| Summary: | 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) are the most common DNA modifications in vertebrate genomes. Promoter-residing 5mC is a well-established transcriptional repression mark. However, the role of gene body DNA modifications, especially those involving 5hmC, remains elusive and controversial. In this study, the role of genic 5mC and 5hmC in transcriptional regulation is investigated. By engineering a cell line with inducible 5hmC it is shown that deposition of 5hmC causes elevated expression of genes with the highest accumulation of this modification. The transcriptional elevating potential of 5hmC is also confirmed via expression analysis of integrated and transfected artificial reporters. Furthermore, it is demonstrated that promoter proximal gene body DNA modifications can modulate transcription initiation in cis. Finally, the ability of methyl-CpG-binding domain protein 1 (MBD1) to interact with genic DNA methylation and repress transcription is analysed. Upon oxidation of 5mC to 5hmC or demethylation, MBD1 binding is abolished and 5mC-mediated repression is relieved. Altogether, this study suggests that 5hmC plays a de-repressive role and proposes a mechanism by which this is achieved.
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