rs27524 — ERAP1

ERAP1 rs27524 — Peptide Trimming, Psoriasis, and the Epistasis Paradigm

Psoriasis is one of the most common chronic autoimmune skin diseases, driven by aberrant activation of CD8+ T cells¹¹ aberrant activation of CD8+ T cells
CD8+ T cells are cytotoxic lymphocytes that recognize short peptides presented by HLA class I molecules on the surface of target cells against skin melanocytes. The key to understanding this SNP lies inside the endoplasmic reticulum (ER) of every cell. Before a peptide can be displayed on the cell surface by an HLA class I molecule, it must first be trimmed to the right length — typically 8-10 amino acids. ERAP1 (Endoplasmic Reticulum Aminopeptidase 1)²² ERAP1 (Endoplasmic Reticulum Aminopeptidase 1)
an enzyme that successively clips amino acids from the N-terminus of peptides, sculpting them to optimal length for MHC class I binding is the enzyme responsible for this trimming step. The rs27524 variant alters how much ERAP1 is produced, and in individuals who also carry HLA-C*06:02, this creates a precise mechanistic path to psoriasis.

The Mechanism

rs27524 is an intronic variant that functions as an expression quantitative trait locus (eQTL)³³ expression quantitative trait locus (eQTL)
a genetic variant that controls how much of a nearby gene's mRNA and protein are produced, rather than changing the protein sequence itself. The A (risk) allele is associated with higher ERAP1 expression and, inversely, lower ERAP2 expression, compared to the G allele. More ERAP1 activity means more aggressive peptide trimming in the ER.

For most people, this has little consequence. But in individuals who carry the HLA-C*06:02 allele, the story changes dramatically. HLA-C*06:02 is the MHC class I allele most strongly associated with psoriasis. It preferentially presents short peptides derived from melanocyte proteins — including the autoantigen ADAMTSL5⁴⁴ ADAMTSL5
a glycoprotein expressed by melanocytes that becomes a self-antigen in psoriasis when presented by HLA-C*06:02. The psoriasis risk ERAP1 haplotype (tagged by the A allele at rs27524) trims the 11-amino acid ADAMTSL5 precursor into a 9-10 amino acid peptide with high efficiency, generating a higher surface density of ADAMTSL5/HLA-C*06:02 complexes⁵⁵ higher surface density of ADAMTSL5/HLA-C*06:02 complexes
more self-peptide displayed per cell on melanocytes. This heightened display activates autoreactive CD8+ T cells that migrate to the skin and drive psoriatic inflammation.

The protective ERAP1 haplotype (G allele) trims the same precursor less efficiently, resulting in fewer functional autoantigen/HLA-C*06:02 complexes and reduced T-cell activation — even in HLA-C*06:02 carriers.

The Evidence

The association between rs27524 and psoriasis was first identified in a landmark 2010 GWAS by the Genetic Analysis of Psoriasis Consortium⁶⁶ landmark 2010 GWAS by the Genetic Analysis of Psoriasis Consortium
2,622 psoriasis cases vs 5,667 controls analyzed for 594,224 SNPs, with replication in 9,079 European samples. This study was notable for demonstrating the first genome-wide significant epistatic interaction between two GWAS loci: the ERAP1 rs27524 effect was only detectable in individuals carrying the HLA-C risk allele rs10484554 (a tag for HLA-C*06:02), with a combined interaction P-value of 6.95×10⁻⁶. In HLA-C*06:02 carriers, the odds ratio for rs27524 rises to approximately 1.43 (95% CI: 1.21-1.69); in non-carriers, no significant association is observed. Homozygosity for risk alleles at both loci substantially increases psoriasis odds compared to the most protective two-locus genotype, reflecting the strong epistatic interaction between ERAP1 and HLA-C.

A meta-analysis of 13 studies⁷⁷ meta-analysis of 13 studies
systematic review and meta-analysis by Zavattaro et al., combining 3,656 psoriasis cases and 3,982 controls across European and Asian populations confirmed the overall association (OR=1.18, 95% CI: 1.08-1.29, p<0.001), with consistent results in both Caucasian and Asian populations.

The functional mechanism was elucidated in 2021⁸⁸ functional mechanism was elucidated in 2021
In vitro peptide trimming assays and CD8+ T-cell activation assays using primary melanocytes and psoriatic T-cell lines: the risk ERAP1 haplotype generates the ADAMTSL5 autoantigen at substantially higher levels than the protective haplotype, and this quantitative difference directly correlates with CD8+ T-cell activation threshold.

The effect shows age-dependent nuance⁹⁹ age-dependent nuance
ERAP1 association was confined to individuals with disease onset between ages 10-20 (OR=1.59, 95% CI: 1.28-1.98); absent in cases with onset under 10 years and not detected in other age groups; within this onset-age-10-20 subgroup specifically, ERAP1 effects appeared independent of HLA-C*06:02 status in that analysis, suggesting that hormonal changes at puberty may interact with the ERAP1 peptide-trimming pathway to influence disease expression timing. This age-subgroup finding does not negate the overall genome-wide significant epistatic interaction between ERAP1 and HLA-C*06:02 from Strange et al. 2010 across the full cohort, which remains the primary framing for this SNP's clinical relevance.

Practical Implications

If you carry one or two copies of the A allele, your ERAP1 enzyme is expressed at higher levels and trims ER peptides more aggressively. This has two main consequences. First, it shifts the peptide repertoire displayed by your HLA class I molecules — potentially altering how your immune system perceives various self-proteins. Second, if you also carry HLA-C*06:02, the combined effect substantially elevates your lifetime risk for plaque psoriasis.

Psoriasis is a chronic condition that alternates between flares and remission. It ranges from mild (a few patches) to severe (widespread plaques, nail involvement, joint disease in psoriatic arthritis). Early diagnosis and consistent management can prevent joint damage and long-term complications.

Environmental triggers for psoriasis flares include skin trauma (Koebner phenomenon), streptococcal throat infections, certain medications (lithium, beta-blockers, NSAIDs, antimalarials), and rapid withdrawal of corticosteroids. Knowing your genetic risk profile allows you to watch for early signs and seek dermatological assessment promptly if characteristic plaques develop.

Interactions

ERAP1 rs27524 × HLA-C*06:02 (rs12191877): Epistasis

The most clinically relevant interaction for rs27524 involves HLA-C*06:02. The rs27524 A allele confers modest unconditional psoriasis risk (OR 1.18 in meta-analysis). The 2010 Strange et al. GWAS¹⁰¹⁰ GWAS
2,622 psoriasis cases vs. 5,667 controls; ERAP1 effect confined to HLA-C risk-allele carriers in this discovery cohort found that ERAP1 effects were confined to HLA-C*06:02 carriers (OR ~1.43 in carriers vs. no significant effect in non-carriers; combined epistasis P=6.95×10⁻⁶), which is mechanistically explained by ERAP1 trimming peptides that are then loaded onto HLA-C*06:02 for presentation. However, a subsequent age-stratified analysis (Lysell et al. 2012¹¹¹¹ Lysell et al. 2012
ERAP1 SNPs rs26653, rs30187, and rs27524 analyzed across age-at-onset groups; ERAP1 association confined to onset ages 10-20, OR 1.59, and found to be independent of HLA-C*06:02 status in contrast to the earlier epistasis model) found that the ERAP1 association in individuals with psoriasis onset between ages 10 and 20 was independent of HLA-C*06:02 status, suggesting the epistasis may be age-specific or population-specific rather than universal. The mechanistic model — ERAP1 trimming peptides for HLA-C*06:02 presentation — remains well-supported by functional data; the degree to which the clinical genetic effect is obligatorily HLA-C*06:02-dependent is not fully resolved across all age and population subgroups.

This interaction is a candidate for compound action documentation. The relevant genotypes are: - rs27524 AA or AG (ERAP1 risk allele present) combined with - rs12191877 CT or TT (HLA-C*06:02 tag SNP risk allele present)

The combined recommendation in this context would center on heightened psoriasis surveillance, early dermatological assessment, awareness of Koebner triggers, and discussion of prophylactic streptococcal management given the strong strep-to-flare link in early-onset psoriasis.

ERAP1 also interacts epistatically with HLA-B27 in ankylosing spondylitis¹²¹² HLA-B27 in ankylosing spondylitis
ERAP1 variants affect AS risk only in HLA-B27 positive individuals, paralleling the psoriasis/HLA-C*06:02 interaction, highlighting a general principle that ERAP1's clinical significance is always conditioned on the specific HLA class I allele background.