Genome plasticity in the mouse oocyte and early embryo.

In dissecting the molecules and molecular mechanisms that control mammalian oocyte-to-embryo transition, we found abundant transcripts representing developmentally regulated ERVs (endogenous retroviruses) in mouse oocyte and two-cell stage embryo cDNA libraries. These retrotransposons can act as alternative promoters and first exons for diverse genes, synchronizing their expression. Heritable genetic change due to replication of these retrotransposons probably occurs specifically in oocytes and early embryos. ERVs are usually epigenetically silenced, through DNA methylation and chromatin-based mechanisms. Their activation and silencing indicates a change in the epigenetic state of the genome. The thousands of endogenous retro-elements in the mouse genome provides potential scope for large-scale co-ordinated epigenetic fluctuations and leads to the hypothesis that differential transposable element expression triggers sequential reprogramming of the embryonic genome during the oocyte-to-embryo transition.