Identification and Characterisation of the Cellular and Molecular Changes in Intestinal Metaplasia Associated with Progression to Gastric Cancer

Abstract

Gastric cancer (GC) patients are mainly asymptomatic and present at an advanced stage with a 5-year survival rate of only 20-30% in most countries. It is crucial to gain greater insight into the process of gastric carcinogenesis in order to develop tools that will allow improved patient stratification and targeted surveillance.

The relationship between intestinal metaplasia (IM), a key premalignant lesion in gastric carcinogenesis, and the diffuse type of gastric cancer (DGC) was investigated using data mining of GC patient pathology reports and existing gene expression data. IM and DGC were shown to be associated both histologically and molecularly in a proportion of cases suggesting that IM might be a precursor lesion to a subset of DGC cases.

Complete and incomplete IM subtypes, the latter being associated with a greater risk of progression, were characterised at the molecular level using gene expression data acquired from macro-dissected IM tissue. Complete IM was associated with genes highly expressed in the small intestine whereas incomplete IM was associated with genes expressed both in the colon and in GC suggesting it is molecularly closer to a state of malignancy.

Using OPAL multiplex immunohistochemistry, the macrophage and T cell landscape in IM was investigated. In addition to traditional “M1-like” (IRF8) and “M2-like” (CD163, CD206) macrophage populations, a novel hybrid subgroup “M1/2” containing macrophages expressing both M1 and M2 markers in different combinations was identified. Overall, complete IM was characterised by M2 macrophages and greater levels of T cell infiltration whereas incomplete IM was characterised by M1/2 macrophages and fewer CD8 and double negative (DN) T cells. Spatial cell analysis showed significantly fewer CD8 and DN T cells in the vicinity of incomplete IM epithelial cells suggesting reduced immune-surveillance may play a key role in progression to dysplasia.

IM subtyping is affected by intra-observer and inter-observer variation. To address this, the potential of CD10 and Das1 as biomarkers for subtyping complete and incomplete IM was investigated. Overall CD10 was shown to be an outstanding biomarker for complete IM and Das1 was shown to have potential as an additional risk associated biomarker.

GC animal models are associated with long duration and high cost. Optimised protocols for growing and characterising gastrointestinal organoids from gastroscopy biopsies were developed. IM organoid cultures were successfully established and characterised. Additionally a human gastrointestinal organoid biobank was created.

In conclusion this thesis offers potential evidence of IM as a precursor lesion to DGC, characterises the molecular differences between complete and incomplete IM and shows key differences in the innate and adaptive immune system between IM subtypes and how these may play a role in progression to GC. It also identifies two biomarkers with potential clinical utility for subtyping IM and describes the methodology for growing IM organoids with future potential as a model for studying gastric carcinogenesis. Finally future studies are required to gain further insight into the cellular and molecular evolution of gastric carcinogenesis which should lead to better management of patients with premalignant lesions and ultimately to the prevention of GC.