Researchers Identify Gut Bacteria That Increases Risk Of Developing Colorectal Cancer
Source: Thailand Medical News Nov 04, 2019 5 years, 2 weeks, 4 days, 21 hours, 23 minutes ago
Researchers from University of Bristol (UK) led by Dr Kaitlin Wade, in the first study to use a technique called Mendelian randomisation to investigate the causal role played by bacteria in the development of
colorectal cancer, found evidence that the presence of an unclassified type of bacteria from a bacterial group called
Bacteroidales increased the risk of
colorectal cancer by between 2-15%.
This infers that people with this type of bacteria within their gut may have a slightly higher risk of
colorectal cancer compared to those who don't. The researchers were able to use Mendelian randomisation to understand the causal role that these bacteria may have on the disease. Their findings support previous studies that have shown that
Bacteroidales bacteria are more likely to be present, and in larger quantities, in individuals with
colorectal cancer compared to those without the disease.
The gut
microbiome is a community of microorganisms, bacteria in this case, that occur naturally in the body. There is increasing evidence that the make-up of the
microbiome plays a role in the human health and the body's susceptibility to disease. The human gut microbiome, which contains approximately three trillion bacteria, aids digestion and provides protection against infections. It is determined by a person's individual genetic makeup and their environment, so is unique to each person. It also remains relatively stable across a person's life, unless it is affected by antibiotics, an illness or a change of diet, among other things.
Dr Kaitlin Wade, in an interview with
Thailand Medical News, commented, "I was interested to see whether variation in the human gut
microbiome, like the number of bacteria or simply the numbers of different types of bacteria, can have an impact on
colorectal cancer. Lots of studies in mice and humans have shown an association between the gut
microbiome and
colorectal cancer but very few have provided convincing evidence for causality. In other words, it's really difficult to discern whether components of the gut
microbiome can cause
colorectal cancer, whether the disease itself leads to variation in the gut
microbiome or whether the association is due to some ot
her factors that cause variation in both."
The Mendelian randomisation protocol uses complex statistical analysis of data from large populations to provide evidence for cause and effect, rather than just the existence of an association.
Dr Kaitlin Wade further added, "With Mendelian randomisation, we use people's natural, randomly inherited genetic variations, which alter levels of bacteria within the gut microbiome in a way that mimics a randomised trial, to see whether people with a different genetic makeup, and therefore different gut
microbiome profiles, have a different risk of
colorectal cancer. In this way, we don't have to edit anyone's gut
microbiome directly by giving antibiotics or probiotics in a randomised trial or waste time waiting to see whether people within the population get
colorectal cancer. We just need studies that have already got this information measured."
The medical researchers used data from 3,890 people taking part in the Flemish Gut Flora Project, the German Food Chain Plus study and the PopGen study, and 120,328 people in the international Genetics and Epidemiology of
Colorectal Cancer Consortium. These studies searched for small variations in the genomes of participants that occur more frequently in people with a particular disease or characteristic than in people without that disease or characteristic - known as genome-wide association studies (GWAS).
The team found that genetic variation in the population in particular parts of the genome were linked to the presence or varying amounts of 13 types of gut bacteria, and that people with an unclassified type of bacteria from the
Bacteroidales group had a higher risk of
colorectal cancer compared to people who did not have these bacteria.
Dr Kaitlin Wade said that her findings need to be replicated by other studies using different sets of data and methods before the implications for human health could be fully understood. She said, "We need to classify the exact species or strain of bacteria in the
Bacteroidales group, and we need to do more work to understand how and why human genetic variation can alter the gut
microbiome. Even if these results show that these bacteria may cause
colorectal cancer, we don't know whether trying to alter them in an effort to reduce the risk of
colorectal cancer might have other, unforeseen effects on other aspects of health. However, I believe that we are at the forefront of understanding and appreciating the complexity of these relationships ,not only those between the human gut microbiome and disease but also between human genetic variation and the gut
microbiome itself - which is required to appropriately use these methods to appraise causalit."
Professor Ian Tomlinson, incoming Director of the Cancer Research UK Edinburgh Centre, University of Edinburgh, UK, a member of the NCRI conference scientific committee commented "The stability of the gut
microbiome is in question and there are complex relationships between the types and numbers of bacteria present, and it is therefore too early to ascribe causality to the findings reported. Nevertheless, similar larger studies have the potential for greatly improving our understanding of how
colorectal cancer develops."
Reference: 2019 NCRI Cancer Conference ,Abstract no: Poster 2532, poster board number 36, area 2. "Exploring the causal role of the human gut microbiome on colorectal cancer: application of Mendelian randomization". Exhibition Hall, Monday 4 November. 
