Targeting Aberrant Transcription in Invasive Lobular Carcinoma Breast Cancer

Invasive lobular carcinoma (ILC) is an understudied breast cancer subtype comprising 8-14% of breast tumours. The majority of ILC (90%) are estrogen receptor (ER)-positive and candidates for endocrine therapy. Unfortunately, de novo resistance 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 bromodomain & 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. RNA sequencing analysis was performed on 61 primary ILC samples and it was found that high expression of BRD3 was associated with poor survival in ILC (log rank test, p=0.037). This finding was validated in a second cohort of 99 ILC primary samples from the METABRIC dataset (log rank test, p=0.0157). Next, it was tested if ILC cell lines were sensitive to BET inhibition using the small molecule inhibitor JQ1, which inhibits all BET family proteins. JQ1 downregulated growth promoting genes in ILC cell lines including ER and MYC. JQ1 inhibited cell growth in all ILC cell lines tested, however apoptosis was only induced in two out of four ILC cell lines. ILC cell lines resistant to JQ1-induced apoptosis had sustained or upregulated expression of the anti-apoptotic BCL-XL following JQ1 treatment by RNA sequencing and qPCR validation. This led us to assess the combination of JQ1 and ABT-263 (BCL-2, BCL-XL and BCL-W inhibitor). The drug combination was synergistic in ILC cell lines and induced apoptosis in ILC cell lines previously apoptotic resistant. The drug combination also inhibited the number and size of 3D spheroids and induced apoptosis in a ILC primary sample grown ex vivo. These findings suggest that the combination of JQ1 and ABT-263 is an effective treatment strategy for ILC.