Schmutte C, Yang A S, Nguyen T T, Beart R W, Jones P A
Department of Biochemistry and Molecular Biology, University of Southern California, School of Medicine, Los Angeles, 90033-0800, USA.
Cancer Res. 1996 May 15;56(10):2375-81.
C --> T transitions at CpG sites are the most prevalent mutations found in the p53 tumor suppressor gene in human colon tumors and in the germline (Li-Fraumeni syndrome). All of the mutational hot spots are methylated to 5-methylcytosine, and it has been hypothesized that the majority of these mutations are caused by spontaneous hydrolytic deamination of this base to thymine. We have previously reported that bacterial methyltransferases induce transition mutations at CpG sites by increasing the deamination rate of C --> U when the concentration of the methyl group donor S-adenosylmethionine (AdoMet) drops below its Km, suggesting an alternative mechanism to create these mutations. Unrepaired uracil pairs with adenine during replication, completing the C --> T transition mutation. To determine whether this mechanism could contribute to the development of human colon cancer, we examined the level of DNA (cytosine-5)-methyltransferase (MTase) expression, the concentration of AdoMet, and the activity of uracil-DNA glycosylase in human colon tissues, and searched for the presence of mutations in the MTase gene. Using reverse transcription-PCR methods, we found that average MTase mRNA expression levels were only 3.7-fold elevated in tumor tissues compared with surrounding normal mucosa from the same patient. Also, no mutations were found in conserved regions of the gene in 10 tumors sequenced. High-performance liquid chromatographic analysis of extracts from the same tissues showed that AdoMet concentrations were not reduced below the Km value for the mammalian enzyme, and the concentration ratio of AdoMet:S-adenosylhomocysteine, the breakdown product of AdoMet and the competitive MTase inhibitor, did not differ significantly. Finally, extracts from the tumor tissue efficiently removed uracil from DNA. Therefore, biochemical conditions favoring a mutagenic pathway of C --> U --> T were not found in a target tissue known to undergo a high rate of C --> T transitions at CpG sites.
在人类结肠肿瘤和种系（李-弗劳梅尼综合征）中，p53肿瘤抑制基因中CpG位点的C→T转换是最常见的突变。所有突变热点都被甲基化为5-甲基胞嘧啶，据推测，这些突变中的大多数是由该碱基自发水解脱氨生成胸腺嘧啶所致。我们之前报道，当甲基供体S-腺苷甲硫氨酸（AdoMet）的浓度降至其Km值以下时，细菌甲基转移酶会通过提高C→U的脱氨速率，在CpG位点诱导转换突变，这提示了产生这些突变的另一种机制。复制过程中未修复的尿嘧啶与腺嘌呤配对，完成C→T转换突变。为了确定该机制是否会促进人类结肠癌的发生，我们检测了人类结肠组织中DNA（胞嘧啶-5）-甲基转移酶（MTase）的表达水平、AdoMet的浓度以及尿嘧啶-DNA糖基化酶的活性，并寻找MTase基因中的突变。使用逆转录PCR方法，我们发现与同一患者周围的正常黏膜相比，肿瘤组织中MTase mRNA的平均表达水平仅升高了3.7倍。此外，在测序的10个肿瘤中，该基因的保守区域未发现突变。对相同组织提取物的高效液相色谱分析表明，AdoMet的浓度并未降至哺乳动物酶的Km值以下，并且AdoMet与其分解产物及竞争性MTase抑制剂S-腺苷高半胱氨酸的浓度比没有显著差异。最后，肿瘤组织提取物能有效从DNA中去除尿嘧啶。因此，在已知在CpG位点发生高频率C→T转换的靶组织中，未发现有利于C→U→T诱变途径的生化条件。
