T2DM has become a global health problem(Ahmad et al., 2022). Several recent studies have suggested that there may be a potential relationship between T2DM and cancer incidence or mortality(Huo et al., 2023; Suh & Kim, 2019; Zhang et al., 2021). However, the relationship between T2DM and the development of GC is unclear, and the clinical features and pathophysiology of overlapping cases of GC with T2DM are unknown(Jenkins et al., 2024). In this study, we assessed the causal relationship between T2DM and GC risk by Mendelian randomization analysis. Our MR results suggest that genetic susceptibility to T2DM is associated with an increased risk of individuals developing GC. The T2DM-associated DEG risk score can classify GC patients into high- and low-risk categories for predicting their survival. The correlations between the T2DM-associated DEG risk score and the level of tumor-infiltrating immune cells, drug sensitivity analysis, and the prediction of immunotherapy response were further validated.
There is growing evidence that T2DM is associated with the incidence of cancer in multiple organs; previous studies have suggested that T2DM may be associated with the development of bladder, breast, colorectal, endometrial, liver, and pancreatic cancers and non-Hodgkin's lymphoma, among other diseases(Georgescu et al., 2023; Jenkins et al., 2024; Sung et al., 2021). A recent large-scale prospective cohort study based on the UK Biobank revealed that T2DM and elevated blood glucose levels may be risk factors for the development of upper gastrointestinal tumors (esophageal and gastric cancers) and that individuals with a high genetic risk of T2DM have a significantly increased risk of developing upper gastrointestinal tumors (Cao et al., 2024). These results are to some extent consistent with our findings that T2DM is a risk factor for GC. Compared with the results of previous observational studies, our MR study may provide stronger evidence. Several potential confounders in observational studies, including time period, environmental exposure, and population-specific genetics, may affect the results to a greater or lesser extent. However, in MR studies, SNP loci of genes are considered natural instrumental variables because they are not altered by external environmental or behavioral factors(Davey Smith & Ebrahim, 2005). The use of SNP loci associated with exposure factors can avoid the influence of confounding factors and ensure the accuracy of the results; moreover, the application of MR controls the influence of confounding factors and reverse causation on the estimation so that reliable causal effect estimations can be obtained based on observational studies(Bastarache et al., 2022; Xu et al., 2022).
In addition to MR analysis, we also performed transcriptome analysis, a genetic validation method used to analyze the relationship between two diseases. We identified 169 DEGs associated with T2DM and GC by differential expression analysis and then used the LASSO algorithm to identify three key therapeutic targets (CST2, PSAPL1, and C4orf48). Among the three genes mentioned above, CST2 and PSAPL1 were shown to be correlated with the prognosis of patients with GC (Li et al., 2022) (Luo et al., 2023). The present study supports the above experimental findings, but a correlation between C4orf48 and GC has not been previously reported. We also constructed a risk score to predict the prognosis of patients with GC combined with T2DM based on T2DM-related DEGs and nomograms. Univariate and multivariate Cox models, KM survival analysis and ROC curve analysis further confirmed the predictive accuracy of the risk score. Recent studies have shown that ARID1A-deficient GC is a unique type of GC that is sensitive to both para-chemotherapy and anti-PD-1 immunotherapy. In this study, the rates of ARID1A mutation were greater than 10% in both the high- and low-risk groups, indicating that ARID1A may be a potential target for treating GC combined withT2DM.
An increasing number of studies have confirmed that the development of GC is closely related to the TME(Zhang et al., 2023). Our study revealed that the risk score was negatively correlated with the degrees of cellular infiltration of CD8+ T cells, CTLs and other cells, which act as tumor suppressors, and the current results of TME cellular infiltration are consistent with the results of the survival analyses of the above molecules. We then performed an analysis of immune genes and found that the expression levels of genes such as CD276 and HHLA2 were significantly elevated in the high-risk group according to the prognostic risk score model. CD276 promotes the migration of GC cells, and HHLA2 overexpression is a novel biomarker of malignant status and poor prognosis in GC, similar to the results of this study (Q. Gao et al., 2023; Mansorunov et al., 2022).
There are several flaws and shortcomings in this study. First, the database of patients with GC in our study included only Europeans. Although we compared the results of data from populations with diabetes in East Asia and Europe, and no significant differences were observed in the study outcomes and meta-analyses of the two populations, the heterogeneity of the exposure and outcome populations may still introduce an ethnographic bias. Second, T2DM is associated with an increased risk of developing GC, but the underlying mechanisms are unclear and require further study. Third, similar to other studies, individual-level data were not obtained for further exploration based on the pooled data analysis used by GWAS.