Epigenetic silencing of maspin gene expression in human breast cancers

Maspin is a tumor suppressor whose expression is lost in many advanced brea st cancers, Maspin has been shown to inhibit cell motility, invasion and me tastasis; however, its precise role in normal mammary epithelium remains to be elucidated. Although expression of maspin mRNA is low or absent in most human breast cancer cells, the maspin gene is rarely re-arranged or delete d, We hypothesized that aberrant cytosine methylation and chromatin condens ation of the maspin promoter participates in the silencing of maspin expres sion during neoplastic progression. To test this hypothesis, we compared cu ltured normal human mammary epithelial cells (HMECs) to 9 cultured human br east cancer cell lines, HMECs expressed maspin mRNA and displayed a complet ely non-methylated maspin gene promoter with an open chromatin structure, I n contrast, 7 of 9 breast cancer cell lines had no detectable maspin expres sion and 6 of these 7 maspin-negative breast cancer cell lines also display ed an aberrant pattern of cytosine methylation of the maspin promoter. Inte restingly, the maspin promoter was completely methylated in maspin-negative normal peripheral blood lymphocytes, This indicates that the maspin promot er is not a functional CpG island and that cytosine methylation of this reg ion may contribute to normal tissue-restricted gene expression. Chromatin a ccessibility studies with MCF-7 cells, which lack maspin expression and hav e a methylated maspin promoter, showed a closed chromatin structure compare d with HMECs, Moreover, maspin gene expression could be re-activated in MCF -7 cells by treatment with 5-aza-2'-deoxycytidine, a DNA demethylating agen t, Thus, aberrant cytosine methylation and heterochromatinization of the ma spin promoter may silence maspin gene expression, thereby contributing to t he progression of human mammary cancer. (C) 2000 Wiley-Liss, Inc.