ALBERT

Description: Abstract 3365 Background: A recent study suggested an increased rate of inhibitor development among Hispanic compared to White hemophilia A (HA) patients; however, a possible mechanism was not proposed or explored. In light of findings implicating a role for race-specific distributions of factor VIII (F8) and class-II human-leukocyte antigen (HLA-II) haplotypes on the disparate risk of inhibitor development in the Black HA population, we sought to determine the distributions of F8 and HLA-II haplotypes among Hispanic Americans and compare them to that of Black and White Americans. Methods: Using archived genomic DNA samples from the 194 unrelated founders of the San Antonio Family Heart Study (SAFHS), a collection of 1431 people from 41 large Mexican American families, we re-sequenced all exons of F8 to genotype the known nonsynonymous-single nucleotide polymorphisms (ns-SNPs) and identify any novel ns-SNPs. We then performed high-resolution HLA-II genotyping to identify each founders' pair of HLA-DRB1 alleles to compare them against those found in other ethnic groups. Results: Among the 291 potentially distinct Mexican American X-chromosomes evaluated, we identified the H2 F8 haplotype, defined by D1241E, in 25.0% of the subjects, which is in between that observed in the White (7.4%) and Black (37.4%) populations. We also found H3 F8, defined by D1241E and M2238V, in two subjects, who represent the first non-Black individuals reported to carry this haplotype. Furthermore, we discovered a previously unreported ns-SNP (H1919N), whose minor allele was found in only one male and defines a ninth wild-type F8 haplotype (H9). Regarding HLA-II alleles, the distribution among Mexican Americans in the present study was quite different from those found in Black and White individuals in the National Marrow Donor Program registry (see Figure). Three of the 4 most common HLA-II alleles in the Mexican Americans (DRB1*0802, DRB1*0407 and DRB1*1406) were seen in 〈 1% of both Blacks and Whites. Discussion: This is the first study to report the haplotypic prevalence of F8 and HLA-II alleles in Mexican Americans, the largest Hispanic ethnic group in the United States. By informing specific wild-type factor VIII (FVIII) peptides for use in HLA-II binding and T-cell stimulation assays, these results may help to identify high risk combinations of FVIII therapeutics and individual HLA-II repertoires that contribute to the higher rate of inhibitor development observed in Mexican versus Caucasian American HA patients. Disclosures: Viel: Histonis, Incorporated: Employment. Howard:Haplomics, Inc.: Equity Ownership.