Oncogenic BRAF mutation induces DNA methylation changes in a murine model for human serrated colorectal neoplasia

Abstract

Colorectal cancer is a major cause of cancer death and approximately 20% arises within serrated polyps, which are under-recognized and poorly understood. Human serrated colorectal polyps frequently exhibit both oncogenic BRAF mutation and widespread DNA methylation changes, which are important in silencing genes restraining neoplastic progression. Here, we investigated whether in vivo induction of mutant Braf is sufficient to result in coordinated promoter methylation changes for multiple cancer-related genes. The BrafV637E mutation was induced in murine intestine on an FVB;C57BL/6J background and assessed for morphological and DNA methylation changes at multiple time points from 10 days to 14 months. Extensive intestinal hyperplasia developed by 10 days post-induction of the mutation. By 8 months, most mice had murine serrated adenomas with dysplasia and invasive cancer developed in 40% of mice by 14 months. From 5 months onwards, Braf mutant mice showed extensive, gene-specific increases in DNA methylation even in hyperplastic mucosa without lesions. This demonstrates that persistent oncogenic Braf signaling is sufficient to induce widespread DNA methylation changes. This occurs over an extended period of time, mimicking the long latency followed by rapid progression of human serrated neoplasia. This study establishes for the first time that DNA methylation arises slowly in direct response to prolonged oncogenic Braf signaling in serrated polyps; this finding has implications both for chemoprevention and for understanding the origin of DNA hypermethylation in cancer generally.

Genome Scale Epigenetic Profiling Reveals Five Distinct Subtypes of Colorectal Cancer

“…This hypothesis is supported by our recent finding that BRAF mutant sessile serrated adenomas of the colorectum rarely exhibit the classic methylator phenotype until after 50 years of age (31). This is also consistent with our animal model for serrated neoplasia where we observe a slow accumulation of DNA methylation changes over time, however these are dramatically accelerated by mutating BRAF, congruent with development of serrated neoplasia (32). This may explain why BRAF mutant sessile serrated adenomas are often identified in younger patients, despite the cancers arising from them occurring primarily in older patients (12, 33, 34).…”

Reduced aquaporin-1 transcript expression in colorectal carcinoma is associated with promoter hypermethylation

“…In melanoma, BRAF V600E mutation was associated with an increase in AQP1 expression, and increased expression was associated with decreased progression-free and overall survival [31]. In CRC, BRAF V600E mutation was associated with a high CpG island methylator phenotype (H-CIMP) [32,33]. However, we did not find a consistent association between BRAF V600E mutation and AQP1 promoter hypermethylation in CRC tissues and colon cancer cell lines.…”

Extracellular Vesicles and Epigenetic Modifications Are Hallmarks of Melanoma Progression

“…However, further mechanisms regulating the de-methylation of DNA remain to be elucidated. These speculations seem to be confirmed by recent discoveries obtained in murine models bearing the BRAF mutated colorectal cancer that proved that the persistent oncogenic BRAF signalling is sufficient to induce a progressive widespread DNA methylation [55].…”