EZH2-mediated epigenetic repression of DNA repair in promoting breast tumor initiating cells

Members of the Polycomb-group (PcG) family of proteins, including EZH2 (enhancer of zeste homolog 2), are involved in establishing epigenetic silencing of developmental genes in adult and embryonic stem cells, and their deregulation has been implicated in cancer. In a recent report, EZH2-mediated epigenetic repression of DNA damage repair in breast tumor initiating cells (BTICs) was identified as a mechanism that could promote expansion of BTICs, and may contribute to cancer progression.


The article
Chang and colleagues [8] demonstrate that increased EZH2 in CD44 + /CD24 -/low cells correlates with their increased abundance, that is, the proportion of CD44 + / CD24 -/low cells. Increased EZH2 in CD44 + /CD24 -/low cells was correlated with decreased RAD51 levels. Using chromatin immunoprecipitation (ChIP), the authors show that EZH2, and other PRC1/PRC2 proteins, are recruited to a putative Ploycomb response element on the RAD51 promoter. Here, hypoxia in the micro environment was identifi ed as a potential cause of EZH2 upregulation. Th e RAD51 gene has a well established role in DNA repair through homologous recombination and, accordingly, EZH2-mediated epigenetic repression of RAD51 associates with accumulation of DNA damage and chromosome abnormalities.
Using protein arrays, the authors identifi ed increased expression of the RAF1 gene as a potential change associated with EZH2-induced genomic instability. Importantly, this association was found to be a consequence of acquired DNA copy number gains over the RAF1 gene at 3p25.2, resulting in activation of ERK and β-catenin, shown to confer survival and proliferative advantages for CD44 + /CD24 -/low cells. Importantly, inhibition of RAF1-MEK-ERK-β-catenin activation by small molecule inhibitors (AZD6244, Sorafenib) leads to eff ective elimina tion of CD44 + /CD24 -/low cells -that is, BTICs -suggesting potential implications with respect to clinical manage ment of the disease.

Viewpoint
Th e most important insight from Chang and colleagues [8] is the observation that genomic instability induced by EZH2-mediated epigenetic repression of DNA repair, that is, RAD51, results in expansion of the BTIC population. Further, the order in which these changes occur did not infl uence the eventual outcome -that is, overexpression of EZH2 has the same eff ect as knock-down of

Abstract
Members of the Polycomb-group (PcG) family of proteins, including EZH2 (enhancer of zeste homolog 2), are involved in establishing epigenetic silencing of developmental genes in adult and embryonic stem cells, and their deregulation has been implicated in cancer. In a recent report, EZH2mediated epigenetic repression of DNA damage repair in breast tumor initiating cells (BTICs) was identifi ed as a mechanism that could promote expansion of BTICs, and may contribute to cancer progression. RAD51. In this context, RAF1 amplifi cation was described as a recurrent oncogenic consequence of downregulated DNA repair leading to activation of ERK and β-catenin and the consequent promotion of the BTIC population. In breast cancer, EZH2 overexpression has previously been linked to aggressive and poorly diff erentiated carcinomas [9], and downregulated DNA repair [10]. Th e insights provided by Chang and colleagues relate to the identifi cation of EZH2 in augmenting BTICs through epigenetic repression of DNA repair, leading to aggressive disease. Th ese results are consistent with clonal evolution of CSCs, that is, the notion that aggressive secondary CSCs can arise from the primary population of CSCs. Th e clinical relevance relates to potential benefi ts from RAF1/MEK/ERK inhibitors in the treatment of breast cancer patients, and in preventing disease progression. Further, poly ADP ribose polymerase (PARP) inhibitors are highly eff ective in targeting cancer cells with homologous recombination defects and therefore hold unexplored potential in this regard, that is, in targeting secondary CSCs [11].
Although somatic mutations in DNA repair genes are rare in sporadic cancers, it is well established that susceptibility to cancer is commonly linked to inherited mutations in DNA repair genes, for example, BRCA1, BRCA2, PALB2, WRN and MLH1 [12]. Th is has led some to speculate that defects in DNA repair genes cannot explain instability in sporadic cancers [12], whereas others have emphasized epigenetic mechanisms [13]. Chang and colleagues [8] describe results suggesting a causa tive link between epigenetic inactivation of DNA repair, the emergence of genomic instability, and cancer progres sion. In breast cancer, epigenetic inactivation of the BRCA1 gene occurs through CpG island hypermethylation [14,15]. BRCA1 is essential in DNA repair of double-strand breaks, with RAD51 as a crucial partner for error-free repair by homologous recombination [16]. Recently, Lim and colleagues [17] demonstrated that BRCA1 defective breast cancers represent an expanded population of luminal progenitor cells. Th is is consistent with the results described in Chang and colleagues [8], demonstrating DNA repair defects and expansion of tumor cells with progenitor/stem-cell characteristics. In conclusion, an important task in future research will be to explore the relationship between epigenetic repression of DNA repair, the induction of genomic instability, and markers of CSCs.