Chemoprevetion of serrated colorectal neoplasia

Abstract

Colorectal cancer is a complex, heterogeneous disease that develops via two distinct pathways: conventional or serrated. The serrated neoplasia pathway describes the histological and molecular alterations that contribute to the development of one-fifth of colorectal cancers. Initiation of this pathway occurs by activating mutation of the BRAF oncogene and accumulation of DNA methylation ensues leading to the development of sessile serrated lesions, the main precursor lesion in this pathway. High levels of microsatellite instability, as a result of methylation-induced silencing of MLH1, commonly occurs with the onset of dysplasia and drives progression to cancer due to high mutation burden. Although, this mechanism does not account for all cases of BRAF mutant colorectal cancer with a portion retaining microsatellite stability. The mechanisms through which BRAF mutant microsatellite stable colorectal cancer develops is unknown.Chapter one identifies molecular pathways altered at different points along the serrated neoplasia pathway to provide a greater understanding of the underlying mechanisms driving microsatellite stable colorectal cancer development. In order to assess serrated neoplasia from initiation to formation of cancer, a mouse model of intestine-specific oncogenic Braf was utilised. Exome and RNA sequencing was performed on various tissues that emulate different stages of cancer development (normal mucosa, hyperplastic mucosa, serrated lesions, carcinoma). Mutation and gene expression analysis highlighted the importance of aberrant activation of WNT and transforming growth factor-β signalling, which acts as an early cue directing serrated lesions to microsatellite stable carcinoma. The comprehensive understanding of the mechanisms that contribute to the development of sessile serrated lesions and BRAF mutant colorectal cancer will greatly benefit the design of targeted chemoprevention and chemotherapeutic strategies.Chemoprevention is the use of safe and inexpensive agents to reduce the risk of colorectal cancer in the population. Aspirin is a promising chemopreventive agent for the primary prevention of colorectal cancer. Consistent evidence shows a benefit in reducing the incidence of conventional colorectal cancer and mortality. It is unknown if aspirin is also beneficial in preventing colorectal cancer arising via the serrated neoplasia pathway. Chapter two shows that aspirin’s prevention of serrated neoplasia is different to conventional. We performed a preclinical study in the Braf mutant mouse model assessing how low-dose aspirin supplementation alters the risk of carcinoma. Although no reduction in the incidence of carcinoma was found with aspirin, we did find that carcinoma was less likely to present with synchronous metastasis. Comparison of differentially expressed genes from cancers arising in mice fed a control or aspirin supplemented diet found that metastasis was associated with increased expression of Hedgehog, NOTCH, fibroblast growth factor receptor and phosphoinositide- 3 kinase signalling. We hypothesise that aspirin delayed or prevented the onset of these mechanisms.Naturally-derived therapies have gained attention as novel preventive agents for cancer. Curcumin has been identified as a new promising agent for colorectal cancer, demonstrating numerous anti- cancer mechanisms. Chapter three evaluates its potential as a primary preventive agent for serrated neoplasia, which is yet to be tested. Preclinical testing in the Braf mutant mouse model found that curcumin significantly reduced the incidence of carcinoma arising from serrated lesions. This study provides reliable preclinical data to support the clinical investigation of curcumin in human chemoprevention trials for primary prevention of colorectal cancer.In summary, this thesis has defined the mechanisms that drive the evolution of serrated neoplasia. Aspirin may not protect against the 20% of colorectal cancers that arise from sessile serrated lesion. Instead aspirin reduces metastatic disease and will decrease the incidence of fatal colorectal cancer. Curcumin demonstrated effective prevention of BRAF mutant colorectal cancer. Ultimately, the effective use of chemopreventive agents will alleviate some of the clinical burden associated with managing this prevalent disease.