K-ras mutations in patients with early colorectal cancers

Abstract

Background: Published data are contradictory about the importance of K-ras mutations in advanced tumours and are not available for early cancers.

Aims: To establish whether specific K-ras mutations are prognostic markers in early stage colorectal adenocarcinoma.

Methods: The presence of K-ras exon 1 mutations were correlated with tumour recurrence in two groups of patients: group 1 was a consecutive series of patients with resected colorectal adenocarcinoma at low risk of recurrence; group 2 were patients referred for chemotherapy after relapse of previously resected early stage tumours. K-ras mutations were detected by direct sequencing of whole tissue samples in all patients and in some, the leading edge and centre of the tumour were also microdissected out individually and sequenced.

Results: Mutations were present in 26 (26.5%) of 98 patients in group 1; 14 patients developed a recurrence, four (28.5%) of whom had a K-ras mutation. Seventy nine patients have not developed tumour recurrence, 22 (28%) of whom had a mutation (p = 0.84). K-ras mutations were present in five of 14 patients in group 2. Microdissection did not increase the number of mutations detected.

Conclusions: Individual K-ras genotypes are distributed homogeneously throughout early stage colorectal adenocarcinomas, but detection of a mutation has no apparent prognostic value.

Figure 1

: Polyacrylamide sequencing gel using the downstream primer showing serial dilution of wild type K-ras DNA with DNA containing a heterozygous codon 13 guanine to adenine mutation. The arrow indicates the level of an additional band which can be seen in the lanes marked "T" when 5% or more of the DNA contained the codon 13 mutation. This extra band is not seen when 100% wild type DNA is sequenced. Wild type DNA was extracted from HT29 cells and mutated DNA from LoVo cells.

Figure 2

: Polyacrylamide sequencing gel showing K-ras exon 1 gene sequence from patient 42. The DNA has been sequenced using both the upstream and downstream primers. An arrow indicates a G to C mutation at position 2 of codon 12 giving an amino acid change from glycine to alanine.

Figure 3

: Toluene blue stained 15 µm paraffin wax section of a Dukes' stage A, Astler-Coller's modification B1 colorectal adenocarcinoma with the leading edge (1) and the core (2) of the tumour removed by microdissection.