Detailed analysis of the variability of peptidylarginine deiminase type 4 in German patients with rheumatoid arthritis: a case-control study

Association of PADI2 and PADI4 polymorphisms in COVID-19 host severity and non-survival

Background

Enzymes of the peptidylarginine deiminase family (PADs) play a relevant role in the pathogenesis of COVID-19. However, the association of single nucleotide polymorphisms (SNPs) in their genes with COVID-19 severity and death is unknown.

Methodology

We included 1045 patients who were diagnosed with COVID-19 between October 2020 and December 2021. All subjects were genotyped for PADI2 (rs1005753 and rs2235926) and PADI4 (rs11203366, rs11203367, and rs874881) SNPs by TaqMan assays and their associations with disease severity, death, and inflammatory biomarkers were evaluated.

Results

291 patients presented had severe COVID-19 according to PaO2/FiO2, and 393 had a non-survival outcome. Carriers of the rs1005753 G/G genotype in the PADI2 gene presented susceptibility for severe COVID-19, while the heterozygous carriers in rs11203366, rs11203367, and rs874881 of the PADI4 gene showed risk of death. The GTACC haplotype in PADI2-PADI4 was associated with susceptibility to severe COVID-19, while the GCACC haplotype was a protective factor. The GCGTG haplotype was associated with severe COVID-19 but as a protective haplotype for death. Finally, the GTACC haplotype was associated with platelet-to-lymphocyte ratio (PLR), the GCACC haplotype with neutrophil-to-hemoglobin and lymphocyte and the GCGTG haplotype as a protective factor for the elevation of procalcitonin, D-dimer, CRP, LCRP, NHL, SII, NLR, and PLR.

Conclusions

Our results suggest that the haplotypic combination of GTACC and some individual genotypes of PADI2 and PADI4 contribute to the subjects' susceptibility for severity and death by COVID-19.

Peptidyl Arginine Deiminases in Chronic Diseases: A Focus on Rheumatoid Arthritis and Interstitial Lung Disease

Protein citrullination is accomplished by a broad enzyme family named Peptidyl Arginine Deiminases (PADs), which makes this post-translational modification in many proteins that perform physiological and pathologic mechanisms in the body. Due to these modifications, citrullination has become a significant topic in the study of pathological processes. It has been related to some chronic and autoimmune diseases, including rheumatoid arthritis (RA), interstitial lung diseases (ILD), multiple sclerosis (MS), and certain types of cancer, among others. Antibody production against different targets, including filaggrin, vimentin, and collagen, results in an immune response if they are citrullinated, which triggers a continuous inflammatory process characteristic of autoimmune and certain chronic diseases. PAD coding genes (PADI1 to PADI4 and PADI6) harbor variations that can be important in these enzymes' folding, activity, function, and half-life. However, few studies have considered these genetic factors in the context of chronic diseases. Exploring PAD pathways and their role in autoimmune and chronic diseases is a major topic in developing new pharmacological targets and valuable biomarkers to improve diagnosis and prevention. The present review addresses and highlights genetic, molecular, biochemical, and physiopathological factors where PAD enzymes perform a major role in autoimmune and chronic diseases.

SNP in PTPN22, PADI4, and STAT4 but Not TRAF1 and CD40 Increase the Risk of Rheumatoid Arthritis in Polish Population

Single nucleotide polymorphisms in non-HLA genes are involved in the development of rheumatoid arthritis (RA). SNPS in genes: PADI4 (rs2240340), STAT4 (rs7574865), CD40 (rs4810485), PTPN22 (rs2476601), and TRAF1 (rs3761847) have been described as risk factors for the development of autoimmune diseases, including RA. This study aimed to assess the prevalence of polymorphisms of these genes in the Polish population of patients with rheumatoid arthritis as compared to healthy controls. 324 subjects were included in the study: 153 healthy subjects and 181 patients from the Department of Rheumatology, Medical University of Lodz who fulfilled the criteria of rheumatoid arthritis diagnosis. Genotypes were determined by Taqman SNP Genotyping Assay. rs2476601 (G/A, OR = 2.16, CI = 1.27-3.66; A/A, OR = 10.35, CI = 1.27-84.21), rs2240340 (C/T, OR = 4.35, CI = 2.55-7.42; T/T, OR = 2.80, CI = 1.43-4.10) and rs7574865 (G/T, OR = 1.97, CI = 1.21-3.21; T/T, OR = 3.33, CI = 1.01-11.02) were associated with RA in the Polish population. Rs4810485 was also associated with RA, however after Bonferroni's correction was statistically insignificant. We also found an association between minor alleles of rs2476601, rs2240340, and rs7574865 and RA (OR = 2.32, CI = 1.47-3.66; OR = 2.335, CI = 1.64-3.31; OR = 1.88, CI = 1.27-2.79, respectively). Multilocus analysis revealed an association between CGGGT and rare (below 0.02 frequency) haplotypes (OR = 12.28, CI = 2.65-56.91; OR = 3.23, CI = 1.63-6.39). In the Polish population, polymorphisms of the PADI4, PTPN22, and STAT4 genes have been detected, which are also known risk factors for RA in various other populations.

Novel functional polymorphism on PADI-4 gene and its association with arthritis onset

Background

Citrullinated proteins formed by peptidyl arginine deiminases (PADIs) deimination of arginine residues in proteins are of particular interest in arthritis pathogenesis. Polymorphisms on the PADI-4 gene lead to the malfunctioning of PADIs leading to the onset of arthritis.

Objective

The present study was conducted to determine the polymorphisms on the PADI-4 gene and their association with rheumatoid arthritis (RA) as well as Osteoarthritis (OA).

Methodology

To achieve the above-mentioned objective a case-control study was conducted. Blood samples were collected from RA, OA, and control subjects. DNA was extracted from each blood sample by modified organic method and was quantified as well as qualified by DNA gel electrophoresis and Nanodrop. Patients were tested for rs874881, rs11203366, rs11203367, rs2240336, rs2240337, rs2240339, rs1748033 and rs2240340 polymorphic sites by amplifying targeted regions through PCR with site-specific primers. Genotyping was performed by Restriction Fragment Length Polymorphism and direct sequencing method. Mutations were identified by analyzing sequences on BioEdit software. Allelic, genetic, and multiple site analysis were performed by SHEsis and PLINK software. Change in the amino acid sequence was identified by MEGA 6.0 software.

Results

Polymorphisms were identified on all targeted polymorphic sites except rs2240337 in both RA and OA individuals. In addition, two novel mutations were also identified in exon 4 identified i-e SCV000804840: c.218T > C and SCV000807675: c.241G > T. All the SNPs except rs11203366 were found to be significantly associated with RA at an allelic level whereas all SNP’s have been significant risk factors in the onset of OA. At genotypic level rs874881, rs11203366, rs2240339, SCV000804840 and SCV000807675 were significantly associated to RA development whereas rs874881, rs11203366, rs11203367, rs2240339, SCV000804840 and SCV000807675 were genetic risk factors in OA onset. Haplotype analysis indicated that GACCACGCC and GACCACGCT were highly significant in disease development. Polymorphisms identified altered the functioning of PADIs by altering their amino acid sequence.

Conclusion

In conclusion, it was found that PADI-4 gene polymorphism was not only involved in the onset of RA but was also found to be a significant risk factor in OA onset.

PADI4 Polymorphisms Confer Risk of Anti-CCP-Positive Rheumatoid Arthritis in Synergy With HLA-DRB1*04 and Smoking

Peptidylarginine deiminases (PADs) catalyze citrullination, a post-translational modification playing a pathogenic role in anti-citrullinated protein antibody (ACPA)-positive rheumatoid arthritis (RA). The interplay between single nucleotide polymorphisms (SNPs) in the PADI genes and known risk factors for ACPA-positive RA, including smoking, HLA-DR4 and -1, and the PTPN22 R620W polymorphism, was investigated. We typed four PADI2 SNPs, four PADI4 SNPs, and the PTPN22 R620W SNP in 445 Danish RA patients and 533 age-matched healthy controls, as well as in 200 North American RA patients and 100 age- and sex-matched controls. The HLA-DRB1 locus was typed in the Danish cohort. Logistic regression analyses, adjusted for age, sex, smoking status, and PTPN22 R620W, revealed increased risk of anti-CCP-positive RA in carriers of rs11203367(T) (OR: 1.22, p=0.03) and reduced risk in carriers of rs2240335(A) in PADI4 (OR: 0.82, p=0.04). rs74058715(T) in PADI4 conferred reduced risk of anti-CCP-negative RA (OR: 0.38, p=0.003). In HLA-DRB1*04-positive individuals, specifically, the risk of anti-CCP-positive RA was increased by carriage of PADI4 rs1748033(T) (OR: 1.54, p=0.007) and decreased by carriage of PADI4 rs74058715(T) (OR: 0.44, p=0.01), and we observed an interaction between these SNPs and HLA-DRB1*04 (p=0.004 and p=0.008, respectively) Thus, PADI4 polymorphisms associate with ACPA-positive RA, particularly in HLA-DRB1*04-positive individuals, and with ACPA-negative RA independently of HLA-DRB1*04.

PAD enzymes in rheumatoid arthritis: pathogenic effectors and autoimmune targets

Peptidylarginine deiminases (PADs) have an important role in the pathogenesis of rheumatoid arthritis (RA) owing to their ability to generate citrullinated proteins - the hallmark autoantigens of RA. Of the five PAD enzyme isoforms, PAD2 and PAD4 are the most strongly implicated in RA at both genetic and cellular levels, and PAD inhibitors have shown therapeutic efficacy in mouse models of inflammatory arthritis. PAD2 and PAD4 are additionally targeted by autoantibodies in distinct clinical subsets of patients with RA, suggesting anti-PAD antibodies as possible biomarkers for RA diagnosis and prognosis. This Review weighs the evidence that supports a pathogenic role for PAD enzymes in RA as both promoters and targets of the autoimmune response, as well as discussing the mechanistic and therapeutic implications of these findings in the wider context of RA pathogenesis. Understanding the origin and consequences of dysregulated PAD enzyme activity and immune responses against PAD enzymes will be important to fully comprehend the pathogenic mechanisms involved in this disease and for the development of novel strategies to treat and prevent RA.

Peptidylarginine deiminase-4 gene polymorphisms are associated with systemic lupus erythematosus and lupus nephritis

OBJECTIVE

Peptidylarginine deiminase-4 (PAD4) is highly expressed by neutrophils and essential for citrullination occurring during the formation of neutrophil extracellular traps, which have been implicated in the pathogenesis of systemic lupus erythematosus (SLE) and lupus nephritis (LN). Single-nucleotide polymorphisms (SNPs) in PADI4 influence PAD4 expression and functionality. Here, we investigate whether SNPs in PADI4 influence the risk of SLE or LN.

METHOD

Altogether, 234 SLE patients and 484 controls were genotyped for nine PADI4 SNPs known to alter PAD4 functionality and/or expression, or to be associated with other autoimmune diseases, using an in-house multiplex Luminex assay. All analyses were adjusted for age and gender.

RESULTS

Heterozygosity for rs1748033, and heterozygosity and homozygosity for rs1635564, were associated with increased occurrence of SLE [odds ratio (OR) 1.55, 95% confidence interval (CI) 1.08-2.23; OR 1.52, 95% CI 1.06-2.19; and OR 2.06, 95% CI 1.08-3.93, respectively]. Homozygosity for rs1635564 was also associated with increased occurrence of LN (OR 3.35, 95% CI 1.2-10.97). Notably, gene dose effects of the rs1635564 variant allele were observed for SLE (p = 0.005) and LN (p = 0.01). Carriage of minor alleles of five other SNPs (rs11203366, rs11203367, rs874881, rs2240340, and rs11203368) was associated with increased occurrence of LN and hypertension.

CONCLUSION

The rs1635564 polymorphism of PADI4 is a candidate risk factor for SLE, particularly with renal involvement. Additional PADI4 polymorphisms also conferred increased risk of LN. Overall, these findings support the notion of PAD4 contributing to the pathogenesis of SLE and LN.

Hypogalactosylation of immunoglobulin G in rheumatoid arthritis: relationship to HLA-DRB1 shared epitope, anticitrullinated protein antibodies, rheumatoid factor, and correlation with inflammatory activity

Background

Galactosylation of immunoglobulin G (IgG) is reduced in rheumatoid arthritis (RA) and assumed to correlate with inflammation and altered humoral immunity. IgG hypogalactosylation also increases with age. To investigate dependencies in more detail, we compared IgG hypogalactosylation between patients with RA, patients with axial spondyloarthritis (axSpA), and healthy control subjects (HC), and we studied it in RA on the background of HLA-DRB1 shared epitope (SE), anticitrullinated protein antibodies (ACPA), and/or rheumatoid factor (RF) status.

Methods

Patients with RA (n = 178), patients with axSpA (n = 126), and HC (n = 119) were characterized clinically, and serum IgG galactosylation was determined by capillary electrophoresis. Markers of disease activity, genetic susceptibility, and serologic response included C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), DAS28, SE, HLA-B27, ACPA, and RF. Expression of glycosylation enzymes, including beta 1–4 galactosyltransferase (B4GALT3) activity, were estimated from transcriptome data for B-cell development (GSE19599) and differentiation to plasma cells (GSE12366).

Results

IgG hypogalactosylation was restricted to RA and associated with increasing CRP levels (p < 0.0001). In axSpA, IgG hypogalactosylation was comparable to HC and only marginally increased upon elevated CRP. Restriction to RA was maintained after correction for CRP and age. Treatment with sulfasalazine resulted in significantly reduced IgG hypogalactosylation (p = 0.003) even after adjusting for age, sex, and CRP (p = 0.009). SE-negative/ACPA-negative RA exhibited significantly less IgG hypogalactosylation than all other strata (vs SE-negative/ACPA-positive, p = 0.009; vs SE-positive/ACPA-negative, p = 0.04; vs SE-positive/ACPA-positive, p < 0.02); however, this indicated a trend only after Bonferroni correction for multiple testing. In SE-positive/ACPA-negative RA IgG hypogalactosylation was comparable to ACPA-positive subsets. The relationship between IgG hypogalactosylation and disease activity was significantly different between strata defined by SE (CRP, p = 0.0003, p_Bonferroni = 0.0036) and RF (CRP, p < 0.0001, p_Bonferroni < 0.0012), whereas ACPA strata revealed only a nonsignificant trend (p = 0.15). Gene expression data indicated that the key enzyme for galactosylation of immunoglobulins, B4GALT3, is expressed at lower levels in B cells than in plasma cells.

Conclusions

Increased IgG hypogalactosylation in RA but not in axSpA points to humoral immune response as a precondition. Reduced B4GALT3 expression in B cells compared with plasma cells supports relatedness to early B-cell triggering. The differential influence of RA treatment on IgG hypogalactosylation renders it a potential diagnostic target for further studies.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1540-0) contains supplementary material, which is available to authorized users.

Role of citrullination modification catalyzed by peptidylarginine deiminase 4 in gene transcriptional regulation

Peptidylarginine deiminase 4 (PADI4), a new histone modification enzyme, which converts both arginine and monomethyl-arginine to citrulline, has gained massive attention in recent years as a potential regulator of gene transcription. Recent studies have shown that arginine residues R2, R8, R17, and R26 in the H3 tail and R3 in the H4 tail can be deiminated by PADI4. This kind of histone post-translational modification has the potential to antagonize histone methylation and coordinate with histone deacetylation to regulate gene transcription. PADI4 also deiminates non-histone proteins, such as p300, NPM1, ING4, RPS2, and DNMT3A. PADI4 has been shown to involve in cell apoptosis and differentiation. Moreover, PADI4 can interact with tumor suppressor p53 and regulate the transcriptional activity of p53. Dysregulation of PADI4 is implicated in a variety of diseases, including rheumatoid arthritis, tumor development, and multiple sclerosis. A wide variety of PADI4 inhibitors have been identified. Further understanding of PADI4 functions may lead to novel diagnostic and therapeutic approaches in these diseases. This review summarizes the recent progress in the study of the regulation mechanism of PADI4 on gene transcription and the major physiological functions of PADI4 in human diseases.

PADI4 polymorphisms and the functional haplotype are associated with increased rheumatoid arthritis susceptibility: A replication study in a Southern Mexican population

Rheumatoid arthritis (RA) is a common autoimmune disease with a complex genetic background. The peptidyl arginine deiminase type IV (PADI4) gene has been associated with RA susceptibility in several populations. We addressed the relationship between three exonic PADI4 gene single nucleotide polymorphisms (SNPs) PADI4_89 (rs11203366), PADI4_90 (rs11203367) and PADI4_92 (rs874881) and related haplotypes with RA in a population from Southern México. This study included 200 RA patients and 200 control subjects. The SNPs were evaluated using the polymerase chain reaction-restriction fragment-length polymorphism (PCR-RFLP) technique, and antibodies to cyclic citrullinated peptides (anti-CCP) were measured by enzyme-linked immunosorbent assay (ELISA). In this population, the minor alleles of PADI4_89∗G, PADI4_90∗T and PADI4_92∗G gene polymorphisms were associated with RA susceptibility (OR=1.34, p=0.04; OR=1.35, p=0.03; OR=1.34, p=0.04; respectively). The GTG haplotype was also significantly associated with RA (OR=2.27 95%CI=1.18-4.41; p=0.008), but did not show association with levels of anti-CCP antibodies and clinical parameters. In conclusion, our replication study in a Southern Mexican population suggests that PADI4 individual polymorphisms and the related susceptibility haplotype (GTG) are also genetic risk markers for RA.

PAD4: pathophysiology, current therapeutics and future perspective in rheumatoid arthritis

INTRODUCTION

Peptidyl arginine deiminase 4 (PAD4) is an enzyme that plays an important role in gene expression, turning out genetic code into functional products in the body. It is involved in a key post translational modification, which involves the conversion of arginine to citrulline. It regulates various processes such as apoptosis, innate immunity and pluripotency, while its dysregulation has a great impact on the genesis of various diseases. Over the last few years PAD4 has emerged as a potential therapeutic target for the treatment of rheumatoid arthritis (RA). Areas covered: In this review, we discuss the basic structure and function of PAD4, along with the role of altered PAD4 activity in the onset of RA and other maladies. We also elucidate the role of PAD4 variants in etiology of RA among several ethnic groups and the current pre-clinical inhibitors to regulate PAD4. Expert opinion: Citrullination has a crucial role in RA and several other disorders. Since PAD4 is an initiator of the citrullination, it is an important therapeutic target for inflammatory diseases. Therefore, an in depth knowledge of the roles and activity of PAD4 is required to explore more effective ways to conquer PAD4 related ailments, especially RA.

Decreased severity of experimental autoimmune arthritis in peptidylarginine deiminase type 4 knockout mice

Background

Peptidylarginine deiminase type 4 (PADI4) has been identified as a susceptibility gene for rheumatoid arthritis (RA) by genome-wide association studies. PADI4 is highly expressed in the bone marrow, macrophages, neutrophils, and monocytes. Peptidyl citrulline is an interesting molecule in RA because it is a target antigen for anti-citrullinated peptide antibodies, and only PADs (translated proteins from PADI genes) can provide peptidyl citrulline via the modification of protein substrates. The aim of this study was to evaluate the importance of the PADI4 gene in the progression of RA.

Methods

We generated Padi4 knockout (Padi4^−/−) DBA1J mice. The Padi4^−/− DBA1J and wild-type mice were immunized with bovine type II collagen (CII) to develop collagen-induced arthritis (CIA). The expression of various inflammatory cytokines and Padi genes in immune cells was detected by the real-time TaqMan assay. Cytokine concentrations in sera were measured by enzyme-linked immunosorbent assays. Localization of the PAD4 and PAD2 proteins was indicated by immunohistochemistry.

Results

We demonstrated that the clinical disease score was significantly decreased in the Padi4^−/− mice and Padi4 expression was induced by CII immunization. In the Padi4^−/− mice, serum anti-type II collagen (CII) immunoglobulin M (IgM), IgG, and inflammatory cytokine levels were significantly decreased compared with those in the wild-type mice. Padi2 expression was induced in the immune cells of the Padi4^−/− mice as a compensation for the defect in Padi4.

Conclusions

Padi4 affected disease severity in the CIA mice and was involved in the enhancement of the collagen-initiated inflammatory responses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-016-1055-2) contains supplementary material, which is available to authorized users.

Increased levels of peptidylarginine deiminase 2 in synovial fluid from anti-CCP-positive rheumatoid arthritis patients: Association with disease activity and inflammatory markers

OBJECTIVE

To quantify peptidylarginine deiminase 2 (PAD2) in SF of RA patients and OA patients, and to determine the association between PAD2 levels, disease activity and inflammatory markers in RA.

METHODS

Blood and SF samples were obtained from 39 RA patients and 40 patients with OA. PAD2 content and PAD activity were measured by means of in-house assays. TNF-α, IL-1β, IL-6, IL-8, IL-10 and IL-12 were measured using flow cytometry.

RESULTS

PAD2 levels and PAD activity were higher in SF from RA than OA patients (P < 0.0001 and P = 0.03, respectively), as were all cytokine levels (P < 0.0001-0.05). SF PAD2 levels were higher among anti-CCP-positive patients than among anti-CCP-negative patients (P = 0.005). PAD2 levels in SF from RA patients correlated with disease activity, as assessed by DAS28 (P < 0.005). Moreover, SF PAD2 levels correlated with circulating CRP and anti-CCP levels (P < 0.0006), as well as with leucocyte count, IL-6, IL-8 and IL-10 levels in SF (P < 0.0001-0.02). PAD activity in SF was higher in RA patients than in OA patients, and correlated with PAD2 concentration.

CONCLUSION

Extracellular PAD2 levels in SF correlate with disease activity in RA patients. Anti-CCP-positive RA patients have higher PAD2 levels in SF than anti-CCP-negative RA patients and OA patients. Since we could demonstrate enzymatically active PADs in SF, we propose that free, extracellular PAD is of pathogenic relevance.

Associations Between PADI4 Gene Polymorphisms and Rheumatoid Arthritis: An Updated Meta-analysis

BACKGROUND AND AIMS

Studies investigating the association between the peptidylarginine deiminase 4 (PADI4) gene polymorphisms and rheumatoid arthritis (RA) reported conflicting results. The aim of this meta-analysis was to assess the association between PADI4 gene polymorphisms and RA.

METHODS

A systematic literature search was conducted to identify relevant studies. Pooled odds ratios (ORs) with 95% confidence intervals (CIs) were used to estimate the strength of the association.

RESULTS

A total of 34 studies from 28 articles involving 19859 patients with RA and 25771 healthy controls were included. Significant association of PADI4-94G/A polymorphism and RA was observed (OR = 0.891, 95% CI = 0.833-0.954, p = 0.001) in the overall study population and in the Asian populations (OR = 0.824, 95% CI = 0.759-0.894, p = 0.000) respectively. For the -92C/G polymorphism, a significant association was observed (OR = 1.481, 95% CI = 1.166-1.882, p = 0.001) in Africans. For the -90C/T polymorphism, a significant association was observed (OR = 0.576, 95% CI = 0.381-0.872, p = 0.009) in the Latin American population. The pooled estimates for the other polymorphisms were not statistically significantly associated with RA (PADI4-104C/T, -89A/G, -96T/C).

CONCLUSIONS

This meta-analysis demonstrates that PADI4-94G/A polymorphism is associated with susceptibility to RA in the overall population and in the Asian population. The PADI4 -92C/G polymorphism confers susceptibility to RA in Africans and the PADI4-90C/T polymorphism was associated with RA in the Latin American population.

Polymorphisms and functional haplotype in PADI4: further evidence for contribution on rheumatoid arthritis susceptibility and anti-cyclic citrullinated peptide antibodies in a western Mexican population

Peptidyl arginine deiminase IV (PADI4) enzyme catalyzes the citrullination of proteins, which are recognized by anti-cyclic citrullinated peptide antibodies (anti-CCP) in rheumatoid arthritis (RA) patients. Here, we determined the association between PADI4 gene polymorphisms and haplotypes with RA susceptibility and clinical characteristics in a western Mexican population. The relationship of PADI4 polymorphisms with anti-CCP and PADI4 mRNA expression was also evaluated. PADI4_89, PADI4_90 and PADI4_92 polymorphisms were individually associated with RA susceptibility. The GTG haplotype was significantly associated with: RA susceptibility; disease onset at ≤ 40 years and anti-CCP antibodies. PADI4 expression was three fold higher in RA patients carrying the susceptibility haplotype (GTG) than in non-susceptibility haplotype carriers (ACC). In conclusion, polymorphisms and functional haplotype (GTG) in PADI4 are associated with RA susceptibility as well as anti-CCP antibodies in a Mexican population. This supports the role of PADI4 early in RA pathogenesis by promoting the generation of citrullinated autoantigens.

PADI4 haplotypes in association with RA Mexican patients, a new prospect for antigen modulation

Peptidyl arginine deiminase IV (PAD 4) is the responsible enzyme for a posttranslational modification called citrullination, originating the antigenic determinant recognized by anti-cyclic citrullinated peptide antibodies (ACPA). Four SNPs (single nucleotide polymorphisms) have been described in PADI4 gene to form a susceptibility haplotype for rheumatoid arthritis (RA); nevertheless, results in association studies appear contradictory in different populations. The aim of the study was to analyze if the presence of three SNPs in PADI4 gene susceptibility haplotype (GTG) is associated with ACPA positivity in patients with RA. This was a cross-sectional study that included 86 RA patients and 98 healthy controls. Polymorphisms PADI4_89, PADI4_90, and PADI4_92 in the PADI4 gene were genotyped. The susceptibility haplotype (GTG) was more frequent in RA patients; interestingly, we found a new haplotype associated with RA with a higher frequency (GTC). There were no associations between polymorphisms and high scores in Spanish HAQ-DI and DAS-28, but we did find an association between RARBIS index and PADI4_89, PADI4_90 polymorphisms. We could not confirm an association between susceptibility haplotype presence and ACPA positivity. Further evidence about proteomic expression of this gene will determine its participation in antigenic generation and autoimmunity.

PADI2 is significantly associated with rheumatoid arthritis

Citrullination, a posttranslational modification of peptidyl arginine to citrulline, plays an essential role in rheumatoid arthritis (RA). Citrullination is catalyzed by a group of peptidylarginine deiminases (PADs) including PADI 1, 2, 3, 4 and 6. Many studies have indicated that the gene encoding PADI4 is a factor in susceptibility to RA. Some studies have detected PADI2 expression in RA synovial tissues, suggesting that PADI2 also plays an important role in the disease. This study evaluated the possible association between the PADI2-encoding gene and RA. Seventeen tag SNPs across the PAD locus were genotyped using a custom-designed Illumina 96-SNP VeraCode microarray. Peripheral blood samples were collected from patients with RA (n = 267), ankylosing spondylitis (AS, n = 51) and healthy controls (n = 160). The results of genotyping were verified using Sequenom MassARRAY in an independent cohort of 307 patients with RA, 324 patients with AS and 509 healthy controls. A western blot analysis was performed using synovial tissue from patients with RA (n = 7), osteoarthritis (OA, n = 7) and AS (n = 5) to determine the levels of expression of PADI2. A microarray analysis revealed a significant association between three selected PADI2 SNPs (rs2235926, rs2057094, rs2076616) and the presence of RA. The increased susceptibility to RA associated with rs2235926 (OR = 1.706733, 95% CI = [1.576366–1.866587], p = 0.000839) and rs2057094 (OR = 1.360432, 95% CI = [1.065483–1.869482], p = 0.003291) was further confirmed by the Sequenom MassARRAY. No tag SNPs in the PADI2 locus showed a significant association with AS. Increased expression of PADI2 was detected in RA synovial tissues compared with samples from patients with OA and AS. PADI2 is significantly associated with RA and may be involved in the pathogenesis of the disease.

Polymorphisms in peptidylarginine deiminase associate with rheumatoid arthritis in diverse Asian populations: evidence from MyEIRA study and meta-analysis

Introduction

The majority of our knowledge regarding disease-related mechanisms of uncontrolled citrullination and anti-citrullinated protein antibody development in rheumatoid arthritis (RA) was investigated in Caucasian populations. However, peptidylarginine deiminase (PADI) type 4 gene polymorphisms are associated with RA in East Asian populations and weak or no association was found in Caucasian populations. This study explores the association between the PADI4 polymorphisms and RA risk in a multiethnic population residing in South East Asia with the goal of elucidating generalizability of association in non-Caucasian populations.

Methods

A total of 320 SNPs from the PADI locus (including PADI1, PADI2, PADI3, PADI4 and PADI6 genes) were genotyped in 1,238 RA cases and 1,571 control subjects from the Malaysian Epidemiological Investigation of Rheumatoid Arthritis (MyEIRA) case-control study. Additionally, we conducted meta-analysis of our data together with the previously published studies of RA from East Asian populations.

Results

The overall odds ratio (OR_overall) for the PADI4 (rs2240340) allelic model was 1.11 (95% confidence interval (CI) = 1.00 to 1.23, P = 0.04) and for the genotypic model was 1.20 (95% CI = 1.01 to 1.44, P = 0.04). Haplotype analysis for four selected PADI4 SNPs revealed a significant association of one with susceptibility (P = 0.001) and of another with a protective effect (P = 0.02). The RA susceptibility was further confirmed when combined meta-analysis was performed using these data together with data from five previously published studies from Asia comprising 5,192 RA cases and 4,317 control subjects (OR_overall = 1.23 (95% CI = 1.16 to 1.31, P_{heterogeneity} = 0.08) and 1.31 (95% CI = 1.20 to 1.44, P_{heterogeneity} = 0.32) in allele and genotype-based models, respectively). In addition, we also detected a novel association of PADI2 genetic variant rs1005753 with RA (OR_overall = 0.87 (95% CI = 0.77 to 0.99)).

Conclusion

Our study demonstrates an association between PADI4 and RA in the multiethnic population from South East Asia and suggests additional association with a PADI2 gene. The study thus provides further support for the notion that polymorphisms in genes for enzymes responsible for citrullination contribute to RA development in multiple populations of Asian descent.

PADI4 polymorphisms and susceptibility to rheumatoid arthritis: a meta-analysis

The objective of the study was to investigate the association between peptidylarginine deiminase 4 (PADI4) polymorphism and susceptibility to rheumatoid arthritis (RA). An electronic searching strategy was employed to collect relevant studies on the association between PADI4 polymorphism and susceptibility to RA. The odds ratio (OR) with the 95 % confidence interval (95 % CI) was used to evaluate the RA risk presented by PADI4 polymorphism. Fixed or random effects models were selected based on heterogeneity. Publication bias was assessed using funnel plots, Begg's test, and Egger's test. A total of 27 studies from 21 articles were included. Six gene loci (padi4_94, 104, 92, 90, 89, and 100) were chosen for the meta-analysis. The pooled ORs (95 % CI) for allele 2 versus 1 were 1.08 (1.05-1.12), 1.17 (1.12-1.23), 1.26 (1.18-1.36), 1.17 (1.10-1.24), 1.30 (1.17-1.44), and 1.25 (1.11-1.40), respectively. All six SNPs were significantly associated with RA in Asian populations. Three SNPs (PADI4_104, 90, 89) showed significant associations, while the other three SNPs (PADI4_94, 92, 100) exhibited no associations in the European population. A dose-response relationship between allele 2 of PADI4 and the risk of RA was also identified. In conclusion, this meta-analysis suggests that PADI4 polymorphisms represent a significant risk factor for RA, especially in Asians.

Peptidylarginine deiminase type 4 and methyl-CpG binding domain 4 polymorphisms in Chinese patients with rheumatoid arthritis

OBJECTIVE

Peptidylarginine deiminase type 4 (PADI4) and methyl-CpG binding domain 4 (MBD4) are closely related with rheumatoid arthritis (RA). We hypothesized that PADI4 and MBD4 polymorphisms may contribute to RA susceptibility.

METHODS

We studied PADI4 rs2240340 G/A, PADI4 rs874881 C/G, MBD4 rs140693 G/A, and MBD4 rs2005618 T/C gene polymorphisms in 329 patients with RA and 697 controls in a Chinese population. Genotyping was done using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS).

RESULTS

When the PADI4 rs2240340 GG homozygote genotype was used as the reference group, the AA genotype was associated with a significantly increased risk of RA. In the recessive model, when PADI4 rs2240340 GG/GA genotypes were used as the reference group, the AA homozygote genotype was associated with a significant increased susceptibility to RA. PADI4 rs874881 C/G was in complete linkage disequilibrium with PADI4 rs2240340 G/A. MBD4 rs140693 G/A and MBD4 rs2005618 T/C polymorphisms were not associated with the risk of RA. In stratification analyses, a significantly increased risk for RA associated with the PADI4 rs2240340 AA genotype was evident among older patients and patients who were anticitrullinated protein antibody (ACPA)-positive compared with the PADI4 rs2240340 GG/GA genotype.

CONCLUSION

These findings suggest that the functional single-nucleotide polymorphism PADI4 rs2240340 G/A variant allele is associated with RA development, especially among older patients and ACPA-positive patients. However, our results were obtained from a moderate-sized sample, and therefore this is a preliminary conclusion. Validation by a larger study from a more diverse ethnic population is needed to confirm these findings.

Insight from genome-wide association studies in rheumatoid arthritis and multiple sclerosis

Autoimmune diseases are caused by multiple genes and environmental effects. In addition, genetic contributions and the number of associated genes differ among different diseases and ethnic populations. Genome-wide association studies (GWAS) on rheumatoid arthritis (RA) and multiple sclerosis (MS) show that these diseases share many genetic factors. Recently, in addition to the major histocompatibility complex (MHC) gene, other genetic loci have been found to be associated with the risk for autoimmune diseases. This review focuses on the search for genetic variants that influence the susceptibility to RA and MS as typical autoimmune diseases and discusses the future of GWAS.

Anti-PAD4 autoantibodies in rheumatoid arthritis: levels in serum over time and impact on PAD4 activity as measured with a small synthetic substrate

Isoform 4 of the human peptidylarginine deiminase (hPAD4) enzyme may be responsible for the citrullination of antigens in rheumatoid arthritis (RA) and has been shown to be itself the target of disease-specific autoantibodies. Here, we have tested whether the level of serum anti-hPAD4 antibodies in RA patients is stable over a period of 10 years and whether the antibodies influence hPAD4-mediated deimination of the small substrate N-α-Benzoyl-l-arginine ethyl ester. RA sera (n = 128) obtained at baseline and after 10 years were assessed for anti-hPAD4 antibodies by a specific immunoassay. For 118 RA patients, serum anti-hPAD4 IgG levels were stable over 10 years. Seven patients who were negative for anti-PAD4 IgG at baseline had become positive after 10 years. Further, total IgG from selected RA patients and controls were purified, and a fraction was depleted for anti-hPAD4 antibodies. Kinetic deimination assays were performed with total IgG and depleted fractions. The k_cat and K_m values of hPAD4-mediated deimination of N-α-Benzoyl-l-arginine ethyl ester were not affected by the depletion of the anti-hPAD4 antibodies from the total IgG pool. In conclusion, RA patients remain positive for anti-hPAD4 antibodies over time and some patients who are initially anti-hPAD4 negative become positive later in the disease course. The anti-hPAD4 antibodies did not affect the enzymatic activity of hPAD4 when the small substrate N-α-Benzoyl-l-arginine ethyl ester was used. However, this finding may not exclude an effect of these autoantibodies on citrullination of protein substrates in RA.

Autocitrullination of human peptidyl arginine deiminase type 4 regulates protein citrullination during cell activation

OBJECTIVE

To address mechanisms that control the activity of human peptidyl arginine deiminase type 4 (PAD-4).

METHODS

PAD-4 autocitrullination was determined by anti-modified citrulline immunoblotting, using purified recombinant and endogenous PAD-4 from activated human primary neutrophils and cell lines expressing PAD-4. The citrullination sites in PAD-4 were determined by mass spectrometry. Mechanisms of autocitrullination-induced inactivation and the functional consequences of autocitrullination in PAD-4 polymorphic variants were addressed using purified components and cell lines expressing PAD-4 wild-type, PAD-4 mutant, and PAD-4 polymorphic variants relevant to rheumatoid arthritis (RA).

RESULTS

PAD-4 is autocitrullinated in vitro and during activation of primary cells and cell lines expressing PAD-4. Interestingly, this modification inactivated the function of the enzyme. The efficiency of inactivation differed among genetically defined PAD-4 variants relevant to RA. PAD-4 was citrullinated at 10 sites, which are clustered into 3 distinct regions, including a cluster of arginines around the active site cleft where Arg-372 and -374 were identified as the potential autocitrullination targets that inactivate the enzyme. Autocitrullination also modified the structure of PAD-4, abrogating its recognition by multiple rabbit antibodies, but augmenting its recognition by human anti-PAD-4 autoantibodies.

CONCLUSION

Our findings suggest that autocitrullination regulates the production of citrullinated proteins during cell activation, and that this is affected by structural polymorphisms in PAD-4. Autocitrullination also influences PAD-4 structure and immune response.

The PADI4 gene does not contribute to genetic susceptibility to rheumatoid arthritis in Chinese Han population

The Peptidyl arginine deiminase, type IV (PADI4) gene has been suggested to have an association with rheumatoid arthritis (RA) in several populations. But its role in Chinese RA is not clarified. We investigated five single-nucleotide polymorphisms (SNPs) of PADI4 as PADI4-89 (rs11203366), PADI4-90 (rs11203367), PADI4-92 (rs874881) PADI4-94 (rs2240340), and PADI4-104 (rs1748033) in Chinese Han population. A total of 378 unrelated RA patients and 204 healthy controls were genotyped for the five SNPs. Individual allele, genotype and haplotype frequencies were compared between patients and controls. No significant differences in the frequency of PADI4 alleles, genotypes and haplotypes were observed between the patients and controls except PADI4-92. These data indicated that PADI4 polymorphisms were unlikely to play an important role in the susceptibility to RA in Chinese Han population.

Peptidyl arginine deiminase type IV (PADI4) haplotypes interact with shared epitope regardless of anti-cyclic citrullinated peptide antibody or erosive joint status in rheumatoid arthritis: a case control study

INTRODUCTION

Anti-cyclic citrullinated peptide autoantibodies (anti-CCP) are the most specific serologic marker for rheumatoid arthritis (RA). Genetic polymorphisms in a citrullinating (or deiminating) enzyme, peptidyl arginine deiminase type IV (PADI4) have been reproducibly associated with RA susceptibility in several populations. We investigated whether PADI4 polymorphisms contribute to anti-CCP-negative as well as -positive RA, whether they influence disease severity (erosive joint status), and whether they interact with two major risk factors for RA, Human Leukocyte Antigen-DRB1 (HLA-DRB1) shared epitope (SE) alleles and smoking, depending on anti-CCP and erosive joint status.

METHODS

All 2,317 unrelated Korean subjects including 1,313 patients with RA and 1,004 unaffected controls were genotyped for three nonsynonymous (padi4_89, padi4_90, and padi4_92) and one synonymous (padi4_104) single-nucleotide polymorphisms (SNPs) in PADI4 and for HLA-DRB1 by direct DNA sequence analysis. Odds ratios (OR) were calculated by multivariate logistic regression. Interaction was evaluated by attributable proportions (AP), with 95% confidence intervals (CI).

RESULTS

A functional haplotype of the three fully correlated nonsynonymous SNPs in PADI4 was significantly associated with susceptibility to not only anti-CCP-positive (adjusted OR 1.73, 95% CI 1.34 to 2.23) but also -negative RA (adjusted OR 1.75, 95% CI 1.15 to 2.68). A strong association with both non-erosive (adjusted OR 1.62, 95% CI 1.29 to 2.05) and erosive RA (adjusted OR 1.62, 95% CI 1.14 to 2.31) was observed for PADI4 haplotype. Gene-gene interactions between the homozygous RA-risk PADI4 haplotype and SE alleles were significant in both anti-CCP-positive (AP 0.45, 95% CI 0.20 to 0.71) and -negative RA (AP 0.61, 95% CI 0.29 to 0.92). Theses interactions were also observed for both non-erosive (AP 0.48, 95% CI 0.25 to 0.72) and erosive RA (AP 0.46, 95% CI 0.14 to 0.78). In contrast, no interaction was observed between smoking and PADI4 polymorphisms.

CONCLUSIONS

A haplotype of nonsynonymous SNPs in PADI4 contributes to development of RA regardless of anti-CCP or erosive joint status. The homozygous PADI4 haplotype contribution is affected by gene-gene interactions with HLA-DRB1 SE alleles.

Pathogenic role of antibodies to citrullinated proteins in rheumatoid arthritis

In the last 10 years, the discovery that antibodies to citrullinated proteins are highly specific for rheumatoid arthritis has led to a model of pathogenesis that ties together the genetic and environmental risk factors for susceptibility and severity of disease. The authors propose that the chronic inflammation is driven by two phases of an immune response. The first phase is the priming of autoimmunity, which may occur many years before the onset of disease and is caused by environmental factors, such as smoking and infectious agents, in the context of disease susceptibility alleles. This may occur in sites outside the joint, such as the respiratory tract. The second phase is the induction of arthritis, which is associated with the generation of citrullinated proteins within the joint, which is then perpetuated as the erosive disease by a local chronic immune response. The identity of candidate synovial citrullinated antigen(s), whether fibrin, vimentin, alpha-enolase, collagen type II or others yet to be described, may be the key to the pathogenesis of the destructive disease characteristic of rheumatoid arthritis. There is emerging evidence that citrullination may already be modified by established therapy in rheumatoid arthritis, but more specific inhibitors of deimination may provide new agents for future treatments.

Pharmacogenetics of rheumatoid arthritis: Potential targets from susceptibility genes and present therapies

Rheumatoid arthritis (RA) is a chronic heterogeneous autoimmune disorder of unknown etiology resulting in inflammation in the synovium, cartilage, and bone. Genetic factors play an important role in susceptibility to RA as the heritability of RA is between 50% and 60%, with the human leukocyte antigen (HLA) locus accounting for at least 30% of overall genetic risk. Outside the major histocompatibility complex (MHC) region, six additional risk loci have been identified and validated including PTPN22, STAT4, PADI4, CTLA4, TNFAIP3-OLIG3, and TRAF1/C5. Genetic factors are also important in RA pharmacotherapy due to the gene-dependent activity of enzymes involved in the pharmacokinetics and/or pharmacodynamics of RA medications. Indeed, there is great variability in drug efficacy as well as adverse events associated with any anti-rheumatic therapy and genetics is thought to contribute significantly to this inter-individual variability in response. This review will summarize the genetic factors that have been implicated in the pathogenesis of RA, and how these determinants may factor into the potential pharmacogenetics of this disease. We will also review the therapeutic agents that are currently being utilized or presently being evaluated in the treatment of RA, along with potential pharmacogenetic markers that have been proposed for such medications.

[Familial clustering, genetic roots and insights into the pathogenesis of autoimmune diseases]

Genome-wide association studies have dramatically increased our knowledge about the genetic contribution to autoimmune diseases. The identified genes are indicators for signal transduction pathways involved in disease pathogenesis and could contribute to potential new therapeutic approaches.

Peptidylarginine deiminase 4 and citrullination in health and disease

Deimination is catalyzed by a family of calcium binding enzymes, called peptidylarginine deiminases (PADs). Among these, the PAD4 isoform has been more extensively studied for its role in some autoimmune diseases. PAD4 is localized in the cytoplasm of monocytes, T and B cells, neutrophils, eosinophils and NK cells and can move to the nucleus upon cell activation. PAD4 plays a physiological role in gene regulation via citrullination of histones. In rheumatoid arthritis (RA), PAD4 contributes to the generation of ACPA specific substrates and is itself a target of autoantibodies; alleles of the PADI4 gene confer susceptibility to RA in Asians but not in Caucasians. In multiple sclerosis, extensive deimination of brain proteins is observed in active lesions, but no role for the PADI4 gene in susceptibility to MS has been so far described.

PADI4 genotype is not associated with rheumatoid arthritis in a large UK Caucasian population

Background

Polymorphisms of the peptidylarginine deiminase type 4 (PADI4) gene confer susceptibility to rheumatoid arthritis (RA) in East Asian people. However, studies in European populations have produced conflicting results. This study explored the association of the PADI4 genotype with RA in a large UK Caucasian population.

Methods

The PADI4_94 (rs2240340) single nucleotide polymorphism (SNP) was directly genotyped in a cohort of unrelated UK Caucasian patients with RA (n=3732) and population controls (n=3039). Imputed data from the Wellcome Trust Case Control Consortium (WTCCC) was used to investigate the association of PADI4_94 with RA in an independent group of RA cases (n=1859) and controls (n=10 599). A further 56 SNPs spanning the PADI4 gene were investigated for association with RA using data from the WTCCC study.

Results

The PADI4_94 genotype was not associated with RA in either the present cohort or the WTCCC cohort. Combined analysis of all the cases of RA (n=5591) and controls (n=13 638) gave an overall OR of 1.01 (95% CI 0.96 to 1.05, p=0.72). No association with anti-CCP antibodies and no interaction with either shared epitope or PTPN22 was detected. No evidence for association with RA was identified for any of the PADI4 SNPs investigated. Meta-analysis of previously published studies and our data confirmed no significant association between the PADI4_94 genotype and RA in people of European descent (OR 1.06, 95% CI 0.99 to 1.13, p=0.12).

Conclusion

In the largest study performed to date, the PADI4 genotype was not a significant risk factor for RA in people of European ancestry, in contrast to Asian populations.

A functional haplotype of PADI4 gene in rheumatoid arthritis: positive correlation in a French population

OBJECTIVE

A functional haplotype of peptidyl arginine deiminase 4 (PADI4) was associated with susceptibility to rheumatoid arthritis (RA) in Asian populations, but the results are contradictory in Europeans. We investigated (1) the association of 2 single-nucleotide polymorphisms (SNP) located in exon 2 of PADI4 with RA in another Caucasian population; and (2) the association between PADI4 and anti-citrullinated protein (anti-CCP) antibodies.

METHODS

DNA samples were obtained from 405 French RA patients and 275 controls. All RA patients met the revised criteria of the American College of Rheumatology. PADI4_89 163(G-->A) and PADI4_90 245(T-->C) SNP were genotyped using a PCR-RFLP method confirmed by direct sequencing. All patients and controls were genotyped for HLA-DRB1. The presence of anti-CCP antibodies was tested in 243 RA patients using an ELISA technique.

RESULTS

We focused on PADI4_89 163(G-->A) and PADI4_90 245(T-->C) SNP that distinguished 2 main haplotypes: AC haplotype (PADI4_89*A PADI4_90*C) and GT haplotype (PADI4_89*G PADI4_90*T), described, respectively, as "nonsusceptible" and "susceptible." A positive association between RA and presence of the GT haplotype was found in the heterozygous state (p = 0.002) and the homozygous state (RA patients 22%, controls 13%; p = 0.005). A correlation was observed between the presence but not the level of anti-CCP antibodies and the GT heterozygous (p = 0.03) and homozygous (p = 0.05) haplotypes. No correlation was found between the HLA-DRB1 shared epitope and any of the PADI4 haplotypes.

CONCLUSION

Our findings confirm those of Japanese, Korean, and Canadian studies and suggest that PADI4 may be a new susceptibility gene independent of HLA-DRB1 for RA in Caucasian populations.

Association of autoimmunity to peptidyl arginine deiminase type 4 with genotype and disease severity in rheumatoid arthritis

OBJECTIVE

Protein citrullination is an important posttranslational modification recognized by rheumatoid arthritis (RA)-specific autoantibodies. One of the citrullinating enzymes, peptidyl arginine deiminase type 4 (PAD-4), is genetically associated with development of RA in some populations, although the mechanism(s) mediating this effect are not yet clear. There have been descriptions of anti-PAD-4 autoantibodies in different rheumatic diseases. This study was undertaken to investigate whether anti-PAD-4 antibodies are specific to RA, are associated with disease phenotype or severity, and whether PAD-4 polymorphisms influence the anti-PAD-4 autoantibody response.

METHODS

Sera from patients with established RA, patients with other rheumatic diseases, and healthy adults were assayed for anti-PAD-4 autoantibodies by immunoprecipitation of in vitro-translated PAD-4. The epitope(s) recognized by PAD-4 autoantibodies were mapped using various PAD-4 truncations. PAD-4 genotyping was performed on RA patients with the TaqMan assay. Joint erosions were scored from hand and foot radiographs using the Sharp/van der Heijde method.

RESULTS

PAD-4 autoantibodies were found in 36-42% of RA patients, and were very infrequent in controls. Recognition by anti-PAD-4 autoantibodies required the 119 N-terminal amino acids, which encompass the 3 nonsynonymous polymorphisms associated with disease susceptibility. Strikingly, the anti-PAD-4 immune response was associated with the RA susceptibility haplotype of PADI4. Anti-PAD-4 antibodies were associated with more severe joint destruction in RA.

CONCLUSION

Our findings indicate that anti-PAD-4 antibodies are specific markers of RA, independently associated with more severe disease, suggesting that an anti-PAD-4 immune response may be involved in pathways of joint damage in this disease. Polymorphisms in the PADI4 gene influence the immune response to the PAD-4 protein, potentially contributing to disease propagation.

Citrullinated fibrinogen inhibits thrombin-catalysed fibrin polymerization

Citrullination is the post-translational modification of arginine residues by peptidylarginine deiminases (PADIs). Fibrinogen is one substrate of PADIs under physiological conditions. Fibrinogen is an important factor for blood coagulation and inducing inflammation. The citrullinated form of fibrinogen appears in rheumatoid arthritis synovial tissue together with the production of autoantibodies that target self-peptides containing citrulline. However, whether the function of fibrinogen changes after citrullination remains unclear. We found that citrullinated fibrinogen markedly impairs the function of thrombin-catalysed fibrin polymerization and also inhibits fibrin formation. Increased citrullinated fibrinogen might thus affect the balance between coagulation and fibrinolysis and alter antigenicity under physiological conditions. These data suggest that citrullination of proteins could physiologically change functions and subsequently generate pro-inflammatory conditions and autoimmune reactions.

Advances in the genetics of rheumatoid arthritis point to subclassification into distinct disease subsets

In the past few years considerable advances have been made in the genetics of susceptibility to rheumatoid arthritis (RA). For decades the HLA-DRB1 alleles were the only extensively replicated genetic factor, but more genetic risk factors have now been identified that predispose to RA. Interestingly, several of the observed genetic variants conferred risk to anticitrulline-peptide antibody (ACPA)-positive RA and two variants may be restricted to ACPA-negative RA, pointing to the need for subclassification of RA. The current manuscript reviews recently identified genetic factors predisposing to ACPA-positive RA and ACPA-negative RA. Additionally, although being scarcely explored, genetic variants affecting the severity of disease course are discussed.

Replication of reported genetic associations of PADI4, FCRL3, SLC22A4 and RUNX1 genes with rheumatoid arthritis: results of an independent Japanese population and evidence from meta-analysis of East Asian studies

We conducted population-based association tests for the four selected SNPs (rs2240340/padi4_94, rs7528684/fcrl3_3, rs3792876/slc2F2 and rs2268277/runx1) previously reported to be associated with rheumatoid arthritis (RA). The study population consisted of 950 unrelated Japanese subjects with RA and 507 controls, none of whom had previously been tested for these variants. Only the SNP rs2240340/padi4_94 was modestly associated with RA [allele odds ratio (OR) 1.22, 95% confidence interval (CI) 1.04-1.43, P=0.012]. The most significant association effect was found for genotype contrast between minor and major allele homozygotes (OR 1.53, 95% CI 1.10-2.12, P=0.010). No other SNPs showed a statistically significant association with RA in our population. Meta-analysis of published studies and our new data confirmed a highly significant association between PADI4 gene SNPs and increased risk of RA in East Asian populations (allele fixed-effects summary OR 1.31, 95% CI 1.22-1.41, P<0.0001). We found some evidence for an association of either rs7528684/fcrl3_3 or rs3792876/slc2F2 with RA; however, because the magnitudes of effects were apparently much weaker than those reported in the initial positive reports, and there were substantial levels of inter-study OR heterogeneity, we concluded that additional studies are needed to fully understand the present results.

Genetic background of anticyclic citrullinated peptide autoantibody production in Hungarian patients with rheumatoid arthritis

Polymorphisms of the peptidylarginine deiminase 4 (PADI4) gene encoding for the isoenzyme that converts arginyl into citrullyl residues have been shown to contribute to susceptibility to rheumatoid arthritis (RA), depending on the population studied. We aimed at determining whether PADI4 single nucleotide polymorphisms (SNPs) are associated with RA in a Hungarian population. The relationship between anticyclic citrullinated peptide (anti-CCP) production and HLA-DRB1 alleles encoding the shared epitope (SE) was also investigated. DNA samples were obtained from RA (n = 261) patients and from control donors (n = 120). HLA-DRB1 genotyping was carried out by polymerase chain reaction (PCR) with sequence-specific priming. PAD4_92 G/C and PAD4_104 T/C SNPs were genotyped using real-time PCR allele discrimination. Autoantibodies against CCP were detected by ELISA. All healthy controls tested anti-CCP negative, whereas 171 (66%) RA patients were anti-CCP positive. No significant difference in allele or genotype frequencies were found between RA patients and controls for any of the PADI4 SNPs. Anti-CCP seropositivity was unrelated to these two SNPs. No association was found between any of the PADI4 SNPs and HLA-DRB1 subtypes. Presence of the HLA-RB1 SE alleles was significantly associated with anti-CCP seropositivity; HLA-DRB1*0401 and HLA-DRB1*1001 carriers showed the strongest association. In conclusion, our data suggest that polymorphisms of the PADI4 gene are not associated with rheumatoid arthritis and are unlikely to be responsible for the presence of anti-CCP autoantibodies in a white Hungarian population. HLA-DRB1 SE alleles, however, may significantly contribute to the genetic determination of anti-CCP production in Hungarian patients with RA.

A new classification of HLA-DRB1 alleles differentiates predisposing and protective alleles for autoantibody production in rheumatoid arthritis

The HLA-DRB1 gene was reported to be associated with anticitrullinated protein/peptide autoantibody (ACPA) production in rheumatoid arthritis (RA) patients. A new classification of HLA-DRB1 alleles, reshaping the shared epitope (SE) hypothesis, was recently found relevant in terms of RA susceptibility and structural severity.

We investigated the relevance of this new classification of HLA-DRB1 SE⁺ alleles in terms of rheumatoid factor (RF) and ACPA production in a sample of French RA patients.

We studied 160 early RA patients included in a prospective longitudinal cohort of French Caucasian patients with recent-onset arthritis. RF, anticyclic citrullinated peptide 2 (anti-CCP2) and antideiminated human fibrinogen autoantibodies (AhFibA) were assessed in all patients at inclusion. The HLA-DRB1 gene was typed by PCR-sequence specific oligonucleotides probes (PCR-SSOP), and SE+ alleles were classified into four groups (S1, S2, S3P, S3D) according to the new classification.

The new classification of HLA-DRB1 SE+ alleles distinguishes predisposing and protective alleles for RF, anti-CCP2 or AhFibA production. The presence of S2 or S3P alleles is associated with both RF, anti-CCP2 or AhFibA positivity, whereas the presence of S3D or S1 alleles appears to be protective for RF, anti-CCP2 or AhFibA positivity.

The new classification of HLA-DRB1 SE⁺ alleles is relevant in terms of autoantibody production in early RA patients by differentiating predisposing and protective alleles for RF or ACPA production.

Citrullination by peptidylarginine deiminase in rheumatoid arthritis

Rheumatoid arthritis (RA) is a complex, multifactorial disease with genetic and immunological aspects. Because RA is an autoimmune condition, dysregulation of the immune system is implied. Many linkage and association studies have also indicated that multiple genetic factors are associated with RA. Although the contribution of each genetic factor is small, the combination of these factors affects RA development. Previous studies have suggested that genetic changes affect the internal immunological environment, which results in autoimmune diseases. More recent genetic studies indicate that the HLA-DRB gene is the predominant cause of RA and that other non-HLA genes are also involved. We reported that peptidylarginine deiminase (gene name abbreviated to PADI, protein name abbreviated to PAD) type 4 is the one of the non-HLA genetic factors involved in RA via citrullination. Antibodies against citrullinated proteins/peptides are highly specific to RA, but the physiological roles of PADI gene, PAD proteins as their products and citrullinated proteins/peptides are obscure. However, levels of anticitrullinated protein antibodies are apparently also increased and were involved in the pathogenesis of autoimmune arthritis in mice with collagen-induced arthritis (CIA). These data suggested that citrullinated protein and anticitrullinated protein antibodies play important roles in the development of RA. This review summarizes the relationship between RA and citrullination, as well as the role of PADI4 genetics.

Association of anti-cyclic citrullinated peptide antibody levels with PADI4 haplotypes in early rheumatoid arthritis and with shared epitope alleles in very late rheumatoid arthritis

OBJECTIVE

Anti-cyclic citrullinated peptide (anti-CCP) antibodies are rheumatoid arthritis (RA)-specific serologic markers. RA susceptibility has been associated with HLA-DRB1 shared epitope (SE) alleles and single-nucleotide polymorphism (SNP) haplotypes in the peptidyl arginine deiminase 4 gene (PADI4). This study was undertaken to determine whether anti-CCP levels are associated with PADI4 haplotypes and/or SE alleles in Korean patients with RA.

METHODS

Three nonsynonymous SNPs in PADI4 (padi4_89, padi4_90, and padi4_92) and SE alleles were genotyped, and serum anti-CCP levels were measured, in 311 patients with nonerosive or erosive RA. The relationships between anti-CCP levels and PADI4 haplotypes and/or SE alleles were analyzed statistically.

RESULTS

Anti-CCP levels were significantly higher in patients carrying the PADI4 RA risk haplotype than in patients who did not have the risk haplotype, among anti-CCP-positive patients with RA with a disease duration of <or=34 months (P = 0.041), but not among patients with a longer disease duration or among those who had erosive RA versus nonerosive RA. In contrast, the levels were significantly higher in SE carriers than in noncarriers among patients with RA with a disease duration of >or=141 months (P = 0.0037) and among those who had erosive RA (P = 0.000098), but not among patients who had a shorter disease duration or those who had nonerosive RA.

CONCLUSION

The PADI4 RA risk haplotype is associated with increased anti-CCP levels in RA patients with disease of short duration, and PADI4 may play a role in early RA. In contrast, SE alleles are associated with increased anti-CCP levels in RA patients with very longstanding disease and in patients with erosive RA, suggesting that SE alleles play a role in very late RA.

PADI4 polymorphisms and rheumatoid arthritis susceptibility: a meta-analysis

We conducted a comprehensive meta-analysis with all available data on the association of allele and genotype of peptidylarginine deiminases 4 (PADI4) polymorphisms with RA overall and in each ethnic population to explore whether the PADI4 polymorphisms confer susceptibility to RA. Nine comparisons, three Asians and six Europeans, from eight studies were included in this meta-analysis. Overall meta-analysis shows a significant association of PADI4_94, 104 and 90 with RA (OR = 1.20, 1.17, 1.35, P = 0.001, <0.0001, 0.006, respectively). There was a significant association with all of the PADI4 polymorphisms with RA in people of Asian descent. However, there was no significant association of PADI4 polymorphisms with RA in people of European descent, except for PADI_94. The presence of 2/2 genotype of the PADI4 significantly increased the risk for RA in European populations (OR = 2.10, 95% CI, 1.66-2.66, P < 0.0001) without between-study heterogeneity (I (2) = 44.3, P = 0.15). In conclusion, this meta-analysis demonstrates that the PADI4 polymorphisms may represent a significant risk factor for RA in Asians and Europeans and may play a larger role in susceptibility to RA in Asian than in European populations. Further studies are needed to see if the PADI4 gene confers a risk of RA in other ethnic groups.