MUC1 is a large transmembrane glycoprotein, which is overexpressed in the majority of carcinomas. The high expression of MUC1 is associated with aggressive tumors, with the MUC1 antigen used as a marker to monitor disease progression in breast cancer patients. Although the MUC1 tumor marker is both sensitive and specific for predicting a relapse in breast cancer, it is not commonly used during the follow-up of breast cancer patients. The aim of this study was to evaluate whether the differential patterns of MUC1 expression in different histological types of breast carcinoma could be used to distinguish tumors from benign lesions, and determine its prognostic relevance with other biological parameters.

22 normal breast, 7 intraductal hyper-plasia (IDH) and 307 malignant lesions were selected and immunostained with MUC1. The patterns of reaction were classified as intraluminal border (ILB), cytoplasmic, intercellular membrane (ICM), intranuclear or mixed staining.

All the normal breast samples showed weak cytoplasmic staining in the ducts and lobules. All the IDH samples showed moderate cytoplasmic and ILB staining. Of the 307 malignant lesions, only 2 (0.8%) showed negative staining; MUC1 positivity was observed in 4 (1.3%), with only ILB staining; 8 (2.6%) with weak cytoplasmic staining, 16 (5.2%) with weak cytoplasmic and intranuclear staining, 168 (54.7%) with moderate cytoplasmic and ILB staining, and 109 (35.5%) with strong cytoplasmic and ICM staining. MUC1 positivity with a moderate to strong staining intensity was observed in 90.6% of the infiltrating ductal carcinomas (221/244), 96.5% of the intraductal carcinomas (28/29), 87.5% of the infiltrating lobular carcinomas (7/8), 66.6% of the mucinous and secretory carcinomas (10/15), 100.0% of the apocrine carcinomas (5/5) and 100.0% of the medullary carcinomas (6/6). The expression of MUC1 was statistically significant between the histological tumor types (p = 0.034), tumor size (p = 0.046), and HER-2/neu (p = 0.004).

These data suggest the expression of MUC1 was different in normal breast, IDH and malignant breast tumors, and was significantly correlated with the histological tumor types, tumor size and HER-2/neu oncogene.