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Background Lung cancer is the leading cause of cancer death, of which 80-90% are attributed to tobacco smoking. Our understanding of how tobacco exposure affects mutational burden, mutational signatures, driver mutations and clonal dynamics in normal lung tissue is limited. Also, genetic differences between normal cells and cancer cells in lung has not been fully elucidated. Methods To access the landscape of somatic mutations in normal bronchial epithelia, we sequenced whole genomes of 632 single-cell-derived colonies of lung basal cells from 16 patients, including 3 children, 4 never-smokers, 6 ex-smokers and 3 current smokers. Five patients had squamous cell carcinomas or carcinoma in situ, which we also sequenced to compare the genetic alteration between normal and cancer cells. Results A positive correlation between the number of base substitutions and age was observed, and 22 somatic mutations accumulated per year (95% confidence interval:20-25; P=10−8). Previous or current smoking significantly increased mutational burden: 2330 substitutions in ex-smokers and 5300 in current smokers. In addition, tobacco smoking is massively increasing both between-subject and within-subject variance of mutation burden. A population of cells in subjects with smoking history had mutation burdens equivalent to that expected for never-smokers: these cells had less damage from tobacco-specific mutational processes, and were four-fold more frequent in ex-smokers than current smokers. Signature analysis revealed that the same mutational signatures seen in lung cancers operate both in patients with and without smoking history: endogeneous signatures, including COSMIC signatures 1 and 5, and APOBEC-related signatures. Three mutational signatures were largely restricted to current or ex-smokers, including known smoking-related COSMIC signature 4 characterized by C>A transversions, signature 16 characterized by T>C mutations with extremely strong transcription strand bias and a new signature characterized by T>A and T>C mutations. dN/dS method identified driver genes in normal bronchial epithelium, including NOTCH1, TP53 and FAT1, which were overlapped with those seen in squamous cell lung cancers and other normal squamous tissues such as esophagus and skin. Driver mutations increased in frequency with age, affecting 4-14% of cells in middle-aged never-smokers. In current smokers, ≥25% of cells carried driver mutations and 0-6% cells had 2 or even 3 drivers. Layering driver mutations onto phylogenetic trees revealed that driver mutations occurred in early life. Compared to the normal bronchial epithelial cells, lung cancers and precancerous lesions were characterized by extensive copy number changes or structural variants and distinct set of driver mutations. Conclusions Tobacco smoking increases mutation burden, cell-to-cell heterogeneity and driver mutations. Our data of genetic lesions in normal bronchial cells provides insights into genetic alterations that drive carcinogenesis in lung. Citation Format: Kenichi Yoshida, Kate HC Gowers, Henry Lee-Six, Deepak P. Chandrasekharan, Tim Coorens, Elizabeth F. Maughan, Kathryn Beal, Andrew Menzies, Fraser R. Millar, Elizabeth Anderson, Sarah E. Clarke, Adam Pennycuick, Ricky M. Thackeray, Colin R. Butler, Nobuyuki Kakiuchi, Tomonori Hirano, Robert E. Hynds, Michael R. Stratton, Inigo Martincorena, Sam M. Janes, Peter J. Campbell. Tobacco exposure and somatic mutations in normal bronchial epithelia [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2342.