A genetic contribution to inflammatory bowel disease in Iceland: a genealogic approach.

BACKGROUND AND AIMS: Both genetic and environmental factors play a role in the development of Crohn's disease (CD) and ulcerative colitis (UC), collectively known as inflammatory bowel disease (IBD). The aim of this study was to estimate the genetic component in IBD in Iceland. METHODS: A population-based sample, representing everyone diagnosed with IBD in Iceland from 1950 to 1996, was studied using a computerized population-wide genealogic database. The relationships among the patients were analyzed by calculating the kinship coefficient and the relative risk. RESULTS: The kinship coefficients for the patients were significantly greater than the mean kinship coefficient for the controls ( P < 10 -6 ). The risk ratio for siblings of IBD, UC, and CD patients was 5.0 ( P < 0.001), 5.9 ( P < 0.001), and 4.1 ( P = 0.033), respectively. The cross-risk ratio for siblings of UC patients developing CD (or vice versa) was 2.6 ( P = 0.015). CONCLUSIONS: The results indicate that the IBD patients are more closely related than the controls, which strongly supports the involvement of a genetic component in the development of IBD in Icelandic patients. We find that the increase in risk for relatives of UC probands to develop UC, or relatives of CD probands to develop CD, is greater than the increase in risk for relatives of UC probands to develop CD, or relatives of CD probands to develop UC.