Cigarette smoking, antiphospholipid antibodies and vascular events in Systemic Lupus Erythematosus

OBJECTIVE

Smoking can induce autoantibodies in persons who are genetically predisposed to rheumatoid arthritis. We investigated the association between smoking and antiphospholipid antibodies (aPL) in systemic lupus erythematosus (SLE), a question not previously addressed. Further, we explored the relationship between smoking, aPL and vascular events (arterial and venous, VE).

METHODS

In this cross-sectional study, clinical evaluation and questionnaire data were collected from 367 prevalent SLE patients. At the same time, we measured aPL (anticardiolipin (aCL), anti-β2 glycoprotein-1 (aβ2GP1) antibodies IgG/IgM/IgA, and lupus anticoagulant (LA)), and a large set of other SLE-associated autoantibodies for comparison. Association analyses using logistic regression models with smoking, (ever, former and current with never as reference) and antibody status as outcome variable were performed. As a secondary outcome, we investigated the associations between aPL, smoking and VE.

RESULTS

In multivariable-adjusted models ever, and in particular former, cigarette smoking was associated with the most pathogenic aPL; LA, aCL IgG and aβ2GP1 IgG. Other SLE-associated autoantibodies were not associated with smoking. The combination of smoking and aPL was strongly associated with VE. We noted a positive interaction between smoking-LA and smoking-'triple aPL' positivity for previous VE.

CONCLUSIONS

We investigated a large set of commonly occurring autoantibodies in SLE, but only aPL were positively associated with a history of smoking. This association was especially apparent in former smokers. Among ever regular smokers who were aPL positive, we observed a strikingly high frequency of former VE. The underlying mechanisms and temporality between smoking, aPL and VE need further investigations.

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.

Familial risks and heritability of rheumatoid arthritis: role of rheumatoid factor/anti-citrullinated protein antibody status, number and type of affected relatives, sex, and age

OBJECTIVE

To estimate familial aggregation of rheumatoid arthritis (RA) in 3 large population-representative samples and to test if familial aggregation is affected by rheumatoid factor (RF)/anti-citrullinated protein antibody (ACPA) status, type of relative, sex, and age at onset of RA.

METHODS

A register-based nested case-control study was performed in the Swedish total population. Data on patients with RA were ascertained through the nationwide Swedish Patient Register (n = 88,639), the clinical Swedish Rheumatology Quality Register (n = 11,519), and the Epidemiological Investigation of Rheumatoid Arthritis case-control study (n = 2,871). Data on first- and second-degree relatives were obtained through the Swedish Multigeneration Register. Familial risks were calculated using conditional logistic regression.

RESULTS

Consistent across data sources, the familial odds ratio for RA was ∼3 in first-degree relatives of RA patients and 2 in second-degree relatives. Familial risks were similar among siblings, parents, and offspring. Familial aggregation was not modified by sex, but was higher in RA patients with early-onset disease and in RF/ACPA-positive RA patients. The observed familial risks were consistent with a heritability of ∼50% for ACPA-positive RA and ∼20% for ACPA-negative RA.

CONCLUSION

The pattern of risks suggests that familial factors influence RA in men and women equally and that these factors are of less importance for late-onset RA. Familial factors are more important for seropositive RA, but there is significant familial overlap between seropositive RA and seronegative RA. Even if the familial risk is assumed to be completely due to genetics, the observed risks suggest that heritability of RA is lower than previously reported, in particular for ACPA-negative RA.

Genetic risk scores and number of autoantibodies in patients with rheumatoid arthritis

OBJECTIVE

Certain HLA-DRB1 alleles and single-nucleotide polymorphisms (SNPs) are associated with rheumatoid arthritis (RA). Our objective was to examine the combined effect of these associated variants, calculated as a cumulative genetic risk score (GRS) on RA predisposition, as well as the number of autoantibodies (none, one or two present).

METHOD

We calculated four GRSs in 4956 patients and 4983 controls from four European countries. All four scores contained data on 22 non-HLA-risk SNPs, and three scores also contained HLA-DRB1 genotypes but had different HLA typing resolution. Most patients had data on both rheumatoid factor (RF) and anti-citrullinated proteins antibodies (ACPA). The GRSs were standardised (std.GRS) to account for population heterogeneity. Discrimination between patients and controls was examined by receiveroperating characteristics curves, and the four std.GRSs were compared across subgroups according to autoantibody status.

RESULTS

The std.GRS improved its discriminatory ability between patients and controls when HLA-DRB1 data of higher resolution were added to the combined score. Patients had higher mean std.GRS than controls (p=7.9×10(-156)), and this score was significantly higher in patients with autoantibodies (shown for both RF and ACPA). Mean std.GRS was also higher in those with two versus one autoantibody (p=3.7×10(-23)) but was similar in patients without autoantibodies and controls (p=0.12).

CONCLUSIONS

The GRS was associated with the number of autoantibodies and to both RF and ACPA positivity. ACPA play a more important role than RF with regards to the genetic risk profile, but stratification of patients according to both RF and ACPA may optimise future genetic studies.

Ambient air pollution exposures and risk of rheumatoid arthritis

OBJECTIVE

Environmental factors may play a role in the development of rheumatoid arthritis (RA). We previously observed increased RA risk among women living closer to major roads (a source of air pollution). Herein, we examined whether long-term exposures to specific air pollutants were associated with RA risk among women in the Nurses' Health Study (NHS).

METHODS

The NHS is a large US cohort of female nurses followed up prospectively every 2 years since 1976. We studied 111,425 NHS participants with information on air pollution exposures as well as data concerning other lifestyle and behavioral exposures and disease outcomes. Outdoor levels of different size fractions of particulate matter (PM10 and PM2.5 ) and gaseous pollutants (SO2 and NO2 ) were predicted for all available residential addresses using monitoring data from the US Environmental Protection Agency. We examined the association of time-varying exposures 6 and 10 years before each questionnaire cycle and cumulative average exposure with the risk of RA, seronegative (rheumatoid factor and anti-citrullinated peptide antibody negative) RA, and seropositive RA.

RESULTS

Over the 3,019,424 person-years of followup, 858 incident RA cases were validated by medical record review by 2 board-certified rheumatologists. Overall, we found no evidence of increased risk of RA, seronegative RA, or seropositive RA with exposure to the different pollutants and little evidence of effect modification by socioeconomic status or smoking status, geographic region, or calendar period.

CONCLUSION

In this group of socioeconomically advantaged middle-aged and elderly women, adult exposures to air pollution were not associated with an increased RA risk.

Parity and the risk of developing rheumatoid arthritis: results from the Swedish Epidemiological Investigation of Rheumatoid Arthritis study

OBJECTIVE

To study the impact of parity history on the risk of antibodies to citrullinated peptide antigens (ACPA) positive and ACPA-negative rheumatoid arthritis (RA), in different age-groups.

METHOD

Data from a population-based case-control study of female incident RA cases were analysed (2035 cases and 2911 controls, aged 18-70 years ). Parity history was assessed through a questionnaire. Parous women were compared with nulliparous, by calculating odds ratios (ORs) with 95% confidence interval (CI).

RESULTS

Parity was associated with an increased risk of ACPA-negative RA in women aged 18-44 years (OR=2.1, 95% CI 1.4 to 3.2), but not in those aged 45-70 years (OR=0.9, 95% CI 0.7 to 1.3). Among young women, an increased risk of ACPA-negative RA was found in those who gave birth during the year of symptom onset (OR=2.6, 95% CI 1.4 to 4.8) and who were at a young age at first birth (<23) (OR=2.5, 95% CI 1.5 to 4.1). Parity and the postpartum period were not associated with ACPA-positive RA, but older age at first birth was weakly associated with a decreased risk.

CONCLUSIONS

The increased risk of ACPA-negative RA in parous women of reproductive age seemed to be associated with an increased postpartum risk and with young age at first birth. Further research is needed to explore the biological mechanisms behind our findings.

Long-term growth of children with autoantibody-mediated congenital heart block

AIM

To analyse growth of children with and without congenital heart block (CHB) born to anti-Ro/SSA positive mothers from birth to 18 years of age, using a population-based cohort of Swedish CHB patients.

METHODS

Medical records for siblings with (n = 72) and without (n = 60) CHB born 1973-2009 to anti-Ro/SSA positive mothers were retrieved from child healthcare centres and school health services and used to extract data on growth from birth to 18 years.

RESULTS

Compared with reference standards, children with CHB were retarded in weight by 0.75-1.0 SD from birth to 2-3 years of age. Thereafter, the CHB children started to catch up, reaching the reference standards at 9-11 years of age. Pacemaker treatment was not correlated with the catch-up in growth. Individuals with CHB were retarded in both weight and height from birth to 9-11 years of age when compared to siblings without CHB, who did not demonstrate restriction in these measurements.

CONCLUSION

Presence of CHB is a more important predictor of growth restriction than maternal rheumatic disease and foetal anti-Ro/SSA exposure. The restriction persists for several years after birth, despite pacemaker treatment, which highlights the importance of follow-up of children with CHB regarding nutrition and growth.

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.

Thyroxin substitution and the risk of developing rheumatoid arthritis; results from the Swedish population-based EIRA study

OBJECTIVES

Hypothyroidism in iodine-repleted areas is usually of autoimmune nature and leads to chronic thyroxin substitution. It shares some risk factors with anti-citrullinated peptide antibodies (ACPA)-positive rheumatoid arthritis (RA). We asked whether thyroxin substitution associated with risk of ACPA-positive or ACPA-negative RA, and whether interactions with established risk factors were present.

METHODS

Data from a population-based case-control study with incident RA cases were analysed (1998 adult cases, 2252 controls). Individuals reporting thyroxin substitution were compared with those without thyroxin, by calculating OR with 95% CI, excluding participants reporting non-autoimmune causes for thyroxin substitution (thyroid cancer, iodine-containing drugs). Interaction was evaluated by attributable proportion (AP) with 95% CI.

RESULTS

Thyroxin substitution was associated with a twofold risk of both ACPA-positive (OR=1.9, 95% CI 1.4 to 2.6) and ACPA-negative RA (OR=2.1, 95% CI 1.5 to 3.1). For ACPA-positive RA, the risk associated with the combination thyroxin+ HLA-DRB1 shared epitope alleles (SE) was much higher (OR=11.8, 95% CI 6.9 to 20.0) than for thyroxin (OR=1.4, 95% CI 0.7 to 3.0) or SE (OR=5.7, 95% CI 4.6 to 6.9) alone, indicating a strong interaction (AP=0.5, 95% CI 0.2 to 0.8). Thyroxin substitution interacted non-significantly with smoking (AP=0.4, 95% CI 0.0 to 0.7; OR thyroxin+smoking=3.6, thyroxin only=1.5, smoking only=1.8). Thyroxin did not interact with the PTPN22*R620W allele.

CONCLUSIONS

Thyroxin users had a doubled risk of both ACPA-positive and ACPA-negative RA. The risk of ACPA-positive RA was manifold if they smoked or carried the SE. Furthermore, although joint symptoms can be a manifestation of hypothyroidism, physicians might consider whether it could be an early manifestation of RA.

HLA-DRB1*04/*13 alleles are associated with vascular disease and antiphospholipid antibodies in systemic lupus erythematosus

BACKGROUND AND OBJECTIVES

Vascular disease is common in systemic lupus erythematosus (SLE) and patients with antiphospholipid antibodies (aPL) are at high risk to develop arterial and venous thrombosis. Since HLA class II genotypes have been linked to the presence of pro-thrombotic aPL, we investigated the relationship between HLA-DRB1 alleles, aPL and vascular events in SLE patients.

METHODS

665 SLE patients of Caucasian origin and 1403 controls were included. Previous manifestations of ischaemic heart disease, ischaemic cerebrovascular disease (ICVD) and venous thromboembolism (together referred to as any vascular events (AVE)) were tabulated. aPL were measured with ELISA. Two-digit HLA-DRB1 typing was performed by sequence-specific primer-PCR.

RESULTS

HLA-DRB1*04 was more frequent among SLE patients with ICVD compared to unaffected patients. This association remained after adjustment for known traditional cardiovascular risk factors. HLA-DRB1*13 was associated with AVE. All measured specificities of aPL-cardiolipin IgG and IgM, β2-glycoprotein-1 IgG, prothrombin (PT) IgG and a positive lupus anticoagulant test were associated with HLA-DRB1*04-while HLA-DRB1*13 was associated with IgG antibodies (β2-glycoprotein-1, cardiolipin and PT). In patients with the combined risk alleles, HLA-DRB1*04/*13, there was a significant additive interaction for the outcomes AVE and ICVD.

CONCLUSIONS

The HLA-DRB1*04 and HLA-DRB1*13 alleles are associated with vascular events and an aPL positive immune-phenotype in SLE. Results demonstrate that a subset of SLE patients is genetically disposed to vascular vulnerability.

Ambient air pollution exposures and risk of rheumatoid arthritis: results from the Swedish EIRA case-control study

OBJECTIVE

Environmental factors may play a role in the development of rheumatoid arthritis (RA). We examined whether long-term exposures to air pollution were associated with the risk of RA in the Swedish Epidemiological Investigation of Rheumatoid Arthritis Study.

METHODS

We studied 1497 incident RA cases and 2536 controls. Local levels of particulate matter (PM10) and gaseous pollutants (sulphur dioxide (SO2) and nitrogen dioxide (NO2)) from traffic and home heating were predicted for all residential addresses. We examined the association of an IQR increase (2 µg/m3 for PM10, 8 µg/m3 for SO2 and 9 µg/m3 for NO2) in each pollutant at different time points before symptom onset and average exposure with the risk of all RA and the risk of the rheumatoid factor and anti-citrullinated protein antibody (ACPA) RA phenotypes.

RESULTS

There was no evidence of an increased risk of RA with PM10. Total RA risks were modestly elevated for the gaseous pollutants, but were not statistically significant after adjustment for smoking and education (OR 1.18, 95% CI 0.97 to 1.43 and OR 1.09, 95% CI 0.99 to 1.19 for SO2 and NO2 in the 10th year before onset). Stronger elevated risks were observed for individuals with less than a university education and with the ACPA-negative RA phenotype.

CONCLUSIONS

No consistent overall associations between air pollution in the Stockholm area and the risk of RA were observed. However, there was a suggestion of increased risks of RA incidence with increases in NO2 from local traffic and SO2 from home heating sources with stronger associations for the ACPA-negative phenotype.

Usage of skin care products and risk of rheumatoid arthritis: results from the Swedish EIRA study

INTRODUCTION

The aim of this study was to investigate the association between exposure to cosmetics, often containing mineral oil, and the risk of developing rheumatoid arthritis (RA). The study was performed against the background that occupational exposure to mineral oil has recently been shown to be associated with an increased risk for RA in man, and that injection of or percutaneous exposure to mineral-oil-containing cosmetics can induce arthritis in certain rat strains.

METHODS

A population-based case-control study of incident cases of RA was performed among the population aged 18 to 70 years in a defined area of Sweden during May 1996 to December 2003. A case was defined as an individual from the study base, who received for the first time a diagnosis of RA according to the 1987 American College of Rheumatology criteria. Controls were randomly selected from the study base with consideration taken for age, gender and residential area. Cases (n = 1,419) and controls (n = 1,674) answered an extensive questionnaire regarding environmental and lifestyle factors including habits of cosmetic usage. The relative risk of developing RA was calculated for subjects with different cosmetic usage compared with subjects with low or no usage. Analysis was also performed stratifying the cases for presence/absence of rheumatoid factor and antibodies to citrulline-containing peptides.

RESULTS

The relative risks of developing RA associated with use of cosmetics were all close to one, both for women and men, for different exposure categories, and in relation to different subgroups of RA.

CONCLUSION

This study does not support the hypothesis that ordinary usage of common cosmetics as body lotions, skin creams, and ointments, often containing mineral oil, increase the risk for RA in the population in general. We cannot exclude, however, that these cosmetics can contribute to arthritis in individuals carrying certain genotypes or simultaneously being exposed to other arthritis-inducing environmental agents.

Development of heart block in children of SSA/SSB-autoantibody-positive women is associated with maternal age and displays a season-of-birth pattern

OBJECTIVE

Congenital heart block may develop in the fetuses of Ro/SSA-positive and La/SSB-positive mothers. Recurrence rates of only 10-20% despite persisting maternal antibodies indicate that additional factors are critical for the establishment of heart block. The authors investigated the influence of other maternal and fetal factors on heart block development in a Swedish population-based cohort.

METHODS

The influence of fetal gender, maternal age, parity and time of birth on heart block development was analysed in 145 families, including Ro/La-positive (n=190) and Ro/La-negative (n=165) pregnancies.

RESULTS

There was a recurrence rate of 12.1% in Ro/La-positive women, and no recurrence in Ro/La-negative women. Fetal gender and parity did not influence the development of heart block in either group. Maternal age in Ro/La-positive pregnancies with a child affected by heart block was, however, significantly higher than in pregnancies resulting in babies without heart block (p<0.05).Seasonal timing of pregnancy influenced the outcome. Gestational susceptibility weeks 18-24 occurring during January-March correlated with a higher proportion of children with heart block and lower vitamin D levels during the same period in a representative sample of Swedish women and a corresponding higher proportion of children with heart block born in the summer (p<0.02). Maternal age or seasonal timing of pregnancy did not affect the outcome in Ro/La-negative pregnancies.

CONCLUSION

This study identifies maternal age and seasonal timing of pregnancy as novel risk factors for heart block development in children of Ro/La-positive women. These observations may be useful for counselling when pregnancy is considered.

GEIRA: gene-environment and gene-gene interaction research application

The GEIRA (Gene-Environment and Gene-Gene Interaction Research Application) algorithm and subsequent program is dedicated to genome-wide gene-environment and gene-gene interaction analysis. It implements concepts of both additive and multiplicative interaction as well as calculations based on dominant, recessive and co-dominant genetic models, respectively. Estimates of interactions are incorporated in a single table to make the output easily read. The algorithm is coded in both SAS and R. GEIRA is freely available to non-commercial users at http://www.epinet.se. Additional information, including user's manual and example datasets is available online at http://www.epinet.se.

Smoking is a major preventable risk factor for rheumatoid arthritis: estimations of risks after various exposures to cigarette smoke

BACKGROUND

Earlier studies have demonstrated that smoking and genetic risk factors interact in providing an increased risk of rheumatoid arthritis (RA). Less is known on how smoking contributes to RA in the context of genetic variability, and what proportion of RA may be caused by smoking.

OBJECTIVES

To determine the association between the amount of smoking and risk of RA in the context of different HLA-DRB1 shared epitope (SE) alleles, and to estimate proportions of RA cases attributed to smoking.

DESIGN

Setting and Participants Data from the Swedish Epidemiological Investigation of Rheumatoid Arthritis (EIRA) case-control study encompassing 1204 cases and 871 controls were analysed. Main Outcome Measure Estimated OR to develop RA and excess fraction of cases attributable to smoking according to the amount of smoking and genotype.

RESULTS

Smoking was estimated to be responsible for 35% of anticitrullinated protein/peptide antibody (ACPA)-positive cases. For each HLA-DRB1 SE genotype, smoking was dose-dependently associated with an increased risk of ACPA-positive RA (p trend <0.001). In individuals carrying two copies of the HLA-DRB1 SE, 55% of ACPA-positive RA was attributable to smoking.

CONCLUSIONS

Smoking is a preventable risk factor for RA. The increased risk due to smoking is dependent on the amount of smoking and genotype.

Common vaccinations among adults do not increase the risk of developing rheumatoid arthritis: results from the Swedish EIRA study

OBJECTIVE

To investigate the association between vaccinations in adults and the risk of developing rheumatoid arthritis (RA).

METHODS

Data from the Swedish population-based Epidemiological Investigation of RA case-control study encompassing 1998 incident cases of RA aged 18-70 years and 2252 randomly selected controls matched for age, sex and residency were analysed. Those vaccinated within 5 years before disease onset were compared with those not vaccinated by calculating OR with 95% CI.

RESULTS

Vaccinations neither increased the risk of RA overall (OR 1.0, 95% CI 0.9 to 1.1) nor the risk of two major subgroups of RA (antibodies to citrullinated peptide-positive (ACPA-positive) and ACPA-negative disease). Furthermore, vaccinations did not increase the risk of RA in smokers or carriers of HLA-DRB1 shared epitope alleles, two groups with established risk factors for RA.

CONCLUSIONS

In this case-control study of incident cases of newly diagnosed RA, no increased risk of RA following immunisation was observed for vaccinations overall or for any specific vaccination. This indicates that immunological provocation of adults with commonly used vaccines in their present form carries no risk of RA. These findings should be implemented among public healthcare providers in order to encourage vaccinations according to recommended national vaccination schedules.

Evidence for interaction between 5-hydroxytryptamine (serotonin) receptor 2A and MHC type II molecules in the development of rheumatoid arthritis

It has repeatedly been suggested that the development of complex diseases can be elucidated by gene-gene interactions. Recently, we found that HTR2A, a member of the serotonin receptor family, is associated with rheumatoid arthritis (RA). This study was aimed to investigate the possibility of a gene-gene interaction between HTR2A and the major genetic risk factor for RA, HLA-DRB1 shared epitope (SE) alleles. We studied 4095 RA cases and 3223 controls from three different populations - from Sweden, the United States and the Netherlands - to test for interaction between the protective HTR2A haplotype and HLA-DRB1 SE alleles. Further, we analyzed mRNA and/or protein expression of HTR2A and HLA-DR in biopsy samples and in synovial fibroblasts from RA patients. The interaction was defined as departure from additivity of effects using attributable proportion due to interaction. First, we could demonstrate and further replicate an interaction between a protective haplotype in HTR2A and HLA-DRB1 SE alleles regarding risk of developing autoantibody-positive RA. Second, we could show that both genes are constitutively expressed in fibroblasts from synovial tissue of RA patients, and, by double immunofluorescence staining, we demonstrated that these two proteins are colocalized in these cells. In conclusion, our data demonstrate a statistical interaction between HTR2A and HLA-DRB1 SE alleles and colocalization of the product of these two genes in inflamed synovial tissue, which suggest a possible biological relationship between these two proteins. This finding may lead to the development of treatment based on enhancing the protective features of 5-HT2A in individuals with a certain HLA genotype.

Silica exposure among male current smokers is associated with a high risk of developing ACPA-positive rheumatoid arthritis

Objective

To study the association between silica exposure, separately as well as combined with smoking, and the risk of developing rheumatoid arthritis (RA) with or without the presence of antibodies against citrullinated peptide antigens (ACPA).

Methods

This Swedish population based case–control study analysed 577 incident RA cases and 659 randomly selected controls, all men aged 18–70 years, included during May 1996 to May 2006. Self-reported silica exposure, defined as exposure to stone dust, rock drilling or stone crushing and cigarette smoking was registered. ACPA status among cases was analysed.

Results

Silica-exposed subjects were found to have a moderately increased risk of ACPA-positive RA (odds ratio (OR) adjusted for age and residency=1.67 (95% CI 1.13 to 2.48), but not of ACPA-negative RA (OR=0.98 (95% CI 0.57 to 1.66)), compared with subjects unexposed to silica. Subjects exposed to rock drilling were found to have a somewhat more markedly increased risk of ACPA-positive RA (OR=2.34 (95% CI 1.17 to 4.68)). A high risk of developing ACPA-positive RA was observed among silica-exposed current smokers (OR=7.36 (95% CI 3.31 to 16.38)), exceeding the risk expected from the separate effects of silica exposure and current smoking, indicating an interaction between these exposures (attributable proportion due to interaction=0.60 (95% CI 0.26 to 0.95)).

Conclusion

Silica exposure combined with smoking among men is associated with an increased risk of developing ACPA-positive RA. These results suggest that different inhalation exposures may interact in the aetiology of ACPA-positive RA.

Specific interaction between genotype, smoking and autoimmunity to citrullinated alpha-enolase in the etiology of rheumatoid arthritis

Gene-environment associations are important in rheumatoid arthritis (RA) susceptibility, with an association existing between smoking, HLA- DRB1 'shared epitope' alleles, PTPN22 and antibodies to cyclic citrullinated peptides (CCP). Here, we test the hypothesis that a subset of the anti-CCP response, with specific autoimmunity to citrullinated alpha-enolase, accounts for an important portion of these associations. In 1,497 individuals from three RA cohorts, antibodies to the immunodominant citrullinated alpha-enolase CEP-1 epitope were detected in 43-63% of the anti-CCP-positive individuals, and this subset was preferentially linked to HLA-DRB1*04. In a case-control analysis of 1,000 affected individuals and 872 controls, the combined effect of shared epitope, PTPN22 and smoking showed the strongest association with the anti-CEP-1-positive subset (odds ratio (OR) of 37, compared to an OR of 2 for the corresponding anti-CEP-1-negative, anti-CCP-positive subset). We conclude that citrullinated alpha-enolase is a specific citrullinated autoantigen that links smoking to genetic risk factors in the development of RA.

Gene-environment interaction between the DRB1 shared epitope and smoking in the risk of anti-citrullinated protein antibody-positive rheumatoid arthritis: all alleles are important

OBJECTIVE

An interaction effect for developing rheumatoid arthritis (RA) was previously observed between HLA-DRB1 shared epitope (SE) alleles and smoking. We aimed to further investigate this interaction between distinct SE alleles and smoking regarding the risk of developing RA with and without anti-citrullinated protein antibodies (ACPAs).

METHODS

We used data regarding smoking habits and HLA-DRB1 genotypes from 1,319 patients and 943 controls from the Epidemiological Investigation of Rheumatoid Arthritis, in which 972 patients and 488 controls were SE positive. Subsequently, 759 patients and 328 controls were subtyped for specific alleles within the DRB1*04 group. Odds ratios with 95% confidence intervals (95% CIs) were calculated by means of logistic regression. Interaction was evaluated by calculating attributable proportion due to interaction, with 95% CIs.

RESULTS

A strong interaction between smoking and SE alleles in the development of ACPA-positive RA was observed for all DRB1*04 SE alleles taken as a group (relative risk [RR] 8.7 [95% CI 5.7-13.1]) and for the *0401 and *0404 alleles (RR 8.9 [95% CI 5.8-13.5]) and the *01 and *10 alleles (RR 4.9 [95% CI 3.0-7.8]) as specific, separate groups, with similar strength of interaction for the different groups (attributable proportion due to interaction 0.4 [95% CI 0.2-0.6], 0.5 [95% CI 0.3-0.7], and 0.6 [95% CI 0.4-0.8], respectively).

CONCLUSION

There is a statistically significant interaction between distinct DRB1 SE alleles and smoking in the development of ACPA-positive RA. Interaction occurs with the *04 group as well as the *01/*10 group, demonstrating that regardless of fine specificity, all SE alleles strongly interact with smoking in conferring an increased risk of ACPA-positive RA.