Possible human leukocyte antigen-mediated genetic interaction between type 1 and type 2 diabetes

We assessed the prevalence of families with both type 1 and type 2 diabetes in Finland; and we studied, in patients with type 2 diabetes, the association between a family history of type 1 diabetes, glutamic acid decarboxylase (GAD) antibodies (GADab), and type 1 diabetes-associated human leukocyte antigen (HLA) DQB1-genotypes. Further, in mixed type 1/type 2 diabetes families, we investigated whether sharing an HLA haplotype with a family member with type 1 diabetes influenced the manifestation of type 2 diabetes. Among 695 families ascertained through the presence of more than 1 patient with type 2 diabetes, 100 (14%) also had members with type 1 diabetes. Type 2 diabetic patients from the mixed families had, more often, GADab (18% vs. 8%, P < 0.0001) and DQB1*0302/X genotype (25% vs. 12%, P = 0.005) than patients from families with only type 2 diabetes; but they had a lower frequency of DQB1*02/0302 genotype, compared with adult-onset type 1 patients (4% vs. 27%, P < 0.0001). In the mixed families, the...