PRTN3 and rheumatoid arthritis: Among current smokers, minor allele (rs6933349_A) carriers had a lower level of DNA methylation at cg21325723 (Fig. 2b), which was previously reported to be associated with increased risk of developing ACPA-positive RA [12] (P value = 1.49 × 10-9) (Additional file 1: Figure S2).