Activating K-Ras mutations outwith ‘hotspot’ codons in sporadic colorectal tumours – implications for personalised cancer medicine

Abstract

Response to EGFR-targeted therapies in colorectal cancer patients has been convincingly associated with Kirsten-Ras (K-Ras) mutation status. Current mandatory mutation testing for patient selection is limited to the K-Ras ‘hotspot’ codons 12 and 13. Colorectal tumours (n=106) were screened for additional K-Ras mutations, phenotypes compared in transformation and Ras GTPase activating assays and gene and pathway changes induced by individual K-Ras mutants identified by microarray analysis. Taqman-based gene copy number and FISH analyses were used to investigate K-Ras gene amplification. Four additional K-Ras mutations (Leu₁₉Phe (1 out of 106 tumours), Lys₁₁₇Asn (1 out of 106), Ala₁₄₆Thr (7 out of 106) and Arg₁₆₄Gln (1 out of 106)) were identified. Lys₁₁₇Asn and Ala₁₄₆Thr had phenotypes similar to the hotspot mutations, whereas Leu₁₉Phe had an attenuated phenotype and the Arg₁₆₄Gln mutation was phenotypically equivalent to wt K-Ras. We additionally identified a new K-Ras gene amplification event, present in approximately 2% of tumours. The identification of mutations outwith previously described hotspot codons increases the K-Ras mutation burden in colorectal tumours by one-third. Future mutation screening to facilitate optimal patient selection for treatment with EGFR-targeted therapies should therefore be extended to codon 146, and in addition should consider the unique molecular signatures associated with individual K-Ras mutations.