Abstract 4118: The EMT transcription factor TWIST1 mediates resistance to EGFR inhibitors in EGFR-mutant non-small cell lung cancer
Authors: Yochum Zachary A. , Wang Hailun , Cades Jessica A. , Khetarpal Susheel , Huang Eric H. , Tran Phouc T. , Burns Timothy F. 
Published: July 2017

Abstract
Recent advances in the treatment of non-small cell lung cancer (NSCLC) stem from the paradigm shift of classifying patients into subtypes based upon the presence of distinct molecular drivers. Subsets of patients, such as those with EGFR mutations and ALK translocations, have dramatic responses in their tumors to tyrosine kinase inhibitors (TKIs) that specifically inhibit these oncogenic drivers. While many patients initially response to TKIs, therapeutic resistance is inevitable. For EGFR-mutant NSCLC, there are multiple described mechanisms of resistance to EGFR TKIs, including epithelial-mesenchymal transition (EMT). Previous studies have implicated the AXL kinase and ZEB1, an EMT transcription factor (EMT-TF), in EMT-mediated EGFR TKI resistance. We have previously demonstrated that the EMT-TF, TWIST1, is required for oncogene-driven NSCLC tumorigenesis, including those tumors with EGFR mutations. In this study, we investigated the role of TWIST1 in EMT-mediated resistance to EGFR TKIs.
We have demonstrated that genetic or pharmacologic inhibition of TWIST1 resulted in growth inhibition in a panel of EGFR-mutant NSCLC cell lines and apoptosis in a subset of these lines. Interestingly, TWIST1 overexpression in EGFR-mutant NSCLC cell lines led to EGFR TKI resistance. Conversely, knockdown of TWIST1 in an erlotinib resistant EGFR-mutant NSCLC cell line restored erlotinib sensitivity. We found that TWIST1 mediates resistance to EGFR TKIs through suppression of apoptosis possibly through decreasing the expression of the pro-apoptotic Bcl-2 member, BCL2L11 (BIM). We observed that TWIST1 knockdown increased BIM levels, while TWIST1 overexpression decreased BIM expression. Furthermore, TWIST1-mediated resistance was overcome by treatment with the BCL-2/BCL-XL inhibitor, ABT-737. Knockdown of BIM recapitulated the resistance seen following TWIST1 overexpression, suggesting that TWIST1 suppression of BIM is a mechanism through which TWIST1 leads to EGFR TKI resistance. To explore the role of TWIST1 in modulating EGFR inhibitor sensitivity in vivo, we used an inducible EGFR-mutant transgenic mouse model, CCSP-rtTA/tetO-EGFRL858R (CE), which expresses EGFRL858R in the lung and a EGFR-mutant/Twist1 transgenic model, CCSP-rtTA/tetO-EGFRL858R/ Twist1- tetO7-luc (CET), which expresses both Twist1 and EGFRL858R in the lung. CET mice had a significantly increased tumor burden, decreased apoptosis and a decreased overall survival compared to CE mice following erlotinib treatment.
In summary, we found that TWIST1 overexpression leads to EGFR TKI resistance by suppressing EGFR TKI-induced apoptosis through suppressing BIM expression. Future studies aim to establish the mechanisms of TWIST1 suppression of BIM expression and determine if our TWIST1 inhibitor, harmine, is effective in overcoming EMT-mediated resistance.
Citation Format: Zachary A. Yochum, Hailun Wang, Jessica A. Cades, Susheel Khetarpal, Eric H. Huang, Phouc T. Tran, Timothy F. Burns. The EMT transcription factor TWIST1 mediates resistance to EGFR inhibitors in EGFR-mutant non-small cell lung 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 4118. doi:10.1158/1538-7445.AM2017-4118