A genome wide association scan of uric acid levels was performed in 868 Amish participants of the Heredity and Phenotype Intervention (HAPI) Heart Study. The study sample included slightly more men (n=460) than women (n=408). Uric acid levels were higher in men than in women (4.54 ± 1.0 vs. 3.71 ± 0.9 mg/dl, p < 0.0001), see .
Characteristics [mean (standard deviation)] by sex of HAPI Heart Study participants.
A total of 361,034 SNPs passed quality control measures with a minor allele frequency greater than 5% and comprised the genome wide scan. The results of the association tests for those SNPs with very strong evidence for association (n = 246 SNPs with p < 0.0001) are given in Supplemental Table S.1
. The strongest association signal was on chromosome 4, in the same region as reported previously (7
) (). The most strongly associated SNP was rs10489070 (p = 4.2 × 10−11
) with a cluster of 20 SNPs in linkage disequilibrium with rs10489070 that all provided strong evidence (p < 10−7
) for association with uric acid levels. These SNPs encompass an approximately 367 KB region that include GLUT9
Region on chromosome 4 strongly associated with serum uric acid levels. Two clusters of SNPs approximately 300 kb apart are in linkage disequilibrium and showed strong association with uric acid level. The region contains GLUT9 and WDR1.
is a class II member of the facilitated hexose transporter family (SLC2A). Substrate specificity is varied with some able to translocate both glucose and fructose (10
, which codes for a 540 amino acid protein, is expressed primarily in liver, kidney and placenta and to some extent in chondrocytes, brain, lung and leukocytes (11
also has a demonstrated splice variant, GLUT9
which codes for a 512 amino acids protein expressed only in kidney and placenta (11
was shown to be located in the apical membrane of human kidney proximal tubule epithelial cells, the primary site for renal uric acid regulation (12
). The WDR1
gene appears to affect actin disassembly and help regulate cell morphologic changes during mitosis (13
). No potential functional correlation between WDR1
and uric acid are known, and we therefore choose GLUT9
as our target for further study.
GLUT9 contains 12 exons spanning 195 Kb and is described to have four non-synonymous coding SNPs, Ala17Thr (rs6820230), Val253Ile (rs16890979), Arg265His (rs3733591) and Pro321Leu (rs2280205) (dbSNP, build 128). We genotyped all four non-synonymous coding SNPs in our HAPI Heart sample; Val253Ile GLUT9, was in linkage disequilibrium with rs10489070 (D’ = 0.92, r2 = 0.71) and showed the strongest association with uric acid in an additive fashion, p = 1.43 × 10−11 (). The Val253Ile substitution is in exon 8 of GLUT9 and is located in the region between transmembrane domains 6 and 7. Valine at this position is highly conserved among the orthologs of GLUT9 and is found in all known primate, rodent, and even tetraodon GLUT9 proteins, . This variant was the only significant association among the 4 coding SNPs when all were included in a single model providing evidence that it is associated with uric acid levels independently of other coding variants in the gene and thus is the most likely functional variant.
Non-synonymous coding SNPs in GLUT9, linkage disequilibrium with strongest signal in genome wide scan and each other, effect size controlling for age, sex and family structure (simple model) and age, sex, family structure and other SNPs (full model).
Amino acid sequences flanking the uric acid-associated non-synonymous SNP Val253Ile (rs16890979) in several species.
Since women have significantly lower uric acid levels than men, we examined the effect of Val253Ile in sex-stratified analysis. After adjusting for age, each copy of the Ile allele was associated with 0.47 mg /dl lower uric acid (95% confidence interval 0.31 − 0.63) among women and 0.27 mg/dl lower uric acid (95% confidence interval 0.10 − 0.45) among men (sex by genotype interaction p-value = 0.16). Additional analyses were carried out in women. These results were consistent with a potential modifying effect of estrogen on genotype - uric acid association. Among the 153 women reporting that they had reached menopause, the effect of the Ile allele was more similar to that observed in men, 0.35 mg/dl lower uric acid (95% confidence interval 0.05 − 0.64), while the greatest effect was observed among the 227 women who reported not yet reaching menopause, 0.53 mg/dl less uric acid per Ile allele (95% confidence interval 0.35 − 0.72).
Serum uric acid has been shown to be associated with a number of cardiovascular inflammation, and metabolic traits (14
). We similarly found strong associations between uric acid levels and a panel of cardiovascular risk factors, including percent body fat, triglycerides, HDL, LDL, glucose, insulin, and estimated glomerular filtration rate (eGFR) calculated by the MDRD equation (16
) (). However, no consistent significant associations were identified between the Val253Ile GLUT9
variant and these cardiovascular and metabolic traits (). Similarly, inclusion of each risk factor into the model did not affect the relationship between Val253Ile GLUT9
and uric acid. This result suggests that Val253Ile may affect serum uric acid levels independent of eGFR and known cardiovascular risk factors.
Association between uric acid and other quantitative traits in the HAPI Heart Study. Point estimates are effect on trait with each increase of 1 mg/dl of uric acid.
Association between Val253Ile (rs16890979) and other quantitative traits in the HAPI Heart Study.
Subjects of the HAPI Heart Study were relatively healthy and gout phenotypes were not available. We thus sought to examine association between this clinically significant consequence of elevated uric acid levels and GLUT9
genotype in subjects from the Framingham Heart Study (FHS). The Val253Ile GLUT9
variant was not genotyped in the 100K GWAS that is publicly available (17
), however, rs10489070 was on both the 100K FHS GWAS and the 500K Amish GWAS. This SNP is in linkage disequilibrium with Val253Ile GLUT9
in the HapMap CEU samples (D’ = 0.68, r2
= 0.42) and was associated with uric acid levels in FHS (ex1 GEE p = 0.0001, ex2 GEE p = 0.002). The allele associated with increased uric acid levels was also strongly associated with gout in the FHS (GEE ß (SE) = −0.03 (0.009); p = 0.004). This demonstrates that common variation in GLUT9
in addition to being associated with serum levels of uric acid has direct clinical relevance.