The inheritance of rheumatoid arthritis in Iceland

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Abstract
Objective Rheumatoid arthritis (RA) is the most common form of inflammatory arthritis. Although there is a large body of evidence suggesting that RA is immune mediated, the etiology remains unresolved. Twin studies have shown disease concordance rates of ∼15% in monozygotic twins and 4% in dizygotic twins, while the estimated risk ratio for siblings of RA patients ranges from 5 to 8. Our goal was to use genealogic data from Iceland to further investigate the genetic component of RA. Methods Data were obtained from a population‐based, computerized genealogy database that was developed to examine multigenerational relationships among individuals in the relatively homogeneous population of Iceland. Using an algorithm, the minimum founder test, we calculated the least number of founders required to account for a list of RA patients, and compared it with 1,000 sets of same‐sized matched control groups. In addition, we estimated the kinship coefficient and risk ratios for relatives of the RA patients. Results Several familial clustering tests demonstrated that the RA patients were more related to each other than were the average control set of Icelanders. A significantly fewer number of founders was necessary to account for our patient list than for the random sets of matched controls (P < 0.001), and the average pairwise identity‐by‐descent sharing was greater among the patients than among the control sets (P < 0.001). In addition, there was an increased risk of RA in first‐ and second‐degree relatives of the patients; e.g., for siblings, the risk ratio was 4.38 (95% confidence interval 3.26–5.67), and for uncles/aunts, the risk ratio was 1.95 (95% confidence interval 1.52–2.43). Conclusion The familial component of RA is shown to extend beyond the nuclear family, thus providing stronger evidence for a significant genetic component to RA.