[A focus of multiple sclerosis in Bretagne: HLA markers and evaluation of consanguinity].

In a previous study, we established the overall prevalence of multiple sclerosis at 25 per 100,000 inhabitants in the French province of Brittany and found that the geographical distribution was uneven with four circumscribed high prevalence areas with more than 45 per 100,000. We conducted the present study to try to ascertain whether the existence of such clusters of MS could be explained by genetic factors, using two ways: the major histocompatibility markers and the frequency of intermarriage. Among the four areas of high prevalence, we examined the one with the highest prevalence, exempt from migratory movements over the last 100 years. We compared this studied population to a sample of 1005 healthy unrelated individuals coming from all over Brittany, designated as general Brittany population. A large sampling (about 25%) of the population accepted to take part in the study. Regarding HLA markers, we observed in the high prevalence area an increases percentage of B7, B8, B12, DR4 and a decreased percentage of B5, compared with the Brittany control population. The consanguinity coefficient in the population of high MS prevalence area was steadily much higher than the one in neighbouring areas with a low MS prevalence during a long period from 1896 to 1945. Under the reserve of non-randomized sampling of individuals for HLA marker study, our findings yielded a double argument in favor of genetic factors influencing the presence of clusters of MS cases: the particularities of HLA phenotype pattern and the increased consanguinity coefficient. The particularities of HLA polymorphism observed in the high prevalence area are likely to be the consequence of the intermarriage habits of this population.