5-hydroxymethylcytosine is a new epigenetic modification deriving from the oxidation of 5-methylcytosine by the TET hydroxylase enzymes. DNA hydroxymethylation drives DNA demethylation events and is involved in the control of gene expression. Deregulation of TET enzymes causes developmental defects and is associated with pathological conditions such as cancer. Little information thus far is available on the regulation of TET activity by post-translational modifications. Here we show that TET1 protein is able to interact with PARP-1/ARTD1 enzyme and is target of both noncovalent and covalent PARylation. In particular, we have demonstrated that the noncovalent binding of ADP-ribose polymers with TET1 catalytic domain decreases TET1 hydroxylase activity while the covalent PARylation stimulates TET1 enzyme. In addition, TET1 activates PARP-1/ARTD1 independently of DNA breaks. Collectively, our results highlight a complex interplay between PARylation and TET1 which may be helpful in coordinating the multiple biological roles played by 5-hydroxymethylcytosine and TET proteins.
TET enzymes are the epigenetic factors involved in the formation of the sixth DNA base 5-hydroxymethylcytosine, whose deregulation has been associated with tumorigenesis. In particular, TET1 acts as tumor suppressor preventing cell proliferation and tumor metastasis and it has frequently been found down-regulated in cancer. Thus, considering the importance of a tight control of TET1 expression, the epigenetic mechanisms involved in the transcriptional regulation of TET1 gene are here investigated. The involvement of poly(ADP-ribosyl)ation in the control of DNA and histone methylation on TET1 gene was examined. PARP activity is able to positively regulate TET1 expressionmaintaining a permissive chromatin state characterized by DNA hypomethylation of TET1 CpG island as well as high levels of H3K4 trimethylation. These epigenetic modifications were affected by PAR depletion causing TET1 down-regulation and in turn reduced recruitment of TET1 protein on HOXA9 target gene. In conclusion, this work shows that PARP activity is a transcriptional regulator of TET1 gene through the control of epigenetic events and it suggests that deregulation of these mechanisms could account for TET1 repression in cancer.
Aberrant DNA methylation can lead to genome destabilization and to deregulated gene expression. Recently, 5-hydroxymethylcytosine (5hmC), derived from oxidation of 5-methylcytosine (5mC) by the Ten-Eleven Translocation (TET) enzymes, has been detected. 5hmC is now considered as a new epigenetic DNA modification with relevant roles in cell homeostasis regulating DNA demethylation and transcription. Our aim was to investigate possible changes in the DNA methylation/demethylation machinery in MS. We assessed the expression of enzymes involved in DNA methylation/demethylation in peripheral blood mononuclear cells (PBMCs) from 40 subjects with MS and 40 matched healthy controls. We performed also, DNA methylation analysis of specific promoters and analysis of global levels of 5mC and 5hmC. We show that TET2 and DNMT1 expression is significantly down-regulated in MS PBMCs and it is associated with aberrant methylation of their promoters. Furthermore, 5hmC is decreased in MS PBMCs, probably as a result of the diminished TET2 level.
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