Cloning and expression of a human gene encoding an N-acetylgalactosamine-alpha2,6-sialyltransferase (ST6GalNAc I): a candidate for synthesis of cancer-associated sialyl-Tn antigens.

The sialyl-Tn (sTn) antigen is a well known cancer-associated antigen, the expression of which is related to the prognosis of cancer patients. We aimed to isolate a human gene encoding an N -acetylgalactosamine alpha2,6-sialyltransferase which synthesizes sTn antigen, and to characterize the enzyme. Degenerate primers encoding sialyl motifs were used for the polymerase chain reaction to amplify complementary DNAs prepared from RNAs of human pyloric mucosae with intestinal metaplasia, which abundantly expressed sTn antigen, followed by screening of full-length cDNAs using the amplified DNA fragment as a probe. We isolated two human cDNA clones, long-form (2.46 kb) and short-form (2.23 kb) cDNAs. The former encodes an active enzyme with a predicted 600 amino acid sequence. The latter, a splice-variant of the long-form, encodes an inactive enzyme. HCT15 human colorectal cancer cells stably expressing the long-form cDNA expressed sTn epitopes on O -glycans. The long form cDNA was considered to encode a human homologue of chick ST6GalNAc I for the following reasons: (1) the putative amino acid sequence showed greater homology to that of chick ST6GalNAc I (55%) compared to other sialyltransferases, (2) it encodes the extraordinarily long stem region that is a typical feature of chick ST6GalNAc I, and (3) the substrate specificity was very similar to that of chick ST6GalNAc I. In situ hybridization demonstrated that the localization of transcripts correlated well with that of sTn antigen in gastric cancer cells and Goblet cells in intestinal metaplastic glands. Thus, we determined that the long-form cDNA of the human ST6GalNAc I gene encodes the probable candidate for the human sTn synthase(s).