Intracellular protein degradation of BRD4 by dBET1 reveals conserved in vivo cereblon function in human and mouse T-cells

Protein degraders have been developed that can aide in understanding gene function. These chemicals contain complementary bifunctional components that 1) interact with specific proteins, and 2) engage an E3 ligase which leads to polyubiquitination and intracellular degradation by the proteasome. Thalidomide, infamously known for its teratogenic effects, is central to this new technology since it binds cereblon (CRBN) which promiscuously recruits the DDB1/Cul4A/Rbx1 complex to trigger target destruction. Thalidomide, and especially its immunomodulatory derivatives lenalidomide and pomalidomide, induce NK and T-cell activation by degrading IKZF1 which is a transcription factor that suppresses IL-2. Currently, it is debated whether mouse cereblon has conserved function due to a single non-conserved amino acid in mouse CRBN, Ile390 (equivalent to Val388 in human CRBN). First, our work confirms that lenalidomide induces IL-2 and the ubiquitin-dependent degradation of IKZF1 only in human and not mouse T-cells treated with lenalidomide. To further test for species-related effects, a series of theoretical and physical binding assays using mutant CRBN proteins show that the binding of IMiD compounds is not impaired by amino acid differences in mouse CRBN. Using dBET1, a novel thalidomide-JQ1 protein degrader, we show for the first time that mouse CRBN can trigger CRBN-dependent degradation of BRD4 and confirm that the CRBN/DDB1/Cul4A/Rbx1 complex is functional in mouse T-cells. Collectively, our results suggest that IKZF1 has divergent regulation in mouse cells rather than the IMiD complex and provide information relevant to the development of chemical conjugates that induce targeted intracellular protein degradation.