Differential expression of Cosmc, T-synthase and mucins in Tn-positive colorectal cancers

The Tn neoantigen (GalNAcα1-O-Ser/Thr) is an O-glycan expressed in various types of human cancers. Studies in several Tn-expressing cancer cell lines and pancreatic tumors have identified loss of Cosmc expression caused by either mutations or promoter hypermethylation. In this study, we explored the mechanism(s) for Tn expression in human colorectal cancers (CRC).

Tn-expressing cell populations were isolated from CRC cell lines by Fluorescence-associated cell sorting (FACS). The expression of the Tn and sialylated Tn (STn) antigens, Cosmc, T-synthase, and mucins was characterized in paired specimens with CRC and in CRC cell lines by immunostaining, western blot, and qPCR.

Using well-defined monoclonal antibodies, we confirmed prevalent Tn/STn expression in CRC samples. However, a majority of these tumors had elevated T-synthase activity and expression of both Cosmc and T-synthase proteins. Meanwhile, Tn antigen expression was not caused by mucin overproduction. In addition, we found that Tn-expressing CRC cell lines had either loss-of-function mutations in Cosmc or reversible Tn antigen expression, which was not caused by the deficiency of T-synthase activity.

Our results demonstrate multiple mechanisms for Tn expression in CRCs.