by Ding-Ping Chen, Ching-Ping Tseng, Wei-Ting Wang, Chien-Feng Sun

The ABO blood type B₃ is the most common B subtype in the Chinese population with a frequency of 1/900. Although IVS3+5G>A (rs55852701) mutation of B gene has been shown to associate with the development of B₃ blood type, genetic and mechanistic evaluation for the unique mixed-field agglutination phenotype has not yet been completely addressed.

In this study, we analyzed 16 cases of confirmed B₃ individuals and found that IVS3+5G>A attributes to all cases of B₃. RT-PCR analyses revealed the presence of at least 7 types of aberrant B₃ splicing transcripts with most of the transcripts causing early termination and producing non-functional protein during translation. The splicing transcript without exon 3 that was predicted to generate functional B₃ glycosyltransferase lacking 19 amino acids at the N-terminal segment constituted only 0.9% of the splicing transcripts. Expression of the B₃ cDNA with exon 3 deletion in the K562 erythroleukemia cells revealed that the B₃ glycosyltransferase had only 40% of B₁ activity in converting H antigen to B antigen. Notably, the typical mixed-field agglutination of B₃-RBCs can be mimicked by adding anti-B antibody to the K562-B₃ cells.

This study thereby demonstrates that both aberrant splicing of B transcripts and the reduced B₃ glycosyltransferase activity contribute to weak B expression and the mixed-field agglutination of B₃, adding to the complexity for the regulatory mechanisms of ABO gene expression.