Presence of nucleotide substitutions in the ABO promoter in individuals with phenotypes A3 and B3.

Recently, the involvement of mutation and deletion of transcription regulatory elements in the Bm , Am , A3 and B3 phenotypes has been reported. In the present study, we carried out genetic analysis of individuals with A3 and B3 using peptide nucleic acid-clamping PCR to exclude amplification of O alleles. Two single-point mutations, -76G>C and -68G>T, were found in the ABO promoter on the A-allele in three A3 individuals and on the B allele in a B3 individual, respectively. Transient transfection of luciferase reporter plasmids carrying the same mutations into K562 cells revealed decreased luciferase activity in comparison with that carrying the wild-type promoter. These observations suggest that the mutations downregulate the promoter activity, leading to reduction in A- or B-antigen expression on red blood cells in individuals with the A3 and B3 phenotypes.