ABO*cisAB allele with unusual phenotype in a Brazilian family.

Background: Among the alleles of the ABO system, cisAB and B(A) are the most intriguing due to their ability to encode a glycosyltransferase that can synthesize both A and B antigens. This dual activity leads to the formation of the AB phenotype, even in the presence of the O allele; resolution is achieved by molecular analyses.

Case presentation and methods: We describe herein a Brazilian family in which the mother (M42.1) of group AB and the father (M42.2) of group O have two children of group AB (M42.3 and M42.4). Serological characterization involved ABO and H phenotyping tests and serial dilution of ABO monoclonal antibodies. Characterization of ABO genotypes and alleles were performed by PCR-RFLP and sequencing.

Results and discussion: In serological tests, red blood cells from M42.1, M42.3, and M42.4 showed an intermediate reactivity pattern between A₁B and A₂B. Molecular analyses revealed the presence of the ABO*O.01.01/O.01.01 genotype in M42.2 and the ABO*cisAB.05/O.01.01 genotype in M42.1, M42.3, and M42.4. The ABO*cisAB.05 allele encodes a glycosyltransferase able to synthesize A and B antigens in quantities sufficient to cause an agglutination reaction higher than that observed in A₂B phenotypes.

Conclusion: The combination of serological and molecular methods used in this study allowed us to determine the serological pattern, identify the ABO alleles, and explain the inheritance of the AB phenotype in this family.