Early stages of gastric metaplasia: molecular profiling

Gastric cancer precursors initiate as a clonal patch, expand by natural drift, and are characterised by a private set of somatic mutations.

Dr William Waddingham (University College London, United Kingdom) started by stating that intestinal metaplasia (IM) can be easily identified by histopathological identification of goblet cells and expression of markers such as CDX2. However, he warned, “the gap in our understanding of the pre-tumour progression is substantial” [1].

Even though the infection rate of H. pylori is extensive, the rate of progression, even at advance stages of chronic gastritis, is less than 1 in 200 per year. The current study aimed to answer three questions: (1) How is gastric IM initiated? (2) How do metaplastic foci spread and expand?; and (3) What are the molecular differences between patches of IM (diversity)? To address these questions, the researchers developed a technique of harvesting a strip of submucosal tissue along the greater curvature of the stomach from distal to proximal and embedding the tissue en face, to recreate the birds-eye view of the endoscopist. This technique was performed on tissue from cancer patients (n=16) and samples from patients undergoing bariatric surgery as non-cancer controls (n=16).

The researchers noted that there is a patchwork of foci of precursor lesions. In the control stomachs, there were individual glands of IM evident from histopathological analysis. Using serial thin sectioning of individual metaplastic glands and staining for CDX2, the researchers could identify that IM initiates from a single stem cell and is therefore clonal in nature. To explore the expansion of the IM, the researchers scanned for glands where only part of the gland is characterised by IM, and using quantitative pathology, they calculated how the IM expands and contracts. This result was balanced around 0, to determine if the IM was outcompeting healthy neighbouring parenchyma; in other words, metaplasia reaches fixation by chance, and is natural drift. Quantifying the size of the IM patches was used as a surrogate for clonal expansion. Plotting these data using CDX2 versus cytochrome C oxidase (as a neutral control marker), the researchers concluded that IM displays increased clonal fitness and that inflammation is a primary constraint of clonal expansion.

Thirdly, using laser-capture microdissection of IM clones, the researchers performed whole exome sequencing to capture DNA mutations and copy-number changes, and to reconstruct phylogenetic trees. Each patch had a number of private somatic mutations, confirming their clonal origin, and confirming that a large number of mutations occur prior to becoming cancer, calculated about 2-3 mutations per year per patient. Each patch shares very few mutations with their neighbour; each is genetically distinct. There was also an increase in copy-number changes in the metaplastic tissue compared with healthy surrounding tissue. Some preliminary work in the epigenetic changes associated with metaplastic tissue suggests that there are broader similarities between patches at the epigenetic level, which drive the phenotypic changes. For example, almost all of the IM patches showed increased methylation at the promoter of the CDX2 gene.

In summary, intestinal metaplasia in the stomach is derived from a single cell, and each IM patch is genetically distinct, although there may be some epigenetic overlap. Clonal diversity may identify individuals at higher risk for gastric cancer. This study supports the combination of high imaging endoscopy with quantification of the clonal density within a stomach, followed by targeted therapeutic approach based on molecular risk profiling.

1. Waddingham W et al. UEG Week 2019, Abstract OP002.

Posted on 23 October, 201918 March, 2021