Positive histone marks are associated with active transcription from a methylated ICSBP/IRF8 gene

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.