A novel B(weak) hybrid allele lacks three enhancer repeats but generates normal ABO transcript levels

Weak expression of A/B histo-blood group antigens is often explained by single nucleotide substitutions at the ABO locus. However, hybrid alleles containing segments from different ABO alleles can result in unexpected phenotypes and may complicate genotype analysis. We investigated the basis of weak B phenotype in a referred sample.

A healthy young woman was serologically phenotyped as AB(weak) and RBCs were characterized by flow cytometry. All seven ABO exons, five introns plus the 5'-region including the CCAAT-binding factor/Nuclear Factor Y (CBF/NF-Y) binding enhancer were sequenced. ABO transcript levels were measured in fresh peripheral blood samples. Expression of B antigen was semiquantified following transfection of HeLa cells.

A new B(weak) allele with 53G>T resulted in a characteristic pattern of moderately weakened B antigen expression on RBCs. Its sequence revealed a novel hybrid between O(2) [O03] and B [B101] alleles with a crossingover region in intron 4 as defined by allele-specific polymorphisms. B transcript levels were similar to normal controls despite the O(2) -related single CBF/NF-Y-binding 43-bp motif in the enhancer region. Expression of the glycosyltransferase including the O(2) -specific Arg18Leu substitution resulted in a slight decrease in B-antigen-positive cells.

We describe here the first hybrid between an O(2) and a B allele and characterized the associated decrease in B antigen expression. Although it lacks three enhancer repeat units compared to common B alleles, the resulting transcript level was unaltered. This study challenges previous suggestions that the number of 43-bp motifs in the ABO enhancer determines transcription rates in erythroid cells.