Exogenous thymine DNA glycosylase regulates epigenetic modifications and meiotic cell cycle progression of mouse oocytes.

In mammalian cells, 5-methylcytosine (5-meC) can be transformed into 5-hydroxymethylcytosine (5-hmC) by the methylcytosine dioxygenase TET proteins (TET1, TET2 and TET3). Thymine DNA glycosylase (TDG), a downstream enzyme of TET proteins, not only functions in base excision repair, but also acts as a key enzyme that participates in active DNA demethylation. Here we microinjected exogenous TDG-mCherry mRNAs into germinal vesicle (GV) stage mouse oocytes, and found that initially TDG-mCherry localized in the nucleus. Just before GV breakdown (GVBD), TDG-mCherry was released from the nucleus into the cytoplasm. In contrast with TDG, another active DNA demethylation-associated enzyme, activation-induced cytidine deaminase (AID) became localized in the cytoplasm of GV oocytes, but entered the nucleus of oocytes just before GVBD. However, both TDG and AID could enter the G0 stage nuclei of cumulus cells injected into the ooplasm. To analyze the effects of TDG on oocyte maturation, we over-expressed TDG-mCherry in GV oocytes, and found that the rates of both GVBD and polar body extrusion rate were significantly decreased. When the TDG over-expressed oocytes were blocked at the GV stage, the oocyte chromatin became decondensed, and the histone 3 trimethyl lysine 9 (H3K9me3) and H3K9me2 levels were decreased. We also found that TDG could reduce the 5-meC level of oocyte genomic DNA. All these results indicate that aberrant TDG expression causes epigenetic modifications and meiotic cell cycle arrest of mouse oocytes.