CRISPR Screen Reveals BRD2/4 Molecular Glue-like Degrader via Recruitment of DCAF16

Molecular glues (MGs) are monovalent
small molecules
that induce
an interaction between proteins (native or non-native partners) by
altering the protein–protein interaction (PPI) interface toward
a higher-affinity state. Enhancing the PPI between a protein and E3
ubiquitin ligase can lead to degradation of the partnering protein.
Over the past decade, retrospective studies of clinical drugs identified
that immunomodulatory drugs (e.g., thalidomide and analogues) and
indisulam exhibit a molecular glue effect by driving the interaction
between non-native substrates to CRBN and DCAF15 ligases, respectively.
Ensuing reports of phenotypic screens focused on MG discovery have
suggested that these molecules may be more common than initially anticipated.
However, prospective discovery of MGs remains challenging. Thus, expanding
the repertoire of MGs will enhance our understanding of principles
for prospective design. Herein, we report the results of a CRISPR/Cas9
knockout screen of over 1000 ligases and ubiquitin proteasome system
components in a BRD4 degradation assay with a JQ1-based monovalent
degrader, compound 1a. We identified DCAF16, a substrate
recognition component of the Cul4 ligase complex, as essential for
compound activity, and we demonstrate that compound 1a drives the interaction between DCAF16 and BRD2/4 to promote target
degradation. Taken together, our data suggest that compound 1a functions as an MG degrader between BRD2/4 and DCAF16 and
provides a foundation for further mechanistic dissection to advance
prospective MG discovery.