Strong association between DQA1/DQB1 genotype and early-onset IDDM in Chinese: the association is with alleles rather than specific residues

We report on the role of HLA-DQA1 and DQB1 alleles in determining susceptibility to insulin-dependent diabetes mellitus (IDDM) in Hong Kong Chinese and investigate whether these alleles affect the age of onset of the disease. We studied 76 unrelated Chinese patients and 250 controls. There was no apparent predisposing effect of non-aspartic acid residues at position 57 of the DQβ chain (Asp^57–) but there was an excess of homozygous genotypes containing arginine at position 52 of the DQα chain (Arg⁵²⁺). This excess was mainly attributable to the genotype DQA1*0301/ DQA1*05011 in early-onset disease. There was a significant excess of heterodimers of DQα and DQβ carrying Arg⁵²⁺ and Asp^57– in both early-onset and late-onset disease, but the excess in early-onset disease was mainly attributable to a single heterodimer formed by DQA1*05011 and DQB1*0201. Of three DQA1/ DQB1 genotypes containing a double dose of Arg⁵²⁺ and Asp^57–, only one had a strong association with both early-onset and late-onset disease. We show that early-onset IDDM and late-onset IDDM in Chinese may be separated on the basis of their associated DQA1 and DQB1 genotypes and we conclude that previously reported associations of IDDM with Arg⁵²⁺ and Asp^57– residues in Chinese are secondary to specific combinations of DQA1 and DQB1 alleles. We also show that DRB1 molecules play a distinct role in determining susceptibility to early-onset IDDM but the greatest effect is exerted by specific DR/DQ genotypic combinations.