Rare human leukocyte antigen genotype in two siblings with type 1 diabetes in a Japanese family clustered with type 1 diabetes

Multiplex families with type 1 diabetes are important for identification of rare variants that cannot be identified in case–control association studies. The very low incidence of type 1 diabetes in the Japanese population, however, makes identification of such families difficult. We identified a Japanese family in which three members developed type 1 diabetes, and studied the genotype of the human leukocyte antigen. All three members with type 1 diabetes had the DRB1*08:02‐DQB1*03:02 haplotype, which is specific to the Asian population and strongly susceptible for type 1 diabetes. In particular, a proband and his sister had the same genotype, DRB1*08:02‐DQB1*03:02/DRB1*08:02‐DQB1*03:02, which is extremely rare even in the Japanese population. Both parents also had DRB1*08:02‐DQB1*03:02, but in combination with different human leukocyte antigen haplotypes. Weakly susceptible DRB1*13:02‐DQB1*06:04 was present in the affected mother, and resistant DRB1*15:01‐DQB1*06:02 in the unaffected father. These data suggest DRB1*08:02‐DQB1*03:02 to be a contributing factor for familial clustering of type 1 diabetes in this family.