Clark S J, Harrison J, Molloy P L
Kanematsu Laboratories, Royal Prince Alfred Hospital, Camperdown, Australia.
Gene. 1997 Aug 11;195(1):67-71. doi: 10.1016/s0378-1119(97)00164-9.
Previously it has been found that binding of the Sp1 transcription factor is not significantly affected by methylation of the CpG dinucleotide within its binding site, 5'-GGGCGG (lower strand, 5'-CCGCCC). Since it has been established that mammalian cells also have the capacity to methylate cytosines (C) at CpNpG sites we examined the effect of methylation of the outer C of the CpCpG on Sp1 binding. We find that methylation of the outer C is inhibitory and in particular methylation of both cytosines (m)Cp(m)CpG inhibits binding by 95%. Furthermore, we have identified endogenous (m)Cp(m)CpG methylation of an Sp1 site in the CpG island promoter of the retinoblastoma (Rb) gene by genomic sequencing. This occurs in a proportion of retinoblastoma tumors which are extensively CpG methylated in the Rb promoter. The results raise the possibility that (m)Cp(m)CpG methylation could have a biological function in preventing Sp1 binding, thereby contributing to the subsequent abnormal methylation of CpG islands often observed in tumor cells.
此前已发现，Sp1转录因子的结合不受其结合位点（5'-GGGCGG，下链为5'-CCGCCC）内CpG二核苷酸甲基化的显著影响。由于已证实哺乳动物细胞也有能力在CpNpG位点对胞嘧啶（C）进行甲基化，我们研究了CpCpG外侧C的甲基化对Sp1结合的影响。我们发现外侧C的甲基化具有抑制作用，特别是两个胞嘧啶的甲基化（m）Cp（m）CpG会使结合减少95%。此外，我们通过基因组测序确定了视网膜母细胞瘤（Rb）基因CpG岛启动子中Sp1位点的内源性（m）Cp（m）CpG甲基化。这种情况发生在一部分视网膜母细胞瘤肿瘤中，这些肿瘤的Rb启动子存在广泛的CpG甲基化。这些结果提示，（m）Cp（m）CpG甲基化可能具有阻止Sp1结合的生物学功能，从而导致肿瘤细胞中经常观察到的CpG岛随后的异常甲基化。
