BRD4-mediated transactivation activity in AML depends upon its interaction with NSD3-short.
Major finding: BRD4-mediated transactivation activity in AML depends upon its interaction with NSD3-short.
Mechanism: NSD3-short links BRD4 and CHD8 at active promoters and super-enhancer regions in AML.
Impact: BET inhibitors act by removing BRD4–NSD3–CHD8 complexes from chromatin to suppress transcription.
Members of the bromodomain and extraterminal (BET) family of transcriptional coactivators, which bind to acetyl-lysine motifs on nuclear proteins via tandem bromodomain interaction modules, have been implicated as therapeutic targets in different cancers. The BET family member bromodomain-containing 4 (BRD4) is necessary for the expression of driver oncogenes such as MYC in hematologic cancers and thus is a therapeutic target, particularly in acute myeloid leukemia (AML). To identify the effector proteins required for BRD4-mediated transcription, Shen and colleagues investigated the interaction between BRD4 and two splice isoforms of the histone H3 lysine 36 methyltransferase nuclear SET domain–containing protein 3 (NSD3, encoded by WHSC1L1), NSD3-long and NSD3-short, in AML. Immunoprecipitation experiments revealed that BRD4 interacts with both NSD3 isoforms, but knockdown and rescue experiments showed that only NSD3-short was essential for AML proliferation. NSD3-short, which lacks lysine methyltransferase activity, interacted with the BRD4 ET domain at residues required for BRD4-mediated transcriptional activation, indicating that NSD3-short acts as an adaptor for BRD4. Consistent with this finding, immunoprecipitation and mutagenesis studies showed that BRD4 and the chromatin remodeling ATPase chromodomain helicase DNA-binding protein 8 (CHD8) both interacted with NSD3-short, which was necessary for the association of CHD8 with the BRD4 ET domain. NSD3-short contained four independent interaction surfaces that were required for AML proliferation: an N-terminal acidic transactivation domain, the BRD4-interacting region, a PWWP chromatin reader module, and the CHD8-interacting region. Similar to BRD4 inhibition, knockdown of NSD3-short or CHD8 induced differentiation of AML cells and loss of BRD4-mediated gene expression. Furthermore, chromatin immunoprecipitation experiments revealed that BRD4, NSD3, and CHD8 colocalized across the AML genome and BRD4 inhibition induced NSD3 and CHD8 release from super-enhancer regions. Together, these results elucidate the roles of NSD3-short and CHD8 in BRD4-mediated transcriptional activation and provide a rationale for therapeutically targeting NSD3-short and CHD8.
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- ©2016 American Association for Cancer Research.