Cheah M S, Wallace C D, Hoffman R M
J Natl Cancer Inst. 1984 Nov;73(5):1057-65.
Site-specific methylation of the human c-myc oncogene was investigated in a set of cultured human tumor cell lines and normal human fibroblast strains. As previously reported, all of the tumor cell lines in contrast to normal cells were hypomethylated to various degrees in their total genomic DNA. The presence of methylation in specific regions of the c-myc gene was analyzed by use of the restriction endonuclease isoschizomers MspI and HpaII, which recognize the sequence 5'-CCGG-3' (CCGG = DNA sequence of 2 cytosine bases followed by 2 guanine bases) but differ in their abilities to cleave at the internal cytosine residue when it is methylated. The first exon, first intervening sequence, and the second exon were hypomethylated in all cell types, regardless of whether the cells were normal or oncogenically transformed and regardless of the degree of total genomic methylation of the cell. However, the solitary CCGG site in the third exon was fully methylated in normal cell strains. In contrast, in 3 of 5 tumor cell lines measured, this site was hypomethylated. This is the first demonstration of a site-specific DNA methylation defect in a cellular oncogene. Some possible implications relating disruption of DNA methylation to oncogene control and oncogenesis are discussed.
在一组培养的人类肿瘤细胞系和正常人类成纤维细胞系中，对人类c-myc癌基因的位点特异性甲基化进行了研究。如先前报道，与正常细胞相比，所有肿瘤细胞系的总基因组DNA均有不同程度的低甲基化。利用限制性内切酶同裂酶MspI和HpaII分析c-myc基因特定区域的甲基化情况，这两种酶识别序列5'-CCGG-3'（CCGG = 由2个胞嘧啶碱基后跟2个鸟嘌呤碱基组成的DNA序列），但在内部胞嘧啶残基甲基化时切割能力不同。所有细胞类型的第一个外显子、第一个间隔序列和第二个外显子均为低甲基化，无论细胞是正常的还是发生了致癌转化，也无论细胞总基因组甲基化程度如何。然而，第三个外显子中的单个CCGG位点在正常细胞系中完全甲基化。相比之下，在检测的5个肿瘤细胞系中有3个该位点为低甲基化。这是首次在细胞癌基因中证明位点特异性DNA甲基化缺陷。讨论了DNA甲基化破坏与癌基因控制和肿瘤发生相关的一些可能影响。
