A six-fold gradient in the incidence of type 1 diabetes at the eastern border of Finland

Objective. Type I diabetes results from gene-environment interactions in subjects with genetic susceptibility to the disease. We assessed the contribution of environmental and genetic factors to type 1 diabetes by comparing the incidence in two neighboring populations living in conspicuously different socioeconomic circumstances. Research design and methods. We compared the incidence over a 10-year period (1990-99) in children younger than 15 years of age living in the Karelian Republic of Russia and in Finland. The frequency of susceptible and protective human leukocyte antigen (HLA)-DQ alleles was analyzed in 400 non-diabetic schoolchildren from Russian Karelia and 1000 Finnish subjects. Results. The average annual age-adjusted incidence of type I diabetes was lower in Russian Karelia than in Finland: 7.4 per 100 000 (95% confidence interval 3.5 - 11.3) versus 41.4 per 100 000 (37.3 - 45.5), while there were no differences in the frequency of the HLA DQ genotypes predisposing to type I diabetes in the background populations. The incidence rate did not differ significantly between different ethnic groups in Russian Karelia (Finns/Karelians, Russians, others). Conclusions. There is a close to six-fold gradient in the incidence of type 1 diabetes between Russian Karelia and Finland, although the predisposing HLA DQ genotypes are equally frequent in the two populations. This suggests that environmental factors contribute to this steep difference in the incidence rate between these adjacent regions.