T-cell-defined DR4 subtypes as markers for type 1 diabetes

In most populations studied, HLA-DR4, a DRB1 (formerly DR beta I) allele, is increased in frequency among patients with insulin-dependent diabetes mellitus (IDDM). Using T-cells, one can distinguish five subtypes of DR4 (Dw4, Dw10, Dw13, Dw14, and Dw15). Two of these (Dw4 and Dw10) are IDDM-associated in the populations studied here. Therefore, Dw4 and Dw10 could be causative or merely markers for a linked diabetes allele. If they are causative, one might expect them to share some unique structural element or at least to associate consistently with IDDM in different populations. Published sequence data show no structural element unique to Dw4 and Dw10; moreover, the associations of these DR4-Dw subtypes with diabetes vary considerably in different populations. Thus the DRB1 locus probably cannot account for the DR4 association in IDDM. The strong linkage disequilibrium between IDDM and Dw4 and Dw10 suggests that the diabetes susceptibility locus should be in the vicinity of the DR region or the DQ region of the HLA complex. Alternative hypotheses are discussed, relating DR- and DQ-region alleles to IDDM. We further postulate that the evolutionary event that produced the Dw10 allele occurred on a Dw4 haplotype that happened to carry a diabetes susceptibility allele at another locus.