The aim of this study was to conduct an MR study to investigate whether the gut microbiota is causally associated with gastric cancer. In the largest GWAS meta-analysis of pooled data on the gut microbiota to date, conducted by the MiBioGen consortium, we identified strong associated genetic variants. Based on comprehensive genetic data from over 200,000 individuals, the relative abundance of several gut microbes was found to be causally associated with gastric cancer, including the protective role of six gut microbes such as Eubacterium (brachy group) and four gut microbes such as Clostridium sensu stricto1 as potential risk factors. These results may have implications for public health interventions aimed at reducing the risk of gastric cancer.
Previous studies have shown that the gut microbiota can contribute to tumor induction and development through multiple mechanisms (31–33). Because the gut microbiota and gastrointestinal cancers share the same ecosystem, more attention has been devoted to speculating on the potential relationship that exists between them. Although chronic H. pylori infection has been recognized as an important risk factor for GC. However, less than 5% of people infected with H. pylori develop stomach cancer. The imbalance of intestinal flora also plays a complex role in the development of gastric cancer (34). Microbial dysbiosis contributes to gastrointestinal cancer susceptibility through multiple pathways (6). Nevertheless, the evidence obtained from previous conventional observational studies on the gut microbial composition of carcinogenesis remains unconvincing due to the intricate interactions between the gut flora and the human host and the interference of environmental factors such as diet, drugs, and disease (35, 36). By using large-scale GWAS statistics, we made a causal inference on the relationship between gut microbiota and gastric cancer, hoping to validate the conclusions of previous studies and reveal new findings.
Through MR analysis, a group of gut microbes that can produce short-chain fatty acids (SCFAs) was found to reduce the risk of gastric cancer, including Eubacterium (brachy group) and Roseburia. This group of gut microbes is a class of probiotics that can break down dietary fiber and complex carbohydrates that cannot be digested by human enzymes into SCFAs, which play a key role in regulating dietary fiber (37–39). SCFAs include butyrate, propionate, and acetate, and factors such as diet, age, and health status can affect their concentrations and ratios (40). Thus, they regulate the cell cycle, apoptosis and immune response (41). Several studies have confirmed the important role of butyrate in humans as the only known short-chain fatty acid with anticancer activity (41, 42). By inhibiting the activation of the NF-κB signaling pathway and promoting the differentiation of IL10-producing T cells and regulatory T cells, butyrate has the potential to impede carcinogenesis and inflammation (43).
It is reported that SCFAs are able to activate various cellular mechanisms, mainly related to the prevention of carcinogenesis. SCFAs helps to regulate histone deacetylases (HDACs), thereby affecting cell attachment, immune cell migration, cytokine production, chemotaxis, and programmed cell death (44). In addition, SCFAs interact with the metabolism-sensing G protein-coupled receptors GPR41, GPR43 and GPR109A expressed in intestinal epithelial cells and immune cells to mediate protective effects in inhibiting intestinal inflammation and carcinogenicity (45). Maik Luu et al. also found that SCFAs valerate and butyrate enhance the antitumor activity of cytotoxic T lymphocytes through metabolic and epigenetic reprogramming (46). Sodium butyrate treatment also leads to acetylation of p53 and induces p21 (CDKN1A), thereby inhibiting the activity of cyclin-dependent kinase 2 (CDK2) in G1/S phase, resulting in cell cycle arrest at G1 (47). These findings suggest the anticancer effects of SCFAs and a potential therapeutic role for SCFAs-producing bacteria in tumor cellular immunotherapy .
We also found that Clostridium sensu stricto1 has a promotive effect on GC. This may be related to the involvement of Clostridium sensu stricto1 in the degradation and recycling of complex organic compounds and the production of beneficial metabolites (48, 49). However, a retrospective study of cancer patients showed that Clostridium sensu stricto1 bacteraemia had an increased rate of infection in oncology patients and was common in gastrointestinal lesions (50). In addition, MR results also showed that Alloprevotella and Peptococcus have a facilitative effect on GC. Previous studies have shown that Alloprevotella is significantly more abundant in the oral cavity of patients with oral squamous cell carcinoma (51). Another study noted that dysbiosis of multiple flora, including Alloprevotella, was negatively correlated with interleukin-10 (IL − 10) levels (52). Based on these findings, we hypothesized that Alloprevotella promotes tumor progression through the induction of chronic systemic inflammation. In the study of Jorne Ubachs, Peptococcus in the intestine of patients with cachectic cancer showed differential abundance, which also consisted with our MR results (53).
While examining the relationship between the gut microbiota and the presence of cancer, it is important to consider and account for confounding factors that may influence the association, including H. pylori, diet and lifestyle. H. pylori is a recognized risk factor of GC and has been reported to interact with intestinal microbiota in the development of GC (34). Long-term colonization of H. pylori will interact with other microbiota and affect the structure of intestinal microbiota (34). This H. pylori-induced alteration of the intestinal microbiome appears to alter the ecological imbalance of the gastric mucosa and lead to the development of severe gastrointestinal diseases, including GC, by inducing a sustained and prolonged inflammatory response (54, 55). Diet plays an important role in shaping the composition of the gut microbiota. Studies suggest that excessive consumption of saturated fats and added sugars may favor the growth of less desirable microbial species and may lead to inflammation, metabolic disturbances, and imbalances in the gut microbiota (56, 57). In addition, lifestyle factors such as smoking, alcohol consumption and physical activity levels can also contribute to disorders of the gut microbiota (58).
In this study, we determined the causal relationship between gut flora and GC by MR analysis, thus excluding confounding factors and reversing causality on causal inference. Genetic variation in the gut microbiota was obtained by the maximum available GWAS meta-analysis, ensuring the strength of the instrumentation in the MR analysis. A two-sample MR design was used with non-overlapping exposure and outcome summary level data to avoid bias. MR-PRESSO and MR-Egger regression intercept terms were used to test for detection and exclusion level pleiotropy.
However, this study also has some limitations that should be noted when interpreting the results. Because summary statistics rather than raw data were used in the analysis, it was not possible to perform subgroup analyses or explore nonlinear relationships. And, some of the GWAS summary data used in our study were taken from other races, which may lead to bias in the estimates and affect generalizability. Since the lowest taxonomic level in the exposure dataset was genus, this limitation prevented us from further exploring the causal relationship between gut microbiota and GC at the species level. If GWAS had used more advanced metagenomic sequencing analysis, the results would have been more specific and accurate. For sensitivity analysis and horizontal pleiotropy testing, more genetic variants need to be included as instrumental variables. Therefore, the SNPs used in the analysis did not reach the traditional GWAS significance threshold (P < 5 × 10− 8).