In a two-sample MR analysis, we assessed the causal association between T2DM and digestive system diseases. Our research revealed that T2DM was inversely associated with GC. Further research was conducted to explore the causal association between the risk factors and T2DM, and we hoped to find the reason why MR analysis achieved a result different from past observational studies.
T2DM is an endocrine disease that is caused by a combination of genetic and environmental factors [17]. It is reported as a risk factor for cardiovascular disease, chronic kidney disease, and neuropathy [18–20]. The association between T2DM and digestive system diseases was not clarified. T2DM could lead to multiorgan damage; naturally, we considered that it is positively correlated with GC. Prior observational studies have indeed obtained similar results [21]. On the other hand, GC is a multifactorial disease caused by multiple genetic and environmental factors, and they both share common risk factors (e.g., high BMI, physical activity, smoking, alcohol consumption, unhealthy diet) [22–24]. Observational studies may be biased by confounding and reverse causality [25], thereby weakening the reliability of findings. Exploring the relationship between T2DM and GC relying upon the results of observational studies seems inappropriate. Our research considered T2DM as an IV to analyze the association by applying the MR-IVW method. The results showed that the two were inversely correlated (OR, 0.10; 95% CI, 0.01–0.69; P = .02) after the deletion of the discrete value (rs10830963). Analysis of heterogeneity and horizontal pleiotropy indicated that no heterogeneity (P = .09) or horizontal pleiotropy (P = .99) was observed in our research. To ensure the stability of the experiment, IVs from different data sources were included, and a meta-analysis was conducted to observe the combined effect, which indicated a similar correlation between the two. This result differed from previous observational studies that pointed out that insulin resistance, hyperglycemia, and hyperinsulinemia caused by T2DM are closely related to the development of GC [3]. Similarly, some observational studies pointed out that both type 1 diabetes mellitus (T1DM) and T2DM have a higher risk of GC [4]. A reverse MR analysis was conducted to assess the causal relationship between GC and T2DM, and the outcome revealed no obvious correlation (OR, 1.00 (1.00–1.00), P = .65). Theoretically, MR analysis could derive a more reliable result owing to not being affected by confounding factors [26]. Our research would further explore why observational studies ended up with opposite conclusions. It is reported that T2DM could promote the occurrence of GC through different mechanisms [3, 27, 28]. The potential links between T2DM and GC may involve interactions with shared risk factors, HP [29] or other pathogen infections [30], medication intake [31], and complications [3, 32]. Therefore, we took the shared risk factors, HP, and part of the complications into our research.
Furthermore, parts of the risk factors for GC that occurred before the incidence of T2DM would affect T2DM events. We found that high BMI, physical activity, smoking, and alcohol consumption could increase the incidence of T2DM. There was no statistical significance observed in high salt intake and low vegetable and fruit intake; in other words, high BMI, physical activity, smoking, and alcohol consumption could affect the outcomes of observational studies as potential confounding factors. Previous studies have indicated that gastric cancer risk increases for men with a BMI ≥ 27 kg/m2 [33]. It was reported that physical activity is a protective factor for GC development [34]. The potential mechanism is probably the suppression of tumor growth and immunologic control of tumors [35]. Our research showed that smoking increases gastric cancer, which was similar to a previous observational study. A possible mechanism was that smoking could aggravate HP infections by weakening the surveillance and defensive functions of the immune system and increasing the failure rate of H. pylori eradication treatment [36, 37]. In terms of alcohol drinking, previous studies revealed that chronic alcohol drinking could combine HP infection with IL-10 (interleukin-10) inhibition to induce gastric tumorigenesis [38]. Additionally, alcohol consumption could lead to direct cellular injury and gene mutation by enhancing the penetration of carcinogens into cells [39].
We searched the GWAS to find SNPs that met the criteria to predict the factors. As shown in our results, anti-HP IgG and anti-EBV IgG were proven to be positively affected by T2DM. This means that T2DM could increase the occurrence by improving the rate of HP infection and EBV infection. HP infection is categorized as a class I carcinogen for the development of GC [40]. It was reported that HP could drive p53 degradation and induce genome instability to accelerate gastric carcinogenesis [41, 42]. In regard to EBV infection, EBV could promote immune escape through the enhancer-mediated transcription of programmed cell death 1 ligand 1 (PD-L1) and promote the progression of GC via Sam68-dependent activation of methyltransferase-like 3 (METTL3) [43, 44].
In regard to T2DM-related complications and comorbidities, the results revealed that no complications or comorbidities were significantly associated with the incidence of GC.
Nevertheless, this study also has several limitations. First, cases from GWAS were mostly European, so the results might not be applicable across races. Second, the horizontal pleiotropic effect was unavoidable in all MR studies. Third, MR outcomes were obtained by using some genetic variants in most situations to minimize the effect of weak instruments and LD and inevitably lower the potency of the test. Fourth, different types of GC might require a separate discussion because they might share different risk factors. Fifth, some of the risk factors could not match data sources that are compliant with the criteria, and more research needs to be conducted to explore the association with other risk factors.