Chronic ulcerative colitis (UC) is associated with an increased colorectal cancer risk which may be secondary to repetitive mucosal injury. Both epigenetic methylation and the classic adenoma-to-carcinoma sequence have been implicated in this malignant transformation, but the underlying molecular mechanisms remain poorly defined. This study compares the molecular characteristics of colitis-associated and common colorectal cancers.

Nineteen patients with colorectal adenocarcinomas arising within UC were matched for age and cancer site with 54 patients with sporadic adenocarcinomas. Tumor tissue was examined for BRAF mutations, CpG island methylator phenotype (CIMP), and MLH1 promoter methylation. Mutations of KRAS and p53 were assessed by sequencing.

Patient demographics were similar for the two groups. CIMP was observed in 22% of sporadic colorectal cancers and in 5% of UC cancers (P = 0.162). Rates of BRAF mutation (4% vs 5%, P = 1.0), MLH1 methylation (9% versus 5%, P = 0.682), and KRAS mutations (24% versus 32%, P = 0.552) were similar between the groups. However, colitis-associated colorectal cancers were more likely to have a p53 mutation compared to sporadic adenocarcinomas (95% versus 53%, P = 0.001). The dominant mutation for colitis-associated cancers was a mutation in codon 4, representing half of the mutations. Furthermore, colitis-associated cancers had a higher rate of mutation in codon 8 (48% versus 6%, P < 0.001) than sporadic counterparts.

Unlike other inflammatory gastrointestinal cancers, colitis-associated colorectal cancers do not preferentially arise via a methylator pathway when compared to sporadic colorectal cancers. Chromosomal instability remains an important etiology, but with a unique p53 frequency and mutation pattern. (Inflamm Bowel Dis 2010;)