rs27044 — ERAP1 Gln730Glu

ERAP1 Gln730Glu — The C-Terminal Switch in Peptide Length Selection

The immune system's ability to distinguish self from non-self depends on an exquisitely precise editing process deep inside every nucleated cell. ERAP1 (Endoplasmic Reticulum Aminopeptidase 1)¹¹ ERAP1 (Endoplasmic Reticulum Aminopeptidase 1)
an enzyme that trims peptides to the optimal 8–10 amino acid length required for loading onto MHC class I molecules before surface display to immune surveillance cells acts as a molecular ruler, measuring and calibrating the peptide repertoire before it reaches the cell surface. The Gln730Glu variant at position 730 of ERAP1 sits in the C-terminal regulatory domain and alters the enzyme's length-selectivity — specifically modulating whether it preferentially trims longer peptides down or terminates trimming at the 9-mer stage. Though subtler in its isolated effect than the better-studied Lys528Arg (rs30187) variant, Gln730Glu completes the major coding variant profile of ERAP1 and is a consistent member of the high-risk CCT haplotype²² CCT haplotype
the three-SNP haplotype at rs27044/rs10050860/rs30187 with alleles C on coding strand (= G plus-strand), C, and T that is associated with approximately 1.8-fold increased AS risk strongly associated with ankylosing spondylitis.

The Mechanism

Position 730 lies in the C-terminal domain of ERAP1, a region that contributes to substrate binding and influences the enzyme's conformational dynamics during the trimming cycle. The Gln730 (Q, G allele on plus strand) residue is present in ERAP1 allotypes 1 and 2 of the naturally occurring haplotype system. Biochemical characterization has shown that allotypes 1 and 2 — those carrying Q730 — display higher enzymatic activity than E730-containing allotypes for 9-mer substrates with hydrophobic C-terminal residues³³ allotypes 1 and 2 — those carrying Q730 — display higher enzymatic activity than E730-containing allotypes for 9-mer substrates with hydrophobic C-terminal residues
the hydrophilic glutamine at 730 is better suited for accommodating substrates with hydrophobic C-termini compared to the negatively charged glutamate in the E730 form. This means Q730 ERAP1 (the G allele, risk) generates a different peptide repertoire than E730 ERAP1 (the C allele, protective): more active trimming of 9-mer substrates, effectively altering the density and identity of short peptides available for MHC class I loading.

The protective effect of the E730 variant (C allele) has been functionally validated at the cellular level. Studies using ERAP1-expressing antigen-presenting cells⁴⁴ Studies using ERAP1-expressing antigen-presenting cells
Zervoudi et al. 2016, Ann Rheum Dis — examining HLA-B27 free heavy chain (FHC) expression in monocytes from AS patients carrying different ERAP1 genotypes demonstrated that the wild-type Q730 ERAP1 (but not the protective K528R or Q730E variants) increases cell surface HLA-B27 free heavy chain expression — a key mediator of AS pathogenesis through NK cell and innate lymphocyte activation. At the receptor level, E730 (C allele) generates suboptimal pHLA-B27 conformers⁵⁵ suboptimal pHLA-B27 conformers
peptide-HLA class I complexes with suboptimal peptide content that bind less efficiently to immune receptors that reduce the affinity of the natural killer cell receptor KIR3DL1 for HLA-B27, weakening the dysregulated NK cell signaling implicated in AS.

The Evidence

Haplotype-level AS risk. The foundational paper establishing ERAP1's role in AS Evans et al. 2009, Wellcome Trust Canadian cohorts⁶⁶ Evans et al. 2009, Wellcome Trust Canadian cohorts
Interaction between ERAP1 and HLA-B27 in ankylosing spondylitis implicates peptide handling in the mechanism for HLA-B27 in disease susceptibility identified a three-SNP ERAP1 haplotype (rs27044/rs10050860/rs30187 — CCT in coding-strand notation, where C at rs27044 corresponds to the G allele in plus-strand notation) as a potent risk factor for AS. This CCT risk haplotype showed a pooled odds ratio of 1.81 (95% CI 1.46–2.24, P=7×10⁻⁸) across three independent Canadian cohorts. rs27044 was present as a core member of this haplotype, establishing its role as a risk-conferring component of the high-activity ERAP1 allotype system.

Variant-level AS association. A 2019 meta-analysis of 26 case-control studies pooling 17,223 AS patients and 36,915 controls specifically examined rs27044 and rs30187⁷⁷ specifically examined rs27044 and rs30187
Chen et al. 2019, Association of rs27044 and rs30187 polymorphisms in ERAP1 gene with ankylosing spondylitis and found that the G allele (Q730, risk) was significantly associated with increased AS susceptibility versus the C allele (E730): OR=1.24 (95% CI 1.16–1.33, P<0.001), with the effect present in both Caucasian and Asian populations. This is a well-powered finding, though effect sizes at the individual variant level are more modest than at the haplotype level, reflecting that rs27044 shares variance with its co-inherited haplotype partners.

Five-SNP haplotype architecture. A comprehensive haplotype analysis Casserly et al. 2017, PNAS⁸⁸ Casserly et al. 2017, PNAS
ERAP1 AS association attributable to common genotypes rather than rare haplotype combinations characterizing five common ERAP1 SNPs (M349V, K528R, D575N, R725Q, Q730E) identified the MKDRQ haplotype — which carries Q730 (G allele) — as significantly elevated in AS cases versus controls (33% vs. 27%, P=1.2×10⁻²⁴). This study clarified that AS risk derives from common haplotype combinations rather than unusual rare combinations, and established that the Q allele at position 730 consistently co-occurs with disease-associated variants on the highest-frequency risk haplotype.

Protective mechanism of E730. At the functional level, the Glu730 variant (C allele) has been shown to be protective through two independent mechanisms: reducing HLA-B27 free heavy chain surface expression in AS patient monocytes, and generating suboptimal pHLA-B27 conformers that diminish KIR3DL1 receptor binding on NK cells. Both findings support the conclusion that Q730 (G allele) contributes to AS pathogenesis by maintaining conditions that favor HLA-B27 misfolding and immunostimulatory FHC formation.

Practical Implications

For most carriers of the GC genotype (approximately 41% of people), there is a modestly elevated contribution from the G allele to ERAP1 activity in the context of specific co-inherited haplotypes. The clinical relevance of rs27044 is greatest when: (1) it co-occurs with the risk alleles at rs30187 (K528, T allele) and rs10050860 on the same chromosome, forming the full CCT risk haplotype; and (2) the individual carries HLA-B27, since all known ERAP1 effects on AS are conditional on HLA-B27 positivity.

For GG homozygotes (approximately 8% of the population), both copies of ERAP1 carry Q730, and if this co-occurs with the full CCT risk haplotype and HLA-B27, the risk elevation is substantial. The conditions to monitor for — ankylosing spondylitis and potentially psoriasis — have well-established early warning signs and effective early treatments.

Interactions

rs27044 × rs30187 × rs10050860: CCT risk haplotype for AS. These three ERAP1 coding variants define the most extensively validated disease-risk haplotype at the ERAP1 locus. The CCT combination (G at rs27044 + risk alleles at rs10050860 and rs30187) is associated with OR=1.81 for AS, substantially higher than any individual variant effect. Individuals who are homozygous for this haplotype (inheriting CCT from both parents) face the highest ERAP1-attributable AS risk in the population. This is a strong candidate for a compound action at the rs27044 + rs30187 level (the two most commonly genotyped variants in the haplotype).

rs27044 × HLA-B27: Conditional epistasis. Like all ERAP1 coding variants, rs27044's contribution to AS risk operates through its effect on the peptide repertoire available to HLA-B27. The mechanistic evidence (FHC reduction, suboptimal pHLA conformers) directly links Q730 to dysregulated HLA-B27 biology. In HLA-B27-negative individuals, ERAP1 variants including rs27044 do not show a detectable AS association — making HLA-B27 status the key modifier of this variant's clinical significance.

rs27044 × rs26653 (Arg127Pro): Third ERAP1 allotype determinant. rs26653, rs30187, and rs27044 together define the major functional allotype classes of the ERAP1 enzyme. The full allotype classification is needed for accurate interpretation: the risk haplotype carries Arg127 (rs26653 C allele, coding strand), Lys528 (rs30187 C allele), and Gln730 (rs27044 G allele, plus strand), while the protective haplotype carries combinations including Glu730 (E allele, rs27044 C allele).