We conducted this study to examine the frequency of NRAS
mutation in relation to various genetic and epigenetic alterations in colorectal cancer. The RAS
pathway plays an important role in the development of various cancers,17, 20, 43
and frequent activating mutations in the KRAS
oncogene have been identified in colorectal cancer.28
Nevertheless, there are only a few reports on NRAS
mutations in colorectal cancer and none of these studies correlated RAS
mutations with other molecular events. 40, 49
We developed a Pyrosequencing assay to detect NRAS
mutations because this methodology has been shown to be applicable to paraffin-embedded tumors and is more sensitive than Sanger dideoxy sequencing in KRAS
We found that NRAS
mutation in colorectal cancer was rare (2.2%). This observation is consistent with publicly available cancer genome sequencing data from the Sanger Institute COSMIC database (www.sanger.ac.uk/genetics/CGP/cosmic/
), which lists NRAS
mutation frequency at approximately 3% in colon cancer.
Our resource of a population-based sample of colorectal cancer (relatively unbiased samples set compared with retrospective or single-hospital-based samples) derived from two prospective cohorts has enabled us to precisely estimate the frequency of specific molecular events (such as KRAS, BRAF, PIK3CA mutations, CIMP etc.) and to correlate mutations with clinical and pathological features of colon cancer. We detected NRAS mutations in only 5 of 225 colon caners using Pyrosequencing (). Because of the low frequency, NRAS mutation was not significantly associated with any clinical or pathologic features or with patient survival. Nevertheless, there was a trend towards NRAS mutations in left-sided MSS tumors that arise in females.
Mutational activation of RAS
via a point mutation at codon 12, 13 or 61 is well characterized as a marker for progression of normal or benign cells toward malignancy.41, 42
Mutant RAS oncoproteins have decreased GTPase activity, essentially locking them into an activated state, and GTP-bound RAS transmits strong downstream signals that alter normal cellular functions.5
Nevertheless, different cancers select for mutations in different RAS
family members, suggesting that the family members exhibit cell type-specific expression patterns or functions. NRAS
is, perhaps, best characterized in leukemia and melanoma, where mutations are relatively common; NRAS
mutations occur in 30% of melanomas and are mutually exclusive with mutations in BRAF
, suggesting that these two events may be functionally equivalent. Unlike colon cancers, KRAS
mutations are rare in melanomas.
Little is known regarding the impact of NRAS
mutation in colorectal cancer. Some studies have shown that NRAS
mutations seem to arise at a later stage in the development of malignancy, unlike KRAS
mutations, which arise early.8, 49
Recent studies utilizing mouse models have demonstrated clear phenotypic differences between mutant KRAS
in colon cancer.15
has a unique ability to promote tumor proliferation and to suppress differentiation, while activated NRAS
suppressed apoptosis in a developing tumor. These data suggest that KRAS
mutations arise in response to unique selective pressures. KRAS
mutations have recently been shown to arise under conditions of low glucose availability.51
The data from animal studies suggest that NRAS
mutations might arise under conditions of chronic apoptotic stress.
Although the results of our study do not reach statistical significance, the trends in the data may provide insight into NRAS
-mutant colorectal cancers. For example, our data indicate that NRAS
mutations are found in left-sided MSS cancers. The mutational pattern is similar to that of KRAS
, but entirely distinct from BRAF
, which is mutated predominantly in right-sided CIMP-high cancers.22, 24
Thus, while NRAS
may play a similar role in melanoma progression, they appear to play distinct roles in colon cancer progression.
In conclusion, our cohort study shows that the frequency of NRAS activating mutations in colorectal cancers is low. Additional studies are needed to elucidate the mechanisms underlying the oncogenic properties of the RAS oncogenes in colorectal cancers.