Abstract
Endoplasmic reticulum stress (ERS) plays a crucial role in the pathogenesis of various diseases, but its causal involvement and therapeutic potential in cancer remain unclear. In this study, we integrated genome-wide association study (GWAS) data from 18 common cancers with quantitative trait loci (cis-eQTL, cis-mQTL, and cis-pQTL) to explore the causal effects of ERS-related genes on cancer. A total of 1,350 ERS-related genes were retrieved from the GeneCards database. Mendelian randomization (MR) and Bayesian colocalization analyses were conducted to assess causality and shared genetic variants across mRNA expression, DNA methylation, and protein expression levels. External datasets were used for expression validation and diagnostic efficacy assessment. To provide experimental evidence, immunohistochemical (IHC) staining was performed to verify the expression and localization of key ERS-related genes in tumor and adjacent normal tissues. Functional enrichment, cellular localization, and drug sensitivity analyses were further applied to reveal potential biological mechanisms. We identified nine ERS-related genes and 15 methylation sites with potential causal relationships to specific cancer types. Both external validation and IHC analysis consistently confirmed the associations of CBY1, CASP8, PLOD1, and several methylation sites (cg09907170, cg09395195, cg08129017, and cg14808739) with their corresponding cancers. This comprehensive multi-omics and experimental validation study provides evidence supporting a causal role of ERS in cancer development and offers new insights into its molecular regulation and therapeutic potential.