Smoking and colorectal cancer survival in relation to tumor LINE-1 methylation levels: a prospective cohort study

Background Carcinogens in cigarette smoke may cause aberrant epigenomic changes. The hypomethylation of long interspersed nucleotide element-1 (LINE-1) in colorectal carcinoma has been associated with genomic instability and worse clinical outcome. We hypothesized that the association between smoking behavior and colorectal cancer mortality might be stronger in tumors with lower LINE-1 methylation levels. Findings To test our hypothesis, we examined the interaction of tumor LINE-1 methylation levels and smoking status at diagnosis using data of 1208 cases among 4420 incident colorectal cancer cases that were ascertained in two prospective cohort studies. We conducted multivariable Cox proportional hazards regression analyses, using inverse probability weighting with covariate data of the 4420 cases to control for potential confounders and selection bias due to data availability. The prognostic association of smoking status at diagnosis differed by tumor LINE-1 methylation levels ( P interaction = 0.050 for overall mortality and 0.017 for colorectal cancer-specific mortality; with an alpha level of 0.005). In cases with <60% LINE-1 methylation, current smoking (vs. never smoking) was associated with worse overall mortality (multivariable hazard ratio, 1.80; 95% confidence interval, 1.19–2.73). In contrast, smoking status was not associated with mortality in cases with ≥60% LINE-1 methylation. Conclusions Our findings suggest that the association between smoking status and mortality is stronger in colorectal cancer patients with lower tumor LINE-1 methylation levels. These results warrant further investigation into an interactive role of smoking and aberrant DNA methylation in colorectal cancer progression.