Sharyl J. Nass

The Loss of Estrogen and Progesterone Receptor Gene Expression in Human Breast Cancer

Hormone responsiveness is a critical determinantof breast cancer progression and management, and theresponse to endocrine therapy is highly correlated withthe estrogen receptor (ER)3 and progesterone receptor (PR) status of tumor cells. Thus, keyareas of study in breast cancer are those mechanismsthat regulate ER and PR expression in normal andmalignant breast tissues. One-third of all breastcancers lack ER and PR; these conditions are associatedwith less differentiated tumors and poorer clinicaloutcome. In addition, approximately one-half ofER-positive tumors lack PR protein and patients withthis phenotype are less likely to respond tohormonal therapies than those whose tumors express bothreceptors. Since PR is induced by ER; its presence is amarker of a functional ER. In this review, we will discuss possible mechanisms for loss of ER andPR gene expression, especially structural changes withineach gene including deletions, polymorphisms ormethylation. Improved understanding of the pathways that lead to loss of ER and/or PR proteinsshould allow the development of better predictiveindicators as well as novel therapeutic approaches totarget these hormone-independent cancers.

THE BIOLOGY OF BREAST CANCER

There are several established prognostic factors for women who have early breast cancer. These include extent of axillary lymph node involvement and tumor size, histology, and steroid receptor studies. Although clinically quite useful, these factors unfortunately are not perfect predictors of individual outcome. This review summarizes recent advances in the field of breast cancer biology that may ultimately enhance our ability to predict more accurately the prognosis for established breast cancer, select therapy more appropriately, and perhaps even identify prospectively women who are at high risk for breast cancer development.

Expression of DNA methyl-transferase (DMT) and the cell cycle in human breast cancer cells

Estrogen receptor (ER)-negative breast cancer cells display extensive methylation of the ER gene CpG island and elevated DNA methyltransferase (DMT) expression compared to ER-positive cells. The present study demonstrates that DMT protein levels tightly correlate with S phase fraction in ER-positive cells, whereas ER-negative cells express DMT throughout the cell cycle. In addition, levels of p21CIP1, which disrupts DMT binding to PCNA, are inversely correlated with DMT levels. Therefore increased DMT expression in ER-negative cells is not simply due to elevated S-phase fraction, but rather to more complex changes that allow cells to escape normal cell cycle-dependent controls on DMT expression. Because ER-negative breast tumors often have activated growth factor pathways, the impact of these pathways on DMT expression was examined in ER-positive cells. Stable transfection with fibroblast growth factors (FGFs) 1 and 4 led to increased DMT expression that could not be accounted for by a shift in S phase fraction. Elevated DMT protein expression in FGF-transfectants was accompanied by a significant decrease in p21, again suggesting a reciprocal relationship between these two proteins. However, acquisition of an estrogen-independent phenotype, even in conjunction with elevated DMT levels, was not sufficient to promote ER gene silencing via methylation. These results indicate that multiple steps are required for de novo methylation of the ER CpG island.