Interactions between amino acid-defined major histocompatibility complex class II variants and smoking in seropositive rheumatoid arthritis

Update on the genetic architecture of rheumatoid arthritis

Human genetic studies into rheumatoid arthritis (RA) have uncovered more than 100 genetic loci associated with susceptibility to RA and have refined the RA-association model for HLA variants. The majority of RA-risk variants are highly shared across multiple ancestral populations and are located in noncoding elements that might have allele-specific regulatory effects in relevant tissues. Emerging multi-omics data, high-density genotype data and bioinformatic approaches are enabling researchers to use RA-risk variants to identify functionally relevant cell types and biological pathways that are involved in impaired immune processes and disease phenotypes. This Review summarizes reported RA-risk loci and the latest insights from human genetic studies into RA pathogenesis, including how genetic data has helped to identify currently available drugs that could be repurposed for patients with RA and the role of genetics in guiding the development of new drugs.

Mechanisms involved in triggering rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory syndrome with a strong autoimmune component. The autoantigens in RA are neither tissue nor organ-specific, but comprise a broad collection of post-translational modified proteins, such as citrullinated proteins. These modifications are likely to be triggered by innate stimuli. In genetically susceptible hosts, they can lead to a more substantiated secondary autoimmune reaction targeting the joints and precipitating the clinical onset of RA. Both innate and adaptive mechanisms will then closely interplay to promote chronic joint inflammation in the several absence of appropriate treatment. This scenario, is shared with other autoimmune diseases where potentially pathogenic immune responses are present already before disease onset. Better understanding of these processes will allow both earlier diagnosis of RA and identification of those healthy individuals that are at risk of developing disease, opening possibilities for disease prevention. In this review, we discuss the iterative processes of innate and adaptive immunity responsible for the (longitudinal) development of immune reactions that may contribute to the development of RA.

Associations of Smoking and Age With Inflammatory Joint Signs Among Unaffected First-Degree Relatives of Rheumatoid Arthritis Patients: Results From Studies of the Etiology of Rheumatoid Arthritis

OBJECTIVE

To examine whether genetic, environmental, and serologic rheumatoid arthritis (RA) risk factors are associated with inflammatory joint signs in a cohort of first-degree relatives (FDRs) of RA patients.

METHODS

We evaluated RA risk factors and inflammatory joint signs in a prospective cohort of FDRs without RA in the Studies of the Etiology of RA. Genetic factors included 5 HLA-DRB1 shared epitope alleles and 45 RA-associated single-nucleotide polymorphisms; loci were combined using genetic risk scores weighted by RA risk. Environmental factors (smoking, body mass index, education, and parity) and RA-related autoantibodies were assessed at baseline. Physical examination was performed at baseline and 2-year follow-up, by observers who were blinded with regard to autoantibody status, to assess inflammatory joint signs as tender or swollen joints at sites typical for RA. Logistic regression was performed to evaluate associations of genetic, environmental, and serologic factors with inflammatory joint signs.

RESULTS

We analyzed 966 non-Hispanic white FDRs at baseline and 262 at 2-year follow-up after excluding those with inflammatory joint signs at baseline. The mean ± SD age was 47.2 ± 15.5 years, 71% were female, and 55% were shared epitope positive. Smoking >10 pack-years was associated with inflammatory joint signs at baseline (odds ratio [OR] 1.89 [95% confidence interval (95% CI) 1.26-2.82]) and at 2 years (OR 2.66 [95% CI 1.01-7.03]), compared to never smokers. There was a significant interaction between smoking and age with regard to risk of inflammatory joint signs (P = 0.02). FDRs younger than 50 years with >10 pack-years had the highest risk of inflammatory joint signs (OR 4.39 [95% CI 2.22-8.66], compared to never smokers younger than 50 years).

CONCLUSION

In a high-risk cohort of FDRs, smoking and age were associated with both prevalent and incident inflammatory joint signs at sites typical for RA. Further prospective investigations of the factors affecting the transitions between preclinical RA phases are warranted.

Major histocompatibility complex harbors widespread genotypic variability of non-additive risk of rheumatoid arthritis including epistasis

Genotypic variability based genome-wide association studies (vGWASs) can identify potentially interacting loci without prior knowledge of the interacting factors. We report a two-stage approach to make vGWAS applicable to diseases: firstly using a mixed model approach to partition dichotomous phenotypes into additive risk and non-additive environmental residuals on the liability scale and secondly using the Levene's (Brown-Forsythe) test to assess equality of the residual variances across genotype groups per marker. We found widespread significant (P < 2.5e-05) vGWAS signals within the major histocompatibility complex (MHC) across all three study cohorts of rheumatoid arthritis. We further identified 10 epistatic interactions between the vGWAS signals independent of the MHC additive effects, each with a weak effect but jointly explained 1.9% of phenotypic variance. PTPN22 was also identified in the discovery cohort but replicated in only one independent cohort. Combining the three cohorts boosted power of vGWAS and additionally identified TYK2 and ANKRD55. Both PTPN22 and TYK2 had evidence of interactions reported elsewhere. We conclude that vGWAS can help discover interacting loci for complex diseases but require large samples to find additional signals.

Smoking and Air Pollution as Pro-Inflammatory Triggers for the Development of Rheumatoid Arthritis

INTRODUCTION

Smoking is now well recognized not only as a risk factor for rheumatoid arthritis (RA), but also as a determinant of disease activity, severity, response to therapy, and possibly mortality.

METHODS

Studies, mostly recent, which have provided significant insights into the molecular and cellular mechanisms which underpin the pathogenesis of smoking-related RA, as well as the possible involvement of other types of outdoor and indoor pollution form the basis of this review.

RESULTS

Smoking initiates chronic inflammatory events in the lungs. These, in turn, promote the release of the enzymes, peptidylarginine deiminases 2 and 4 from smoke-activated, resident and infiltrating pulmonary phagocytes. Peptidylarginine deiminases mediate conversion of various endogenous proteins to putative citrullinated autoantigens. In genetically susceptible individuals, these autoantigens trigger the production of anti-citrullinated peptide, pathogenic autoantibodies, an event which precedes the development of RA.

CONCLUSIONS

An increasing body of evidence has linked chronic inflammatory events in the lungs of smokers, to the production of anti-citrullinated peptide autoantibodies and development of RA. Creation of awareness of the associated risks, assessment of smoking status and implementation of compelling antismoking strategies must be included in the routine clinical management of patients presenting with suspected RA.

IMPLICATIONS

Chronic inflammatory mechanisms operative in the lungs of smokers lead to the production of anti-citrullinated protein antibodies which, in turn, drive the development of RA. These mechanistic insights not only reinforce the association between smoking and risk for RA, but also the necessity to increase the level of awareness in those at highest risk.

The Roles of Cigarette Smoking and the Lung in the Transitions Between Phases of Preclinical Rheumatoid Arthritis

While the etiology of rheumatoid arthritis (RA) remains to be fully elucidated, recent research has advanced the understanding of RA pathogenesis to the point where clinical trials for RA prevention are underway. The current paradigm for RA pathogenesis is that individuals progress through distinct preclinical phases prior to the onset of clinically apparent RA. These preclinical RA phases consist of genetic risk, local inflammation, presence of RA-related autoantibodies, asymptomatic systemic inflammation, and early non-specific symptoms prior to clinical seropositive RA. Epidemiologic studies have been important in forming hypotheses related to the biology occurring in preclinical RA. Specifically, studies associating cigarette smoking with overall RA risk as well as transitions between phases of preclinical RA were vital in helping to establish the lung as a potential important initiating site in the pathogenesis of seropositive RA. Herein, we review the epidemiology associating smoking with transitions in preclinical phases of RA as well as the recent literature supporting the lung as a critical site in RA pathogenesis.

An HLA-C amino-acid variant in addition to HLA-B*27 confers risk for ankylosing spondylitis in the Korean population

INTRODUCTION

The presence of the HLA-B*27 allele is a major risk factor for the development of ankylosing spondylitis (AS), which causes chronic inflammation of the spine and other sites. We investigated residual effects outside HLA-B within the major histocompatibility complex (MHC) region in the Korean population.

METHODS

Using the Korean HLA reference panel, we inferred the classic HLA alleles and amino-acid residues of the six HLA genes (HLA-A, -B, -C, -DPB1, -DQB1, and -DRB1) and MHC single-nucleotide polymorphisms in 3820 Korean subjects, including 654 Korean cases of AS and 3166 controls, who were genotyped by using Immunochip. Logistic regression and log-likelihood ratio tests were used in AS association tests for imputed markers.

RESULTS

The most significant associations were identified at amino-acid positions in the epitope-binding site of HLA-B (P = 1.71 × 10(-481) at position 70, P = 7.20 × 10(-479) at position 97, and P = 2.54 × 10(-484) at positions 114), highlighting the risk effect of the HLA-B*27 allele and the protective effects of other classic alleles. A secondary effect was located at the leucine at amino-acid position 116 in the epitope-binding site of HLA-C (P = 1.69 × 10(-14)), completely tagging the HLA-C*15:02 allele. This residue had a large effect in HLA-B*27-negative patients (odds ratio = 6.6, 95 % confidence interval = 3.8 to 11.4).

CONCLUSIONS

The four amino-acid positions of HLA-B and -C account for most of the associations between AS and MHC in the Korean population. This finding updates the list of AS susceptibility loci and provides new insight into AS pathogenesis mediated by MHC class I molecules.