Epigenetic Reprogramming of HOXC10 in Endocrine-Resistant Breast Cancer

Genome-wide screen identifies methylation of the estrogen-repressed HOXC10 gene as a determinant of resistance to aromatase inhibitors in breast cancer. Playing Tug-of-War with HOXC10 Aromatase inhibitors are drugs that prevent androgens from being converted into estrogen, and they are frequently used to treat breast cancers that express the estrogen receptor. Unfortunately, some patients’ tumors never respond to these drugs, and others gradually become resistant over time. Although the development of resistance to aromatase inhibitors has been investigated in some previous studies and some potential mechanisms have been proposed, much about this process remains unknown. Pathiraja and colleagues began by performing a genome-wide methylation screen in breast cancer cells, which identified the developmental gene HOXC10 as a target of epigenetic silencing in the context of long-term estrogen withdrawal. When HOXC10 is active, it interferes with proliferation and can stimulate apoptosis, but estrogen suppresses its activity, thereby promoting tumor growth. By decreasing estrogen production, aromatase inhibitors up-regulate HOXC10, accounting for some of their antitumor activity. However, long-term estrogen deprivation eventually has the opposite effect, leading to methylation of HOXC10 and its long-term suppression even in the absence of estrogen. These findings suggest that a rational approach for overcoming aromatase resistance in breast cancer may involve the addition of demethylating drugs to overcome the methylation of HOXC10 and take advantage of its antitumor effects, although this remains to be demonstrated directly. Resistance to aromatase inhibitors (AIs) is a major clinical problem in the treatment of estrogen receptor (ER)–positive breast cancer. In two breast cancer cell line models of AI resistance, we identified widespread DNA hyper- and hypomethylation, with enrichment for promoter hypermethylation of developmental genes. For the homeobox gene HOXC10, methylation occurred in a CpG shore, which overlapped with a functional ER binding site, causing repression of HOXC10 expression. Although short-term blockade of ER signaling caused relief of HOXC10 repression in both cell lines and breast tumors, it also resulted in concurrent recruitment of EZH2 and increased H3K27me3, ultimately transitioning to increased DNA methylation and silencing of HOXC10. Reduced HOXC10 in vitro and in xenografts resulted in decreased apoptosis and caused antiestrogen resistance. Supporting this, we used paired primary and metastatic breast cancer specimens to show that HOXC10 was reduced in tumors that recurred during AI treatment. We propose a model in which estrogen represses apoptotic and growth-inhibitory genes such as HOXC10, contributing to tumor survival, whereas AIs induce these genes to cause apoptosis and therapeutic benefit, but long-term AI treatment results in permanent repression of these genes via methylation and confers resistance. Therapies aimed at inhibiting AI-induced histone and DNA methylation may be beneficial in blocking or delaying AI resistance.