LAUSR

Introduction KRAS is one of the most frequently mutated oncogenes in cancer and KRAS mutations are commonly associated with resistance to therapy and poor prognosis. KRAS is still not directly druggable, therefore current therapeutic strategies for KRAS mutant cancers aim at identifying susceptibilities in downstream signalling pathways. One unresolved aspect of KRAS biology with potential to translate into patient stratification criteria is the difference between distinct KRAS activating mutations in terms of downstream signalling and drug sensitivity. Understanding the biochemical and biological differences among specific KRAS mutants is essential to discover new actionable vulnerabilities for mutant KRAS. Material and methods To study the role of different KRAS mutants in a controlled and reliable genetic setting, we developed an isogenic KRASMUT inducible system that lacks endogenous HRas/NRas and harbours conditional CREERT2-controlled KRaslox alleles (KRaslox KRASMUT system). This system allows direct and robust comparison between different KRAS oncogenic isoforms and rigorous evaluation of the in vitro and in vivo impact on tumour progression and response to MEK/ERK inhibition. Results and discussions Our data confirm differential GTP-hydrolysis properties among different KRAS mutants (G12C, G12D, G12V, G12A, G13D and Q61H). This intrinsic feature is translated into distinct proliferation rates in vitro and in vivo, as well as differential responsiveness to MEK/ERK inhibitors in specific KRaslox KRASMUT cell lines, with up to 12-fold IC50 variability. Interestingly, the Q61H mutant, known to exhibit the lowest intrinsic GTP-hydrolysis rates, is the most sensitive to MEK/ERK inhibition, suggesting that intrinsic biochemical properties of specific KRAS mutants affect drug response. Moreover, in our KRaslox KRASMUT cell lines expressing specific KRAS mutants, we observed differential reactivation of upstream RTKs in response to MAPK inhibition, suggesting the existence of a functional crosstalk between specific KRAS mutations and upstream receptors that may be modulated by oncogenic signalling, with potential implications on drug resistance mechanisms. Conclusion KRAS specific mutants retain unique GTP-hydrolysis features KRAS specific mutants have distinct growth propertied in vitro and in vivo KRAS specific mutants show differential responsiveness to MEK/ERK inhibitors in vitro and in vivo.