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Colorectal cancer (CRC) can be classified as high-level microsatellite instability (MSI-H), low-level MSI (MSI-L) and microsatellite stable (MSS) depending on levels of MSI. MSI-H CRC relies on a distinct molecular pathway due to the mismatch repair (MMR) deficiency and shows methylation in multiple gene promoters. The genetic pathway leading to MSI-L is unknown, although higher levels of promoter methylation are observed in this group compared with MSS CRCs. This study explored how promoter methylation affects MSI phenotype, by analysing the methylation status of eight CRC-related promoters, MSI phenotype and KRAS/BRAFmutations in a series of 234 CRCs. Promoter methylation of p14ARFwas significantly related to MSI-L CRC with KRASmutation. The MSI-H phenotype was related to methylation of MLH1as expected, while the MSS phenotype was related to methylation of p16INK4aand O6-methylguanine-DNA methyltransferase, although this was not statistically significant. Thus, promoter methylation of p14ARFcould be a significant alteration leading to CRC with MSI-L.

Codon 12 and 13 mutations in 170 colorectal cancer (CRC) and 66 gastric cancer (GC) specimens were analysed by an ‘enriched’ polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) method. All identified mutations were verified by direct sequencing of the second PCR products. Among the 170 CRC specimens, mutations were identified in 47 (28%) and 13 (7.6%) cases in codons 12 and 13, respectively. In the 66 GC specimens examined, however, mutations in codons 12 and 13 were only detected in two (3.0%) and one (1.5%) cases, respectively. Mutations in both codon 12 and 13 were found in 3/170 (1.8%) CRCs and 1/66 (1.5%) GCs. Duplicate mutations were never identified in the same allele, which was confirmed by direct sequencing of the second amplified products. The majority of colorectal and gastric cancer cells with KRASmutations are homogeneous because they have the same KRASmutation. A few colorectal or gastric cancers, however, showed heterogeneity, as verified by the fact that single mutations were identified in the same allele.