Differential expression of genes involved in the epigenetic regulation of cell identity in normal human mammary cell commitment and differentiation

The establishment and maintenance of mammary epithelial cell identity depends on the activity of a group of proteins, collectively called maintenance proteins, that act as epigenetic regulators of gene transcription through DNA methylation, histone modification, and chromatin remodeling. Increasing evidence indicates that dysregulation of these crucial proteins may disrupt epithelial cell integrity and trigger breast tumor initiation. Therefore, we explored in silico the expression pattern of a panel of 369 genes known to be involved in the establishment and maintenance of epithelial cell identity and mammary gland remodeling in cell subpopulations isolated from normal human mammary tissue and selectively enriched in their content of bipotent progenitors, committed luminal progenitors, and differentiated myoepithelial or differentiated luminal cells. The results indicated that, compared to bipotent cells, differentiated myoepithelial and luminal subpopulations were both characterized by the differential expression of 4 genes involved in cell identity maintenance: CBX6 and PCGF2, encoding proteins belonging to the Polycomb group, and SMARCD3 and SMARCE1, encoding proteins belonging to the Trithorax group. In addition to these common genes, the myoepithelial phenotype was associated with the differential expression of HDAC1, which encodes histone deacetylase 1, whereas the luminal phenotype was associated with the differential expression of SMARCA4 and HAT1, which encode a Trithorax protein and histone acetylase 1, respectively. The luminal compartment was further characterized by the overexpression of ALDH1A3 and GATA3, and the down-regulation of NOTCH4 and CCNB1, with the latter suggesting a block in cell cycle progression at the G2 phase. In contrast, myoepithelial differentiation was associated with the overexpression of MYC and the down-regulation of CCNE1, with the latter suggesting a block in cell cycle progression at the G1 phase.

Epithelial cell identity in hyperplastic precursors of breast cancer

Introduction:In the adult human breast,hyperplastic enlarged lobular unit(HELU) and atypical ductal hyperplasia(ADH) are two common abnormalities that frequently coexist with ductal carcinoma in situ(DCIS).For this reason,they have been proposed as the early steps in a biological continuum toward breast cancer.Methods:We investigated in silico the expression of 369 genes experimentally recognized as involved in establishing and maintaining epithelial cell identity and mammary gland remodeling,in HELUs or ADHs with respect to the corresponding patient-matched normal tissue.Results:Despite the common luminal origin,HELUs and ADHs proved to be characterized by distinct gene profiles that overlap for 5 genes only.While HELUs were associated with the overexpression of progesterone receptor(PGR),ADHs were characterized by the overexpression of estrogen receptor 1(ESR1) coupled with the overexpression of some proliferation-associated genes.Conclusions:This unexpected finding contradicts the notion that in differentiated luminal cells the expression of estrogen receptor(ER) is dissociated from cell proliferation and suggests that the establishing of an ER-dependent signaling is able to sustain cell proliferation in an autocrine manner as an early event in tumor initiation.Although clinical evidence indicates that only a fraction of HELUs and ADHs evolve to invasive cancer,present findings warn that exposure to synthetic progestins,frequently administered as hormone-replacement therapy,and estrogens,when abnormally produced by adipose cells and persistently present in the stroma surrounding the mammary gland,may cause these hyperplastic lesions.

The role of maintenance proteins in the preservation of epithelial cell identity during mammary gland remodeling and breast cancer initiation

During normal postnatal mammary gland development and adult remodeling related to the menstrual cycle, pregnancy, and lactation, ovarian hormones and peptide growth factors contribute to the delineation of a definite epithelial cell identity. This identity is maintained during cell replication in a heritable but DNAindependent manner. The preservation of cell identity is fundamental, especially when cells must undergo changes in response to intrinsic and extrinsic signals. The maintenance proteins, which are required for cell identity preservation, act epigenetically by regulating gene expression through DNA methylation, histone modification, and chromatin remodeling. Among the maintenance proteins, the Trithorax(TrxG) and Polycomb(PcG) group proteins are the best characterized. In this review, we summarize the structures and activities of the TrxG and PcG complexes and describe their pivotal roles in nuclear estrogen receptor activity. In addition, we provide evidence that perturbations in these epigenetic regulators are involved in disrupting epithelial cell identity, mammary gland remodeling, and breast cancer initiation.

Differential Expression of Genes Involved in Cell Polarity, EMT and Cell-Fate in Breast Cancer and Corresponding Normal Tissue

Objectives: Cell polarity and epithelial morphology are peculiar features of cells forming the terminal ductal lobular unit, and they are early lost during neoplastic transformation because of an epithelial-mesenchymal transition (EMT). To understand these early events we analyzed a set of 125 genes related to cell polarity, EMT and cell-fate decision in 26 breast cancer specimens and corresponding patient-matched normal tissue. Methods: The difference of gene expression was explored by t-paired test. In addition, to evidence latent variables accounting for genes correlations, a Factor Analysis was applied as exploratory technique. Results: Among the 90 differentially expressed genes, those coding for cell polarity complexes, apical-junctional components and luminal cytokeratins were overexpressed in tumor samples (suggesting a terminally differentiated phenotype) whereas those coding for stemness-associated features or related with EMT were expressed in normal tissues but not in tumor samples, suggesting the persistence of stem/progenitor cells. Factor analysis confirmed these findings and indicated that the difference between tumors and normal tissues can be synthesized in three main features representative of specific molecular/morphological alterations. Conclusions: The a priori definition of a selected panel of genes and the application of an exploratory statistical approach, greatly contribute to reduce the intrinsic biological complexity of tumor specimens and to describe the difference between tumor specimens and corresponding histologically normal tissues.