HLA-DQA1 and DQB1 allele and genotype contribution to IDDM susceptibility in an ethnically mixed population.
HLA-DRB1, DQA1 and DQB1 alleles have been determined in 42 families with one IDDM proband and 64 healthy controls, by oligotyping (PCR-SSO) using primers and probes from the XI International Histocompatibility Workshop. A positive DRB1*03 and DRB1*04 association with the disease was observed, whereas DRB1*11 and DRB1*07 showed negative association but 19% of patients carried DRB1 alleles different to DRB1*03 or *04. When single alleles were considered, DQA1*03 showed the strongest association with susceptibility to the disease (RR = 8.2, Pc = 0.00001) but this association was outgrown by 2 and 3 allele combinations, with genotype DRB1*04-DQA1*03-DQB1*0302/DRB1*03- DQA1*0501- DQB1*0201 showing the strongest association (RR = 28, Pc = 0.002). Application of the relative predispositional effect (RPE) method to our data, revealed a further susceptibility risk provided by the DRB1*13-DQA1*0102-DQB1*0604 haplotype once DR3 and DR4 haplotypes were removed. When DQA1-DQB1 genotypes were analysed for presence of Arg 52 (DQ alpha) and absence of Asp 57 (DQ beta), genotypes SS/SS were found significantly increased in diabetics. Interestingly, one of the strongest associations with the disease was observed with the DQA1*03-DQB1*0201 combination encoded mainly by genes in trans (RR = 11.7 Pc = 0.00004). These observations and their comparison with DR-DQ haplotypes in more homogeneous ethnic groups support the stronger influence of the DQ molecule rather than the individual DR or DQ alleles in the susceptibility to IDDM. They also emphasize the need for detailed HLA haplotype studies in non-Caucasian and ethnically mixed populations to gain further insight into the nature of genetic and environmental factors contribution to autoimmunity.