Abstract 1181: PTEN loss-of-function mutations prevalent in HRAS-mutant cancers results in resistance to targeted therapy
Authors: Nigam Aradhya , Chatila Walid K. , Krishnamoorthy Gnana P. , Ho Alan L. , Fagin James A. , Schultz Nikolaus D. , Untch Brian R. 
Published: June 2022

Abstract
Background: The clinical development of farnesyltransferase inhibitors (FTIs) as targeted therapy for HRAS-mutant cancers has demonstrated mixed responses dependent on cancer type. Co-occurring mutations may affect tumor response, supported by previous studies demonstrating that NF1 mutations confer resistance to HRAS inhibition by the FTI tipifarnib in thyroid cancer mouse models. We aimed to determine if PI3K pathway activating mutations altered responses to targeted therapy in HRAS-mutant cancers.
Methods: Targeted sequencing data from MSK-IMPACT cohort and DFCI-GENIE (Version 9.0) database was used to investigate co-mutations amongst HRAS-mutant cancers. Fisher’s exact test was used to determine co-altered mutations found predominantly in HRAS-mutant cancers relative to respective KRAS- and NRAS-mutant cancers. ‘RASless’ (KRASlox/HRASKO/NRASKO) mouse embryonic fibroblasts (MEFs) were obtained that in the presence of 600nM tamoxifen (4OHT) resulted in a KRAS knock-out. ‘Rasless’ MEFs were transfected with HRASG13R to create a system for testing sensitivity to FTIs in the presence or absence of WT KRAS, or with concurrent PTEN loss generated by CRISPR-Cas9 technology.
Results: A greater proportion of HRAS-mutant cancers had co-altered mutations (48.8%) in genes encoding effectors in the MAPK, PI3K or RTK pathways compared to KRAS- and NRAS-mutant cancers (41.4% and 38.4%, respectively; p&lt;0.05). PTEN mutations were more prevalent in HRAS-mutant NSCLC (21%) compared to KRAS- and NRAS-mutant NSCLC (1% and 2%, respectively; p&lt;0.05). Non-transfected MEFs were sensitized to tipifarnib by introduction of a HRASG13R allele in non-4OHT (IC50: MEF= &gt;3uM, HRASG13R = 324.7nM) and 4OHT (IC50: MEF= &gt;3uM, HRASG13R= 0.62nM; p&lt;0.001) conditions, indicating that WT KRAS confers a relative resistance to the inhibitory effects of the FTI on HRAS. PTEN loss-of-function mutations led to tipifarnib resistance in HRASG13R MEFs in the absence (IC50: &gt;3uM; p&lt;0.001) or presence of 4OHT (IC50: 213.6nM; p&lt;0.001). Combined treatment of HRASG13R/PTEN MEFs with the PIK3CB-specific inhibitor AZD8186 and tipifarnib sensitized cells in non-4OHT (IC50- 100nM:100nM Tipifarnib:AZD8186) and 4OHT (IC50- 100nM:10nM Tipifarnib:AZD8186) conditions.
Conclusions: Co-altered mutations of MAPK, PI3K or RTK effectors are found more commonly in HRAS than in KRAS or NRAS-mutant cancers. Co-alteration of PTEN preferentially associated with HRAS-mutations in NSCLC. Deletion of PTEN resulted in resistance to FTI targeted therapy in vitro. Co-altered mutations may predict sensitivity and resistance to FTIs and guide clinical trial design.
Citation Format: Aradhya Nigam, Walid K. Chatila, Gnana P. Krishnamoorthy, Alan L. Ho, James A. Fagin, Nikolaus D. Schultz, Brian R. Untch. PTEN loss-of-function mutations prevalent in HRAS-mutant cancers results in resistance to targeted therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1181.