Investigating the relationship between melatonin patterns and methylation in circadian genes among day shift and night shift workers.

Objectives: Mechanisms underlying the carcinogenicity of night shift work remain uncertain. One compelling yet understudied cancer mechanism may involve altered DNA methylation in circadian genes due to melatonin secretion patterns. The objective of this study was to explore the relationship between melatonin secretion patterns and circadian gene methylation among day and night shift workers.

Methods: Female healthcare employees (n=38 day workers, n=36 night shift workers) for whom we had urinary 6-sulfatoxymelatonin secretion data from a previous study were recontacted. New blood samples were collected and used to measure methylation levels at 1150 CpG loci across 22 circadian genes using the Illumina Infinium MethylationEPIC beadchip. Linear regression was used to examine the association between melatonin (acrophase and mesor) and M values for each CpG site (false discovery rate, q=0.2), while testing for effect modification by shift work status.

Results: Among night shift workers, a higher mesor (24 hours of mean production of melatonin) was associated with increased methylation in the body of (q=0.02) and decreased methylation in the putative promoter region of (q=0.03). Later acrophase (ie, time of peak concentration) was associated with increased methylation in the putative promoter region of (q=0.20) and decreased methylation in the body of (q=0.20). No associations were identified among day workers.

Conclusions: In conclusion, patterns in melatonin secretion were associated with differential circadian gene methylation among night shift workers. Melatonin and alteration of DNA methylation in circadian genes may be one pathway towards increased cancer risk, although larger-scale studies examining multiple time points are needed.