Abstract 7073: Integrating fragment-based crystallographic screening with dTAG-PROTAC degradation and genetic rescue to explore the function of eIF4E
Authors: Clarke Paul A. , Sharp Swee Y. , Martella Marianna , Milton Christopher I. , East Charlotte , Vinkovic Mladen , Wallis Nicola , Ward George , Richardson Caroline , Woodhead Andrew 
Published: March 2024

Abstract
The translation initiation factor eIF4E is a rate-limiting factor for protein synthesis that binds the mRNA m7G-cap to initiate the recruitment and binding of eIF4F components such as eIF4G. Targeting eIF4E has long been considered a promising anticancer strategy but it has remained undruggable using conventional screening approaches. Fragment-based crystallographic screening is a powerful technique for probing the surface of proteins to identify potentially druggable binding sites. Here, we describe fragment-based screening using a combination of Xray crystallography and NMR to identify low affinity fragment hits binding to the mRNA cap-binding site or at an additional site of unknown functional relevance (site 2). Subsequent rounds of iterative structure-based design yielded a lead molecule, with 100nM binding potency for site 2, that could disrupt the eIF4E:eIF4G interaction and inhibit cap-dependent translation in cell lysates. To explore the functional relevance of site 2 we established a cell model expressing an eIF4E-FKBP12F36V fusion protein that was sensitive to degradation following treatment with the heterobifunctional dTAGV-1 molecule. Degradation of eIF4E resulted in inhibition of cell growth and reduced expression of MCL1 protein, that were rescued by re-expression of a non-degradable wild-type eIF4E. A mutation reported to block canonical eIF4G binding (W73F) disrupted binding to eIF4G but to our surprise retained the ability to rescue the eIF4E degradation phenotype. Mutation of site 2 (L85R and L134R) also disrupted the eIF4E:eIF4G interaction. The L85R mutant remained able to partially rescue the cellular responses, in contrast to the L134R mutant that could not rescue eIF4E loss. The W73F/L85R double mutant resulted in a combinatorial loss of eIF4E function. Despite disrupting eIF4E function in lysates, there was a significant drop-off of compound activity in cells. However, treatment of the W73F mutant with compound recapitulated the combination of the W73F canonical eIF4G binding mutation with the L85R site 2 mutation. This approach demonstrates the power of coupling fragment screening with target-degradation and genetic rescue approaches to find and explore novel functional pockets. Our data suggest it may be necessary to disrupt the extended protein-protein interaction made by both the canonical and non-canonical regions of eIF4E to drive a strong functional effect in cells. However, the discovery of a lead compound, the associated structural understanding, and the knowledge that one site 2 mutation inactivates eIF4E provides hope that a significantly more potent small molecule inhibitor or degrader could drive a more profound cellular effect which may lead to improved cellular activity and a deeper understanding of eIF4E biology.
Citation Format: Paul A. Clarke, Swee Y. Sharp, Marianna Martella, Christopher I. Milton, Charlotte East, Mladen Vinkovic, Nicola Wallis, George Ward, Caroline Richardson, Andrew Woodhead. Integrating fragment-based crystallographic screening with dTAG-PROTAC degradation and genetic rescue to explore the function of eIF4E [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7073.