Repair of alkylation DNA damage

基因靶向法获得编码O^6 -甲基鸟嘌呤- dna甲基转移酶的MGMT基因缺陷小鼠。这些MGMT^+小鼠对烷基化致癌物甲基亚硝基脲（MNU）最为敏感；给6周龄小鼠注射不同剂量的MNU，测定第30天存活率，MGMT^+和MGMT^<+/+>小鼠LD_<50S>分别为20和240 mg/kg体重。MGMT^<+/->小鼠的抗性与MGMT^<+/+>小鼠相同，但当两种基因型小鼠暴露于相对高剂量的MNU时，存活时间有所差异。以2.5 mg/kg体重剂量暴露于MNU的MGMT^<-/->小鼠中出现大量胸腺淋巴瘤和肺腺瘤。在相同剂量下，MGMT^<+/+>和MGMT^<+/->小鼠没有或很少发生肿瘤。DNA修复甲基转移酶蛋白似乎保护这些小鼠免受mnu诱导的肿瘤发生。为了研究在小鼠组织DNA中产生的O^6-甲基鸟嘌呤的命运，我们使用了埃森大学Rajewsky小组培养的单克隆抗体。该抗体制剂特异性识别O^6-乙基鸟嘌呤和O^6-甲基鸟嘌呤。为了这项合作工作，九州大学的H.Hayakawa和K.Sakumi访问了埃森实验室，埃森的T.Schweer访问了福冈。我们已经建立了适当剂量的MNU给予MGMT^<+/+ bb0和MGMT^<-/->小鼠，以及条件和程序，以跟踪小鼠组织中甲基化碱基的数量变化。初步结果表明，MGMT^<-/->小鼠在DNA中保留了大量的烷基化碱基，而MGMT^<+/+>小鼠则很快失去了这些碱基。

Gene targeting was used to obtain mice defective in the MGMT gene, encoding O^6 -methylguanine-DNA methyltransferase. These MGMT^+ mice were most sensitive to the alkylating carcinogen, methylnitrosourea (MNU) ; when varied doses of MNU were administered to 6-week-old mice and survivals at the 30th day were determined, LD_<50S> of MGMT^+ and MGMT^<+/+> mice were 20 and 240 mg/kg of body weight, respectively.MGMT^<+/-> mice were as resistant as MGMT^<+/+> mice, but some difference in survival time was noted when the two genotypes of mice were exposed to a relatively high dose of MNU.A large number of thymic lymphomas, as well as lung adenomas, occurred in MGMT^<-/-> mice exposed to MNU at a dose of 2.5 mg/kg of body weight. In case of exposure to the same dose of drub, no or few tumors occureed in the MGMT^<+/+> and MGMT^<+/-> mice. It appears that the DNA repair methyltransferase protein protected these mice from MNU-induced tumorigenesis.To pursue the fate ot O^6-methylguanin produced in the DNA of mouse tissues, we used monoclonal antibodies raised by the Rajewsky's group of University of Essen. This antibody preparation specifically recognizes O^6-ethylguanine as well as O^6-methylguanin. For the collaborative work, H.Hayakawa and K.Sakumi of Kyushu University visited the Essen laboratory and T.Schweer of Essen visited here in Fukuoka. We have established appropriate doses of MNU to be given to MGMT^<+/+> and MGMT^<-/-> mice and also conditions and procedures to follow changes in amounts of methylated bases in mouse tissues. A preliminary result indicates that MGMT^<-/-> mice retain a significant level of alkylated bases in DNA whereas MGMT^<+/+> mice lose these bases rather quickly.