The Evolving Role of Genetic Testing in Monogenic Kidney Stone Disease: Spotlight on Primary Hyperoxaluria.

PURPOSE: Multiple factors are thought to give rise to common, recurrent kidney stone disease, but for monogenic stone disorders a firm diagnosis is possible through genetic testing. The autosomal recessive primary hyperoxalurias (PH) are rare forms of monogenic kidney stone disease. All 3 types of PH are caused by inborn errors of glyoxylate metabolism in the liver, leading to hepatic oxalate overproduction and excessive renal urinary oxalate excretion. These conditions are characterized by kidney stones, nephrocalcinosis, progressive chronic kidney disease, and kidney failure. Systemic oxalosis, the extra-renal deposition of oxalate resulting in severe morbidity and mortality, occurs in chronic kidney disease when oxalate clearance by the kidneys declines. Novel small interfering RNA-based therapeutics targeting the liver to reduce urinary oxalate excretion have been approved, introducing precision medicine to treat primary hyperoxaluria type 1. The goal of this narrative review is to address the benefits and practicalities of genetic testing for suspected monogenic kidney stone disease and the critical roles of a multidisciplinary team. MATERIALS AND METHODS: We collated our procedures, education, training, and workflows to help other clinicians integrate genetic assessment into their diagnostic routines. RESULTS: In our experience, increased access to genetic testing facilitates early detection of PH and other monogenic causes of kidney stone disease so that individualized care can be instituted promptly. CONCLUSIONS: Alongside biochemical assessments, more widespread genetic testing may ensure more timely diagnoses so that patients with suspected monogenic kidney stone disease gain access to an expanded range of services and enrollment in clinical trials and registries.

Forty Years of Oxalobacter formigenes, a Gutsy Oxalate-Degrading Specialist.

Oxalobacter formigenes, a unique anaerobic bacterium that relies solely on oxalate for growth, is a key oxalate-degrading bacterium in the mammalian intestinal tract. Degradation of oxalate in the gut by O. formigenes plays a critical role in preventing renal toxicity in animals that feed on oxalate-rich plants. The role of O. formigenes in reducing the risk of calcium oxalate kidney stone disease and oxalate nephropathy in humans is less clear, in part due to difficulties in culturing this organism and the lack of studies which have utilized diets in which the oxalate content is controlled. Herein, we review the literature on the 40th anniversary of the discovery of O. formigenes, with a focus on its biology, its role in gut oxalate metabolism and calcium oxalate kidney stone disease, and potential areas of future research. Results from ongoing clinical trials utilizing O. formigenes in healthy volunteers and in patients with primary hyperoxaluria type 1 (PH1), a rare but severe form of calcium oxalate kidney stone disease, are also discussed. Information has been consolidated on O. formigenes strains and best practices to culture this bacterium, which should serve as a good resource for researchers.

Disparities in Kidney Stone Disease: A Scoping Review.

PURPOSE: We reviewed the available evidence regarding health disparities in kidney stone disease to identify knowledge gaps in this area. MATERIALS AND METHODS: A literature search was conducted using PubMed®, Embase® and Scopus® limited to articles published in English from 1971 to 2020. Articles were selected based on their relevance to disparities in kidney stone disease among adults in the United States. RESULTS: Several large epidemiological studies suggest disproportionate increases in incidence and prevalence of kidney stone disease among women as well as Black and Hispanic individuals in the United States, whereas other studies of comparable size do not report racial and ethnic demographics. Numerous articles describe disparities in imaging utilization, metabolic workup completion, analgesia, surgical intervention, stone burden at presentation, surgical complications, followup, and quality of life based on race, ethnicity, socioeconomic status and place of residence. Differences in urinary parameters based on race, ethnicity and socioeconomic status may be explained by both dietary and physiological factors. All articles assessed had substantial risk of selection bias and confounding. CONCLUSIONS: Health disparities are present in many aspects of kidney stone disease. Further research should focus not only on characterization of these disparities but also on interventions to reduce or eliminate them.

Dietary oxalate and kidney stone formation.

Dietary oxalate is plant-derived and may be a component of vegetables, nuts, fruits, and grains. In normal individuals, approximately half of urinary oxalate is derived from the diet and half from endogenous synthesis. The amount of oxalate excreted in urine plays an important role in calcium oxalate stone formation. Large epidemiological cohort studies have demonstrated that urinary oxalate excretion is a continuous variable when indexed to stone risk. Thus, individuals with oxalate excretions >25 mg/day may benefit from a reduction of urinary oxalate output. The 24-h urine assessment may miss periods of transient surges in urinary oxalate excretion, which may promote stone growth and is a limitation of this analysis. In this review we describe the impact of dietary oxalate and its contribution to stone growth. To limit calcium oxalate stone growth, we advocate that patients maintain appropriate hydration, avoid oxalate-rich foods, and consume an adequate amount of calcium.

Medical expulsive therapy.

The objective of this review article is to present the current literature on medical expulsive therapy (MET) and help guide practitioners in the appropriate use of MET for treatment of stone disease. Kidney stones can be treated with multiple modalities including medical therapy, ureteroscopy, shock wave lithotripsy (SWL), percutaneous nephrostolithotomy, open/laparoscopic stone removal, and/or combinations of these modalities. The choice of intervention depends on patient factors, anatomical considerations, surgeon preference, and stone location and characteristics. MET is an excellent treatment modality in the appropriately selected patient. The AUA/EAU guidelines suggest MET as a reasonable treatment choice in select patients. A review of the data suggests the use of alpha antagonist and calcium channel blockers can improve stone expulsion rates. Most data suggests alpha antagonists as superior to calcium channel blockers. There are numerous available alpha antagonists, all of which have supporting data for their use in MET. Evidence suggests that MET can decrease colic events, narcotic use, and hospital visits. MET may also reduce medical costs and prevent unnecessary surgeries and the associated risks. Further, there is a role for alpha antagonists and calcium channel blockers in improving stone passage and decreasing pain in those subjects treated with other modalities (i.e. SWL and ureteroscopy). Despite this evidence, MET remains underutilized as a treatment modality.

4-hydroxy-2-oxoglutarate metabolism in a mouse model of Primary Hyperoxaluria Type 3.

Primary Hyperoxaluria Type 3 (PH3) results from 4-hydroxy-2-oxoglutarate (HOG) aldolase (HOGA) deficiency, which causes an increase in endogenous oxalate synthesis leading to calcium oxalate kidney stone disease. The mechanisms underlying HOG metabolism and increased oxalate synthesis in PH3 are not well understood. We used a Hoga1 knock-out mouse model of PH3 to investigate two aspects of HOG metabolism: reduction to dihydroxyglutarate (DHG), a pathway that may limit oxalate synthesis in PH3, and metabolism to glyoxylate, which is a direct precursor to oxalate. The metabolism of HOG to DHG was highest in liver and kidney cortical tissue, enhanced in the cytosolic compartment of the liver, and preferred NADPH as a cofactor. In the absence of HOGA, HOG to glyoxylate aldolase activity was highest in liver mitoplasts, with no activity present in brain tissue lysates. These findings will assist in the identification of enzymes responsible for the metabolism of HOG to DHG and glyoxylate, which may lead to novel therapeutic approaches to limit oxalate synthesis in those afflicted with PH3.

End Point Considerations for Clinical Trials in Enteric Hyperoxaluria.

Enteric hyperoxaluria is a medical condition characterized by elevated urinary oxalate excretion due to increased gastrointestinal oxalate absorption. Causative features include fat malabsorption and/or increased intestinal permeability to oxalate. Enteric hyperoxaluria has long been known to cause nephrolithiasis and nephrocalcinosis, and, more recently, an association with CKD and kidney failure has been shown. Currently, there are no US Food and Drug Administration-approved therapies for enteric hyperoxaluria, and it is unclear what end points should be used to evaluate the efficacy of new drugs and biologics for this condition. This study represents work of a multidisciplinary group convened by the Kidney Health Initiative to review the evidence supporting potential end points for clinical trials in enteric hyperoxaluria. A potential clinical outcome is symptomatic kidney stone events. Potential surrogate end points include ( 1 ) an irreversible loss of kidney function as a surrogate for progression to kidney failure, ( 2 ) asymptomatic kidney stone growth/new stone formation observed on imaging as a surrogate for symptomatic kidney stone events, ( 3 ) urinary oxalate and urinary calcium oxalate supersaturation as surrogates for the development of symptomatic kidney stone events, and ( 4) plasma oxalate as a surrogate for the development of the clinical manifestations of systemic oxalosis. Unfortunately, because of gaps in the data, this Kidney Health Initiative workgroup was unable to provide definitive recommendations. Work is underway to obtain robust information that can be used to inform trial design and medical product development in this space.

The association of cardiovascular disease and metabolic syndrome with nephrolithiasis.

PURPOSE OF REVIEW: This review describes the relationship between nephrolithiasis, vascular disease and metabolic syndrome. RECENT FINDINGS: There is increasing evidence that kidney stone formation is associated with a number of systemic problems including cardiovascular disease, metabolic syndrome and its components. Some of these associations are bidirectional. The reasons for these associations are not totally clear, but potential factors include metabolic responses associated with these disorders that promote a stone forming milieu in urine, environmental factors such as diet, oxidative stress and inflammation and molecular changes impacting the transport of certain analytes in urine. SUMMARY: Urologists need to be cognizant of these associations as they may be able to contribute to an early diagnosis of a significant medical problem, or provide counseling to patients to prevent their occurrence.

Primary hyperoxaluria type 1: time for prime time?

Oxalate crystals in the kidney were first described in 1925. Since then, many major milestones have been reached in the understanding of genetic primary hyperoxaluria(s). Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disease due to a mutation in the AGXT gene, which encodes the hepatic peroxisomal enzyme alanine-glyoxylate aminotransferase (AGT), inducing excess oxalate production and further kidney stones, nephrocalcinosis and chronic kidney disease (CKD). Symptoms and age at diagnosis of PH1 vary dramatically, from the most severe infantile forms leading to end-stage kidney disease (ESKD) during the first months of life to the less severe adult forms with moderate CKD and recurrent kidney stones. In 2020, the Food and Drug Administration (FDA) and European Medicines Agency (EMA) approved a therapy based on RNA interference (RNAi) that profoundly reduces endogenous oxalate synthesis and dramatically changes the treatment algorithm for patients with PH1. The aim of this supplement of Clinical Kidney Journal includes contemporary reviews of the pathophysiology and genetics, (conventional) medical therapeutic management, urological therapeutic management and novel therapies (including not only RNAi, but also other therapeutic perspectives). The specific opinions of both adult and paediatric nephrologists will be compared and the ethical issues, as well as challenges faced by physicians and patients in developing countries, will also be discussed. Despite all the accomplishments, there are still looming questions that require further investigation and discovery.

Progress with RNA Interference for the Treatment of Primary Hyperoxaluria.

Over the last few years, US Food and Drug Administration-approved drugs using RNA interference have come to the market. Many have treated liver-specific diseases utilizing N-acetyl galactosamine conjugation because of its effective delivery and limited off-target effects. The autosomal recessive disorder primary hyperoxaluria, specifically type 1, has benefited from these developments. Primary hyperoxaluria arises from mutations in the enzymes involved in endogenous oxalate synthesis. The severity of disease varies but can result in kidney failure and systemic oxalosis. Until recently, the treatment options were limited and focused primarily on supportive treatments, pyridoxine use in a subset of patients with primary hyperoxaluria type 1, and liver-kidney transplants in those who progressed to kidney failure. Two genes have been targeted with RNA interference; lumasiran targets glycolate oxidase and nedosiran targets lactate dehydrogenase A. Lumasiran was recently approved in the treatment of primary hyperoxaluria type 1 and nedosiran is in the approval process. Unfortunately, despite initial hopes that nedosiran may also be a treatment option for primary hyperoxaluria types 2 and 3, initial data suggest otherwise. The use of RNA interference liver-specific targeting for the treatment of primary hyperoxaluria type 1 will likely transform the natural history of the disease.

Associations of Obesity and Neighborhood Factors With Urinary Stone Parameters.

INTRODUCTION: Obesity is associated with kidney stone disease, but it is unknown whether this association differs by SES. This study assessed the extent to which obesity and neighborhood characteristics jointly contribute to urinary risk factors for kidney stone disease. METHODS: This was a retrospective analysis of adult patients with kidney stone disease evaluated with 24-hour urine collection (2001-2020). Neighborhood-level socioeconomic data were obtained for a principal component analysis, which identified 3 linearly independent factors. Associations between these factors and 24-hour urine measurements were assessed using linear regression as well as groupings of 24-hour urine results using multivariable logistic regression. Finally, multiplicative interactions were assessed testing effect modification by obesity, and analyses stratified by obesity were performed. Analyses were performed in 2021. RESULTS: In total, 1,264 patients met the study criteria. Factors retained on principal component analysis represented SES, family structure, and housing characteristics. On linear regression, there was a significant inverse correlation between SES and 24-hour urine sodium (p=0.0002). On multivariable logistic regression, obesity was associated with increased odds of multiple stone risk factors (OR=1.61; 95% CI=1.15, 2.26) and multiple dietary factors (OR=1.33; 95% CI=1.06, 1.67). No significant and consistent multiplicative interactions were observed between obesity and quartiles of neighborhood SES, family structure, or housing characteristics. CONCLUSIONS: Obesity was associated with the presence of multiple stone risk factors and multiple dietary factors; however, the strength and magnitude of these associations did not vary significantly by neighborhood SES, family structure, and housing characteristics.

Generation of a GLO-2 deficient mouse reveals its effects on liver carbonyl and glutathione levels.

OBJECTIVE: Hydroxyacylglutathione hydrolase (aka as GLO-2) is a component of the glyoxalase pathway involved in the detoxification of the reactive oxoaldehydes, glyoxal and methylglyoxal. These reactive metabolites have been linked to a variety of pathological conditions, including diabetes, cancer and heart disease and may be involved in the aging process. The objective of this study was to generate a mouse model deficient in GLO-2 to provide insight into the function of GLO-2 and to determine if it is potentially linked to endogenous oxalate synthesis which could influence urinary oxalate excretion. METHODS: A GLO-2 knock out mouse was generated using CRISPR/Cas 9 techniques. Tissue and 24-h urine samples were collected under baseline conditions from adult male and female animals for biochemical analyses, including chromatographic measurement of glycolate, oxalate, glyoxal, methylglyoxal, D-lactate, ascorbic acid and glutathione levels. RESULTS: The GLO-2 KO animals developed normally and there were no changes in 24-h urinary oxalate excretion, liver levels of methylglyoxal, glyoxal, ascorbic acid and glutathione, or plasma d-lactate levels. GLO-2 deficient males had lower plasma glycolate levels than wild type males while this relationship was not observed in females. CONCLUSIONS: The lack of a unique phenotype in a GLO-2 KO mouse model under baseline conditions is consistent with recent evidence, suggesting a functional glyoxalase pathway is not required for optimal health. A lower plasma glycolate in male GLO-2 KO animals suggests glyoxal production may be a significant contributor to circulating glycolate levels, but not to endogenous oxalate synthesis.

Effect of alanine supplementation on oxalate synthesis.

The Primary Hyperoxalurias (PH) are rare disorders of metabolism leading to excessive endogenous synthesis of oxalate and recurring calcium oxalate kidney stones. Alanine glyoxylate aminotransferase (AGT), deficient in PH type 1, is a key enzyme in limiting glyoxylate oxidation to oxalate. The affinity of AGT for its co-substrate, alanine, is low suggesting that its metabolic activity could be sub-optimal in vivo. To test this hypothesis, we examined the effect of L-alanine supplementation on oxalate synthesis in cell culture and in mouse models of Primary Hyperoxaluria Type 1 (Agxt KO), Type 2 (Grhpr KO) and in wild-type mice. Our results demonstrated that increasing L-alanine in cells decreased synthesis of oxalate and increased viability of cells expressing GO and AGT when incubated with glycolate. In both wild type and Grhpr KO male and female mice, supplementation with 10% dietary L-alanine significantly decreased urinary oxalate excretion ~30% compared to baseline levels. This study demonstrates that increasing the availability of L-alanine can increase the metabolic efficiency of AGT and reduce oxalate synthesis.

Screening for primary hyperparathyroidism in a tertiary stone clinic, a useful endeavor.

INTRODUCTION AND OBJECTIVES: Primary hyperparathyroidism (1HPT) is associated with the risk of developing kidney stones. Our objective was to determine the prevalence of 1HPT amongst SF evaluated at a tertiary stone clinic and determine if it is cost-effective to screen for this condition. METHODS: We retrospectively reviewed 742 adult SF seen by a single urologic surgeon from 2012 to 2017 all of who were screened for 1HPT with an intact serum PTH (iPTH) and calcium. The diagnosis of 1HPT was based on the presence of hypercalcemia with an inappropriately elevated iPTH or a high normal serum calcium and an inappropriately elevated iPTH. The diagnosis was confirmed by surgical neck exploration. Published cost data and stone recurrence rates were utilized to create a cost-effectiveness decision tree. RESULTS OBTAINED: Fifty-three (7.1%) were diagnosed with 1HPT. 15 (28%) had hypercalcemia and inappropriately elevated iPTH, 38 (72%) had high normal serum calcium levels and inappropriately elevated iPTH. The potential diagnosis was ignored/missed by primary care physicians in 9 (17.0%) based on a review of prior lab results. Cost modeling was undertaken for 5, 10, 15, and 20-year intervals after screening. Based on our prevalence data, historical risks for recurrence and published cost data for stone treatments, cost savings in screening are realized at 10 years. CONCLUSION: These results support screening for primary hyperparathyroidism in patients evaluated in a tertiary referral setting.

The effects of the inactivation of Hydroxyproline dehydrogenase on urinary oxalate and glycolate excretion in mouse models of primary hyperoxaluria.

The major clinical manifestation of the Primary Hyperoxalurias (PH) is increased production of oxalate, as a consequence of genetic mutations that lead to aberrant glyoxylate and hydroxyproline metabolism. Hyperoxaluria can lead to the formation of calcium-oxalate kidney stones, nephrocalcinosis and renal failure. Current therapeutic approaches rely on organ transplants and more recently modifying the pathway of oxalate synthesis using siRNA therapy. We have recently reported that the metabolism of trans-4-hydroxy-L-proline (Hyp), an amino acid derived predominantly from collagen metabolism, is a significant source of oxalate production in individuals with PH2 and PH3. Thus, the first enzyme in the Hyp degradation pathway, hydroxyproline dehydrogenase (HYPDH), represents a promising therapeutic target for reducing endogenous oxalate production in these individuals. This is supported by the observation that individuals with inherited mutations in HYPDH (PRODH2 gene) have no pathological consequences. The creation of mouse models that do not express HYPDH will facilitate research evaluating HYPDH as a target. We describe the phenotype of the Prodh2 knock out mouse model and show that the lack of HYPDH in PH mouse models results in lower levels of urinary oxalate excretion, consistent with our previous metabolic tracer and siRNA-based knockdown studies. The double knockout mouse, Grhpr KO (PH2 model) and Prodh2 KO, prevented calcium-oxalate crystal deposition in the kidney, when placed on a 1% Hyp diet. These observations support the use of the Grhpr KO mice to screen HYPDH inhibitors in vivo. Altogether these data support HYPDH as an attractive therapeutic target for PH2 and PH3 patients.

Contribution of Dietary Oxalate and Oxalate Precursors to Urinary Oxalate Excretion.

Kidney stone disease is increasing in prevalence, and the most common stone composition is calcium oxalate. Dietary oxalate intake and endogenous production of oxalate are important in the pathophysiology of calcium oxalate stone disease. The impact of dietary oxalate intake on urinary oxalate excretion and kidney stone disease risk has been assessed through large cohort studies as well as smaller studies with dietary control. Net gastrointestinal oxalate absorption influences urinary oxalate excretion. Oxalate-degrading bacteria in the gut microbiome, especially Oxalobacter formigenes, may mitigate stone risk through reducing net oxalate absorption. Ascorbic acid (vitamin C) is the main dietary precursor for endogenous production of oxalate with several other compounds playing a lesser role. Renal handling of oxalate and, potentially, renal synthesis of oxalate may contribute to stone formation. In this review, we discuss dietary oxalate and precursors of oxalate, their pertinent physiology in humans, and what is known about their role in kidney stone disease.

Reduction in urinary oxalate excretion in mouse models of Primary Hyperoxaluria by RNA interference inhibition of liver lactate dehydrogenase activity.

The Primary Hyperoxaluria's (PH) are rare autosomal recessive disorders characterized by elevated oxalate production. PH patients suffer recurrent calcium oxalate kidney stone disease, and in severe cases end stage renal disease. Recent evidence has shown that RNA interference may be a suitable approach to reduce oxalate production in PH patients by knocking down key enzymes involved in hepatic oxalate synthesis. In the current study, wild type mice and mouse models of PH1 (AGT KO) and PH2 (GR KO) were treated with siRNA that targets hepatic LDHA. Although siRNA treatment substantially reduced urinary oxalate excretion [75%] in AGT KO animals, there was a relatively modest reduction [32%] in GR KO animals. Plasma and liver pyruvate levels significantly increased with siRNA treatment and liver organic acid analysis indicated significant changes in a number of glycolytic and TCA cycle metabolites, consistent with the known role of LDHA in metabolism. However, siRNA dosing data suggest that it may be possible to identify a dose that limits changes in liver organic acid levels, while maintaining a desired effect of reducing glyoxylate to oxalate synthesis. These results suggest that RNAi mediated reduction of hepatic LDHA may be an effective strategy to reduce oxalate synthesis in PH, and further analysis of its metabolic effects should be explored. Additional studies should also clarify in GR KO animals whether there are alternate enzymatic pathways in the liver to create oxalate and whether tissues other than liver contribute significantly to oxalate production.