DNA Methylation Divergence and Tissue Specialization in the Developing Mouse Placenta

The placental epigenome plays a vital role in regulating mammalian growth and development. Aberrations in placental DNA methylation are linked to several disease states, including intrauterine growth restriction and preeclampsia. Studying the evolution and development of the placental epigenome is critical to understanding the origin and progression of such diseases. Although high resolution studies have found substantial variation between placental methylomes of different species, the nature of methylome variation has yet to be characterized within any individual species. We conducted a study of placental DNA methylation at high resolution in multiple strains and closely related species of house mice (Mus musculus musculus, Mus m. domesticus, and M. spretus), across developmental timepoints (embryonic days 15 to 18), and between the labyrinthine transport and junctional endocrine layers. We observed substantial genome-wide methylation heterogeneity in mouse placenta compared to other differentiated tissues. Species-specific methylation profiles were concentrated in retrotransposon subfamilies, specifically RLTR10 and RLTR20 subfamilies. Regulatory regions such as gene promoters and CpG islands displayed cross-species conservation, but showed strong differences between layers and developmental timepoints. Partially methylated domains exist in the mouse placenta and widen during development. Taken together, our results characterize the mouse placental methylome as a highly heterogeneous and deregulated landscape globally, intermixed with actively regulated promoter and retrotransposon sequences. Two datasets were produced for this study, an inter-specific dataset and intra-specific dataset. Inter-specific samples are whole placental samples, originated from three species, and are composed of reciprocal hybrid crosses between strains in each species. Intra-specific samples are purified methylomes of the junctional and labyrinthine zones with equal numbers of samples from male and female mice and embryonic days 15 and 18.