Toshinori Chiba

Common Defects of ABCG2, a High-Capacity Urate Exporter, Cause Gout: A Function-Based Genetic Analysis in a Japanese Population

Dysfunctional genotype combinations of polymorphic adenosine 5′-triphosphate–binding cassette transporter gene ABCG2/BCRP, which encodes a high-capacity urate secretion transporter in human gut and kidney, are major causes of gout. Gout, the “Disease of Kings” as it is often known, is a painful medical condition characterized by sharp acute pain in bone joints, due to the high deposition of uric acid crystals from the blood serum into the surrounding cartilage. It affects approximately 1% of the U.S. population and remains a significant public health concern. The prevalence of gout is much higher in certain Asian ethnic groups, and is also reportedly rising in African Americans. Current medical treatments are aimed at ameliorating pain severity, but as the underlying genetic etiology of the disease unfolds, new targets for future therapies are likely to be found. Although genome-wide association studies (GWAS) have enabled the calculation of risk predispositions for a wide variety of complex diseases, the relation of gene function to the causality of disease-related mutations has remained largely unclear. A recent U.S. population–based study supported an association between urate levels and gout in individuals carrying variants in a multifunctional transporter gene, ABCG2. This study identified Q141K as a high-risk variant in nearly 10% of gout cases in Caucasians. Now, a team led by Hirotaka Matsuo report that in a Japanese population, another risk variant in ABCG2, namely the Q126X nonfunctional mutation, confers an even higher risk associated with an increase in uric acid deposition in the blood and may cause gout in Asians. Because this gene is responsible for giving rise to a protein that transports harmful waste products and metabolites out of the kidney and gut, they extensively validate the biological activity of ABCG2 using functional assays in vitro that effectively recapitulate human data obtained from Japanese individuals afflicted with the disease. These findings lend weight to previously reported GWAS; moreover, these newly identified specific high-risk variants that block urate secretion may serve as potential intervention points for quelling the disease. Gout based on hyperuricemia is a common disease with a genetic predisposition, which causes acute arthritis. The ABCG2/BCRP gene, located in a gout-susceptibility locus on chromosome 4q, has been identified by recent genome-wide association studies of serum uric acid concentrations and gout. Urate transport assays demonstrated that ABCG2 is a high-capacity urate secretion transporter. Sequencing of the ABCG2 gene in 90 hyperuricemia patients revealed several nonfunctional ABCG2 mutations, including Q126X. Quantitative trait locus analysis of 739 individuals showed that a common dysfunctional variant of ABCG2, Q141K, increases serum uric acid. Q126X is assigned to the different disease haplotype from Q141K and increases gout risk, conferring an odds ratio of 5.97. Furthermore, 10% of gout patients (16 out of 159 cases) had genotype combinations resulting in more than 75% reduction of ABCG2 function (odds ratio, 25.8). Our findings indicate that nonfunctional variants of ABCG2 essentially block gut and renal urate excretion and cause gout.

ABCG2 dysfunction causes hyperuricemia due to both renal urate underexcretion and renal urate overload

Gout is a common disease which results from hyperuricemia. We have reported that the dysfunction of urate exporter ABCG2 is the major cause of renal overload (ROL) hyperuricemia, but its involvement in renal underexcretion (RUE) hyperuricemia, the most prevalent subtype, is not clearly explained so far. In this study, the association analysis with 644 hyperuricemia patients and 1,623 controls in male Japanese revealed that ABCG2 dysfunction significantly increased the risk of RUE hyperuricemia as well as overall and ROL hyperuricemia, according to the severity of impairment. ABCG2 dysfunction caused renal urate underexcretion and induced hyperuricemia even if the renal urate overload was not remarkable. These results show that ABCG2 plays physiologically important roles in both renal and extra-renal urate excretion mechanisms. Our findings indicate the importance of ABCG2 as a promising therapeutic and screening target of hyperuricemia and gout.

Common dysfunctional variants in ABCG2 are a major cause of early-onset gout

Gout is a common disease which mostly occurs after middle age, but more people nowadays develop it before the age of thirty. We investigated whether common dysfunction of ABCG2, a high-capacity urate transporter which regulates serum uric acid levels, causes early-onset gout. 705 Japanese male gout cases with onset age data and 1,887 male controls were genotyped, and the ABCG2 functions which are estimated by its genotype combination were determined. The onset age was 6.5 years earlier with severe ABCG2 dysfunction than with normal ABCG2 function (P = 6.14 × 10−3). Patients with mild to severe ABCG2 dysfunction accounted for 88.2% of early-onset cases (twenties or younger). Severe ABCG2 dysfunction particularly increased the risk of early-onset gout (odds ratio 22.2, P = 4.66 × 10−6). Our finding that common dysfunction of ABCG2 is a major cause of early-onset gout will serve to improve earlier prevention and therapy for high-risk individuals.

Genome-wide association study of clinically defined gout identifies multiple risk loci and its association with clinical subtypes

Objective Gout, caused by hyperuricaemia, is a multifactorial disease. Although genome-wide association studies (GWASs) of gout have been reported, they included self-reported gout cases in which clinical information was insufficient. Therefore, the relationship between genetic variation and clinical subtypes of gout remains unclear. Here, we first performed a GWAS of clinically defined gout cases only. Methods A GWAS was conducted with 945 patients with clinically defined gout and 1213 controls in a Japanese male population, followed by replication study of 1048 clinically defined cases and 1334 controls. Results Five gout susceptibility loci were identified at the genome-wide significance level (p<5.0×10−8), which contained well-known urate transporter genes (ABCG2 and SLC2A9) and additional genes: rs1260326 (p=1.9×10−12; OR=1.36) of GCKR (a gene for glucose and lipid metabolism), rs2188380 (p=1.6×10−23; OR=1.75) of MYL2-CUX2 (genes associated with cholesterol and diabetes mellitus) and rs4073582 (p=6.4×10−9; OR=1.66) of CNIH-2 (a gene for regulation of glutamate signalling). The latter two are identified as novel gout loci. Furthermore, among the identified single-nucleotide polymorphisms (SNPs), we demonstrated that the SNPs of ABCG2 and SLC2A9 were differentially associated with types of gout and clinical parameters underlying specific subtypes (renal underexcretion type and renal overload type). The effect of the risk allele of each SNP on clinical parameters showed significant linear relationships with the ratio of the case–control ORs for two distinct types of gout (r=0.96 [p=4.8×10−4] for urate clearance and r=0.96 [p=5.0×10−4] for urinary urate excretion). Conclusions Our findings provide clues to better understand the pathogenesis of gout and will be useful for development of companion diagnostics.

NPT1/SLC17A1 is a renal urate exporter in humans and its common gain-of-function variant decreases the risk of renal underexcretion gout

Serum uric acid (SUA) levels in humans are mainly regulated by urate transporters. Recent genome‐wide association studies suggested that common variants of the human sodium‐dependent phosphate cotransporter type 1 gene (NPT1/SLC17A1) influence SUA. NPT1 has been reported to mediate urate transport, but its physiologic role in regulating SUA in humans remains unclear. Furthermore, the findings of replication studies of the relationship between NPT1 variants and gout have been inconsistent. The aims of this study were to investigate the effect of NPT1 on gout and to determine its physiologic role.

ABCG2 Dysfunction Increases the Risk of Renal Overload Hyperuricemia

ATP-binding cassette transporter, sub-family G, member 2 (ABCG2/BCRP) is identified as a high-capacity urate exporter, and its dysfunction has an association with serum uric acid levels and gout/hyperuricemia risk. Generally, hyperuricemia has been classified into urate “overproduction type,” “underexcretion type,” and “combined type” based on only renal urate excretion, without considering an extra-renal pathway such as gut excretion. In this study, we investigated the effects of ABCG2 dysfunction on human urate handling and the mechanism of hyperuricemia. Clinical parameters for urate handling including urinary urate excretion (UUE) were examined in 644 Japanese male outpatients with hyperuricemia. The severity of their ABCG2 dysfunction was estimated by genotype combination of two common ABCG2 variants, nonfunctional Q126X (rs72552713) and half-functional Q141K (rs2231142). Contrary to the general understanding that ABCG2 dysfunction leads to decreased renal urate excretion, UUE was significantly increased by ABCG2 dysfunction (P = 3.60 × 10−10). Mild, moderate, and severe ABCG2 dysfunctions significantly raised the risk of “overproduction” hyperuricemia including overproduction type and combined type, conferring risk ratios of 1.36, 1.66, and 2.35, respectively. The present results suggest that common dysfunctional variants of ABCG2 decrease extra-renal urate excretion including gut excretion and cause hyperuricemia. Thus, “overproduction type” in the current concept of hyperuricemia should be renamed “renal overload type,” which is caused by two different mechanisms, “extra-renal urate underexcretion” and genuine “urate overproduction.” Our new concept will lead to a more accurate diagnosis and more effective therapeutic strategy for hyperuricemia and gout.

Common dysfunctional variants of ABCG2 have stronger impact on hyperuricemia progression than typical environmental risk factors

Gout/hyperuricemia is a common multifactorial disease having typical environmental risks. Recently, common dysfunctional variants of ABCG2, a urate exporter gene also known as BCRP, are revealed to be a major cause of gout/hyperuricemia. Here, we compared the influence of ABCG2 dysfunction on serum uric acid (SUA) levels with other typical risk factors in a cohort of 5,005 Japanese participants. ABCG2 dysfunction was observed in 53.3% of the population investigated, and its population-attributable risk percent (PAR%) for hyperuricemia was 29.2%, much higher than those of the other typical environmental risks, i.e. overweight/obesity (BMI ≥ 25.0; PAR% = 18.7%), heavy drinking (>196 g/week (male) or >98 g/week (female) of pure alcohol; PAR% = 15.4%), and aging (≥60 years old; PAR% = 5.74%). SUA significantly increased as the ABCG2 function decreased (P = 5.99 × 10−19). A regression analysis revealed that ABCG2 dysfunction had a stronger effect than other factors; a 25% decrease in ABCG2 function was equivalent to “an increase of BMI by 1.97-point” or “552.1 g/week alcohol intake as pure ethanol” in terms of ability to increase SUA. Therefore, ABCG2 dysfunction originating from common genetic variants has a much stronger impact on the progression of hyperuricemia than other familiar risks. Our study provides a better understanding of common genetic factors for common diseases.

Pathogenic GLUT9 mutations causing renal hypouricemia type 2 (RHUC2).

Renal hypouricemia (MIM 220150) is an inherited disorder characterized by low serum uric acid levels and has severe complications such as exercise-induced acute renal failure and urolithiasis. We have previously reported that URAT1/SLC22A12 encodes a renal urate-anion exchanger and that its mutations cause renal hypouricemia type 1 (RHUC1). With the large health-examination database of the Japan Maritime Self-Defense Force, we found two missense mutations (R198C and R380W) of GLUT9/SLC2A9 in hypouricemia patients. R198C and R380W occur in highly conserved amino acid motifs in the “sugar transport proteins signatures” that are observed in GLUT family transporters. The corresponding mutations in GLUT1 (R153C and R333W) are known to cause GLUT1 deficiency syndrome because arginine residues in this motif are reportedly important as the determinants of the membrane topology of human GLUT1. Therefore, on the basis of membrane topology, the same may be true of GLUT9. GLUT9 mutants showed markedly reduced urate transport in oocyte expression studies, which would be the result of the loss of positive charges in those conserved amino acid motifs. Together with previous reports on GLUT9 localization, our findings suggest that these GLUT9 mutations cause renal hypouricemia type 2 (RHUC2) by their decreased urate reabsorption on both sides of the renal proximal tubule cells. However, a previously reported GLUT9 mutation, P412R, was unlikely to be pathogenic. These findings also enable us to propose a physiological model of the renal urate reabsorption via GLUT9 and URAT1 and can lead to a promising therapeutic target for gout and related cardiovascular diseases.

ABCG2 variant has opposing effects on onset ages of Parkinson's disease and gout

Uric acid (urate) has been suggested to play a protective role in Parkinsons disease onset through its antioxidant activity. Dysfunction of ABCG2, a high‐capacity urate exporter, is a major cause for early‐onset gout based on hyperuricemia. In this study, the effects of a dysfunctional ABCG2 variant (Q141K, rs2231142) were analyzed on the ages at onset of gout patients (N = 507) and Parkinsons disease patients (N = 1015). The Q141K variant hastened the gout onset (P = 0.0027), but significantly associated with later Parkinsons disease onset (P = 0.025). Our findings will be helpful for development of more effective prevention of Parkinsons disease.

Common Variants of cGKII/PRKG2 Are Not Associated with Gout Susceptibility

Objective. Recently, genetic analyses indicated the association between gout and cGMP-dependent protein kinase 2 (cGKII/PRKG2) gene in a Fukien-Taiwanese heritage population. However, no replication study has been reported in other ancestries. Therefore, we investigated this association in a Japanese population. Methods. Genotyping of 4 variants (rs11736177, rs10033237, rs7688672, and rs6837293) of cGKII was performed in 741 male gout patients and 1302 male controls. Results. cGKII variants have no association with gout. Conclusion. Our replication study suggests that cGKII is not involved in gout susceptibility.