rs26653 — ERAP1

ERAP1 rs26653 — The Arg127Pro Switch in Peptide Trimming

Every cell in your body constantly displays a sample of its internal protein inventory on its surface, using molecules of the MHC class I system¹¹ MHC class I system
Major Histocompatibility Complex class I proteins (HLA-A, HLA-B, HLA-C) that present 8-10 amino acid peptides to patrolling CD8+ T cells for immune surveillance as display platforms. Before a peptide can be loaded, it must first be trimmed to the right length inside the endoplasmic reticulum²² endoplasmic reticulum
the cellular compartment where protein folding and quality control occur, and where peptides destined for HLA class I presentation are processed. ERAP1 (Endoplasmic Reticulum Aminopeptidase 1) is the enzyme that performs this trimming — clipping amino acids from the N-terminus of longer precursor peptides until an optimal 8-10-mer is generated. The rs26653 variant subtly modifies the ERAP1 protein at position 127, and the resulting shift in trimming behavior has documented consequences for risk of psoriasis and ankylosing spondylitis.

The Mechanism

rs26653 is a missense variant (c.380G>C in coding notation) that substitutes a proline for an arginine at amino acid position 127 of ERAP1 (p.Arg127Pro). Position 127 sits in an N-terminal regulatory domain of the enzyme, outside the catalytic site and peptide-binding pocket. Structural and biochemical analyses suggest that the proline substitution reduces conformational flexibility at this position³³ proline substitution reduces conformational flexibility at this position
proline is a cyclic amino acid that restricts the backbone dihedral angles available to a peptide chain, potentially affecting interdomain motion and allosteric transitions in the enzyme, consistent with the observation that ERAP1 allotypes carrying Pro127 tend to have modestly lower catalytic activity overall, though no single in vitro study has isolated the effect of this variant alone.

The clinical significance of rs26653 arises not from a dramatic loss of enzymatic activity, but from subtle shifts in the peptide repertoire ultimately loaded onto MHC class I molecules. In individuals who carry the immune-sensitizing HLA alleles HLA-C*06:02 (associated with psoriasis) or HLA-B27 (associated with ankylosing spondylitis), even small changes in which peptides are generated and in what abundance can tip the balance toward aberrant self-recognition. The ERAP1 locus has now been shown to interact [epistatically | a situation where the clinical effect of one gene depends on the genotype at a second, separate locus] with both HLA-C*06:02 and HLA-B27, meaning that rs26653's disease risk operates primarily through — and is amplified by — specific HLA backgrounds.

The Evidence

Psoriasis: Age-stratified signal. A large Swedish psoriasis cohort study first highlighted rs26653 as an independently significant ERAP1 variant, distinct from the previously characterized rs27524 and rs30187. This study⁴⁴ This study
Lysell et al. Genetic association with ERAP1 in psoriasis is confined to disease onset after puberty and not dependent on HLA-C*06. J Invest Dermatol, 2013 found that rs26653 was associated with psoriasis overall (OR=1.31, 95% CI 1.16–1.48), but the association was dramatically concentrated in individuals whose psoriasis appeared between ages 10 and 20 (OR=1.59, 95% CI 1.28–1.98, P=0.00008). Prepubertal onset (ages 0–9) lacked association, as did late-onset disease (>40 years). Importantly, the association was not modulated by HLA-C*06:02 status — making rs26653's age-specific effect mechanistically distinct from rs27524's HLA-C*06:02-dependent effect.

A Chinese case-control study found a similar signal for psoriasis vulgaris: Pro127 homozygotes had nearly double the risk of psoriasis⁵⁵ Pro127 homozygotes had nearly double the risk of psoriasis
OR=1.96 for CC vs GG; C allele OR=1.40, P=0.042; stronger in early-onset subgroup (OR=2.08, P=0.036).

Ankylosing spondylitis: HLA-B27-conditioned epistasis. The original landmark discovery of ERAP1's role in AS came from a genome-wide study demonstrating that ERAP1 variants affect AS risk exclusively in HLA-B27-positive individuals. Evans et al.⁶⁶ Evans et al.
Interaction between ERAP1 and HLA-B27 in ankylosing spondylitis implicates peptide handling in the mechanism for HLA-B27 in disease susceptibility. Nat Genet, 2011 reported a combined interaction P-value of 7.3×10⁻⁶, with no detectable ERAP1 effect in HLA-B27-negative individuals. A Turkish population study found that rs26653 specifically was associated with AS susceptibility (OR=1.609, 95% CI 1.163–2.226, p=0.004), with a population-attributable risk of 23.4%, though notably no stratification by HLA-B27 status revealed a significant interaction in that smaller cohort.

A meta-analysis pooling 24,271 AS patients and 42,666 controls found that rs26653's AS association was population-specific: statistically significant in Caucasian populations, not significant in Asians, which contributed to the overall pooled OR (1.15, 95% CI 0.94–1.42) crossing the null.

Functional context. In biochemical studies examining ERAP1 allotypes, Pro127-containing allotypes show modestly lower rates of substrate processing compared to Arg127-containing allotypes in some contexts. Studies ranking ERAP1 polymorphisms' contribution to HLA-B27 peptidome shaping⁷⁷ Studies ranking ERAP1 polymorphisms' contribution to HLA-B27 peptidome shaping
Kochan et al., PMC6030728 note that prior in vitro work found no isolated effect of R127P on trimming — the functional consequences of rs26653 may be context-dependent, emerging primarily within specific allotype combinations or HLA environments rather than in isolation.

Practical Implications

Carrying one or two copies of the G allele (Pro127) means your ERAP1 enzyme trimming landscape is subtly shifted. The most clinically relevant implication depends on your HLA background:

For psoriasis risk, the association is strongest if your skin condition appeared or appears in adolescence (ages 10–20) — rs26653 may be a contributing factor even in the absence of HLA-C*06:02. For ankylosing spondylitis, the ERAP1 locus primarily matters if you are HLA-B27 positive. If you carry both HLA-B27 and ERAP1 risk variants, your AS risk is substantially higher than either factor alone.

Both conditions are chronic and manageable with early recognition. For psoriasis, prompt treatment prevents skin thickening and joint involvement. For ankylosing spondylitis — inflammatory back pain that is classically worse in the morning, improves with activity, and comes on before age 45 — early diagnosis and treatment with NSAIDs or biologics dramatically reduces the risk of permanent spinal fusion.

Interactions

rs26653 × HLA-C*06:02 (rs12191877): Partially independent in psoriasis. Unlike rs27524, which shows strict dependence on HLA-C*06:02 for its psoriasis effect, rs26653's age-stratified psoriasis signal is not abolished in HLA-C*06:02-negative individuals. This suggests rs26653 may influence a broader set of peptide/HLA interactions, or that adolescent-onset psoriasis involves different immunological substrates than adult-onset plaque psoriasis.

rs26653 × HLA-B27: Epistasis in ankylosing spondylitis. The ERAP1 locus broadly interacts with HLA-B27 in AS pathogenesis. The mechanism is mechanistic necessity⁸⁸ The mechanism is mechanistic necessity
ERAP1 trims peptides that are then loaded onto HLA-B27; altered trimming changes which self-peptides are presented, potentially generating arthritogenic peptides or increasing free heavy chain formation. Compound risk in HLA-B27-positive individuals carrying multiple ERAP1 risk alleles (including rs26653 + rs30187) is documented.

rs26653 × rs30187 (ERAP1 Lys528Arg): Same-gene haplotype effects. rs26653 and rs30187 are both missense variants in ERAP1 and travel together on haplotypes. The protective haplotype (rs30187/rs26618/rs26653 with alleles C-T-G, where G = Arg127 coding-strand notation) is associated with reduced AS risk (OR=0.77). This means the combination of Arg127 (rs26653) and Lys528 (rs30187) residues in the same ERAP1 protein defines a high-risk allotype, while Arg528 (rs30187 protective variant) tends to reduce trimming activity regardless of rs26653 status.

The interaction between rs26653 and rs30187 is a strong candidate for a compound action: individuals carrying risk alleles at both sites have a different ERAP1 allotype with distinct immunopeptidome characteristics compared to those with mixed or protective allotypes at both sites.