A twin study in Behçet's syndrome.

OBJECTIVES: Case reports on monozygotic (MZ) twins with Behçet's syndrome (BS) have been few and we are not aware of formal twin studies. We sought the frequency of MZ and dizygotic (DZ) twin births in BS and compared it to a healthy population sample from the same geography. We also looked for the concordance rate among the MZ and DZ twins. METHODS: 1705 (1039M/666F) patients attending a dedicated BS outpatient clinic and 7761 (3848M/3913F) medical school students were asked about having a MZ or DZ twin sibling. MZ and DZ twins thus identified among both patients and controls were individually seen at the clinic. In addition, HLA, DNA microsatellite markers and blood groups were typed to further confirm twin- ship. All twins were contacted 8 years later for new emergence of disease. RESULTS: There were 14 (0.82%) patients with BS and 120 (1.55%) controls who had a twin sibling (p=0.022). Of these, 8 (0.47%) patients with BS and 92 (1.19%) controls had a DZ twin sibling (p=0.009). MZ twin frequency was similar between BS patients (6/1705; 0.35%) and control population (28/7761; 0.36%). The pairwise concordance rate for BS was 2/6 (95% CI: -0.21-0.88) for MZ and 1/8 (95% CI: -0.17-0.42) for DZ twins (p=0.538). Genetic effects accounted for 41% of the phenotypic variance for BS among twins. After 8 years of follow-up, 4 of 6 MZ and 6 of 7 DZ twin pairs were still discordant. CONCLUSIONS: The frequency of MZ twin births in BS is not different than that in the general population while the DZ twins were seen less frequently among the BS patients. The concordances for BS were higher in MZ compared with DZ twins, suggesting genetic predisposition. On the other hand, the persistence of discordance after 8 years of follow up among the remaining MZ twins demands further research to understand non- genetic factors in causation of BS.