Low somatic K- ras mutation frequency in colorectal cancer diagnosed under the age of 45 years

Somatic mutation of K- ras is known to be a common event in colorectal cancer tumourigenesis however its association with age at onset has not been widely explored. In this study, we have analyzed tumours from a population-based study of colorectal cancer diagnosed before the age of 45 years, in which cases had been previously screened for germ-line mismatch repair gene mutations and for microsatellite instability. We used a micro-dissection and sequencing approach to search for somatic K- ras mutations in codons 12, 13 and 61 in 101 early-onset colorectal cancers. Six (6%) somatic K- ras mutations were detected; five in codon 12 (4 G > T transitions and 1 G > A) and one in codon 13 (G > A transition). All codon 12 mutations were identified in microsatellite stable tumours and the codon 13 mutation was identified in a MSI-high tumour. Four cases with K- ras mutations had no reported family history of colorectal cancer and two had some family history of colorectal cancer. None were known to carry a germ-line mutation in hMSH2, hMLH1, hMSH6 or hPMS2. The role of somatic K- ras mutations in early-onset colorectal cancer carcinogenesis appears to be minor, in contrast to its significant role in colorectal cancer of later age of onset.