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Abstract
Objective To test the association between ARTS1 polymorphisms and Koreans with ankylosing spondylitis (AS).
Methods All patients and controls were Korean. 872 patients with AS fulfilling the modified New York criteria and 403 healthy controls were genotyped for five single nucleotide polymorphisms (SNPs), rs27044, rs17482078, rs10050860, rs30107 and rs2287987, known to be associated with AS in Caucasians.
Results SNPs rs27044 (p=9.37 × 10−7) and rs30187 (p=7.16 × 10−6) of ARTS1 were significantly associated with AS in Koreans. There was no significant association for rs17482078, rs10050860 and rs2287987. Two four-marker haplotypes were found to be associated with AS (GCCT: p=4.71×10−7, CCCC: p=8.56×10−6).
Conclusions This is first confirmation in a non-Caucasian population that genetic polymorphisms in ARTS1 are associated with AS, implicating common pathogenetic mechanisms in Korean and Caucasian patients with AS.
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Introduction
Ankylosing spondylitis (AS) is a chronic inflammatory disorder primarily affecting the spine. The aetiology is unknown, but multiple genetic factors are involved and account for over 90% of the susceptibility. It is regarded as an oligogenic disease involving three to nine genes, with HLA-B27 undoubtedly being the most significant.1 HLA-B27, by a mechanism still unknown, accounts for up to 40% of the genetic susceptibility and the major histocompatibility complex (MHC) as a whole contributes up to 50%. There is, however, also good evidence to support the role of non-MHC genes in susceptibility to AS.2 Case–control studies using candidate gene approaches have identified CYP2D6 and IL1 as susceptibility genes, but some of these findings have been inconsistent, the effect size small and the level of significance modest.3 4 Genome-wide linkage studies and subsequent meta-analyses have shown that other non-MHC genes on chromosomes 10q, 16q, 1q, 3q, 5q, 6q, 9q, 17q and 19q also contribute to susceptibility for AS.5 6
In 2007, the Wellcome Trust Case Control Consortium (WTCCC) published a genome-wide association study involving a scan of 14 500 non-synonymous single nucleotide polymorphisms (SNPs) in 1500 Caucasian controls and 1000 Caucasian patients with AS. As expected, MHC genes showed the strongest association with AS, but two non-MHC genes, ARTS1 (rs27044, rs17482078, rs10050860, rs30187, rs2287987) and IL23R (rs11209026, rs1004819, rs10489629, rs11465804, rs11465804, rs11209026, rs1343151, rs10889677, rs11209032, rs1495965) also showed strong association.7 The association of ARTS1 and IL23R with AS susceptibility was demonstrated in the British in the WTCCC, and it was also replicated in a North American study with comparable significance; their contribution to population-attributable risk was 26% and 9%, respectively. IL23R has been also implicated in susceptibility to AS in Canadian populations.8 But ARTS1 was first discovered in the genome-wide association study as a susceptibility gene in AS with substantial population-attributable risk.7 The two known functions of ARTS1—trimming of peptides to complement MHC class I presentation in the endoplasmic reticulum9 and modulating shedding of IL1RII,10 IL6Rα,11 and TNFR112 thus downregulating proinflammatory cytokine signalling—also make it a plausible candidate. The association with AS has been found in Caucasians and we sought to examine it in Koreans.
Patients and methods
Subjects
A total of 872 unrelated patients with AS and 403 healthy controls, both groups of Korean ethnic origin, were recruited from the Hospital for Rheumatic Diseases, Hanyang University Medical Centre in Seoul, Korea. All patients with AS fulfilled the modified New York criteria (76.3%) or the criteria for juvenile spondyloarthritis (23.7%).13 All patients provided written informed consent and the Institutional Review Board of Hanyang University Medical Centre approved the study.
The patients were aged 34.9±9.9 years (mean±SD), 88.6% were male, 96.1% were positive for HLA-B27, 49.9% had peripheral joint involvement and 27.8% had a history of uveitis. The control subjects were aged 32.8±6.3 years and 83.4% were male.
Genotyping of ARTS1 SNPs
The five SNPs investigated were rs27044 (C/G polymorphism), rs30187 (C/T polymorphism), rs2287987 (C/T polymorphism), rs10050860 (C/T polymorphism) and rs17482078 (C/T polymorphism). The MassARRAY system (Sequenom, San Diego, California, USA) was used to genotype each study participant in a two-well reaction designed using AssayDesigner 3.1. Genotypes were determined using MassARRAY Typer v.4.0 software.
Statistical analysis
The study was designed to have over 90% power to detect the relative risk of the SNP in the WTCCC study (OR=1.40)7 at a significance level of 5%. We applied Hardy–Weinberg equilibrium in the control group for each SNP. Association tests for allele frequencies of each SNP and reconstructed haplotypes in cases and controls, and LD blocks were determined using Haploview version 3.32. p<0.05 was considered significant.
Results
Genotypes were all in Hardy–Weinberg equilibrium in the controls. Significant positive association of AS with ARTS1 SNPs rs27044 (p=9.37×10−7) and rs30187 (p=7.16×10−6) was seen. The association remained significant after correction for multiple comparisons by Bonferroni method. However, SNPs rs17482078 (p=0.70), rs10050860 (p=0.38) and rs2287987 (p=0.76) showed no association with AS in Korean (table 1).
Association of ARTS1 single nucleotide polymorphisms and haplotypes with ankylosing spondylitis in Koreans
Pairwise D′ values of the five SNPs and haplotype analysis were obtained using Haploview. The four SNPs rs27044, rs17482078, rs10050860 and rs30187 formed an LD block with D′ ranging from 95 to 100, indicating strong or complete LD (figure 1). Four haplotypes were identified and two haplotypes among them showed association with AS (GCCT: p=4.71×10−7, CCCC: p=8.56×10−6) (table 1).
Figure showing tight link between the ARTS1 single nucleotide polymorphisms (SNPs) that is displayed with LD colour scheme of standard (D′/LOD). All D′ values of these five SNPs are over 0.95 as indicated by grey shading. However, r2 values are variable (range 0.03–0.98) as indicated by digits inside the squares. LD block was defined by the use of the confidence interval (Gabriel et al16) method (default method) using Haploview version 3.32.
Statistical power was >80% for rs27044 and rs30187, and <10% for rs17482078, rs100050860 and rs2287987.
In the subset analysis, the significant association of these alleles and haplotype variants remained in the various clinical subsets stratified by the presence of uveitis, peripheral joint involvement and juvenile onset AS.
Discussion
With completion of the Human Genome Project and International HapMap Projects, genome-wide association studies are offering an efficient and effective method of discovering genetic associations with human diseases. Almost 100 loci for over 40 diseases have already been studied, producing new associations as well as confirming previous ones.14 The WTCCC study, which examined 14 500 non-synonymous SNPs, is the most comprehensive study to date in AS.7 It confirmed the undisputable association of MHC genes represented predominantly by HLA-B27 and found an association with the IL23R and ARTS1 genes, which are newly implicated in the susceptibility to AS with high population-attributable risk. The association with ARTS1 in British subjects was replicated in North American subjects in the same study. ARTS1 is an attractive candidate gene in AS because of its biological functions of trimming peptides in the endoplasmic reticulum before assembly with MHC class I molecules presentation and of regulating proinflammatory cytokine receptors.
The G allele frequency of rs270444, the SNP with the strongest association with AS, was 0.53 in Korean patients with AS and 0.43 in Korean controls, while it was 0.34 and 0.27 in British patients and controls, respectively. This is consistent with the results of the International HapMap Project, in which it was 0.43 in Japanese and 0.41 in Chinese, while it was 0.25 in Caucasian and 0.30 in African.15 The relative abundance in Asian controls implies that there must be requisite interactions with other genes, the environment or both. But the discovery of the ARTS1 relationship is shedding a new light on a disease in which there have been few confirmed genetic associations since the discovery of HLA-B27.
In the Korean population, the ARTS1 gene association with AS was replicated with strong significance. To our knowledge this is the first replication in a non-Caucasian population. Genetic associations are not always replicated across racial and ethnic boundaries. This may be due to number of factors, including true ethnic differences, population stratification, lack of type I error control and lack of statistical power. When associations are replicated across wide geographical and racial differences, especially with strong significance as in ARTS1 with AS, it can provide meaningful insights into the pathogenesis of the disease. ARTS1 is a new significant finding in AS, providing new avenues to explore the pathogenic significance of this gene.
References
Footnotes
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Competing interests None.
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Ethics approval This study was conducted with the approval of the Hanyang University Medical Centre.
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Provenance and peer review Not commissioned; externally peer reviewed.