DNA Damage and Tobacco-Induced Lung Cancer

描述(申请人提供)：烟草烟雾(TS)中产生的致癌物质能够引起DNA损伤和突变，可能引发肺癌。有趣的是，尽管TS中存在大量的DNA损伤剂，但终生吸烟者中只有10%-15%会患肺癌。尽管TS已被发现导致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。在吸烟者和非吸烟者的肺癌中，这两个基因的突变特征不同。以培养的人肺细胞系统为模型，我们发现：致癌物优先在P53突变热点和K-ras基因第12密码子形成DNA加合物，在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序列的位置，C5胞嘧啶甲基化导致这些位置优先形成加合物。这些发现使我们假设TS诱导的DNA损伤在肺癌的发生中起着核心作用。在肺细胞中，p53基因的胞嘧啶甲基化状态和K-ras基因未知的表观遗传因素可能决定了个体对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产物可以极大地降低细胞DNA修复能力。由于TS含有大量的这些重金属，而且还会诱导过度的氧化应激，因此TS可能会导致DNA修复的抑制。我们认为，每个人反应的不同导致了TS诱发肺癌的不同易感性。TS可能会引起不同程度的DNA损伤，一是对肺癌发生至关重要的基因，如p53和K-ras基因的DNA损伤；二是不同个体之间DNA修复能力的抑制。为了验证这些假说，我们建议确定三个因素在患有和不患有肺癌的烟草吸烟者的肺细胞中：一是DNA损伤在P53和K-ras基因中的分布，二是P53基因中的C5胞嘧啶甲基化状态，三是修复能力。我们还将确定这些肺组织样本中的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基因的表观遗传修饰导致该基因第12密码子的优先致癌物结合和修复不良。这些研究的结果将加深我们对肺癌易感性和DNA损伤诱导的人类癌变的理解，并使我们能够开发生物标记物。

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.