Translation of the carcinogenic DNA breakage-repair inhibition mechanism for hexavalent chromium to its key targets: Lung epithelial cells, lung tissue and impacts on the genome.

Lung cancer is the leading cause of cancer death worldwide. Hexavalent chromium [Cr(VI)] is a human lung carcinogen with widespread occupational and environmental exposure. Despite the known health risks, how Cr(VI) causes lung cancer remains unclear. This dissertation investigates the mechanisms of Cr(VI)-induced carcinogenesis. Cr(VI) causes DNA damage, specifically DNA double strand breaks, and inhibits homologous recombination repair, a high-fidelity repair pathway. Unrepaired DNA double strand breaks or those repaired through error-prone pathways, progress to chromosomal damage, and chromosome instability. The latter is a common characteristic of Cr(VI)-exposed cells, and a hallmark of lung cancer. Although the mechanism of Cr(VI)-induced DNA double strand breaks/repair inhibition/chromosome instability is the most studied mechanism to date, important data-gaps remain. This mechanism has been demonstrated in human bronchial fibroblast cells, which accumulate Cr(VI), yet only bronchial epithelial cells form Cr(VI)-induced tumors. In this dissertation we have successfully translated the mechanism of Cr(VI) to immortalized and primary human bronchial epithelial cells. Specifically, we found Cr(VI) causes DNA double strand breaks in human bronchial epithelial cells after acute and prolonged exposures. However, at prolonged exposures, we observed an inhibition in homologous recombination repair. Another data gap is the translation of Cr(VI)-induced carcinogenesis mechanism to human-relevant animal models. We used guinea pigs, as the anatomy of their bronchial bifurcations resembles that of humans. We exposed guinea pigs to Cr(VI) for acute and subchronic exposures, and found Cr accumulated in the lungs. We also translated the mechanism of DNA double strand breaks and repair inhibition to this model, specifically in the bronchioles and not the alveoli, consistent with pathology data. Finally, we characterized Cr(VI)-induced changes in the genomic sequence and observed large numbers of structural variants, specifically, inversions, deletions, duplications and translocations. To a lesser extent, we also found small insertions and deletions and single base substitution mutations. In comparison, the larger structural variants likely have a more significant impact on the cells. In summary, we translated Cr(VI)-induced DNA double strand breaks and homologous recombination repair inhibition to human bronchial epithelial cells and guinea pig lungs and further characterized Cr(VI)-induced genomic changes, providing new insights into the mechanism of Cr(VI)-carcinogenesis.