High frequency of SLC22A12 variants causing renal hypouricemia 1 in the Czech and Slovak Roma population; simple and rapid detection method by allele-specific polymerase chain reaction

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.

Pathogenic Variants of SLC22A12 (URAT1) and SLC2A9 (GLUT9) in Spanish Patients with Renal Hypouricemia: Founder Effect of SLC2A9 Variant c.374C>T; p.(T125M)

Renal hypouricemia (RHUC) is a rare inherited disorder characterized by impaired urate reabsorption in the proximal tubule resulting in low urate serum levels and increased urate excretion. Some patients may present severe complications such as exercise-induced acute renal failure and nephrolithiasis. RHUC is caused by inactivating mutations in the SLC22A12 (RHUC type 1) or SLC2A9 (RHUC type 2) genes, which encode urate transporters URAT1 and GLUT9, respectively. In this study, our goal was to identify mutations associated with twenty-one new cases with RHUC through direct sequencing of SLC22A12 and SLC2A9 coding exons. Additionally, we carried out an SNPs-haplotype analysis to determine whether the rare SLC2A9 variant c.374C>T; p.(T125M), which is recurrent in Spanish families with RHUC type 2, had a common-linked haplotype. Six intragenic informative SNPs were analyzed using PCR amplification from genomic DNA and direct sequencing. Our results showed that ten patients carried the SLC22A12 mutation c.1400C>T; p.(T467M), ten presented the SLC2A9 mutation c.374C>T, and one carried a new SLC2A9 heterozygous mutation, c.593G>A; p.(R198H). Patients carrying the SLC2A9 mutation c.374C>T share a common-linked haplotype, confirming that it emerged due to a founder effect.

Hypouricemia and Urate Transporters

Hypouricemia is recognized as a rare disorder, defined as a serum uric acid level of 2.0 mg/dL or less. Hypouricemia is divided into an overexcretion type and an underproduction type. The former typical disease is xanthinuria, and the latter is renal hypouricemia (RHUC). The frequency of nephrogenic hypouricemia due to a deficiency of URAT1 is high in Japan, accounting for most asymptomatic and persistent cases of hypouricemia. RHUC results in a high risk of exercise-induced acute kidney injury and urolithiasis. It is vital to promote research on RHUC, as this will lead not only to the elucidation of its pathophysiology but also to the development of new treatments for gout and hyperuricemia.

Genetic Association Between SLC22A12 Variants and Susceptibility to Hyperuricemia: A Meta-Analysis

Aims: Gout is a form of inflammatory arthritis characterized by the deposition of monosodium urate crystals. An important risk factor for gout is hyperuricemia. The relationship between SLC22A12 gene variants and the susceptibility to hyperuricemia has been reported, but these findings have been inconsistent. Thus, we aimed to assess the relationship between SLC22A12 gene variants and hyperuricemia susceptibility through a meta-analysis. Methods: The meta-analysis was performed by searching PubMed, Embase, Web of Science, and Chinese National Knowledge Infrastructure (CNKI) databases. The relationship between hyperuricemia risk and the SLC22A12 rs11602903, rs524023, rs3825018, rs3825016, rs11231825, rs7932775, rs893006, and rs475688 variants was assessed by odds ratios and 95% confidence intervals. Results: In total, 20 eligible publications with 4817 cases and 6819 controls were included in the meta-analysis. Hyperuricemia risk was significantly associated with the SLC22A12 alleles rs3825018, rs7932775, and rs475688 under both the dominant and recessive models and with rs3825016 under the allelic and dominant models. Conclusions: Under the allelic model SLC22A12 rs3825018 and rs3825016 were risk factors for hyperuricemia and gout as was rs7932775 under dominant and recessive models, while the SLC22A12 rs475688 was protective against hyperuricemia under both dominant and recessive models.

Renal Hypouricemia 1: Rare Disorder as Common Disease in Eastern Slovakia Roma Population

Renal hypouricemia (RHUC) is caused by an inherited defect in the main reabsorption system of uric acid, SLC22A12 (URAT1) and SLC2A9 (GLUT9). RHUC is characterized by a decreased serum uric acid concentration and an increase in its excreted fraction. Patients suffer from hypouricemia, hyperuricosuria, urolithiasis, and even acute kidney injury. We report clinical, biochemical, and genetic findings in a cohort recruited from the Košice region of Slovakia consisting of 27 subjects with hypouricemia and relatives from 11 families, 10 of whom were of Roma ethnicity. We amplified, directly sequenced, and analyzed all coding regions and exon–intron boundaries of the SLC22A12 and SLC2A9 genes. Sequence analysis identified dysfunctional variants c.1245_1253del and c.1400C>T in the SLC22A12 gene, but no other causal allelic variants were found. One heterozygote and one homozygote for c.1245_1253del, nine heterozygotes and one homozygote for c.1400C>T, and two compound heterozygotes for c.1400C>T and c.1245_1253del were found in a total of 14 subjects. Our result confirms the prevalence of dysfunctional URAT1 variants in Roma subjects based on analyses in Slovak, Czech, and Spanish cohorts, and for the first time in a Macedonian Roma cohort. Although RHUC1 is a rare inherited disease, the frequency of URAT1-associated variants indicates that this disease is underdiagnosed. Our findings illustrate that there are common dysfunctional URAT1 allelic variants in the general Roma population that should be routinely considered in clinical practice as part of the diagnosis of Roma patients with hypouricemia and hyperuricosuria exhibiting clinical signs such as urolithiasis, nephrolithiasis, and acute kidney injury.

The impact of an URAT1 polymorphism on the losartan treatment of hypertension and hyperuricemia

BACKGROUND

This study was designed to evaluate the impact of polymorphisms in the urate transporter 1 (URAT1) gene on the uricosuric action of losartan therapy in hypertensive patients suffering from hyperuricemia.

METHODS

A MassARRAY approach was used to detect single nucleotide polymorphism (SNP) loci in the URAT1 and CYP2C9 genes (16 and 2 loci, respectively) in 111 patients with hypertension and hyperuricemia taking losartan and in 121 healthy controls. In addition, we compared serum urate (SUA) levels and other key clinical biochemistry indices between these two patient groups.

RESULTS

We detected significant differences between the two patient groups with respect to age, SUA, urea, creatine, triglycerides, high-density lipoprotein, low-density lipoprotein, and fasting plasma glucose (all p < 0.05). In addition, we found that hypertensive patients with hyperuricemia were more likely to exhibit the rs3825016(C/T) (36.9% vs 21.5%, p = 0.03), and we determined that a 2-week treatment course with losartan was associated with significant decreases in SUA values (p < 0.001).

CONCLUSION

Our findings indicate that the URAT1 rs3825016 polymorphism may influence the uricosuric action of losartan.

A Proposal for Practical Diagnosis of Renal Hypouricemia: Evidenced from Genetic Studies of Nonfunctional Variants of URAT1/SLC22A12 among 30,685 Japanese Individuals

BACKGROUND

Renal hypouricemia (RHUC) is characterized by a low serum uric acid (SUA) level and high fractional excretion of uric acid (FE_UA). Further studies on FE_UA in hypouricemic individuals are needed for a more accurate diagnosis of RHUC.

METHODS

In 30,685 Japanese health-examination participants, we genotyped the two most common nonfunctional variants of URAT1 (NFV-URAT1), W258X (rs121907892) and R90H (rs121907896), in 1040 hypouricemic individuals (SUA ≤ 3.0 mg/dL) and 2240 individuals with FE_UA data. The effects of NFV-URAT1 on FE_UA and SUA were also investigated using linear and multiple regression analyses.

RESULTS

Frequency of hypouricemic individuals (SUA ≤ 3.0 mg/dL) was 0.97% (male) and 6.94% (female) among 30,685 participants. High frequencies of those having at least one allele of NFV-URAT1 were observed in 1040 hypouricemic individuals. Furthermore, NFV-URAT1 significantly increased FE_UA and decreased SUA, enabling FE_UA and SUA levels to be estimated. Conversely, FE_UA and SUA data of hypouricemic individuals are revealed to be useful to predict the number of NFV-URAT1.

CONCLUSIONS

Our findings reveal that specific patterns of FE_UA and SUA data assist with predicting the number of nonfunctional variants of causative genes for RHUC, and can also be useful for practical diagnosis of RHUC even before genetic tests.

Genetic-epidemiological analysis of hypouricemia from 4,993 Japanese on nonfunctional variants of URAT1/SLC22A12 gene

OBJECTIVES

Up to 0.3% of Japanese have hypouricemia. Most cases appear to result from a hereditary disease, renal hypouricemia (RHUC), which causes exercise-induced acute kidney injury and urolithiasis. However, to what extent RHUC accounts for hypouricemia is not known. We therefore investigated its frequency and evaluated its risks by genotyping a general Japanese population.

METHODS

A cohort of 4,993 Japanese was examined by genotyping the nonfunctional variants R90H (rs121907896) and W258X (rs121907892) of URAT1/SLC22A12, the two commonest causative variants of RHUC in Japanese.

RESULTS

Participants' fractional excretion of uric acid and risk allele frequencies markedly increased at lower SUA levels. Ten participants (0.200%) had a serum uric acid (SUA) level of ≤ 2.0 mg/dl and nine had R90H or W258X, likely to have RHUC. Logistic regression analysis revealed these URAT1 variants to be significantly and independently associated with the risk of hypouricemia and mild hypouricemia (SUA ≤ 3.0 mg/dl) as well as sex, age, and BMI, but these URAT1 variants were the only risks in the hypouricemia population (SUA ≤ 2.0 mg/dl). W258X was only the risk in males with SUA of ≤ 3.0 mg/dl.

CONCLUSION

Our study accurately reveals the prevalence of RHUC and provides genetic evidence for its definition (SUA ≤ 2.0 mg/dl). We also show that individuals with SUA of ≤ 3.0 mg/dl, especially males, are prone to RHUC. Our findings will help to promote a better epidemiological understanding of RHUC as well as more accurate diagnosis, especially in males with mild hypouricemia.

Substantial anti-gout effect conferred by common and rare dysfunctional variants of URAT1/SLC22A12

Objectives

Gout, caused by chronic elevation of serum uric acid levels, is the commonest form of inflammatory arthritis. The causative effect of common and rare variants of ATP-binding cassette transporter G2 (ABCG2/BCRP) on gout risk has been studied, but little attention has been paid to the effect of common (rs121907892, p.W258X) and rare variants of urate transporter 1 (URAT1/SLC22A12) on gout, despite dysfunctional variants of URAT1 having been identified as pathophysiological causes of renal hypouricaemia.

Methods

To address this important but overlooked issue, we investigated the effects of these URAT1 variants on gout susceptibility, using targeted exon sequencing on 480 clinically defined gout cases and 480 controls of Japanese males in combination with a series of functional analyses of newly identified URAT1 variants.

Results

Our results show that both common and rare dysfunctional variants of URAT1 markedly decrease the risk of gout (OR 0.0338, reciprocal OR 29.6, P = 7.66 × 10⁻⁸). Interestingly, we also found that the URAT1-related protective effect on gout eclipsed the ABCG2-related causative effect (OR 2.30–3.32). Our findings reveal only one dysfunctional variant of URAT1 to have a substantial anti-gout effect, even in the presence of causative variants of ABCG2, a ‘gout gene’.

Conclusion

Our findings provide a better understanding of gout/hyperuricaemia and its aetiology that is highly relevant to personalized health care. The substantial anti-gout effect of common and rare variants of URAT1 identified in the present study support the genetic concept of a ‘Common Disease, Multiple Common and Rare Variant’ model.

Roma Ethnicity and Sex-Specific Associations of Serum Uric Acid with Cardiometabolic and Hepatorenal Health Factors in Eastern Slovakian Population: The HepaMeta Study

Background: Health characteristics associated with uric acid (UA) in the Roma minority remain less well known. The study sought to determine the ethnicity- and sex-specific associations of serum UA with health factors in Eastern Slovakian Roma and non-Roma populations. Methods: Data from the comparative cross-sectional HepaMeta study conducted in Slovakia in 2011 were used. The study enrolled 452 Roma subjects (35.2% men) and 403 non-Roma individuals (45.9% men) aged 18-55 years. Results: All study parameters differed between the sexes in both the Roma and non-Roma participants (p < 0.05). UA was related to sex with odds ratio for female sex 0.873, 95% CI 0.853-0.893 (p < 0.0001) per 10-unit increase of UA. Average level of UA ± standard deviation was lower in Roma than in non-Roma (226.54 ± 79.8 vs. 259.11 ± 84.53 umol/L; p < 0.0001). The Roma population presented with greater levels of high-sensitivity C-reactive protein (hsCRP) (3.07 ± 4 mg/L vs. 1.98 ± 2.83 mg/L; p < 0.0001) and ferritin in Roma males (403.78 ± 391.84 vs. 302.67 ± 236.26 mg/L; p < 0.0001). Conclusions: Serum UA is sex- and ethnicity specific. Elevated levels of hsCRP and ferritin particularly in Roma males can reflect low-grade systemic inflammation and thus serve as a marker of an increased cardiovascular risk.

Polygenic analysis of the effect of common and low-frequency genetic variants on serum uric acid levels in Korean individuals

Increased serum uric acid (SUA) levels cause gout and are associated with multiple diseases, including chronic kidney disease. Previous genome-wide association studies (GWAS) have identified more than 180 loci that contribute to SUA levels. Here, we investigated genetic determinants of SUA level in the Korean population. We conducted a GWAS for SUA in 6,881 Korean individuals, calculated polygenic risk scores (PRSs) for common variants, and validated the association of low-frequency variants and PRS with SUA levels in 3,194 individuals. We identified two low-frequency and six common independent variants associated with SUA. Despite the overall similar effect sizes of variants in Korean and European populations, the proportion of variance for SUA levels explained by the variants was greater in the Korean population. A rare, nonsense variant SLC22A12 p.W258X showed the most significant association with reduced SUA levels, and PRSs of common variants associated with SUA levels were significant in multiple Korean cohorts. Interestingly, an East Asian-specific missense variant (rs671) in ALDH2 displayed a significant association on chromosome 12 with the SUA level. Further genetic epidemiological studies on SUA are needed in ethnically diverse cohorts to investigate rare or low-frequency variants and determine the influence of genetic and environmental factors on SUA.

A population-specific low-frequency variant of SLC22A12 (p.W258*) explains nearby genome-wide association signals for serum uric acid concentrations among Koreans

Prolonged hyperuricemia is a cause of gout and an independent risk factor for chronic health conditions including diabetes and chronic kidney diseases. Genome-wide association studies (GWASs) for serum uric acid (SUA) concentrations have repeatedly confirmed genetic loci including those encoding uric acid transporters such as ABCG2 and SLC9A2. However, many single nucleotide polymorphisms (SNPs) found in GWASs have been common variants with small effects and unknown functions. In addition, there is still much heritability to be explained. To identify the causative genetic variants for SUA concentrations in Korean subjects, we conducted a GWAS (1902 males) and validation study (2912 males and females) and found four genetic loci reaching genome-wide significance on chromosomes 4 (ABCG2) and 11 (FRMD8, EIF1AD and SLC22A12-NRXN2). Three loci on chromosome 11 were distributed within a distance of 1.3 megabases and they were in weak or moderate linkage disequilibrium (LD) states (r² = 0.02–0.68). In a subsequent association analysis on the GWAS loci of chromosome 11 using closely positioned markers derived from whole genome sequencing data, the most significant variant to be linked with the nearby GWAS signal was rs121907892 (c.774G>A, p.W258*) of the SLC22A12 gene. This variant, and each of the three GWAS SNPs, were in LD (r² = 0.06–0.32). The strength of association of SNPs with SUA concentration (negative logarithm of P-values) and the genetic distance (r² of LD) between rs121907892 and the surrounding SNPs showed a quantitative correlation. This variant has been found only in Korean and Japanese subjects and is known to lower the SUA concentration in the general population. Thus, this low-frequency variant, rs121907892, known to regulate SUA concentrations in previous studies, is responsible for the nearby GWAS signals.

Coexistence of autosomal dominant polycystic kidney disease type 1 and hereditary renal hypouricemia type 2: A model of early-onset and fast cyst progression

Autosomal dominant polycystic kidney disease (ADPKD) is a heterogeneous inherited disease characterized by renal and extrarenal manifestations with progressive fluid-filled cyst development leading to end-stage renal disease. The rate of disease progression in ADPKD exhibits high inter- and intrafamilial variability suggesting involvement of modifier genes and/or environmental factors. Renal hypouricemia (RHUC) is an inherited disorder characterized by impaired tubular uric acid transport with severe complications, such as acute kidney injury and chronic kidney disease (CKD). However, the two disorders have distinct and well-delineated genetic, biochemical, and clinical findings. Only a few cases of coexistence of ADPKD and RHUC (type 1) in a single individual have been reported. We report a family with two members: an ADPKD 24-year-old female which presented bilateral renal cysts in utero and hypouricemia since age 5, and her mother with isolated hypouricemia. Next-generation sequencing identified two mutations in two genes PKD1 and SLC2A9 in this patient and one isolated SLC2A9 mutation in her mother, showing RHUC type 2, associated to CKD. The coexistence of these two disorders provides evidence of SLC2A9 variant could act as a modifier change, with synergistic actions, that could promote cystogenesis and rapid ADPKD progression. This is the first case of coexistence of PKD1 and SLC2A9 mutations treated with tolvaptan.

Donor-derived hypouricemia in irrelevant recipients caused by kidney transplantation

Background

Hereditary renal hypouricemia (HRH) is a genetically heterogenetic disease. Patients with HRH are almost asymptomatic; but some may experience exercise-induced acute kidney injury (EAKI) and nephrolithiasis which may bring concerns regarding the risk-benefit ratio as marginal kidney donors. This study examined the pathogenic mutations of hypouricemia in two recipients after receiving kidney transplantation, providing preliminary evidence for the mechanism of hypouricemia.

Methods

Two participants underwent detailed biochemical examinations. DNA and RNA were extracted from transplant specimens for sequencing. The whole-genome sequencing and polymerase chain reaction (PCR) amplification were performed to confirm the pathogenic genes. Functional effects of mutant proteins were verified by bioinformatics analysis. RNA-sequencing (RNA-seq) was used to study the transcriptome of hypouricemia.

Results

Both of the recipients had the low serum uric acid (UA) (45–65 µmol/l), high fraction excretion of UA (44% and 75%) and an increase in the UA clearance (35.9 and 73.3 mL/min) with a functioning graft. The sequencing analyses revealed 7 kinds of potential mutational genes in this case, two novel mutations p.R89H and p.L181V in SLC22A12 gene which were revealed by bioinformatics could be pathogenic in nature.

Conclusions

Two novel mutations of SLC22A12 were identified. Preliminary functional analysis revealed a potential deleterious effect of these mutations in the grafts derived from the donor and sequencing analysis expand the molecular mechanisms of renal hypouricemia.

Hypouricemia: what the practicing rheumatologist should know about this condition

We presented an update in the field of hypouricemia, which is defined as a serum urate concentration of < 2 mg/dL (119 μmol/L), for the practicing rheumatologist, who usually is the consulting physician in cases of disorders of urate metabolism. We performed a narrative review through a literature search for original and review articles in the field of human hypouricemia published between January 1950 and July 2018. We divided the etiology of hypouricemia into two main categories: those associated with a decrease in urate production and those promoting the elimination of urate via the kidneys. The most common conditions associated with these categories are discussed. Furthermore, the etiology of hypouricemia may be associated with certain medications prescribed by the practicing rheumatologists, such as the following: urate-lowering drugs (allopurinol and febuxostat); recombinant uricase (pegloticase); uricosuric agents (probenecid, benzbromarone); urate transporter URAT1 inhibitor (lesinurad); angiotensin II receptor blocker (losartan); fenofibrate; high-dose trimethoprim-sulfamethoxazole; some NSAID; and high-dose salicylate therapy. The rheumatologist is considered an expert in the metabolism of urate and its associated pathological conditions. Therefore, specialists must recognize hypouricemia as a biomarker of various pathological and potentially harmful conditions, highlighting the importance of conducting a deeper clinical investigation to reach a more accurate diagnosis and treatment.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Clinical and Functional Characterization of a Novel URAT1 Dysfunctional Variant in a Pediatric Patient with Renal Hypouricemia

Renal hypouricemia (RHUC) is caused by an inherited defect in the main (reabsorptive) renal urate transporters, URAT1 and GLUT9. RHUC is characterized by decreased concentrations of serum uric acid and an increase in its excretion fraction. Patients suffer from hypouricemia, hyperuricosuria, urolithiasis, and even acute kidney injury. We report the clinical, biochemical, and genetic findings of a pediatric patient with hypouricemia. Sequencing analysis of the coding region of SLC22A12 and SLC2A9 and a functional study of a novel RHUC1 variant in the Xenopus expression system were performed. The proband showed persistent hypouricemia (67–70 µmol/L; ref. range 120–360 µmol/L) and hyperuricosuria (24–34%; ref. range 7.3 ± 1.3%). The sequencing analysis identified common non-synonymous allelic variants c.73G > A, c.844G > A, c.1049C > T in the SLC2A9 gene and rare variants c.973C > T, c.1300C > T in the SLC22A12 gene. Functional characterization of the novel RHUC associated c.973C > T (p. R325W) variant showed significantly decreased urate uptake, an irregular URAT1 signal on the plasma membrane, and reduced cytoplasmic staining. RHUC is an underdiagnosed disorder and unexplained hypouricemia warrants detailed metabolic and genetic investigations. A greater awareness of URAT1 and GLUT9 deficiency by primary care physicians, nephrologists, and urologists is crucial for identifying the disorder.

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.

Long-term effects of the SLC2A9 G844A and SLC22A12 C246T variants on serum uric acid concentrations in children

BACKGROUND

We evaluated the effects of two single-nucleotide polymorphisms on UA concentrations in the first decade of life using repeated-measures data.

METHODS

We included all subjects who were followed-up at least once and for whom we had both UA and genotypic data (i.e., 375, 204, 307, and 363 patients aged 3, 5, 7, and 9 years, respectively). All participated in the Ewha Birth and Growth Cohort study. We used a mixed model analysis to estimate the longitudinal association of serum UA concentration due to the rs3825017 (SLC22A12 c. 246C > T) and rs16890979 (SLC2A9 c. 844G > A) genotypes.

RESULTS

Overall, the tracking coefficient of UA concentrations in children 3 to 9 years of age was 0.31, and was higher in boys than in girls (0.34 vs. 0.29, respectively). Regarding individual variance, serum UA concentrations decreased as age increased (β = - 0.07, p < 0.05), but there were no significant differences by sex. The effects of rs3825017 on UA concentration were significant in boys, but not in girls. Boys with the T allele of rs3825017 had higher concentrations than their counterparts regardless of the time of follow-up. The rs16890979 genotypes were not significantly associated with serum UA concentration in either sex.

CONCLUSION

This study showed that rs3825017 in the SLC22A12 gene was associated with UA concentration in childhood.

Renal hypouricemia caused by novel compound heterozygous mutations in the SLC22A12 gene: a case report with literature review

Background

Renal hypouricemia (RHUC) is a heterogeneous genetic disorder that is characterized by decreased serum uric acid concentration and increased fractional excretion of uric acid. Previous reports have revealed many functional mutations in two urate transporter genes, SLC22A12 and/or SLC2A9, to be the causative genetic factors of this disorder. However, there are still unresolved patients, suggesting the existence of other causal genes or new mutations. Here, we report an RHUC patient with novel compound heterozygous mutations in the SLC22A12 gene.

Case presentation

A 27-year-old female presenting with recurrent hypouricemia during routine checkups was referred to our hospital. After obtaining the patient’s consent, both the patient and her healthy parents were analyzed using whole-exome sequencing (WES) and Sanger sequencing to discover and validate causal mutations, respectively. The prioritization protocol of WES screened out two mutations of c.269G > A/p.R90H and c.1289_1290insGG/p.M430fsX466, which are both located in the SLC22A12 gene, in the patient. Sanger sequencing further confirmed that the patient’s heterozygous c.269G > A/p.R90H mutation, which has been reported previously, derived from her mother, and the heterozygous c.1289_1290insGG/p.M430fsX466 mutation, which was found for the first time, derived from her father. p.R90H, which is highly conserved among different species, may decrease the stability of this domain and was considered to be almost damaging in silicon analysis. p.M430fsX466 lacks the last three transmembrane domains, including the tripeptide motif (S/T)XΦ (X = any amino acid and Φ = hydrophobic residue), at the C-terminal, which interact with scaffolding protein PDZK1 and thus will possibly lead to weak functioning of urate transport through the disruption of the “transporter complex” that is formed by URAT1 and PDZK1.

Conclusions

We report a Chinese patient with RHUC, which was caused by compound heterozygous mutations of the SLC22A12 gene, using WES and Sanger sequencing for the first time. Mutation-induced structural instability or malfunction of the urate transporter complex may be the main mechanisms for this hereditary disorder.

Electronic supplementary material

The online version of this article (10.1186/s12881-018-0595-8) contains supplementary material, which is available to authorized users.

Serum Uric Acid in Roma and Non-Roma-Its Correlation with Metabolic Syndrome and Other Variables

Background: The Roma population is one of the major marginalized groups in Europe, having higher incidence of all spectrums of disease and a shorter life expectancy. Yet, the reasons for higher morbidity and its exact prevalence were not properly studied. Objectives: The objective of our study was to compare the frequency of metabolic syndrome (MetS) in Roma people to the non-Roma population in Slovakia, and to compare levels of uric acid and its correlation with components of metabolic syndrome. Methods: A group of 452 Roma people aged 18–55 years, was compared to a control group of 403 non-Roma people. The data were obtained by questionnaire, anthropometric measures, and analyzed blood and urine samples Results: The prevalence of MetS was significantly higher among Roma participants (131; 29.6%) compared with non-Roma participants (80; 20.1%), p = 0.001. Roma people significantly more often fulfilled obesity and low high-density lipoprotein (HDL) criteria of MetS (257, 58.9% vs. 180, 45.8%, p < 0.0001, and 312, 70.0% vs. 140, 34.9%, p < 0.0001). There was no difference in the triacylglycerols (TG), glycemia or blood pressure (BP) criteria of MetS. The Roma also presented with greater levels of high sensitivity C-reactive protein (hs-CRP). Baseline levels of uric acid (UA) among the Roma population were significantly lower compared with the majority population (226.54 ± 79.8 vs. 259.11 ± 84.53) (p < 0.001). The levels of UA significantly correlated with fulfilled criteria of MetS. Univariate regression showed that UA is a significant predictor of MetS in the whole cohort (unadjusted odds ratio (OR) 1.005; 95% CI 1.004–1.007; p < 0.0001) also after the adjustment for age, sex, and ethnicity (adjusted OR 1.008; 95% CI 1.005–1.010; p < 0.0001). Conclusions: We were able to show that prevalence of MetS among the Roma is higher than in the majority population. Moreover, the uric acid levels are significantly lower in the Roma group as well as when it comes to a cohort with MetS. Levels of UA, besides others, depend on ethnicity, age, and sex.

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).

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.

Urate levels predict survival in amyotrophic lateral sclerosis: Analysis of the expanded Pooled Resource Open-Access ALS clinical trials database

INTRODUCTION

Urate has been identified as a predictor of amyotrophic lateral sclerosis (ALS) survival in some but not all studies. Here we leverage the recent expansion of the Pooled Resource Open-Access ALS Clinical Trials (PRO-ACT) database to study the association between urate levels and ALS survival.

METHODS

Pooled data of 1,736 ALS participants from the PRO-ACT database were analyzed. Cox proportional hazards regression models were used to evaluate associations between urate levels at trial entry and survival.

RESULTS

After adjustment for potential confounders (i.e., creatinine and body mass index), there was an 11% reduction in risk of reaching a survival endpoint during the study with each 1-mg/dL increase in uric acid levels (adjusted hazard ratio 0.89, 95% confidence interval 0.82-0.97, P < 0.01).

DISCUSSION

Our pooled analysis provides further support for urate as a prognostic factor for survival in ALS and confirms the utility of the PRO-ACT database as a powerful resource for ALS epidemiological research. Muscle Nerve 57: 430-434, 2018.

Uric acid, an important screening tool to detect inborn errors of metabolism: a case series

Background

Uric acid is the metabolic end product of purine metabolism in humans. Altered serum and urine uric acid level (both above and below the reference ranges) is an indispensable marker in detecting rare inborn errors of metabolism. We describe different case scenarios of 4 Sri Lankan patients related to abnormal uric acid levels in blood and urine.

Case 1

A one-and-half-year-old boy was investigated for haematuria and a calculus in the bladder. Xanthine crystals were seen in microscopic examination of urine sediment. Low uric acid concentrations in serum and low urinary fractional excretion of uric acid associated with high urinary excretion of xanthine and hypoxanthine were compatible with xanthine oxidase deficiency.

Case 2

An 8-month-old boy presented with intractable seizures, feeding difficulties, screaming episodes, microcephaly, facial dysmorphism and severe neuro developmental delay. Low uric acid level in serum, low fractional excretion of uric acid and radiological findings were consistent with possible molybdenum cofactor deficiency. Diagnosis was confirmed by elevated levels of xanthine, hypoxanthine and sulfocysteine levels in urine.

Case 3

A 3-year-10-month-old boy presented with global developmental delay, failure to thrive, dystonia and self-destructive behaviour. High uric acid levels in serum, increased fractional excretion of uric acid and absent hypoxanthine–guanine phosphoribosyltransferase enzyme level confirmed the diagnosis of Lesch–Nyhan syndrome.

Case 4

A 9-year-old boy was investigated for lower abdominal pain, gross haematuria and right renal calculus. Low uric acid level in serum and increased fractional excretion of uric acid pointed towards hereditary renal hypouricaemia which was confirmed by genetic studies.

Conclusion

Abnormal uric acid level in blood and urine is a valuable tool in screening for clinical conditions related to derangement of the nucleic acid metabolic pathway.

URAT1 inhibition by ALPK1 is associated with uric acid homeostasis

Objective

The aim of this study was to identify a protein for urate transporter 1 (URAT1) regulation.

Methods

The clinical dataset consisted of 492 case-control samples of Han Chinese (104 gout and 388 controls). Three alpha kinase 1 ( ALPK1 ) and SLC22A12 loci associated with high gout risk and uric acid levels were genotyped. The overexpression of ALPK1 on URAT1 protein expression was evaluated in vivo in h ALPK1 transgenic mice. The in vitro protein levels of ALPK1 and URAT1 in ALPK1 small interfering RNA-transfected human kidney-2 cells with MSU crystal stimulation were examined.

Results

ALPK1 , which is a single nucleotide polymorphism (SNP) of rs11726117 (M861T; T), reduced the risk of gout via the SLC22A12 gene SNPs rs3825016 and rs475688, as compared with the subject of ALPK1 rs11726117 (C) allele {rs11726117 [CT + TT] vs rs3825016, odds ratio [OR] 0.39 [95% confidence interval (CI) 0.23, 0.67]; rs11726117 [CT + TT] vs rs475688, OR 0.39 [95% CI 0.23, 0.67]}. ALPK1-overexpressed mice demonstrated lower levels of URAT1 protein ( P = 0.0045). Mouse endogenous ALPK1 proteins were detected in renal proximal tubule cells. MSU crystals inhibited URAT1 expressions through an upregulation of ALPK1 in human kidney-2 cells.

Conclusion

Elevated ALPK1 expression decreased URAT1 expression. ALPK1 might prevent the impact of urate reuptake via SLC22A12 and appeared to be negatively associated with gout. ALPK1 is a potential repressor of URAT1 protein expression.

The genetics of gout: towards personalised medicine?

Over the last decade, there have been major advances in the understanding of the genetic basis of hyperuricaemia and gout as well as of the pharmacogenetics of urate-lowering therapy. Key findings include the reporting of 28 urate-associated loci, the discovery that ABCG2 plays a central role on extra-renal uric acid excretion, the identification of genes associated with development of gout in the context of hyperuricaemia, recognition that ABCG2 variants influence allopurinol response, and the impact of HLA-B*5801 testing in reducing the prevalence of allopurinol hypersensitivity in high-risk populations. These advances, together with the reducing cost of whole genome sequencing, mean that integrated personalised medicine approaches may soon be possible in clinical practice. Genetic data may inform assessment of disease prognosis in individuals with hyperuricaemia or established gout, personalised lifestyle advice, selection and dosing of urate-lowering therapy, and prevention of serious medication adverse effects. In this article, we summarise the discoveries from genome-wide association studies and discuss the potential for translation of these findings into clinical practice.

Prevalence of URAT1 allelic variants in the Roma population

The Roma represents a transnational ethnic group, with a current European population of 8-10 million. The evolutionary process that had the greatest impact on the gene pool of the Roma population is called the founder effect. Renal hypouricemia (RHUC) is a rare heterogenous inherited disorder characterized by impaired renal urate reabsorption. The affected individuals are predisposed to recurrent episodes of exercise-induced nonmyoglobinuric acute kidney injury and nephrolithiasis. To date, more than 150 patients with a loss-of-function mutation for the SLC22A12 (URAT1) gene have been found, most of whom are Asians. However, RHUC 1 patients have been described in a variety of ethnic groups (e.g., Arab Israelis, Iraqi Jews, Caucasians, and Roma) and in geographically noncontiguous countries. This study confirms our previous findings regarding the high frequency of SLC22A12 variants observed. Frequencies of the c.1245_1253del and c.1400C>T variants were found to be 1.92% and 5.56%, respectively, in a subgroup of the Roma population from five regions in three countries: Slovakia, Czech Republic, and Spain. Our findings suggested that the common dysfunction allelic variants of URAT1 exist in the general Roma population and thus renal hypouricemia should be kept in differential diagnostic algorithm on Roma patients with defect in renal tubular urate transport. This leads to confirm that the genetic drift in the Roma have increased the prevalence of hereditary disorders caused by very rare variants in major population.

Discovery of URAT1 SNPs and association between serum uric acid levels and URAT1

OBJECTIVES

Human urate transporter 1 (URAT1) is a member of the organic anion transporter family (SLC22A12) that primarily regulates the renal tubular reabsorption of uric acid. This case-control study was designed to analyse whether hURAT1 might also be a candidate gene for hyperuricaemia or hypouricaemia.

SETTING

We recruited 68 healthy volunteers and divided them into two groups: a normal uric acid group and a hyperuricaemia group. We analysed the sequence of the URAT1 gene and found five significant single nucleotide polymorphisms (SNPs). We then selected 900 male subjects from the 262,200 enrolled in the Korean Cancer Prevention Study-II (KCPS-II) cohort for further genetic analysis.

PARTICIPANTS

DNA samples from 36 individuals with normal uric acid (<4.5 mg/dL) and 32 individuals with hyperuricaemia (>8.5 mg/dL) were sequenced. Five significant SNPs (rs7929627, rs75786299, rs3825017, rs11602903 and rs121907892) were identified. We then chose 900 subjects from the KCPS-II cohort consisting of 450 subjects with normal uric acid (UA <4.1 mg/dL) and 450 subjects with hyperuricaemia (UA >8.7 mg/dL). The groups were matched by age, body mass index, metabolic syndrome and use of anti-hypertensive medication.

PRIMARY OUTCOME MEASURES

We compared the OR of the incidence of hyperuricaemia by URAT1 genotype.

RESULTS

The strongest association with hyperuricaemia was observed for rs75786299 (IVS3+11A/G) with an OR of 32.05. rs7929627 (IVS7-103A/G) and rs3825017 (N82N) showed an association with hyperuricaemia with ORs of 2.56 and 2.29, respectively. rs11602903 (788A/T) and rs121907892 (W258X) were negatively correlated with hyperuricaemia with ORs of 0.350 and 0.447, respectively. Individuals carrying the GATAG haplotype (n=32)-a relatively common variant consisting of rs7929627, rs75786299 and rs3825017-showed the highest risk for hyperuricaemia with an OR of 92.23 (p=9.55×10(-3)).

CONCLUSIONS

These results indicate that five newly described SNPs in the hURAT1 gene are significantly associated with uric acid level (4-2008-0318 and 4-2011-0277).