Plasma lipid species, immune cell traits, and gastric cancer risk: A Mendelian randomization study

Plasma lipid composition has been linked to multiple cancers, yet its causal contribution to gastric cancer and the potential intermediary role of immune cells remain unclear. We aimed to clarify these relationships and identify specific lipid and immune cell traits that either protect against or promote gastric cancer. We performed a 2-sample, 2-step Mendelian randomization analysis using summary statistics from large genome-wide association studies of gastric cancer (1423 cases, 3,14,193 controls), plasma lipidomics (179 molecular species), and 731 immune cell phenotypes. Independent, genome-wide significant single-nucleotide variants served as instrumental variables. First, we estimated the causal effects of each plasma lipid on gastric cancer. Then, we explored the potential intermediary role of lipid-associated immune cell traits using a 2-step Mendelian randomization framework. Two lipids - phosphatidylethanolamine (18:0 0:0) and phosphatidylcholine (O-18:0 16:1) - were causally associated with a lower risk of gastric cancer. Three immune cell traits (CD8br and CD8dim %leukocyte, IgD on IgD+ CD38- and CD3 on CD28- CD8br) similarly showed protective effects. In contrast, phosphatidylcholine (O-16:1 18:2), triacylglycerol (49:1), triacylglycerol (56:3), and triacylglycerol (56:4) increased gastric cancer risk, as did immune traits such as TD DN (CD4-CD8-)AC, CD19 on memory B cell, CD28 on CD39+ activated Treg, CD45 on CD4+, CD127 on CD28+DN(CD4-CD8-) and CCR2 on CD14+CD16+ monocyte. Exploratory mediation analyses found no statistically significant evidence that immune cell traits mediated the effects of plasma lipids on gastric cancer risk. Specific phosphatidylethanolamines and phosphatidylcholines confer protection against gastric cancer, whereas several triacylglycerols increase risk. However, exploratory mediation analyses provided no statistically significant evidence that immune cell traits mediated these associations.