Project Summary/Abstract Increased ribosome biogenesis is a hallmark of cancer and reflects the increased and pervasive protein synthetic needs of the cancer cells. Targeting this process by curbing the rate-limiting step, RNA polymerase I (Pol I) transcription, using specific inhibitors abrogating this activity, is a promising strategy for cancer therapy. BMH-21 is a first-in-class small molecule that inhibits Pol I transcription and large-scale cancer cell line screens demonstrated its potent efficacy across broad cancer lineages. However, a heterogeneous response was observed emphasizing the presence of mechanisms dampening the therapeutic response. Genome-wide positive selection CRISPR Cas9 knock-out screens were performed in human colorectal carcinoma cells to identify genes that cause resistance to the selective inhibition of Pol I. The screens identified all key positive regulators of the mTORC1 pathway accounting for the resistance. mTOR is a major driver of ribosome biogenesis and cellular translational programs and considered essential for cancer growth. The inactivation of mTOR as a resistance mechanism is counterintuitive, as this mechanism requires a reduced translational state by the cancer cells and inactivation of two key pathways enabling protein synthesis. These unexpected findings led to the hypothesis that cancer cells evade severe ribosome biogenesis stress by switching off mTOR-dependent translational pathways. This premise stipulates that bypass mechanisms have evolved to facilitate translation of essential proteins for survival and growth when cancer cells face obstacles in protein translation. This concept will be tested using pharmacological and genetic approaches that block mTOR. The goal of the project is to gain knowledge on the mechanisms that enable unabated ribosome translation in cancer cells. The following aims will be implemented to achieve this goal: Aim 1. Identify genes driving resistance to Pol I and translation inhibitor, Aim 2. Investigate the translational activity maintained under translation stress in cancer cells and Aim 3. Identify the regulators of ribosome activity that support survival by translation stress. By executing these aims we will gain knowledge how cancer cells survive severe translational stress and the mechanisms of this escape. These studies will identify critical processes and proteins required for survival and how their translation is maintained with limited ribosome numbers. These findings have implications in therapeutic strategies that target ribosome biogenesis, protein synthesis and translation and the approaches are designed to use this knowledge to develop new combination strategies to overcome these resistance mechanisms.

Project Narrative Targeting RNA polymerase I is a potentially effective therapeutic strategy for the treatment of many cancer types. This study investigates and identifies mechanisms of therapeutic resistance resulting from rewiring of protein translation. The alterations in translation and ribosome function will be determined to facilitate new therapeutic strategies to overcome this resistance.