Downregulation of Glycosyltransferase a Expression By miRNA-331-3p Is Mediated By the Inhibition of the Transcription Factor SP1

More than 100 ABO subgroup-related variations were detected in the coding region of glycosyltransferases, which may be causative for a weak blood group antigen A or B expression. Most variation in expression is explained genetically by mutations that reduce transferase activity. However, a substantial number of weak variants have not yet been explained by current methods. We recently postulated a role of miRNA in the regulation of blood group A antigen expression levels. By using different approaches, including gene array analysis, luciferase reporter assay and overexpression of glycosyltransferase specific miRNAs in primary hematopoietic stem cells (HSCs), we found that miR-331-3p directly targets glycosyltransferase A and B mRNA. Now we have been further embarking on the underlying mechanisms of miRNA and glycosyltransferase interactions and show that the effects of miR-331-3p are mediated by inhibition of transcription factor SP1, which is a major regulator of the ABO gene, thereby resulting in downregulation of blood group A antigen expression in A₂O individuals by up to 80% and in A₁O individuals by up to 50%. Using microRNA target prediction tools we also identified Sp1 as a potential target gene for miR-331-3p. Western blot analyses of glycosyltransferase protein expression showed that overexpression of miR-331 led to a decreased glycosyltransferase and SP1 protein expression. Further approaches with the SP1 inhibitor, mithramycin A, revealed similar results. Analysis of Aw04/O_1v genotypes revealed elevated miRNA levels, thus confirming microRNAs as regulators of blood group glycosyltransferase expression. Our findings extend our understanding of blood group regulation in carriers of weak blood group variants. Involvement of miRNAs in the downregulation of ABO blood group antigen expression may also provide an explanation for observed changes of ABO antigen expression in abnormal processes such as tumorigenesis, pregnancy and aging. Furthermore, this pathway may play a role in the regulation of other blood group variants (for instance Rhesus, KELL, amongst others).