Abstract
The small-molecule drug BI-3802 induced the formation of BCL6 filaments leading to degradation.
Major Finding: The small-molecule drug BI-3802 induced the formation of BCL6 filaments, leading to degradation.
Concept: Cryo-electron microscopy showed how BI-3802 facilitates BCL6 dimerization and assembly into helices.
Impact: This work reveals the mechanism behind a BCL6-degrading agent that may be of use in B-cell cancers.
Targeting nonenzymatic oncogenic drivers such as BCL6, a transcriptional repressor that can promote B-cell malignancies such as diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma when mutated or dysregulated, has proven challenging. However, a recent screen for small-molecule BCL6 inhibitors revealed that BI-3802 could induce specific and potent ubiquitination and subsequent proteasomal degradation of BCL6 to a greater extent than other inhibitors or proteolysis-targeting chimeras (PROTAC). To determine the mechanism of BI-3802–mediated BCL6 degradation, Słabicki, Yoon, Koeppel, and colleagues began by establishing that BI-3802 was highly selective for BCL6 degradation and specifically depended on a region of 275 amino acid residues containing the BTB domain, which mediates BCL6′s homodimerization and interactions with corepressors. Interestingly, treatment of cells derived from DLBCL cells with BI-3802 led to the reversible formation of BCL6 foci visible using live-cell fluorescence microscopy, suggesting that BI-3802 treatment may cause assembly of supramolecular BCL6 structures. Further investigation using negative-stain electron microscopy revealed that BI-3802 treatment triggered BCL6 polymerization into regular helical structures, and cryo-electron microscopy analysis was used to determine the structure of these filaments. Inspection of this structure revealed that BI-3802 was located at the interfaces between BCL6 dimers, where BI-3802 had specific contacts to a tyrosine residue in the BTB domain of one monomer and a cysteine residue in the BTB domain of the adjacent monomer while also facilitating the formation of an arginine–glutamate salt bridge between the two monomers. Mutating the BCL6 residues important for these interactions to alanine residues prevented the formation of BI-3803–induced BCL6 foci in cells, indicating that BCL6 polymerization is necessary for the appearance of these foci. CRISPR–Cas9-mediated genome-scale genetic screens revealed that the non-cullin E3 ubiquitin ligase SIAH1 was responsible for BI-3802–mediated BCL6 ubiquitination and degradation, with BI-3802 promoting interactions between SIAH1 and BCL6 as assessed through in vitro assays and in cells. Collectively, these findings clearly demonstrate the mechanism by which BI-3802 potently and selectively induces BCL6 degradation.
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