‘Placeholder’ nucleosomes underlie germline-to-embryo DNA methylation reprogramming [RRBS]

The function and retention/reprogramming of epigenetic marks during the germline-to-embryo transition is a key issue in developmental and cellular biology, with relevance to stem cell programming and trans-generational inheritance. In zebrafish, DNAme patterns are programmed in transcriptionally-quiescent early cleavage embryos; paternally-inherited patterns are maintained, whereas maternal patterns are reprogrammed to match the paternal pattern. Here we show that a ‘placeholder’ nucleosome, containing the histone H2A variant H2A.Z(FV) and H3K4me1, occupies virtually all regions lacking DNAme in both sperm and cleavage embryos – residing at promoters encoding housekeeping and early embryonic transcription factors. Upon genome-wide transcriptional onset, genes with the Placeholder become either active H3K4me3-marked or silent H3K4me3/K27me3-marked (bivalent). Importantly, functional perturbation causing Placeholder loss confers DNAme acquisition, whereas acquisition/expansion of Placeholder confers DNA hypomethylation and improper gene activation. Thus, during transcriptionally quiescent stages (gamete-zygote-cleavage), an H2A.Z(FV)/H3K4me1-containing Placeholder nucleosome deters DNAme, poising parental genes for either gene-specific activation or facultative repression. DNA methylation was analyzed for Sperm and Sphere stage embryos that were either mutant for the anp32e gene, or were injected with either an SRCAP morpholino or recombinant Anp32e protein.