Genetic adaptations of Helicobacter pylori during gastric disease progression

Abstract

Helicobacter pylori colonizes the stomachs of approximately half of all humans. In the absence of treatment, H. pylori can persist throughout the life of the host. Although most infected individuals are asymptomatic, a significant proportion develops peptic ulcer disease, chronic atrophic gastritis (ChAG) or gastric adenocarcinoma. ChAG is considered to be a precursor to gastric cancer and is characterized by loss of acidproducing parietal cells and pepsinogen-secreting zymogenic cells, and an accompanying amplification of gastric stem cells in the gastric mucosa. The precise mechanisms by which H. pylori causes gastric cancer are unknown.

This thesis aimed to identify genetic adaptations of H. pylori to the atrophic gastric environment of hosts with ChAG during disease progression, and to examine the impact of H. pylori isolates on gastric stem cell biology. To obtain information concerning the long-term adaptation of H. pylori to an acid-free gastric environment, we examined genomic and transcriptional adaptations of the ChAG-associated strain HPAG1, which was isolated from a Swedish patient with ChAG. We found that the 1,596,366 bp HPAG1 genome is smaller than the two previously sequenced H. pylori genomes 26695 and J99 due to deletions of strain-specific plasticity zone-associated genes. Interestingly, a number of the missing genes have been shown to be acidregulated. The loss of these genes might illustrate a genomic adaptation and a streamlining of the genome size to an acid-free gastric environment where these genes are no longer needed. Whole-genome genotyping of additional ChAG-associated strains revealed that outer membrane proteins (OMPs) and genes involved in metal utilization were over-represented among ChAG-associated genes. The enrichment of these genes indicates that they play important roles in the adhesive adjustments and nutrient adaptations of H. pylori in the setting of ChAG.

Further, we assessed the diversity and structural adjustments of the surface-exposed LPS-molecule and of Lewis antigens to varying gastric conditions in individuals during disease progression and in two mouse models. We observed extensive diversity in H. pylori Lewis antigen expression both within and between individuals, however the proportions of presented Lewis epitopes appeared relatively stable over a time-period of four years. Interestingly, we noted that Lewis antigen expression differed between H. pylori isolates obtained after an experimental infection of two types of mice with distinct gastric pH. Thus, the LPS diversity within the bacterial population provides a means for H. pylori to adapt to changes in the gastric environment and enables modulation of the inflammatory response. A genome-wide comparison of deep draft assemblies of 24 H. pylori genomes collected from six patients at two time points with a four-year interval was performed. We found that genomic variations clustered by host rather than disease state, and that intra-individual strains exhibited remarkable stability over the four years. Functional genomic studies of both host and microbial responses to H. pylori infection of a mouse gastric epithelial progenitor (mGEP) cell line, disclosed host transcriptional responses enriched in genes related to cell proliferation and gastric carcinogenesis, as well as bacterial genes encoding OMPs and genes involved in metal ion binding.

Our findings have provided new insights on H. pylori´s genetic and functional adaptations to ChAG. Varying expression and composition of surfaceexposed structures as well as regulating genes involved in metal ion utilization, are associated with this disease state. The intimate association with GEP cells may be important for survival in a gastric ecosystem that lacks parietal cells. However, this dangerous liaison may also increase the risk of tumorigenesis.