AbstractObjectiveAnti–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.MethodsThree 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.ResultsAnti‐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 ≤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 ≥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.ConclusionThe 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.