Investigating breast cancer O-linked glycosylation changes and their immunotherapeutic potential
Changes that occur in malignant cells can take a variety of forms and include changes in cellular glycosylation. Aberrant glycosylation is considered to be a universal feature of malignant transformation and tumour progression. Mucins are major carriers of altered glycans in most carcinomas, and mainly belong to the MUC family, with MUC1 considered one of its most prestigious affiliates, as it is upregulated and aberrantly glycosylated in the majority of breast and many other adenocarcinomas. The large domain of the extracellular region of MUC1 is made up of tandemly repeated amino acids that form a scaffold for the attachment of the Olinked glycans. The aberrant glycosylation of MUC1 observed in breast cancer results in the expression of simple, unbranched sugars that are often sialylated and this is partly due to the upregulation of the sialyltransferase, ST3Gal-I. These consistent and conserved changes suggest that aberrant glycosylation is advantageous for the tumour, and data generated by this thesis suggest that indeed this is the case. Attention has been focused on MUC1 as a target molecule for the immunotherapy of cancer. To investigate specific MUC1 tumour-associated glycans, recombinant MUC1 glycoproteins carrying specific and defined O-linked glycans were purified from engineered CHO cells. The immune responses to these MUC1 glycoforms were analysed using wild-type and MUC1 transgenic mice. Different tumour-associated glycoforms elicited different immune responses, with MUC1 carrying the disaccharide known as T inducing a cellular response in MUC1 Tg mice, while the MUC1-ST glycoform was incapable of inducing an immune response in MUC1 Tg mice or even wild-type mice. To determine whether the changes in glycosylation, observed in breast carcinomas, confer an advantage to cancer cells, we generated transgenic mice over-expressing ST3Gal-I in the mammary gland and other epithelial tissues, and crossed these with mice that develop spontaneous mammary tumours. The tumours that developed from the ST3Gal-I mice appeared significantly earlier than the controls. Thus overexpression of ST3Gal-I in the mammary gland promotes tumorigenesis in this model, and the MUC1-ST glycoform formed by the action of ST3Gal-I on the T antigen is not immunogenic and may even inhibit an immune response.
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