Abstract 1897: RAS signaling pathway gene mutations and acquired resistance to EGFR tyrosine-kinase inhibitors in EGFR mutant lung cancer

Abstract

Abstract EGFR mutant lung cancers are highly sensitive to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs), but in the metastatic setting, acquired drug resistance is inevitable. Second-site EGFR mutations, MET amplification, PIK3CA mutations, and histologic changes (transformation to small cell lung cancer, epithelial-mesenchymal transition (EMT)) have been reported as resistance mechanisms. Up to 40% of cases are still mechanistically unexplained. Surprisingly, despite the importance of the RAS signaling pathway downstream of EGFR, no RAS pathway signaling gene mutations have been reported to occur in acquired resistance. Here, we investigated further the significance of this pathway in preclinical models and tumor specimens. First, we screened 21 lung tumors from 11 mice with acquired resistance to erlotinib (Politi et al. Dis Model Mech. 2010; 3(1-2): 111-9) and found one tumor with a KrasG12V mutation. Next, we established 6 new cell-line based models of acquired erlotinib resistance in 5 different drug-sensitive EGFR mutant cell lines. While 5 lines developed known mechanisms of resistance (i.e. EGFR T790M, MET amplification, EMT), 1 (11-18 cells) developed de novo a NRASQ61K mutation. Single cell cloning suggested that both the EGFR mutation (L858R) and NRASQ61K mutations were in the same cell. Consistent with these findings, NRAS activity was higher in 11-18 resistant cells (11-18R) compared to parental cells. In growth inhibition assays, 11-18R were sensitive only to combined EGFR inhibition with siRNA knockdown of NRAS or with MEK inhibitors. Expression of NRASQ61K in drug-sensitive PC-9 cells conferred resistance to erlotinib. Finally, we screened 127 tumor samples from patients with acquired resistance patients to EGFR-TKIs. No NRAS or KRAS mutations were found, but one patient's tumor harbored in addition to EGFR L858R both EGFRT790M and BRAFV600E mutations. BRAFV600E expression also conferred resistance to EGFR inhibition, both in PC-9 (EGFR exon 19 deletion) and PC-9R cells (EGFR exon 19 deletion plus T790M). Collectively, these data shown that mutations in RAS pathway signaling genes may be infrequent but occur and could influence therapeutic outcomes in trials for patients with acquired resistance to EGFR TKIs. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1897. doi:1538-7445.AM2012-1897