Abstract 4674: Epigenetic modulators for the treatment of invasive lobular carcinoma breast cancer
Authors: Walsh Louise , Moran Bruce , Das Sudipto , Tarrant Finbarr , Schouten Philip , Chin Suet-Feung , Bernards Rene , Caldas Carlos , Gallagher William , Chonghaile* Tríona Ní , O'Connor* Darran 
Published: July 2017

Invasive lobular carcinoma (ILC) is a breast cancer subtype comprising 10% of breast tumors. The majority of ILC (90%) are estrogen receptor (ER)-positive and therefore candidates for endocrine therapy. Unfortunately, de novo resistant to endocrine therapies occurs in 33% of women and a further 40% will relapse on treatment. Therefore, novel therapeutic targets are required for ILC. Deregulated transcription is a recurring theme in cancer, which can be due to epigenetic events. The bromdomain & extra-terminal domain (BET) family of proteins (BRD2, BRD3, BRD4, BRDT) function as chromatin readers that bind acetylated lysine residues on histones and regulate transcription. We performed RNA-Sequencing analysis on 61 primary ILC samples and found that high expression of BRD3 is associated with poor survival in ILC (log rank test, p=0.037). We validated this finding, that high expression of BRD3 is associated with poor survival, in a second cohort of 99 ILC primary samples from the METABRIC dataset (log rank test, p=0.0157). Next, we tested if ILC cell lines were sensitive to BET inhibition using the small molecule inhibitor JQ1, which inhibits all BET family proteins. JQ1 downregulates growth and survival genes in ILC cell lines including MYC, ER and BCL-XL. Pathway analysis following RNA sequencing revealed that JQ1 targets the apoptotic and Wnt signalling pathways in ILC cell lines. Interestingly, JQ1 inhibited the cell growth in all ILC cell lines tested, however apoptosis was only induced in two ILC cell lines. Furthermore, ILC cell lines which were relatively resistant to JQ1-induced apoptosis expressed both the BCL-2 and BCL-XL anti-apoptotic proteins. This led us to assess the combination of JQ1 and the BH3 mimetics, ABT-199 and ABT-263. ABT-199 is a selective small molecule inhibitor of BCL-2, whereas ABT-263 is an inhibitor of BCL-2, BCL-XL and BCL-W proteins. We found the combination of JQ1 and ABT-263, but not the combination of JQ1 & ABT-199, to be synergistic and enhance apoptosis in ILC cell lines. This is in accordance with BH3 profiling of ILC cell lines which indicated that ILC cell lines are dependent on BCL-2/ BCL-XL proteins for cell survival. As JQ1 is a pan-BET family inhibitor, we also wish to determine which BET protein is responsible for sensitivity to JQ1. Following knockdown of each BET protein in the CAMA-1 cell line we found that BRD3 and BRD4 were responsible for loss of cell viability. Future work will include determining the specific role of BRD3 in ILC and the effectiveness of the JQ1 and ABT-263 combination in vivo. Our work suggests that inhibition of BET proteins in combination with BH3 mimetics may be a rational therapeutic combination for ILC.

Citation Format: Louise Walsh, Bruce Moran, Sudipto Das, Finbarr Tarrant, Philip Schouten, Suet-Feung Chin, Rene Bernards, Carlos Caldas, William Gallagher, Tríona Ní Chonghaile*, Darran O'Connor*. Epigenetic modulators for the treatment of invasive lobular carcinoma breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4674. doi:10.1158/1538-7445.AM2017-4674