A genome-wide association study of rheumatoid arthritis without antibodies against citrullinated peptides.

INTRODUCTION: Rheumatoid arthritis (RA) patients can be classified based on presence or absence of anticitrullinated peptide antibodies (ACPA) in their serum. This heterogeneity among patients may reflect important biological differences underlying the disease process. To date, the majority of genetic studies have focused on the ACPA-positive group. Therefore, our goal was to analyse the genetic risk factors that contribute to ACPA-negative RA. METHODS: We performed a large-scale genome-wide association study (GWAS) in three Caucasian European cohorts comprising 1148 ACPA-negative RA patients and 6008 controls. All patients were screened using the Illumina Human Cyto-12 chip, and controls were genotyped using different genome-wide platforms. Population-independent analyses were carried out by means of logistic regression. Meta-analysis with previously published data was performed as follow-up for selected signals (reaching a total of 1922 ACPA-negative RA patients and 7087 controls). Imputation of classical HLA alleles, amino acid residues and single nucleotide polymorphisms was undertaken. RESULTS: The combined analysis of the studied cohorts resulted in identification of a peak of association in the HLA-region and several suggestive non-HLA associations. Meta-analysis with previous reports confirmed the association of the HLA region with this subset and an observed association in the CLYBL locus remained suggestive. The imputation and deep interrogation of the HLA region led to identification of a two amino acid model (HLA-B at position 9 and HLA-DRB1 at position 11) that accounted for the observed genome-wide associations in this region. CONCLUSIONS: Our study shed light on the influence of the HLA region in ACPA-negative RA and identified a suggestive risk locus for this condition.

The influence of polygenic risk scores on heritability of anti-CCP level in RA.

The objective of this study was to study genetic factors that influence quantitative anticyclic citrullinated peptide (anti-CCP) antibody levels in RA patients. We carried out a genome-wide association study (GWAS) meta-analysis using 1975 anti-CCP+ RA patients from three large cohorts, the Brigham Rheumatoid Arthritis Sequential Study (BRASS), North American Rheumatoid Arthritis Consortium (NARAC) and the Epidemiological Investigation of RA (EIRA). We also carried out a genome-wide complex trait analysis (GCTA) to estimate the heritability of anti-CCP levels. GWAS-meta-analysis showed that anti-CCP levels were most strongly associated with the human leukocyte antigen (HLA) region with a P-value of 2 × 10(-11) for rs1980493. There were 112 SNPs in this region that exceeded the genome-wide significance threshold of 5 × 10(-8), and all were in linkage disequilibrium (LD) with the HLA- DRB1*03 allele with LD r(2) in the range of 0.25-0.88. Suggestive novel associations outside of the HLA region were also observed for rs8063248 (near the GP2 gene) with a P-value of 3 × 10(-7). None of the known RA risk alleles (∼52 loci) were associated with anti-CCP level. Heritability analysis estimated that 44% of anti-CCP variation was attributable to genetic factors captured by GWAS variants. In summary, anti-CCP level is a heritable trait, and HLA-DR3 and GP2 are associated with lower anti-CCP levels.

Bounds on causal interactions for binary outcomes.

A common goal of epidemiologic research is to study how two exposures interact in causing a binary outcome. Causal interaction is defined as the presence of subjects for which the causal effect of one exposure depends on the level of the other exposure. For binary exposures, it has previously been shown that the presence of causal interaction is testable through additive statistical interaction. However, it has also been shown that the magnitude of causal interaction, defined as the proportion of subjects for which there is causal interaction, is generally not identifiable. In this article, we derive bounds on causal interactions, which are applicable to binary outcomes and categorical exposures with arbitrarily many levels. These bounds can be used to assess the magnitude of causal interaction, and serve as an important complement to the statistical test that is frequently employed. The bounds are derived both without and with an assumption about monotone exposure effects. We present an application of the bounds to a study of gene-gene interaction in rheumatoid arthritis.

Regional differences regarding risk of developing rheumatoid arthritis in Stockholm County, Sweden: results from the Swedish Epidemiological Investigation of Rheumatoid Arthritis (EIRA) study.

OBJECTIVES: Rheumatoid arthritis (RA) is a complex disease that is associated with genetic and environmental factors. We have investigated geospatial variation in the risk of developing RA within Stockholm County, Sweden, with respect to established environmental risk factors for RA, as well as serologically defined subgroups of RA. METHOD: Information regarding geographical location for 1432 cases and 2529 controls from the Epidemiological Investigation of Rheumatoid Arthritis (EIRA) study, living in Stockholm County at RA symptom onset, or matched date for controls, was used to estimate geospatial variation in risk. We used generalized additive models (GAMs) to create a risk surface, calculate odds ratios (ORs), and adjust for potential confounding by smoking, education level, and RA within family. We performed a stratified analysis based on the presence/absence of anti-citrullinated peptide antibodies (ACPA). RESULTS: We found significant spatial variation in the odds of developing RA in Stockholm County. After adjustment for smoking, education level, and family history of RA, this geospatial variation remained. The stratified analysis showed areas with higher ORs for ACPA-positive RA and ACPA-negative RA, after adjusting for smoking, education level, and having a family history of RA. Living in the city of Stockholm was associated with decreased risk of RA. CONCLUSIONS: The risk of developing RA in Stockholm County is not distributed evenly and there are areas of increased risk that could not be explained by known factors. Further investigations of local exposures or social factors are warranted.

Alcohol consumption is associated with decreased risk of rheumatoid arthritis: results from two Scandinavian case-control studies.

OBJECTIVES: To determine the association between risk of rheumatoid arthritis (RA) and alcohol consumption in combination with smoking and HLA-DRB1 shared epitope (SE). METHODS: Data from two independent case-control studies of RA, the Swedish EIRA (1204 cases and 871 controls) and the Danish CACORA (444 cases and 533 controls), were used to estimate ORs of developing RA for different amounts of alcohol consumed. RESULTS: Alcohol consumption was significantly more common in controls (p<0.05) and dose-dependently associated with reduced risk of RA (p for trend <0.001) in both studies. Among alcohol consumers, the quarter with the highest consumption had a decreased risk of RA of the order of 40-50% compared with the half with the lowest consumption (EIRA, OR = 0.5 (95% CI 0.4 to 0.6); CACORA, OR = 0.6 (95% CI 0.4 to 0.9)). For the subset of RA that is seropositive for antibodies to citrullinated peptide antigens, alcohol consumption reduced the risk most in smokers carrying HLA-DRB1 SE alleles. CONCLUSIONS: The observed inverse association between alcohol intake and risk of RA and the recent demonstration of a preventive effect of alcohol in experimental arthritis indicate that alcohol may protect against RA. This highlights the potential role of lifestyle in determining the risk of developing RA, and emphasises the advice to stop smoking, but not necessarily to abstain from alcohol in order to diminish risk of RA. The evidence of potential RA prevention should prompt additional studies on how this can be achieved.

Silica exposure is associated with increased risk of developing rheumatoid arthritis: results from the Swedish EIRA study.

OBJECTIVE: To study the association between silica exposure and rheumatoid arthritis and how it is modified by cigarette smoking. METHODS: Data were analysed from 276 male cases and 276 male controls aged 18 to 70 years, included in a Swedish population based study between May 1996 and June 2001. A case was defined as a person recently diagnosed with rheumatoid arthritis according to the ACR criteria. Controls were selected from the study base as a stratified random sample accounting for age, sex, and residency. Men with a self reported history of work with rock drilling, stone crushing, or exposure to stone dust in general were defined as silica exposed. Rheumatoid factor (RF) status among cases was recorded. RESULTS: Silica exposed men had increased risk of rheumatoid arthritis, with an odds ratio (OR), adjusted for age, residential area, and smoking, of 2.2 (95% confidence interval, 1.2 to 3.9) among men aged 18 to 70 years, and 2.7 (1.2 to 5.8) among those aged 50 to 70 years. Men who had worked with rock drilling or stone crushing (regarded as highly exposed) had a slightly greater increase in risk of rheumatoid arthritis than silica exposed men in general, with an OR of 3.0 (1.2 to 7.6). The joint effects of silica exposure and smoking were compatible with synergy between these two exposures in the development of rheumatoid arthritis but this was not conclusive. CONCLUSIONS: Silica exposure is associated with increased risk of developing rheumatoid arthritis. This association is not explained by smoking habits.