Investigation of Crosstalk Between GSK3 and the MAPK Pathway in V600EBRAF-Driven Colorectal Cancer

Colorectal cancer (CRC) is the third most prevalent cancer in advanced countries which mostly falls within the traditional class involving APC mutation. However, a recently classified V600EBRAF-driven class, sessile serrated CRCs (SSCRCs) was discovered to develop in the proximal colon. SSCRCs often evade detection because of their location and flat pale appearance. They have poor prognosis and are resistant to BRAF inhibition – necessitating the development of novel therapies for this devastating disease affecting~3000 people in the UK each year. A greater understanding of disease initiation and progression is needed for this purpose. A conditional, Cre-regulated, V600EBraf (BrafLSL-V600E/WT; Villin-CreERT (BVE/Cre)) knockin mouse model was therefore characterized. We show that initiation and progression of tumour development, from hyperplastic crypts to serrated adenomas and carcinomas, is similar to that reported in humans. A 3 day V600EBRaf expression results in the formation of hyperplastic crypts, which is surprisingly not associated with activation of the downstream MAPK pathway, and is not reversed by treating mice with MEK inhibitors. Instead, gene expression profiling shows upregulation of negative regulators of the MAPK pathway, a subset of Wnt pathway target genes, and genes involved in cholesterol biosynthesis and glycolysis, possibly accounting for the effect on crypt hyperplasia. GSK3 phosphorylation role in V600EBRAF effects were investigated by intercrossing BVE mice with Gsk3a/b S9A/S21A mice. The combined mutant showed increased early-stage crypt elongation – suggesting GSK3 phosphorylation suppresses V600EBRAF-driven tumour initiation. Consistently, these mice demonstrated increased tumour burden at later stages, with no differences in tumour stage. Gene expression profiling shows this suppressive effect of GSK3 phosphorylation is most likely due to suppression of a subset of MAPK target genes. Altogether, these results provide novel insight into the role of V600EBraf at early stages of serrated CRC neoplasia.