CD66a, also known as biliary glycoprotein (BGP), is a member of the carcinoembryonic antigen (CEA) family and the human homologue of the rat cell-CAM. There is evidence that aberrant expression or loss of CD66a in tumor tissue is of biological significance. No data about its expression in breast carcinoma cells and only sparse information about the expression of CD66a in normal breast are available thus far. In this study we used monoclonal antibodies to analyze the expression of CD66a and CEA in normal tissue, benign lesions, and in noninvasive and invasive carcinomas of the mammary gland. In normal tissue and benign lesions, CD66a was consistently expressed at the apical sites of epithelial cells and in myoepithelia, whereas CEA was absent or was restricted only to some apical membranes within the ductal tree. The specific staining of myoepithelia was most evident in pseudoinfiltrative radial scars and sclerosing adenosis. However, the apical expression of CD66a disappeared with the development of the malignant phenotype in noninvasive and invasive carcinomas, and changed gradually from low- to high-grade noninvasive carcinomas into a predominant uniform membrane staining all around the atypical cells. CEA expression was irregular in intensity and distribution. The native apical CD66a staining was partially preserved in some highly differentiated invasive carcinomas with a better prognosis, such as tubular and papillary carcinomas. These findings indicate that loss of CD66a expression rather than a change in staining patterns coincides with the development of the malignant phenotype.
