Mechanisms of Endocrine Therapy Resistance in Estrogen Receptor Positive Breast Cancer by ZNF217 and ERBB3 Signaling

Around 70% of breast cancer tumors are estrogen receptor positive (ER+). Although endocrine therapies like tamoxifen and fulvestrant have been the mainstay for ER+ breast cancer patient treatment, about 33% of these patients have tumors that eventually relapse and develop as recurrent metastatic tumors that are resistant to endocrine therapy. The molecular mechanisms of endocrine therapy resistance are poorly understood.

Our lab identified the oncogene and transcription factor ZNF217 (Human)/Zfp217 (Mouse) as an ER modulating protein which drives tamoxifen resistance in vivo. However, no studies have described mechanistic information on how ZNF217 causes tamoxifen resistance. Ligand independent activation of ER by growth factor signaling pathways is a main mechanism of tamoxifen resistance. Significantly, ZNF217 directly activates the ERBB3/PI3K/AKT signaling pathway. In this study, I identified a novel and prognostic ZNF217- and NRG1-(ligand of ERBB3) dependent gene expression signature by RNA-Seq and non-canonical ER binding sites in the genome by ChIP-Seq in MCF7 cells. Through CUT&RUN sequencing, I also discovered ER- and NRG1-dependent ZNF217 binding sites in the MCF7 genome. I discovered transcriptomic and genomic changes that promote lower survival in ER+ breast cancer patients receiving tamoxifen. These data identified key pathways altered in a ZNF217-dependent manner after NRG1 induction. Through integration of ER ChIP-Seq & ZNF217 CUT&RUN data, I identified PCK1 and PHGDH as two novel inducers of tamoxifen resistance that rewire metabolism. Both can drive endocrine therapy resistance in cell culture. Pck1 deficiency reduces tumor burden in vivo in mouse PyMT mammary cancer cells ± shRNA Pck1 knockdown. Significantly, tumor overexpression of Pck1 made tumors resistant to tamoxifen, while the genetic and pharmacological inhibition of PCK1 overcame tamoxifen resistance in cell culture and in vivo.

Moreover, by analyzing cell culture media in MCF7 cells ± siRNA ZNF217, I discovered that ZNF217 alters the cytokine/chemokine/growth factor profile secreted by ER+ breast cancer cells. This suggests a role for ZNF217 in modulating the immune cell landscape in the tumor microenvironment.

Together, my results suggest that Zfp217 overexpression in ER+ breast cancer cells causes endocrine therapy resistance and that growth factor induction via NRG1 induces non-canonical ER binding to the genome and a ZNF217-dependent gene expression signature. I have identified new targets that are regulated by ZNF217 and NRG1 signaling and a possible new role for ZNF217 in regulating the crosstalk between ER+ breast cancer cells and immune cells. This study unraveled new mechanisms of endocrine therapy resistance, unveiling new therapeutic options for ER+ breast cancer patients.