Transcription starting from an alternative promoter leads to the expression of the human ABO histo-blood group antigen

BACKGROUND : Using the 5′‐rapid amplification of cDNA ends technique with the ex vivo culture of AC133 –CD34+ cells, a transcription start site was recently identified approximately 0.7 kb upstream from the transcription start sites previously determined. The transcripts from the alternative starting exon 1a were demonstrated in the cells of both erythroid and epithelial lineages. Because the nucleotide sequence of exon 1a does not contain an ATG codon, we examined whether transcription starting from exon 1a leads to production of a functional glycosyltransferase. STUDY DESIGN AND METHODS : Stable transfection experiments into the human gastric cancer MKN28 cells were performed using the various A transferase expression plasmids. RESULTS : Large amounts of A antigens were demonstrated on the cells transfected with the A transferase expression plasmid containing the entire cDNA from exon 1a or the 5′‐truncated cDNA leading to the production of the N‐truncated protein with deletion of the cytoplasmic tail and a portion of the transmembrane domain. However, negligible amounts of A antigens were observed on the cells transfected with the A transferase expression plasmids containing the 5′‐truncated cDNA leading to the production of the N‐truncated proteins without the cytoplasmic tail and the transmembrane domain. CONCLUSION : This study suggests that a functional A transferase could be produced by the transcription from exon 1a.