C29
Novel mutations in the K-Ras oncogene: implications for personalised medicine in the treatment of colorectal cancer
Gillian Smith1, Rebecca Bounds1, Helga Wolf1, Robert Steele1, Francis Carey2, Roland Wolf1
1University of Dundee, UK, 2NHS Tayside, Dundee, UK
Ras mutations are found in up to one third of human tumours. The Kirsten-Ras (K-Ras) gene is most frequently mutated in human colorectal tumours, where activating mutations have been described at codons 12, 13 and 61.
We have used a combination of WAVE denaturing HPLC analysis and direct sequencing to confirm the presence of mutations at codons 12 (21.5%) and 13 (5.6%), and to identify additional K-Ras mutations at codons 19 (<1%), 117 (<1%), 146 (6.5%) and 164 (<1%) in a cohort of 106 unselected sporadic colorectal tumours. Comparison of associated phenotypes in transformation and Ras GTPase activating assays suggests that the Leu19Phe, Lys117Asn and Ala146Thr mutations have phenotypes similar to previously described activating Ras mutations, while the Arg164Gln mutation is phenotypically equivalent to wild type K-Ras. We have used comparative genomic hybridisation (CGH) analysis to identify common chromosomal aberrations in our colorectal tumour series, and have described an amplification of chromosome 12p in a subset of tumours (Leslie et al, Cancer Res, 2003 63: 4656-4661). In order to investigate whether the K-Ras gene locus was amplified, we developed a novel real-time Taqman PCR-based gene copy number assay, with which we identified tumour-specific amplification of the K-Ras gene in 2.4% of our colorectal tumour series. K-Ras mutation status has recently been convincingly associated with response to EGFR antagonists including cetuximab (Erbitux), where response is preferentially observed in wt K-Ras tumours (e.g. Livre et al, J Clin Oncol 2008 26: 374-379; Karapetis et al, N Engl J Med 2008 359: 1757-1765).
Our data demonstrates that previous studies significantly underestimate K-Ras mutation frequency in colorectal tumours, and suggests that future K-Ras mutation screening to facilitate optimal patient selection for treatment with cetuximab and related therapies should be extended beyond previously described mutation hotspots.
