Transcriptional plasticity promotes primary and acquired resistance to BET inhibition

BET bromodomain inhibitors are being explored as potential therapeutics in cancer; here, AML cells are shown to evade sensitivity to BET inhibition through rewiring the transcriptional regulation of BRD4 target genes such as MYC in a process that is facilitated by suppression of PRC2 and WNT signalling activation. Emergence of resistance to BET inhibitors BET inhibitors that target bromodomain chromatin readers such as BRD4 are being explored as potential therapeutics in cancer. Two papers published in this issue of Nature identify mechanisms that may be involved in resistance to BET inhibition in models of leukaemia. In an MLL–AF9 model, Mark Dawson and colleagues find that resistance emerges from leukaemic stem cells and is, in part, a consequence of increased Wnt signalling. Johannes Zuber and colleagues find that suppression of the PRC2 complex renders acute myeloid leukaemia cells resistant to BET inhibition by rewiring the transcriptional regulation of BRD4 target genes such as MYC . Wnt signalling is also implicated as a key driver of resistance. Following the discovery of BRD4 as a non-oncogene addiction target in acute myeloid leukaemia (AML) 1 , 2 , bromodomain and extra terminal protein (BET) inhibitors are being explored as a promising therapeutic avenue in numerous cancers 3 , 4 , 5 . While clinical trials have reported single-agent activity in advanced haematological malignancies 6 , mechanisms determining the response to BET inhibition remain poorly understood. To identify factors involved in primary and acquired BET resistance in leukaemia, here we perform a chromatin-focused RNAi screen in a sensitive MLL–AF9;Nras G12D -driven AML mouse model, and investigate dynamic transcriptional profiles in sensitive and resistant mouse and human leukaemias. Our screen shows that suppression of the PRC2 complex, contrary to effects in other contexts, promotes BET inhibitor resistance in AML. PRC2 suppression does not directly affect the regulation of Brd4-dependent transcripts, but facilitates the remodelling of regulatory pathways that restore the transcription of key targets such as Myc . Similarly, while BET inhibition triggers acute MYC repression in human leukaemias regardless of their sensitivity, resistant leukaemias are uniformly characterized by their ability to rapidly restore MYC transcription. This process involves the activation and recruitment of WNT signalling components, which compensate for the loss of BRD4 and drive resistance in vari