Mitochondrial DNA breaks and copy number and the risk of lung cancer in never-smoking women

Introduction Lung cancer is the leading cause of cancer-related mortality worldwide, with approximately 25% of cases occurring in never-smokers, particularly East Asian women. Mitochondria are critical regulators of cellular energy production and oxidative stress responses, and mitochondrial DNA (mtDNA) is especially susceptible to damage from environmental exposures. While findings from previous studies on mtDNA copy number (mtDNAcn) and lung cancer risk have been mixed, mtDNA fraction with breaks (mtDNAfb) has recently emerged as a potential marker of oxidative stress and mitochondrial integrity. We investigated associations between prediagnostic mtDNAfb, mtDNAcn, and lung cancer risk in never-smoking women in the prospective Shanghai Women's Health Study (SWHS). Methods This nested case-control study included 789 incident lung cancer cases and 789 controls individually matched on birth year and blood collection date from the SWHS, a prospective cohort of 74,942 women enrolled between 1996 and 2000. DNA was extracted from white blood cells or buccal cells, and mtDNAfb was quantified using a high-throughput qPCR assay comparing amplification signals from TaqI-treated (digested) and untreated samples. Relative mtDNAcn was derived from qPCR measurements normalized to a reference gene. Conditional logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for the associations of mtDNAfb and mtDNAcn with lung cancer risk, adjusting for age, body mass index, and assay plate. Sensitivity analyses further controlled for education, environmental tobacco smoke exposure, family history of lung cancer, and mutual adjustment for mtDNAfb and mtDNAcn. Interaction between mtDNAfb and mtDNAcn was tested on a multiplicative scale. Results We observed higher mtDNAfb was associated with lower risk of lung cancer with participants in the highest mtDNAfb tertile having 34% lower odds compared with those in the lowest tertile ( p trend = 0.026). No independent association was observed for mtDNAcn; however, a multiplicative interaction between mtDNAfb and mtDNAcn was identified ( p interaction = 8.3 × 10⁻⁶) where the association between mtDNAfb and lung cancer was more pronounced among participants with low mtDNAcn (OR = 0.47, 95% CI: 0.29-0.78) than those with high mtDNAcn (OR = 1.02, 95% CI: 0.58-1.79). Conclusions Lower mtDNAfb was associated with increased lung cancer risk among never-smoking women, particularly among women with low mtDNAcn, suggesting possible pathologic reductions in mitochondrial biogenesis and functioning, and consequently, reduced innate immune activation. These findings provide novel insights into mitochondrial dysfunction as a potential mechanism underlying lung cancer in never-smokers and warrant replication and mechanistic investigation in future studies.