The overall
KRAS mutation frequency of 39% is in line with the results from previous studies
http://www.sanger.ac.uk/genetics/CGP/cosmic, though the different mutation rates - 33% in colon cancer and 55% in rectal cancer (p = 0.02) - has not been reported in conjunction with treatment-predictive testing. Distinct molecular pathways are preferentially involved in the adenoma-carcinoma sequence in the proximal and the distal colon with
TP53 and
KRAS mutations being favored in rectal cancer, whereas mismatch repair defects are more common in proximal colon tumors [
7]. A higher mutation frequency in rectal cancers compared to colon cancer has previously been described, although the magnitude has been lower than identified herein [
8]. The effect seems to be limited to females with rectal cancer, 67% of whom harbored
KRAS mutations. This observation is in line with the findings in a large population-based series from the Netherlands in which Brink et al. reported a higher
KRAS mutation rate in female rectal cancer patients [
8]. Furthermore, in the subset of females with rectal cancer the KRAS mutant tumors were diagnosed mean 10 years earlier (mean age 48 years) behaviour than the wild-type tumors (mean age 58). Though this observation is new and needs confirmation, a recent study has demonstrated that early-onset colorectal cancers frequently show pathologic features associated with aggressive clinical features [
9]. The tumors here investigated were selected based on potential EGFR treatment, which may bias the results toward tumors prone to metastatic development, though the overall
KRAS mutation frequency of 39% and the relation between colon cancer and rectal cancer are as expected. The differences in
KRAS mutation rates identified suggest that sex and tumor location influence the chance of clinical usefulness of EGFR inhibitors with a particularly low likelihood of benefit among female rectal cancer patients.
A stepwise accumulation of mutations in different pathways constitutes a hallmark of colorectal cancer development. The observation of coexisting
KRAS mutations in a small number of colorectal cancers, however, suggests that some tumors benefit from repeated targeting of the same pathway. This observation is in line with the association between
KRAS mutations and 12p gain in colorectal adenomas [
10]. Recently, the "two-hit-hypothesis" was challenged in a study of inactivating
APC gene mutations in colorectal cancer, in which three hits - including mutations, deletions, and copy number gain - were demonstrated in a subset of the tumors [
11].
APC gene mutations, as well as
KRAS mutations, are recognized as early events in colorectal cancer development. Based on our results and previous observations, concurrent
KRAS mutations seem to occur in about 3% of colorectal cancers. The majority of these cases show coexisting mutations in codon 12, whereas a smaller number have mutations in different codons (12 and 13 or 12 and another codon) [
8,
12‐
15]. Different
KRAS mutations have shown variable efficacy in downstream signaling transduction and also seem to involve different effector molecules [
16,
17]. The specific
KRAS mutation may thereby influence different aspects of tumor progression, which could explain why repeated KRAS targeting is observed in a subset of colorectal cancer. Interestingly the p.Gly12Val mutation was found in 10 rectal cancers and 4 colon cancers, suggesting an overrepresentation in the former tumor type. This mutation has also been linked to a worse prognosis [
16]. The addition of a p.Gly12Val mutation in the invasive component of a tumor already containing dual
KRAS mutations (figure
1, case 3) suggests a role for repeated KRAS targeting during tumor progression and serves as a reminder to consider coexisting mutations when unexpected patterns occur in treatment predictive testing.