ASXL3 bridges BRD4 to BAP1 complex and governs enhancer activity in small cell lung cancer

The bromodomain and extra-terminal (BET) protein BRD4 functions as a transcriptional activator and is a therapeutic target for different human cancers. Here, we report a critical link between Polycomb repressive deubiquitinase-BAP1 (PR-DUB) complex and BRD4, which is bridged by the physical interaction between additional sex combs-like protein 3 (ASXL3) in small cell lung cancer (SCLC). We further showed that ASXL3 functions as an adaptor protein, which directly interacts with BRD4-extra-terminal (ET) domain via a novel BRD4 binding motif (BBM), and maintains chromatin occupancy of BRD4 at active enhancers. Genetic depletion of ASXL3 leads to a genome-wide reduction of histone H3K27Ac levels as well as BRD4 occupancy, resulting in a similar change of gene expression profile induced by BET inhibitors. Pharmacologically-induced inhibition with BET specific chemical degrader (dBET6) selectively inhibits cell proliferation of a subtype of SCLC, characterized with high expression of ASXL3. Collectively, this study provides mechanistic insight into the oncogenic function of ASXL3-BRD4 axis in SCLC. Overall design: To determine gene expression and histone modification change in ASXL3 depleted cells