CHICAGO – Does eradicating Helicobacter pylori prevent gastric cancer?
The answer is yes, sometimes, but it depends on where you live, and what other bacteria coexist in your gut microbiome.
Dr. Richard M. Peek Jr.
The overall view is a positive one, Richard M. Peek Jr., MD, said at the at the meeting sponsored by the American Gastroenterological Association. A very large, recent meta-analysis confirms it (Gastroenterology. 2016. doi:10.1053/j.gastro.2016.01.028). Comprising 24 studies and 48,000 subjects, the meta-analysis determined that eradicating the bacteria in infected people cut gastric cancer incidence significantly.
That’s great news – but there’s a big caveat, said Dr. Peek of Vanderbilt University, Nashville. “The benefit was dependent on what your baseline risk was. For those with a high baseline risk, the benefit was tremendous. For those with a low baseline risk, it was not statistically significant.”
There are long-term data suggesting that treating H. pylori sooner rather than later is the way to go. A 2005 study followed more than 700 patients with preneoplastic gastric lesions for 12 years. It found that the treatment effect was cumulative: The longer the patient was free of H. pylori, the more reliably healing occurred (Gut. 2005. doi:10.1136/gut.2005.072009).
At baseline, the patients were randomized to nutritional supplements or to a combination of amoxicillin, metronidazole, and bismuth subsalicylate. At 6 years, the trial was unblinded and all patients were offered treatment. Patients were followed for another 6 years. Those who were H. pylori negative at 12 years had 15% more regression and 14% less progression than subjects who were positive at 12 years. Among those who received anti–H. pylori treatment at the 6-year mark, the effect was smaller and nonsignificant.
Perhaps surprisingly, though, the biggest bang for H. pylori treatment is seen in the antrum of the stomach, not in the corpus. Another meta-analysis, this one of 16 studies, found very consistent reductions in the severity of intestinal metaplasia in the antrum after antibiotic treatment – but no difference at all in corpus metaplasia. The reason for that finding isn’t at all clear, the authors of that paper noted (World J Gastro. 2014. doi:10.3748/wjg.v20.i19.5903).
The bacteria-metaplasia cancer link gets even more complicated when H. pylori is viewed as a contributing member of society, rather than a hermit. The bacterium seems to be a bully in the neighborhood, radically altering the normal gastric microbiome, Dr. Peek said.
In the absence of H. pylori, the gastric microbiome is much more diverse, consisting of about 50% Actinobacteria and 25% Firmicutes species. Bacteroides and Proteobacteria species make up the remainder, with a small population of Cyanobacteria as well. In its presence, Proteobacteria – a gram-negative genus that includes a wide variety of pathogens – almost completely subsume beneficial bacteria.
Researchers saw this change in action in 2011, when a group at the Massachusetts Institute of Technology, Cambridge, inoculated two mouse populations with H. pylori and followed them for gastric neoplasms (Gastroenterology. 2011. doi:10.1053/j.gastro.2010.09.048). All the mice were genetically engineered to overexpress human gastrin, a characteristic that invariably leads them to develop gastric cancers.
One group comprised germ-free mice raised in sterile environments. The control group was free of pathogens, but lived in a conventional environment and so had normal gastric flora. Both groups were inoculated with H. pylori.
By 11 months, the microbiome of the control group was strikingly different. It showed a significant increase in the number of Firmicutes bacteria in the stomach, with an associated decrease in the number and variety of other bacteria including Bacteroides. This was especially interesting when viewed in relation to the rate of gastric neoplasia, Dr. Peek said.
These mice are programmed to develop gastric cancer by 6 months of age – and this is what happened in the control mice, which had H. pylori plus other gastric microbes. But the germ-free mice who were monoinfected with H. pylori showed a much different progression of disease. At 7 months, most showed only a mild hypergastrinemia. Conversely, at 7 months, all of the H. pylori–infected control mice had developed gastric intraepithelial neoplasia, 80% of it high grade. Only 10% of the monoinfected mice developed cancer, and all of it was low grade.
“It looks like there is active collaboration between H. pylori and other bacteria in the stomach,” resulting in this increased cancer risk, Dr. Peek said.
It’s a collaboration that reaches deep into the tumors themselves, he said. “A very interesting study a couple of years ago searched cancer genomes for the presence of bacterial DNA, and found that gastric cancers incorporated the second-highest amount of microbial DNA into their cancer genomes. But it wasn’t just H. pylori. Many other species had integrated their DNA into these tumors.”