Structural mimicry of UM171 and neomorphic cancer mutants co-opts E3 ligase KBTBD4 for HDAC1/2 recruitment

Neomorphic mutations and drugs can elicit unanticipated effects that hinder mechanistic understanding for clinical practice. Recurrent indel mutations in the Kelch domain of the KBTBD4 E3 ligase rewire epigenetic programs for stemness in medulloblastoma by recruiting LSD1-CoREST-HDAC1/2 complexes as neo-substrates for ubiquitination and degradation. Remarkably, UM171, an investigational drug for haematopoietic stem cell transplantation, was found to degrade LSD1-CoREST-HDAC1/2 complexes in a wild-type KBTBD4-dependent manner, suggesting a potential common mode of action. We identified that these neomorphic interactions were mediated by the HDAC deacetylase domain. Cryo-EM studies of both wild-type and mutant KBTBD4 captured 2:1 and 2:2 KBTBD4-HDAC2 complexes at resolutions spanning 2.7 Å to 3.1 Å. The mutant and drug-induced complexes adopted similar structural assemblies requiring both Kelch domains in the KBTBD4 dimer for each HDAC2 interaction. UM171 was identified as a bona fide molecular glue binding across the ternary interface. Most strikingly, the indel mutation reshaped the same surface of KBTBD4 providing the first example of a natural mimic of a molecular glue. Together, the structures provide mechanistic understanding of neomorphic KBTBD4 and help to explain the structure-activity relationships of UM171 derivatives for future drug design.