Different HLA haplotypes in Mexican Americans with IDDM

The study of HLA histocompatibility antigens and insulin-dependent diabetes mellitus (IDDM) in non-White populations may provide a unique opportunity to more accurately define the diabetes susceptibility gene(s) located within the HLA region. To determine whether HLA haplotypes differ between ethnic groups, we compared 105 HLA haplotypes from 55 Mexican-American IDDM patients with 272 haplotypes from 136 IDDM patients of non-Hispanic White descent. The accurate determination of genotypes and haplotypes requires the study of family units. Therefore, all diabetic patients in this study were from studies of families having one or more siblings with IDDM. In the Mexican-American group, HLA-DR3 and -DR4 were the most common HLA-DR alleles and were present in comparable frequencies in the non-Hispanic White group (HLA-DR3, 27% of Mexican-American and 29% of non-Hispanic White haplotypes; DR4, 46% of Mexican-American and 43% of non-Hispanic White haplotypes). However, the HLA-B/DR-containing haplotypes and haplotype frequencies differed between the two groups. Several common haplotypes (B8/DR3, B15/DR4) in the non-Hispanic White group occurred less frequently in the Mexican-American group. In contrast, uncommon haplotypes in the non-Hispanic White group comprised nearly 50% of the DR4-containing haplotypes (B35/DR4, B40/DR4, B44/DR4) in the Mexican-American group. Although both DR3- and DR4-haplotype frequencies differed significantly between the two groups, the relative frequency of DR3- but not DR4-containing haplotypes was similar in both ethnic groups. This adds to the evidence suggesting that different susceptibilities are provided by the haplotypes carrying the DR3 and DR4 alleles.