Cellular Resistance Mechanisms to Targeted Protein Degradation Converge Toward Impairment of the Engaged Ubiquitin Transfer Pathway.

Proteolysis targeting chimeras are bifunctional small molecules capable of recruiting a target protein of interest to an E3 ubiquitin ligase that facilitates target ubiquitination followed by proteasome-mediated degradation. The first molecules acting on this novel therapeutic paradigm have just entered clinical testing. Here, by using Bromodomain Containing 4 (BRD4) degraders engaging cereblon and Von Hippel-Lindau E3 ligases, we investigated key determinants of resistance to this new mode of action. A loss-of-function screen for genes required for BRD4 degradation revealed strong dependence on the E2 and E3 ubiquitin ligases as well as for members of the COP9 signalosome complex for both cereblon- and Von Hippel-Lindau-engaging BRD4 degraders. Cancer cell lines raised to resist BRD4 degraders manifested a degrader-specific mechanism of resistance, resulting from the loss of components of the ubiquitin proteasome system. In addition, degrader profiling in a cancer cell line panel revealed a differential pattern of activity of Von Hippel-Lindau- and cereblon-based degraders, highlighting the need for the identification of degradation-predictive biomarkers enabling effective patient stratification.