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Endoplasmic reticulum aminopeptidase-1 polymorphism increases the risk of rheumatoid arthritis

Objectives: Endoplasmic reticulum aminopeptidase-1 ( ERAP1 ) polymorphic changes cause autoimmunity. To understand the contribution of ERAP1 to the occurrence of rheumatoid arthritis (RA) disease, we investigated the relationship between ERAP1 and RA. Methods: This study was conducted with 201 patients and 171 healthy controls. The rs26653, rs27044, rs27582, rs28096, and rs30187 polymorphic regions of ERAP1 were investigated. The comparison was done with Arlequin software and logistic regression. Haplotypes were analyzed with Phylogenetic Network software. ERAP1 was modeled using Promod3.Topologicalchangesin ERAP1 wereanalyzedwith TM-Score. Results: The results showed that rs26653G>C (p=0.002, OR=2.001, 95%CI=1.276 – 3.137), rs27044C>G (p=0.037, OR=1.583, 95%CI=1.028 – 2.440), rs27582G>A (p<0.05, OR=0.348, 95%CI=0.194 – 0.622) and rs30187C>T (p=0.006, OR=1.849, 95%CI=1.191 – 2.870) polymorphisms are associated with RA disease risk. The relationship between rs28096 polymorphism and RA disease risk could not be determined (p=0.509). The risk haplotype for rheumatoid arthritis was determined as [CGAAT]. It was determined that polymorphisms of ERAP1 cause changes in the entry pocket of substrate and ligand. Conclusions: We report a haplotype [CGAAT] that is associated with RA risk from Turkey that has not been described before. These data will make important contributions to elucidating the molecular mechanism of RA.


Introduction
Rheumatoid arthritis (RA) is a systemic inflammatory disorder that affects between 0.5 and 1% of the population and ultimately induces persistent synovial inflammation, resulting in joint destruction and disablement [1]. The disease affects women more frequently than men 2 to 3 times and appears at every age. The highest incidence emerged during the sixth decade [2]. A highly specific RA biomarker linked to disease seriousness, the plurality of infected RA cases (about 70 percent) are seropositive for anti-citrullinated protein antibodies (ACPA) [3]. An increased risk for RA has been associated with multiple genetic and environmental factors. The highest connections have been seen with female gender, a familial history of RA, genetic factor, and tobacco smoke exposure [4]. RA inheritance is estimated to be approximately 60% [5]. Genetic factors including class II major histocompatibility antigens and non-human leukocyte antigen (HLA) genes have been implicated in the pathogenesis of RA [6,7]. While the pathogenesis of RA is complicated and remains uncertain, it is necessary to consider and acknowledge its pathogenesis in disclosing the effective therapeutic strategy that could contribute to meaningful clinical benefits [8]. Early detection is critical to proper clinical progress. Especially in patients with proven risk factors, including high disease activity and antibodies, or early joint injury [9].
Endoplasmic reticulum aminopeptidase-1 (ERAP1) is a zinc metallopeptidases that are active in trimming peptides in the endoplasmic reticulum (ER) to load them into the HLA class I antigen-binding groove [10]. ERAP1 cleavages proinflammatory cytokine receptors, such as interleukin 1 receptor type 2 (IL1R2), interleukin 6 receptor subunit alpha (IL6RA), and tumor necrosis factor receptor 1 (TNFR1), leading to a decrease in cell transmission of inflammatory signaling [11]. ERAP1 activity affects cell surface immunopeptidome and epitope immunodominance models, although its molecular mechanism is not fully elucidated. Studies have shown that ERAP1 is an essential determinant of the immune response by producing mature antigenic epitopes from peptides that are extracted from degraded proteins [12]. Resolving the particulars of how ERAP1 generates antigenic peptides would have significant implications about how medications may be used to control the immunopeptidome and how ERAP1 biology causes a predisposition to autoimmune disease [13].
Single nucleotide polymorphism (SNP) of ERAP1 has been shown to be associated with cancer, rheumatic and viral diseases [14]. A genome-wide association study showed that polymorphic changes in ERAP1 can result in genetic predisposition to the disease and changes in the immune response. Antigen presentation is influenced by SNPs which trigger changes to enzyme activity [14,15].

Patients and ethical compliance
This case-control study was conducted with 201 patients who were clinically and serologically diagnosed with RA and 171 healthy controls. European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) classification criteria were used in the diagnosis of RA. The individuals without any autoimmune and endocrine diseases were included in the control group. Table 1 depicts the clinical and demographic specifications of study subjects and their values. Ethical rules were followed in this study and written consent of volunteers was obtained. The study protocol was approved by the Ethics Committee of Malatya Clinical Investigations (2016/12). The study followed the ethical standards of the Helsinki Declaration.

SNP selection and genotyping
We first retrieved the reference SNPs (rs) defined for ERAP1 from the NCBI database and selected potential functional SNPs based on the following criteria: (i) located in the exon and intron regions; (ii) not studied in the published genome-wide association studies; (iii) potential functional SNPs identified using SNP info software; (iv) SNPs reported to be associated with different autoimmune diseases were selected. Linkage disequilibrium (LD) was checked. Finally, NM_016442.5:p.Arg127Pro (rs26653), NM_016442.5:p.Gln730Glu (rs27044), NC_000005.10:g.96762509G>A (rs27582), NC_000005.10: g.96773539G>A (rs28096) and NM_016442.5:p.Lys527Arg (rs30187) polymorphic regions were included in the study. DNA isolation from blood samples was performed using Invisorb Spin Blood Mini Kit™ (Catalog Number: 1031100300). Polymorphic regions were amplified by multiplex Polymerase Chain Reaction (PCR) using designed primer pairs. The genotype analysis was performed according to Gabriel et al. [16]. Primer pairs and probe information used in the single-base elongation reaction and multiplex PCR are presented in Supplemental Table 1.

Sequence data and modeling of polymorphic protein
Reference nucleotide and protein sequence information of ERAP1 were obtained from the NCBI (NM_016442.5) and Uniprot portal (Q9NZ08), respectively. The polymorphic residues information of ERAP1 was processed with MEGAX software. Models were built based on the target-template alignment using ProMod3 (ver3.1.1). 6rqx (RCSB protein data bank code) was selected as template. ProSA and MolProbity tools were used for structural validation of ERAP1 polymorphic model [17,18]. Conformational analyzes of polymorphic ERAP1 were performed with PyMOL (ver2.4.1). Topological differences of wild and polymorphic ERAP1 were calculated with the i-Tasser TM-Score and root mean square deviation (RMSD) algorithm [19].

Statistical analysis
The association of SNP alleles and genotypes with disease predisposition was analyzed by a logistic regression test (SPSS 24.0, Chicago). Binary Logistic Regression model was established in which polymorphic changes, smoking, family history, diabetes, blood pressure, surgery, mechanical trauma, infection history, and gender variables represent covariates if the group represents the patient and control variable is dependent. Associations of RA with polymorphisms are shown as odds ratios (ORs), and risks were estimated with 95% confidence intervals (CIs). Power analysis was performed by using G*Power 3.1. Demographic data were given as mean ± standart deviation and min/max values. p-Value smaller than 0.05 was regarded as statistically significant. The associated haplotype varieties for healthy control samples and RA population were calculated with Arlequin 3.5 software [20]. LD analysis was performed using Haploview v. 4.2 program [21]. Hardy-Weinberg equilibrium (HWE) test was performed at each locus for HWE analysis in populations. Arlequin software was used to calculate p values in these analyzes. Arlequin software uses Fisher's exact test to calculate the significance of the deviation from the HWE. The probably historical movement model of RA was estimated by neutrality tests and mismatch distribution analysis method. For this purpose, tau (τ) and theta initial (θ 0 ) were estimated assuming theta final (θ 1 ) to calculate the possible mode of mismatch distribution using Arlequin software. The parameters of the infinity and the raggedness index and the effects of the Tajima and Fu neutrality tests were determined using Arlequin.

Results
The clinical and demographic data for the 201 patients and 171 controls evaluated in this study are shown in Table 1. The percentages of the patients for anti-CCP (+), rheumatoid factor-RF (+), erythrocyte sedimentation rate-ESR (+), C-reactive protein-CRP (+) were 58.71%, 64.18%, 50.75%, 49.75%, respectively. It was found that the diagnosis of the disease was delayed for 2.4 years. Gender was associated with the risk of disease, and the incidence of the disease in women was 2.61 times higher than men (p<0.05). There was a familial RA disease story of 22.39% of the patients (p=0.030). Before the onset of disease complaints, surgical intervention (p<0.05), severe infectious disease (p=0.009) and presence of diabetes (p=0.020) may be associated with the risk of disease (Table 1). Hosmer-Lemeshow statistics was used to test the goodness of fit in the model established to predict the group variable of polymorphic, clinical and demographic variables, and it was found to be statistically sufficient (χ2=6.362, SD=8, p=0.607>0.05).
Genotyping results showed that rs26653 (p=0.002), rs27044(p=0.037), rs27582 (p<0.05) and rs30187 (p=0.006) polymorphisms are associated with RA disease risk ( Table 2). The individuals carrying the C allele in the rs26653 polymorphic region had a 2-fold higher risk of RA disease (OR: 2.001). It was determined that the G allele for rs27044 increased the disease risk 1.6 fold. The C>G polymorphic change at this position results in Gln730 Glu amino acid change in the protein structure. Similarly, T allele for rs30187 increased the disease risk 1.9 fold. Similarly, it was shown that rs30187C>T polymorphic change causing Lys528Arg amino acid change increased   Populations diversity parameters (h, π, k) are shown at Supplemental Table 2. The h parameter is highly calculated and seems to be compatible with the control population. (0.91 ± 0.005) and the RA population (0.92 ± 0.004). Nucleotide diversity (π) has low and similar values; 2.15 ± 1.32 (healthy control population) and 2.37 ± 1.43 (RA population). Fu's Fs test results show that both populations have common growth rates (Supplemental Table 2). Tajima's D results (p>0.05) indicate that there is neutral equilibrium among the populations. These compatible data show that unlikely haplotype diversity is not possible in future generations. On the other hand, Tajima's D values indicate that heterozygotes have a selective advantage in these populations. Estimations of time τ and the current and historical population parameters θ imply a similar historical episode of expansion for the two populations (τ>0 and θ 1 >θ 0 ; Supplemental Table 2). The demographic histories of the two populations were examined using a mismatch distribution analysis (Supplemental Table 3). The null hypothesis of the neutrality test expresses the population balance. The opposite hypothesis describes   the population growth according to the results of SSD and rg tests. The distribution of pairwise differences in control and RA is significantly different (p<0.05) from the expected distribution based on both SSD and rg analysis results performed with the sudden population expansion model (Supplemental Table 3). According to the SSD and rg tests, the null hypothesis is not accepted and the distribution is defined as unimodal. In our results, distribution parameters, the control and RA population is departure from the unimodal distribution (Supplemental Table 3). The reason for this difference in distribution is that the control and RA populations presented in Supplemental Table 4 are departure from HWE of the 3., 4. and 3. locus, respectively [22,23]. We performed the phylogenetic network analysis of the haplotypes linked with the RA population by calculating the frequency and possible link estimations with NETWORK software (Supplemental Figure 1). We tested the LD using Haploview software. LD analysis showed weak allelic combinations in the rs27582 and rs26653 polymorphic regions in the RA and control populations ( Figure 3).
Missense changes caused by polymorphic substitutions in tertiary structure were modeled. The molprobity score of polymorphic ERAP1 was 1.25. The Z-score of the polymorphic model was −13.17 and the model was found in the native proteins range that were identified by X-ray crystallography. It was determined that polymorphic ERAP1 has topological and conformational differences compared to reference ERAP1 (RMSD=0.13 Å and TM-score=0.97). The data obtained from wild and polymorphic model comparisons indicate topological and conformational changes.

Discussion
ERAP1 modulates immune responses by regulating the peptide repertory eligible for presenting by MHC molecules [24]. In doing so, ERAP1 clips peptides by a unique molecular-scale mechanism that recognizes both the N-and C-ends of the peptides to read the length of the substrate [25]. The polymorphic changes of ERAP1 cause HLA-dependent inflammatory autoimmunity [15]. Some polymorphic modifications, including the Lys528Arg and Gln730Glu, are located close substrate binding and regulatory sites and may influence the processing and clipping specificity of peptides [26].
The rs30187 polymorphism was associated with RA disease risk (p=0.006, OR=1.849, 95%CI=1.191-2.870). Until today, the relationship of rs30187 polymorphism with susceptibility to different diseases other than RA has been shown in many studies. In the ERAP1-ankylosing spondylitis (AS) association study by Harvey et al. with 730 patients and 1,021 controls, the rs30187 polymorphism was shown to be strongly associated with disease risk (p=0.0034) [27]. While Gureini et al. showed the genetic predisposition of rs30187 to multiple sclerosis, Gianchecchi et al. showed the contribution of rs30187 to the risk of type 1 diabetes mellitus [28,29].
One of the important processes in ERAP1 catalytic activity is to increase the efficiency of the catalytic activity by closing domain 4, which consists of alpha helices, on the active site. In the transition between the closed-open formation, Gly529 and its surrounding residues act as a hinge, moving it 28°away from the protease area of domain 4 and performing 12°of lateral rotation [30,31]. The transformation of the open-closed formation transition, which has an important role in the immune response, after the mutation in the Lys528 position from Lys containing 2 nitrogen atoms to Arg containing 4 nitrogen atoms results in an increase in protonation capacity. In this case, two alternative scenarios will result in a change in catalytic activity. Firstly, a decrease in catalytic activity can be seen as the full transition from open to closed formation cannot be achieved and the activity distance (29 Å) between the N-terminal catalytic region and the C-terminal regulatory region will change. Secondly, when the complete transition from closed to open formation cannot be achieved, sustained catalytic activity and antigenic peptide presentation may result in unwanted changes and products in the immune response.
It has been shown that ERAP1 Lys528Arg polymorphism will cause a significant change in peptide processing properties as a result of disruption in interdomain interactions. ERAP1 is a multifunctional enzyme involved in the regulation of immune and inflammatory responses [32].
The Gln730 is one of the polymorphic residues of ERAP1, which has been shown to be associated with disease susceptibility and antigen presentation [26]. Gln730 is located in the interior of the catalytic cavity of ERAP1 and its interaction with catalytic residues makes it important in ERAP1 efficacy. The polymorphic change of Gln730Glu will change the surface electrostatic potential of the inner cavity, affecting substrate specificity and/or the conformational dynamics of the enzyme. Sui et al. conducted a study with the -IINFEKL-substrate to understand the roles of the C-terminal regulator region of ERAP1 in catalytic activity. ERAP1 acts by binding to the substrate placed in the catalytic cavity, the N terminal catalytic region and the C terminal regulator region. In this mechanism, Gln730 located at the midpoints of the substrate interacts with the asparagine (N) of the peptide by hydrogen bonding [25]. On the other hand, the Gln730Glu polymorphic residue is located positionally next to the C-terminal specificity pocket (Figure 1). Electrostatic charge change beside the C-terminal specificity pocket, which has an important role in peptide specificity, may affect the binding affinity of the C-terminal region. It is thought that the rs27044 polymorphic modification may change the hydrogen bonding pattern and this may affect the substrate affinity and peptide presentation mechanism. The data obtained in this study showed that the rs27044 polymorphism was associated with a genetic predisposition to RA disease (p=0.037, OR=1.583, 95%CI=1.028-2.440).
The ERAP1 rs26653 polymorphism has been associated with the risk of many autoimmune and infectious diseases [33,34]. Macieja et al. conducted a study with 148 patients and 146 healthy controls showed that the rs26653 polymorphism was associated with the risk of psoriasis (p=3.1 × 10 −5 ) [34]. In another study, Li et al. showed the association of ERAP1 rs26653 (p=0.001) and rs27044 (p=0.003) polymorphisms with cervical cancer risk [35]. In this study, the rs26653 Arg127Pro polymorphism was shown to be associated with the risk of rheumatoid arthritis disease (p=0.002, OR=2.001, 95%CI=1.276-3.137). It was determined that the Arg127Pro polymorphic change caused a change in the ERAP1 substrate entry pocket, and the distance between 127th residues and 900th residues in the entrance region expanded from 7.5 to 10.3 Å (Figure 2). It was observed that Arg→Pro change at residue 127 transformed the loop secondary formation into a semi-helical form. The association of the rs26653 polymorphism with the RA disease risk indicates that this formal change in the entry pocket of ligand and substrate exerts effects on the activity of the enzyme that will affect the peptide processing mechanism.
Intronic variant rs27582 was found to be associated with RA disease risk in the Turkish population (p<0.05, OR=0.348, 95%CI=0.194-0.622). The rs27582 polymorphic change has been associated with an autoimmune disease, AS, in different populations. The ERAP1 rs27582 polymorphism was associated with the risk of AS disease in a  study conducted with 602 patients and 619 controls in the Beijing Han community [36]. Despite the link established in East Asia, the association between ERAP1 and rs27582 could not be shown in terms of susceptibility to AS in a study conducted with 730 patients and 1,021 controls in white European cohorts [27].
The rs28096 polymorphism in the intron region of the ERAP1 gene was not associated with RA disease risk (p=0.509, OR=1.167, 95%CI=0.738-1.847). As in other polymorphic regions investigated in this study, although the rs28096 region has not been previously examined in terms of RA disease risk, its relationship with different diseases was examined. While Dargahi et al. could not correlate the risk of preeclampsia disease with rs28096 polymorphism, Harvey et al. stated that the rs28096 polymorphic change increased ERAP1 gene expression excessively and was associated with AS disease risk (p=1.6 × 10 −32 ) [27,37]. It is known that the 3′untranslated regions of genes are important structures that affect the post-transcriptional regulation and translation rate [38]. This basic function lies behind the rs28096 polymorphic change within this region causing a change in the expression levels of the gene.
Evaluating these data contributes to approaches phylogeographical and genetic basis and risk factors for diseases. In addition, expected to provide valuable contributions to discuss the mechanisms of variations and mechanisms of modeling to the potential of gene therapy approaches. The data obtained for both healthy control and RA populations are in HWE (p>0.05) for except three loci of the five polymorphic loci as shown in Supplemental Table 4. However, utilizing a mismatch distribution method, the demographic histories of the two groups are examined. For these two populations, the mismatch distribution seemed to be multimodal (multimodal distribution; the exponential increase is not regular) (Supplemental Table 3, p≤0.05). These results show that there may be genetic drift-induced transitions such as population motions, emigrations, and environmental impacts in the Malatya province gene pool that lead to the formation of the RA population. Historical population parameters θ (θ 0 , and θ 1 ) and τ values show a similar historical growth period for the populations (Supplemental Table 2). Supplemental Table 2 demonstrated high and peer haplotypic diversity (h), weak nucleotide diversity (π) and peer average number of pairwise nucleotide differences (k) between the two populations [39]. In addition, for the two groups, the Fu's Fs stats revealed a strong negative value, suggesting a comparable population expansion for both communities across history. For two populations, Tajima's D values were negligible (p>0.05), indicating that populations are at neutral equilibrium (Supplemental Table 2). These results demonstrate that in the historical period, the molecular diversity of both groups has genetically similar growth and expansion [40]. These data also indicate that in Malatya, Turkey, the RA population does not derive from the healthy control population. Despite similar genetic heritage, both populations have distinct haplotype variants. The reason for these differences is thought to be due to the departure from HWE at the third and fourth loci (Supplemental Table 2). The probable impact of the RA on polymorphic loci in the rs26653, rs27044, rs27582, rs28096, and rs30187 may reason for this haplotypic alteration between healthy control and RA populations. LD analysis revealed weak allelic combinations at positions (rs27582 and rs26653) in the RA and control populations ( Figure 3). This result supports the low frequency of the risk haplotype [CGAAT] in the RA population (Table 3).
In conclusion, it is known that polymorphic changes, which were examined for the first time in this study in terms of the RA relationship, are also associated with different diseases. It was determined that four exonic and intronic polymorphic regions (rs26653, rs27044, rs27582, and rs30187) were associated with disease risk. We report a haplotype [CGAAT] that is associated with RA risk from Turkey that has not been described before. It has been shown that polymorphic missense changes cause alterations in protein tertiary structure that may affect functional properties. The association of ERAP1 with the risk of many autoimmune diseases of polymorphic regions involved in RA disease risk does not explain its specific role in RA development. It is thought that other biological structures (genes, ligands, etc.) that interact with peptide products resulting from ERAP1 polymorphic changes may contribute to the development of the disease. It is thought that studies to be carried out with large sample groups including different polymorphic regions of ERAP1 will contribute to the elucidation of the RA-ERAP1 relationship.