Defining the translational landscape of MYC-driven cancer cells in response to therapeutic targeting of the ribosome

Abstract

Recent studies by our group and others have demonstrated that oncogene- driven hyper-activation of ribosome synthesis and activity is a vulnerability that can be targeted for cancer treatment. Specifically, combined inhibition of ribosome synthesis using the Pol I inhibitor CX-5461 and mTORC1-dependent mRNA translation using everolimus (EV) synergistically improved survival benefit of a MYC-driven mouse model of B-lymphoma [1]. Despite this promising outcome, a few questions remain: 1) what are the molecular mechanisms that trigger the pro-death pathways in these lymphoma cells in response to CX-5461-everolimus co-treatment (CX-5461+EV); 2) what mechanisms confer resistance to this combination therapy; 3) how can we improve the efficacy of this ribosome targeting therapy. Since both inhibitors target the ribosome, this thesis applies poly(ribo)some profiling analysis as the principal technique to address these research questions.

mRNA translation is an important step in gene expression and can be examined in a genome-wide and high-throughput manner using polysome profiling analysis. Firstly, multiple aspects of this method were optimised to enable robust characterisation of the cellular translational landscape in this MYC-driven model of B-lymphoma (Chapter 3). Polysome profiling analysis was then utilised to evaluate changes in the translatome of CX-5461+EV- treated MYC-driven B-lymphoma cells in vivo (Chapter 4). The analysis revealed that the synergistic improvement in therapeutic efficacy in response to CX-5461+EV treatment is due to selective reduction in the translation of mRNAs encoding component of translational apparatus, which is associated with decreased translation efficiency of metabolism-related mRNAs.

We have also demonstrated that the resistance to CX-5461+EV treatment was not due to the lack of on target activity of the drugs. Rather, it is due to a translation-driven activation of a metabolism-dependent, pro-survival response via the cAMP-EPAC1/2-Rap1 pathway. Importantly, the inhibition of this pro-survival pathway by the anti-diabetic drug metformin, which inhibits energy metabolism, re-sensitise the CX-5461+EV-resistant cells to CX- 5461+EV in vitro and in vivo. Together, this thesis highlights the role of translational changes in drug response and resistance development in the context of MYC-driven cancer. Furthermore, this work also identifies the potential of combined therapeutic targeting of the ribosome and metabolism for the treatment of MYC-driven haematological cancers.