Introduction: To date, no group has studied the epigenetic effects of e-cigarettes (EC) on the urothelium. DNA methylation changes appear to be one of the earliest events in bladder tumorigenesis and can be readily found in normal urothelium adjacent to bladder tumors. EC exposure causes urothelial hyperplasia in mice but not bladder tumors. We therefore asked whether EC exposure results in early field alterations of DNA methylation in the bladder urothelium.
Methods: Mice (4 females, 4 males; 35 weeks old) were exposed to 12% nicotine in 1:1 polyethylene glycol: vegetable glycerol or vehicle daily: 3 hr sessions, 3 sec vape duration, 10 min interval between vapes. Plasma nicotine and cotinine serum concentrations were measured using LC-MS. Reduced-representation bisulfite sequencing (RRBS) was performed using DNA from microdissected urothelium harvested at 4 weeks (Zymo Research Kit). Number of loci covered by each sample was determined and data was filtered to only include loci with >10 reads. Differentially methylated loci were determined (p < 0.01). Gene Set Enrichment Analysis was performed to contextualize the DNA methylation changes.
Results: No significant adverse events were observed in the mice during the exposure period to either vehicle or nicotine. Acute and repeated exposure resulted in significantly higher levels of nicotine and cotinine serum concentration in the nicotine vs. the vehicle group (Figure 1). No bladder tumors formed. RRBS analysis demonstrated that EC exposure resulted in hypermethylation of the promoters of 665 genes and hypomethylation of 285 genes after accounting for sex (p < 0.05) Pathway analysis showed enrichment of MYC target genes in hypomethylated regions and enrichment of genes associated with oxidative phosphorylation, cilium assembly, and ciliopathies in hypermethylated loci.
Conclusions: Significant changes of DNA methylation were observed in mice exposed to ECS vapor vs. vehicle control. EC metabolites in urine may result in aberrant methylation patterns in the bladder urothelium. Results are suggestive of a pre-malignant phenotype driven by EC exposure via stimulation of MYC targets and silencing of genes responsible for establishing primary cilia. Loss of primary cilia in epithelial cells has been associated with loss of cell polarity and increased cell migration.
Source of Funding: NCI Geographical Management of Cancer Health Disparities Program (3P30-CA016086), UNC Lineberger Innovation Award 2020.
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