Study of Defense Mechanisms against Cellular Damage Caused by Free Radicals

对于活着的生物体来说，最基本的生物学功能是保持其包含遗传信息的基因组DNA的完整性，并将它们在细胞之间以及从父母传给后代的情况下准确地传递。基因组DNA及其前体核苷酸总是处于氧呼吸和其他正常代谢过程中产生的超氧化物或一氧化氮等自由基的氧化危险中。自由基在DNA或核苷酸池中形成各种氧化的碱基和核苷酸，如果不修复，这种氧化的DNA损伤可能会导致突变或细胞死亡。突变可以诱发癌症，细胞死亡可能与神经退行性疾病等多种退行性疾病有关。在本项目中，我们旨在揭示保护基因组完整性免受自由基破坏的分子机制。我们特别关注非增殖性细胞氧化损伤导致的神经细胞死亡，以及被认为是增殖性细胞氧化损伤所导致的癌症。我们鉴定并鉴定了四种人类酶，(1)氧化嘌呤核苷三磷酸酶(MTH1)，(2)2-羟基腺嘌呤/腺嘌呤DNA糖基酶(MYH)，(3)8-氧鸟嘌呤DNA糖基酶(OGG1)，和(4)新的AP核酸内切酶(APE2)。对每种基因敲除小鼠的研究表明，氧化DNA损伤的自发积累会增加突变和致癌的发生。我们还发现，在神经退行性疾病患者中，随着保护酶表达的改变，这种DNA氧化损伤在变性神经元中积累，这表明DNA氧化损伤可能参与了神经退行性疾病。

For living organisms, the most fundamental biological function is maintaining the integrity of their genomic DNAs harboring the genetic information and transmitting them precisely from cell to cell, as well as from parents to their offsprings. The genomic DNA and its precursor nucleotides, are always in danger of oxidation by free radicals such as superoxide or NO produced during the oxygen respiration and other normal metabolisms. Various oxidized bases and nucleotides are formed in DNA or nucleotide pools by the free radicals, and such oxidative DNA damage may cause mutations or cell death if they are not repaired. Mutations may induce cancers, and cell death may be related to various degenerative disorders such as neurodegenarative diseases.In this project, we have aimed to unveil the molecular mechanisms protecting genomic integrity from damage caused by free radicals. We have been especially focusing on neuronal cell death as a consequence of oxidative damages in non-proliferative cells, as well as on cancer that is believed to be a consequence of such damages in proliferative cells.We have identified and characterized four human enzymes, (1) oxidized purine nucleoside triphosphatase (MTH1), (2) 2-hydroxyadenine/adenine DNA glycosylase (MYH), (3) 8-oxoguanine DNA glycosylase (OGG1), and (4) novel AP endonuclease (APE2). Characterization of knockout mice for each gene revealed that spontaneous accumulation of oxidative DNA damage causes increased occurrence of mutation as well as carcinogenesis. We also found that such oxidative DNA damage accumulates in degenerative neurons along with altered expression of the protective enzymes in patients with neurodegenerative diseases, suggesting that oxidative DNA damage may be involved in neurodegeneration.