Dynamic expression of chromatin modifiers during developmental transitions in preimplantation embryos

There are two major developmental transitions that take place during the first four days of mouse embryogenesis. The first one is the maternal-to-zygotic transition in the zygote and 2-cell embryo, the second one is the first cell lineage commitment at the 32-cell stage. A number of transcription factors govern zygotic expression and the specification of pluripotent versus trophectoderm cells, while at the same time there are dynamic changes in DNA methylation and chromatin organization. It is not fully understood which chromatin modifying complexes take part in these processes and whether there is a particular spatiotemporal distribution of certain chromatin players in preimplantation development. To address this question, we performed gene expression profiling of single cells from the oocyte to the blastocyst stage. We describe the patterns of expression of over 100 genes involved in histone methylation, DNA methylation and chromatin remodelling. For a number of these genes we observed differential maternal versus zygotic expression. This is particularly the case for homologous genes like Tet1 and Tet3 (zygotic and maternal respectively), or Suv39h1 and Suv39h2 (zygotic and maternal respectively). In contrast to the maternal-to-zygotic switch, most chromatin modifying genes are ubiquitously expressed in the subsequent lineage specification at the 32-cell stage. The few exceptions that showed a differential expression pattern are Kdm1b, Tet1 and Prdm14. Our data suggests that genes coding for histone modifying complexes are ubiquitously expressed in early embryos and that there are maternal and zygotic variants of most of these complexes. Furthermore, the method that we applied, allows the distinction of female versus male embryos based on the expression of Xist (X-linked, expressed in female embryos) and Kdm5d (Y-linked, expressed strongly in male embryo). With the exception of the X- and Y-linked genes, we did not observe sex-specific gene expression patterns of chromatin modifiers or transcription factors. 189 samples screened for the expression of 192 genes by Fluidigm qPCR