A cancer-unique glycan: de-N-acetyl polysialic acid (dPSA) linked to cell surface nucleolin depends on re-expression of the fetal polysialyltransferase ST8SIA2 gene

Background: Polysialic acid (PSA) modifies a few cell surface proteins in humans mainly during fetal development and some blood cells in adults. Two genes in humans, ST8SIA2 and ST8SIA4, code for polysialyltransferases that synthesize PSA. The product of ST8SIA2, STX, is highly expressed during fetal development and in many cancers but not in adult normal human cells. The product of ST8SIA4, PST1, is expressed in fetal and some adult tissues and also in many cancers. We identified a derivative of PSA containing de-N-acetyl neuraminic acid residues (dPSA), which is expressed on the cell surface of human cancer cell lines and tumors but not normal cells. Methods: dPSA-modified proteins in several human cancer cell lines and normal blood cells were identified using co-immunoprecipitation with anti-dPSA antibodies and mass spectroscopy. RNAi and CRISPR were used to knockdown and knockout, respectively, the polysialyltransferase genes in two different cell lines to determine effect on production of cell surface dPSA measured by flow cytometry and fluorescence microscopy. Results: We found that dPSA is linked to nucleolin, a nuclear protein reported to be on the cell surface of many cancers but not normal cells. Knocking down expression of ST8SIA2 with RNAi or knocking out each gene individually and in combination using CRISPR showed that cell surface dPSA depended on expression of ST8SIA2 and not ST8SIA4. Conclusions: The presence of dPSA specifically in a broad range of human cancers offers novel possibilities for targeting the dPSA antigen and synthetic pathway for detection, treatment, and prevention of cancer.