A Unique FOXA1-Associated Chromatin State Dictates Therapeutic Resistance in Lobular Breast Cancer

Abstract

Invasive lobular carcinomas (ILC) are the second most common histologic subtype of breast cancer, accounting for up to 15% of cases. ILC is estrogen receptor (ER) positive, yet its biology is distinct from invasive ductal carcinomas (IDC), and retrospective analyses have indicated a poorer outcome with endocrine therapy. In this issue of Cancer Research, Nardone and colleagues investigated the mechanisms of this differential therapy response in ILC, which cannot be solely explained by the genetic profile of these tumors. The authors conducted a thorough examination of the epigenome of ILC compared with IDC in clinical and preclinical models and revealed an alternative chromatin accessibility state in ILC driven by the pioneer factor FOXA1. FOXA1 regulates its own expression in a feed-forward mechanism by binding to an ILC-unique FOXA1 enhancer site. This results in a FOXA1-ER axis that promotes the transcription of genes associated with tumor progression and tamoxifen resistance. Targeting the FOXA1 enhancer region blocks this transcriptional program and inhibits ILC proliferation. These results shed light on a new epigenetic mechanism driving ILC tumor progression and treatment resistance, which may have profound therapeutic implications. See related article by Nardone et al., p. 3673.

Figure 1.

A unique FOXA1-associated chromatin state influences therapeutic resistance in lobular breast cancer. ILC tumors have a unique autoregulatory binding site in the FOXA1 enhancer region that does not exist in IDC. This allows for a feed-forward loop that upregulates FOXA1 and leads to subsequent alterations in global chromatin accessibility and ER-dependent transcription (left). ER target genes downregulated with tamoxifen treatment in IDC are maintained by FOXA1 binding in ILC, leading to tumor resistance (right).