The view of irritable bowel syndrome (IBS) as a disorder of altered interactions within the brain-gut-microbiome system is supported today by “extensive clinical, brain and microbiome-derived evidence,” Emeran A. Mayer, MD, of the University of California, Los Angeles, said at the annual Gut Microbiota for Health World Summit.
A recently published genome-wide analysis from the United Kingdom of more than 53,000 people with IBS found shared genetic pathways with mood and anxiety disorders and “is one of the most convincing studies to date [showing] that we’re not dealing with separate disorders, but that we’re dealing with the brain-gut-microbiome system,” said Dr. Mayer, director of the G. Oppenheimer Center for Neurobiology of Stress and Resilience, and codirector of the Cure: Digestive Diseases Research Center at UCLA.
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Meanwhile, multi-omics analyses from his team have established significant differences in the composition and function of the gut microbiome between IBS and healthy controls and, significantly, between IBS subgroups, he reported.
The genome analysis, published in Nature Genetics, utilized the UK Biobank, which contains genome-wide SNP genotyping data and health data for a half a million individuals. People with IBS were identified with a digestive health questionnaire that included Rome III symptom criteria.
In their cohort of 53,400 IBS cases and 433,201 healthy controls, the researchers identified 6 genetic susceptibility loci for IBS, 3 of which have previously been shown to be associated with depression, neuroticism, and other psychiatric disorders. (Significant associations were replicated in a 23andMe panel.) “The study emphasizes that GI symptoms [of IBS] and mood and anxiety disorders are two sides of the same coin,” Dr. Mayer said.
Differences in IBS subtypes
The team’s multi-omics profiles of the intestinal microbiota in IBS and its subtypes, based on bowel habits, have shown that IBS is characterized by “altered abundances of certain bacterial taxa, transcripts, and metabolites,” he said.
The research, awaiting publication, has also shown that IBS metabolites, transcripts, and transcript/gene ratios differentiate IBS with diarrhea (IBS-D) and IBS with constipation (IBS-C) with high accuracy. In addition, the IBS-D subtype is differentiated from IBS-C by “diverse functional shifts including increased polyamines, bile acids, glutamate synthesis, and ethanolamine utilization,” Dr. Mayer said.
In related multi-omics research incorporating brain imaging data, Dr. Mayer’s team has identified greater alterations in measures of brain connectivity in the IBS-D group, “just as we saw for the microbiome parameters as well,” he said at the meeting, which was sponsored by the American Gastroenterological Association and the European Society for Neurogastroenterology & Motility.
These “are all associations,” he noted. The microbiome is an established integral player in gut-brain communication, but “a casual role between the gut microbiome and IBS remains to be established.”
Although mechanistic studies are still in earlier stages, it’s clear that the microbiome is a potentially important site for therapeutic interventions. “Microbiome-targeted therapies are likely to be effective in subsets of IBS patients, based on their bowel habits, and on microbiome features,” said Dr. Mayer.