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 A 3 and B 3 , 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 A m , B m and AB m . We investigated the molecular mechanisms involved in the A 3 and B 3 phenotypes. Materials and methods Genomic DNA s were prepared from peripheral blood of seven A 3 individuals and twelve B 3 or AB 3 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 A 3 individuals, while promoter assays revealed decreased activity at the site as a result of each substitution. In two B 3 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 A 3 and B 3 , respectively.