Understanding epigenetic remodeling in primordial germ cells

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Summary Germ cells are responsible for the passage of a parent's genome and epigenome from one generation to the next. Although the genome does not change after fertilization (except in instances of DNA damage) the epigenome in mammals is substantially altered through a process known as epigenetic remodeling. After embryo implantation, a second major wave of epigenetic remodeling occurs, this time in newly specified germ cells of the embryo called primordial germ cells (PGCs). The second wave of epigenetic remodeling, most notably erasure of DNA methylation from the epigenome is speculated to erase any acquired epialleles that could cause disease in future generations. In the last five years, my lab together with colleagues in the field discovered that DNA methylation remodeling in mouse and human PGCs is incomplete, involving stage- dependent combinations of DNA methylation erasure and DNA methylation protection. In our previous funding period, we showed that disrupting stage-dependent DNA methylation remodeling in PGCs results in germ cell loss and infertility. Given the importance of correctly staged DNA methylation remodeling to the biology of PGCs and the ability to reproduce, we are next interested in the underlying chromatin landscape responsible for dynamic DNA methylation protection and erasure. Results from this work will significantly enhance our knowledge of the epigenetic basis of reproduction. In this renewal, our overall hypothesis is that Polycomb repressor complex 2 upstream of Histone H3 Lysine 27 trimethylation (H3K27me3) (aim 1) and Tripartite motif 28 (Trim28) upstream of H3K9me3 (aim 2) play major roles in stage-dependent DNA methylation remodeling in PGCs. To address these hypotheses, we aim to use a combination of genomics, epigenomics and mouse modeling. In addition, we also aim to use single cell sequencing technologies (aim 3) to define the true epigenetic ground state of PGCs. In summary, identifying the epigenetic landscape of PGCs and the enzymes required to maintain it are critical to prioritizing future studies that disrupt the epigenome in PGCs during pregnancy, or the identification of epigenetic hot-spots in PGCs that could be tested for specific roles in infertility or transgenerational epigenetic inheritance in the future.