Background: Electronic cigarettes (e-cigs) are one of the most popular tobacco products in the US. Little is known regarding their pulmonary effects. E-cigs induce similar oxidant reactivity as cigarette smoke and promote oxidative damage/inflammation in airway cells. Given that mtDNA is more prone to oxidative stress than nuclear DNA because of a less effective proofreading system, mtDNA alterations may be important indicators of e-cigs' toxic effects. Clinically, mitochondrial DNA alteration is an emerging biomarker of respiratory diseases.

Methods: We compared mtDNA copy number (mtDNA-CN) from lung brushings in a cross-sectional bronchoscopy study of healthy young adults, e-cig users (EC)(n=15), non-smokers, non-EC users (NS)(n=43), and smokers (SM)(n=26). We examined associations of mtDNA-CN with immune response (differential cell counts and cytokines in bronchoalveolar lavage), DNA methylation and gene expression brushings. Associations for: 1) EC vs NS vs SM, and 2) tobacco product users (EC+SM) vs NS for MtDNA-CN with immune response, methylation, and expression were made using linear regression. Further, significant features by group interactions were followed up by within-group tests. False Discovery Rate (FDR) at 0.1 was considered significant. Ingenuity pathway analysis was used to identify the most significantly enriched pathways/molecular functions/diseases.

Results: MtDNA-CN was not significantly different among the three groups (P=0.06). MtDNA-CN was higher in SM than NS (P=0.02), and in tobacco product users than NS (P=0.02); EC mtDNA-CN tended to be intermediate between the 2 other groups. There were significantly positive associations of IL-2 and IL-4 with mtDNA-CN in EC, but not in SM or NS (Interaction FDR=0.06 for both). We found 147 transcripts (60 genes) and 1,153 CpGs (713 genes) to be significantly associated with mtDNA-CN in all three groups. The most common canonical pathway of the signatures for both expression and methylation were immune responses. The top molecular and cellular functions for both included cell death and survival. Ten transcripts (LINC01184, SNU13, RPL35A, COLCA1, HLA-DRB1, LOC105379655, TRIM9, TCIRG1, CLPB, MIR2114) and 3,929 CpGs (top: ULK4, STARD13, HLCS, FLT1, TMEM91, CYP2J2) were associated with mtDNA-CN in E-cig users only. Some of these genes are known to play a role in lung diseases, including cancer. For the signatures associated in all groups, we found many more significant signatures (236 vs 147 transcripts and 40,830 vs 1,153 CpGs) in the two group vs. three group comparisons, respectively.

Conclusion: While the sample size was small, this study is the first to suggest that mtDNA-CN is a site of pulmonary toxic effects. We found associations of mtDNA-CN with inflammatory markers among EC users, and with a number of biological signatures, particularly genes related to immune response, in the lungs of EC, SM, and NS, but differently by groups for some.

Citation Format: Kellie M. Mori, Joseph P. McElroy, Daniel Y. Weng, Sangwoon Chung, Sarah A. Reisinger, Kevin L. Ying, Quentin A. Nickerson, Theodore M. Brasky, Mark D. Wewers, Jo L. Freudenheim, Peter G. Shields, Min-Ae Song. Lung mitochondrial DNA copy number variations: E-cig users, smokers, and never-smokers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 751.