The 2 chief pathways implicated in colorectal carcinogenesis, microsatellite instability and chromosomal instability, are not present in 20% to 37% of cases.
To determine whether the CpG island methylator phenotype (CIMP) pathway, characterized by simultaneous methylation of several known tumor suppressor genes, is the principal underlying mechanism in cases without chromosomal or microsatellite instability, and to determine the significance of CIMP pathway and BRAF mutations in microsatellite-stable (MSS) cases.
Clinicopathologic features and chromosomal instability status by loss of heterozygosity analysis were determined in 83 cases of colorectal cancer in which microsatellite instability, CIMP status, BRAF mutations, and KRAS mutations were previously known.
Microsatellite instability was present in 14 cases (17%). Of the 69 MSS cases (83%), chromosomal instability manifested by LOH involving at least one locus was observed in 53 cases (64%). Hence, 16 (19%) of 83 colorectal cancer cases showed neither microsatellite instability nor chromosomal instability. These cases had a low incidence of CIMP (3/16; 19%) and BRAF mutation (1/16; 6%). The 5-year survival in these cases was significantly better compared with MSS cases with chromosomal instability (80% vs 54%, P = .02). BRAF mutations were identified in 10 MSS cases (15%). BRAF mutation in MSS cases correlated significantly with high-level chromosomal instability (P = .009) and poor 5-year survival (0% vs 70%, P < .001).
CIMP does not appear to play a key role in colorectal cancer without microsatellite instability and chromosomal instability. These cases have a better survival, probably related to absence of significant chromosomal instability. BRAF mutations in MSS cases are associated with high levels of chromosomal instability that are likely responsible for the adverse outcomes in these cases.