Epigenetic regulation of germ cell and early embryonic development by Polycomb group proteins

Mammalian development begins at fertilization, when two highly specialized gametes, sperm and oocyte fuse to form a zygote. It is well established that early development of the embryo is driven by factors that are inherited from the oocyte. A complex maternal program, consisting of mRNAs, proteins and chromatin modifications is prepared in the oocyte that bears the intimate potential to execute events that will ultimately lead to the formation of a totipotent embryo. The components of this maternal program and the players involved in establishing it are poorly characterized.
Polycomb group proteins (PcG) are evolutionarily conserved chromatin–modifying factors that maintain cellular identity during many rounds of cell division by transcriptionally repressing developmental regulator genes that are inappropriate for the given cell lineage. Lack of Polycomb function has been shown to result in de-repression of differentiation specific factors in embryonic stem (ES) cells and interfere with proper differentiation. Similarly in vivo, embryonic deficiency for some core PcG members in the mouse results in embryonic lethality around gastrulation. PcG proteins are known to function in at least two major complexes, termed Polycomb Repressive Complex 1 (PRC1) and 2 (PRC2). Silencing is thought to be in part a consequence of the ubiquitin ligase activity of the PRC1 complex towards histone H2A, inhibiting productive transcriptional elongation of genes. Previous data from our laboratory indicated that PcG proteins were present in the oocyte and transmitted to the embryo.
The aim of this PhD project is to address the role of the Polycomb silencing pathway, focusing on the PRC1 complex, during oogenesis and early embryonic development.
We show that embryos lacking the maternal contribution of Rnf2 (Ring1b) -the main catalytic subunit of PRC1- and its paralog Rnf1 (Ring1) fail to develop beyond the 2-cell stage. Expression profiling of fully grown oocytes revealed de-repression of numerous developmental regulator genes, most of which are established Polycomb targets in other cellular systems. We show however, that these differentiation-specific transcripts are only translated after fertilization, resulting in the inappropriate presence of differentiation factors during the otherwise totipotent stage of early embryogenesis. Additionally, maternal pro-nuclear transfer experiments between control and maternal Rnf1/Rnf2 double mutant zygotes revealed that the developmental block is not only due to inappropriate cytoplasmic factors, but also to a defective chromatin setup inherited from the PRC1 deficient oocyte.
Our findings demonstrate that PRC1-mediated silencing during oocyte growth is an essential component in the preparation of the maternal to embryonic transition program required for proper initiation of embryonic development.