Abstract 660: Modelling conventional and serrated colorectal tumorigenesis by altering the Wnt and MAPK signaling pathways

Abstract

Abstract Background. The majority of conventional colorectal adenomas are initiated by activation of the Wnt signal, whilst activation of the MAPK pathway is associated with progression to cancer. In contrast, sessile serrated adenomas are initiated by activation of the MAPK signaling pathway due to mutation of the BRAF oncogene, whilst the Wnt pathway is activated at the transition to dysplasia. We hypothesised that whilst both the Wnt and MAPK pathways play key roles in colorectal tumorigenesis, the order in which these signaling pathways are activated would determine the morphology of the lesion arising. Methods. To test this hypothesis, we utilised a conditional BRAF mutant mouse model crossed with an inducible Cre driven by the murine Villin gene promoter, to restrict Cre expression and induction of the BRAF mutation to the intestine. The MAPK pathway was activated two weeks after birth, whilst activation of the Wnt signal was achieved using APCMin mice or the intestinal carcinogen azoxymethane. Morphology and number of lesions was confirmed by histologically examination. Nuclear β-catenin immunostaining provided a surrogate for activation of the Wnt signal. Results. Induction of the BRAF mutation resulted in the rapid and sustained development of widespread intestinal hyperplasia. Murine serrated adenomas (mSA) mimicking the morphology of human traditional serrated adenomas were observed by 5 months and by 10 months 8/18 (44.4%) mice had developed mSA, primarily in the duodenum. Activation of the Wnt pathway using azoxymethane dramatically accelerated the serrated phenotype in both the small intestine (2.9 mSA per mouse, P<0.01) and colon (3.1 mSA per mouse, P<0.05) by 3 months. The MAPK and Wnt pathways also synergized in APCMin mice, increasing the average number of conventional adenomas per mouse from 2.1 to 78.8 (P<0.001). Conclusion. The rapid induction of hyperplasia and exclusive development of serrated lesions demonstrates that BRAF mutation is a key molecular event underlying initiation of serrated neoplasia. Importantly, we have demonstrated that activation of the MAPK pathway through BRAF mutation is sufficient for commitment to the serrated pathway and that whilst subsequent activation of the Wnt signal can accelerate tumorigenesis, it is not sufficient to override the existing serrated morphology. Citation Format: Vicki Whitehall, Winnie Fernando, Diane McKeone, Mark Bettington, Sally Pearson, Catherine Bond, Barbara Leggett. Modelling conventional and serrated colorectal tumorigenesis by altering the Wnt and MAPK signaling pathways. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 660.