Characterization of the Histo-Blood Group O2 Gene and Its Protein Product

Background and Objectives: This study aimed to show the full sequence and function of the O2 allele, and investigate whether it accounts for the incompatible expression of A antigens in gastric carcinomas of blood group O persons. Materials and Methods: By PCR, we determined the ABO genotype of group O subjects (76 gastric carcinoma patients and 165 blood donors). Two expression constructs, encoding either the putative soluble or full-length O2 protein, were used to transfect Sf9 cells. The expression and the activity of the O2 protein were analysed by immunohistochemistry and enzymatic assays, respectively. Results: No significant difference was detectable between the O2 allele frequency in gastric carcinoma patients (3.9%) and blood donors (4.2%). Sequencing analysis of the O2 allele revealed an intact reading frame identical to that of A transferase except for four nucleotide substitutions. O2-transfected Sf9 cells and gastric carcinomas genotyped as O1O2 both expressed a protein recognized by anti-A/B transferase monoclonal antibodies. In enzymatic assays, the O2 protein failed to show measurable A transferase activity. Conclusion: The O2 allele has an intact reading frame encoding a protein immunologically related to A/B transferases and enzymatically inactive. Further, our data gave no indication that the O2 allele is related to the phenomenon of incompatible A antigen expression in gastric cancer.