Prof Martin Bushell - RNA and Translational Control in Cancer

Introduction

Bushell

The Bushell laboratory's work focuses on the interplay between two key mRNA regulatory hubs, the eIF4F and CCR4-NOT complexes, which control the fate and expression of mRNA and are fundamentally dysregulated and a hallmark of cancer. These complexes act as master switches, ensuring that the necessary proteins are made to support cell growth and survival. However, when dysregulated, these factors can drive malignant programmes. We are examining how these complexes are dysregulated and how this dysregulation mechanistically results in the production of oncogenic gene expression programmes. We use an array of approaches, spanning from reconstitution of purified recombinant components with enzymology to advanced bespoke next-generation sequencing methods, coupled with bioinformatics analysis and machine learning approaches, to cover control mechanisms and information embedded within the oncogenic expression landscapes.

eIF4F complex: Driving Translation and Oncogenic Programmes

The eIF4F complex is the central regulator of translation initiation, functioning at the 5' end of the mRNA. It is composed of the cap-binding protein eIF4E, the scaffold protein eIF4G, and the RNA helicase eIF4A1. Its primary role is to unwind secondary structures in the mRNA's 5'-untranslated region (UTR), enabling the ribosome to associate and begin ribosomal scanning to find the translation start codon. Dysregulated eIF4F activity is a known driver of malignant transformation, since it lies at the nexus of multiple oncogenic signalling cascades. Oncogenes like c-Myc are especially dependent on eIF4F because their 5'-UTR often has highly structured RNA elements that under healthy conditions act to repress translation, limiting their oncogenic potential. We are dissecting how the eIF4F complex is driving these oncogenic gene expression programmes, focusing on understanding why oncogenic mRNAs are specifically commanded by this complex in the oncogenic setting. Additionally, we are exploring how different subunits and associated components are involved in delivering the different oncogenic activities of this complex.

CCR4-NOT complex: Sensing the mRNA Code and Directing Fate

The CCR4-NOT complex controls mRNA turnover and stability and is recruited by miRNAs and an array of RNA binding proteins that act to initiate mRNA decay through its associated deadenylases. It is a large complex with many associated factors, several of which are known to function in both translational repression and mRNA decay. It has been known for many years that its function in mRNA decay is critically intertwined with protein synthesis, but how has remained elusive. We and others have recently found that this complex senses the speed of ribosome decoding through the CNOT3 subunit, which physically interacts with the ribosome only when it is moving slowly across the mRNA. Ribosome decoding speed is intimately connected to the availability of amino acid charged tRNA. Thus, through the CCR4-NOT complex, decoding speed dictated by available tRNAs is coupled to mRNA decay. However, our data suggest that this level of regulation is much more complicated and is critical to allow correct and functional proteome establishment at many levels. We are investigating the role of this CCR4-NOT complex in cancer, where we know tRNA deployment can alter and change the amount of protein and the decoding rate at which it is made. How does it control and sculpt the oncogenic gene expression landscape? How do changes in protein production change protein functionality?

The Therapeutic Opportunity

These opposing and distinct functions of eIF4F and CCR4-NOT in controlling gene expression programmes open opportunities for therapeutic targeting. We are working to understand the molecular details that determine which mRNAs are controlled by which complex, enabling the rational design of compounds that can selectively interfere with the aberrant oncogenic programmes driven by these two powerful master regulators at different stages of cancer development.