Objective To investigate the role of genetic and environmental factors in the development of anticitrullinated protein antibodies (ACPA) and ACPA-positive rheumatoid arthritis (RA) in a twin cohort.

Methods A total of 12 590 twins were analysed for the presence of ACPAs (CCP2 ELISA), HLA-DRB1 shared epitope (SE) gene alleles, and exposure to smoking. Twins with established RA were identified in national public care registers. Antibody reactivities against citrullinated and native forms of α-enolase, vimentin, fibrinogen and type II collagen peptides were tested by ELISA in anti-CCP2-positive subjects and their cotwins. Structural equation models and ORs for the development of ACPA and ACPA-positive RA were computed for smokers and SE carriers.

Results A total of 2.8% (350/12 590) of the twins were ACPA positive, and 1.0% (124/12 590) had ACPA-positive RA. Most of the variability in the ACPA status was accounted for by non-shared environmental or stochastic factors (78%, 95% CI 55% to 100%) rather than shared environmental and genetic factors. Analysis of specific risk factors revealed an association between smoking and SE and the presence of ACPAs. Twins with ACPA-positive RA were more frequently SE positive than twins with ACPAs without RA. Reactivities against multiple citrullinated peptides were present in most twins with ACPA-positive RA but in fewer twins with ACPAs without RA.

Conclusions Environment, lifestyle and stochastic factors may be more important than genetics in determining which individuals develop ACPAs. Genetic factors (particularly SE) may have a relatively larger role in determining which ACPA-positive individuals will ultimately develop arthritis.