Differential DNA methylation in blood as a mediator of the association between cigarette smoking and bladder cancer risk among postmenopausal women

ABSTRACT

Smoking accounts for approximately 52% of bladder cancer incidence among postmenopausal women, but the underlying mechanism is poorly understood. Our study investigates whether changes in DNA methylation, as measured in blood, mediate the impact of smoking on bladder cancer risk among postmenopausal women. We conducted analyses among 206 cases and 251 controls that were current or never smokers at baseline from a previous case-control study of bladder cancer and genome-wide DNA methylation nested within the Women’s Health Initiative. Separate mediation analyses were conducted for three CpG sites demonstrating robust associations with smoking in prior methylome-wide association studies: cg05575921 (AhRR), cg03636183 (F2RL3), and cg19859270 (GPR15). We estimated causal effects using the regression-based, four-way decomposition approach, which addresses the interaction between smoking and each CpG site. The overall proportion of the excess relative risk mediated by cg05575921 was 92% (p-value = 0.004) and by cg19859270 was 79% (p-value = 0.02). The largest component of the excess relative risk of bladder cancer due to 30 pack-years of smoking history in current smokers was the mediated interaction for both cg05575921 (72%, p = 0.02) and cg19859270 (72%, p-value = 0.04), where the mediated interaction is the effect of smoking on bladder cancer that both acts through differential methylation and depends on smoking history. There was little evidence that smoking was mediated through cg03636183. Our results suggest that differential methylation of cg05575921 and cg19859270 mediate the effects of smoking on bladder cancer, potentially revealing downstream effects of smoking relevant for carcinogenesis.

KEYWORDS:

• Cigarette smoking
• DNA methylation
• bladder cancer
• aryl-hydrocarbon receptor repressor gene (AhRR)
• G protein-coupled receptor 15 gene (GPR15)
• F2R like thrombin or trypsin receptor 3 gene (F2RL3)

Acknowledgments

We would like to acknowledge Xiaoling Song for performing the bisulfite conversion and plating of DNA samples and Cassandra Sather for running the DNA methylation assays.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was supported by the American Cancer Society under Grant 125299-RSG-13-100-01-CCE; and Kristina M. Jordahl was supported by the National Cancer Institute (NCI) at the National Institutes of Health (NIH) under Training Grants R25 CA094880 and T32 CA094880. The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of the NCI, NIH. The WHI program is funded by the National Heart, Lung, and Blood Institute, National Institutes of Health, U. S. Department of Health and Human Services through contracts HHSN268201600018C, HHSN268201600001C, HHSN268201600002C, HHSN268201600003C, and HHSN268201600004C.