“Stomach infection with the bacterium Helicobacter pylori leads to gastric cancer, but we still have many questions about how and why that process actually happens,” stated Dr. Valerie O’Brien, a postdoctoral research fellow in the Salama Lab at Fred Hutchinson Cancer Center. “A lot of research has focused on how H. pylori is able to set up shop in the healthy stomach, but we were interested to ask how H. pylori handles the changing stomach environment during the onset of harsh pre-cancerous changes, which include influx of immune cells and altered gene expression in gastric epithelial cells.” The lab of Dr. Nina Salama, a professor in the Human Biology Division, has generated a mouse model of gastric metaplasia or altered structure and composition of the tissues lining the stomach, that develops as a precursor to gastric cancer. By utilizing this animal model infected with one of several H. pylori strains, “we found that the bug can genetically modify a particular protein called SabB, and this modification helps the bug stick better to pre-cancerous gastric tissues,” stated Dr. O’Brien. Their work characterizing how H. pylori establishes infection in gastric tissues was recently published in mBio.
In an infected person, H. pylori can be found in the mucus lining the stomach and secretes factors which activate inflammation and cause tissue metaplasia, dysplasia, and progression to gastric cancer. Despite these significant changes to gastric tissues, H. pylori infection persists. To determine how H. pylori adapts to these changes, the Salama lab investigated several H. pylori strains for their ability to colonize gastric tissue and analyzed mutations that may represent genetic adaptation to the changing environment.
The researchers initially characterized the colonization of multiple H. pylori strains in control mice or mice with genetically induced tissue metaplasia. Several of the tested strains could infect both groups equally well, suggesting no preference for the pre-cancerous tissue. They also tested H. pylori isolates from a single human patient at two different stages during infection: at initial presentation of a stomach ulcer driven by H. pylori, and again six years later when the patient returned to the clinic with worse disease, after having refused to take the prescribed antibiotics six year prior. One strain from the 6-year time point, called “C2,” colonized healthy and metaplastic tissue for half of the mice, exhibited significant variation in total bacteria abundance, and grew extremely well in some mice with metaplasia. The researchers found that individual C2 bacteria that grew well in the metaplastic stomach could be collected and used to infect new mice, where they again grew very well. Thus, this C2 strain was able to adapt to the altered tissue environment.
Because genomic diversity can enhance H. pylori persistence, the researchers analyzed the genomes of the H. pylori strains. Comparing the adapted C2 strain to the original C2 isolate, the researchers discovered mutations in sabB, a gene that may encode an outer membrane protein. Paralogs of this gene facilitate bacteria binding to transmembrane proteins on the surface of host cells. Therefore, mutations in SabB may increase bacteria tethering to the host cells lining the stomach. In support of this hypothesis, the researchers found that deletion of sabB from the adapted C2 strain significantly inhibited adherence of the bacteria to healthy and metaplastic gastric tissue. This genetic approach identified SabB as an important factor in bacterial persistence during changes to the gastric tissue environment.
Research conducted at Fred Hutch frequently utilizes the expertise from more than one lab. Dr. Salama highlights this aspect of the project saying, “This was a collaborative effort with our colleagues Chris Johnston and Dakota Jones in the Vaccine and Infectious Disease Division, and we couldn’t have done the work without the support of the outstanding Shared Resources at Fred Hutch. We did some sophisticated bacterial genetic sequencing in collaboration with the Johnston lab and the Genomics & Bioinformatics Core to characterize how H. pylori changed in the pre-cancerous stomach environment, which led us to SabB. We also did some beautiful imaging in the Cellular Imaging Shared Resource to show H. pylori interactions with pre-cancerous tissue. Experimental Histopathology and Comparative Medicine were also essential to this work.” The collaborative work and critical resources at Fred Hutch enabled this important discovery.
Mr. Jacob Frick, a graduate student in the Salama Lab, shared the lab’s future directions in which he will be a key element. “We are very interested in learning more about SabB. Right now, we think it’s an adhesin, a protein that lets the H. pylori stick to a tissue surface, which means it could be a potential drug target. I have started the process of expressing, purifying and characterizing SabB so we can learn more about its role in gastric infection. We also identified other H. pylori genes that change during the onset of severe gastric disease and are excited to pursue those leads as well.”
The spotlighted research was funded by an Innovation Grant from the PAM-IRC at Fred Hutchinson Cancer Center, the National Institutes of Health, Cancer Research Institute, Debbie’s Dream Foundation–AACR Gastric Cancer Research, and the M. J. Murdock Charitable Trust.
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium members Christopher Johnston and Nina Salama contributed to this work.
O'Brien VP, Jackson LK, Frick JP, Rodriguez Martinez AE, Jones DS, Johnston CD, Salama NR. Helicobacter pylori Chronic Infection Selects for Effective Colonizers of Metaplastic Glands. mBio. 2023 Jan 4:e0311622. Online ahead of print.