SUN-012 Investigating the Effect of GRHL2 on pS118-ER Transcriptional Activity in Breast Cancer

In estrogen receptor (ER) positive breast cancers, there is a continual balance between growth and differentiation. Upon activation with estrogen, ER is phosphorylated at serine 118 (pS118-ER). High pS118-ER expression has been observed in low grade and well differentiated tumors. However, this phospho-variant presents a paradox as high expression has also been detected in therapy resistant metastatic tumors. Recently our lab conducted a comprehensive genome wide analysis of ER and pS118-ER through a ChIP-seq study. DNA motif analysis revealed enrichment of the GRHL2 motif near pS118-ER occupancy sites over ER occupancy sites. GRHL2 is a transcription factor that has also been implicated in epithelial differentiation and has been shown to have roles as both a tumor suppressor and oncogene in breast cancer. However, its role in facilitating pS118-ER transcriptional activity is unclear. In these studies, we examine the role of GRHL2 in pS118-ER occupancy through conventional ChIP at GRHL2/pS118-ER co-occupancy sites. GRHL2 occupancy can increase upon estrogen treatment at sites where pS118-ER is also present, but that increase has not been seen at sites lacking pS118-ER occupancy. Using a CRISPR knockout of GRHL2, the role of GRHL2 in pS118-ER DNA binding is being investigated further. In addition, we are using luminal cultures to model breast ducts and investigate the role of GRHL2 in breast cancer epithelial differentiation. These lumens maintain their estrogen responsiveness and GRHL2 expression, which will allow for the study of GRHL2 in a biologically relevant structure. These studies provide the foundation for understanding GRHL2 function in breast cancer.