Common defects of ABCG2, a high-capacity urate exporter, cause gout: a function-based genetic analysis in a Japanese population

Novel function of Niemann-Pick C1-like 1 as a negative regulator of Niemann-Pick C2 protein

The hepatic expression of Niemann-Pick C1-like 1 (NPC1L1), which is a key molecule in intestinal cholesterol absorption, is high in humans. In addition to NPC1L1, Niemann-Pick C2 (NPC2), a secretory cholesterol-binding protein involved in intracellular cholesterol trafficking and the stimulation of biliary cholesterol secretion, is also expressed in the liver. In this study, we examined the molecular interaction and functional association between NPC1L1 and NPC2. In vitro studies with adenovirus-based or plasmid-mediated gene transfer systems revealed that NPC1L1 negatively regulated the protein expression and secretion of NPC2 without affecting the level of NPC2 messenger RNA. Experiments with small interfering RNA against NPC1L1 confirmed the endogenous association of these proteins. In addition, endocytosed NPC2 could compensate for the reduction of NPC2 in NPC1L1-overexpressing cells, and this demonstrated that the posttranscriptional regulation of NPC2 was dependent on a novel ability of NPC1L1 to inhibit the maturation of NPC2 and accelerate the degradation of NPC2 during its maturation. Furthermore, to confirm the physiological relevance of NPC1L1-mediated regulation, we analyzed human liver specimens and found a negative correlation between the protein levels of hepatic NPC1L1 and hepatic NPC2.

CONCLUSION

NPC1L1 down-regulates the expression and secretion of NPC2 by inhibiting its maturation and accelerating its degradation. NPC2 functions as a regulator of intracellular cholesterol trafficking and biliary cholesterol secretion; therefore, in addition to its role in cholesterol re-uptake from the bile by hepatocytes, hepatic NPC1L1 may control cholesterol homeostasis via the down-regulation of NPC2.

Extra-renal elimination of uric acid via intestinal efflux transporter BCRP/ABCG2

Urinary excretion accounts for two-thirds of total elimination of uric acid and the remainder is excreted in feces. However, the mechanism of extra-renal elimination is poorly understood. In the present study, we aimed to clarify the mechanism and the extent of elimination of uric acid through liver and intestine using oxonate-treated rats and Caco-2 cells as a model of human intestinal epithelium. In oxonate-treated rats, significant amounts of externally administered and endogenous uric acid were recovered in the intestinal lumen, while biliary excretion was minimal. Accordingly, direct intestinal secretion was thought to be a substantial contributor to extra-renal elimination of uric acid. Since human efflux transporter BCRP/ABCG2 accepts uric acid as a substrate and genetic polymorphism causing a decrease of BCRP activity is known to be associated with hyperuricemia and gout, the contribution of rBcrp to intestinal secretion was examined. rBcrp was confirmed to transport uric acid in a membrane vesicle study, and intestinal regional differences of expression of rBcrp mRNA were well correlated with uric acid secretory activity into the intestinal lumen. Bcrp1 knockout mice exhibited significantly decreased intestinal secretion and an increased plasma concentration of uric acid. Furthermore, a Bcrp inhibitor, elacridar, caused a decrease of intestinal secretion of uric acid. In Caco-2 cells, uric acid showed a polarized flux from the basolateral to apical side, and this flux was almost abolished in the presence of elacridar. These results demonstrate that BCRP contributes at least in part to the intestinal excretion of uric acid as extra-renal elimination pathway in humans and rats.

Null alleles of ABCG2 encoding the breast cancer resistance protein define the new blood group system Junior

The breast cancer resistance protein, also known as ABCG2, is one of the most highly studied ATP-binding cassette (ABC) transporters because of its ability to confer multidrug resistance. The lack of information on the physiological role of ABCG2 in humans severely limits cancer chemotherapeutic approaches targeting this transporter. We report here that ABCG2 comprises the molecular basis of a new blood group system (Junior, Jr) and that individuals of the Jr(a-) blood type have inherited two null alleles of ABCG2. We identified five frameshift and three nonsense mutations in ABCG2. We also show that the prevalence of the Jr(a-) blood type in the Japanese and European Gypsy populations is related to the p.Gln126* and p.Arg236* protein alterations, respectively. The identification of ABCG2(-/-) (Jr(a-)) individuals who appear phenotypically normal is an essential step toward targeting ABCG2 in cancer and also in understanding the physiological and pharmacological roles of this promiscuous transporter in humans.

ABCG2 null alleles define the Jr(a-) blood group phenotype

The high-incidence erythrocyte blood group antigen Jr(a) has been known in transfusion medicine for over 40 years. To identify the gene encoding Jr(a), we performed SNP analysis of genomic DNA from six Jr(a-) individuals. All individuals shared a homozygous region of 397,000 bp at chromosome 4q22.1 that contained the gene ABCG2, and DNA sequence analysis showed that ABCG2 null alleles define the Jr(a-) phenotype.

Gout, genetics and ABC transporters

Gout is a chronic arthritic disease associated with high levels of urate in blood. Recent advances in research have permitted the identification of several new and common genetic factors underlying the disease. Among them, a polymorphism in the ABC transporter gene ATP-binding cassette transporter isoform G2 has been highlighted. ATP-binding cassette transporter isoform G2 was found to be involved in renal urate elimination, and the presence of the Q141K polymorphism to induce a 2-fold decrease in urate efflux. The Q141K variant has been shown to have impaired trafficking, leading to its intracellular retention, whereas the wild type protein is expressed on the cell surface. Several agents are being studied for the purpose of improving folding, trafficking and function of various ABC transporters, including ATP-binding cassette transporter isoform G2. If successful, this strategy opens doors to potential new therapies for gout.

PDZK1 regulates breast cancer resistance protein in small intestine

Transporter adaptor protein PDZK1 regulates several influx transporters for xenobiotics and nutrients in small intestine, and their expression on the apical membrane is diminished in pdzk1 gene knockout [pdzk1(-/-)] mice. In the present study, we initially attempted to use pdzk1(-/-) mice to functionally identify influx transporters responsible for intestinal absorption of cimetidine. Contrary to our expectation, the plasma concentration of cimetidine after oral administration to pdzk1(-/-) mice was higher than that in wild-type mice, and the double peaks of plasma concentration found in wild-type mice were not observed in pdzk1(-/-) mice. Western blot analysis of intestinal brush-border membranes revealed that expression of breast cancer resistance protein (BCRP) but not of P-glycoprotein is reduced in pdzk1(-/-) mice. This result was compatible with the reduction of apical localization of BCRP in pdzk1(-/-) mice assessed by immunohistochemical analysis. Transcellular transport of cimetidine in the basal-to-apical direction in Madin-Darby canine kidney II (MDCKII) cells stably expressing both BCRP and PDZK1 (MDCKII/BCRP/PDZK1) was higher than that in MDCKII cells stably expressing BCRP (MDCKII/BCRP) cells. Moreover, MDCKII/BCRP/PDZK1 cells are more resistant than MDCKII/BCRP cells to the cytotoxicity of the anticancer agent 7-ethyl-10-hydroxycamptothecin (SN-38), which is a substrate of BCRP. These results were consistent with the higher expression of BCRP on apical membranes in MDCKII/BCRP/PDZK1 cells. Pull-down and immunoprecipitation studies revealed a physical interaction between BCRP and PDZK1. Taken together, these findings demonstrate that PDZK1 plays a pivotal role in the apical localization of BCRP. This is the first identification of a regulatory protein that physically interacts with and regulates BCRP in small intestine in vivo.

Transporter-Mediated Drug Interaction Strategy for 5-Aminolevulinic Acid (ALA)-Based Photodynamic Diagnosis of Malignant Brain Tumor: Molecular Design of ABCG2 Inhibitors

Photodynamic diagnosis (PDD) is a practical tool currently used in surgical operation of aggressive brain tumors, such as glioblastoma. PDD is achieved by a photon-induced physicochemical reaction which is induced by excitation of protoporphyrin IX (PpIX) exposed to light. Fluorescence-guided gross-total resection has recently been developed in PDD, where 5-aminolevulinic acid (ALA) or its ester is administered as the precursor of PpIX. ALA induces the accumulation of PpIX, a natural photo-sensitizer, in cancer cells. Recent studies provide evidence that adenosine triphosphate (ATP)-binding cassette (ABC) transporter ABCG2 plays a pivotal role in regulating the cellular accumulation of porphyrins in cancer cells and thereby affects the efficacy of PDD. Protein kinase inhibitors are suggested to potentially enhance the PDD efficacy by blocking ABCG2-mediated porphyrin efflux from cancer cells. It is of great interest to develop potent ABCG2-inhibitors that can be applied to PDD for brain tumor therapy. This review article addresses a pivotal role of human ABC transporter ABCG2 in PDD as well as a new approach of quantitative structure-activity relationship (QSAR) analysis to design potent ABCG2-inhibitors.

ABCG transporters and disease

ATP-binding cassette (ABC) transporters form a large family of transmembrane proteins that facilitate the transport of specific substrates across membranes in an ATP-dependent manner. Transported substrates include lipids, lipopolysaccharides, amino acids, peptides, proteins, inorganic ions, sugars and xenobiotics. Despite this broad array of substrates, the physiological substrate of many ABC transporters has remained elusive. ABC transporters are divided into seven subfamilies, A-G, based on sequence similarity and domain organization. Here we review the role of members of the ABCG subfamily in human disease and how the identification of disease genes helped to determine physiological substrates for specific ABC transporters. We focus on the recent discovery of mutations in ABCG2 causing hyperuricemia and gout, which has led to the identification of urate as a physiological substrate for ABCG2.

The effect of female hormones upon urate transport systems in the mouse kidney

Increased levels of serum urate in postmenopausal women are thought to be caused by a change in renal urate elimination associated with the loss of female hormones. In this study, we investigated the regulation of renal urate transporter expression by female hormones using ovariectomized mice with or without hormone replacement. Estradiol suppressed the protein levels of urate reabsorptive transporters urate transporter 1 and glucose transporter 9 (Urat1 and Glut9), and that of urate efflux transporter ATP-binding cassette sub-family G member 2 (Abcg2). Progesterone suppressed protein levels of sodium-coupled monocarboxylate transporter 1 (Smct1). However, neither estradiol nor progesterone influenced the respective levels of mRNA.

Significant association of serum uric acid levels with SLC2A9 rs11722228 among a Japanese population

Genome-wide association studies identified that SLC2A9 (GLUT9) gene polymorphisms were associated with serum uric acid (SUA) levels. Among the Japanese, a C/T polymorphism in intron 8 (rs11722228) was reported to be highly significant, though the function and strength of association were unknown. This study aimed to confirm the association, estimating the means of SUA according to the genotype, as well as OR of the genotype. Subjects were 5024 health checkup examinees (3413 males and 1611 females) aged 35 to 69 years with creatinine <2.0 mg/dL. Since SLC22A12 258X allele and ABCG2 126X allele are known to influence SUA levels strongly, the subjects with SLC22A12 258WW and ABCG2 126QQ (3082 males and 1453 females, in total 4535 subjects) were selected. The genotype frequency of SLC2A9 rs11722228 was 2184 for CC, 1947 for CT, and 404 for TT, being in Hardy-Weinberg equilibrium (p=0.312). Mean SUA was 6.10 mg/dL for CC, 6.25 mg/dL for CT, and 6.45 mg/dL for TT among males (p=1.5E-6), and 4.34 mg/dL, 4.59 mg/dL, and 4.87 mg/dL among females (p=4.6E-11), respectively. Males with SUA less than 5.0 mg/dL were 14.7% for CC, 10.6% for CT, and 7.8% for TT (p=2.3E-4), and females with SUA less than 4.0 mg/dL were 34.1%, 25.5%, and 15.4% (p=3.7E-6), respectively. This study was the first report to estimate the impact of SLC2A9 rs11722228 on SUA levels. Since the allele frequency of rs11722228 is similar among different ethnic groups, the impact remains to be examined in other ethnic groups.

Pharmacogenomics of human ABC transporter ABCC11 (MRP8): potential risk of breast cancer and chemotherapy failure

Some genetic polymorphisms of human ABC transporter genes are reportedly related to the risk of certain diseases and patients’ responses to medication. Human ABCC11 functions as an ATP-dependent efflux pump for amphipathic anions. One non-synonymous SNP 538G>A (Gly180Arg) has been found to greatly affect the function and stability of de novo synthesized ABCC11 (Arg180) variant protein. The SNP variant lacking N-linked glycosylation is recognized as a misfolded protein in the endoplasmic reticulum (ER) and readily undergoes proteasomal degradation. This ER-associated degradation of ABCC11 protein underlies the molecular mechanism of affecting the function of apocrine glands. On the other hand, the wild type (Gly180) of ABCC11 is associated with wettype earwax, axillary osmidrosis, colostrum secretion from the mammary gland, and the potential susceptibility of breast cancer. Furthermore, the wild type of ABCC11 reportedly has ability to efflux cyclic nucleotides and nucleoside-based anticancer drugs. The SNP (538G>A) of the ABCC11 gene is suggested to be a clinical biomarker for prediction of chemotherapeutic efficacy. Major obstacle to the successful chemotherapy of human cancer is development of resistance, and nucleoside-based chemotherapy is often characterized by inter-individual variability. This review provides an overview about the discovery and the genetic polymorphisms in human ABCC11. Furthermore, we focus on the impact of ABCC11 538G>A on the apocrine phenotype, patients’ response to nucleoside-based chemotherapy, and the potential risk of breast cancer.

Genome-wide association study for serum urate concentrations and gout among African Americans identifies genomic risk loci and a novel URAT1 loss-of-function allele

Serum urate concentrations are highly heritable and elevated serum urate is a key risk factor for gout. Genome-wide association studies (GWAS) of serum urate in African American (AA) populations are lacking. We conducted a meta-analysis of GWAS of serum urate levels and gout among 5820 AA and a large candidate gene study among 6890 AA and 21 708 participants of European ancestry (EA) within the Candidate Gene Association Resource Consortium. Findings were tested for replication among 1996 independent AA individuals, and evaluated for their association among 28 283 EA participants of the CHARGE Consortium. Functional studies were conducted using (14)C-urate transport assays in mammalian Chinese hamster ovary cells. In the discovery GWAS of serum urate, three loci achieved genome-wide significance (P< 5.0 × 10(-8)): a novel locus near SGK1/SLC2A12 on chromosome 6 (rs9321453, P= 1.0 × 10(-9)), and two loci previously identified in EA participants, SLC2A9 (P= 3.8 × 10(-32)) and SLC22A12 (P= 2.1 × 10(-10)). A novel rare non-synonymous variant of large effect size in SLC22A12, rs12800450 (minor allele frequency 0.01, G65W), was identified and replicated (beta -1.19 mg/dl, P= 2.7 × 10(-16)). (14)C-urate transport assays showed reduced urate transport for the G65W URAT1 mutant. Finally, in analyses of 11 loci previously associated with serum urate in EA individuals, 10 of 11 lead single-nucleotide polymorphisms showed direction-consistent association with urate among AA. In summary, we identified and replicated one novel locus in association with serum urate levels and experimentally characterize the novel G65W variant in URAT1 as a functional allele. Our data support the importance of multi-ethnic GWAS in the identification of novel risk loci as well as functional variants.

Avian renal proximal tubule urate secretion is inhibited by cellular stress-induced AMP-activated protein kinase

Urate is a potent antioxidant at high concentrations but it has also been associated with a wide variety of health risks. Plasma urate concentration is determined by ingestion, production, and urinary excretion; however, factors that regulate urate excretion remain uncertain. The objective of this study was to determine whether cellular stress, which has been shown to affect other renal transport properties, modulates urate secretion in the avian renal proximal tubule. Chick kidney proximal tubule epithelial cell primary culture monolayers were used to study the transepithelial transport of radiolabeled urate. This model allowed examination of the processes, such as multidrug resistance protein 4 (Mrp4, Abcc4), which subserve urate secretion in a functional, intact, homologous system. Our results show that the recently implicated urate efflux transporter, breast cancer resistance protein ( ABCG2), does not significantly contribute to urate secretion in this system. Exposure to a high concentration of zinc for 6 h induced a cellular stress response and a striking decrease in transepithelial urate secretion. Acute exposure to zinc had no effect on transepithelial urate secretion or isolated membrane vesicle urate transport, suggesting involvement of a cellular stress adaptation. Activation of AMP-activated protein kinase (AMPK), a candidate modulator of ATP-dependent urate efflux, by 5′-aminoimidazole-4-carboxamide 1-β-d-ribo-furanoside caused a decrease in urate secretion similar to that seen with zinc-induced cellular stress. This effect was prevented with the AMPK inhibitor compound C. Notably, the decrease in urate secretion seen with zinc-induced cellular stress was also prevented by compound C, implicating AMPK in regulation of renal uric acid excretion.

Ubiquitin-mediated proteasomal degradation of ABC transporters: a new aspect of genetic polymorphisms and clinical impacts

The interindividual variation in the rate of drug metabolism and disposition has been known for many years. Pharmacogenomics dealing with heredity and response to drugs is a part of science that attempts to explain variability of drug responses and to search for the genetic basis of such variations or differences. Genetic polymorphisms of drug metabolizing enzymes and drug transporters have been found to play a significant role in the patients' responses to medication. Accumulating evidence demonstrates that certain nonsynonymous polymorphisms have great impacts on the protein stability and degradation, as well as the function of drug metabolizing enzymes and transporters. The aim of this review article is to address a new aspect of protein quality control in the endoplasmic reticulum and to present examples regarding the impact of nonsynonymous single-nucleotide polymorphisms on the protein stability of thiopurine S-methyltransferase as well as ATP-binding cassette (ABC) transporters including ABCC4, cystic fibrosis transmembrane conductance regulator (CFTR, ABCC7), ABCC11, and ABCG2. Furthermore, we will discuss the molecular mechanisms underlying posttranslational modifications (intramolecular and intermolecular disulfide bond formation and N-linked glycosylation) and ubiquitin-mediated proteasomal degradation of ABCG2, one of the major drug transporter proteins in humans.

Uremic toxins inhibit transport by breast cancer resistance protein and multidrug resistance protein 4 at clinically relevant concentrations

During chronic kidney disease (CKD), there is a progressive accumulation of toxic solutes due to inadequate renal clearance. Here, the interaction between uremic toxins and two important efflux pumps, viz. multidrug resistance protein 4 (MRP4) and breast cancer resistance protein (BCRP) was investigated. Membrane vesicles isolated from MRP4- or BCRP-overexpressing human embryonic kidney cells were used to study the impact of uremic toxins on substrate specific uptake. Furthermore, the concentrations of various uremic toxins were determined in plasma of CKD patients using high performance liquid chromatography and liquid chromatography/tandem mass spectrometry. Our results show that hippuric acid, indoxyl sulfate and kynurenic acid inhibit MRP4-mediated [³H]-methotrexate ([³H]-MTX) uptake (calculated Ki values: 2.5 mM, 1 mM, 25 µM, respectively) and BCRP-mediated [³H]-estrone sulfate ([³H]-E1S) uptake (Ki values: 4 mM, 500 µM and 50 µM, respectively), whereas indole-3-acetic acid and phenylacetic acid reduce [³H]-MTX uptake by MRP4 only (Ki value: 2 mM and IC₅₀ value: 7 mM, respectively). In contrast, p-cresol, p-toluenesulfonic acid, putrescine, oxalate and quinolinic acid did not alter transport mediated by MRP4 or BCRP. In addition, our results show that hippuric acid, indole-3-acetic acid, indoxyl sulfate, kynurenic acid and phenylacetic acid accumulate in plasma of end-stage CKD patients with mean concentrations of 160 µM, 4 µM, 129 µM, 1 µM and 18 µM, respectively. Moreover, calculated Ki values are below the maximal plasma concentrations of the tested toxins. In conclusion, this study shows that several uremic toxins inhibit active transport by MRP4 and BCRP at clinically relevant concentrations.

Serum uric acid distribution according to SLC22A12 W258X genotype in a cross-sectional study of a general Japanese population

Background

Although SLC22A12 258X allele was found among those with hypouricemia, it was unknown that serum uric acid distribution among those with SLC22A12 258X allele. This study examined serum uric acid (SUA) distribution according to SLC22A12 W258X genotype in a general Japanese population.

Methods

Subjects were 5,023 health checkup examinees (3,413 males and 1,610 females) aged 35 to 69 years with creatinine < 2.0 mg/dL, who were participants of a cohort study belonging to the Japan Multi-Institutional Collaborative Cohort Study (J-MICC Study). SLC22A12 W258X was genotyped with a polymerase chain reaction with confronting two-pair primers.

Results

The genotype frequency was 4,793 for WW, 225 for WX, and 5 for XX, which was in Hardy-Weinberg equilibrium (p = 0.164) with X allele 0.023 (95% confidence interval [0.021-0.027]). Mean (range) SUA was 6.2 (2.1-11.4) mg/dL for WW, 3.9 (0.8-7.8) mg/dL for WX, and 0.8 (0.7-0.9) mg/dL for XX among males, and 4.5 (1.9-8.9) mg/dL, 3.3 (2.0-6.5) mg/dL, and 0.60 (0.5-0.7) mg/dL among females, respectively. Six individuals with SUA less than 1.0 mg/dL included two males with XX genotype, one male with WX genotype, and three females with XX genotype. Subjects with WX genotype were 14 (77.8%) of 18 males with a SUA of 1.0-2.9 mg/dL, and 28 (34.6%) of 81 females with the same range of SUA. The corresponding values were 131 (25.1%) of 522 males and 37 (3.5%) of 1,073 females for SUA 3.0-4.9 mg/dL, and 8 (0.4%) of 2,069 males and 5 (1.1%) of 429 females for SUA 5.0-6.9 mg/dL. The X allele effect for SUA less than 3 mg/dL was significantly (p < 0.001) higher in males (OR = 102.5, [33.9-309.8]) than in females (OR = 25.6 [14.4-45.3]).

Conclusions

Although SLC22A12 W258X was a determining genetic factor on SUA, SUA of those with WX genotype distributed widely from 0.8 mg/dL to 7.8 mg/dL. It indicated that other genetic traits and/or lifestyle affected SUA of those with WX genotype, as well as those with WW genotype.

Posttranslational negative regulation of glycosylated and non-glycosylated BCRP expression by Derlin-1

Human breast cancer resistance protein (BCRP)/MXR/ABCG2 is a well-recognized ABC half-transporter that is highly expressed at the apical membrane of many normal tissues and cancer cells. BCRP facilitates disposition of endogenous and exogenous harmful xenobiotics to protect cells/tissues from xenobiotic-induced toxicity. Despite the enormous impact of BCRP in the physiological and pathophysiological regulation of the transport of a wide variety of substrates, little is known about the factors that regulate posttranslational expression of BCRP. Here, we identified Derlin-1, a member of a family of proteins that bears homology to yeast Der1p, as a posttranslational regulator of BCRP expression. Overexpression of Derlin-1 suppressed ER to Golgi transport of wild-type (WT) BCRP that is known to be efficiently trafficked to the plasma membrane. On the other hand, protein expression of N596Q variant of BCRP, N-linked glycosylation-deficient mutant that preferentially undergoes ubiquitin-mediated ER-associated degradation (ERAD), was strongly suppressed by the overexpression of Derlin-1, whereas knockdown of Derlin-1 stabilized N596Q protein, suggesting a negative regulatory role of Derlin-1 for N596Q protein expression. Notably, knockdown of Derlin-1 also stabilized the expression of tunicamycin-induced deglycosylated WT BCRP protein, implying the importance of glycosylation state for the recognition of BCRP by Derlin-1. Thus, our data demonstrate that Derlin-1 is a negative regulator for both glycosylated and non-glycosylated BCRP expression and provide a novel posttranslational regulatory mechanism of BCRP by Derlin-1.

New and improved strategies for the treatment of gout

The Western world appears to be in the midst of the third great gout epidemic of all time. In this century, gout is increasing in prevalence despite an increased understanding of its risk factors and pathophysiology, and the availability of reasonably effective treatment. The main cultural factors responsible for this appear to be diet, obesity, ethanol use and medications. Excess fructose consumption is a newly recognized modifiable risk factor. The debate has been renewed concerning hyperuricemia as an independent risk factor for renal insufficiency and cardiovascular disease. Prevention is still rooted in lifestyle choices. Existing treatments have proven to be unsatisfactory in many patients with comorbidities. New treatments are available today and on the horizon for tomorrow, which offer a better quality of life for gout sufferers. These include febuxostat, a nonpurine inhibitor of xanthine oxidase with a potentially better combination of efficacy and safety than allopurinol, and investigational inhibitors of URAT-1, an anion exchanger in the proximal tubule that is critical for uric acid homeostasis. New abortive treatments include interleukin-1 antagonists that can cut short the acute attack in 1 to 2 days in persons who cannot take nonsteroidal anti-inflammatory drugs, colchicine or corticosteroids. Lastly, newer formulations of uricase have the ability to dissolve destructive tophi over weeks or months in patients who cannot use currently available hypouricemic agents. Diagnostically, ultrasound and magnetic resonance imaging offer advanced ways to diagnose gout noninvasively, and just as importantly, a way to follow the progress of tophus dissolution. The close association of hyperuricemia with metabolic syndrome, hypertension and renal insufficiency ensures that nephrologists will see increasing numbers of gout-afflicted patients.

The rs2231142 variant of the ABCG2 gene is associated with uric acid levels and gout among Japanese people

OBJECTIVES

Recent genome-wide association and functional studies have shown that the ABCG2 gene encodes for a urate transporter, and a common causal ABCG2 variant, rs2231142, leads to elevated uric acid levels and prevalent gout among Whites and Blacks. We examined whether this finding is observed in a Japanese population, since Asians have a high reported prevalence of the T-risk allele.

METHODS

A total of 3923 Japanese people from the Circulatory Risk in Communities Study aged 40-90 years were genotyped for rs2231142. Associations of the rs2231142 variant with serum uric acid levels and prevalence of gout and hyperuricaemia were examined.

RESULTS

The frequency of the T-risk allele was 31% in this Japanese sample. Multivariable adjusted mean uric acid levels were 7-9 micromol/l higher for TG and TT than GG carriers (P-additive = 0.0006). The multivariable-adjusted odds ratio (OR) of prevalent gout was 1.37 (95% CI 0.68, 2.76) for TG and 4.37 (95% CI 1.98, 9.62) for TT compared with the GG carriers (P-additive = 0.001). When evaluating the combined outcome of hyperuricaemia and gout, the respective ORs were 1.40 (95% CI 1.04, 1.87) for TG and 1.88 (95% CI 1.23, 2.89) for TT carriers. The population attributable risk was 29% for gout and 19% for gout and/or hyperuricaemia.

CONCLUSIONS

The association of the causal ABCG2 rs2231142 variant with uric acid levels and gout was confirmed in a sample of Japanese ancestry. Our study emphasizes the importance of this common causal variant in a population with a high risk allele frequency, especially as more Japanese adopt a Western lifestyle with a concomitant increase in mean serum uric acid levels.

What lies behind serum urate concentration? Insights from genetic and genomic studies

Many factors, including genetic components and acquired factors such as obesity and alcohol consumption, influence serum uric acid (urate) concentrations. Since serum urate concentrations are determined by the balance between renal urate excretion and the volume of urate produced via purine metabolism, urate transporter genes as well as genes coding for enzymes involved in purine metabolism affect serum urate concentrations. URAT1 was the first transporter affecting serum urate concentrations to be identified. Using the characterization of this transporter as an indicator, several transporters have been shown to transport urate, allowing the construction of a synoptic renal urate transport model. Notable re-absorptive urate transporters are URAT1 at apical membranes and GLUT9 at basolateral membranes, while ABCG2, MRP4 (multidrug resistance protein 4) and NPT1 are secretive transporters at apical membranes. Recent genome-wide association studies have led to validation of the in vitro model constructed from each functional analysis of urate transporters, and identification of novel candidate genes related to urate metabolism and transport proteins, such as glucokinase regulatory protein (GKRP), PDZK1 and MCT9. However, the function and physiologic roles of several candidates, as well as the influence of acquired factors such as obesity, foods, or alcoholic beverages, remain unclear.