Abstract IA05: Dimerization is critical for the functions of wild-type and mutant KRAS
Authors: Ambrogio Chiara 
Published: June 2021

Abstract
Background: Mutations in KRAS are among the most frequent RAS alterations in human cancers and the prevalent driver event in lung adenocarcinoma (LUAD). There are no effective targeted therapies for KRAS-driven LUAD and chemotherapy remains the standard of care. Small-molecule inhibitors of the MAPK pathway, one of the prominent downstream KRAS mediators, show minimal clinical activity either as single agents or in combination with chemotherapy. Recent data demonstrated that loss of wild-type KRAS enhances tumor fitness in KRAS mutant cancer cells while concomitantly increasing sensitivity to MEK inhibition. We hypothesized that dimerization/oligomerization between KRAS proteins could be a key regulator for lung adenocarcinoma biology and determinant of treatment response.
Methods: To study the role of KRAS dimerization, we selected and characterized a dimerization-impaired mutant KRAS (KRASD154Q). KRAS-driven murine and human LUAD cell lines lacking the wild-type KRAS allele were reconstituted with either KRASWT or KRASD154Q to evaluate their impact on tumor progression and response to MEK inhibition. In parallel, we generated an inducible system to force either wild-type/mutant or mutant/mutant KRAS dimerization by taking advantage of the rapamycin-analog mediated heterodimerization of FK506 binding protein (FKBP) and FKBP-rapamycin binding domain (FRB). The rapamycin-analog AP21967 induces FKBP/FRB dimerization, hence allowing a tunable control of wild-type/mutant versus mutant/mutant KRAS dimers formation.
Results: We demonstrated, using FRET (Fluorescence Resonance Energy Transfer) assays, that KRASD154Q mutant disrupts KRAS dimerization without affecting membrane localization or GTPase activity. Introduction of KRASWT into LUAD cells that solely express mutant KRAS decreased their growth both in vitro and in vivo. In the presence of KRASWT both the IC50 and drug concentration required to inhibit pERK1/2 by MEK inhibitors increased in vivo and in vitro. These findings were not observed in the presence of the dimerization-impaired wild-type KRAS protein (KRASD154Q). Similarly, forced dimerization between wild-type/mutant KRAS resulted in impaired cell growth as compared to forced mutant/mutant KRAS dimerization. Interestingly, introduction of the D154Q in cis with an oncogenic KRAS mutation (G12C or G12D), in the absence of the wild-type KRAS allele led to a significant reduction of growth both in vitro and in vivo without impacting GTPase activity. Mechanistically, impaired mutant KRAS dimerization resulted in inefficient CRAF/BRAF heterodimers formation.
Conclusions: Dimerization of wild-type KRAS with mutant KRAS results in growth inhibition and changes the therapeutic index for MEK inhibitors. Mutant-mutant KRAS dimerization is critical for the full oncogenic properties of mutant KRAS. Collectively, these observations suggest that strategies designed to interfere with KRAS dimerization should be evaluated as a therapeutic approach in KRAS-mutant cancers.
Citation Format: Chiara Ambrogio. Dimerization is critical for the functions of wild-type and mutant KRAS [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr IA05.