Recurrent EIARF and PRES with severe renal hypouricemia by compound heterozygous SLC2A9 mutation

Recurrent exercise-induced acute kidney injury associated with hypouricemia: a case report and literature review

BACKGROUND

Hereditary renal hypouricemia (RHUC) is a heterogenous disorder characterized by defective uric acid (UA) reabsorption resulting in hypouricemia and increased fractional excretion of UA. RHUC is an important cause of exercise-induced acute kidney injury (EIAKI), nephrolithiasis and posterior reversible encephalopathy syndrome (PRES). We present here an unusual case of a patient with RHUC who presented with recurrent EIAKI and had two heterozygous mutations in the SLC2A9 gene.

CASE PRESENTATION

A 43-year old man was admitted to our clinic because of bilateral loin pain, nausea and sleeplessness for 3 days after strenuous exercise. The laboratory results revealed increased levels of blood urea nitrogen (BUN) (15 mmol/l) and serum creatinine (Scr) (450 μmol/l), while the UA level was extremely low at 0.54 mg/dl, and his fractional excretion of urate (FE-UA) was 108%. The patient had an episode of acute kidney injury after playing soccer approximately 20 years ago, and on routine physical examination, his UA was less than 0.50 mg/dl. In view of the marked hypouricemia and high FE-UA, a diagnosis of RHUC was suspected, which led us to perform mutational screening of the SLC22A12 and SLC2A9 genes. DNA sequencing revealed no mutation in SLC22A12 gene, but two heterozygous mutations in the SLC2A9 gene.

CONCLUSIONS

This is a rare report of a patient with RHUC2 due to the mutation of SLC2A9. And this unique symptom of EIAKI and decreased or normal serum concentrations of UA warrant more attention as an early cue of RHUC.

New SLC22A12 (URAT1) Variant Associated with Renal Hypouricemia Identified by Whole-Exome Sequencing Analysis and Bioinformatics Predictions

Renal hypouricemia (RHUC) is a rare hereditary disorder caused by loss-of-function mutations in the SLC22A12 (RHUC type 1) or SLC2A9 (RHUC type 2) genes, encoding urate transporters URAT1 and GLUT9, respectively, that reabsorb urate in the renal proximal tubule. The characteristics of this disorder are low serum urate levels, high renal fractional excretion of urate, and occasional severe complications such as nephrolithiasis and exercise-induced acute renal failure. In this study, we report two Spanish (Caucasian) siblings and a Pakistani boy with clinical characteristics compatible with RHUC. Whole-exome sequencing (WES) analysis identified two homozygous variants: a novel pathogenic SLC22A12 variant, c.1523G>A; p.(S508N), in the two Caucasian siblings and a previously reported SLC2A9 variant, c.646G>A; p.(G216R), in the Pakistani boy. Our findings suggest that these two mutations cause RHUC through loss of urate reabsorption and extend the SLC22A12 mutation spectrum. In addition, this work further emphasizes the importance of WES analysis in clinical settings.

Hyperuricemia, Gout, and the Brain-an Update

PURPOSE OF REVIEW

This review aims to summarize recent evidence regarding the complex relationship between uric acid (UA), gout, and brain diseases.

RECENT FINDINGS

Observational studies have suggested that patients with hyperuricemia or gout might have a decreased risk of neurodegenerative diseases. Conversely, they may be at increased risk of cerebrovascular disease. Mendelian randomization (MR) studies use a genetic score as an instrumental variable to address the causality of the association between a risk factor (here, UA or gout) and an outcome. So far, MR analyses do not support a causal relationship of UA or gout with Alzheimer's disease and dementia, and of UA with Parkinson's disease or stroke. Observation studies indicate a U-shaped association between UA and brain diseases, but MR studies do not support that this association is causal. Further studies should address the causal role of gout as well as the impact of urate-lowering therapy on these outcomes.

Identification of two novel heterozygous SLC2A9 mutations in a Chinese woman and review of literature

OBJECTIVE

This study is aimed to describe the clinical and genetic characteristics of a Chinese woman diagnosed with renal hypouricemia type 2 (RHUC2). We also summarize the advances in research on RHUC2 by reviewing related literature.

METHODS

We measured clinical parameters of a 57-year-old female and performed whole-exome sequencing to screen for mutations. Human embryonic kidney 293 cells were transiently transfected with plasmids containing wild-type or mutants. Relative mRNA quantification was determined by real-time quantitative reverse transcription polymerase chain reaction (RT-qPCR).

RESULTS

This patient was diagnosed with diabetes and coronary heart disease. In addition, a decrease in 24-hour urinary chloride was observed. Two novel heterozygous variants of SLC2A9 (NM_020041.2): c.682-2_682-1insC and c.267C > G (p.Y89X) were identified. The mini-gene splicing assay revealed that c.682-2_682-1insC variant resulted in a frameshift mutation p. E228PfsX23. There was a statistically significant difference in mRNA expression level between the two mutants and the wild-type.

CONCLUSIONS

These findings strongly suggest that the two novel mutations are the causative agents of RHUC2. In particular, our findings provide further insights into the function of SLC2A9 and mechanisms of the complications.

Non-urate transporter 1, non-glucose transporter member 9-related renal hypouricemia and acute renal failure accompanied by hyperbilirubinemia after anaerobic exercise: a case report

Background

Renal hypouricemia (RHUC) is an inherited heterogenous disorder caused by faulty urate reabsorption transporters in the renal proximal tubular cells. Anaerobic exercise may induce acute kidney injury in individuals with RHUC that is not caused by exertional rhabdomyolysis; it is called acute renal failure with severe loin pain and patchy renal ischemia after anaerobic exercise (ALPE).

RHUC is the most important risk factor for ALPE. However, the mechanism of onset of ALPE in patients with RHUC has not been elucidated. The currently known genes responsible for RHUC are SLC22A12 and SLC2A9.

Case presentation

A 37-year-old man presented with loin pain after exercising. Despite having a healthy constitution from birth, biochemical examination revealed hypouricemia, with a uric acid (UA) level of < 1 mg/dL consistently at every health check. We detected acute kidney injury, with a creatinine (Cr) level of 4.1 mg/dL, and elevated bilirubin; hence, the patient was hospitalized. Computed tomography revealed no renal calculi, but bilateral renal swelling was noted. Magnetic resonance imaging detected cuneiform lesions, indicating bilateral renal ischemia. Fractional excretion values of sodium and UA were 0.61 and 50.5%, respectively. Urinary microscopy showed lack of tubular injury. The patient’s older sister had hypouricemia. The patient was diagnosed with ALPE. Treatment with bed rest, fluid replacement, and nutrition therapy improved renal function and bilirubin levels, and the patient was discharged on day 5. Approximately 1 month after onset of ALPE, his Cr, UA, and TB levels were 0.98, 0.8, and 0.9 mg/dL, respectively. We suspected familial RHUC due to the hypouricemia and family history and performed genetic testing but did not find the typical genes responsible for RHUC. A full genetic analysis was opposed by the family.

Conclusions

To the best of our knowledge, this is the first report of ALPE with hyperbilirubinemia. Bilirubin levels may become elevated as a result of heme oxygenase-1 activation, occurring in exercise-induced acute kidney injury in patients with RHUC; this phenomenon suggests renal ischemia-reperfusion injury. A new causative gene coding for a urate transporter may exist, and its identification would be useful to clarify the urate transport mechanism.

Amplicon targeted resequencing for SLC2A9 and SLC22A12 identified novel mutations in hypouricemia subjects

Background

To identify potential causative mutations in SLC2A9 and SLC22A12 that lead to hypouricemia or hyperuricemia (HUA).

Methods

Targeted resequencing of whole exon regions of SLC2A9 and SLC22A12 was performed in three cohorts of 31 hypouricemia, 288 HUA and 280 normal controls.

Results

A total of 84 high‐quality variants were identified in these three cohorts. Eighteen variants were nonsynonymous or in splicing region, and then included in the following association analysis. For common variants, no significant effects on hypouricemia or HUA were identified. For rare variants, six single nucleotide variations (SNVs) p.T21I and p.G13D in SLC2A9, p.W50fs, p.Q382L, p.V547L and p.E458K in SLC22A12, occurred in totally six hypouricemia subjects and were absent in HUA and normal controls. Allelic and genotypic frequency distributions of the six SNVs differed significantly between the hypouricemia and normal controls even after multiple testing correction, and p.G13D in SLC2A9 and p.V547L in SLC22A12 were newly reported. All these mutations had no significant effects on HUA susceptibility, while the gene‐based analyses substantiated the significant results on hypouricemia.

Conclusion

Our study first presents a comprehensive mutation spectrum of hypouricemia in a large Chinese cohort.

Uric acid and cognitive decline: a double-edge sword?

PURPOSE OF REVIEW

This narrative review aims to highlight recent findings on the relation between uric acid level and cognitive decline or dementia.

RECENT FINDINGS

The antioxidant properties of uric acid, which have supported the hypothesis that uric acid may be neuroprotective, have been questioned by preclinical data. Studies investigating the relation between serum uric acid (SUA) level and Alzheimer disease are mostly cross-sectional, and results are often inconclusive. Similarly, data for an association between uric acid level and cognitive performance are inconsistent. There is some evidence that low SUA level might be associated with Parkinson disease, but studies are limited by methodological heterogeneity and risk of bias. Patients with gout may have decreased risk for Alzheimer disease, but the impact of treatment is unclear. Recent data suggest an increased risk of vascular dementia with high SUA level via increased cerebrovascular burden in older patients. The relation between SUA level and neurologic disorders may be U-shaped.

SUMMARY

We lack strong evidence for an association between low SUA level and cognitive decline over time. Conversely, high SUA level might increase the cerebrovascular burden and the risk of vascular dementia; physicians should continue to treat hyperuricemia when appropriate.

Multiple Membrane Transporters and Some Immune Regulatory Genes are Major Genetic Factors to Gout

Gout is a common form of inflammatory arthritis caused by hyperuricemia and the deposition of Monosodium Urate (MSU) crystals. It is also considered as a complex disorder in which multiple genetic factors have been identified in association with its susceptibility and/or clinical outcomes. Major genes that were associated with gout include URAT1, GLUT9, OAT4, NPT1 (SLC17A1), NPT4 (SLC17A3), NPT5 (SLC17A4), MCT9, ABCG2, ABCC4, KCNQ1, PDZK1, NIPAL1, IL1β, IL-8, IL-12B, IL-23R, TNFA, MCP-1/CCL2, NLRP3, PPARGC1B, TLR4, CD14, CARD8, P2X7R, EGF, A1CF, HNF4G and TRIM46, LRP2, GKRP, ADRB3, ADH1B, ALDH2, COMT, MAOA, PRKG2, WDR1, ALPK1, CARMIL (LRRC16A), RFX3, BCAS3, CNIH-2, FAM35A and MYL2-CUX2. The proteins encoded by these genes mainly function in urate transport, inflammation, innate immunity and metabolism. Understanding the functions of gout-associated genes will provide important insights into future studies to explore the pathogenesis of gout, as well as to develop targeted therapies for gout.

A heterozygous variant in the SLC22A12 gene in a Sri Lanka family associated with mild renal hypouricemia

Background

Renal hypouricemia is a rare heterogeneous inherited disorder characterized by impaired tubular uric acid transport, reabsorption insufficiency and /or acceleration of secretion. The affected individuals are predisposed to nephrolithiasis and recurrent episodes of exercise-induced acute kidney injury. Type 1 is caused by dysfunctional variants in the SLC22A12 gene (URAT1), while type 2 is caused by defects in the SLC2A9 gene (GLUT9). To date, more than 150 patients with the loss-of-function mutations for the SLC22A12 gene have been found (compound heterozygotes and/or homozygotes), most of whom are Japanese and Koreans.

Case presentation

Herein, we report a nine year old Sri Lankan boy with renal hypouricemia (serum uric acid 97 μmol/L, fractional excretion of uric acid 33%).The sequencing analysis of SLC22A12 revealed a potentially deleterious missense variant c.1400C > T (p.T467 M, rs200104135) in heterozygous state. This variant has been previously identified in homozygous and/or compound heterozygous state with other causative SLC22A12 variant c.1245_1253del (p.L415_G417del) in Roma population.

Conclusions

This is the first identification of a family with mild renal hypouricemia1 associated to the p.T467 M variant. Detailed investigations of urate blood and urine concentrations in patients with unexplained hypouricemia are needed and renal hypouricemia should also be considered in patients other than those from Japan and/or Korea. Our finding confirms an uneven geographical and ethnic distribution of Romany prevalent SLC22A12 variant that need to be considered in Asian patients (population data Genome Aggregation Database: allele frequency in South Asia 0.007055, in East Asia 0.001330).

Physiology of Hyperuricemia and Urate-Lowering Treatments

Gout is the most common form of inflammatory arthritis and is a multifactorial disease typically characterized by hyperuricemia and monosodium urate crystal deposition predominantly in, but not limited to, the joints and the urinary tract. The prevalence of gout and hyperuricemia has increased in developed countries over the past two decades and research into the area has become progressively more active. We review the current field of knowledge with emphasis on active areas of hyperuricemia research including the underlying physiology, genetics and epidemiology, with a focus on studies which suggest association of hyperuricemia with common comorbidities including cardiovascular disease, renal insufficiency, metabolic syndrome and diabetes. Finally, we discuss current therapies and emerging drug discovery efforts aimed at delivering an optimized clinical treatment strategy.

URAT1 and GLUT9 mutations in Spanish patients with renal hypouricemia

BACKGROUND

Renal hypouricemia (RHUC), a rare inherited disorder characterized by impaired uric acid (UA) reabsorption in the proximal tubule, is caused by mutations in SLC22A12 or SLC2A9. Most mutations have been identified in Japanese patients, and only a few have been detected in Europeans.

METHODS

We report clinical, biochemical and genetics findings of fourteen Spanish patients, six Caucasians and eight of Roma ethnia, diagnosed with idiopathic RHUC. Two of the patients presented exercise-induced acute renal failure and another one had several episodes of nephrolithiasis and four of them had progressive deterioration of renal function, while the rest were asymptomatic.

RESULTS

Molecular analysis revealed SLC22A12 mutations in ten of the patients, and SLC2A9 mutations in the other four. A new heterozygous SLC22A12 missense mutation, c.1427C>A (p.A476D), was identified in two affected members of the same family. The rest of the patients presented homozygous, heterozygous or compound heterozygous mutations that have been previously identified in patients with RHUC; SLC22A12 p.T467M and p.L415_G417del, and SLC2A9 p.T125M. Expression studies in Xenopus oocytes revealed that c.1427C>A reduced UA transport but did not alter the location of URAT1 protein on the plasma membrane.

CONCLUSIONS

The biochemical and clinical features of our patients together with the genetic analysis results confirmed the diagnosis of RHUC. This is the first report describing SLC22A12 and SLC2A9 mutations in Spanish patients.

A novel homozygous GLUT9 mutation cause recurrent exercise-induced acute renal failure and posterior reversible encephalopathy syndrome

Renal hypouricemia (RHU) is an autosomal recessive hereditary disease characterized by impaired renal urate reabsorption and subsequent profound hypouricemia. There are two types of RHU, type 1 and type 2, caused by the loss-of-function mutation of SLC22A12 and SLC2A9 genes, respectively. RHU predisposes affected people to exercise-induced acute renal failure (EIARF), posterior reversible encephalopathy syndrome (PRES) and nephrolithiasis. A Chinese patient had experienced three episodes of EIARF and one episode of PRES. The investigations showed profound hypouricemia and significantly increased renal excretion of UA. Cranial magnetic resonance imaging showed communicating hydrocephalus. Renal biopsy displayed interlobular artery intimal thickening with reduction of lumen and acute tubulointerstitial injury. The mutational analysis revealed a homozygous splice-site mutation in the SLC2A9 gene encoding glucose transporter 9. The patient was diagnosed as RHU type 2 caused by a loss-of-function mutation of the SLC2A9 gene. Consequently, he was strictly prohibited from strenuous exercise. During the 5-year follow-up, EIARF and PRES never recurred. Strenuous exercise may induce systemic (including renal and cerebrovascular) vasoconstriction eventually resulting in EIARF and PRES in patients with RHU. To our knowledge, this is the first report of a homozygous splice-site mutation in the SLC2A9 gene, renal arteriolar chronic lesion, concurrence of RHU and communicating hydrocephalus.

Recurrent exercise-induced acute kidney injury by idiopathic renal hypouricemia with a novel mutation in the SLC2A9 gene and literature review

Background

Idiopathic renal hypouricemia (iRHUC) is an autosomal recessive hereditary disorder, characterized by impaired tubular uric acid transport, re-absorption insufficiency and/or the acceleration of secretions. Some patients present with severe complications, such as exercise-induced acute kidney injury (EIAKI) and nephrolithiasis.

Case presentation

Herein, we report the case of a girl with severe iRHUC (serum urate 0.05 mg/dL, fractional excretion of uric acid 295.99%) associated with recurrent EIAKI, in whom the disease was caused by a homozygous mutation (g.68G > A in exon 3) in the SLC2A9 gene. Her family members (father, mother and brother) carried the same mutation but were heterozygous, without any signs of severe hypouricemia.

Conclusions

Our findings indicate that iRHUC is a rare disorder but that it should also be considered in patients with EIAKI, especially in those patients who manifest with moderately elevated or normal serum concentrations of uric acid during the acute phase of AKI. Mutational screening of the SLC2A9 gene is necessary for the diagnosis of iRHUC, and homozygous mutations of the SLC2A9 alleles can cause severe hypouricemia. Careful attention should be paid to any signs of hypouricemia during the recovery phase of AKI and long-term follow-up.

Urate transporters in the genomic era

PURPOSE OF REVIEW

Recent advances in genome technology have provided us with a list of molecules affecting urate handling in humans, many of which are unlikely to be identified through traditional physiological approach alone. Although this article is focused on urate, this can be viewed as a successful model of genomics-physiology collaboration.

RECENT FINDINGS

URATv1/GLUT9 (SLC2A9) is shown to play a critical role in urate reabsorption at the proximal tubule, probably more prominent than its partner URAT1 (SLC22A12). The major site of action of ABCG2 (ABCG2), an influential urate secretion transporter, has been shown to be the intestine rather than the kidney proximal tubule. Accordingly, hypofunction of ABCG2 leads to increased fractional excretion of urate, a finding traditionally interpreted as overproduction hyperuricemia. Some SLC17 family members secrete urate in the kidney or intestine. OAT2 (SLC22A7) may take up urate from blood to the proximal tubular cell. In addition, how a common single-nucleotide polymorphisms in ABCG2 affects its function has been elucidated.

SUMMARY

A finer grained picture of urate handling in the human body is now emerging, which will help choosing novel targets for urate-lowering therapy.

Effect of pegloticase on renal function in patients with chronic kidney disease: a post hoc subgroup analysis of 2 randomized, placebo-controlled, phase 3 clinical trials

BACKGROUND

Pegloticase is approved in the US for treatment of refractory chronic gout. Since chronic kidney disease (CKD) is common in these patients, we conducted a post-hoc analysis of 2 replicate phase 3 trials and the subsequent open-label extension study to determine the effects of pegloticase on renal function in patients with CKD stages 3 and 4, as well as the effects of renal dysfunction on pegloticase efficacy and safety.

FINDINGS

Patients with renal insufficiency were randomized to pegloticase 8 mg every 2 weeks (n = 42), pegloticase 8 mg every 4 weeks (n = 41), or placebo (n = 20) for 6 months as defined by the study protocols. Renal function was assessed by estimated glomerular filtration rate (eGFR). All patients completing the randomized trials could participate in an open-label extension study for a further 2.5 years. Uric acid response, the primary end point in the trials, was plasma uric acid <6.0 mg/dl for 80% of months 3 and 6.Mean eGFR in both pegloticase dosing cohorts remained constant over the randomized treatment phase and long-term open-label extension study. The number of patients achieving uric acid response was similar across CKD stages (32% stage 1, 23% stage 2, 35% stage 3, and 39% stage 4, respectively, P = 0.3). There was no difference in the pegloticase safety profile based on CKD stage.

CONCLUSIONS

Pegloticase treatment does not impact eGFR in CKD patients and response to pegloticase is independent of CKD stage.

TRIAL REGISTRATION

Clinical trial identifier: NCT00325195.

Recurrent exercise-induced acute renal failure in a young Pakistani man with severe renal hypouricemia and SLC2A9 compound heterozygosity

Background

Familial renal hypouricemia (RHUC) is a hereditary disease characterized by hypouricemia, high renal fractional excretion of uric acid (FE-UA) and can be complicated by acute kidney failure and nephrolithiasis. Loss-of-function mutations in the SLC22A12 gene cause renal hypouricemia type 1 (RHUC1), whereas renal hypouricemia type 2 (RHUC2) is caused by mutations in the SLC2A9 gene.

Case presentation

We describe a 24-year-old Pakistani man who was admitted twice to our hospital for severe exercise-induced acute renal failure (EIARF), abdominal pain and fever; he had very low serum UA levels (0.2 mg/dl the first time and 0.09 mg/dl the second time) and high FE-UA (200% and 732% respectively), suggestive of RHUC. Mutational analyses of both urate transporters revealed a new compound heterozygosity for two distinct missense mutations in the SLC2A9 gene: p.Arg380Trp, already identified in heterozygosity, and p.Gly216Arg, previously found in homozygosity or compound heterozygosity in some RHUC2 patients. Compared with previously reported patients harbouring these mutations, our proband showed the highest FE-UA levels, suggesting that the combination of p.Arg380Trp and p.Gly216Arg mutations most severely affects the renal handling of UA.

Conclusions

The clinical and molecular findings from this patient and a review of the literature provide new insights into the genotype-phenotype correlation of this disorder, supporting the evidence of an autosomal recessive inheritance pattern for RHUC2. Further investigations into the functional properties of GLUT9, URAT1 and other urate transporters are required to assess their potential research and clinical implications.

Posterior reversible encephalopathy syndrome with exercise-induced acute kidney injury in renal hypouricemia type 1

Renal hypouricemia type 1 is caused by mutations in the SLC22A12 gene, whereas type 2 is caused by defects in the SLC2A9 gene. Although both subtypes predispose to exercise-induced acute kidney injury (EIAKI), posterior reversible encephalopathy syndrome (PRES) occurring with this disorder is an uncommon phenomenon that has only been reported to date in a patient with renal hypouricemia type 2. We describe a 13-year-old boy with renal hypouricemia type 1 (serum uric acid, 0.9 mg/dL) with a homozygous W258X mutation in the SLC22A12 gene, presenting with EIAKI and PRES. On admission, his body weight was 61 kg (11 kg above the dry weight), and blood pressure was 153/88 mmHg. Cranial magnetic resonance imaging revealed high-intensity areas in the cortical and subcortical white matter of the occipital lobe. After admission, the patient responded well to a combination of hemodialysis and intravenous nicardipine. This is the first case of concurrent PRES and EIAKI in a patient with renal hypouricemia type 1. We suggest that PRES is not due to severe hypouricemia caused by SLC2A9 mutation but is a manifestation of severe EIAKI associated with renal hypouricemia.

Molecular background of urate transporter genes in patients with exercise-induced acute kidney injury

BACKGROUND/AIMS

Exercise-induced acute renal failure [exercise-induced acute kidney injury (EI-AKI)] is defined as AKI due to heavy anaerobic exercise. Although hypouricemia is known to be a risk factor for the onset of EI-AKI, a direct causal link between EI-AKI and serum uric acid has not been established. This study aimed to analyze urate transporter genes in patients with EI-AKI and its molecular mechanism.

METHODS

Genomic DNA and total RNA were isolated from peripheral blood leukocytes of patients with a history of EI-AKI. Mutations were analyzed by PCR and a direct sequencing method. We first analyzed the SLC22A12 gene, and then the SLC2A9 gene if no mutations were found in SLC22A12.

RESULTS

Seventeen patients were enrolled in this study and 16 had mutations: 15 in SLC22A12 and 1 in SLC2A9. Fourteen (82.4%) patients showed hypouricemia, and all of the patients with hypouricemia had either homozygous or compound heterozygous mutations in SLC22A12 or SLC2A9, which confirmed that all of them had renal hypouricemia. Two patients had heterozygous mutations of SLC22A12, and they were not accompanied by hypouricemia. One patient was found to have no mutations in SLC22A12 or SLC2A9.

CONCLUSION

We were able to determine the genetic background of urate transporter genes in patients with EI-AKI. Decreased function of urate transporters, rather than decreased serum uric acid levels, may be of great importance for the onset of EI-AKI.

Novel allelic variants and evidence for a prevalent mutation in URAT1 causing renal hypouricemia: biochemical, genetics and functional analysis

Renal hypouricemia (RHUC) is a heterogeneous inherited disorder characterized by impaired tubular uric acid (UA) transport with severe complications, such as acute kidney injury (AKI). Type 1 is caused by a loss-of-function mutation in the SLC22A12 gene (URAT1), type 2 in the SLC2A9 gene (GLUT9). This article describes three Czech families with RHUC type 1. The serum UA in the probands was 0.9, 1.1 and 0.5 mg/dl and expressed as an increase in the fractional excretion of UA (48, 43 and 39%). The sequencing analysis of SLC22A12 revealed three novel variants: p.G366R, p.T467M and a deletion p.L415_G417del. A detailed metabolic investigation in proband C for progressive visual failure supported suspicion of neuronal ceroid lipofuscinosis type 7 conditioned by the mutation in the MFSD8 gene. Functional studies showed significantly decreased urate uptake and a mis-localized URAT1 signal in p.G366R, p.L415_G417del and p.T467M. Furthermore, colocalization studies showed accumulation of URAT1 protein in the endoplasmic reticulum. The findings suggest that loss-of-function mutations cause RHUC via loss of UA absorption partly by protein misfolding. However, they do not necessarily lead to AKI and a possible genotype-phenotype correlation was not proposed. Furthermore, results confirm an uneven geographical and ethnic distribution of SLC22A12 variants; the p.L415_G417del mutation predominates in the Roma ethnic group in the Czech Republic.

Clinical characteristics of acute renal failure with severe loin pain and patchy renal vasoconstriction

BACKGROUND

Acute renal failure (ARF) with severe loin pain and patchy renal vasoconstriction (PRV) is a syndrome presenting with sudden loin pain after anaerobic exercise. We aimed to investigate the clinical characteristics and the efficacy of diagnostic imaging studies of patients with this syndrome.

METHODS

We retrospectively selected 17 patients with ARF accompanied by loin or abdominal pain who showed multiple patchy wedge-shaped delayed contrast enhancements on a computerized tomography scan. Information about the clinical characteristics, including the nature of pain and combined symptoms, suspected causes, such as exercise, drug or alcohol intake, and renal hypouricemia, and the results of laboratory and imaging tests were gathered.

RESULTS

The mean age of patients with episodes of ARF accompanied by loin pain was 23.0±6.5 (range 16-35) years old. Pain was mainly located in the loin (70.6%) or abdominal area (76.5%) and continued for approximately 3.5±4.0 days. Exercise was suspected as a primary cause of disease in 12 (70.6%) patients. Maximal serum creatinine was 5.42±3.16 (1.4-12.1) mg/dL 3.1±1.8 (1-7) days after the onset of pain. The peak level of serum uric acid was 9.41±2.91 (6.0-15.8) mg/dL. All of the patients recovered to near-normal renal function, and one patient showed hypouricemia after recovery.

CONCLUSION

ARF with severe loin pain and PRV can present with loin or abdominal pain, even without a history of anaerobic exercise. Careful history taking and appropriate imaging studies are critical in the diagnosis and management of this syndrome.

Acute kidney injury in two children caused by renal hypouricaemia type 2

BACKGROUND

Renal hypouricaemia is a heterogeneous inherited disorder characterized by impaired tubular uric acid transport with severe complications, such as acute kidney injury and nephrolithiasis. Type 1 is caused by a loss-of-function mutation in the SLC22A12 gene (OMIM #220150), while type 2 is caused by defects in the SLC2A9 gene (OMIM #612076).

CASE-DIAGNOSIS/TREATMENT

The cases of two children, a 12- and a 14-year-old boy with acute kidney injury (proband 1: urea 9.4 mmol/l, creatinine 226 μmol/l; proband 2: urea 11.7 mmol/l, creatinine 202 μmol/l) are described. Both are offspring of nonconsanguineous couples in the UK. The concentrations of serum uric acid were consistently below the normal range (0.03 and 0.04 mmol/l) and expressed as an increase in the fractional excretion of uric acid (46 and 93 %).

CONCLUSIONS

A sequencing analysis of the coding region of uric acid transporters SLC22A12 and SLC2A9 was performed. Analysis of genomic DNA revealed two unpublished missense transitions, p.G216R and p.N333S in the SLC2A9 gene. No sequence variants in SLC22A12 were found. Our findings suggest that homozygous and/or compound heterozygous loss-of-function mutations p.G216R and p.N333S cause renal hypouricaemia via loss of uric acid absorption and do lead to acute kidney injury.