An amino acid substitution in the putative second extracellular loop of RBC band 3 accounts for the Froese blood group polymorphism

BACKGROUND: The low incidence RBC antigen Fr-a has been excluded from 17 of the 25 established blood group systems. Previous genetic analysis assigned the gene controlling Fr-a expression to the same chromosomal region as the solute carrier family 4, anion exchanger member 1 gene (SLC4A1). Because SLC4A1 encodes RBC band 3 and controls the expression of Diego blood group system antigens, the possible relationship of Fr-a to the Diego blood group system was investigated by molecular analysis of SLC4A1. STUDY DESIGN AND METHODS: Blood samples were obtained from the members of two unrelated Mennonite kindreds segregating for Fra. DNA was extracted, amplified by PCR using intronic primer sets flanking exons 11-20 of SLC4A1, and screened by single-strand conformation polymorphism (SSCP) analysis. Those exons displaying SSCPs were subjected to DNA sequence analysis. RESULTS: An exon 13 SSCP mobility shift was observed in the DNA from all Fr(a+) persons that was not seen in the DNA from Fr(a-) family members or control subjects. Linkage between the exon 13 SSCP and Fa was established, with peak lods = 3.62 at theta = 0.00 for combined paternal and maternal meioses. DNA sequencing revealed a GAG --> AAG mutation that underlies a Glu480Lys substitution in RBC band 3. CONCLUSIONS: A point mutation in exon 13 of SLC4A1 accounting for a Glu480Lys substitution in band 3 controls Fr-a expression. On the basis of these our results, the International Society of Blood Transfusion Working Party on Terminology for Red Cell Surface Antigens has assigned Fr-a to the Diego blood group system, with the designation D120.