Evaluation in mammalian oocytes of gene transcripts linked to epigenetic reprogramming

The mature mammalian metaphase II (MII) oocyte has a unique ability to reprogram sperm chromatin and support early embryonic development. This feature even extends to the epigenetic reprogramming of a terminally differentiated cell nucleus as observed in connection with somatic cell nuclear transfer. Epigenetic nuclear reprogramming is highly linked to chromatin structure and includes covalent modifications of DNA and core histone proteins as well as reorganization of higher-order chromatin structure. A group of conserved enzymes mediating DNA methylation, methyl-CpG-binding protein (MeCP), histone acetylation and methylation, and chromatin remodeling are extensively involved in epigenetic reprogramming in mammalian cells. Using the oligonucleotide microarray technique, the present study compared the expression levels of 86 genes associated with epigenetic reprogramming in murine in vivo matured MII oocytes with that of germinal vesicle oocytes. Correlation between biological replicates was high. A total of 57 genes with potential reprogramming effect were detected. In MII oocytes, four genes were significant up-regulated, whereas 18 were down-regulated and 35 unchanged. The significantly regulated genes were validated by real-time quantitative RT-PCR. For example, MII oocytes showed a significant down-regulation of oocyte-specific maintenance DNA methyltransferase, Dnmt1o, and up-regulation of MeCP transcript, methyl-CpG binding domain protein 2. Furthermore, histone acetyltransferases were proportionally overrepresented when compared with histone deacetylases. These data elucidate for the first time some of the mechanisms that the oocyte may employ to reprogram a foreign genome either in form of a spermatozoa or a somatic nucleus and may thus be of importance for advancing the fields of stem cell research and regenerative medicine.