BACKGROUND: The serrated colorectal neoplasia pathway describes the progression of morphologically serrated polyps to cancer and accounts for approximately one third of all colorectal cancer cases. Human serrated polyps are characterised by activating mutation of the BRAF oncogene and widespread DNA methylation changes termed the CpG Island Methylator Phenotype. A causative versus synergistic relationship between BRAF mutation and this methylator phenotype has not been determined. We aimed to address this by developing a murine model for serrated neoplasia driven by BRAF mutation.
METHODS: BrafV637E conditionally active mice were crossed with intestine-specific, inducible Villin-CreERT2 mice to direct the BRAF mutation to the intestine at 2 weeks of age. The proximal ilieum or proximal colon were sampled at defined time points including 10 days, 10 weeks, 5 months, 8 months, 10 months, 12 months and 14 months. Macroscopic lesions larger than 10mm were bisected for molecular and histological assessment. The entire remaining intestine was fixed and examined histologically. DNA methylation was investigated for 94 genes known to by methylated in colorectal cancer using Epitect MethylII Complete PCR Arrays (Qiagen).
RESULTS: Braf mutant mice displayed histologic changes analogous to the human serrated neoplasia pathway. Extensive intestinal hyperplasia developed by 10 days post induction of the BRAF mutation. By 10 weeks, 50% mice had developed areas of crypt dilation reminiscent of human sessile serrated adenomas. By 8 months, the majority of mice had murine serrated adenomas with dysplasia and invasive cancer developed in 40% of mice by 14 months. Compared to age-matched control mice, Braf mutant mice showed significant, gene-specific increases in DNA methylation from 5 months (p<0.0001).
CONCLUSIONS: Using an in vivo model we observed the temporal accumulation of DNA methylation changes in hyperplastic epithelium in direct response to mutation of the BRAF oncogene. This murine model morphologically and molecularly recapitulates the human serrated neoplasia pathway and establishes a causative role for BRAF mutation in establishing a methylator phenotype.