Unreprogrammed H3K9me3 prevents minor zygotic genome activation and lineage commitment in SCNT embryos

Abstract

Somatic cell nuclear transfer (SCNT) can be used to reprogram differentiated somatic cells to a totipotent state but has poor efficiency in supporting full-term development. H3K9me3 is considered to be an epigenetic barrier to zygotic genomic activation in 2-cell SCNT embryos. However, the mechanism underlying the failure of H3K9me3 reprogramming during SCNT embryo development remains elusive. Here, we perform genome-wide profiling of H3K9me3 in cumulus cell-derived SCNT embryos. We find redundant H3K9me3 marks are closely related to defective minor zygotic genome activation. Moreover, SCNT blastocysts show severely indistinct lineage-specific H3K9me3 deposition. We identify MAX and MCRS1 as potential H3K9me3-related transcription factors and are essential for early embryogenesis. Overexpression of Max and Mcrs1 significantly benefits SCNT embryo development. Notably, MCRS1 partially rescues lineage-specific H3K9me3 allocation, and further improves the efficiency of full-term development. Importantly, our data confirm the conservation of deficient H3K9me3 differentiation in Sertoli cell-derived SCNT embryos, which may be regulated by alternative mechanisms.

Epigenetic reprogramming in the transition from pluripotency to totipotency

“…After fertilization, histone modifications associated with constitutive heterochromatin such as H3K9me3, H3K64me3, and H4K20me3 decreased sharply at the genome-wide level (Burton et al, 2020). De novo H3K9me3 assists in creating "mature" constitutive heterochromatin during implantation and is believed to hinder cell fate change (Li, Sun et al, 2023;Xu et al, 2022Xu et al, , 2023. If H3K9me2/3 is not fully demethylated, it can severely affect ZGA and early embryonic development (Matoba et al, 2014).…”

The Dynamics of Histone Modifications during Mammalian Zygotic Genome Activation

“…Recent revelations from investigations into Somatic Cell Nuclear Transfer (SCNT) embryos add a layer of complexity to our understanding of H3K9me3 dynamics [84,85]. SCNT enables animal resurrection and the treatment of human diseases by reprogramming somatic cells into pluripotent states.…”

Heterochromatin protein ERH represses alternative cell fates during early mammalian differentiation

“…Following fertilization, the human zygote divides three times to 8 cells while the cells remain totipotent -possessing the ability to contribute to all embryonic and extraembryonic cell lineages 5 . Zygotic genome activation (ZGA), marked by initial transcription of key genes and transposable elements 6 , occurs at the 1-2 cell stage in mice and the 4-8 cell stage in humans 6 , and requires demethylation of H3K9me3 marks in oocytes 7,8 . Protein-coding genes and a subset of repeats activated in ZGA are subsequently silenced through H3K9me3 establishment as the embryo loses totipotency and differentiates into the first two developmentally-specified lineages, the inner cell mass (ICM) and the trophectoderm, forming the blastocyst 4,9,10 .…”