The identification of the cellular targets of small molecules with anticancer activity is crucial to their further development
as drug candidates. Here, we present the application of a large-scale RNA interference–based short hairpin RNA
(shRNA) barcode screen to gain insight in the mechanism of action of nutlin-3. Nutlin-3 is a small-molecule inhibitor of MDM2, which can activate the p53 pathway. Nutlin-3 shows strong antitumor effects in mice, with surprisingly few side effects on normal tissues1. Aside from p53, we here identify
53BP1 as a critical mediator of nutlin-3–induced cytotoxicity. 53BP1 is part of a signaling network induced by DNA damage
that is frequently activated in cancer but not in healthy tissues. Our results suggest that nutlin-3's tumor specificity
may result from its ability to turn a cancer cell–specific property (activated DNA damage signaling) into a weakness that can be exploited therapeutically.