Molecular Analysis of Epitopic Heterogeneity of the Breast Mucin
The breast mucin is a highly glycosylated large molecular weight glycoprotein on the surface of breast epithelial cells, in breast carcinoma cells, and is a major component of the human milk fat globule membrane. By epitope mapping we have determined that 5 different monoclonal antibodies (Mc1, Mc5, BrEl, BrE2, BrE3) raised against human milk fat globule and selected for breast specificity recognize 4 distinct but overlapping linear amino acid sequences encompassing the most hydrophilic region of the 20 amino acid tandem repeat that makes up a large part of polypeptide core of the breast mucin. Although these MoAbs bind to overlapping polypeptide epitopes they have different tissue and tumor specificities in histopathology, differ quantitatively in the binding to breast carcinoma cell lines by flow cytometry, and they have distinct competition patterns for bind-ng to the native antigen on breast carcinoma cells. Even though BrE2 nd BrE3 have the same polypeptide epitope, they differ in their relative binding to breast carcinoma cell as determined by flow cytometry and binding to fusion proteins containing mimotopes produced by cDNA clone isolated from a breast cDNA library. There is considerable variation in the breast mucin mRNA, but the expression of the different epitopes shows little correlation with mRNA levels. Surface expression of an epitope and detection of the epitope in secreted material is associated with its presence on a larger size molecular specie than the exclusively cytoplasmic expression of an epitope. This detailed analysis of the epitopic heterogeneity of an immunodominant region of the tandem repeat segment of the core polypeptide of the breast mucin, possibly involving altered glycosylation, reveals an epitopic heterogenity that may have functional significance and suggests approaches for preparing new monoclonal antibodies with improved qualities for breast cancer diagnosis and/or therapy.
KeywordsSugar Cellulose Agar Carbohydrate Estrogen
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