Concordance of inflammatory bowel disease among Danish twins - Results of a nationwide study

Background: Previous studies have shown an increased risk of inflammatory b owel disease (IBD) among relatives of patients with Crohn disease and ulcer ative colitis. In the present study the probandwise concordance rates for u lcerative colitis and Crohn disease among mono- and dizygotic twins were es timated. Further we aimed to evaluate whether smoking habits might influenc e the concordance, and to look for clinical characteristics of concordant v ersus discordant twin pairs, Methods: Among the 38,507 identified twins bor n in Denmark from 1953 to 1982, a questionnaire was sent to the 34,076 who previously had accepted to participate in studies. For twins reporting IBD, the diagnosis was verified by applying standard criteria to records reques ted from hospitals or practitioners. Results: Among the 29,421 (86.3%) twin s answering the questionnaire, 103 pairs had at least one twin who suffered from IBD. In the Crohn disease group five of 10 monozygotic pairs, but non e of 27 dizygotic pairs were concordant. In the ulcerative colitis group th ree of 21 monozygotic, and two of 44 dizygotic pairs were concordant. The p robandwise concordance rate among monozygotic pairs was 58.3% for Crohn dis ease and 18.2% for ulcerative colitis; among the dizygotic pairs the rates were 0 and 4.5%, respectively. The frequency of smokers was higher among tw ins with Crohn disease and lower among twins with ulcerative colitis compar ed to the frequency in the twin register. Furthermore, smoking habits were found to be of significance for discordance for disease. Regarding the clin ical characteristics no homogenous pattern was observed within the concorda nt pairs and the differences between concordant and discordant pairs were n ot significant. Conclusion: The observation of a significantly higher conco rdance rate among monozygotic than among dizygotic twin pairs strongly poin ts to a genetic influence on occurrence of IBD, which seems to be more pron ounced with regard to Crohn disease than to ulcerative colitis. Differences in smoking habits among the members of the discordant twin pairs may influ ence the discordance.