Meta-analysis of 28,141 individuals identifies common variants within five new loci that influence uric acid concentrations

Serum uric acid is more strongly associated with impaired fasting glucose in women than in men from a community-dwelling population

Serum uric acid (SUA) levels are associated with metabolic syndrome (MetS) and its components such as glucose intolerance and type 2 diabetes. It is unknown whether there are gender-specific differences regarding the relationship between SUA levels, impaired fasting glucose (IFG) and newly detected diabetes. We recruited 1,209 men aged 60±15 (range, 19–89) years and 1,636 women aged 63±12 (range, 19–89) years during their annual health examination from a single community. We investigated the association between SUA levels and six categories according to fasting plasma glucose (FPG) level {normal fasting glucose (NFG), <100 mg/dL; high NFG-WHO, 100 to 109 mg/dL; IFG-WHO, 110 to 125 mg/dL; IFG-ADA, 100 to 125 mg/dL; newly detected diabetes, ≥126 mg/dL; known diabetes} SUA levels were more strongly associated with the different FPG categories in women compared with men. In women, the associations remained significant for IFG-WHO (OR, 1.23, 95% CI, 1.00–1.50) and newly detected diabetes (OR, 1.33, 95% CI, 1.03–1.72) following multivariate adjustment. However, in men all the associations were not significant. Thus, there was a significant interaction between gender and SUA level for newly detected diabetes (P = 0.005). SUA levels are associated with different categories of impaired fasting glucose in participants from community-dwelling persons, particularly in women.

Lack of associations of ten candidate coronary heart disease risk genetic variants and subclinical atherosclerosis in four US populations: the Population Architecture using Genomics and Epidemiology (PAGE) study

BACKGROUND

A number of genetic variants have been discovered by recent genome-wide association studies for their associations with clinical coronary heart disease (CHD). However, it is unclear whether these variants are also associated with the development of CHD as measured by subclinical atherosclerosis phenotypes, ankle brachial index (ABI), carotid artery intima-media thickness (cIMT) and carotid plaque.

METHODS

Ten CHD risk single nucleotide polymorphisms (SNPs) were genotyped in individuals of European American (EA), African American (AA), American Indian (AI), and Mexican American (MA) ancestry in the Population Architecture using Genomics and Epidemiology (PAGE) study. In each individual study, we performed linear or logistic regression to examine population-specific associations between SNPs and ABI, common and internal cIMT, and plaque. The results from individual studies were meta-analyzed using a fixed effect inverse variance weighted model.

RESULTS

None of the ten SNPs was significantly associated with ABI and common or internal cIMT, after Bonferroni correction. In the sample of 13,337 EA, 3809 AA, and 5353 AI individuals with carotid plaque measurement, the GCKR SNP rs780094 was significantly associated with the presence of plaque in AI only (OR = 1.32, 95% confidence interval: 1.17, 1.49, P = 1.08 × 10(-5)), but not in the other populations (P = 0.90 in EA and P = 0.99 in AA). A 9p21 region SNP, rs1333049, was nominally associated with plaque in EA (OR = 1.07, P = 0.02) and in AI (OR = 1.10, P = 0.05).

CONCLUSIONS

We identified a significant association between rs780094 and plaque in AI populations, which needs to be replicated in future studies. There was little evidence that the index CHD risk variants identified through genome-wide association studies in EA influence the development of CHD through subclinical atherosclerosis as assessed by cIMT and ABI across ancestries.

Effect of Genetic Polymorphisms on Development of Gout

Objective.

To validate the association between genetic polymorphisms and gout in Japanese patients, and to investigate the cumulative effects of multiple genetic factors on the development of gout.

Methods.

Subjects were 153 Japanese male patients with gout and 532 male controls. The genotypes of 11 polymorphisms in the 10 genes that have been indicated to be associated with serum uric acid levels or gout were determined. The cumulative effects of the genetic polymorphisms were investigated using a weighted genotype risk score (wGRS) based on the number of risk alleles and the OR for gout. A model to discriminate between patients with gout and controls was constructed by incorporating the wGRS and clinical factors. C statistics method was applied to evaluate the capability of the model to discriminate gout patients from controls.

Results.

Seven polymorphisms were shown to be associated with gout. The mean wGRS was significantly higher in patients with gout (15.2 ± 2.01) compared to controls (13.4 ± 2.10; p < 0.0001). The C statistic for the model using genetic information alone was 0.72, while the C statistic was 0.81 for the full model that incorporated all genetic and clinical factors.

Conclusion.

Accumulation of multiple genetic factors is associated with the development of gout. A prediction model for gout that incorporates genetic and clinical factors may be useful for identifying individuals who are at risk of gout.

Association of TLR4 Gene rs2149356 polymorphism with primary gouty arthritis in a case-control study

Background

The toll-like receptor (TLR)4-interleukin1β (IL1β) signaling pathway is involved in the monosodium urate (MSU)-mediated inflammation. The aim of this present study was to determine whether the TLR4 gene rs2149356 SNP is associated with gouty arthritis (GA) susceptibility and whether rs2149356 SNP impacts the TLR4-IL1β signaling pathway molecules expression.

Methods and Findings

The rs2149356 SNP was detected in 459 GA patients and 669 control subjects (containing 459 healthy and 210 hyperuricemic subjects). Peripheral blood mononuclear cells (PBMCs) TLR4 mRNA and serum IL1β were measured in different genotype carriers, and correlations between TLR4 gene SNP and TLR4 mRNA, IL1β were investigated. The frequencies of the genotype and allele were significantly different between the GA and control groups (P<0.01, respectively). The TT genotype was associated with a significantly increased risk of GA (OR = 1.88); this finding was not influenced by making adjustments for the components of possible confounders (adjusted OR = 1.96). TLR4 mRNA and IL1β were significantly increased in the TT genotype from acute GA patients (P<0.05, respectively), and lipids were significantly different among three genotypes in the GA patients (P<0.05, respectively).

Conclusions

The TLR4 gene rs2149356 SNP might be associated with GA susceptibility, and might participate in regulating immune, inflammation and lipid metabolism. Further studies are required to confirm these findings.

Genetic variability related to serum uric acid concentration and risk of Parkinson's disease

BACKGROUND

Low serum uric acid (UA) levels have been associated with increased Parkinson's disease (PD) risk and accelerated disease progression. We analyzed the effect of polymorphisms in 9 genes influencing serum UA concentration on the risk of PD.

METHODS

We genotyped SLC2A9 rs734553, ABCG2 rs2231142, SLC17A1 rs1183201, SLC22A11 rs17300741, SLC22A12 rs505802, GCKR rs780094, PDZK1 rs12129861, LRRC16A+SCGN rs742132, and SLC16A9 rs12356193 in 1061 PD patients and 754 controls. For each subject we calculated a cumulative genetic risk score (GRS), defined as the total number of PD-risk alleles (range, 2-15) associated to lower serum UA levels. Serum UA levels were measured in a subgroup of 365 PD cases and 132 controls.

RESULTS

Serum UA levels were significantly lower in men with PD than in controls. Subjects (both men and women) carrying more than 9 risk alleles (third GRS tertile) had a 1.5 higher risk of developing PD than subjects with less than 8 risk alleles (first GRS tertile). An inverse correlation was observed between higher GRS and lower serum UA concentration in both men and women.

CONCLUSIONS

Genetic variability influencing serum UA levels might modify susceptibility to PD.

Impact of polymorphisms in drug pathway genes on disease-free survival in adults with acute myeloid leukemia

Acute myeloid leukemia (AML) is a clinically heterogeneous disease, with 5-year disease-free survival (DFS) ranging from under 10% to over 70% for distinct groups of patients. At our institution, cytarabine, etoposide and busulfan are used in first or second remission patients treated with a 2-step approach to autologous stem cell transplantation (ASCT). In this study, we tested the hypothesis that polymorphisms in the pharmacokinetic and pharmacodynamic pathway genes of these drugs are associated with DFS in AML patients. A total of 1659 variants in 42 genes were analyzed for their association with DFS using a Cox proportional hazards model. 154 genetically European patients were used for the primary analysis. An intronic SNP in ABCC3 (rs4148405) was associated with a significantly shorter DFS (HR=3.2, p=5.6 x 10(-6)) in our primary cohort. In addition a SNP in the GSTM1-GSTM5 locus, rs3754446, was significantly associated with a shorter DFS in all patients (HR=1.8, p=0.001 for 154 European ancestry; HR=1.7, p=0.028 for 125 non-European patients). Thus for the first time, genetic variants in drug pathway genes are shown to be associated with DFS in AML patients treated with chemotherapy-based autologous ASCT.

Association of functional polymorphism rs2231142 (Q141K) in the ABCG2 gene with serum uric acid and gout in 4 US populations: the PAGE Study

A loss-of-function mutation (Q141K, rs2231142) in the ATP-binding cassette, subfamily G, member 2 gene (ABCG2) has been shown to be associated with serum uric acid levels and gout in Asians, Europeans, and European and African Americans; however, less is known about these associations in other populations. Rs2231142 was genotyped in 22,734 European Americans, 9,720 African Americans, 3,849 Mexican Americans, and 3,550 American Indians in the Population Architecture using Genomics and Epidemiology (PAGE) Study (2008-2012). Rs2231142 was significantly associated with serum uric acid levels (P = 2.37 × 10(-67), P = 3.98 × 10(-5), P = 6.97 × 10(-9), and P = 5.33 × 10(-4) in European Americans, African Americans, Mexican Americans, and American Indians, respectively) and gout (P = 2.83 × 10(-10), P = 0.01, and P = 0.01 in European Americans, African Americans, and Mexican Americans, respectively). Overall, the T allele was associated with a 0.24-mg/dL increase in serum uric acid level (P = 1.37 × 10(-80)) and a 1.75-fold increase in the odds of gout (P = 1.09 × 10(-12)). The association between rs2231142 and serum uric acid was significantly stronger in men, postmenopausal women, and hormone therapy users compared with their counterparts. The association with gout was also significantly stronger in men than in women. These results highlight a possible role of sex hormones in the regulation of ABCG2 urate transporter and its potential implications for the prevention, diagnosis, and treatment of hyperuricemia and gout.

Uric acid transporter ABCG2 is increased in the intestine of the 5/6 nephrectomy rat model of chronic kidney disease

BACKGROUND

Uric acid (UA) remains a risk factor of chronic kidney disease (CKD). Therefore, it is important to clarify the mechanism of UA excretion in CKD. The specific mechanisms of extrarenal excretion from the intestine are unknown. We evaluated the expression of the UA transporter in the intestinal tract--the ATP-binding cassette transporter G2 (ABCG2)--in a 5/6 nephrectomy rat model of CKD.

METHODS

Male Wistar rats (6 weeks old) were randomly assigned to the 5/6 nephrectomized (Nx) group or the sham-operated control group. Urine and blood samples were collected every 4 weeks. All the rats were killed at 8 weeks to obtain liver, duodenum, jejunum, ileum, and transverse colon tissues. Uricase activity was measured in the liver. Expression of ABCG2 in intestinal mucosa was measured with real time polymerase chain reaction (PCR).

RESULTS

The Nx group showed significantly decreased urine UA excretion/body weight and UA clearance compared to the control group at 4 and 8 weeks after nephrectomy. In contrast, serum UA and uricase activity were not significant. The expression of ABCG2 in the ileum of the Nx group showed significantly increased upregulation, while no changes were seen in the intestines of the control group.

CONCLUSIONS

The Nx rats exhibited lower excretion of urine UA and over-expression of ABCG2 in the ileum. The fact that serum UA did not increase despite the decrease in UA excretion suggests that an excretory pathway other than the kidney, probably the intestine, may operate in a complementary role that corroborates the increase in ABCG2 expression in the ileum.

ALPK1 genetic regulation and risk in relation to gout

BACKGROUND

The present study investigated whether single nucleotide polymorphisms (SNPs) in the alpha-protein kinase 1 (ALPK1) gene are associated with gout in aboriginal and Han Chinese Taiwanese.

METHODS

A total of 1351 aborigines from the community (511 cases and 840 controls) and 511 Han people from hospital (104 cases and 407 controls) were recruited. SNPs in potentially functional regions of the 38 genes within 4q25 were identified and genotypes determined by direct sequencing. Quantitation of blood ALPK1 mRNA expression levels and luciferase assay of gout-associated rs231253 pGL3-SNP constructs cotransfected with hsa-miR-519e were examined.

RESULTS

We found that ALPK1 gene was the most determinant of gout. Three SNPs of rs11726117 M861T [C], rs231247 [G] and rs231253 [G] were most associated with gout risk [odd ratios (OR) ≥1.44, P ≤ 3.78 × 10(-6)) in aborigines. A replication set using Han people had risk at rs11726117 and rs231247 (OR ≥1.72, P ≤ 4.08 × 10(-3)). From pooled analysis (Breslow-Day test, P > 0.33) assuming an additive model, each increasing copy of the risk allele of rs11726117 [C], rs231247 [G] and rs231253 [G] showed significantly elevated OR for gout ≥1.42 (P ≥ 1.53 × 10(-6)). Consistently, the composite homozygous of linked 3 SNPs (versus wild-type, OR = 1.83, P = 8.21 × 10(-4)) had strong associations with ALPK1 mRNA expression. Luciferase showed reduced hybridization between hsa-miR-519e and construct carrying gout-associated rs231253 [G] than the wild-type [C] (P = 6.19 × 10(-4)).

CONCLUSIONS

Our study found that a newly identified ALPK1 gene can effectively interfere with microRNA target recognition and modulates the mRNA expression; and the varying distribution of the implicated SNPs among cases and controls in the two studied populations suggests a significant role in gout susceptibility.

Association between gout and vertigo in a Taiwanese population

There are reports of an association between benign paroxysmal positional vertigo and hyperuricemia. We sought to determine the risk of vertigo among patients with gout compared with the general population, using a nationwide Taiwanese population-based claims database. Our study cohort consisted of patients with a diagnosis of gout disorders in 2004 (N = 18773). Four age- and gender-matched controls for every patient in the study cohort were selected using random sampling as the comparison cohort (N = 75092). All subjects were followed from the date of cohort entry until they developed vertigo or to the end of 2006. Cox proportional hazard regressions were performed to evaluate the 3-year vertigo-free survival rates. Of the total sample, 2563 (incidence, 10.09 per 1000 person-years) had vertigo during the 3-year follow-up period: 570 (incidence, 11.78 per 1000 person-years) from the study cohort and 1993 (incidence, 9.69 per 1000 person-years) from the comparison cohort. The adjusted hazard ratios (HR) of peripheral and central vertigo in patients with gout compared with controls during the 2-3-year follow-up were 1.17 (95% confidence interval [CI] = 1.05-1.29, p = 0.003) and 1.08 (95% CI = 0.86-1.36, p = 0.53), respectively. This is the first population-based study performed to suggest that patients with gout may have an increased risk of peripheral vertigo but not central vertigo. Benign paroxysmal positional vertigo may be the reason for the observed association; however, future studies are required to further ascertain the relationship between gout and the various causes of peripheral vertigo.

No association between MTHFR C677T and serum uric acid levels among Japanese with ABCG2 126QQ and SLC22A12 258WW

Several genome-wide association studies (GWAS) have revealed that single nucleotide polymorphisms (SNPs) of ABCG2 and SLC22A12 were strongly associated with serum uric acid (SUA), but those of methylene tetrahydrofolate reductase (MTHFR) were not. However, there were several studies indicating the association with MTHFR C677T polymorphism. This study examined the association with the polymorphism, taking into account the genotypes of ABCG2 Q126X and SLC22A12 W258X. Subjects were 5,028 health checkup examinees of Seirei Preventive Health Care Center (3,416 males and 1,612 females) aged 35 to 69 years, who participated in the Japan Multi-Institutional Collaborative Cohort Study (J-MICC Study). Hyperuricemia was defined as SUA equal to 7 mg/dL or over. The genotype frequency was 35.9% for CC, 48.1% for CT, and 16.0% for TT, being in Hardy-Weinberg equilibrium (p=0.90). Among 4,425 participants with ABCG2 126QQ and SLC22A12 258WW who were not under medication for hyperuricemia, the mean SUA was 5.6 mg/dL, 5.6 mg/dL, and 5.7 mg/dL, respectively. When 114 participants with ABCG2 126QQ and SLC22A12 258WW under medication for hyperuricemia were included in hyperuricemia cases, the sex-age adjusted odds ratio (OR) of hyperuricemia was not significant; OR=1.00 (95% confidence interval, 0.89-1.24) for CT genotype and OR=0.98 (0.84-1.32) for TT genotype, relative to CC genotype. The present study indicated no association between SUA and MTHFR C677T genotype, after the influences of ABCG2 Q126X and SLC22A12 W258X were removed.

Genome-wide association analyses identify 18 new loci associated with serum urate concentrations

Elevated serum urate concentrations can cause gout, a prevalent and painful inflammatory arthritis. By combining data from >140,000 individuals of European ancestry within the Global Urate Genetics Consortium (GUGC), we identified and replicated 28 genome-wide significant loci in association with serum urate concentrations (18 new regions in or near TRIM46, INHBB, SFMBT1, TMEM171, VEGFA, BAZ1B, PRKAG2, STC1, HNF4G, A1CF, ATXN2, UBE2Q2, IGF1R, NFAT5, MAF, HLF, ACVR1B-ACVRL1 and B3GNT4). Associations for many of the loci were of similar magnitude in individuals of non-European ancestry. We further characterized these loci for associations with gout, transcript expression and the fractional excretion of urate. Network analyses implicate the inhibins-activins signaling pathways and glucose metabolism in systemic urate control. New candidate genes for serum urate concentration highlight the importance of metabolic control of urate production and excretion, which may have implications for the treatment and prevention of gout.

The UK Adult Twin Registry (TwinsUK Resource)

TwinsUK is a nation-wide registry of volunteer twins in the United Kingdom, with about 12,000 registered twins (83% female, equal number of monozygotic and dizygotic twins, predominantly middle-aged and older). Over the last 20 years, questionnaire and blood/urine/tissue samples have been collected on over 7,000 subjects, as well as three comprehensive phenotyping assessments in the clinical facilities of the Department of Twin Research and Genetic Epidemiology, King's College London. The primary focus of study has been the genetic basis of healthy aging process and complex diseases, including cardiovascular, metabolic, musculoskeletal, and ophthalmologic disorders. Alongside the detailed clinical, biochemical, behavioral, and socio-economic characterization of the study population, the major strength of TwinsUK is availability of several 'omics' technologies for the participants. These include genome-wide scans of single nucleotide variants, next-generation sequencing, exome sequencing, epigenetic markers (MeDIP sequencing), gene expression arrays and RNA sequencing, telomere length measures, metabolomic profiles, and gut flora microbiomics. The scientific community now can freely access parts of the phenotype data from the 'TwinsUK', and interested researchers are encouraged to contact us via our Web site (www.twinsuk.ac.uk) for future collaborations.

Racial and gender disparities among patients with gout

Gout affects 8.3 million Americans according to NHANES 2007-2008, approximately 3.9 % of the US population. Gout has substantial effect on physical function, productivity, health-related quality of life (HRQOL), and health care costs. Uncontrolled gout is also associated with significant use of emergency care services. Women are less likely to have gout than men, but in the postmenopausal years the gender difference in disease incidence decreases. Compared with whites, racial and/or ethnic minorities, especially blacks, have higher prevalence of gout. Blacks are also less likely to receive quality gout care, leading to disproportionate morbidity. Women are less likely than men to receive allopurinol, and less likely to undergo joint aspirations for crystal analysis to establish diagnosis, but those on urate-lowering therapy are as likely as, or more likely than, men to undergo serum urate check within six months of initiation. Although a few studies provide the knowledge related to gender and race and/or ethnicity disparities for gout, several knowledge gaps exist in gout epidemiology and outcomes differences by gender and race and/or ethnicity. These should be investigated in future studies.

Effect of the ATP-binding cassette transporter ABCG2 on pharmacokinetics: experimental findings and clinical implications

INTRODUCTION

The ATP-binding cassette transporter ABCG2 can actively extrude a broad range of endogenous and exogenous substrates across biological membranes. Thereby, ABCG2 limits oral drug bioavailability, mediates hepatobiliary and renal excretion and participates functionally in the blood-brain barrier.

AREAS COVERED

The paper provides a review of the clinical evidence of the role of ABCG2 in the bioavailability and brain disposition of drugs. It also sheds light on the value of experimental/preclinical data in predicting the role of ABCG2 in pharmacokinetics in humans.

EXPERT OPINION

Experimental studies indicate that ABCG2 may limit the oral bioavailability and brain penetration of many drugs. ABCG2 has also been recognized as an important determinant of the disposition of some drugs in humans. For example, loss-of-function variants of ABCG2 affect the pharmacokinetics and pharmacodynamics of rosuvastatin in a clinically significant manner. Moreover, clinically relevant pharmacokinetic drug-drug interactions have been attributed to ABCG2 inhibition. However, examples from human studies are still rare compared with the overwhelming evidence from experimental studies. The large degree of functional redundancy of ABCG2 with other transporters such as P-glycoprotein may explain the rare occurrence of ABCG2-dependent drug-drug interactions in humans. Providing clinicians with consolidated information on the clinically relevant interactions of drugs with ABCG2 remains a matter of future exploration.

The pharmacokinetics of oxypurinol in people with gout

AIMS

Our aim was to identify and quantify the sources of variability in oxypurinol pharmacokinetics and explore relationships with plasma urate concentrations.

METHODS

Non-linear mixed effects modelling was applied to concentration-time data from 155 gouty patients with demographic, medical history and renal transporter genotype information.

RESULTS

A one compartment pharmacokinetic model with first order absorption best described the oxypurinol concentration-time data. Renal function and concomitant medicines (diuretics and probenecid), but not transporter genotype, significantly influenced oxypurinol pharmacokinetics and reduced the between subject variability in the apparent clearance of oxypurinol (CL/F(m)) from 65% to 29%. CL/F(m) for patients with normal, mild, moderate and severe renal impairment was 1.8, 0.6, 0.3 and 0.18 l h(-1), respectively. Model predictions showed a relationship between plasma oxypurinol and urate concentrations and failure to reach target oxypurinol concentrations using suggested allopurinol dosing guidelines.

CONCLUSIONS

In conclusion, this first established pharmacokinetic model provides a tool to achieve target oxypurinol plasma concentrations, thereby optimizing the effectiveness and safety of allopurinol therapy in gouty patients with various degrees of renal impairment.

Plasma uric acid is associated with increased risk of type 2 diabetes independent of diet and metabolic risk factors

Current evidence suggests a direct association of uric acid with diabetes risk, but it is still unclear whether this is independent of risk factors such as obesity and diet. We aimed to investigate whether plasma uric acid concentrations are independently associated with incident type 2 diabetes and to investigate the role of a uric acid-related dietary pattern in this association. We used a case-cohort nested in the European Prospective Investigation into Cancer and Nutrition-Netherlands study. The study included 2318 subcohort members and 845 incident diabetes cases, with a mean follow-up of 10 y. At baseline, blood samples were taken and diet was assessed using a validated FFQ. A uric acid-related dietary pattern was derived with reduced rank regression. Diabetes was mainly self-reported and verified against general practitioner records. Plasma uric acid was (mean ± SD) 231 ± 54.6 μmol/L in the subcohort. After adjustment for established diabetes risk factors such as age, the HR (highest vs. lowest quartile of uric acid) for diabetes was 4.36 (95% CI: 3.22, 5.90). Further adjustment for adiposity attenuated the HR to 1.86 (95% CI: 1.32, 2.62). Additional adjustment for hypertension and biochemical markers, such as TG, slightly attenuated the association [HR = 1.43 (95% CI: 0.97, 2.10)]. A uric acid-related dietary pattern did not confound the association. In conclusion, this study supports that high uric acid concentrations are associated with increased diabetes risk, although a large part of the association can be explained by the degree of adiposity.

Gene-centric meta-analyses of 108 912 individuals confirm known body mass index loci and reveal three novel signals

Recent genetic association studies have made progress in uncovering components of the genetic architecture of the body mass index (BMI). We used the ITMAT-Broad-Candidate Gene Association Resource (CARe) (IBC) array comprising up to 49 320 single nucleotide polymorphisms (SNPs) across ~2100 metabolic and cardiovascular-related loci to genotype up to 108 912 individuals of European ancestry (EA), African-Americans, Hispanics and East Asians, from 46 studies, to provide additional insight into SNPs underpinning BMI. We used a five-phase study design: Phase I focused on meta-analysis of EA studies providing individual level genotype data; Phase II performed a replication of cohorts providing summary level EA data; Phase III meta-analyzed results from the first two phases; associated SNPs from Phase III were used for replication in Phase IV; finally in Phase V, a multi-ethnic meta-analysis of all samples from four ethnicities was performed. At an array-wide significance (P < 2.40E-06), we identify novel BMI associations in loci translocase of outer mitochondrial membrane 40 homolog (yeast) - apolipoprotein E - apolipoprotein C-I (TOMM40-APOE-APOC1) (rs2075650, P = 2.95E-10), sterol regulatory element binding transcription factor 2 (SREBF2, rs5996074, P = 9.43E-07) and neurotrophic tyrosine kinase, receptor, type 2 [NTRK2, a brain-derived neurotrophic factor (BDNF) receptor gene, rs1211166, P = 1.04E-06] in the Phase IV meta-analysis. Of 10 loci with previous evidence for BMI association represented on the IBC array, eight were replicated, with the remaining two showing nominal significance. Conditional analyses revealed two independent BMI-associated signals in BDNF and melanocortin 4 receptor (MC4R) regions. Of the 11 array-wide significant SNPs, three are associated with gene expression levels in both primary B-cells and monocytes; with rs4788099 in SH2B adaptor protein 1 (SH2B1) notably being associated with the expression of multiple genes in cis. These multi-ethnic meta-analyses expand our knowledge of BMI genetics.

Changes in uric acid levels following bariatric surgery are not associated with SLC2A9 variants in the Swedish Obese Subjects Study

Context and Objective

Obesity and SLC2A9 genotype are strong determinants of uric acid levels. However, data on SLC2A9 variants and weight loss induced changes in uric acid levels are missing. We examined whether the changes in uric acid levels two- and ten-years after weight loss induced by bariatric surgery were associated with SLC2A9 single nucleotide polymorphisms (SNPs) in the Swedish Obese Subjects study.

Methods

SNPs (N = 14) identified by genome-wide association studies and exonic SNPs in the SLC2A9 gene locus were genotyped. Cross-sectional associations were tested before (N = 1806), two (N = 1664) and ten years (N = 1201) after bariatric surgery. Changes in uric acid were compared between baseline and Year 2 (N = 1660) and years 2 and 10 (N = 1172). A multiple testing corrected threshold of P = 0.007 was used for statistical significance.

Results

Overall, 11 of the 14 tested SLC2A9 SNPs were significantly associated with cross-sectional uric acid levels at all three time points, with rs13113918 showing the strongest association at each time point (R² = 3.7−5.2%, 3.9×10⁻²²≤p≤7.7×10⁻¹¹). One SNP (rs737267) showed a significant association (R² = 0.60%, P = 0.002) with change in uric acid levels from baseline to Year 2, as common allele homozygotes (C/C, N = 957) showed a larger decrease in uric acid (−61.4 µmol/L) compared to minor allele carriers (A/X: −51.7 µmol/L, N = 702). No SNPs were associated with changes in uric acid from years 2 to 10.

Conclusions

SNPs in the SLC2A9 locus contribute significantly to uric acid levels in obese individuals, and the associations persist even after considerable weight loss due to bariatric surgery. However, we found little evidence for an interaction between genotype and weight change on the response of uric acid to bariatric surgery over ten years. Thus, the fluctuations in uric acid levels among the surgery group appear to be driven by the weight losses and gains, independent of SLC2A9 genotypes.

Genetic impact on uric acid concentration and hyperuricemia in the Japanese population

AIM

Using general Japanese populations, we performed a replication study of genetic loci previously identified in European-descent populations as being associated with uric acid and gout. The relative contribution of non-genetic and genetic factors to the variances in serum uric acid concentration was then evaluated.

METHODS

Seven single nucleotide polymorphisms (SNPs) were genotyped from 7 candidate loci robustly confirmed in Europeans. Genotyping was performed in up to 17,226 individuals, from which 237 hyperuricemia cases and 3,218 controls were chosen for a case-control study. For 6 SNPs showing a replication of uric acid association in 17,076 general population samples, we further tested the associations with other metabolic traits (n≤5,745) and with type 2 diabetes (931 cases and 1404 controls) and coronary artery disease (806 cases and 1337 controls).

RESULTS

Significant uric acid associations (one-tailed p<0.05) were replicated for 6 loci in Japanese. The strongest association was detected at SLC22A12 rs505802 for uric acid (p=2.4×10(-50)) and ABCG2 rs2231142 for hyperuricemia (p3.6×10(-10)). The combined genetic effect could explain some proportion of inter-individual variation in uric acid (R(2)=0.03) and was more or less comparable to the effect of well-recognized risk factors -BMI (R(2)=0.04) and alcohol intake (R(2)=0.01). The tested SNPs were not significantly associated with cardiovascular risk traits except for GCKR rs780094.

CONCLUSION

Our results confirm that 6 common uric acid variant loci are reproducible in Japanese. Further investigation is warranted to efficiently use the knowledge about genetic factors in combination with modifiable risk factors when we decide an individual's treatment strategy for hyperuricemia.

Does uric acid qualify as an independent risk factor for cardiovascular mortality?

UA (uric acid) is the final product of purine metabolism in humans and is implicated in many disease conditions. Sustained hyperuricaemia has putative adverse roles in cardiovascular diseases. Despite strong evidence emerging from large epidemiological studies supporting the hypothesis that UA independently influences cardiovascular disease outcomes and mortality, a causal role is yet to be established. Serum UA is also considered as a useful biomarker for mortality in high-risk patients with acute coronary syndromes, heart failure and hypertension and in patients with Type 2 diabetes mellitus. Post-hoc analyses of clinical trial data suggest beneficial effects of reducing serum UA. However, these findings are inconclusive and are only hypothesis-generating. In the present issue of Clinical Science, Ndrepepa and co-workers have investigated the prognostic role of UA in high-risk Type 2 diabetic patients with established coronary artery disease in predicting 1-year survival and cardiovascular mortality. These results support the independent role of serum UA in predicting survival in Type 2 diabetic patients. However, long-term follow-up studies are required with serial UA measurement to establish the time-dependent association of UA with mortality outcomes.

Gout: why is this curable disease so seldom cured?

Gout is the most common inflammatory arthritis and one in which pathogenesis and risk factors are best understood. One of the treatment objectives in current guidelines is 'cure'. However, audits show that only a minority of patients with gout receive adequate advice and treatment. Suboptimal care and outcomes reflect inappropriately negative perceptions of the disease, both in patients and providers. Historically, gout has been portrayed as a benign and even comical condition that is self-inflicted through overeating and alcohol excess. Doctors often focus on managing acute attacks rather than viewing gout as a chronic progressive crystal deposition disease. Urate-lowering treatment is underprescribed and often underdosed. Appropriate education of patients and doctors, catalysed by recent introduction of new urate-lowering treatments after many years with no drug development in the field, may help to overcome these barriers and improve management of this easily diagnosed and curable form of potentially severe arthritis.

Renal transport of uric acid: evolving concepts and uncertainties

In addition to its role as a metabolic waste product, uric acid has been proposed to be an important molecule with multiple functions in human physiologic and pathophysiologic processes and may be linked to human diseases beyond nephrolithiasis and gout. Uric acid homeostasis is determined by the balance between production, intestinal secretion, and renal excretion. The kidney is an important regulator of circulating uric acid levels by reabsorbing about 90% of filtered urate and being responsible for 60% to 70% of total body uric acid excretion. Defective renal handling of urate is a frequent pathophysiologic factor underpinning hyperuricemia and gout. Despite tremendous advances over the past decade, the molecular mechanisms of renal urate transport are still incompletely understood. Many transport proteins are candidate participants in urate handling, with URAT1 and GLUT9 being the best characterized to date. Understanding these transporters is increasingly important for the practicing clinician as new research unveils their physiologic characteristics, importance in drug action, and genetic association with uric acid levels in human populations. The future may see the introduction of new drugs that act specifically on individual renal urate transporters for the treatment of hyperuricemia and gout.

SHAVE: shrinkage estimator measured for multiple visits increases power in GWAS of quantitative traits

Measurement error and biological variability generate distortions in quantitative phenotypic data. In longitudinal studies with repeated measurements, the multiple measurements provide a route to reduce noise and correspondingly increase the strength of signals in genome-wide association studies (GWAS).To optimize noise correction, we have developed Shrunken Average (SHAVE), an approach using a Bayesian Shrinkage estimator. This estimator uses regression toward the mean for every individual as a function of (1) their average across visits; (2) their number of visits; and (3) the correlation between visits. Computer simulations support an increase in power, with results very similar to those expected by the assumptions of the model. The method was applied to a real data set for 14 anthropomorphic traits in ∼6000 individuals enrolled in the SardiNIA project, with up to three visits (measurements) for each participant. Results show that additional measurements have a large impact on the strength of GWAS signals, especially when participants have different number of visits, with SHAVE showing a clear increase in power relative to single visits. In addition, we have derived a relation to assess the improvement in power as a function of number of visits and correlation between visits. It can also be applied in the optimization of experimental designs or usage of measuring devices. SHAVE is fast and easy to run, written in R and freely available online.

Localising loci underlying complex trait variation using Regional Genomic Relationship Mapping

The limited proportion of complex trait variance identified in genome-wide association studies may reflect the limited power of single SNP analyses to detect either rare causative alleles or those of small effect. Motivated by studies that demonstrate that loci contributing to trait variation may contain a number of different alleles, we have developed an analytical approach termed Regional Genomic Relationship Mapping that, like linkage-based family methods, integrates variance contributed by founder gametes within a pedigree. This approach takes advantage of very distant (and unrecorded) relationships, and this greatly increases the power of the method, compared with traditional pedigree-based linkage analyses. By integrating variance contributed by founder gametes in the population, our approach provides an estimate of the Regional Heritability attributable to a small genomic region (e.g. 100 SNP window covering ca. 1 Mb of DNA in a 300000 SNP GWAS) and has the power to detect regions containing multiple alleles that individually contribute too little variance to be detectable by GWAS as well as regions with single common GWAS-detectable SNPs. We use genome-wide SNP array data to obtain both a genome-wide relationship matrix and regional relationship (“identity by state" or IBS) matrices for sequential regions across the genome. We then estimate a heritability for each region sequentially in our genome-wide scan. We demonstrate by simulation and with real data that, when compared to traditional (“individual SNP") GWAS, our method uncovers new loci that explain additional trait variation. We analysed data from three Southern European populations and from Orkney for exemplar traits – serum uric acid concentration and height. We show that regional heritability estimates are correlated with results from genome-wide association analysis but can capture more of the genetic variance segregating in the population and identify additional trait loci.

PM10-induced hospital admissions for asthma and chronic obstructive pulmonary disease: the modifying effect of individual characteristics

BACKGROUND

Evidence suggests that oxidative stress is a unifying feature underlying the toxic actions of particulate matter (PM). We have investigated whether individual plasma antioxidant concentrations (uric acid and vitamins C, A, and E) and 10 antioxidant genes modify the response to PM with respect to hospital admissions for chronic obstructive pulmonary disease (COPD) or asthma.

METHODS

Using a bidirectional, hospital-based, case-crossover study, 209 patients admitted for asthma or COPD to the Chelsea and Westminster Hospital (London), with 234 admissions, were recruited between May 2008 and July 2010. PM10 levels in the area of Kensington and Chelsea at the time of admission were compared with the levels 14 days before and 14 days after the event. Conditional logistic regression was used to estimate the effect of PM10 at several temporal lags, while controlling for confounders.

RESULTS

An increase in asthma/COPD admission rate was related to a 10 μg/m increase in PM10, with the highest effect noted 0-3 days before the exacerbation (for lag 0-3, odds ratio = 1.35 [95% confidence interval = 1.04-1.76]). Serum vitamin C modified the effect of PM10 on asthma/COPD exacerbations. A similar (although weaker) influence was observed for low levels of uric acid and vitamin E, whereas vitamin A showed no effect modification. GSTP1 (rs1695), SOD2 (rs4880), and Nrf2 (rs1806649) were associated with a trend toward an increased risk of hospital admissions during periods of high PM10 levels.

CONCLUSIONS

Our study suggests that the concentration of antioxidants in patients' serum modifies the short-term effects of PM10 on asthma and COPD exacerbations.

Genetic variation in metabolic phenotypes: study designs and applications

Many complex disorders are linked to metabolic phenotypes. Revealing genetic influences on metabolic phenotypes is key to a systems-wide understanding of their interactions with environmental and lifestyle factors in their aetiology, and we can now explore the genetics of large panels of metabolic traits by coupling genome-wide association studies and metabolomics. These genome-wide association studies are beginning to unravel the genetic contribution to human metabolic individuality and to demonstrate its relevance for biomedical and pharmaceutical research. Adopting the most appropriate study designs and analytical tools is paramount to further refining the genotype-phenotype map and eventually identifying the part played by genetic influences on metabolic phenotypes. We discuss such design considerations and applications in this Review.

High-carbohydrate diets induce hepatic insulin resistance to protect the liver from substrate overload

In population studies hepatic steatosis in subjects with Non-alcoholic fatty liver disease (NAFLD) is strongly associated with insulin resistance. This association has encouraged debate whether hepatic steatosis is the cause or the consequence of hepatic insulin resistance? Although genome-wide studies have identified several gene variants associated with either hepatic steatosis or type 2 diabetes, no variants have been identified associated with both hepatic steatosis and insulin resistance. Here, the hypothesis is proposed that high-carbohydrate diets contribute to the association between hepatic steatosis and insulin resistance through activation of the transcription factor ChREBP (Carbohydrate response element binding protein). Postprandial hyperglycaemia raises the hepatic concentrations of phosphorylated intermediates causing activation of ChREBP and induction of its target genes. These include not only enzymes of glycolysis and lipogenesis that predispose to hepatic steatosis but also glucose 6-phosphatase (G6PC) that catalyses the final reaction in glucose production and GCKR, the inhibitor of hepatic glucokinase that curtails hepatic glucose uptake. Induction of G6PC and GCKR manifests as hepatic glucose intolerance or insulin resistance. Induction of these two genes by high glucose serves to safeguard intrahepatic homeostasis of phosphorylated intermediates. The importance of GCKR in this protective mechanism is supported by "less-active" GCKR variants in association not only with hepatic steatosis and hyperuricaemia but also with lower fasting plasma glucose and decreased insulin resistance. This supports a role for GCKR in restricting hepatic glucose phosphorylation to maintain intrahepatic homeostasis. Pharmacological targeting of the glucokinase-GCKR interaction can favour either glucose clearance by the liver or intrahepatic metabolite homeostasis.

The genetics of hyperuricaemia and gout

Gout is a common and very painful inflammatory arthritis caused by hyperuricaemia. This review provides an update on the genetics of hyperuricaemia and gout, including findings from genome-wide association studies. Most of the genes that associated with serum uric acid levels or gout are involved in the renal urate-transport system. For example, the urate transporter genes SLC2A9, ABCG2 and SLC22A12 modulate serum uric acid levels and gout risk. The net balance between renal urate absorption and secretion is a major determinant of serum uric acid concentration and loss-of-function mutations in SLC2A9 and SLC22A12 cause hereditary hypouricaemia due to reduced urate absorption and unopposed urate secretion. However, the variance in serum uric acid explained by genetic variants is small and their clinical utility for gout risk prediction seems limited because serum uric acid levels effectively predict gout risk. Urate-associated genes and genetically determined serum uric acid levels were largely unassociated with cardiovascular-metabolic outcomes, challenging the hypothesis of a causal role of serum uric acid in the development of cardiovascular disease. Strong pharmacogenetic associations between HLA-B*5801 alleles and severe allopurinol-hypersensitivity reactions were shown in Asian and European populations. Genetic testing for HLA-B*5801 alleles could be used to predict these potentially fatal adverse effects.

Serum uric Acid level and diverse impacts on regional arterial stiffness and wave reflection

BACKGROUND

Both increased arterial stiffness and hyperuricaemia are associated with elevated cardiovascular risks. Little is known about the relations of serum uric acid (UA) level to regional arterial stiffness and wave reflection. The aim of the study was to investigate the gender-specific association of serum UA and indices of arterial function in a community-based investigation in China.

METHODS

Cross-sectional data from 2374 adults (mean age 58.24 years) who underwent routine laboratory tests, regional pulse wave velocity (PWV) and pulse wave analysis measurements were analyzed in a gender-specific manner. None of the participants had atherosclerotic cardiovascular disease, chronic renal failure, systemic inflammatory disease, gout, or were under treatment which would affect serum UA level.

RESULTS

Men had higher serum UA level than women. Subjects with hyperuricaemia had significantly higher carotid-ankle PWV in both genders (P< 0.05), and the carotid-femoral PWV (PWVc-f) was higher in women (P< 0.001) while the augmentation index was marginally lower in men (P = 0.049). Multiple regression analysis showed that serum UA was an independent determinant only for PWVc-f in women (β = 0.104, P = 0.027) when adjusted for atherogenic confounders. No other independent relationship was found between UA level and other surrogates of arterial stiffness.

CONCLUSIONS

Serum UA levels are associated with alterations in systemic arterial stiffness that differ in men and women. Women might be more susceptible to large vascular damage associated with hyperuricaemia.

Regulation of uric acid excretion by the kidney

It has been known for many years that the kidney plays a major role in uric acid homeostasis, as more than 70% of urate excretion is renal. Furthermore, hyperuricemia in gout is most commonly the result of relative urate underexcretion, as the kidney has enormous capacity for urate reabsorption. A clear understanding of the mechanisms of renal handling of urate has been hampered by the differences between humans and animal models. The power of human genetics and genome-wide association studies has now provided new insight into the molecular mechanisms of urate transport by identifying the transporters that have critical roles in urate transport. This review surveys the new evidence for a molecular model of urate transport in the renal proximal tubule and uses these data to refute the popular four-component model for urate transport that has long been in vogue. It also discusses data that help us understand the relation of diuretics to hyperuricemia, losartan-induced uricosuria, variations in uric acid levels in hyperglycemia, and the effects of dairy diets on serum urate levels. In the end, several of these clinical findings are explained, and the remaining gaps in our knowledge will become evident.

Meta-analysis identifies multiple loci associated with kidney function-related traits in east Asian populations

Chronic kidney disease (CKD), impairment of kidney function, is a serious public health problem, and the assessment of genetic factors influencing kidney function has substantial clinical relevance. Here, we report a meta-analysis of genome-wide association studies for kidney function-related traits, including 71,149 east Asian individuals from 18 studies in 11 population-, hospital- or family-based cohorts, conducted as part of the Asian Genetic Epidemiology Network (AGEN). Our meta-analysis identified 17 loci newly associated with kidney function-related traits, including the concentrations of blood urea nitrogen, uric acid and serum creatinine and estimated glomerular filtration rate based on serum creatinine levels (eGFRcrea) (P < 5.0 × 10(-8)). We further examined these loci with in silico replication in individuals of European ancestry from the KidneyGen, CKDGen and GUGC consortia, including a combined total of ∼110,347 individuals. We identify pleiotropic associations among these loci with kidney function-related traits and risk of CKD. These findings provide new insights into the genetics of kidney function.

Organic anion transporters and their implications in pharmacotherapy

Organic anion transporters play an essential role in the distribution and excretion of numerous endogenous metabolic products and exogenous organic anions, including a host of widely prescribed drugs. The expression and activity of these transporters is influenced by several conditions, including transcriptional regulation, gender-dependent regulation, and genetic variation. In addition, the interaction of these transporters with several drugs and endogenous substrates has been well documented and may play a significant role in drug disposition and development of various disease states, such as nephrotoxicity and familial idiopathic hypouricemia. Members of this family of transporters have been localized mainly to the renal epithelia of various species. Much of the early research in this field has focused on their role in renal drug transport, yet increasing research on this family of transporters has localized them to various other epithelial tissues, including liver, brain, and placenta. Thus, an understanding of the role of these transporters in drug interaction and disposition in the kidney and other tissues may help in the determination of individual drug response, susceptibility to drug toxicity, and chemical carcinogenesis. This review seeks to summarize current knowledge of the molecular function and substrate profile of cloned organic anion transporters and to discuss recent progress in the understanding of the impact of interindividual variability, transcriptional regulation, and tissue distribution on individual drug response.

Serum uric acid levels are associated with polymorphism in the SAA1 gene in Chinese subjects

OBJECTIVE

Serum uric acid (SUA) is a cardiovascular risk marker associated with inflammation. The serum amyloid A protein (SAA) is an inflammatory factor and is associated with cardiovascular disease (CVD). However, the relationship between genetic polymorphisms of SAA and SUA levels has not been studied. The objective of this study was to investigate the association between SUA levels and SAA genetic polymorphisms.

METHODS

All participants were selected from subjects participating in the Cardiovascular Risk Survey (CRS) study. The single nucleotide polymorphism (SNP) rs12218 of the SAA1 gene was genotyped by using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. The association of SUA levels with genotypes was assessed by using the general liner mode.

RESULTS

The SNP rs12218 was associated with SUA levels by analyses of a dominate model (P = 0.002) and additive model (P = 0.005), and the difference remained significant after adjustment of sex, age, obesity, ethnicity, HDL-C, alcohol intake, smoking, and creatinine (P = 0.006 and P = 0.023, respectively). The TT genotype was associated with an increased SUA concentration of 39.34 mmol/L (95% confidence interval [CI], 3.61-75.06, P = 0.031) compared with the CC genotype, and the TT genotype was associated with an increased SUA concentration of 2.48 mmol/L (95% CI, 6.86-38.10; P = 0.005) compared with the CT genotype.

CONCLUSIONS

The rs12218 SNP in the SAA1 gene was associated with SUA levels in Chinese subjects, indicating that carriers of the T allele of rs12218 have a high risk of hyperuricemia.

Identification and validation of copy number variants using SNP genotyping arrays from a large clinical cohort

BACKGROUND

Genotypes obtained with commercial SNP arrays have been extensively used in many large case-control or population-based cohorts for SNP-based genome-wide association studies for a multitude of traits. Yet, these genotypes capture only a small fraction of the variance of the studied traits. Genomic structural variants (GSV) such as Copy Number Variation (CNV) may account for part of the missing heritability, but their comprehensive detection requires either next-generation arrays or sequencing. Sophisticated algorithms that infer CNVs by combining the intensities from SNP-probes for the two alleles can already be used to extract a partial view of such GSV from existing data sets.

RESULTS

Here we present several advances to facilitate the latter approach. First, we introduce a novel CNV detection method based on a Gaussian Mixture Model. Second, we propose a new algorithm, PCA merge, for combining copy-number profiles from many individuals into consensus regions. We applied both our new methods as well as existing ones to data from 5612 individuals from the CoLaus study who were genotyped on Affymetrix 500K arrays. We developed a number of procedures in order to evaluate the performance of the different methods. This includes comparison with previously published CNVs as well as using a replication sample of 239 individuals, genotyped with Illumina 550K arrays. We also established a new evaluation procedure that employs the fact that related individuals are expected to share their CNVs more frequently than randomly selected individuals. The ability to detect both rare and common CNVs provides a valuable resource that will facilitate association studies exploring potential phenotypic associations with CNVs.

CONCLUSION

Our new methodologies for CNV detection and their evaluation will help in extracting additional information from the large amount of SNP-genotyping data on various cohorts and use this to explore structural variants and their impact on complex traits.

A Na+-phosphate cotransporter homologue (SLC17A4 protein) is an intestinal organic anion exporter

The SLC17 anion transporter family comprises nine members that transport various organic anions in membrane potential (Δψ)- and Cl−-dependent manners. Although the transport substrates and physiological relevance of the majority of the members have already been determined, little is known about SLC17A4 proteins known to be Na+-phosphate cotransporter homologue (NPT homologue). In the present study, we investigated the expression and transport properties of human SLC17A4 protein. Using specific antibodies, we found that a human NPT homologue is specifically expressed and present in the intestinal brush border membrane. Proteoliposomes containing the purified protein took up radiolabeled p-aminohippuric acid (PAH) in a Cl−-dependent manner at the expense of an electrochemical gradient of protons, especially Δψ, across the membrane. The Δψ- and Cl−-dependent PAH uptake was inhibited by diisothiocyanostilbene-2,2′-disulfonic acid and Evans blue, common inhibitors of SLC17 family members. cis-Inhibition studies revealed that various anionic compounds, such as hydrophilic nonsteroidal anti-inflammatory drugs, pravastatin, and urate inhibited the PAH uptake. Proteoliposomes took up radiolabeled urate, with the uptake having properties similar to those of PAH uptake. These results strongly suggested that the human NPT homologue acts as a polyspecific organic anion exporter in the intestines. Since SLC17A1 protein (NPT1) and SLC17A3 protein (NPT4) are responsible for renal urate extrusion, our results reveal the possible involvement of a NPT homologue in urate extrusion from the intestinal duct.

Human SLC2A9a and SLC2A9b isoforms mediate electrogenic transport of urate with different characteristics in the presence of hexoses

Human SLC2A9 (GLUT9) is a novel high-capacity urate transporter belonging to the facilitated glucose transporter family. In the present study, heterologous expression in Xenopus oocytes has allowed us to undertake an in-depth radiotracer flux and electrophysiological study of urate transport mediated by both isoforms of SLC2A9 (a and b). Addition of urate to SLC2A9-producing oocytes generated outward currents, indicating electrogenic transport. Urate transport by SLC2A9 was voltage dependent and independent of the Na(+) transmembrane gradient. Urate-induced outward currents were affected by the extracellular concentration of Cl(-), but there was no evidence for exchange of the two anions. [(14)C]urate flux studies under non-voltage-clamped conditions demonstrated symmetry of influx and efflux, suggesting that SLC2A9 functions in urate efflux driven primarily by the electrochemical gradient of the cell. Urate uptake in the presence of intracellular hexoses showed marked differences between the two isoforms, suggesting functional differences between the two splice variants. Finally, the permeant selectivity of SLC2A9 was examined by testing the ability to transport a panel of radiolabeled purine and pyrimidine nucleobases. SLC2A9 mediated the uptake of adenine in addition to urate, but did not function as a generalized nucleobase transporter. The differential expression pattern of the two isoforms of SLC2A9 in the human kidney's proximal convoluted tubule and its electrogenic transport of urate suggest that these transporters play key roles in the regulation of plasma urate levels and are therefore potentially important participants in hyperuricemia and hypouricemia.

Pathway analysis of genome-wide association studies on uric acid concentrations

OBJECTIVE

The aims of this study were to identify the candidate causal single nucleotide polymorphisms (SNPs) and candidate causal mechanisms influencing uric acid level and to generate hypotheses for the SNP to gene to pathways that influence uric acid concentrations.

METHODS

Meta-analysis data of 954 SNPs with genome-wide significance in 14 genome-wide association studies (GWASs) comprising 28,141 individuals of European ancestry was subjected to ICSNPathway (Identify candidate Causal SNPs and Pathways) analysis to establish associations between pathways and uric acid concentrations.

RESULTS

ICSNPathway analysis identified 14 candidate causal SNPs, five genes, and two candidate causal pathways, which provided two hypothetical biologic mechanisms: (1) rs2728121 (regulatory region) to polycystic kidney disease 2 (PKD2) to ion transmembrane transporter activity; (2) rs942377, rs3799346, rs3799344, rs2762353, rs13197601, rs3757131, rs1165215, rs1165196 to SLC17A1 to ion transmembrane transporter activity and secondary active transmembrane transporter activity. SLC17A1, SLC17A3, SLC17A4, SLC22A11, and SLC2A9 were involved in both pathways, and PKD2 and SLC16A9 in one pathway.

CONCLUSION

By applying ICSNPathway analysis to GWAS data on uric acid levels, 14 SNPs, five genes, and two pathways involving the PKD2, SLC17A1, SLC17A3, SLC17A4, and SLC2A9 genes were identified that might contribute to the condition in Europeans.

Nature versus nurture in gout: a twin study

BACKGROUND

Gouty arthritis (gout) is the most common inflammatory arthritis in the United States and several other countries. Some rare forms of gout have a known genetic basis, but the relative importance of genetic factors on the risk for the lifetime prevalence of gout is not clear.

METHODS

We performed a heritability analysis for hyperuricemia and gout among 514 unselected, all-male twin pairs who were a part of the National Heart, Lung, and Blood Institute twin study, a prospective observational cohort study. Statistical analyses were performed using structural equation models and maximum likelihood methods. The covariates used for adjustment in the structural equation models were identified using bivariate logistic regressions.

RESULTS

The study population included 253 monozygotic (MZ) and 261 dizygotic (DZ) twin pairs, aged 48 (±3) years at baseline and followed for a mean of 34 years. The lifetime prevalence of gout did not differ between MZ and DZ twins. The concordance of hyperuricemia was 53% in MZ and 24% in DZ twin pairs (P<.001). Models that quantified the relative contribution of genetic and environmental factors on phenotypic variance showed that individual variability in gout was substantially influenced by environmental factors shared between co-twins and not by genetic factors. In contrast, individual differences in hyperuricemia were influenced significantly by genetic factors.

CONCLUSION

Hyperuricemia is a genetic trait. Outside the context of rare genetic disorders, risk for gout is determined by the environment. This has implications for prevention and treatment approaches.

Significant interaction between LRP2 rs2544390 in intron 1 and alcohol drinking for serum uric acid levels among a Japanese population

A genome-wide association study identified that LRP2 rs2544390 in intron 1 was associated with serum uric acid (SUA) levels among Japanese, as well as polymorphisms of SLC22A12, ABCG2, and SLC2A9. This study aimed to confirm the association of rs2544390 C/T with SUA, as well as another LRP2 polymorphism (rs3755166 G/A) in the promoter. Subjects were 5016 health checkup examinees (3409 males and 1607 females) aged 35 to 69years with creatinine<2.0mg/dL. The subjects with SLC22A12 258WW, SLC2A9 rs11722228C allele, ABCG2 126QQ and 141Q allele (2546 males and 1199 females) were selected for analysis. Mean SUA was 6.03mg/dL for CC, 6.18mg/dL for CT, and 6.19mg/dL for TT among males (p=0.012), and 4.49mg/dL, 4.45mg/dL, and 4.42mg/dL among females (not significant), respectively. No association was observed for rs3755166. The association with rs2544390 was stronger among male drinkers. The odds ratio of drinking ≥5/week relative to no drinking for hyperuricemia (SUA≥7mg/dL and/or under medication for hyperuricemia) was 1.11 (95% confidence interval, 0.67-1.84) among CC males, 1.75 (1.22-2.51) among CT males, and 3.13 (1.80-5.43) among TT males. The interaction terms with drinking ≥5/week were 1.56 (p=0.156) for CT and 2.87 (p=0.005) for TT. This was the first report on the interaction between LRP2 genotype and alcohol drinking for SUA. Since the low density lipoprotein-related protein 2 (megalin) encoded by LRP2 is a multi-ligand endocytic receptor expressed in many tissues including the kidney proximal tubules, the association/interaction remained to be confirmed both epidemiologically and biologically.

The renal urate transporter SLC17A1 locus: confirmation of association with gout

INTRODUCTION

Two major gout-causing genes have been identified, the urate transport genes SLC2A9 and ABCG2. Variation within the SLC17A1 locus, which encodes sodium-dependent phosphate transporter 1, a renal transporter of uric acid, has also been associated with serum urate concentration. However, evidence for association with gout is equivocal. We investigated the association of the SLC17A1 locus with gout in New Zealand sample sets.

METHODS

Five variants (rs1165196, rs1183201, rs9358890, rs3799344, rs12664474) were genotyped across a New Zealand sample set totaling 971 cases and 1,742 controls. Cases were ascertained according to American Rheumatism Association criteria. Two population groups were studied: Caucasian and Polynesian.

RESULTS

At rs1183201 (SLC17A1), evidence for association with gout was observed in both the Caucasian (odds ratio (OR) = 0.67, P = 3.0 × 10-6) and Polynesian (OR = 0.74, P = 3.0 × 10-3) groups. Meta-analysis confirmed association of rs1183201 with gout at a genome-wide level of significance (OR = 0.70, P = 3.0 × 10-8). Haplotype analysis suggested the presence of a common protective haplotype.

CONCLUSION

We confirm the SLC17A1 locus as the third associated with gout at a genome-wide level of significance.

Association of common variants in TNFRSF13B, TNFSF13, and ANXA3 with serum levels of non-albumin protein and immunoglobulin isotypes in Japanese

We performed a genome-wide association study (GWAS) on levels of serum total protein (TP), albumin (ALB), and non-albumin protein (NAP). We analyzed SNPs on autosomal chromosomes using data from 9,103 Japanese individuals, followed by a replication study of 1,600 additional individuals. We confirmed the previously- reported association of GCKR on chromosome 2p23.3 with serum ALB (rs1260326, P(meta) = 3.1 × 10(-9)), and additionally identified the significant genome-wide association of rs4985726 in TNFRSF13B on 17p11.2 with both TP and NAP (P(meta) = 1.2 × 10(-14) and 7.1 × 10(-24), respectively). For NAP, rs3803800 and rs11552708 in TNFSF13 on 17p13.1 (P(meta) = 7.2 × 10(-15) and 7.5 × 10(-10), respectively) as well as rs10007186 on 4q21.2 near ANXA3 (P(meta) = 1.3 × 10(-9)) also indicated significant associations. Interestingly, TNFRSF13B and TNFSF13 encode a tumor necrosis factor (TNF) receptor and its ligand, which together constitute an important receptor-ligand axis for B-cell homeostasis and immunoglobulin production. Furthermore, three SNPs, rs4985726, rs3803800, and rs11552708 in TNFRSF13B and TNFSF13, were indicated to be associated with serum levels of IgG (P<2.3 × 10(-3)) and IgM (P<0.018), while rs3803800 and rs11552708 were associated with IgA (P<0.013). Rs10007186 in 4q21.2 was associated with serum levels of IgA (P = 0.036), IgM (P = 0.019), and IgE (P = 4.9 × 10(-4)). Our results should add interesting knowledge about the regulation of major serum components.

Decreased extra-renal urate excretion is a common cause of hyperuricemia

ABCG2, also known as BCRP, is a high-capacity urate exporter, the dysfunction of which raises gout/hyperuricemia risk. Generally, hyperuricemia has been classified into urate 'overproduction type' and/or 'underexcretion type' based solely on renal urate excretion, without considering an extra-renal pathway. Here we show that decreased extra-renal urate excretion caused by ABCG2 dysfunction is a common mechanism of hyperuricemia. Clinical parameters, including urinary urate excretion, are examined in 644 male outpatients with hyperuricemia. Paradoxically, ABCG2 export dysfunction significantly increases urinary urate excretion and risk ratio of urate overproduction. Abcg2-knockout mice show increased serum uric acid levels and renal urate excretion, and decreased intestinal urate excretion. Together with high ABCG2 expression in extra-renal tissues, our data suggest that the 'overproduction type' in the current concept of hyperuricemia be renamed 'renal overload type', which consists of two subtypes-'extra-renal urate underexcretion' and genuine 'urate overproduction'-providing a new concept valuable for the treatment of hyperuricemia and gout.

ABCG2/BCRP dysfunction as a major cause of gout

Recent genome-wide association studies showed that serum uric acid (SUA) levels relate to ABCG2/BCRP gene, which locates in a gout-susceptibility locus revealed by a genome-wide linkage study. Together with the ABCG2 characteristics, we hypothesized that ABCG2 transports urate and its dysfunction causes hyperuricemia and gout. Transport assays showed ATP-dependent transport of urate via ABCG2. Kinetic analysis revealed that ABCG2 mediates high-capacity transport of urate (Km: 8.24 ± 1.44 mM) even under high-urate conditions. Mutation analysis of ABCG2 in 90 Japanese hyperuricemia patients detected six nonsynonymous mutations, including five dysfunctional variants. Two relatively frequent dysfunctional variants, Q126X and Q141K, were then examined. Quantitative trait locus analysis of 739 Japanese individuals showed that Q141K increased SUA as the number of minor alleles of Q141K increased (p = 6.60 × 10(-5)). Haplotype frequency analysis revealed that there is no simultaneous presence of Q126X and Q141K in one haplotype. Becuase Q126X and Q141K are assigned to nonfunctional and half-functional haplotypes, respectively, their genotype combinations are divided into four functional groups. The association study with 161 male gout patients and 865 male controls showed that all of those with dysfunctional ABCG2 increased the gout risk, especially those with ≤1/4 function (OR, 25.8; 95% CI, 10.3-64.6; p = 3.39 × 10(-21)). These genotypes were found in 10.1% of gout patients, but in only 0.9% of control. Our function-based clinicogenetic (FBCG) analysis showed that combinations of the two dysfunctional variants are major causes of gout, thereby providing a new approach for prevention and treatment of the gout high-risk population.

Urate transporters: an evolving field

Urate (uric acid) is the end product of purine metabolism in human beings owing to the genetic loss of hepatic urate oxidase (uricase). Despite its potential advantage as an antioxidant, sustained hyperuricemia is associated with gout, renal diseases, hypertension, and cardiovascular diseases. Because the kidney plays a dominant role in maintaining serum urate levels through its excretion, it is important to understand the molecular mechanism of renal urate handling. Although molecular identification of the urate/anion exchanger URAT1 (SLC22A12) in 2002 paved the way for successive identification of several urate transport-related proteins, the entire picture of effective renal urate handling in human beings has not yet been clarified. Recently, several genome-wide association studies have revealed close associations between serum urate levels and single nucleotide polymorphisms in at least 10 genetic loci including eight transporter-related genes. These findings led us to consider the roles of urate transporters in extrarenal tissues such as the intestine. In this review, we discuss various aspects of transmembrane transport of urate in the human body.