BET inhibitor resistance emerges from leukaemia stem cells

BET inhibitors that target bromodomain chromatin readers such as BRD4 are being explored as potential therapeutics in cancer; here, in a MLL–AF9 mouse leukaemia model, resistance to BET inhibitors is shown to emerge from leukaemia stem cells, and be partly due to increased Wnt/β-catenin signalling. 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. Bromodomain and extra terminal protein (BET) inhibitors are first-in-class targeted therapies that deliver a new therapeutic opportunity by directly targeting bromodomain proteins that bind acetylated chromatin marks 1 , 2 . Early clinical trials have shown promise, especially in acute myeloid leukaemia 3 , and therefore the evaluation of resistance mechanisms is crucial to optimize the clinical efficacy of these drugs. Here we use primary mouse haematopoietic stem and progenitor cells immortalized with the fusion protein MLL–AF9 to generate several single-cell clones that demonstrate resistance, in vitro and in vivo , to the prototypical BET inhibitor, I-BET. Resistance to I-BET confers cross-resistance to chemically distinct BET inhibitors such as JQ1, as well as resistance to genetic knockdown of BET proteins. Resistance is not mediated through increased drug efflux or metabolism, but is shown to emerge from leukaemia stem cells both ex vivo and in vivo . Chromatin-bound BRD4 is globally reduced in resistant cells, whereas the expression of key target genes such as Myc remains unaltered, highlighting the existence of alternative mechanisms to regulate transcription. We demonstrate that resistance to BET inhibitors, in human and mouse leukaemia cells, is in part a consequence of increased Wnt/β-catenin signalling, and negative regulation of this pathway results in restoration of sensitivity to I-BET in vitro and in vivo