Antibodies to novel myoepithelium-associated proteins distinguish benign lesions and carcinoma in situ from invasive carcinoma of the breast

The presence of an intact layer of myoepithelial cells (MECs) in benign breast lesions and in carcinoma in situ is an important feature for distinguishing these neoplasms from invasive tumors of the breast. Immunohistochemical detection of MECs, therefore, may be an important ancillary test in the analysis of breast lesions. However, existing myoepithelial markers are imperfect: most notably, antibodies to muscle and smooth muscle actins also identify stromal myofibroblasts (MFBs) and may thus result in great difficulties in interpretation. The recent availability of monoclonal antibodies to three novel smooth muscle-specific proteins-smooth muscle myosin heavy chains (SM-MHCs), calponin, and heavy caldesmon-prompted us to test their potential diagnostic utility in routine surgical specimens of breast. Preliminary studies with combined enzyme digestion and heat-induced epitope retrieval techniques revealed that specific expression of all three proteins may be demonstrated in routine, formalin-fixed, deparaffinized sections of smooth muscle tissue in a highly sensitive manner. We performed a retrospective immunohistochemical study employing these antibodies on a series of 100 breast specimens, including 70 invasive carcinomas of various types (invasive cribriform and tubular), 11 carcinomas in situ (ductal and lobular types), 10 sclerosing carcinomas, and nine papillary lesions. The antibody to SM-MHCs immunostained MECs of ducts and acini in normal breast lobules in all cases, those preserved at the periphery of involved ducts and acini of carcinomas in situ in 11 of II cases, and those in the lobules of all 10 sclerosing lesions tested. A uniform bimorphic pattern with basally located MECs along papillary fronds was clearly delineated in three of three solitary intraductal papillomas, whereas MECs were absent in six of six intracystic papillary carcinomas. Rarely did the antibody io SM-MHCs react with MFBs in the desmoplastic stroma of invasive carcinoma (five of 70 cases). This is in contrast to the anti-smooth muscle actin antibody 1A4, which showed strong and uniform reactivity with MFBs in 70 of 70 cases examined. The antibody to calponin exhibited a immunostaining pattern of MECs similar to that of SM-MHCs. This antibody, however, reacted with a minor subset of MFBs present in invasive carcinoma, although the extent of immunostaining was much less than that with 1A4. Although nonreactive with MFBs, the antibody to heavy caldesmon labeled only a small fraction of MECs, mostly those located in the major ducts and lactiferous tubules. MECs in the acini showed only weak and variable immunostaining with this antibody. It is concluded that monoclonal antibodies to SM-MHCs and calponin, in contrast to antibodies to muscle actin, can better discriminate MECs from MFBs and can be instrumental in distinguishing benign and in situ lesions from invasive carcinoma of the breast.