Behçet's disease, the Silk Road and HLA-B51: historical and geographical perspectives.

Behçet's disease (BD), also known as the Silk Road disease, is a blinding inflammatory disorder of young adults found predominantly between the Mediterranean basin and the Orient, and is strongly associated with the major histocompatibility complex (MHC) antigen HLA-B51. In this article we review the history of Behçet's disease since its first description by Hippocrates, the development of the trading routes collectively known as the Silk Road and the effect of population movement on the distribution of HLA-B51. The global distribution of this antigen among healthy control populations bears a striking similarity both to the ancient trading routes and the distribution of Behçet's disease, suggesting a genetic risk that migrated in parallel with population movement between the Mediterranean and Asia. However, certain indigenous Amerindian peoples have a high prevalence of HLA-B51 but no reported cases of BD. Furthermore, a clear genealogical relationship exists between eastern, but not central, Siberian populations with the Amerindians. Since a high level of recombination within the MHC is known to have occurred in these eastern populations before their migration into Beringia, we suggest that disruption of genetic loci in linkage disequilibria with HLA-B51 may be one reason for the absence of disease in these high HLA-B51-bearing populations. However, a contributory influence of environmental factors is not excluded by this data, and the wide variation that exists in relative risk of HLA-B51 even within Europe would support other non-genetic risk factors on the Silk Road which may be absent, or non-contributory to disease, in the Americas.