Functional haplotypes in citrullinating enzyme peptidylarginine deiminase 4 are associated with rheumatoid arthritis

Introduction

Previous studies of rheumatoid arthritis (RA) indicated that the risk of disease in the siblings of affected individuals (lsib) is increased twofold to 17-fold, suggesting that genetic factors are important. Recently, five sib-paid linkage studies from Europe, North America and Japan were published. Although there was no locus suggested by all the studies in common, some loci were suggested by multiple studies. Chromosome 1p36 was one such loci. Within 10 Mb centromeric from these microsatellite markers, there was a region containing clusters of enzymes that were very likely to be functionally relevant to RA-specific autoantibody production. These enzymes were peptidylarginine deiminases (PADIs). PADIs are enzymes that post-translationally convert arginine residues to citrulline. Citrullination and citrullinated peptides have been recognized to be one of the most RA-specific phenomena because citrullination was revealed to be related to the most RA-specific autoantibodies. The clinical importance of the measurement of anti-citrullinated peptide antibodies and the specificity of the autoantibodies suggest a specific role of citrullination and PADIs in RA pathophysiology.

Methods and results

We performed a case–control association study, using single nucleotide polymorphisms discovered by the Japanese Millennium Genome Project in the 1p36 region, containing the genes for PADI1, PADI2, PADI3 and PADI4. We identified a RA-susceptibility haplotype in the PADI4 gene, but no RA association in the neighboring PADI genes. Expression of the PADI4 gene was confirmed in hematological cells by northern blot hybridization and in synovial tissue of RA patients by in situ RT-PCR and immunohistochemistry. Moreover, the susceptibility haplotype of PADI4 was related to anticitrullinated filaggrin antibody levels in sera of RA patients. We then found a difference in mRNA stability between nonsusceptibility and susceptibility variants.

Conclusions

Our results imply that the RA-susceptibility haplotype of PADI4 increases production of the citrullinated peptides that act as autoantigens, resulting in an increase of the risk of developing RA.