DNA Damage and Tobacco-Induced Lung Cancer

描述（由申请人提供）：烟草烟雾（TS）中产生的致癌物能够引起可能引发肺癌的DNA损伤和突变。有趣的是，尽管TS中存在大量的DNA损伤因子，但只有10-15%的终身吸烟者患肺癌。虽然已发现TS引起许多与肺癌相关的基因突变，但尚未建立明确的因果关系。p53和K-ras基因是ts相关肺癌中两个常见的突变基因。这两个基因在吸烟者和非吸烟者肺癌中的突变特征是不同的。以培养的人肺细胞系统为模型，我们有三个发现：TS致癌物优先在p53突变热点和K-ras基因密码子12上形成DNA加合物，这些序列上形成的加合物修复不良，大多数p53突变热点发生在含有CpG序列的位点，C5胞嘧啶甲基化导致这些位点优先形成加合物。这些发现使我们假设ts诱导的DNA损伤在肺癌发生中起核心作用。在肺细胞中，p53基因中的胞嘧啶甲基化状态和K-ras基因中未知的表观遗传因子可能决定个体对ts诱导的DNA损伤的易感性。我们最近发现镍、铬和脂质过氧化代谢物可以大大降低细胞DNA修复能力。由于TS含有大量的这些重金属，并且还会引起过度的氧化应激，因此TS可能会导致DNA修复的抑制。我们认为，每个个体的反应差异导致了对ts诱导的肺癌的易感性差异。TS可能导致不同程度的DNA损伤，一是对肺癌发生至关重要的基因，如p53和K-ras基因，二是不同个体的DNA修复能力受到抑制。为了验证这些假设，我们建议确定吸烟者肺细胞中有和没有肺癌的三个因素：一，p53和K-ras基因中的DNA损伤分布，二，p53基因中的C5胞嘧啶甲基化状态，三，修复能力。我们还将确定这些肺组织样本中肿瘤细胞和“正常”细胞中与肺癌相关的基因突变。最后，我们将确定K-ras基因中导致优先致癌物质结合和该基因密码子12修复不良的表观遗传修饰。这些研究的结果将增强我们对人类肺癌易感性和DNA损伤诱导的致癌作用的理解，并使我们能够开发生物标志物。

DESCRIPTION (provided by applicant): Carcinogens generated in tobacco smoke (TS) are able to cause DNA damage and mutations that may initiate lung carcinogenesis. Intriguingly, despite the presence of substantial amounts of DNA damaging agents in TS, only 10-15% lifetime tobacco smokers develop lung cancer. Although TS has been found to cause mutations in many genes related to lung cancer, a definable cause-effect relationship has not been established. The p53 and K-ras genes are two frequently mutated genes in TS-related lung cancers. Mutational features in these two genes in the lung cancers of smokers and non-smokers are different. Using a cultured human lung cell system as a model, we have made three discoveries: TS carcinogens preferentially form DNA adducts at p53 mutational hotspots and at codon 12 of the K-ras gene, adducts formed at these sequences are poorly repaired, and most of p53 mutational hotspots occur at sites that contain a CpG sequence and C5 cytosine methylation causes preferential adduct formation at these sites. These findings led us to hypothesize that TS-induced DNA damage plays a central role in lung carcinogenesis. In lung cells, both the cytosine methylation status in the p53 gene and an unknown epigenetic factor in the K-ras gene may determine an individual's susceptibility to TS-induced DNA damage. We recently found that nickel, chromium, and lipid peroxidation metabolites can greatly reduce cellular DNA repair capacity. Since TS contains significant amounts of these heavy metals and also induces excessive oxidative stress, it is possible that TS may cause inhibition of DNA repair. We propose that variations in every individual's response contribute to the differences in susceptibility to TS-induced lung cancer. TS may induce different levels of one, DNA damage at genes crucial for developing lung cancer, such as p53and K-ras genes, and two, inhibition of DNA repair capacity among different individuals. To test these hypotheses we proposed to determine three factors in the lung cells of tobacco smokers with and without lung cancer: one, DNA damage distribution in the p53 and K-ras genes, two, C5 cytosine methylation status in the p53 gene, and three, the repair capacity. We will also determine the mutations in genes related to lung cancer in both tumor and "normal" cells in these lung tissue samples. Finally, we will determine the epigenetic modification in the K-ras gene that causes preferential carcinogen binding and poor repair at codon12 of this gene. Results from these studies will enhance our understanding of lung cancer susceptibility and DNA damage-induced carcinogenesis in humans, and enable us to develop biomarkers.