rs9268839 — HLA-DRA

HLA-DRA rs9268839 — The Class II Sentinel for Rheumatoid Arthritis

The region just upstream of HLA-DRA on chromosome 6p21.32 is home to one of the most powerful common genetic risk signals for rheumatoid arthritis (RA) in the human genome. rs9268839 is an intergenic variant located approximately 16 kilobases upstream of HLA-DRA¹¹ HLA-DRA
HLA class II histocompatibility antigen, DR alpha chain — the gene encoding the alpha subunit of the HLA-DR heterodimer on antigen-presenting cells, within a dense cluster of HLA class II genes including HLA-DRB5, HLA-DRB9, and TSBP1. The G allele carries an odds ratio of approximately 2.47 per copy for RA in European populations — among the largest effect sizes of any common non-coding variant discovered in autoimmune disease genetics.

The Mechanism

HLA-DRA encodes the invariant alpha chain of the HLA-DR molecule, a class II major histocompatibility complex (MHC) protein expressed on the surface of antigen-presenting cells — dendritic cells, macrophages, and B cells. HLA-DR is a heterodimer: the alpha chain (encoded by HLA-DRA) pairs with a beta chain (encoded by HLA-DRB1, DRB3, DRB4, or DRB5 depending on the haplotype). The combined molecule presents peptide fragments from pathogens and self-proteins to CD4+ helper T cells, initiating adaptive immune responses. The critical insight is that HLA-DRA is largely non-polymorphic²² HLA-DRA is largely non-polymorphic
Unlike HLA-DRB1, which has hundreds of functionally distinct alleles, HLA-DRA varies very little between individuals — its alpha chain is nearly identical across all people. rs9268839 is therefore not a coding variant in HLA-DRA itself, but a tag SNP marking an extended haplotype block that includes the HLA-DRB5 locus and influences local regulatory architecture through linkage disequilibrium with functional variants in flanking HLA-DRB genes.

The precise functional variant this SNP tags has not been fully resolved — a common situation in the HLA region, where extreme polymorphism and LD make fine-mapping challenging. What is clear is that the G-allele haplotype is enriched for HLA-DR conformations that present citrullinated self-peptides particularly efficiently to autoreactive CD4+ T cells, driving the production of anti-citrullinated protein antibodies (ACPA)³³ anti-citrullinated protein antibodies (ACPA)
The hallmark autoantibody in seropositive RA; detected by the anti-CCP test and predictive of joint erosion — the defining pathological antibodies in seropositive RA.

The Evidence

Rheumatoid arthritis. The GWAS signal at rs9268839 is among the most replicated findings in autoimmune genetics. In the landmark Okada et al. 2014 meta-analysis⁴⁴ Okada et al. 2014 meta-analysis
Genetics of rheumatoid arthritis contributes to biology and drug discovery. Nature 2014; 29,880 RA cases and 73,758 controls spanning European and Asian populations, the HLA class II locus at chromosome 6p21.32 (of which rs9268839 is a primary tag) showed the strongest association in the entire genome. In the Laufer et al. 2019 trans-ethnic fine-mapping study⁵⁵ Laufer et al. 2019 trans-ethnic fine-mapping study
Genetic influences on susceptibility to rheumatoid arthritis in African-Americans. Hum Mol Genet 2019; 916 AA cases + >100,000 European/East Asian participants, the HLA-DRB1/DRA region reached p<1×10⁻²⁵⁰ in Europeans — the maximum reportable significance threshold. The rs9268839-G allele carries an OR of approximately 2.47 (95% CI 2.39–2.55) in European cohorts and 1.90 (95% CI 1.81–1.99) in East Asian cohorts, both at p=1×10⁻²⁵⁰.

Sarcoidosis. The G allele at rs9268839 has also been implicated in sarcoidosis susceptibility, a granulomatous inflammatory disease driven by dysregulated antigen presentation in the lung and lymph nodes. In Levin et al. 2013⁶⁶ Levin et al. 2013
Association of ANXA11 genetic variation with sarcoidosis in African Americans and European Americans. Genes Immun 2013; 1,689 cases and 1,252 controls, rs9268839 near HLA-DRA was identified as a sarcoidosis risk SNP showing significant SNP-SNP interaction with ANXA11 rs1049550, suggesting that the HLA-DRA region risk variant amplifies disease susceptibility when combined with ANXA11 variants that impair calcium-dependent immune regulation.

Practical Actions

Carrying one or two G alleles does not mean RA or sarcoidosis will develop — these are probabilistic risk elevations, not deterministic outcomes. However, G carriers should be alert to early symptoms and pursue earlier investigation when musculoskeletal symptoms appear. The most actionable step for G carriers is anti-CCP antibody testing: ACPA can appear years before clinical RA and predicts severity and progression. G carriers who are also ACPA-positive should seek rheumatology review proactively.

Smoking is the most important environmental co-trigger: the HLA class II region and cigarette smoke interact to produce citrullinated lung antigens that drive ACPA production, multiplying RA risk superadditively in G-allele carriers.

Interactions

rs9268839 operates in the same antigen-presentation pathway as rs660895 (HLA-DRB1 shared epitope tag), rs6910071 (TSBP1/C6orf10 intronic variant), and rs2476601 (PTPN22 R620W). These variants act at distinct steps — peptide groove composition, MHC region haplotype structure, T-cell receptor signalling — and their co-occurrence is associated with multiplicative rather than additive RA risk. rs9268839 and rs660895 are in moderate LD in Europeans and may partly tag the same underlying haplotype, but they are not fully redundant: rs9268839 explains additional variation in RA susceptibility beyond the shared epitope alone.