This study employed a two-sample MR approach to systematically investigate the causal relationship between serum UA levels and digestive system cancers (including esophageal, gastric, hepatic, pancreatic, and colorectal cancers). None of the different MR analysis methods and the meta-analysis using the IVW method supported the susceptibility of serum UA levels to GI cancers risks.
Our findings are consistent with some previous studies and can be explained from the perspective of the pathogenic mechanism. Ames et al. proposed that UA can clear singlet oxygen and hydroxyl radicals, protecting red cell membranes from oxidative damage, reducing the oxidants produced by hemoglobin reacting with peroxides, thus avoiding aging and cancer caused by oxidants and free radicals(Ames et al. 1981). This viewpoint has been supported by other studies(Becker 1993; Song et al. 2019). Additionally, Itahana et al. proposed that the UA transporter SLC2A9 is a direct target gene of the tumor suppressor p53. By transporting UA as a source of antioxidants, it participates in antioxidant defense, reduces the generation of ROS, prevents DNA damage and cell death, and may thus contribute to suppressing cancer development(Itahana et al. 2015). Recent experimental results in hepatocellular carcinoma further confirmed this conclusion, demonstrating that SLC2A9 could be a novel tumor suppressor gene and a potential target for treating liver cancer(Han et al. 2019).
As of now, a unanimous conclusion has not been reached regarding whether UA can increase or decrease the risk of digestive system cancers. Observational studies frequently yield inconsistent results. For instance, in a recent cohort study with an average follow-up of 6.6 years, involving 444,000 participants without a history of cancer and 920 cancer patients. This study found that in men, serum UA levels exhibited a U-shaped relationship with the risk of liver cancer, while in women, serum UA levels were positively correlated with the risk of pancreatic cancer(Huang et al. 2020). Another cohort study, which included 8,408 newly diagnosed gout patients and 25,010 controls, found a significant positive correlation between gout and colorectal, liver, and stomach cancers but no significant association with esophageal and pancreatic cancers(Chen et al. 2014). Boffetta et al. reported an increased risk of colon, liver, and pancreatic cancer in patients with gout(Boffetta et al. 2009). However, another cohort study, which included 256 patients with colorectal cancer, failed to find a significant association between serum UA levels and colorectal cancer(Kühn et al. 2017). Observational studies are susceptible to reverse causality and confounding factors, making them prone to bias.
In the past, there have been inconsistent results in MR studies regarding the relationship between serum UA levels and cancers. Kobylecki et al. used two epidemiological designs, namely observational studies and MR studies based on the SLC2A9 rs7442295 genotype, to assess the potential causal effect of high serum UA levels on cancer incidence and overall mortality. They found that high serum UA levels were associated with high cancer incidence and overall mortality in both observational and genetic aspects, which contradicts our conclusions(Kobylecki et al. 2017). However, this difference may be due to the fact that they only used one genetic variant (SLC2A9 rs7442295) as an IV for serum UA levels and did not fully consider other genetic factors influencing serum UA levels. Furthermore, SLC2A9 has varying expression levels in different organs, but the study did not perform MR analysis for specific organ cancers(Kobylecki et al. 2017). Another study conducted by Jiang et al. investigated the relationship between serum UA levels and eight specific cancers, including bladder cancer, breast cancer, colorectal cancer, lung cancer, prostate cancer, renal cancer, skin cancer, and thyroid cancer. Consistent with our research findings, this study did not find a causal relationship between serum UA levels and colorectal cancer(Jiang et al. 2021). However, it is worth noting that this study only used data from the UK Biobank, and individuals in this database are generally healthier than the general population, so selection bias cannot be ruled out. In addition, the study did not cover other GI cancers(Jiang et al. 2021).
Compared to previous studies, our research is the first to utilize a Meta-analysis approach in MR to investigate the causal relationship between serum UA levels and specific GI cancers. Our study has several advantages. Firstly, we extensively leveraged GWAS data from multiple large genetic consortia, including thousands of patients and hundreds of thousands of control individuals, making our MR study one of the most comprehensive and largest-scale investigations into the association between serum UA levels and GI cancer risk. Secondly, we positioned the MR method at the core of our study to mitigate potential confounding factors and reverse causality that could affect outcomes in observational studies. Finally, we employed various MR analytical strategies, including sensitivity analyses and supplementary analyses, to combine results from GI cancers data derived from multiple databases using meta-analysis, ensuring the robustness of our research findings.
However, our study also has some limitations to consider. Firstly, serum UA levels vary by gender and age, with males and postmenopausal females typically having higher serum UA levels (Fini et al. 2012). Since our dataset does not provide sufficiently detailed individual information, we were unable to perform gender and age-stratified analyses. Additionally, to avoid selective bias resulting from population stratification, our sample primarily consisted of individuals of European descent, which may impact the external generalizability of our study results. Therefore, further research is needed to validate the applicability of these findings in other racial populations.