Abstract 1020: BRD9 defines a novel mammalian SWI/SNF (BAF) complex configuration which supports proliferation in AML
Authors: Michel Brittany C. , Pan Joshua , Meyers Robin M. , Grandi Paola , Humphreys Phillip G. , Garton Neil G. , Prinja Rab K. , Kadoch Cigall 
Published: July 2017

Genes encoding subunits of the mammalian SWI/SNF (BAF) ATP-dependent chromatin remodeling complexes are mutated in over 20% of human cancer. Specific subunits are mutated in specific malignancies, highlighting their tissue-specific protective roles; moreover, synthetic lethal screens have uncovered genetic- and lineage-based features which confer dependence on specific mSWI/SNF subunits. As combinatorial complexity represents a major challenge, identification of specialized mSWI/SNF configurations, subunit-specific functions, binding restrictions, and exclusivity relationships is critical for understanding oncogenic mechanisms and for the selection of appropriate therapeutic agents targeting mSWI/SNF complex subunits.

Here, we discover that BRD9, a recently identified mSWI/SNF subunit, defines a novel complex configuration distinct from BAF and PBAF, which we term non-canonical BAF, or ncBAF. We used biochemical methods to isolate BRD9-containing complexes and find that BRD9 selectively marks a sub-stoichiometric group of mSWI/SNF complexes of smaller molecular weight that lack several members of canonical BAF complexes such as BAF47 and ARID1A. Moreover, chemoproteomics using a BRD9 inhibitor (GSK) isolates only ncBAF and does not resolve BAF-specific or PBAF-specific components, including the highly related bromodomain-containing subunit BRD7. We further identified regions of BRD9 and BRD7 that confer specificity of these subunits to ncBAF and PBAF complexes, respectively, resolving their mSWI/SNF binding domains.

Using genome-wide ChIP-seq and RNA-seq experiments, we determined that ncBAF complexes target a distinct subset of all mSWI/SNF complex target genes and, consistent with previous studies, maintain proliferation of AML cells. Finally, we applied a newly- generated approach to deriving functional relationships within and between protein complex families from shRNA and CRISPR-based genetic screening datasets across hundreds of cancer cell lines to explore ncBAF-specific subunits. We find that ncBAF-specific complex subunits form a distinct functional module, supporting biochemical studies and pointing to the specific and divergent functions of the ncBAF configuration. Cancers of hematologic origin collectively exhibit the most significant responses to perturbation of three ncBAF subunits including BRD9, substantiating previous small molecule screening efforts using BRD9 bromodomain inhibitors. These data demonstrate that ncBAF complexes represent a novel BAF complex composition with distinct function in cancer.

Citation Format: Brittany C. Michel, Joshua Pan, Robin M. Meyers, Paola Grandi, Phillip G. Humphreys, Neil G. Garton, Rab K. Prinja, Cigall Kadoch. BRD9 defines a novel mammalian SWI/SNF (BAF) complex configuration which supports proliferation in AML [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1020. doi:10.1158/1538-7445.AM2017-1020