Phenotypic alterations in myoepithelial cells associated with benign sclerosing lesions of the breast.

Myoepithelial cells surrounding spaces involved by ductal carcinoma in situ show phenotypic differences from normal myoepithelial cells. Myoepithelial cells are also present around entrapped glandular spaces in benign sclerosing lesions of the breast, but the immunophenotype of these myoepithelial cells has not been characterized. We evaluated myoepithelial cell immunophenotype in 48 benign sclerosing lesions using antibodies to 7 myoepithelial cell markers (smooth muscle actin, calponin, smooth muscle myosin heavy chain, p63, CD10, cytokeratin 5/6, and p75). Staining intensity of the myoepithelial cells surrounding entrapped glands was compared with that of myoepithelial cells surrounding normal ducts and lobules on the same section. When compared with normal breast ducts and lobules on the same slide, myoepithelial cells associated with benign sclerosing lesions showed reduced expression of cytokeratin 5/6 in 31.8% of cases, smooth muscle myosin heavy chain in 20.9%, CD10 in 15.2%, p63 in 9.3%, and calponin in 6.4%. In 15.9% of cases, myoepithelial cells surrounding entrapped glands showed complete absence of staining for cytokeratin 5/6. None of the cases showed reduced myoepithelial cell expression of smooth muscle actin or p75. The proportion of radial scars/complex sclerosing lesions and sclerosing adenosis with reduced expression was significantly different for CD10 (26.9% and 0% respectively; P=0.01) and p63 (17.4% and 0% respectively; P=0.05). We conclude that myoepithelial cells associated with benign sclerosing lesions of the breast may show immunophenotypic differences from normal myoepithelial cells. This needs to be taken into consideration when selecting myoepithelial markers to help distinguish benign sclerosing lesions from invasive breast cancer.