Structural basis for recognition of breast and colon cancer epitopes Tn antigen and Forssman disaccharide by Helix pomatia lectin

Helix pomatia agglutinin (HPA) is a lectin that has been used extensively in histopathology, since its binding to tissue sections from breast and colon cancers is correlated with the worst prognosis for the patients. The lectin recognizes alpha-D-N-acetylgalactosamine (alpha GalNAc) containing epitopes which are only present in cancer cell lines having a high likelihood to undergo metastasis, such as the HT29 cancer colon cell line. Several breast cancer cell lines have also been shown to be labeled, although IGROV1, an ovarian cancer cell line, is not. Inhibition studies, using GalNAc monosaccharides, are reported here, showing that the labeling is dependent upon the presence of carbohydrate epitopes. The crystal structures of the lectin complexed with two GaINAc containing epitopes associated with cancer, the Tn (alpha GaINAc-Ser) and Forssman (alpha GalNAc1-3GaINAc) antigens, show the lectin's specificity for GaINAc is due to a particular network of hydrogen bonds. A histidine residue makes hydrophobic contact with the aglycon, rationalizing the preference for GaINAc bearing an additional sugar or amino acid in the a position. These structures provide the molecular basis for the use of HPA in metastasis research.