Genome-scale CRISPR/Cas9 screening identifies Hippo pathway as key determinant for susceptibility to BET inhibitors in lung cancer

Lung cancer is the main cause of cancer-related mortality worldwide. Despite the availability of different therapeutic options, including chemotherapy, target therapy, and immunotherapy, prognosis for lung cancer patients remains poor. Among innovative therapeutic approaches, inhibitors of the bromodomain and extraterminal domain containing proteins (BETi) have proven efficacy in preclinical settings and are currently in clinical trials for hematologic and solid tumors, including lung cancer. BETi downregulate key oncogenes to which cancer cells are addicted, causing a decrease in proliferation and an increase in apoptosis and differentiation. However, biomarkers that can help to select patients for this treatment are still lacking. To discover the mechanisms responsible of susceptibility and/or resistance to BETi, we performed a genome-scale knockout screening using the CRISPR/Cas9 technology in lung cancer cells. Through this approach, we identified three genes belonging to Hippo pathway, LATS2, TAOK1, and NF2, as mediators of susceptibility to BETi. Hippo pathway is an oncosuppressor pathway that converges on the phosphorylation of YAP1 and TAZ. YAP1 and TAZ are two transcriptional coactivators that interact with TEAD family proteins in nucleus for the activation of pro-oncogenic transcriptional programs. Phosphorylated YAP1 and TAZ are excluded from nucleus, degraded, and unable to enhance the expression of their target genes. We showed that LATS2, TAOK1, and NF2 support susceptibility to BETi, restraining TAZ nuclear localization and activity. We observed that TAZ knockout increases sensitivity whereas TAZ overexpression supports resistance to BETi. Furthermore, we showed that TAZ, YAP1, and TEAD are direct targets of BRD4 and their expression is downregulated by BETi treatment in a large panel of cancer cell lines. Noticeably, molecular alterations in YAP1, TAZ or in the Hippo genes LATS2, TAOK1, and NF2 are present in 19% of all non-small cell lung cancer patients, and overexpression or amplification of TAZ correlates with a worse outcome in lung adenocarcinoma patients. Our data demonstrate that Hippo pathway genes are required for susceptibility to BETi by restraining TAZ activity as transcriptional coactivator, and suggest that this mechanism can be exploited to detect and overcome drug resistance in lung cancer. BETi-mediated downregulation of the YAP1/TAZ/TEAD transcriptional activity provides a rationale for using these drugs to counteract the activity of these pro-oncogenic transcription factors.