Molecular mechanisms that lead to reduced expression of duffy antigens

In the Duffy blood group system, the null phenotype Fy(a-b-) has been classically associated with a mutated GATA box, while the Fy(x) phenotype weak Fy(b) is associated with Arg89Cys and Ala100Thr mutations. This report assesses the prevalence of the Duffy GATA box and the Fy(x)-associated mutations in white and African American (black) donors and investigates the molecular mechanism underlying the Fy(x) phenotype.

PCR RFLP Duffy genotyping was performed on blood samples from blacks and whites. Duffy antigen expression (Fy(a), Fy(b), Fy6, Fy3) on RBCs was measured by flow cytometry. By site-directed mutagenesis, the relevance of each Fy(x)-associated mutation to Duffy (mRNA, antigen, and protein) expression was analyzed in transfectants by Northern blotting, flow cytometry, and immunoblotting.

The mutated GATA box occurred at a high allele frequency (0.8) in blacks and was rare among whites. Conversely, the Fy(x)-associated mutations were absent in blacks, but present in 3.5 percent of whites. By flow cytometry, Duffy antigens (Fy(a) or Fy(b), Fy6 and Fy3) showed a dosage effect in RBC samples that were transcriptionally silenced by the GATA box mutation in one allele. By contrast, the reduced (10%) Duffy protein in Fy(x) RBCs was shown by heterologous expression analysis not to be due to reduced RNA levels, but to protein instability caused by Arg89Cys.

Reduced Duffy expression can result from mutations affecting transcription (mutated GATA box in one allele) or instability of the translated protein (Arg89Cys). The frequencies of these mutations vary among populations.