Tamoxifen induces G:C→T:A mutations in the cII gene in the liver of lambda/lacI transgenic rats but not at 5′-CpG-3′ dinucleotide sequences as found in the lacI transgene
Tamoxifen induces G:C→T:A mutations in the cII gene in the liver of lambda/lacI transgenic rats but not at 5′-CpG-3′ dinucleotide sequences as found in the lacI transgene
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DOI：
10.1093/carcin/20.7.1351
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发表时间：
1999-07-01
期刊：
影响因子：
4.7
通讯作者：
Styles, JA
中科院分区：
文献类型：
--
作者：
Davies, R;Gant, TW;Styles, JA
他莫昔芬，一种大鼠肝脏致癌物，可以诱导λ/lacI转基因大鼠肝脏中lacI基因的突变。然而，肝脏DNA上存在持久性他莫昔芬加合物，这可能导致无法排除体外lad试验期间离体突变对诱变的某些贡献。为了解决这个问题，使用不受他莫昔芬-DNA加合物的存在影响的基于选择的测定来确定转基因穿梭载体的cll基因处的诱变。雌性λ/lacI转基因大鼠口服他莫昔芬（20毫克/公斤体重），每天6周，导致3.2倍增加的突变频率（MF）的cII基因与溶剂处理的动物相比。这与先前在lad基因发现的MF相似，并证实了他莫昔芬在体内具有致突变性。他莫昔芬在cll基因中诱导的主要类型的突变是G：C->T：A颠换，如先前在lad基因中发现的。然而，在一项由他莫昔芬诱导的肝肿瘤p53基因突变的未重复研究中，没有发现G：C->T：A颠换;探讨了正常组织和肿瘤组织中诱变之间的可能差异。G：C->：A颠换的主要部分发生在lacI基因的5 '-CpG-3'二核苷酸（CpG）位点，而不是在cII基因的此类位点。CpG位点的甲基化极大地增强了致癌物对脱氧鸟苷的靶向作用，因此这一发现可能是由其各自CpG位点的甲基化模式的差异来解释的;然而，对于该转基因大鼠中lacI和cll基因的甲基化状态一无所知。这项研究提出了一个重要的问题，即靶基因（哺乳动物或转基因）应作为哺乳动物诱变试验的终点。
Tamoxifen, a rat liver carcinogen, can induce mutations in the lacI gene in the livers of lambda/lacI transgenic rats. However, the presence of persistent tamoxifen adducts on the liver DNA raises the possibility that some contribution to the mutagenesis from ex vivo mutations during the in vitro lad assay cannot be ruled out. To address this issue, mutagenesis at the cll gene of the transgenic shuttle vector was determined using a selection based assay which is unaffected by the presence of tamoxifen-DNA adducts. Female lambda/lacI transgenic rats were dosed orally with tamoxifen (20 mg/kg body wt) daily for 6 weeks, causing a 3.2-fold increase in the mutant frequency (MF) in the cII gene compared with that obtained with solvent treated animals. This was similar to the MF found previously at the lad gene and confirms that tamoxifen is mutagenic in vivo. The major class of mutation induced by tamoxifen in the cll gene was G:C-->T:A transversions as was found previously in the lad gene. However, in the one unreplicated study of mutations in the p53 gene of liver tumours induced by tamoxifen, no G:C-->T:A transversions were found; possible differences between mutagenesis in normal and tumour tissues are explored, The major proportion of the G:C-->:A transversions occurred at 5'-CpG-3' dinucleotide (CpG) sites in the lacI gene, but not at such sites in the cll gene. The methylation of CpG sites greatly enhances the targeting of deoxyguanosine by carcinogens, thus this finding might be explained by differences in the methylation patterns at their respective CpG sites; however nothing is known about the methylation status of either the lacI nor the cll gene in this transgenic rat. This study raises the important issue of which target genes (mammalian or transgenic) should be used as endpoints in mammalian mutagenesis assays.