Tshuikina Marina, Nilsson Kenneth, Oberg Fredrik
Department of Genetics and Pathology, Rudbeck Laboratory, Uppsala University, Sweden.
Gene. 2008 Mar 15;410(2):259-67. doi: 10.1016/j.gene.2007.12.013. Epub 2008 Feb 1.
Epigenetic modifications are critical for regulating many different aspects of normal cell biology and tumourigenesis. Gene expression may be epigenetically silenced by DNA-methylation and histone modifications, resulting in remodelling of chromatin into a repressed state. We performed DNA-methylation analysis of the ICSBP/IRF8 gene, a member of the IRF family of transcriptional regulators expressed in monocytic and lymphocytic cells, in human monoblastic U-937 cells. We found complete methylation of all 39 CpG positions located in a 308 bp sequence encompassing the proximal promoter and transcriptional start site of the ICSBP/IRF8 gene. However, strikingly, the ICSBP/IRF8 gene is still expressed. Chromatin Immuno-precipitation (ChIP) showed that RNA-Polymerase II was present at the major transcriptional start site. Investigating the histone modifications across the ICSBP/IRF8 gene we found the positive histone marks H3K9ac and H3K4me3 to be enriched at the promoter, whereas the level of H3K9me3 was low. This suggests that an active chromatin structure, indicated by histone H3 modifications and enrichment for RNA Pol II, can over-ride the silencing effect of DNA-methylation at the promoter, thereby permitting transcription of the ICSBP/IRF8 gene.
表观遗传修饰对于调控正常细胞生物学和肿瘤发生的许多不同方面至关重要。基因表达可通过DNA甲基化和组蛋白修饰在表观遗传上被沉默，导致染色质重塑为抑制状态。我们对人单核细胞U-937细胞中的ICSBP/IRF8基因进行了DNA甲基化分析，该基因是在单核细胞和淋巴细胞中表达的转录调节因子IRF家族的成员。我们发现位于一个308 bp序列中的所有39个CpG位点完全甲基化，该序列包含ICSBP/IRF8基因的近端启动子和转录起始位点。然而，令人惊讶的是，ICSBP/IRF8基因仍在表达。染色质免疫沉淀（ChIP）显示RNA聚合酶II存在于主要转录起始位点。研究ICSBP/IRF8基因上的组蛋白修饰，我们发现组蛋白阳性标记H3K9ac和H3K4me3在启动子处富集，而H3K9me3的水平较低。这表明由组蛋白H3修饰和RNA Pol II富集所指示的活跃染色质结构可以克服启动子处DNA甲基化的沉默效应，从而允许ICSBP/IRF8基因转录。
