PI3K/mTOR activity sensitizes cancer cells to nucleolar stress

Drugs targeting nucleoli and generating nucleolar stress (NS) such as RNA Polymerase I inhibitors have shown anticancer properties with some progressing to clinical trials. However, the mechanisms that modulate the sensitivity to NS remain poorly understood, which has limited the design of clinical trials on patients most likely responding to these therapies. To get a panoramic view of the genetic determinants that shape the response to NS in cancer cells, we conducted genome-wide CRISPR screens in cells treated with 2 independent RNA Polymerase I inhibitors: Actinomycin D (ActD) and BMH-21. As expected, mutations known to regulate the response to NS such as P53 or RB1 increased the resistance to both drugs. On the other hand, the toxicity of RNA Pol I inhibitors was increased in the context of mutations that enhance PI3K/mTOR signaling. Surprisingly, we found mutations that sensitized to ActD but increased the resistance to BMH-21, revealing that, while both drugs are used as RNA Pol I inhibitors, the must have additional unknown targets. Together our study provides a global landscape of the mechanisms that modulate the sensitivity to NS-inducing agents and illustrate the need for an in-depth analysis of the mechanism of toxicity of drugs, particularly when these are advancing to clinical use.