Bromodomain and extra-terminal motif (BET) inhibition is synthetic lethal with loss of SMAD4 in colorectal cancer cells via restoring the loss of MYC repression.

The tumor suppressor SMAD4 is frequently mutated in colorectal cancer (CRC). However, no effective targeted therapies exist for CRC with SMAD4 loss. Here, we employed a synthetic lethality drug screening in isogenic SMAD4+/+ and SMAD4-/- HCT116 CRC cells and found that bromodomain and extra-terminal motif (BET) inhibitors, as selective drugs for the growth of SMAD4-/- HCT116 cells. BET inhibition selectively induced G1 cell cycle arrest in SMAD4-/- cells and this effect was accompanied by the reprogramming of the MYC-p21 axis. Mechanistically, SMAD4 is a transcription repressor of MYC, and MYC in turn represses p21 transcription. SMAD4-/- cells lost MYC repression ability, thereby causing the cells addicted to the MYC oncogenic signaling. BET inhibition significantly reduced MYC level and restored p21 expression in SMAD4-/- cells, inducing the selective growth arrest. The ectopic overexpression of MYC or the silencing of p21 could rescue the BET inhibitor-induced growth arrest in SMAD4-/- cells, verifying this model. Tumor xenograft mouse experiments further demonstrated the synthetic lethality interaction between BET and SMAD4 in vivo. Taken together, our data suggest that BET could be a potential drug target for the treatment of SMAD4-deficient CRC.