First example of an FY*01 allele associated with weakened expression of Fy^a on red blood cells

Duffy antigens are important in immunohematology. The reference allele for the Duffy gene (FY) is FY*02, which encodes Fy^b. An A>G single nucleotide polymorphism (SNP) at coding nucleotide (c.) 125 in exon 2 defines the FY*01 allele, which encodes the antithetical Fy^a. A C>T SNP at c.265 in the FY*02 allele is associated with weakening of Fy^b expression on red blood cells (RBCs) (called Fy^X). Until recently, this latter change had not been described on a FY*01 background allele. Phenotype-matched units were desired for a multi-transfused Vietnamese fetus with α-thalassemia. Genotyping of the fetus using a microarray assay that interrogates three SNPs (c.1-67, c.125, and c.265) in FY yielded indeterminate results for the predicted Duffy phenotype. Genomic sequencing of FY exon 2 showed that the fetal sample had one wild-type FY*01 allele and one new FY*01 allele with the c.265C>T SNP, which until recently had only been found on the FY*02 allele. Genotyping performed on samples from the proband’s parents indicated that the father had the same FY genotype as the fetus. Flow cytometry, which has been previously demonstrated as a useful method to study antigen strength on cells, was used to determine if this new FY*01 allele was associated with reduced Fy^a expression on the father’s RBCs. Median fluorescence intensity of the father’s RBCs (after incubation with anti-Fy^a and fluorescein-labeled anti-IgG) was similar to known FY*01 heterozygotes and significantly weaker than known FY*01 homozygotes. In conclusion, the fetus and father both had one normal FY*01 allele and one new FY*01 allele carrying c.265C>T. This new FY*01 allele, named FY*01W.01, is associated with weakened expression of Fy^a on RBCs. Immunohematology2015;31:103–107.