—  SYMPOSIUM #55  —

New Frontiers in Breast Pathology
Moderator: Dr. Sunil Lakhani

Section 2 - Epigenetics of Breast Cancer

José Palacios
Spain


During the last years, it has been established the relevance of aberrant epigenetic patterns in human tumours. For DNA methylation, it is known that two apparently contrasting phenomena coexist in the cancer cell: a profound loss of global 5-methylcytosine genomic content with discrete areas of dense hypermethylation. Overall hypomethylation takes place predominantly in DNA repetitive and endoparasitic sequences and has been linked to the generation of chromosomal instability. On the other hand, hypermethylation occurs in the CpG islands located in the promoters of certain tumour-suppressor genes, leading to gene silencing (Esteller 2006).

It is well known that these two kinds of changes in the DNA methylation pattern occur in breast cancer. This paradoxical coexistence of a global decrease in methylation with regional hypermethylation implies that independent and different processes are responsible for hypomethylation and hypermethylation. It has been suggested that global hypomethylation is caused by a global increase in demethylase activity whereas regional hypermethylation results from local changes in chromatin structure, which prevent access to the abundant demethylase(s) (Szyf et al, 2004).

The phenomenon of regional DNA hypermethylation and silencing of tumor suppressor genes in cancer has been the focus of attention in the last decade. A significant amount of data has established a list of genes hypermethylated in cancer and recently whole genome approaches have identified methylation signatures of breast cancer cells (Yan et al, 2000). These methylation signatures, which are the unique combination of methylated CpG islands in a cancer cell were correlated with breast cancer stage and have been proposed to be a diagnostic marker of breast cancer cells. In addition to their diagnostic value in breast cancer it is clear from the repertoire of methylated genes that silencing of these genes by DNA methylation plays a role in the transformation process. Amongst the methylated genes are tumor suppressor genes such as p16 whose methylation is proposed to silence this gene and override cell growth regulatory signals (Silva et al, 2003). p16 methylation in DNA prepared from plasma of breast cancer patients was associated with nodal metastasis. Another group of methylated genes in breast cancer is composed of damage response genes such as BRCA1 (Esteller et al, 2000), which is also mutated in familial breast cancer. Disruption of repair genes might increase sporadic mutations frequency, a hallmark of cancer cells. Steroid receptor genes family members such as the estrogen receptor (Ottaviano et al, 2000; Widschwendter et al, 2004)) and retinoic acid beta 2 (RARβ 2) receptor are methylated and silenced in a fraction of breast cancers (Widschwendter et al, 2000). Interaction of RAR β2 receptor with retinoic acid might have an antiproliferative effect and its silencing confers a selective advantage on advanced breast cancer cells. Cell adhesion and cell surface molecules such as E-cadherin (Nass et al, 2000) and inhibitors of proteases such as TIMP-3, whose silencing might promote metastasis are also found to be methylated in breast cancer. In addition, E-cadherin hyemethylation occurs in around 40% of lobular carcinomas and is implicated in the absence of protein expression that is characteristic of this histotype of breast cancer (Sarrió et al, 2003).

The methylation status of several genes has been analyzed on samples obtained from nipple aspiration, ductal lavage, and duct endoscopy, to evaluate their diagnostic potential. A seminal study conducted by Evron et al examined fluid derived from cancer-bearing ducts, identified by concomitant ductoscopy. Exfoliated ductal cells were tested by methylation-specific PCR. Methylated alleles of cyclin D2, RARß, and TWISTgenes frequently were detected in cells isolated from cancer-bearing ducts (17 of 20), but rarely in cells from healthy ducts (five of 45). In a study conducted by Krassenstein et al, the methylation status of a panel of six tumor suppressor and other cancer genes (GSTP1, RARß, P16inka, p14ARF, RASSFIA, and DAPK) known to be hypermethylated in breast neoplasia was analyzed in 22 NAF DNA samples and compared with their respective tumor samples. These investigators found that each of the 22 tumors had hypermethylation of at least one gene from the panel, thereby providing a target that was successfully detected in 18 (82%) of the matched aspirate fluids with 100% specificity.

Recently, promoter methylation in the non-neoplastic cells of breast cancer stroma has been reported. This alteration in tumor microenvironment may advance our understanding of breast cancer development and progression and lead to new diagnostic and prognostic markers and therapeutic targets.

References
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