, alkali therapy, or allopurinol). We then stratified patients into subgroups based on the presence of a concomitant condition or other factors that raised their stone recurrence risk. Finally, we fit multivariable regression models to measure the association between stone-related events (emergency department visit, hospitalization, and surgery) and 24-hour urine testing before PPT prescription by high-risk subgroup. RESULTS: Overall, 8369 adults with USD had a concomitant condition that raised their recurrence risk. Thirty-three percent (n = 2722) of these patients were prescribed PPT after 24-hour urine testing (median follow-up, 590 days), and 67% (n = 5647) received PPT empirically (median follow-up, 533 days). Compared to patients treated empirically, those with a history of recurrent USD had a significantly lower hazard of a subsequent stone-related event if they received selective PPT (hazard ratio, 0.83; 95% confidence interval, 0.71-0.96). No significant associations were noted for selective PPT in the other high-risk subgroups. CONCLUSION: Patients with a history of recurrent USD benefit from PPT when guided by findings from 24-hour urine testing.

Bariatric surgery in a patient with cystinuria.

We recently encountered concern about the safety of bariatric surgery for a patient with cystinuria. Bariatric surgery procedures include those that cause malabsorption, like the Roux-en-Y gastric bypass procedure, and restrictive operations, such as the sleeve gastrectomy. These procedures produce beneficial effects on health and life expectancy, though whether kidney stones are prevented, as well as promoted, is not established. Although the importance of body weight to metabolic stone activity in patients with cystinuria is not established, the patient's physicians were concerned about whether any bariatric surgery procedure would affect her ability to drink sufficient quantities of water in order to reduce stone activity. Here we report the experience of a genetically defined patient with cystinuria who underwent a gastric sleeve procedure. In the months after the procedure, she lost 45 kg, though with time she regained 23 kg of that loss. She was able to maintain a urine volume of 4.0 L per day and has had no stone recurrence.

Urinary stone disease prevalence and associations in cystic fibrosis.

Cystic fibrosis (CF) may predispose patients to urinary stone disease (USD), but reported prevalence of USD in patients with CF in previous small studies is variable. To date, analysis of risk factors for USD within the CF population has been limited. We studied 29,396 patients in the Cystic Fibrosis Foundation Patient Registry to calculate age and sex-stratified prevalence of USD. For adult patients, we examined age and multivariable-adjusted cross-sectional associations between demographic and clinical factors, CFTR mutation class, and prevalent USD. Prevalence of USD was 0.4% (95% CI 0.3-0.5%) under age 18 years, 3.1% (2.7-3.6%) at 18-24 years, 6.4% (5.8-7.1%) at 25-34 years, 7.5% (6.5-8.5%) at 35-44 years, and 6.7% (5.8-7.8%) at 45 years and older. Prevalence for women was higher than men at younger (< 45 years) but not older ages (P value for interaction < 0.0005). Multivariable odds of prevalent USD were significantly increased for severe CFTR mutations, OR 1.53 (1.14-2.06), diabetes, OR 1.24 (1.03-1.50), hypertension, OR 1.58 (1.29-1.93), and chronic macrolide therapy, OR 1.27 (1.07-1.52). BMI was not associated with USD. USD prevalence in CF is similar to that in the general population. With the exception of BMI, known risk factors for USD in the general population also appear to be important for patients with CF. We identified several novel associations in CF patients, including greater prevalence of USD in individuals with severe CFTR mutations and among young women.

Comparison of Selective Versus Empiric Pharmacologic Preventative Therapy With Kidney Stone Recurrence.

OBJECTIVE: To assess the effectiveness of an empiric approach to metabolic stone prevention. METHODS: Using medical claims from a cohort of working age adults with kidney stone diagnoses (2008-2017), we identified the subset who were prescribed thiazides, alkali therapy, or allopurinol-collectively known as preventive pharmacologic therapy (PPT). We distinguished between those who had 24-hour urine testing prior to initiating PPT (selective therapy) from those without it (empiric therapy). We conducted a survival analysis for time to first recurrence for stone-related events, including ED visits, hospitalizations, and surgery, up to 2 years after initiating PPT. RESULTS: Of 10,125 patients identified, 2744 (27%) and 7381 (73%) received selective and empiric therapy, respectively. The overall frequency of any stone-related event was 11%, and this did not differ between the 2 groups on bivariate analysis (P = .29). After adjusting for sociodemographic factors, comorbidities, medication class, and adherence, there was no difference in the hazard of a stone-related event between the selective and empiric therapy groups (hazard ratio, 0.97; 95% confidence interval, 0.84-1.12). When considered individually, the frequency of ED visits, hospitalizations, and surgeries did not differ between groups. Greater adherence to PPT and older age were associated with a lower hazard of a stone-related event (both P < .05). CONCLUSION: Compared to empiric therapy, PPT guided by 24-hour urine testing, on average, is not associated with a lower hazard of a stone-related event. These results suggest a need to identify kidney stone patients who benefit from 24-hour urine testing.

Evaluation and Medical Management of Patients with Cystine Nephrolithiasis: A Consensus Statement.

Purpose: Cystinuria is a genetic disorder with both autosomal recessive and incompletely dominant inheritance. The disorder disrupts cystine and other dibasic amino acid transport in proximal tubules of the kidney, resulting in recurrent kidney stone formation. Currently, there are no consensus guidelines on evaluation and management of this disease. This article represents the consensus of the author panel and will provide clinicians with a stepwise framework for evaluation and clinical management of patients with cystinuria based on evidence in the existing literature. Materials and Methods: A search of MEDLINE®/PubMed® and Cochrane databases was performed using the following key words: "cystine nephrolithiasis," "cystinuria," "penicillamine, cystine," and "tiopronin, cystine." In total, as of May 2018, these searches yielded 2335 articles, which were then evaluated for their relevance to the topic of evaluation and management of cystinuria. Evidence was evaluated by the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) system. Results: Twenty-five articles on the topic of cystinuria or cystine nephrolithiasis were deemed suitable for inclusion in this study. The literature supports a logical evaluation process and step-wise treatment approach beginning with conservative measures: fluid intake and dietary modification. If stone formation recurs, proceed to pharmacotherapeutic options by first alkalinizing the urine and then using cystine-binding thiol drugs. Conclusions: The proposed clinical pathways provide a framework for efficient evaluation and treatment of patients with cystinuria, which should improve overall outcomes of this rare, but highly recurrent, form of nephrolithiasis.

Acute Peritoneal Dialysis During the COVID-19 Pandemic at Bellevue Hospital in New York City.

Background: The COVID-19 pandemic strained hospital resources in New York City, including those for providing dialysis. New York University Medical Center and affiliations, including New York City Health and Hospitals/Bellevue, developed a plan to offset the increased needs for KRT. We established acute peritoneal dialysis (PD) capability, as usual dialysis modalities were overwhelmed by COVID-19 AKI. Methods: Observational study of patients requiring KRT admitted to Bellevue Hospital during the COVID surge. Bellevue Hospital is one of the largest public hospitals in the United States, providing medical care to an underserved population. There were substantial staff, supplies, and equipment shortages. Adult patients admitted with AKI who required KRT were considered for PD. We rapidly established an acute PD program. A surgery team placed catheters at the bedside in the intensive care unit; a nephrology team delivered treatment. We provided an alternative to hemodialysis and continuous venovenous hemofiltration for treating patients in the intensive-care unit, demonstrating efficacy with outcomes comparable to standard care. Results: From April 8, 2020 to May 8, 2020, 39 catheters were placed into ten women and 29 men. By June 10, 39% of the patients started on PD recovered kidney function (average ages 56 years for men and 59.5 years for women); men and women who expired were an average 71.8 and 66.2 years old. No episodes of peritonitis were observed; there were nine incidents of minor leaking. Some patients were treated while ventilated in the prone position. Conclusions: Demand compelled us to utilize acute PD during the COVID-19 pandemic. Our experience is one of the largest recently reported in the United States of which we are aware. Acute PD provided lifesaving care to acutely ill patients when expanding current resources was impossible. Our experience may help other programs to avoid rationing dialysis treatments in health crises.

Assessment of health-related quality of life in patients with cystinuria on tiopronin therapy.

Cystinuria comprises less than 1% of kidney stones and is associated with impaired health-related quality of life (HRQOL). Limited evidence is available regarding HRQOL of patients with cystinuria treated with tiopronin (Thiola®). The objective of this study was to assess the HRQOL of patients with or without tiopronin treatment. For this cross-sectional survey, patients on tiopronin treatment were recruited through the "Thiola® Total Care Hub," a specialty pharmacy used to dispense tiopronin, and compared with patients not taking tiopronin (non-tiopronin group) who were identified from the Cystinuria Contact Registry at New York University School of Medicine. Consented patients responded to a survey that included questions about their experiences with kidney stones, the Wisconsin stone quality of life (WISQOL) (disease-specific) questionnaire, and the short form-36 version 2 (SF-36v2) (generic) HRQOL questionnaire. Statistical analyses included independent-sample t tests, one-way analysis of variance (ANOVA), and correlations. The survey was completed by 312 patients: 267 in the tiopronin group (144 male, 123 female; mean 49 years) and 45 in the non-tiopronin group (10 male, 35 female; mean 48 years). Both groups utilized pain medications similarly (24% overall). Patients on tiopronin had a significantly better HRQOL than patients not on tiopronin for all WISQOL domains (p < 0.001) and all but the physical functioning SF-36v2 domain (p < 0.001), where both groups approached the US normative mean, when controlling for the last stone event. Compared with patients in the non-tiopronin group, patients taking tiopronin reported better HRQOL on both the WISQOL and SF-36v2.

Nephrolithiasis in women: how different from men?

PURPOSE OF REVIEW: Men have more kidney stones compared with women; however, the difference is progressively decreasing. The reasons for higher prevalence of stones in men, as well as increasing prevalence in women, is a subject of ongoing speculation. In this review, we summarize the evidence of differences between men and women and expand on the speculative causes. RECENT FINDINGS: Stone incidence is rising in women and adolescent girls. Stone disease is more heritable among men than women, and women demonstrate greater influence of the unique environment. Women under the age of 50 years who have been pregnant, have more than double the odds of kidney stones compared with those who have never been pregnant. Women are more burdened with obesity, bariatric surgery and dieting, all associated with increased stones. Women have higher urinary pH because of greater absorption of dietary organic anions leading to increased urinary citrate, compared with men, and they differ in tubular calcium handling. SUMMARY: It is obvious that the cause of stones in men and women is complex and requires further study. Potential clues offered are in the change of the female environment, influencing increasing incidence in stones, particularly of younger women and female adolescents.

Effect of increasing doses of cystine-binding thiol drugs on cystine capacity in patients with cystinuria.

Appropriate dosing of cystine-binding thiol drugs in the management of cystinuria has been based on clinical stone activity. When new stones form, the dose is increased. Currently, there is no method of measuring urinary drug levels to guide the titration of therapy. Increasing cystine capacity, a measure of cystine solubility, has been promoted as a method of judging the effects of therapy. In this study, we gave increasing doses of tiopronin or D-penicillamine, depending on the patients' own prescriptions, to ten patients with cystinuria and measured cystine excretion and cystine capacity. The doses were 0, 1, 2, 3 g per day, given in two divided doses, and administered in a random order. Going from 0 to 1 g/day led to an increase in cystine capacity from - 39.1 to 130.4 mg/L (P < 0.009) and decreased 24 h cystine excretion from 1003.9 to 834.8 mg/day (P = 0.039). Increasing the doses from 1 to 2 to 3 g/day had no consistent or significant effect to further increase cystine capacity or decrease cystine excretion. Whether doses higher than 1 g/day have additional clinical benefit is not clear from this study. Limiting doses might be associated with fewer adverse effects without sacrificing the benefit of higher doses if higher doses do not offer clinical importance. However, trials with stone activity as an outcome would be desirable.

Urine proteomic profiling in patients with nephrolithiasis and cystinuria.

PURPOSE: The purpose of the study was to assess the differences in the concentration and function of urinary proteins between patients with cystine stones (CYS) and healthy controls (HC). We postulated that CYS and HC groups would demonstrate different proteomic profiles. METHODS: A pilot study was performed comparing urinary proteomes of 10 patients with CYS and 10 age- and gender-matched HC, using liquid chromatography-mass spectrometry. Proteins which met the selection criteria (i) ≥ 2 unique peptide identifications; (ii) ≥ twofold difference in protein abundance; and (iii) ≤ 0.05 p value for the Fisher's Exact Test were analyzed using Gene Ontology classifications. RESULTS: Of the 2097 proteins identified by proteomic analysis, 398 proteins were significantly different between CYS and HC. Of those, 191 were involved in transport processes and 61 in inflammatory responses. The majority were vesicle-mediated transport proteins (78.5%), and 1/3 of them were down-regulated; of those, 12 proteins were involved in endosomal transport (including 6 charged multivesicular body proteins (CHMP) and 3 vacuolar sorting-associated proteins) and 9 in transmembrane transport. Myosin-2 and two actin-related proteins were significantly up-regulated in the vesicle-mediated transport group. CONCLUSION: We provide proteomic evidence of impaired endocytosis, dysregulation of actin and myosin cytoskeleton, and inflammation in CYS. Endosomal transport proteins were down-regulated mainly through defective CHMP. These findings may contribute to further understanding of the pathogenesis of CYS, potentially affecting its management.

Cystinuria: genetic aspects, mouse models, and a new approach to therapy.

Cystinuria, a genetic disorder of cystine transport, is characterized by excessive excretion of cystine in the urine and recurrent cystine stones in the kidneys and, to a lesser extent, in the bladder. Males generally are more severely affected than females. The disorder may lead to chronic kidney disease in many patients. The cystine transporter (b0,+) is a heterodimer consisting of the rBAT (encoded by SLC3A1) and b0,+AT (encoded by SLC7A9) subunits joined by a disulfide bridge. The molecular basis of cystinuria is known in great detail, and this information is now being used to define genotype-phenotype correlations. Current treatments for cystinuria include increased fluid intake to increase cystine solubility and the administration of thiol drugs for more severe cases. These drugs, however, have poor patient compliance due to adverse effects. Thus, there is a need to reduce or eliminate the risks associated with therapy for cystinuria. Four mouse models for cystinuria have been described and these models provide a resource for evaluating the safety and efficacy of new therapies for cystinuria. We are evaluating a new approach for the treatment of cystine stones based on the inhibition of cystine crystal growth by cystine analogs. Our ongoing studies indicate that cystine diamides are effective in preventing cystine stone formation in the Slc3a1 knockout mouse model for cystinuria. In addition to crystal growth, crystal aggregation is required for stone formation. Male and female mice with cystinuria have comparable levels of crystalluria, but very few female mice form stones. The identification of factors that inhibit cystine crystal aggregation in female mice may provide insight into the gender difference in disease severity in patients with cystinuria.

Tamm-Horsfall protein/uromodulin deficiency elicits tubular compensatory responses leading to hypertension and hyperuricemia.

Expression of Tamm-Horsfall protein (THP or uromodulin) is highly restricted to the kidney thick ascending limb (TAL) of loop of Henle. Despite the unique location and recent association of THP gene mutations with hereditary uromodulin-associated kidney disease and THP single nucleotide polymorphisms with chronic kidney disease and hypertension, the physiological function(s) of THP and its pathological involvement remain incompletely understood. By studying age-dependent changes of THP knockout (KO) mice, we show here that young KO mice had significant salt and water wasting but were partially responsive to furosemide, due to decreased luminal translocation of Na-K-Cl cotransporter 2 (NKCC2) in the TAL. Aged THP KO mice were, however, markedly oliguric and unresponsive to furosemide, and their NKCC2 was localized primarily in the cytoplasm as evidenced by lipid raft floatation assay, cell fractionation, and confocal and immunoelectron microscopy. These aged KO mice responded to metolazone and acetazolamide, known to target distal and proximal tubules, respectively. They also had marked upregulation of renin in juxtaglomerular apparatus and serum, and they were hypertensive. Finally, the aged THP KO mice had significant upregulation of Na-coupled urate transporters Slc5a8 and Slc22a12 as well as sodium-hydrogen exchanger 3 (NHE3) in the proximal tubule and elevated serum uric acid and allantoin. Collectively, our results suggest that THP deficiency can cause progressive disturbances in renal functions via initially NKCC2 dysfunction and later compensatory responses, resulting in prolonged activation of the renin-angiotensin-aldosterone axis and hyperuricemia.