rs660895 — HLA-DRB1

Regulatory tag SNP in the HLA-DRB1 region; the G allele marks the HLA-DRB1*04:01 shared epitope haplotype, conferring elevated susceptibility to anti-citrullinated protein antibody (ACPA)-positive rheumatoid arthritis and accelerated joint damage progression.

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HLA-DRB1*04:01 — The Shared Epitope and Rheumatoid Arthritis Risk

The human leukocyte antigen (HLA) region on chromosome 6 is the single largest genetic risk factor for rheumatoid arthritis (RA), and within it the HLA-DRB1 gene dominates. rs660895 is a regulatory tag SNP that travels almost exclusively with the HLA-DRB1*04:01 allele11 HLA-DRB1*04:01 allele
HLA alleles encode the antigen-binding groove of class II molecules; *04:01 is one of the alleles carrying the "shared epitope" motif at positions 70–74 of the beta chain. The G allele marks this high-risk haplotype; homozygosity for G roughly doubles the shared-epitope dose and substantially amplifies RA risk and severity.

The Mechanism

HLA-DRB1 encodes the beta chain of a class II MHC molecule that sits on the surface of antigen-presenting cells and presents peptide fragments to CD4+ T cells. The shared epitope is a five-amino-acid sequence (QKRAA, QRRAA, or RRRAA) in the peptide-binding groove22 peptide-binding groove
specifically at positions 70–74 of the DRβ1 hypervariable region-3. This motif is particularly efficient at binding and displaying citrullinated peptides — proteins where arginine residues have been chemically modified (citrullinated) by the enzyme PAD4. CD4+ T cells recognising these citrullinated epitopes drive B cells to produce anti-citrullinated protein antibodies (ACPA/anti-CCP), the hallmark autoantibody of seropositive RA. The SE alleles explain roughly 18% of the genetic variance in ACPA-positive RA — by far the largest single-locus contribution of any common variant.

The Evidence

Disease susceptibility. The SE gene-dose effect on ACPA-positive RA susceptibility is one of the most replicated findings in autoimmune genetics.

Joint damage progression. rs660895 does more than predict susceptibility — it predicts how fast joints will erode. In the CARDERA trial cohort (524 early RA patients followed for 2 years), Canhão et al. 201533 Canhão et al. 2015
Do Genetic Susceptibility Variants Associate with Disease Severity in Early Active Rheumatoid Arthritis? J Rheumatol 2015 found that each G allele of rs660895 was associated with a 1.07-fold greater annual increase in Larsen radiographic score (p=0.0003). Critically, this association held only in ACPA-positive patients (β = 1.07, p = 0.0037); no effect was seen in ACPA-negative patients (β = 1.03, p = 0.4571). A subsequent meta-analysis of seven RA radiographic GWAS44 meta-analysis of seven RA radiographic GWAS
Genetic associations with radiological damage in RA: meta-analysis of 2,775 cases. Arthritis Res Ther 2019 confirmed rs660895 as the most consistently replicated genetic predictor of joint damage.

Practical Actions

Carrying one or two G alleles does not mean RA is inevitable — lifestyle and environmental factors (notably smoking, which dramatically amplifies SE risk) are co-required triggers. However, G carriers who are ACPA-positive have substantially higher odds of rapid radiographic progression and should pursue aggressive disease management early. ACPA testing and baseline hand/feet X-rays (or ultrasound/MRI) are the key monitoring tools. Smoking cessation is uniquely important in G-allele carriers because gene-environment interaction with smoking multiplies RA risk superadditively.

Interactions

rs660895 acts in the same antigen-presentation pathway as rs2476601 (PTPN22 R620W), the second largest RA risk locus. When both are present (a common scenario given both are relatively frequent), RA risk is multiplicative rather than additive — each amplifies T-cell autoreactivity through distinct mechanisms (peptide presentation vs. T-cell receptor signalling). The interaction between SE alleles and rs2476601 has been confirmed in multiple large European cohorts.

The G allele also shows a protective association with Parkinson's disease (PD): in a meta-analysis of 44,451 individuals55 meta-analysis of 44,451 individuals
Hamza et al. Mov Disord 2012, rs660895-G was associated with reduced PD risk (OR 0.86).

Genotype Interpretations

What each possible genotype means for this variant.

AA No Shared Epitope Normal

No HLA-DRB1*04:01 shared epitope — baseline RA risk

While the AA genotype at rs660895 indicates absence of the DRB1*04:01 shared epitope, RA risk is not zero — other HLA alleles (DRB1*01, *10), non-HLA GWAS loci, and environmental factors (smoking, infections) all contribute. The absence of the shared epitope also means reduced predisposition to ACPA production, making ACPA-negative RA the more likely subtype if RA were to develop.

AG Single Shared Epitope Copy Intermediate

One copy of HLA-DRB1*04:01 shared epitope — moderately elevated RA risk

HLA-DRB1*04:01 carriers present citrullinated peptides more efficiently to CD4+ T cells, promoting ACPA production. In the CARDERA cohort, each G allele was associated with a 1.07-fold annual increase in Larsen radiographic score (p=0.0003) in ACPA-positive patients. This effect is exclusive to the seropositive subset — if you are ACPA-negative, the G allele does not predict faster joint erosion. One SE allele substantially increases ACPA production odds; ORs for ACPA-positive RA with a single SE allele are typically 3–5× compared to non-SE individuals.

GG Double Shared Epitope High Risk

Two copies of HLA-DRB1*04:01 shared epitope — substantially elevated RA risk and progression

Homozygosity for the shared epitope (GG) compounds antigen-presentation efficiency: both HLA-DRB1 molecules in the cell surface repertoire carry the *04:01 groove, maximising presentation of citrullinated peptides to autoreactive CD4+ T cells. This leads to higher ACPA titres and more aggressive disease in most studies. Radiographic progression studies consistently show a linear dose-response: GG > AG > AA for annual Larsen score increase in ACPA-positive patients.