Dissecting the embryonic and extraembryonic requirements for DNA methylation in mice

Abstract

DNA methylation is a remarkably dynamic repressive DNA modification that can be stably propagated over successive cell divisions and forms an essential layer of the epigenetic landscape that defines each cell. In the early mammalian embryo, DNA methylation is largely erased after zygote formation and established anew during the process of uterine implantation by the coordinated activities of the DNA methyltransferases (Dnmts). In mice, by the time implantation is complete the Dnmts have established two distinct DNA methylation patterns in the embryo and its extraembryonic lineages. Disrupting any Dnmt at the single-cell zygote stage results in lethality demonstrating the importance of methylation establishment. Previous work has largely focused on the role of the Dnmts in the embryo without much interest in their role in the placenta, a transient support organ critical for the growth and survival of the embryo. In this study, we investigate the developmental consequences of selectively disrupting all active Dnmts in the placenta and the embryo. To do so, we applied tetraploid complementation as a means to generate reciprocal knockouts of the Dnmts in either the embryo alone or the extraembryonic lineages alone. We found that the extraembryonic loss of the Dnmts results in restricted growth and lethality of an otherwise wildtype embryo, and that embryos lacking Dnmt expression are nonviable, indicating an essential role in both the embryonic and extraembryonic lineages. Finally, we systematically dissected the tolerance of the developing embryo and its placenta to a range of global DNA methylation levels and find that the embryo proper has a stringent DNA methylation requirement needed for gastrulation while the placenta has a broader range of methylation permissible for its differentiation. Together, our results define an essential role for the Dnmts in generating and maintaining distinct methylation patterns in the developing embryo and placenta, and this work establishes a framework for studying a gene’s function in the embryonic and extraembryonic lineages independently.