Relative importance of genetic effects in rheumatoid arthritis: historical cohort study of Danish nationwide twin population Commentary: Do genes or environment influence development of rheumatoid arthritis?

Abstract Objective: To determine the relative importance of environmental and genetic effects in the development of rheumatoid arthritis. Design: Historical cohort study with record linkage between a twin registry and the Danish discharge registry as well as the Danish national registry of deaths used to estimate completeness. Setting: Two population based nationwide twin birth cohorts. Participants: 37 338 twins were sent a questionnaire about rheumatic diseases. Self reported rheumatoid arthritis was verified by clinical examination and from medical records. Main outcome measures: The probandwise concordance rate of rheumatoid arthritis in monozygotic and dizygotic twins. Results: The response rate was 84.7%. Rheumatoid arthritis was verified in 13 monozygotic and 36 dizygotic twins. There were no concordant monozygotic twin pairs and two concordant dizygotic twin pairs. Based on capture-recapture methods the probability of ascertainment was 78.3%. The probandwise concordance rate was 0 (95% confidence interval 0 to 24.7) in monozygotic twins and 8.8 (1.9 to 23.7) in dizygotic twins. Conclusion: Genes are of minor importance in the development of rheumatoid arthritis. What is already known on this topic Rheumatoid arthritis is a multifactorial disease determined by both genetic and environmental factors Previous twin studies have shown a higher concordance for rheumatoid arthritis in monozygotic than in dizygotic twins, but the results have been biased in favour of genetic effects What this paper adds As concordance for rheumatoid arthritis in this study was no more common in monozygotic twins than in dizygotic twins environmental effects may be more important than genetic effects in the development of rheumatoid arthritis