MUC1 isoform specific monoclonal antibody 6E6/2 detects preferential expression of the novel MUC1/Y protein in breast and ovarian cancer

The products of the MUC1 gene are known to be highly expressed in human bre ast cancer cells. The best characterized MUC1 protein is a polymorphic, typ e 1 transmembrane molecule containing a large extracellular domain composed primarily of a variable number of 20 amino acid tandem repeats. We have re cently identified a novel protein product of the MUC1 gene, the MUC1/Y prot ein, that is also a transmembrane protein but is devoid of the tandem repea t array and its immediate flanking sequences. To analyze its expression in tumor cells we generated monoclonal antibodies directed against the MUC1/Y extracellular domain (anti-MUC1/Yex MAbs). Epitope mapping identified the M Ab, 6E6, which recognized the MUC1/Y isoform with exquisite specificity- th e repeat-array-containing MUC1 isoform could not compete out this immunorea ctivity. A 30mer peptide which is unique for MUC1/Y and corresponds to the "join" region generated by the MUC1/Y specific splice, abrogated all 6E6 MA b immunoreactivity towards MUC1/Y. Immunoprecipitation of the MUC1/Y protei n with 6E6 MAbs revealed that, in contrast with the proteolytic cleavage of the tandem-repeat-array-containing MUC1 isoform, MUC1/Y is not cleaved. Fl ow cytometry analyses using the 6E6 MAbs demonstrated that the MUC1/Y isofo rm is expressed on the cell surface of both MCF-7 breast cancer cells and m alignant epithelial cells present in effusions obtained from breast and ova rian cancer patients. Our results unequivocally establish that the MUC1/Y p rotein is expressed on the surface of breast cancer cells and cells of othe r epithelial malignancies, The anti-MUC1/Y MAbs described here can target M UC1/Y expressing tumor cells in vivo and are likely to be important reagent s both for epithelial tumor diagnosis and immunotherapy. (C) 1999 Wiley-Lis s, Inc.