ABSTRACT
Early embryo development begins with the fusion of the spermatozoon and the oocyte to originate a pluripotent embryo. The gamete-specific epigenetic marks disappear temporarily to, shortly after, be re-written differentially according with the cell type. Overall, there is a global massive demethylation wave, with some exceptions (imprinted genes and some active retrotransposons) that maintain DNA methylation marks (5mC). DNA methyltransferase (DNMT) DNMT1 and UHRF1 are responsible of the DNA methylation maintenance to avoid dilution effect (passive demethylation) during replication. DNMT3A, DNMT3B and DNMT3L act as de novo DNMT. TET proteins produce hydroxymethylation and participate in the active demethylation through oxidative demethylation coupled with replication. Overlapping demethylation and remethylation processes occur almost from the fusion of the gametes, and the result depends on the balance from these two processes. In addition, also other epigenetic marks as histone methylation, histone acetylation or histone ubiquitination undergo changes during this period in the life cycle. Main repressive (H3K27me3, H3K9me2/me3) and active marks (H3K4me3) are deleted and subsequently reacquired, happening H3K4me3 earlier than that of H3K27me3. Recent genome-wide analysis of histone marks have provided detailed and novel information about their dynamic, and will be also helpful to understand connections between histone marks and DNA methylation.
