Deletion of the RUNX 1 binding site in the erythroid cell‐specific regulatory element of the ABO gene in two individuals with the A m phenotype

Background and Objectives An erythroid cell‐specific regulatory element, referred to as the +5·8‐kb site, had been identified in the first intron of the human ABO blood group gene. Subsequent studies revealed that either a 5·8‐kb deletion including the +5·8‐kb site or disruption of a GATA factor binding motif at the site was present in all B m and AB m individuals examined. We investigated the molecular mechanism of the A m phenotype, which is analogous to the B m phenotype. Materials and Methods Genomic DNA s were prepared from peripheral blood of two A m individuals, and the nucleotide sequences were investigated using PCR and direct sequencing. Electrophoretic mobility shift assay ( EMSA ) and promoter assay with K 562 cells were carried out. Results A novel 23‐bp nucleotide deletion was found at the +5·8‐kb site in both individuals. EMSA s demonstrated binding of the transcription factor RUNX 1 to the nucleotides within the deletion. Promoter assays showed that the deletion reduced the transcriptional activity of the +5·8‐kb site. Conclusion Deletion of the 23‐bp nucleotides including the RUNX 1 binding site decreases transcription of the A allele, resulting in the reduction in A antigen expression in the A m phenotype.