DNA methylation instability by BRAF-mediated TET silencing and lifestyle-exposure divides colon cancer pathways

Background Aberrations in DNA methylation are widespread in colon cancer (CC). Understanding origin and progression of DNA methylation aberrations is essential to develop effective preventive and therapeutic strategies. Here, we aimed to dissect CC subtype-specific methylation instability to understand underlying mechanisms and functions. Methods We have assessed genome-wide DNA methylation in the healthy normal colon mucosa (HNM), precursor lesions and CCs in a first comprehensive study to delineate epigenetic change along the process of colon carcinogenesis. Mechanistically, we used stable cell lines, genetically engineered mouse model of mutant BRAFV600Eand molecular biology analysis to establish the role of BRAFV600E-mediated-TET inhibition in CpG-island methylator phenotype (CIMP) inititation. Results We identified two distinct patterns of CpG methylation instability, determined either by age–lifestyle (CC-neutral CpGs) or genetically (CIMP-CpGs). CC-neutral-CpGs showed age-dependent hypermethylation in HNM, all precursors, and CCs, while CIMP-CpGs showed hypermethylation specifically in sessile serrated adenomas/polyps (SSA/Ps) and CIMP-CCs.BRAFV600E-mutated CCs and precursors showed a significant downregulation ofTET1andTET2DNA demethylases. Stable expression ofBRAFV600Ein nonCIMP CC cells and in a genetic mouse model was sufficient to repress TET1/TET2 and initiate hypermethylation at CIMP-CpGs, reversible byBRAFV600Einhibition.BRAFV600E-driven CIMP-CpG hypermethylation occurred at genes associated with established CC pathways, effecting functional changes otherwise achieved by genetic mutation in carcinogenesis. Conclusions Hence, while age–lifestyle-driven hypermethylation occurs generally in colon carcinogenesis,BRAFV600E-driven hypermethylation is specific for the “serrated” pathway. This knowledge will advance the use of epigenetic biomarkers to assess subgroup-specific CC risk and disease progression.