Clinical features and genotype of adenine phosphoribosyltransferase deficiency in iceland

Optimization and validation of a UPLC-MS/MS assay for simultaneous quantification of 2,8-dihydroxyadenine, adenine, allopurinol, oxypurinol and febuxostat in human plasma

Adenine phosphoribosyltransferase (APRT) deficiency is a rare , hereditary disorder characterized by renal excretion of 2,8-dihydroxyadenine (DHA), leading to kidney stone formation and chronic kidney disease (CKD). Treatment with a xanthine oxidoreductase inhibitor, allopurinol or febuxostat, reduces urinary DHA excretion and slows the progression of CKD. The method currently used for therapeutic monitoring of APRT deficiency lacks specificity and thus, a more reliable measurement technique is needed. In this study, an ultra-performance liquid chromatography-tandem mass spectrometry method for simultaneous quantification of DHA, adenine, allopurinol, oxypurinol and febuxostat in human plasma was optimized and validated. Plasma samples were prepared with protein precipitation using acetonitrile followed by evaporation. The chemometric approach design of experiments was implemented to optimize gradient steepness, amount of organic solvent, flow rate, column temperature, cone voltage, desolvation temperature and desolvation flow rate. Experimental screening was conducted using fractional factorial design with addition of complementary experiments at the axial points for optimization of peak area, peak resolution and peak width. The assay was validated according to the US Food and Drug Administration guidelines for bioanalytical method validation over the concentration range of 50 to 5000 ng/mL for DHA, allopurinol and febuxostat, 100 to 5000 ng/mL for adenine and 50 to 12,000 ng/mL for oxypurinol, with r2 ≥ 0.99. The analytical assay achieved acceptable performance of accuracy (-10.8 to 8.3 %) and precision (CV < 15 %). DHA, adenine, allopurinol, oxypurinol and febuxostat were stable in plasma samples after five freeze-thaw cycles at -80 °C and after storage at -80 °C for 12 months. The assay was evaluated for quantification of the five analytes in clinical plasma samples from six APRT deficiency patients and proved to be both efficient and accurate. The proposed assay will be valuable for guiding pharmacotherapy and thereby contribute to improved and more personalized care for patients with APRT deficiency.

Adenine phosphoribosyl transferase (APRT) deficiency and a novel sequence variant in APRT with phenotypic diversity and a literature review

Adenine phosphoribosyl transferase (APRT) deficiency is an autosomal recessive disorder and a rare cause of urolithiasis due to mutations in APRT (OMIM #102600). APRT deficiency results in increased urinary excretion of 2,8-dihydroxyadenine (DHA) which can cause urolithiasis and kidney failure. However, with prompt diagnosis, patients with APRT deficiency can be treated with xanthine oxidoreductase inhibitors which decrease urinary DHA excretion and improve outcomes. We report a pair of siblings, an 11-year-old brother and his 14-year-old sister with compound heterozygous variants c.270del (p.Lys91Serfs*46) and c.484_486del (p.Leu162del) in APRT with variable clinical presentation of APRT deficiency. The brother presented at 17 months of age with urolithiasis and severe acute kidney injury. His elder sister remained well and asymptomatic with normal kidney function and did not develop renal calculi. Brownish disk or sphere-like crystals with both concentric and radial markings were reported on urine microscopy in the sister on screening. The sister's diagnosis was confirmed with further laboratory evidence of absent red cell lysate APRT activity with corresponding elevated levels of urinary DHA. In conclusion, we identified a novel mutation in the APRT gene in a pair of siblings with greater phenotypic severity in the male.

A case of 2,8-DHA crystalline nephropathy caused by adenine phosphoribosyltransferase deficiency: diagnosis and treatment

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disorder that leads to the accumulation of poorly soluble 2,8-dihydroxyadenine (DHA) in the kidneys, resulting in a variety of renal presentations including nephrolithiasis, acute kidney injury, and chronic kidney disease (CKD) caused by crystal nephropathy. Here, we report a case of a 43-year-old man with 2,8-DHA crystalline nephropathy caused by APRT deficiency strongly suspected by renal biopsy results and definitively diagnosed by a urine gas chromatography-mass spectrometry (GC/MS)-based plasma metabolomic assessment. This case represents the importance of awareness and recognition of the signs and symptoms of this rare condition and its progression to CKD, which can be prevented by the early administration of xanthine oxidoreductase inhibitors.

The genetics of kidney stone disease and nephrocalcinosis

Kidney stones (also known as urinary stones or nephrolithiasis) are highly prevalent, affecting approximately 10% of adults worldwide, and the incidence of stone disease is increasing. Kidney stone formation results from an imbalance of inhibitors and promoters of crystallization, and calcium-containing calculi account for over 80% of stones. In most patients, the underlying aetiology is thought to be multifactorial, with environmental, dietary, hormonal and genetic components. The advent of high-throughput sequencing techniques has enabled a monogenic cause of kidney stones to be identified in up to 30% of children and 10% of adults who form stones, with ~35 different genes implicated. In addition, genome-wide association studies have implicated a series of genes involved in renal tubular handling of lithogenic substrates and of inhibitors of crystallization in stone disease in the general population. Such findings will likely lead to the identification of additional treatment targets involving underlying enzymatic or protein defects, including but not limited to those that alter urinary biochemistry.

A Rare Genetic Mutation in a Stone Former

A 30-year-old woman with history of passage of stones since childhood presented with oliguria and pedal edema for 10 days. She had hypertension with a creatinine of 4.1 mg/dL. Evaluation showed presence of bilateral multiple renal calculi with features of chronicity of kidney disease. Metabolic work-up for nephrolithiasis turned out to be negative and eventually renal biopsy revealed features of chronic interstitial nephritis with greenish brown refractile crystals in the tubular lumen and interstitium. The possibility of dihydroxy adenine crystalline nephropathy was considered. Spectrophotometry of RBC lysates revealed decreased activity of Adenine phosphoribosyl-transferase enzyme. Gene amplification by PCR and sequential analysis identified a missense mutation in exon 3 region of APRT gene in the patient and her family members. This case report highlights the need to contemplate the diagnosis of DHA crystalline nephropathy in young patients with nephrolithiasis and the identification of a rare genetic mutation, which is being reported for the first time in India.

Allele frequency of variants reported to cause adenine phosphoribosyltransferase deficiency

Adenine phosphoribosyltransferase deficiency is a rare, autosomal recessive disorder of purine metabolism that causes nephrolithiasis and progressive chronic kidney disease. The small number of reported cases indicates an extremely low prevalence, although it has been suggested that missed diagnoses may play a role. We assessed the prevalence of APRT deficiency based on the frequency of causally-related APRT sequence variants in a diverse set of large genomic databases. A thorough search was carried out for all APRT variants that have been confirmed as pathogenic under recessive mode of inheritance, and the frequency of the identified variants examined in six population genomic databases: the deCODE genetics database, the UK Biobank, the 100,000 Genomes Project, the Genome Aggregation Database, the Human Genetic Variation Database and the Korean Variant Archive. The estimated frequency of homozygous genotypes was calculated using the Hardy-Weinberg equation. Sixty-two pathogenic APRT variants were identified, including six novel variants. Most common were the missense variants c.407T>C (p.(Met136Thr)) in Japan and c.194A>T (p.(Asp65Val)) in Iceland, as well as the splice-site variant c.400 + 2dup (p.(Ala108Glufs*3)) in the European population. Twenty-nine variants were detected in at least one of the six genomic databases. The highest cumulative minor allele frequency (cMAF) of pathogenic variants outside of Japan and Iceland was observed in the Irish population (0.2%), though no APRT deficiency cases have been reported in Ireland. The large number of cases in Japan and Iceland is consistent with a founder effect in these populations. There is no evidence for widespread underdiagnosis based on the current analysis.

A Rare Case of APRT Deficiency with End-stage Renal Failure and Successful Renal Transplant

Renal calculus disease is a common cause of renal injury. However, crystal nephropathy (uric acid, oxalate, and dihydroxyadenine) can present as chronic kidney disease without any evidence of renal stones. If left undiagnosed, there is a potential chance of recurrence in the allograft leading to graft failure after transplantation. Pretransplant identification and management can avoid such complications. Here, we describe a case of APRT deficiency leading to crystal nephropathy and end-stage renal failure in a patient who underwent a successful kidney transplant.

Recurrent DHA nephropathy in renal allograft-revisiting clinicopathological aspects of a rare entity

Background

Adenine phosphoribosyltransferase (APRT) enzyme deficiency is a rare autosomal recessive disorder of purine metabolism affecting mainly the kidneys. It can present at any age with varying degrees of acute and chronic renal damage. Though xanthine dehydrogenase inhibitors offer effective control over the disease process, delay in diagnosis and treatment often lead to compromised function of native and even graft kidneys.

Methods

We have done a retrospective search of records of renal biopsies reported at our center during the 5-year period from 2014 to 2018 to identify biopsies with 2,8-dihydroxyadenine crystal deposits. The demographic, clinical, and histopathological findings in these cases were studied and reviewed in the light of available literature.

Results

Of 9059 renal biopsies received during the study period, 3 cases had the rare 2,8- dihydroxyadenine (DHA) crystals. All of them were diagnosed for the first time on allograft biopsies.

Conclusion

A high index of clinical suspicion together with the characteristic microscopic appearance of crystals on renal biopsy and urine microscopy can clinch the diagnosis of this rare disease. Hence, improving awareness about this entity among clinicians and pathologists is extremely important.

Urinary mulberry bodies as a potential biomarker for early diagnosis and efficacy assessment of enzyme replacement therapy in Fabry nephropathy

BACKGROUND

The inability of enzyme replacement therapy (ERT) to prevent progression of Fabry nephropathy (FN) in the presence of >1 g/day proteinuria underscores the necessity of identifying effective biomarkers for early diagnosis of FN preceding proteinuria. Here we attempted to identify biomarkers for early detection of FN.

METHODS

Fifty-one Fabry disease (FD) patients were enrolled. Urinary mulberry bodies (uMBs) were immunostained for globotriaosylceramide (Gb3) and renal cell markers to determine their origin. The association between semiquantitative uMB excretion and the histological severity of podocyte vacuolation was investigated in seven patients using the vacuolated podocyte:glomerular average area ratio. The association between the semiquantitative estimate of uMB excretion and duration of ERT was analyzed. A longitudinal study was conducted to assess the effect of ERT on uMB excretion.

RESULTS

Thirty-two patients (63%) had uMBs, while only 31% showed proteinuria. The uMBs were positive for Gb3, lysosomal-associated membrane protein 1 and podocalyxin, suggesting they were derived from lysosomes with Gb3 accumulation in podocytes. We observed more severe podocyte vacuolation with increased uMB excretion (P = 0.03 for trend); however, the same was not observed with increased proteinuria. The percentage of patients with substantial uMB excretion increased with shorter ERT duration (P = 0.018). Eighteen-month-long ERT reduced uMB excretion (P = 0.03) without affecting proteinuria.

CONCLUSIONS

uMB excretion, implying ongoing podocyte injury, preceded proteinuria in most patients. Semiquantitative uMB estimates can serve as novel biomarkers for early FN diagnosis and for monitoring the efficacy of FD-specific therapies.

Crystalline Nephropathy due to APRT Deficiency: A Preventable Cause of Renal and Renal Allograft Failure

Adenine phosphororibosyl transferase (APRT) deficiency, a rare inborn error of metabolism is inherited as an autosomal recessive trait. It presents with 2,8-dihydroxyadenine (2,8-DHA) crystal nephropathy and recurrent nephrolithiasis and often progresses to end stage renal disease (ESRD). After transplant, it can recur in the allograft. If APRT deficiency is recognized early, renal failure can be prevented, arrested or reversed in native kidney and in allograft by treatment with allopurinol, which inhibits xanthine oxidase and reduces 2,8-DHA formation. We report two cases of APRT deficiency from our center. DNA sequencing of APRT gene performed in one of the cases revealed a pathogenic variant in Exon1 of APRT gene (c.3G>C; p.Met1). This variant affects the translation initiation codon and results in a start loss. The variant has previously been reported in two cases with APRT deficiency.

Are conventional stone analysis techniques reliable for the identification of 2,8-dihydroxyadenine kidney stones? A case series

We have recently encountered patients incorrectly diagnosed with adenine phosphoribosyltransferase (APRT) deficiency due to misidentification of kidney stones as 2,8-dihydroxyadenine (DHA) stones. The objective of this study was to examine the accuracy of stone analysis for identification of DHA. Medical records of patients referred to the APRT Deficiency Research Program of the Rare Kidney Stone Consortium in 2010-2018 with a diagnosis of APRT deficiency based on kidney stone analysis were reviewed. The diagnosis was verified by measurement of APRT enzyme activity or genetic testing. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra of pure crystalline DHA and a kidney stone obtained from one of the confirmed APRT deficiency cases were generated. The ATR-FTIR spectrum of the kidney stone matched the crystalline DHA spectrum and was used for comparison with available infrared spectra of stone samples from the patients. Of 17 patients referred, 14 had sufficient data available to be included in the study. In all 14 cases, the stone analysis had been performed by FTIR spectroscopy. The diagnosis of APRT deficiency was confirmed in seven cases and rejected in the remaining seven cases. Comparison of the ATR-FTIR spectrum of the DHA stone with the FTIR spectra from three patients who did not have APRT deficiency showed no indication of DHA as a stone component. Misidentification of DHA as a kidney stone component by clinical laboratories appears common among patients referred to our program. Since current clinical protocols used to interpret infrared spectra for stone analysis cannot be considered reliable for the identification of DHA stones, the diagnosis of APRT deficiency must be confirmed by other methods.

Cellular and Molecular Mechanisms of Kidney Injury in 2,8-Dihydroxyadenine Nephropathy

BACKGROUND

Hereditary deficiency of adenine phosphoribosyltransferase causes 2,8-dihydroxyadenine (2,8-DHA) nephropathy, a rare condition characterized by formation of 2,8-DHA crystals within renal tubules. Clinical relevance of rodent models of 2,8-DHA crystal nephropathy induced by excessive adenine intake is unknown.

METHODS

Using animal models and patient kidney biopsies, we assessed the pathogenic sequelae of 2,8-DHA crystal-induced kidney damage. We also used knockout mice to investigate the role of TNF receptors 1 and 2 (TNFR1 and TNFR2), CD44, or alpha2-HS glycoprotein (AHSG), all of which are involved in the pathogenesis of other types of crystal-induced nephropathies.

RESULTS

Adenine-enriched diet in mice induced 2,8-DHA nephropathy, leading to progressive kidney disease, characterized by crystal deposits, tubular injury, inflammation, and fibrosis. Kidney injury depended on crystal size. The smallest crystals were endocytosed by tubular epithelial cells. Crystals of variable size were excreted in urine. Large crystals obstructed whole tubules. Medium-sized crystals induced a particular reparative process that we term extratubulation. In this process, tubular cells, in coordination with macrophages, overgrew and translocated crystals into the interstitium, restoring the tubular luminal patency; this was followed by degradation of interstitial crystals by granulomatous inflammation. Patients with adenine phosphoribosyltransferase deficiency showed similar histopathological findings regarding crystal morphology, crystal clearance, and renal injury. In mice, deletion of Tnfr1 significantly reduced tubular CD44 and annexin two expression, as well as inflammation, thereby ameliorating the disease course. In contrast, genetic deletion of Tnfr2, Cd44, or Ahsg had no effect on the manifestations of 2,8-DHA nephropathy.

CONCLUSIONS

Rodent models of the cellular and molecular mechanisms of 2,8-DHA nephropathy and crystal clearance have clinical relevance and offer insight into potential future targets for therapeutic interventions.

Kidney Transplant Outcomes in Patients With Adenine Phosphoribosyltransferase Deficiency

BACKGROUND

Adenine phosphoribosyltransferase (APRT) deficiency is a rare, hereditary cause of kidney stones and chronic kidney disease (CKD) which is characterized by 2,8-dihydroxyadenine renal parenchymal crystal deposition. The aim of this study was to examine outcomes of kidney transplantation in APRT deficiency patients.

METHODS

Included were 13 patients in the APRT Deficiency Registry of the Rare Kidney Stone Consortium, 2 from Westmead Hospital in Sydney, Australia, and 2 from Necker Hospital in Paris, France. The CKD-EPI and CKiD equations were used to calculate glomerular filtration rate estimates. Allograft survival was analyzed employing the Kaplan-Meier method. The Wilcoxon-Mann-Whitney test was used to compare alllograft outcomes according to xanthine oxidoreductase (XOR) inhibitor treatment status at transplantation.

RESULTS

Seventeen patients (9 females) received 22 kidney transplants. Age at first transplantation was 47.2 (14.9-67.0) years. Ten patients received XOR inhibitor therapy pretransplant (11 allografts), while 8 patients did not receive such treatment before transplantation (11 allografts). Two-year allograft survival was 91% and 55% in the 2 groups, respectively (P = 0.16). The median (range) estimated glomerular filtration rate at 2 years posttransplant was 61.3 (24.0-90.0) mL/min/1.73 m when XOR inhibitor therapy was initiated before transplantation, and 16.2 (10.0-39.0) mL/min/1.73 m (P = 0.009) when such treatment was not administered pretransplant.

CONCLUSIONS

Kidney allograft outcomes are good in APRT deficiency patients beginning XOR inhibitor therapy pretransplant. Delay in such treatment is a major cause of premature graft loss in these patients. Increased awareness among clinicians is imperative, promoting early diagnosis of APRT deficiency and pharmacotherapy initiation before kidney transplantation.

Rare crystalline nephropathy leading to acute graft dysfunction: a case report

BACKGROUND

Adenine phosphoribosyl transferase (APRT) deficiency is a rare genetic form of kidney stones and/or kidney failure characterized by intratubular precipitation of 2,8 dihydroxyadenine crystals. Early diagnosis and prompt management can completely reverse the kidney injury.

CASE PRESENTATION

44 year old Indian male, renal transplant recipient got admitted with acute graft dysfunction. Graft biopsy showed light brown refractile intratubular crystals with surrounding giant cell reaction, consistent with APRT deficiency. Patient improved after receiving allopurinol and hydration.

CONCLUSION

APRT forms a reversible cause of crystalline nephropathy. High index of suspicion is required for the correct diagnosis as timely diagnosis has therapeutic implications.

Urinary 2,8-dihydroxyadenine excretion in patients with adenine phosphoribosyltransferase deficiency, carriers and healthy control subjects

BACKGROUND

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disorder of adenine metabolism that results in excessive urinary excretion of the poorly soluble 2,8-dihydroxyadenine (DHA), leading to kidney stones and chronic kidney disease. The purpose of this study was to assess urinary DHA excretion in patients with APRT deficiency, heterozygotes and healthy controls, using a recently developed ultra-performance liquid chromatography - tandem mass spectrometry (UPLC-MS/MS) assay.

METHODS

Patients enrolled in the APRT Deficiency Registry and Biobank of the Rare Kidney Stone Consortium (http://www.rarekidneystones.org/) who had provided 24-h and first-morning void urine samples for DHA measurement were eligible for the study. Heterozygotes and healthy individuals served as controls. Wilcoxon-Mann-Whitney test was used to compare 24-h urinary DHA excretion between groups. Associations were examined using Spearman's correlation coefficient (r_s).

RESULTS

The median (range) 24-h urinary DHA excretion was 138 (64-292) mg/24 h and the DHA-to-creatinine (DHA/Cr) ratio in the first-morning void samples was 13 (4-37) mg/mmol in APRT deficiency patients who were not receiving xanthine oxidoreductase inhibitor therapy. The 24-h DHA excretion was highly correlated with the DHA/Cr ratio in first-morning void urine samples (r_s = 0.84, p < .001). DHA was detected in all urine samples from untreated patients but not in any specimens from heterozygotes and healthy controls.

CONCLUSIONS

High urinary DHA excretion was observed in patients with APRT deficiency, while urine DHA was undetectable in heterozygotes and healthy controls. Our results suggest that the UPLC-MS/MS assay can be used for diagnosis of APRT deficiency.

Long-term renal outcomes of APRT deficiency presenting in childhood

BACKGROUND

Adenine phosphoribosyltransferase (APRT) deficiency is a hereditary purine metabolism disorder that causes kidney stones and chronic kidney disease (CKD). The purpose of this study was to examine the course of APRT deficiency in patients who presented in childhood.

METHODS

The disease course of 21 (35%) patients in the APRT Deficiency Registry of the Rare Kidney Stone Consortium, who presented with manifestations of APRT deficiency and/or were diagnosed with the disorder before the age of 18 years, was studied. The effect of pharmacotherapy on renal manifestations and outcomes was thoroughly assessed.

RESULTS

Fourteen children were placed on allopurinol, 100 (25-200) mg/day, at the age of 2.6 (0.6-16.5) years. Six of these patients had experienced kidney stone events and three had developed acute kidney injury (AKI) prior to allopurinol treatment. During 18.9 (1.7-31.5) years of pharmacotherapy, stones occurred in two patients and AKI in three. Six adult patients started allopurinol treatment, 200 (100-300) mg/day, at age 29.8 (20.5-42.4) years. Five of these patients had experienced 28 stone episodes and AKI had occurred in two. Stone recurrence occurred in four patients and AKI in two during 11.2 (4.2-19.6) years of allopurinol therapy. Lack of adherence and insufficient dosing contributed to stone recurrence and AKI during pharmacotherapy. At latest follow-up, estimated glomerular filtration rate (eGFR) was 114 (70-163) and 62 (10-103) mL/min/1.73 m² in those who initiated treatment as children and adults, respectively. All three patients with CKD stages 3-5 at the last follow-up were adults when pharmacotherapy was initiated.

CONCLUSION

Timely diagnosis and treatment of APRT deficiency decreases renal complications and preserves kidney function.

APRT deficiency: the need for early diagnosis

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disorder which leads to accumulation of poorly soluble 2,8-dihydroxyadenine in kidneys resulting in nephrolithiasis as well as chronic kidney disease from crystal nephropathy. This report describes a 55-year-old previously fit man who presented with shortness of breath and the investigative pathway that eventually led to a diagnosis of APRT deficiency. Early diagnosis has aided in timely institution of allopurinol, thereby improving his renal function and possibility of weaning off renal replacement therapy. Genetic testing has enabled early identification of other family members at risk and prevention of renal failure by commencing xanthine oxidoreductase (XOR) inhibitors. The issues surrounding kidney donation by a member of this family are also discussed. This case represents the importance of awareness and recognition of the signs and symptoms of this rare condition, complications of which can be easily prevented by early institution of XOR inhibitor therapy.

Fish Oil Supplementation Reduces Inflammation but Does Not Restore Renal Function and Klotho Expression in an Adenine-Induced CKD Model

Background: Chronic kidney disease and inflammation promote loss of Klotho expression. Given the well-established anti-inflammatory effects of omega-3 fatty acids, we aimed to investigate the effect of fish oil supplementation in a model of CKD. Methods: Male C57BL/6 mice received supplementation with an adenine-enriched diet (AD, n = 5) or standard diet (CTL, n = 5) for 10 days. Two other experimental groups were kept under the adenine diet for 10 days. Following adenine withdrawal on the 11th day, the animals returned to a standard diet supplemented with fish oil (Post AD-Fish oil, n = 9) or not (Post AD-CTL, n = 9) for an additional period of 7 days. Results: Adenine mice exhibited significantly higher mean serum urea, creatinine, and renal expression of the pro-inflammatory markers Interleukin-6 (IL-6), C-X-C motif chemokine 10 (CXCL10), and Interleukin-1β (IL-1β), in addition to prominent renal fibrosis and reduced renal Klotho gene expression compared to the control. Post AD-Fish oil animals demonstrated a significant reduction of IL-6, C-X-C motif chemokine 9 (CXCL9), and IL-1β compared to Post AD-CTL animals. However, serum creatinine, renal fibrosis, and Klotho were not significantly different in the fish oil-treated group. Furthermore, renal histomorphological changes such as tubular dilatation and interstitial infiltration persisted despite treatment. Conclusions: Fish oil supplementation reduced renal pro-inflammatory markers but was not able to restore renal function nor Klotho expression in an adenine-induced CKD model.

Noninvasive Immunohistochemical Diagnosis and Novel MUC1 Mutations Causing Autosomal Dominant Tubulointerstitial Kidney Disease

BACKGROUND

Autosomal dominant tubulointerstitial kidney disease caused by mucin-1 gene (MUC1) mutations (ADTKD-MUC1) is characterized by progressive kidney failure. Genetic evaluation for ADTKD-MUC1 specifically tests for a cytosine duplication that creates a unique frameshift protein (MUC1fs). Our goal was to develop immunohistochemical methods to detect the MUC1fs created by the cytosine duplication and, possibly, by other similar frameshift mutations and to identify novel MUC1 mutations in individuals with positive immunohistochemical staining for the MUC1fs protein.

METHODS

We performed MUC1fs immunostaining on urinary cell smears and various tissues from ADTKD-MUC1-positive and -negative controls as well as in individuals from 37 ADTKD families that were negative for mutations in known ADTKD genes. We used novel analytic methods to identify MUC1 frameshift mutations.

RESULTS

After technique refinement, the sensitivity and specificity for MUC1fs immunostaining of urinary cell smears were 94.2% and 88.6%, respectively. Further genetic testing on 17 families with positive MUC1fs immunostaining revealed six families with five novel MUC1 frameshift mutations that all predict production of the identical MUC1fs protein.

CONCLUSIONS

We developed a noninvasive immunohistochemical method to detect MUC1fs that, after further validation, may be useful in the future for diagnostic testing. Production of the MUC1fs protein may be central to the pathogenesis of ADTKD-MUC1.

Diagnosis and management of non-calcium-containing stones in the pediatric population

Compared to adults, urolithiasis is less common in children, with a definite rise in incidence, especially among young adults (Tasian et al. in Clin J Am Soc Nephrol 11:488, 2016). In the last 25 years, the incidence in children has increased by approximately 6-10% annually, for reasons still unknown, with an associated significant increase in related health care-related expenditures (Hyams and Matlaga in Transl Androl Urol 3(3):278-83, 2014). It has been shown that there is twice as high a risk of chronic kidney disease (CKD) or end stage renal disease (ESRD) in stone formers compared to non-stone formers (Tasian et al. 2016). While calcium-containing stones are by far the most common category of stone encountered in both children and adults, non-calcium stones are more common in children than adults and have been shown in several studies to be associated with greater morbidity and lower renal function than calcium stones (Issler et al. in BMC Nephrol 18(1):136, 2017; Gambaro et al. in J Urol 198:268-273, 2017). This could be related to the challenges in the management of non-calcium-containing stones due to associated infection or metabolic derangements, further leading to recurrence and loss of renal function. There is currently a gap in our understanding of how to appropriately and effectively encounter and manage patients with non-calcium-containing stones, as such cases are encountered less frequently. Identification of stone composition and appropriate management is very important to reduce serious complications and recurrence, especially in non-calcium stones. We present a review of diagnosis and management of non-calcium-containing stones in the pediatric population, in hopes of providing more clarity to providers and promoting a consideration of non-calcium stone composition with all children presenting with urolithiasis.

Comparison of the effect of allopurinol and febuxostat on urinary 2,8-dihydroxyadenine excretion in patients with Adenine phosphoribosyltransferase deficiency (APRTd): A clinical trial

INTRODUCTION

Adenine phosphoribosyltransferase (APRT) deficiency is a rare, but significant, cause of kidney stones and progressive chronic kidney disease. The optimal treatment has not been established. The purpose of this pilot study was to compare the effect of the xanthine oxidoreductase inhibitors allopurinol and febuxostat on urinary 2,8-dihydroxyadenine (DHA) excretion in APRT deficiency patients.

MATERIALS AND METHODS

Patients listed in the APRT Deficiency Registry of the Rare Kidney Stone Consortium, currently receiving allopurinol therapy, were invited to participate. The trial endpoint was the 24-h urinary DHA excretion following treatment with allopurinol (400mg/day) and febuxostat (80mg/day). Urinary DHA was measured using a novel ultra-performance liquid chromatography - electrospray tandem mass spectrometry assay.

RESULTS

Eight of the 10 patients invited completed the study. The median (range) 24-h urinary DHA excretion was 116 (75-289) mg at baseline, and 45 (13-112) mg after 14days of allopurinol therapy (P=0.036). At the end of the febuxostat treatment period, 4 patients had urinary DHA below detectable limits (<20ng/mL) compared with none of the participants following allopurinol treatment (P=0.036). The other 4 participants had a median 24-h urinary DHA excretion of 13.2 (10.0-13.4) mg at the completion of febuxostat therapy (P=0.036).

CONCLUSION

Urinary DHA excretion in APRT deficiency patients decreased with conventional doses of both allopurinol and febuxostat. Febuxostat was, however, significantly more efficacious than allopurinol in reducing DHA excretion in the prescribed doses. This finding, which may translate into improved outcomes of patients with APRT deficiency, should be confirmed in a larger sample.

Febuxostat for the Prevention of Recurrent 2,8-dihydroxyadenine Nephropathy due to Adenine Phosphoribosyltransferase Deficiency Following Kidney Transplantation

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disorder that results in irreversible renal damage due to 2,8-dihydroxyadenine (DHA) nephropathy. A 28-year-old man underwent living-related kidney transplantation for chronic kidney disease of unknown etiology. Numerous spherical brownish crystals observed in his urinary sediment on postoperative day 3 and were observed within the tubular lumen of renal allograft biopsy specimens on postoperative day 7. After a genetic diagnosis, febuxostat treatment was started on postoperative day 7, with the dosage gradually increased to 80 mg/day until complete the disappearance of 2,8-DHA crystals. Febuxostat prevented secondary 2,8-DHA nephropathy after kidney transplantation.

Adenine phosphoribosyltransferase deficiency in the United Kingdom: two novel mutations and a cross-sectional survey

Background

Adenine phosphoribosyltransferase deficiency is an inborn error of metabolism that can cause kidney disease from crystalline nephropathy or kidney stones.

Methods

We present three cases from a single centre with varied presentations to illustrate how increasing awareness led to better patient identification. We then undertook a cross-sectional survey of all the patients identified from the Purine Research Laboratory in the UK since 1974.

Results

Our index case presented with recurrent nephrolithiasis and was diagnosed on stone analysis, the second case presented with acute kidney injury and the third case was identified from a biopsy undertaken for acute on chronic kidney injury. Genetic studies identified two novel mutations. Twenty patients were retrospectively identified. The mean age at diagnosis was 25 years (range 2–70); eight were <20 years, seven were 20–40 years and five were >40 years. Five of the 20 patients were deceased, 3 after end-stage renal disease (ESRD). Twelve have normal renal function, one had CKD stage 3, one had severe kidney disease and one was on dialysis.

Conclusions

Adenine phosphoribosyltransferase deficiency presents in a wide spectrum in all age groups. Patients can be completely asymptomatic and kidney disease may be incorrectly attributed to other conditions. Outcome is poor in late diagnosis and there is a high prevalence of ESRD. Patients with unexplained renal stone disease or deterioration in kidney function should be considered for screening. Identification and surveillance of patients in the UK can improve. There is now a rare disease registry with meetings organized that include patients, families and health care providers to improve awareness.

Quantitative UPLC-MS/MS assay of urinary 2,8-dihydroxyadenine for diagnosis and management of adenine phosphoribosyltransferase deficiency

Adenine phosphoribosyltransferase (APRT) deficiency is a hereditary disorder that leads to excessive urinary excretion of 2,8-dihydroxyadenine (DHA), causing nephrolithiasis and chronic kidney disease. Treatment with allopurinol or febuxostat reduces DHA production and attenuates the renal manifestations. Assessment of DHA crystalluria by urine microscopy is used for therapeutic monitoring, but lacks sensitivity. We report a high-throughput assay based on ultra-performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS) for quantification of urinary DHA. The UPLC-MS/MS assay was optimized by a chemometric approach for absolute quantification of DHA, utilizing isotopically labeled DHA as an internal standard. Experimental screening was conducted with D-optimal design and optimization of the DHA response was performed with central composite face design and related to the peak area of DHA using partial least square regression. Acceptable precision and accuracy of the DHA concentration were obtained over a calibration range of 100 to 5000ng/mL on three different days. The intra- and inter-day accuracy and precision coefficients of variation were well within ±15% for quality control samples analyzed in replicates of six at three concentration levels. Absolute quantification of DHA in urine samples from patients with APRT deficiency was achieved wihtin 6.5min. Measurement of DHA in 24h urine samples from three patients with APRT deficiency, diluted 1:15 (v/v) with 10mM ammonium hydroxide (NH4OH), yielded a concentration of 3021, 5860 and 10563ng/mL and 24h excretion of 816, 1327 and 1649mg, respectively. A rapid and robust UPLC-MS/MS assay for absolute quantification of DHA in urine was successfully developed. We believe this method will greatly facilitate diagnosis and management of patients with APRT deficiency.

Obstructive uropathy and severe acute kidney injury from renal calculi due to adenine phosphoribosyltransferase deficiency

BACKGROUND

Adenine phosphoribosyltransferase (APRT) deficiency is an uncommon genetic cause of chronic kidney disease due to crystalline nephropathy.

METHODS

A case of a Chinese boy with APRT deficiency presenting with severe acute kidney injury secondary to obstructive uropathy from multiple renal calculi was reviewed.

RESULTS

The patient underwent staged removal of the calculi. Infrared spectrometry of the renal calculi showed 2,8-dihydroxyadenine. APRT deficiency was confirmed with abolished APRT enzyme activity in red blood cells. He was started on allopurinol and low purine diet with complete resolution of the residual calculi.

CONCLUSION

APRT deficiency should be considered in patients with multiple radiolucent renal calculi.

Kidney Disease in Adenine Phosphoribosyltransferase Deficiency

BACKGROUND

Adenine phosphoribosyltransferase (APRT) deficiency is a purine metabolism disorder causing kidney stones and chronic kidney disease (CKD). The course of nephrolithiasis and CKD has not been well characterized. The objective of this study was to examine long-term kidney outcomes in patients with APRT deficiency.

STUDY DESIGN

An observational cohort study.

SETTING & PARTICIPANTS

All patients enrolled in the APRT Deficiency Registry of the Rare Kidney Stone Consortium.

OUTCOMES

Kidney stones, acute kidney injury (AKI), stage of CKD, end-stage renal disease, estimated glomerular filtration rate (eGFR), and changes in eGFR.

MEASUREMENTS

Serum creatinine and eGFR calculated using creatinine-based equations.

RESULTS

Of 53 patients, 30 (57%) were females and median age at diagnosis was 37.0 (range, 0.6-67.9) years. Median duration of follow-up was 10.3 (range, 0.0-31.5) years. At diagnosis, kidney stones had developed in 29 (55%) patients and 20 (38%) had CKD stages 3 to 5, including 11 (21%) patients with stage 5. At latest follow-up, 33 (62%) patients had experienced kidney stones; 18 (34%), AKI; and 22 (42%), CKD stages 3 to 5. Of 14 (26%) patients with stage 5 CKD, 12 had initiated renal replacement therapy. Kidney stones recurred in 18 of 33 (55%) patients. The median eGFR slope was -0.38 (range, -21.99 to 1.42) mL/min/1.73m(2) per year in patients receiving treatment with an xanthine dehydrogenase inhibitor and -5.74 (range, -75.8 to -0.10) mL/min/1.73m(2) per year in those not treated prior to the development of stage 5 CKD (P=0.001).

LIMITATIONS

Use of observational registry data.

CONCLUSIONS

Progressive CKD and AKI episodes are major features of APRT deficiency, whereas nephrolithiasis is the most common presentation. Advanced CKD without a history of kidney stones is more prevalent than previously reported. Our data suggest that timely therapy may retard CKD progression.

Do kidney stone formers have a kidney disease?

Nephrolithiasis is a highly prevalent disorder affecting approximately one in eleven people and is associated with multiple complications including hypertension, cardiovascular disease, and chronic kidney disease. Significant epidemiologic associations with chronic kidney disease and ESRD have been noted and are reviewed herein, but debate persists in the literature as to whether kidney stone formation is a pathogenic process contributing to kidney disease. Corroborating evidence supporting the presence of kidney disease in stone formers includes the variability of renal function by stone type, the positive association of stone size with renal dysfunction, the presence of markers of renal injury in the urine of even asymptomatic stone formers, and direct evidence of renal tissue injury on histopathology. Proposed pathogenic mechanisms include recurrent obstruction and comorbid conditions such as recurrent urinary tract infections and structural abnormalities. Recent work evaluating the renal histopathology of different groups of stone formers adds further granularity, suggesting variability in mechanisms of renal injury by stone type and confirming the pathogenic effects of crystal formation. Genetic abnormalities leading to stone formation including cystinuria and primary hyperoxaluria, among others, contribute to the burden of disease in the stone-forming population.

Recurrence of crystalline nephropathy after kidney transplantation in APRT deficiency and primary hyperoxaluria

Purpose of review

To provide transplant physicians with a summary of the pathogenesis and diagnosis of adenine phosphoribosyl transferase (APRT) deficiency and primary hyperoxaluria and, focussed on kidney transplantation, and to discuss interventions aimed at preventing and treating the recurrence of crystalline nephropathy in renal transplant recipients.

Source of information

Pubmed literature search.

Setting

Primary hyperoxaluria and APRT deficiency are rare inborn errors of human metabolism. The hallmark of these diseases is the overproduction and urinary excretion of compounds (2,8 dihydroxyadenine in APRT deficiency, oxalate in primary hyperoxaluria) that form urinary crystals. Although recurrent urolithiasis represents the main clinical feature of these diseases, kidney injury can occur as a result of crystal precipitation within the tubules and interstitium, a condition referred to as crystalline nephropathy. Some patients develop end-stage renal disease (ESRD) and may become candidates for kidney transplantation. Since kidney transplantation does not correct the underlying metabolic defect, transplant recipients have a high risk of recurrence of crystalline nephropathy, which can lead to graft loss. In some instances, the disease remains undiagnosed until after the occurrence of ESRD or even after kidney transplantation.

Key messages

Patients with APRT deficiency or primary hyperoxaluria may develop ESRD as a result of crystalline nephropathy. In the absence of diagnosis and adequate management, the disease is likely to recur after kidney transplantation, which often leads to rapid loss of renal allograft function. Primary hyperoxaluria, but not APRT deficiency, becomes a systemic disease at low GFR with oxalate deposition leading to malfunction in non-renal organs (systemic oxalosis). We suggest that these diagnoses should be considered in patients with low glomerular filtration rate (GFR) and a history of kidney stones. In APRT deficiency, stones may be confused with uric acid stones, unless specialized techniques are used (infrared spectroscopy or X-ray crystallography for urinary crystals or stone analysis; Fourier transform infrared microscopy for crystals in kidney biopsy). Where these are unavailable, and for confirmation, the diagnosis can be made by measurement of enzyme activity in red blood cell lysates or by genetic testing. In patients with primary hyperoxaluria, levels of urinary and plasma oxalate; and the presence of nearly pure calcium oxalate monohydrate in stones, which often also have an unusually pale colour and unorganized structure, increase diagnostic suspicion. Molecular genetic testing is the criterion measure. Lifelong allopurinol therapy, with high fluid intake if appropriate, may stabilize kidney function in APRT deficiency; if ESRD has occurred or is near, results with kidney transplantation after initiation of allopurinol are excellent. In primary hyperoxaluria recognized before ESRD, pyridoxine treatment and high fluid intake may lead to a substantial decrease in urinary calcium oxalate supersaturation and prevent renal failure. In non-responsive patients or those recognized later in their disease, liver transplantation cures the underlying defect and should be considered when the GFR falls below 30 ml/min/1.73 m²; in those which or near ESRD, liver transplantation and intensive dialysis before kidney transplantation may be considered to reduce the total body oxalate burden before kidney transplantation.

Limitations

The availability of diagnostic tests varies between countries and centres. Data on long term outcomes after kidney transplantation are limited, especially for APRT deficiency patients.

Implications

Increasing transplant physicians knowledge of APRT deficiency and primary hyperoxaluria should enable them to implement adequate diagnostic and therapeutic interventions, thereby achieving good outcomes after kidney transplantation.

Common and rare variants associated with kidney stones and biochemical traits

Kidney stone disease is a complex disorder with a strong genetic component. We conducted a genome-wide association study of 28.3 million sequence variants detected through whole-genome sequencing of 2,636 Icelanders that were imputed into 5,419 kidney stone cases, including 2,172 cases with a history of recurrent kidney stones, and 279,870 controls. We identify sequence variants associating with kidney stones at ALPL (rs1256328[T], odds ratio (OR)=1.21, P=5.8 × 10⁻¹⁰) and a suggestive association at CASR (rs7627468[A], OR=1.16, P=2.0 × 10⁻⁸). Focusing our analysis on coding sequence variants in 63 genes with preferential kidney expression we identify two rare missense variants SLC34A1 p.Tyr489Cys (OR=2.38, P=2.8 × 10⁻⁵) and TRPV5 p.Leu530Arg (OR=3.62, P=4.1 × 10⁻⁵) associating with recurrent kidney stones. We also observe associations of the identified kidney stone variants with biochemical traits in a large population set, indicating potential biological mechanism.

Kidney stone formation is influenced by genetic factors and recurrent stone formation places a significant burden on health care systems. Here Oddsson et al. perform a large-scale genome-wide association study and uncover new genetic variants associated with kidney stone susceptibility and associated biochemical traits.

2,8-Dihydroxyadenine urolithiasis: a not so rare inborn error of purine metabolism

Adenine phosphoribosyltransferase (APRT) deficiency is a rare inherited metabolic disorder that leads to the formation and hyperexcretion of 2,8-dihydroxyadenine (DHA) into urine. The low solubility of DHA results in precipitation and formation of urinary crystals and kidney stones. The disease can be present as recurrent urolithiasis or nephropathy secondary to crystal precipitation into renal parenchyma (DHA nephropathy). The diagnostic tools available, including stone analysis, crystalluria, and APRT activity in red blood cells, make the diagnosis easy to confirm when APRT deficiency is suspected. However, the lack of recognition of this metabolic disorder frequently resulted in a delay in diagnosis and treatment with grave consequences. The early recognition and treatment of APRT deficiency are of crucial importance to prevent irreversible loss of renal function. This review summarizes the genetic and metabolic mechanisms underlying DHA stones formation and chronic kidney disease, along with the issues of diagnosis and management of APRT deficiency. Moreover, we report the mutations in the APRT gene responsible for APRT deficiency in 51 French patients (43 families) including 22 pediatric cases (18 families) among the 64 patients identified in the biochemistry laboratories of Necker Hospital, Paris (1978-2013).

Adenine phosphoribosyltransferase deficiency: Leave no stone unturned

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disease leading to generation of large amounts of 2,8-dihydroxyadenine (DHA). DHA is excreted in urine, where it precipitates into crystals due to its low solubility. DHA crystals can aggregate into stones or cause injury to the renal parenchyma (DHA nephropathy). Recurrent urolithiasis and DHA nephropathy are the two clinical manifestations of APRT deficiency. Diagnosis of APRT deficiency can be made during childhood as well as adulthood. Diagnosis mainly relies on the recognition of DHA in stones or urine crystals. Measurement of APRT activity and genetic testing are useful for confirmation of diagnosis, for family screening and should be considered in difficult cases of urolithiasis or crystalline nephropathy. Allopurinol therapy is the cornerstone of treatment and is highly effective in preventing recurrence of stones and kidney disease. High fluid intake and dietary modifications are also recommended. Early diagnosis and treatment are of paramount importance to prevent renal damage. Unfortunately, diagnosis of APRT deficiency is often overlooked and irreversible renal failure still occurs in a substantial proportion of patients. Clinicians must be alert to the possibility of APRT deficiency and consider the appropriate diagnostic tests in certain cases. This review discusses the genetic and biochemical mechanisms of APRT deficiency, and the issues of diagnosis and management.

Recurrent 2,8-dihydroxyadenine nephropathy: a rare but preventable cause of renal allograft failure

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive enzyme defect of purine metabolism that usually manifests as 2,8-dihydroxyadenine (2,8-DHA) nephrolithiasis and more rarely chronic kidney disease. The disease is most often misdiagnosed and can recur in the renal allograft. We analyzed nine patients with recurrent 2,8-DHA crystalline nephropathy, in all of whom the diagnosis had been missed prior to renal transplantation. The diagnosis was established at a median of 5 (range 1.5-312) weeks following the transplant procedure. Patients had delayed graft function (n=2), acute-on-chronic (n=5) or acute (n=1) allograft dysfunction, whereas one patient had normal graft function at the time of diagnosis. Analysis of allograft biopsies showed birefringent 2,8-DHA crystals in renal tubular lumens, within tubular epithelial cells and interstitium. Fourier transformed infrared microscopy confirmed the diagnosis in all cases, which was further supported by 2,8-DHA crystalluria, undetectable erythrocyte APRT enzyme activity, and genetic testing. With allopurinol therapy, the allograft function improved (n=7), remained stable (n=1) or worsened (n=1). At last follow-up, two patients had experienced allograft loss and five had persistent chronic allograft dysfunction. 2,8-DHA nephropathy is a rare but underdiagnosed and preventable disorder that can recur in the renal allograft and may lead to allograft loss.

Adenine phosphoribosyltransferase (APRT) deficiency: identification of a novel nonsense mutation

Background

Adenine phosphoribosyltransferase deficiency (APRTD) is an under estimated genetic form of kidney stones and/or kidney failure, characterized by intratubular precipitation of 2,8-dihydroxyadenine crystals (2,8-DHA). Currently, five pathologic allelic variants have been identified as responsible of the complete inactivation of APRT protein.

Case presentation

In this study, we report a novel nonsense mutation of the APRT gene from a 47- year old Italian patient. The mutation, localized in the exon 5, leads to the replacement of a cytosine with a thymine (g.2098C > T), introducing a stop codon at amino acid position 147 (p.Gln147X).

This early termination was deleterious for the enzyme structural and functional integrity, as demonstrated by the structure analysis and the activity assay of the mutant APRT protein.

Conclusion

These data revealed that the p.Gln147X mutation in APRT gene might be a new cause of APRT disease.

An APRT mutation is strongly associated with and likely causative for 2,8-dihydroxyadenine urolithiasis in dogs

2,8-Dihydroxyadenine (2,8-DHA) urolithiasis in people is caused by autosomal recessive mutations in the adenine phosphoribosyltransferase gene (APRT). 2,8-DHA urolithiasis has recently been reported in two dogs, but, to the authors' knowledge, no studies have yet investigated the genetic basis for susceptibility to the development of 2,8-DHA urolithiasis in this species. Our aim was to sequence APRT in dogs affected by 2,8-DHA urolithiasis and compare the results to clinically healthy dogs of similar ancestral lineages. Our hypothesis was that we would identify an autosomal recessive mutation in APRT that is associated with the disease. The case population consisted of six dogs with a history of 2,8-DHA urolithiasis: five Native American Indian Dogs (NAIDs) and a mixed breed. The control population consisted of adult NAIDs with no history of urolithiasis. We sequenced APRT and identified a missense mutation in a highly conserved codon of APRT (c.260G>A; p.Arg87Gln). The c.260A mutation was present in a homozygous state in all six dogs with 2,8-DHA urolithiasis, and it was strongly associated with the disease. This exact missense mutation has been previously reported to cause loss of APRT enzyme function in a human cell line, and it is likely a causative mutation in dogs. Therefore, the dog offers a naturally-occurring genetic animal model for 2,8-DHA urolithiasis.

Adenine phosphoribosyltransferase deficiency as a rare cause of renal allograft dysfunction

Adenine phosphoribosyltransferase deficiency is a rare autosomal recessive disorder manifesting as urolithiasis or crystalline nephropathy. It leads to the generation of large amounts of poorly soluble 2,8-dihydroxyadenine excreted in urine, yielding kidney injury and in some patients, kidney failure. Early recognition of the disease, institution of xanthine analog therapy to block the formation of 2,8-dihydroxyadenine, high fluid intake, and low purine diet prevent CKD. Because of symptom variability and lack of awareness, however, the diagnosis is sometimes extremely deferred. We describe a patient with adenine phosphoribosyltransferase deficiency who was diagnosed during evaluation of a poorly functioning second kidney allograft. This report highlights the risk of renal allograft loss in patients with undiagnosed adenine phosphoribosyltransferase deficiency and the need for improved early detection of this disease.

Hereditary causes of kidney stones and chronic kidney disease

Adenine phosphoribosyltransferase (APRT) deficiency, cystinuria, Dent disease, familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), and primary hyperoxaluria (PH) are rare but important causes of severe kidney stone disease and/or chronic kidney disease in children. Recurrent kidney stone disease and nephrocalcinosis, particularly in pre-pubertal children, should alert the physician to the possibility of an inborn error of metabolism as the underlying cause. Unfortunately, the lack of recognition and knowledge of the five disorders has frequently resulted in an unacceptable delay in diagnosis and treatment, sometimes with grave consequences. A high index of suspicion coupled with early diagnosis may reduce or even prevent the serious long-term complications of these diseases. In this paper, we review the epidemiology, clinical features, diagnosis, treatment, and outcome of patients with APRT deficiency, cystinuria, Dent disease, FHHNC, and PH, with an emphasis on childhood manifestations.

Triamterene crystalline nephropathy

Medications can cause a tubulointerstitial insult leading to acute kidney injury through multiple mechanisms. Acute tubular injury, a dose-dependent process, occurs due to direct toxicity on tubular cells. Acute interstitial nephritis characterized by interstitial inflammation and tubulitis develops from drugs that incite an allergic reaction. Other less common mechanisms include osmotic nephrosis and crystalline nephropathy. The latter complication is rare but has been associated with several drugs, such as sulfadiazine, indinavir, methotrexate, and ciprofloxacin. Triamterene crystalline nephropathy has been reported only rarely, and its histologic characteristics are not well characterized. We report 2 cases of triamterene crystalline nephropathy, one of which initially was misdiagnosed as 2,8-dihydroxyadenine crystalline nephropathy.

When to suspect a genetic disorder in a patient with renal stones, and why

Nephrolithiasis is a common disorder, with a rising prevalence in the general population. Its pathogenesis is still unclear, but a role for genetics has long been recognized, especially in cases of the more common calcium nephrolithiasis. Although relatively rare, monogenic causes of hypercalciuria and nephrolithiasis do exist and their timely recognition is important from a prognostic and therapeutic viewpoint. This article reviews the clinical and laboratory findings characterizing inherited causes of nephrolithiasis with a view to helping clinicians to recognize and manage these rare conditions.

Kidney stones and kidney function loss: a cohort study

OBJECTIVE

To investigate whether the presence of kidney stones increase the risk of end stage renal disease (ESRD) or other adverse renal outcomes.

DESIGN

A registry cohort study using validated algorithms based on claims and facility utilisation data. Median follow-up of 11 years.

SETTING

Alberta, Canada, between 1997 and 2009.

PARTICIPANTS

3,089,194 adult patients without ESRD at baseline or a history of pyelonephritis. Of these, 1,954,836 had outpatient serum creatinine measurements and were included in analyses of chronic kidney disease and doubling of serum creatinine level.

EXPOSURE

One or more kidney stones during follow-up.

MAIN OUTCOME MEASURES

Incident ESRD, development of stage 3b-5 chronic kidney disease (estimated glomerular filtration rate <45 mL/min/1.73 m(2)), and sustained doubling of serum creatinine concentration from baseline.

RESULTS

23,706 (0.8%) patients had at least one kidney stone, 5333 (0.2%) developed ESRD, 68,525 (4%) developed stage 3b-5 chronic kidney disease, and 6581 (0.3%) experienced sustained doubling of serum creatinine. Overall, one or more stone episodes during follow-up was associated with increased risk of ESRD (adjusted hazard ratio 2.16 (95% CI 1.79 to 2.62)), new stage 3b-5 chronic kidney disease (hazard ratio 1.74 (1.61 to 1.88)), and doubling of serum creatinine (hazard ratio 1.94 (1.56 to 2.43)), all compared with those without kidney stones during follow-up. The excess risk of adverse outcomes associated with at least one episode of stones seemed greater in women than in men, and in people aged <50 years than in those aged ≥ 50. However, the risks of all three adverse outcomes in those with at least one episode of stones were significantly higher than in those without stones in both sexes and age strata. The absolute increase in the rate of adverse renal outcomes associated with stones was small: the unadjusted rate of ESRD was 2.48 per million person days in people with one or more episodes of stones versus 0.52 per million person days in people without stones.

CONCLUSION

Even a single kidney stone episode during follow-up was associated with a significant increase in the likelihood of adverse renal outcomes including ESRD. However, the increases were small in absolute terms.

Adenine phosphoribosyltransferase deficiency

Complete adenine phosphoribosyltransferase (APRT) deficiency is a rare inherited metabolic disorder that leads to the formation and hyperexcretion of 2,8-dihydroxyadenine (DHA) into urine. The low solubility of DHA results in precipitation of this compound and the formation of urinary crystals and stones. The disease can present as recurrent urolithiasis or nephropathy secondary to crystal precipitation into renal parenchyma (DHA nephropathy). The diagnostic tools available-including stone analysis, crystalluria, and APRT activity measurement-make the diagnosis easy to confirm when APRT deficiency is suspected. However, the disease can present at any age, and the variability of symptoms can present a diagnostic challenge to many physicians. The early recognition and treatment of APRT deficiency are of crucial importance for preventing irreversible loss of renal function, which still occurs in a non-negligible proportion of cases. This review summarizes the genetic and metabolic mechanisms underlying stone formation and renal disease, along with the diagnosis and management of APRT deficiency.

Adenine phosphoribosyltransferase deficiency in children

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disorder characterized by 2,8-dihydroxyadenine (2,8-DHA) crystalluria that can cause nephrolithiasis and chronic kidney disease. The aim of our study was to assess the clinical presentation, diagnosis, and outcome of APRT deficiency in a large pediatric cohort. All pediatric cases of APRT deficiency confirmed at the same French reference laboratories between 1978 and 2010 were retrospectively reviewed. Twenty-one patients from 18 families were identified. The median age at diagnosis was 3 years. Diagnosis was made after one or more episodes of nephrolithiasis (17 patients), after urinary tract infection (1 patient), and by family screening (3 patients). The diagnosis was based on stone analysis and microscopic examination of urine and/or enzymatic determination of APRT on red blood cells. All children had null APRT enzyme activity in erythrocytes. APRT gene sequencing was performed on 18 patients, revealing six homozygous and 12 compound heterozygous mutations. At diagnosis, half of the patients had decreased kidney function, and two children presented with acute renal failure. Allopurinol treatment was given to all patients at a median dose of 9 mg/kg/day. After a median follow-up of 5 years, all patients showed stabilization or improvement of kidney function, normal growth and development, and six patients had recurrence of nephrolithiasis. Based on these results, we conclude that an excellent outcome can be achieved in children with APRT deficiency who receive the proper treatment.

Adenine phosphoribosyltransferase deficiency: an underdiagnosed cause of lithiasis and renal failure

We describe an infant affected by adenine phosphoribosyltransferase (APRT) deficiency diagnosed at 18 months of age with a de novo mutation that has not been previously reported. APRT deficiency is a rare defect of uric acid catabolism that leads to the accumulation of 2,8 dihydroxyadenine (2,8-DHA), a highly insoluble substance excreted by the kidneys that may precipitate in urine and form stones. The child suffered from renal colic due to a stone found in the peno-scrotal junction of the bulbar urethra. Stone spectrophotometric analysis allowed us to diagnose the disease and start kidney-saving therapy in order to avoid irreversible chronic kidney damage. APRT deficiency should always be considered in the differential diagnosis of pediatric urolithiasis.

Genetic determinants of urolithiasis

Urolithiasis affects approximately 10% of individuals in Western societies by the seventh decade of life. The most common form, idiopathic calcium oxalate urolithiasis, results from the interaction of multiple genes and their interplay with dietary and environmental factors. To date, considerable progress has been made in identifying the metabolic risk factors that predispose to this complex trait, among which hypercalciuria predominates. The specific genetic and epigenetic factors involved in urolithiasis have remained less clear, partly owing to the candidate gene and linkage methods that have been available until now, being inherently low in their power of resolution and in assessing modest effects in complex traits. However, together with investigations of rare, Mendelian forms of urolithiasis associated with various metabolic risk factors, these methods have afforded insights into biological pathways that seem to underlie the development of stones in the urinary tract. Monogenic diseases account for a greater proportion of stone formers in children and adolescents than in adults. Early diagnosis of monogenic forms of urolithiasis is of importance owing to associated renal injury and other potentially treatable disease manifestations, but diagnosis is often delayed because of a lack of familiarity with these rare disorders. In this Review, we will discuss advances in the understanding of the genetics underlying polygenic and monogenic forms of urolithiasis.