Determining the role of Helicobacter pylori and the microbiota during gastric cancer progression

Giddings, Harriet Jayne (2025). Determining the role of Helicobacter pylori and the microbiota during gastric cancer progression. University of Birmingham. Ph.D.

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Abstract

The global burden of gastric cancer (GC) is increasing and approximately 89% of non-cardia GC cases are linked to Helicobacter pylori infection. H. pylori-induced gastric adenocarcinoma (GAC) is typically preceded by a progression of histological changes within the gastric mucosa, which begins with chronic gastritis (CG) and advances to gastric intestinal metaplasia (GIM). Sequencing studies have also shown that the gastric microbiota undergoes significant alterations during carcinogenesis, with H. pylori gradually becoming displaced by non-H. pylori bacteria. However, studying H. pylori infection in vitro presents several challenges and sequencing data often lack spatial information regarding bacterial distribution and location. Therefore, although H. pylori genotypes encoding the virulence factors VacA, CagA and HtrA are associated with increased risk of GAC, the precise role of H. pylori and the gastric microbiota during carcinogenesis remains unclear. The aim of this study was to explore the relationship between H. pylori and the gastric microbiota across the early, middle, and late stages of gastric carcinogenesis. Patient-derived organoid monolayers were used to study the vacuolating effect of H. pylori VacA in vitro. Whilst VacA induced prominent vacuolation in the AGS cell line, vacuolation was not observed in monolayers exposed to different VacA allelic forms, raising questions over the significance of vacuolation during infection. To further understand the relationship between H. pylori and the gastric microbiota in carcinogenesis, multiplex imaging using RNAscope in situ hybridisation (ISH) combined with immunohistochemistry (IHC) of CG, GIM, and GAC histological gastric tissue was conducted. Although E-cadherin and MUC5AC expression remained relatively stable in CG, MUC2 expression was elevated in both GIM and GAC. Additionally, a significant correlation was found between H. pylori presence and invasion of non-H. pylori bacteria into the lamina propria. In order to identify these bacteria, tissue regions containing invasive bacteria were isolated using laser capture microdissection (LCM) followed by full-length 16S ribosomal rRNA sequencing. Although multiple non-H. pylori bacterial genera were identified in CG, GIM and GAC samples, low sample biomass and amplicon contamination posed a significant challenge, highlighting the need for improved methods in future studies. Finally, a modified Gram stain was validated as a rapid, cost-effective tool for identification of non-H. pylori bacteria in histological samples, which confirmed previous findings and holds promise as a surveillance tool for GAC. In conclusion, in addition to H. pylori, the gastric microbiota likely also contribute to gastric carcinogenesis and future studies should therefore focus on confirming the identity and functional role of these bacteria.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):