Abstract
Background: Accurate ABO typing is essential for transfusion safety, yet weak A/B subgroups continue to present diagnostic challenges. Although most weakened ABO antigen expressions are linked to missense mutations or small indels within coding regions, the contribution of non-coding regulatory variants -especially large structural alterations-remains insufficiently characterized. Standard genotyping methods, including Sanger sequencing and targeted next-generation sequencing, often fail to detect large upstream deletions, leaving some serological-genotypic discrepancies unresolved in donors and patients. Such gaps hinder accurate transfusion risk assessment. To clarify these mechanisms, this study performed detailed ABO blood typing in a donor with an A subgroup and his family and explored the molecular basis for weakened A antigen expression. Materials and methods: ABO phenotypes of the proband and his parents, spouse, two daughters, and son were determined using serological testing. Full-length ABO gene sequencing was then performed using third-generation single-molecule sequencing. Results: The proband, his father, and his son displayed the Aweak phenotype. All three carried a heterozygous 12,309-bp deletion located upstream of the ABO gene promoter, along with a normal ABOA1.02 allele. The remaining family members carried genotypes ABOO.01.01/ABOO.01.01 (mother), ABOB.01/ABOO.01.02 (wife), ABOB.01/ABOO.01.01 (daughter 1), and ABOO.01.01/ABOO.01.02 (daughter 2). No variants were detected in coding regions, introns, or splice sites. Pedigree analysis confirmed that the proband inherited the ABOA1.02 allele with the 12.3-kb deletion from his father and passed it to his son. Discussion: This study identifies, for the first time, a large 12.3-kb upstream deletion of the ABO promoter responsible for weakened erythrocyte A antigen expression. The finding expands current understanding of regulatory mechanisms underlying weak ABO phenotypes and highlights the value of advanced sequencing for transfusion safety.