Presence of nucleotide substitutions in transcriptional regulatory elements such as the erythroid cell-specific enhancer-like element and the ABO promoter in individuals with phenotypes A3 and B3, respectively.

Background And Objectives: An erythroid cell-specific regulatory element, referred to as the +5.8-kb site, has been identified in the first intron of the human ABO blood group gene. Subsequent studies have revealed involvement of deletion or mutation at the site in phenotypes Am , Bm and ABm . We investigated the molecular mechanisms involved in the A3 and B3 phenotypes.

Materials And Methods: Genomic DNAs were prepared from peripheral blood of seven A3 individuals and twelve B3 or AB3 individuals, and the nucleotide sequences were investigated using PCR and sequencing. Promoter assays were performed with K562 cells.

Results: Two single point-mutations at +5893 or +5909 in the site on the A-allele were found in A3 individuals, while promoter assays revealed decreased activity at the site as a result of each substitution. In two B3 individuals, a single point-mutation at -77 in the ABO promoter on the B-allele was found, and the substitution was demonstrated to reduce the promoter activity.

Conclusion: Nucleotide substitutions in the transcriptional regulatory elements such as the +5.8-kb site and the ABO promoter appear to decrease transcription from the A- and B-alleles, resulting in reduction in A- and B-antigen expression in A3 and B3, respectively.