Franchina M, Kay P H
Department of Pathology, The University of Western Australia, Nedlands, Perth.
DNA Cell Biol. 2000 Sep;19(9):521-6. doi: 10.1089/104454900439755.
In contrast to the complex sequence specificities of the prokaryotic DNA methylating systems, the mammalian machinery identified thus far methylates cytosine residues within the context of a 5'-CG-3' dinucleotide. To explore the possibility that cytosine residues that do not precede guanine may be independently methylated in mammalian DNA, we have examined a region of the human myogenic gene, Myf-3, which is not targeted by the methylating system that methylates 5'-CG-3' dinucleotides. Our investigations have revealed cytosine methylation within the 5'-CCTGG-3' pentanucleotides specified by the 0.8-kb Myf-3 probe. We have also found that in DNA from neoplastic cells, in which 5'-CG-3' dinucleotides within Myf-3 become abnormally hypermethylated, cytosine residues within 5'-CCTGG-3' pentanucleotides are not methylated. Moreover, methylation of 5'-CCTGG-3' pentanucleotides was not detected within the closely related Myf-4 gene, which is normally 5'-CG-3' hypermethylated. These findings indicate the existence of a system that methylates 5'-CCTGG-3' pentanucleotides independently of the system that methylates cytosine residues within 5'-CG-3' dinucleotides. It is possible that the 5'-CCTGG-3' methylating system influences the fate of foreign integrated DNA.
与原核生物DNA甲基化系统复杂的序列特异性不同，迄今为止所鉴定的哺乳动物机制会在5'-CG-3'二核苷酸的背景下使胞嘧啶残基发生甲基化。为了探究在哺乳动物DNA中，鸟嘌呤之前的胞嘧啶残基是否可能独立发生甲基化，我们研究了人类成肌基因Myf-3的一个区域，该区域并非是使5'-CG-3'二核苷酸发生甲基化的甲基化系统的作用靶点。我们的研究揭示了由0.8 kb的Myf-3探针所确定的5'-CCTGG-3'五核苷酸内存在胞嘧啶甲基化。我们还发现，在肿瘤细胞的DNA中，Myf-3内的5'-CG-3'二核苷酸发生异常超甲基化，但5'-CCTGG-3'五核苷酸内的胞嘧啶残基并未发生甲基化。此外，在密切相关的Myf-4基因中未检测到5'-CCTGG-3'五核苷酸的甲基化，该基因通常是5'-CG-3'超甲基化的。这些发现表明存在一种独立于使5'-CG-3'二核苷酸内胞嘧啶残基发生甲基化的系统之外，使5'-CCTGG-3'五核苷酸发生甲基化的系统。5'-CCTGG-3'甲基化系统有可能影响外源整合DNA的命运。
