Hormones and progeny of breast tumor cells.

The rudimentary human glandular breast, with the approach of puberty, starts to grow both at glandular and stromal sites. Full differentiation is a gradual process and takes many years, and is only fully attained by pregnancy. The risk of breast cancer is inversely related to parity. Women during adolescence have the highest susceptibility to breast cancer development. This appears to be the period when the mammary gland has the highest number of stem cells. Stem cells may represent important targets for transformational events. Immunohistochemistry allows for identification of the lineage-specific precursor glandular and myoepithelial cells and their differentiated progeny. Both estrogen receptor subtypes are found in epithelial cells of alveoli and ducts as well as in stromal cells. Immunophenotypia of benign proliferative breast disease favors a fundamentally different epithelial composition from that of most malignant epithelial proliferations such as atypical ductal hyperplasia, ductal carcinoma in situ, lobular neoplasia and invasive breast carcinoma. Immunophenotypical characterization of these lesions assists in distinguishing benign from malignant disease. Based on the observation of bilateral risks and frequent multifocality with atypical ductal hyperplasia, atypical lobular hyperplasia and lobular carcinoma in situ, it is suggested that these may represent risk factors as well as precursors. One should, however, realize that ductal as well as lobular premalignant breast lesions ultimately arise from stem cells in the terminal duct lobular units. Estrogen receptor-beta (ERbeta)-positive and ERalpha-negative expression characterizes the highest levels of proliferative cancer cell activity. Point mutations and alterations of co-activators and co-repressors will also determine hormone sensitivity. There is evidence for different genetic pathways in the development of ductal carcinoma in situ and lobular carcinoma in situ. While they share recurrent 16q losses, a second hit in the E-cadherin gene explains the advent of lobular lesions and their common existence with the primary ductal type. Based on our immunocytochemical observations, the most likely target cell of malignant transformation is the Ck18/18-positive and ER-negative transient cell of normal breast epithelium. Pregnancy confers a different genomic imprint to breast epithelial stem cells. Further elucidation of this mechanism may assist in developing appropriate means of breast cancer prevention. This paper is the Pieter van Keep Memorial Lecture given by Professor Hermann Schneider at the 11th World Congress on the Menopause, October 18-22, 2005, in Buenos Aires, Argentina.