Abstract 1146: Targeted degradation of the enhancer lysine acetyltransferases CBP and p300

The chromatin regulators CREB-binding protein (CBP) and p300 are factors critical for establishing and activating enhancer-mediated transcription. These large proteins are made up of at least ten distinct functional domains and are known to interact with over 400 other proteins including DNA-binding transcription factors, other chromatin regulators, and components of the general transcription apparatus. In addition to mediating protein-protein interactions on chromatin, p300/CBP exert enzymatic activity through a lysine acetyltransferase (KAT) domain. p300/CBP can dynamically acetylate ~21,000 lysines on over 5,000 proteins including more than three dozen known enhancer-associated regulators. In cancer, p300/CBP have been implicated as both oncogenes and tumor suppressors in a large number of malignant cell types and active drug development projects by a number of groups have focused on the discovery of potent and selective compounds. Small molecules that target the p300/CBP bromodomain and KAT domains have provided a unique toolset for chromatin and chemical biologists to probe various p300/CBP functions at enhancers. However, it is clear that simultaneous inhibition of multiple functional domains, or even complete protein depletion, is required to ablate the enhancer activity mediated by p300/CBP. To pursue this goal further and to provide the research community with additional tools to probe the function of this important class of chromatin regulators, we have designed and characterized the first heterobifunctional chemical degrader of p300/CBP, dCBP-1. Bifunctional, or chimeric, degraders that leverage the activity of E3 ligases such as cereblon (CRBN) through chemical-induced proximity to proteins of interest have transformed the capabilities for chemical degradation. Aided by in silico modeling of a ternary complex of ligand-bound CRBN with the KAT and bromodomains of p300/CBP, we were able to efficiently synthesize a highly potent and selective degrader, dCBP-1. Using dCBP-1 as a tool, we profile the consequences of complete p300/CBP loss in cellular models of multiple myeloma, a tumor type uniquely addicted to the activity of both p300 and CBP. Proteomics profiling reveals selective effects on p300 and CBP, with loss of oncogenic MYC expression. Epigenomic profiles of gene expression and chromatin structures with dCBP-1 treatment reveals augmented effects of oncogene expression compared to inhibitors alone or in combination, with complete ablation