Characterization of the histo-blood group O(2) gene and its protein product

BACKGROUND AND OBJECTIVES: This study aimed to show the full sequence and function of the O(2) 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 O(2) protein, were used to transfect Sf9 cells. The expression and the activity of the O(2) protein were analysed by immunohistochemistry and enzymatic assays, respectively. RESULTS: No significant difference was detectable between the O(2) allele frequency in gastric carcinoma patients (3.9%) and blood donors (4.2%). Sequencing analysis of the O(2) allele revealed an intact reading frame identical to that of A transferase except for four nucleotide substitutions. O(2)-transfected Sf9 cells and gastric carcinomas genotyped as O(1)O(2) both expressed a protein recognized by anti-A/B transferase monoclonal antibodies. In enzymatic assays, the O(2) protein failed to show measurable A transferase activity. CONCLUSION: The O(2) 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 O(2) allele is related to the phenomenon of incompatible A antigen expression in gastric cancer.