Reprogramming the E3 ubiquitin ligase CRL4CRBN to degrade Helios

The zinc finger transcription factor Helios is critical for maintaining the identity and suppressive phenotype of regulatory T cells (Tregs) and as such is an attractive target to enhance the efficacy of currently approved immunotherapies. However, no small molecules exist which can directly modulate Helios activity or abundance. Here, we report the structure-guided development of novel small molecules capable of inducing neo-interactions between Helios and the E3 ubiquitin ligase substrate adaptor Cereblon (CRBN), thereby promoting Helios degradation. Crystallographic studies reveal that these new compounds accommodate a key histidine residue in the β-hairpin loop of its second zinc finger domain. Finally, pharmacological Helios degradation destabilized the anergic phenotype of regulatory T cells, demonstrating that Helios is a new druggable target that may enhance anti-tumor immunity. Our study provides a road-map for developing small molecule degraders of other difficult-to-drug targets through ligase reprogramming that is complementary to the bi-valent proteolysis targeting chimera (PROTACs) approach.