Expanded KRAS and NRAS assays increase detection of mutations that predict for resistance to therapy in colorectal cancer patients.

Abstract

519 Background: The selection of targeted therapies is guided by the analysis of somatic mutations. The identification of RAS activating mutations can be used to examine tyrosine kinase inhibitor therapeutic eligibility and prognosis. Beyond known mutations in RAS exon 2 (codons 12 and 13), the identification of additional mutations in RAS exons 3 and 4 (codons 61, 117 and 146) also predict for resistance to EGFR therapy in colorectal cancer (CRC). Meta-analysis supports screening for these additional mutations in any screening strategy prior to administration of EGFR mAb therapy in metastatic CRC patients (Sorich, MJ, et al., Ann Oncol, Aug 12, 2014). Methods: We expanded our existing allele-specific KRAS and sequencing-based NRAS assays to include codons 61, 117, and 146 and analytically validated these assays to CAP/CLIA standards. DNA, from microdissected colon tumor tissue that was wild-type for RAS exon 2, was tested for exon 3 and 4 mutations and included over 15 additional mutations. Results: Forty-two (9.1%) samples were identified as bearing either an exon 3 or 4 KRAS mutation amongst 461 colon cancer specimens. Exon 4 codon 146 mutations were more prevalent than three of the commonly screened exon 2 mutations: G12A, G12R, and G12S. Five (1.8%) of samples were identified as carrying exon 3 or 4 NRAS mutations amongst 272 colon cancer specimens. This included a single codon 146 mutation in exon 4. As is seen with exon 2, RAS mutations at exons 3 and 4 were mutually exclusive of activating BRAF mutations with ~10% of patients harboring V600E. The collection of additional data studying KRAS and NRAS mutation status is currently ongoing. Conclusions: The KRAS expanded coverage contributed an additional 5.5% to overall burden of specimens bearing mutations. The NRAS expanded coverage contributed an additional 1.8% to the mutational burden. These analyses in clinical cohorts support the observations made in a trial population (Douillard JV, et al. NEJM 369:1023-34, 2013). The expanded RAS coverage identifies additional patients unlikely to respond to EGFR-targeted therapies that would otherwise have been assessed as “no mutation detected” in using assays restricted to RAS exon 2.