ABSTRACT
Background There is growing evidence that paternal pre-conception cigarette smoke (CS) exposure is associated with increased risk of behavioral disorders and cancer in offspring. The aim of the current study was to evaluate the impact of paternal pre-conception CS exposure on sperm DNA methylation and offspring phenotype.
Methodology/Principal To characterize the effects of CS exposure on the sperm epigenome and offspring neurodevelopment, we exposed male mice to CS and bred exposed and control males to unexposed females and subsequently evaluated sperm DNA methylation in sires and frontal cortex DNA methylation and gene expression in offspring. We further investigated the role of oxidative stress on sperm epigenetic changes using a mouse model (Nrf2-/-) with impaired antioxidant capacity. Lastly, we evaluated the capacity for sperm DNA methylation to recover following removal of CS for 1-5 spermatogenic cycles (28-171 days).
Conclusions/Significance Smoking significantly impacts sperm DNA methylation as well as DNA methylation and gene expression in offspring. These changes were largely recapitulated in Nrf2-/- mice independent of smoke exposure. Recovery experiments indicated that about half of differentially methylated regions returned to normal within 28 days of removal from smoke exposure, however additional recovery following longer periods was not observed. We present strong evidence that cigarette smoke exposure induces paternally mediated, heritable epigenetic changes. Parallel studies performed in Nrf2-/- mice provide evidence for oxidative stress as the predominant underlying mechanism for smoke-induced epigenetic changes to sperm along with the associated effects in offspring. Lastly, recovery experiments indicate that while many epigenetic changes are corrected following removal from smoke exposure, aberrant methylation persists at a significant number of regions even after five spermatogenic cycles
