Hendrich B, Hardeland U, Ng H H, Jiricny J, Bird A
Institute of Cell and Molecular Biology, University of Edinburgh, UK.
Nature. 1999 Sep 16;401(6750):301-4. doi: 10.1038/45843.
In addition to its well-documented effects on gene silencing, cytosine methylation is a prominent cause of mutations. In humans, the mutation rate from 5-methylcytosine (m5C) to thymine (T) is 10-50-fold higher than other transitions and the methylated sequence CpG is consequently under-represented. Over one-third of germline point mutations associated with human genetic disease and many somatic mutations leading to cancer involve loss of CpG. The primary cause of mutability appears to be hydrolytic deamination. Cytosine deamination produces mismatched uracil (U), which can be removed by uracil glycosylase, whereas m5C deamination generates a G x T mispair that cannot be processed by this enzyme. Correction of m5CpG x TpG mismatches may instead be initiated by the thymine DNA glycosylase, TDG. Here we show that MBD4, an unrelated mammalian protein that contains a methyl-CpG binding domain, can also efficiently remove thymine or uracil from a mismatches CpG site in vitro. Furthermore, the methyl-CpG binding domain of MBD4 binds preferentially to m5CpG x TpG mismatches-the primary product of deamination at methyl-CpG. The combined specificities of binding and catalysis indicate that this enzyme may function to minimize mutation at methyl-CpG.
除了其对基因沉默的显著影响外，胞嘧啶甲基化还是突变的一个主要原因。在人类中，从5-甲基胞嘧啶（m5C）突变为胸腺嘧啶（T）的突变率比其他转换高10至50倍，因此甲基化序列CpG的含量较低。超过三分之一的与人类遗传疾病相关的种系点突变以及许多导致癌症的体细胞突变都涉及CpG的缺失。突变性的主要原因似乎是水解脱氨。胞嘧啶脱氨产生错配的尿嘧啶（U），尿嘧啶糖基化酶可以将其去除，而m5C脱氨产生一个G×T错配，该酶无法处理。m5CpG×TpG错配的校正可能由胸腺嘧啶DNA糖基化酶TDG启动。在这里，我们表明MBD4是一种不相关的哺乳动物蛋白，含有一个甲基-CpG结合结构域，它也可以在体外有效地从错配的CpG位点去除胸腺嘧啶或尿嘧啶。此外，MBD4的甲基-CpG结合结构域优先结合m5CpG×TpG错配——甲基-CpG脱氨的主要产物。结合和催化的综合特异性表明，这种酶可能起到最小化甲基-CpG处突变的作用。
