Importance of DNA repair in carcinogenesis : Insight for transgene or gene targeting experiments.

本课题利用转基因动物研究了DNA修复系统与肿瘤发生的关系（ada小鼠）引入细菌DNA修复酶基因，O^6-甲基鸟嘌呤-DNA甲基转移酶（MGMT ; ada）基因Ada基因在转基因小鼠的肝脏中有高表达，其酶活性是非转基因小鼠的3倍使用ADA小鼠和非转基因对照小鼠，我们已经报道了ADA小鼠在用肝致癌物二甲基亚硝胺或二乙基亚硝胺处理时对肿瘤发生具有抗性。来源于C3 H/HeN小鼠（C3 H小鼠品系）的表达ada基因的转基因小鼠通常已知在以后的生活中发生自发性肝癌的高发病率。在这个项目中，我们比较了ada小鼠和非转基因小鼠之间自发性肝肿瘤的发生率。结果，两组的肝脏肿瘤发生率相同（48%）。但 关于我们 组织学检查显示ada小鼠肝癌发生率明显低于非转基因小鼠。2）着色性干皮病（Xeroderma pigmentosum，XP）患者的DNA修复能力明显不足。由大坂大学的Tanaka博士的小组产生的XPA（XP A组互补）基因敲除小鼠的可用性使我们能够使用小鼠皮肤作为模型系统来研究XPA核苷酸切除修复在体内基因起始和促进癌发生中的功能作用。XPA无效小鼠在DMBA处理后5至7天表现出皮肤溃疡。随后的实验中，XPA裸小鼠反复与DMBA在每周的时间间隔，揭示了乳头状瘤首先出现在XPA裸小鼠之间的10和16周，其中达到100%的发病率。杂合子和野生型小鼠显示几乎为零或非常低的乳头状瘤产量。从上述两种不同的动物模型中获得的结果提供了DNA修复保护小鼠免受化学致癌物引起的DNA损伤的直接证据。3）众所周知，p53基因缺陷小鼠在生命早期发生自发性肿瘤。纯合子中最常见的肿瘤类型是恶性淋巴瘤，杂合子中以骨肉瘤和软组织肉瘤为主。在我们的项目中，p53缺陷小鼠（由Aizawa等人，（熊本大学）在妊娠12-16天经胎盘给予乙基亚硝基脲。70%的纯合子小鼠和4%的杂合子小鼠在3至4个月大时患上了脑肿瘤。在野生小鼠中未观察到。少

In our research project, we studied about relationship between DNA repair system and carcinogenesis using transgenic animals.1) We generated the transgenic mice (ada mouse) introduced with bacterial DNA repair enzyme gene, O^6-methylguanine-DNA methyltransferase (MGMT ; ada) gene The ada gene was highly expressed in the liver and the enzyme activity in the liver was 3 times higher in transgenic mouse than in the nontransgenic mouse.Using ada mice and nontransgenic control mice, we already reported that ada mouse was resistant to tumorigenesis when treated with liver carcinogens, dimethylnitrosamine or diethylnitrosamine. Transgenic mice expressing ada gene derived from C3H/HeN mouse, C3H mouse strain is generaly known to develop spontaneous liver tumors at high incidence in later life. In this project, we compared the incidences of spontaneous liver tumors between ada mice and nontransgenic mice. In results, both groups developed liver tumours at the same incidence (48%). However, the … More histological examination showed the incidence of hepatocellular carcinoma in ada mouse was significantly lower than in nontransgenic mouse. These obserbations suggest that MGMT may also protect mice from tumor progression.2) It is well known that Xeroderma pigmentosum (XP) patients are deficient in DNA repair. The availability of XPA (XP group A complementing) gene knockout mice generated by Dr.Tanaka's group at Osaka University has enabled us to investigate that functional role of XPA nucleotide excision repair in gene initiation and promotion of carcinogenesis in vivo, using the mouse skin as a model system. XPA null mice demonstrated skin ulcers 5 to 7 days after DMBA treatment. Subsequent experiment in which XPA null mice were repeatedly trated with DMBA at weekly intervals revealed that papillomas first arrise in XPA null mice between 10 and 16 weeks in which a 100% incidence was achieved. Heterozygous and wild-type mice showed an almost zero or very much lower yield of papillomas. Results obtained from the above two separate animal models provide direct evidence that DNA repair protects mice from DNA damage elicited by chemical carcinogens.3) It is well known that the p53 gene deficient mice develop spontaneous tumors in early life. The most frequent tumor type in homozygotes was malignant lymphoma ; in heterozytotes, osteosarcomas and soft tissue sarcomas predominated. In our project, the p53 deficient mice (generated by Aizawa et al., Kumamoto Univ.) were treated with ethylnitrosourea transplacentaly at 12-16 pregnant day. Seventy percent of homozygous and 4% of heterozyous mice developed brain tumors by the age of 3 to 4 months. None were observed in wild mice. Less