Multiple sclerosis in western Finland: evidence for a founder effect.

We have previously demonstrated that there is a high-risk focus for multiple sclerosis (MS) in the southern Ostrobothnian region of western Finland (population 376121 in 1993). Of the two southern Ostrobothnian health-care districts, Vaasa and Seinäjoki, the incidence and prevalence of MS were especially high in the latter. In recent genetic studies, we identified haplotypes of the myelin basic protein (MBP) gene in a group of MS patients originating from southern Ostrobothnia, suggesting a founder effect. This finding led us to explore the population history of the southern Ostrobothnia and correlate it with MS epidemiology. Southern Ostrobothnia can be divided into three distinct regions with respect to its historical settlement: Vaasa, Seinäjoki-south, and Seinäjoki-north. Vaasa, the coastal region was settled by Swedes, who immigrated during the 13th century. In Vaasa, the prevalence of clinically definite MS (CDMS) in 1993 was 107/10(5) (95% CI 90-124). Seinäjoki-south was populated from the 13th century onwards from southwestern Finland, a region which has been recognised as a high-risk focus of MS. In Seinäjoki-south, the prevalence of CDMS in 1993 was 219/10(5) (95% CI 190-247). Seinäjoki-north was inhabited rather late starting in the 16th century from eastern Finland. In Seinäjoki-north the prevalence of CDMS in 1993 was 136/10(5) (95% CI 108-164). The historical settlement pattern of the southern Ostrobothnia indicates that its population is quite heterogeneous. Seinäjoki-south has a very high prevalence of MS, significantly higher than its two neighbouring regions. The distinctive settlement history of Seinäjoki-south, the historical link with the other southwestern high-risk foci and molecular genetic evidence, suggest that a founder effect plays an important role in the high-risk of MS in western Finland.