Expression of epithelial mucins Muc1, Muc2, and Muc3 in ductal carcinoma in situ of the breast.

Epithelial mucins are glycoproteins secreted by epithelial cells and their carcinomas. At least nine mucin genes have been identified, and their products (MUC1-MUC9) are expressed in various epithelia. MUC1 is a mucin expressed in breast epithelial cells, whereas MUC2 and MUC3 are primarily intestinal mucins. Although MUC1 and MUC2 expression has been documented in invasive ductal carcinoma of the breast, mucin expression in pure ductal carcinoma in situ (DCIS) has not been investigated. Sixty-one of 105 cases of DCIS without coexisting infiltrating carcinoma diagnosed during a 30-month period were selected as having sufficient tissue for study. Paraffin-embedded tissue sections were stained using immunohistochemical techniques with mouse monoclonal anti-MUC1, anti-MUC2, and rabbit-specific polyclonal anti-MUC3 antibodies. Immunoreactive epitopes of MUC1, MUC2, and MUC3 were expressed in DCIS in 61, 19, and 16 of 61 cases, respectively. MUC2 and MUC3 staining intensity in DCIS was markedly less than that observed for MUC1. Luminal and/or cytoplasmic patterns of staining were observed for MUC1. MUC2 and MUC3 showed only cytoplasmic staining. Cytoplasmic-only staining of MUC1 was associated with a higher grade of DCIS. Any MUC2 staining was also associated with a higher grade of DCIS. Coexpression of MUC2 and MUC3 was present in only 6 of 61 cases, and MUC3 staining was unrelated to the grade of DCIS. Cytoplasmic expression of MUC1 and MUC2 appears to be associated with a higher grade of DCIS. MUC3 expression appears to be independent of grade and expression of MUC1 and MUC2. The relationship of mucin expression and grade warrants further study.