The study was conducted by a team of researchers led Dr. Bjarni Thjodleifsson at Landspitali University Hospital, Reykjavik.
As part of the study researchers assessed the occurrence of IBD and AS among relatives and the risk of inheriting either and both disorders in over 790,000 Icelanders.
The study found significant clustering of each condition within families, extending over 6 generations.
Researchers further calculated the risk ratio of each disorder for a relative of an affected person, including spouses, as compared with the risk in the population as a whole, for 10,000 sets of matched control subjects to strengthen the findings and performed cross-risk ratios of AS for relatives of patients with IBD and of IBD for relatives of patients with AS.
Researchers found that first-, second-, and third-degree relatives of patients with AS had risk ratios of 94, 25, and 3.5, respectively, indicating an increased risk of developing AS, while first-, second-, and third-degree relatives of patients with IBD had risk ratios for IBD of 4.4, 2.2, and 1.4, respectively.
Similar to the findings from the risk analyses, even stronger relationships among AS patients were evident throughout the first three generations. Furthermore, patients with IBD and patients with AS were more closely related to each other, even after exclusion of second-degree relatives - grandparents, grandchildren, and first cousins - than were population controls.
"Our results provide strong evidence that a molecular-genetic approach should be utilized in patients with these diseases," Dr. Thjodleifsson said.
The findings of the study were published in the August issue of Arthritis and Rheumatism.