Key points

Introduction

Gout is an inflammatory arthritis caused by an extremely painful but self-limiting innate immune response to monosodium urate (MSU) crystals deposited in synovial fluid. Without effective management, in some individuals, gout can become chronic, with the development of tophi (organized lumps of urate and immune cells ) and permanent bony erosion and disability. Gout is also comorbid with other metabolic-based conditions, such as heart and kidney disease and type 2 diabetes, with the causal relationships that are of much clinical interest, remaining unclear. An elevated concentration of serum urate (hyperuricemia) is necessary, but not sufficient, for the development of gout with host-specific and environmental factors required for the progression from hyperuricemia to gout. Approximately 30 genetic loci, including SLC2A9 and ABCG2 that have major effects, influence serum urate concentrations with less understood about the genetic control of the formation of MSU crystals and the subsequent inflammatory response. Urate-lowering therapy, in particular use of the xanthine oxidase inhibitor allopurinol, is a cornerstone of gout management, but for a variety of reasons it is often not effective.

Hyperuricemia and gout are more prevalent in men than women, with the prevalence of both increasing with age and particularly in women after menopause. The prevalence of gout is typically 3% to 4% in people of European ancestry, up to 1% in populations of Asian ancestry, and 6% to 8% in Taiwanese Aboriginals and Polynesian people (Maori and Pacific Islanders) living in New Zealand. The increased prevalence of gout in the latter populations has a strong contribution from the inherent hyperuricemia in these groups. Serum urate levels are a balance of overproduction and renal and gut underexcretion, with renal excretion particularly important. The renal fractional excretion of uric acid is reduced in hyperuricemia compared with normouricemia, in men compared with women, and in Pacific Islanders and New Zealand Maori compared with Europeans. Regarding overproduction, urate is the end product of purine metabolism, with the liver a major site of production. For example, the liver produces urate as a by-product of fructose and alcohol-induced purine nucleotide degradation.

Monogenic disorders of purine metabolism including hypoxanthine-guanine phosphoribosyltransferase deficiency (Lesch-Nyhan syndrome) and 5-phosporibosyl-1-pyrophosphate synthetase superactivity generate rare pediatric syndromes of hyperuricemia, early-onset gout, and kidney stones. Familial juvenile hyperuricemic nephropathy is a disorder of renal uric acid underexcretion caused by mutations in uromodulin leading to hyperuricemia, early-onset gout, and chronic kidney disease. These rare disorders provide insights into purine metabolism and renal uric acid excretion mechanisms but account for an extremely small proportion of hyperuricemia and gout in the general population. Genome-wide association studies (GWAS) for common genetic variants contributing to the polygenic component of hyperuricemia and gout in the general population exhibit very little to no overlap (with the possible exception of a locus containing the PRPSAP1 gene) with monogenic disorders. This review, therefore, focuses on insights into the common genetic variants contributing to the development of gout, in particular on recent and other pertinent findings regarding the 2 major urate and gout loci, SLC2A9 and ABCG2 .

Introduction

Gout is an inflammatory arthritis caused by an extremely painful but self-limiting innate immune response to monosodium urate (MSU) crystals deposited in synovial fluid. Without effective management, in some individuals, gout can become chronic, with the development of tophi (organized lumps of urate and immune cells ) and permanent bony erosion and disability. Gout is also comorbid with other metabolic-based conditions, such as heart and kidney disease and type 2 diabetes, with the causal relationships that are of much clinical interest, remaining unclear. An elevated concentration of serum urate (hyperuricemia) is necessary, but not sufficient, for the development of gout with host-specific and environmental factors required for the progression from hyperuricemia to gout. Approximately 30 genetic loci, including SLC2A9 and ABCG2 that have major effects, influence serum urate concentrations with less understood about the genetic control of the formation of MSU crystals and the subsequent inflammatory response. Urate-lowering therapy, in particular use of the xanthine oxidase inhibitor allopurinol, is a cornerstone of gout management, but for a variety of reasons it is often not effective.

Hyperuricemia and gout are more prevalent in men than women, with the prevalence of both increasing with age and particularly in women after menopause. The prevalence of gout is typically 3% to 4% in people of European ancestry, up to 1% in populations of Asian ancestry, and 6% to 8% in Taiwanese Aboriginals and Polynesian people (Maori and Pacific Islanders) living in New Zealand. The increased prevalence of gout in the latter populations has a strong contribution from the inherent hyperuricemia in these groups. Serum urate levels are a balance of overproduction and renal and gut underexcretion, with renal excretion particularly important. The renal fractional excretion of uric acid is reduced in hyperuricemia compared with normouricemia, in men compared with women, and in Pacific Islanders and New Zealand Maori compared with Europeans. Regarding overproduction, urate is the end product of purine metabolism, with the liver a major site of production. For example, the liver produces urate as a by-product of fructose and alcohol-induced purine nucleotide degradation.

Monogenic disorders of purine metabolism including hypoxanthine-guanine phosphoribosyltransferase deficiency (Lesch-Nyhan syndrome) and 5-phosporibosyl-1-pyrophosphate synthetase superactivity generate rare pediatric syndromes of hyperuricemia, early-onset gout, and kidney stones. Familial juvenile hyperuricemic nephropathy is a disorder of renal uric acid underexcretion caused by mutations in uromodulin leading to hyperuricemia, early-onset gout, and chronic kidney disease. These rare disorders provide insights into purine metabolism and renal uric acid excretion mechanisms but account for an extremely small proportion of hyperuricemia and gout in the general population. Genome-wide association studies (GWAS) for common genetic variants contributing to the polygenic component of hyperuricemia and gout in the general population exhibit very little to no overlap (with the possible exception of a locus containing the PRPSAP1 gene) with monogenic disorders. This review, therefore, focuses on insights into the common genetic variants contributing to the development of gout, in particular on recent and other pertinent findings regarding the 2 major urate and gout loci, SLC2A9 and ABCG2 .

Genome-wide association studies in urate

In the context of medically important metabolites, urate is very tractable to research on etiology. It is easily measured and levels typically do not fluctuate over short periods; this allows good quality of phenotyping for studies of genetic and environmental risk factors. Approximately 90% of variance in renal uric acid handling and approximately 60% of variance in serum urate concentrations are explained by inherited genetic variants. The largest GWAS to date was carried out in people of European ancestry. A total of 110,347 individuals were genotyped at 2.45 million single nucleotide polymorphism (SNP) markers. Of these markers, 2201 were associated with serum urate concentrations at an experiment-wide level of significance ( P <5 × 10 ⁻⁸ ) that accounts for the multiple testing inherent in a GWAS. These markers were spread over 28 distinct regions of the genome; each region can be considered a locus containing one or more genetic variants with a causal role in determining serum urate concentrations. Predictably, most of these loci are also associated with the risk of gout in multiple ancestral groups. Within the 28 loci, renal and gut uric acid excretion genes are prominent, some with a very strong effect, particularly SLC2A9 and ABCG2 . The urate-raising allele at SLC2A9 associates with an average 0.37 mg/dL increase in serum urate, the urate-raising allele at ABCG2 with an average 0.22 mg/dL increase. The amount of variance in serum urate concentrations explained by SLC2A9 (GLUT9) is 2% to 3% and ABCG2 is approximately 1%, both very strong effects in the context of complex phenotype loci. In comparison, the established and strongest effect weight locus ( FTO/IRX3 ) in Europeans explains approximately 0.3% of variance in body mass index. Other loci with renal and gut uric acid transporters or auxiliary molecules identified are SLC17A1-A3 (NPT1, NPT3, NPT4), SLC22A12 (URAT1), SLC22A11 (OAT4), and PDZK1 . Smaller GWAS for serum urate concentrations have been performed in people of East Asian, African, and Pacific ancestry. The East Asian study included 51,327 participants and approximately 2.4 million SNPs with 4 loci identified, all overlapping with those discovered in the European study ( SLC2A9, ABCG2, SLC22A12 , and MAF ). Two African American GWAS, both published in 2011, included 5820 and 1017 participants, so were inadequately powered to detect loci of moderate to weak effect. SLC2A9 was detected in both studies. The larger study also detected SLC22A12 and a novel locus containing the SLC2A12 and SGK1 genes that encode the GLUT12 transporter and serine/threonine protein kinase serum/glucocorticoid-regulated kinase 1, respectively. Finally, the study by Kenny and colleagues had 2906 participants from the Micronesian island of Kosrae. The only locus detected at a genome-wide level of significance was SLC22A12 (URAT1). Interestingly, the signal at SLC22A12 was a considerable distance (500 kb) upstream of SLC22A12 , suggesting it may represent a novel locus.

SLC2A9

Hundreds of single nucleotide variants are associated with urate concentrations and the risk of gout over a several hundred kilobase region on chromosome 4 encompassing the SLC2A9 gene. SLC2A9 confers an extremely strong single genetic effect in the context of complex phenotypes, reminiscent of the human leukocyte antigen (HLA) locus in autoimmune disease. Like the HLA locus in autoimmunity, dissection of the complex genetic architecture at SLC2A9 and identification of causal genetic variant(s) is challenging, with relatively little progress yet at SLC2A9 . Circumstantial evidence suggests that the major SLC2A9 genetic effect is associated with isoform expression, whereby the urate-raising causal genetic variant increases the expression of an SLC2A9 isoform (SLC2A9-S) that has a 28-residue portion missing from the N-terminus. This isoform is expressed on the apical (urine) side of the collecting duct, where it presumably increases reuptake of secreted uric acid, whereas the full-length version (SLC2A9-L) is expressed on the basolateral side where it is the major basolateral uric acid exit route into the circulation. There are multiple independent causal genetic effects at SLC2A9 . Aside from mutations that cause type 2 hereditary renal hypouricemia, the precise common genetic variants that causally control serum urate concentrations are yet to be pinpointed.

ABCG2

The ATP binding cassette G2 (ABCG2) protein is one of a superfamily of 48 human ABC transporters that transport a wide array of substrates. A GWAS first identified the missense rs2231142 (Q141K) variant to be associated with serum urate concentration in Europeans, with the 141K allele associated with increased urate concentration. This genome-wide significant level of association has been consistently replicated in other GWAS in people of European and East Asian ancestry but not in people of African American ancestry. However, there is nominal evidence for association of the Q141K variant with serum urate concentration in African American individuals, with the 141K allele also associated with increased urate and the risk of gout. ABCG2 transports uric acid, with the 141K variant reducing by approximately 50% the ability to secrete uric acid and highly likely to be the lead causal variant at ABCG2 .

There is evidence for a second genetically and statistically independent effect at the ABCG2 locus, marked by SNP rs2622629 ( Table 1 ). This second effect is of clinical relevance, as the rs2622629 -correlated SNPs (ie, those SNPs in linkage disequilibrium) include rs10011796 ( r ² = 0.84 in Europeans); rs10011796 is also implicated in gout and in allopurinol response (see Table 1 ). The molecular mechanism whereby rs2622629 (or tightly correlated variants) influence serum urate levels is not known, but this variant maps within a DNaseI hypersensitivity cluster of approximately 150 bp ( www.genome.ucsc.edu ) identified by the Encode project ( www.encodeproject.org ), consistent with the effect being mediated through control of gene expression and/or mRNA editing.

Aside from Q141K, the only other common (>1%) missense variant in ABCG2 is V12M ( rs2231137 ), situated in the intracellular portion of ABCG2. This variant is genetically independent of both Q141K ( r ² = 0.002 in Europeans) and rs2622629 ( r ² = 0.009 in Europeans). This variant is associated with serum urate and gout and can be considered a third independent effect on gout in ABCG2 , after Q141K and rs10011796/rs2622629 . The GWAS by Köttgen and colleagues in serum urate in European individuals reported an increase in serum urate of 0.077 mg/dL and the GWAS by Okada and colleagues in East Asian individuals reported an increase of 0.108 mg/dL per copy of the 12V allele. In gout, the major 12V allele consistently confers risk in 3 separate samples drawn from East Asian populations: Taiwanese Aborigines (odds ratio [OR] = 1.36), Han Chinese in Taiwan (OR = 1.33), and Han Chinese in Shanghai (OR = 1.82). Any impact of this variant on the uric acid transport activity of ABCG2 is currently unknown.

Expression of most uric acid transporters is relatively high in the kidney or, for SLC22A12/URAT1, restricted to the kidney. However, expression of ABCG2 is also relatively high in the gut. Recent work in a well-defined Japanese gout sample set has demonstrated the role of ABCG2 uric acid excretion in both the gut and kidney. It was possible to create grades of ABCG2 dysfunction based on Q141 K and Q126X genotype combinations, with individuals positive for the dysfunctional variants 126X and 141K having the highest serum urate concentrations and highest risk for gout, and those homozygous for 141Q and 126Q having the lowest serum urate concentrations and lowest risk for gout. The presence of the 141K (and 126X) alleles reduces excretion through the gut and adds to the circulating urate, overloading the kidney excretion system, resulting in increased urinary uric acid levels.

Restoring ABCG2 expression and function in people with detrimental genetic polymorphisms may be an important next step to limit urate levels and inflammatory responses. Recently, it has been shown that histone deacetylase (HDAC) inhibitors and colchicine can restore the function of the 141K ABCG2 variant by restoring trafficking and dimer expression. HDAC inhibitors and colchicine may impede targeting of 141K ABCG2 to the aggresome (an aggregation of misfolded proteins formed when the protein degradation system of a cell is overloaded) and promote relocalization on the cell surface. Colchicine is an anti-inflammatory agent that acts by inhibiting microtubule polymerization via binding to tubulin. This suggests that HDAC inhibitors, as is the case with colchicine, inhibit trafficking of ABCG2 to the aggresome along microtubules. These results should encourage further research into the use of HDAC inhibitors and small molecules to restore defective ABCG2 function in patients with the 141K polymorphism.