Sensitivity and engineered resistance of myeloid leukemia cells to BRD9 inhibition

Here we show that acute myeloid leukemia (AML) cells require the BRD9 subunit of the SWI/SNF chromatin remodeling complex to sustain MYC transcription, rapid cell proliferation, and a block in differentiation. Based on these observations, we derived small-molecule inhibitors of the BRD9 bromodomain, which selectively suppressed the proliferation of mouse and human AML cell lines. To establish these effects as on-target, we engineered a bromodomain-swap allele of BRD9, which retains functionality despite a radically altered bromodomain pocket. Expression of this allele in AML cells conferred resistance to the anti-proliferative effects of our compound series, thus establishing BRD9 as the relevant cellular target. Furthermore, we used an analogous domain-swap strategy to generate an inhibitor-resistant allele of EZH2. Our study provides the first evidence for a role of BRD9 in cancer and reveals a simple genetic strategy for constructing resistance alleles to demonstrate on-target activity of chemical probes in cells.