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 Fr(a). 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 FR:(a) 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 DI20.