Investigating the role of BRAF[superscript V600E] in the serrated neoplastic pathway of colorectal cancer

Over 40 missense mutations in the BRAF gene have been identified in human cancers. Of these, 90% harbour the BRAF[superscript V600E] mutation, which results in a constitutively active protein with a 500-fold kinase activity compared to wildtype. This is the most common genetic mutation in melanoma, however it is also found in a subset of colorectal cancers, which progress through the serrated neoplastic pathway. This study was set out to investigate the role of BRAF[superscript V600E] in serrated lesions of Colorectal cancer, using a mouse model that expresses this mutation in the cells of the mouse GI tract. Expression of the Braf[superscript V600E] gene is under the control of Cre recombinase, which is controlled at the transcriptional and post-transcriptional levels. This form of regulated expression, mimics the sporadic onset of human serrated CRCs. After 3 days of Braf[superscript V600E] expression, the number of cells in the crypt increased up to ~200 cells per crypt compared to the controls. However this was not sustained and at ~1wk p.i. oncogene-induced-senescence (OIS) occurred. This was in association with elevated expression levels of p16[superscript INK4a] and p15[superscript INK4b] TSGs. At around 12wks p.i. OIS was evaded with the growth of serrated adenomas in a small number of mice. These were devoid of p16[superscript INK4a] expression. Analysis of the CpG island in exon 1α of the gene revealed elevated levels of methylation in the tumours. In support of this, levels of the DNA methyltransferases Dnmt3b and the histone methylase Ezh2 were highly expressed in the tumours. The Ezh2 is known to target the p16INKa gene locus, suggesting a role for methylation in the inactivation of this gene as a mechanism for malignant progression. These results provide strong evidence that BRAF[superscript V600E] has an oncogenic role in the early stages of serrated CRC and furthermore, as suggested in equivalent human lesions, inactivation of TSGs through targeted methylation is associated with this mutation in CRC development.