K-RAS mutation, loss of heterozygosity, and MLH1 and MGMT hypermethylation analyses were performed, as well as clinical features being scrutinized, to examine possible differences between MSI-L and MSS tumors using all of the possible cutoff levels for MSI-L.
Loss of MGMT protein expression was associated with the MSI-L phenotype but was not a prognostic factor for overall survival in colon cancer. p16 methylation was significantly less frequent in MSI-L than in MSI-H and MSS tumors and was not associated with survival.
Among the CIMP-low group, tumours with both MGMT methylation and loss were far more frequent in MSI-low tumours (67%, 12/18) than MSI-high tumours (5.6%, 1/18; p = 0.0003) and microsatellite stable (MSS) tumours (33%, 52/160; p = 0.008).
The MSI-H phenotype was related to methylation of MLH1 as expected, while the MSS phenotype was related to methylation of p16(INK4a) and O(6)-methylguanine-DNA methyltransferase, although this was not statistically significant.
Both MGMT promoter hypermethylation and negative MGMT expression were noted only in cases of mucinous or clear cell carcinoma in which MSI status were mostly MSS-phenotyped; however, no significant correlation was found between MSI status and clinicopathological parameters.