Nucleosome Reconfiguration During The Development Of Mouse Somatic Cell Nuclear Transfer Embryos

Terminally differentiated somatic cells can be reprogrammed into totipotent embryos by somatic cell nuclear transfer(SCNT).However,the reprogramming of SCNT embryos is defective compared with that of fertilized embryos,which results in low SCNT efficiency and birth defects of SCNT embryos.The nucleosome,which is the fundamental structural unit of chromatin in eukaryotes,is closely related to gene expression.After fertilization,nucleosome remodeling is one of the earliest reprogramming events and is essential for embryonic development.How the nucleosome position changes during early SCNT embryonic development is largely unexplored.Here,we performed genome-wide profiling of nucleosome occupancy and positioning in early mouse SCNT embryos using ultra-low-input MNase-seq(ULIMNase-seq)method.We found that the nucleosome-depleted regions(NDRs)around transcription start sites(TSSs)disappeared as early as 1 hour after nuclear injection and were re-established until 6 hours after activation of the SCNT embryos,which is consistent with the dynamics of the cell cycle.The dynamics of the nucleosome position in SCNT embryos were delayed compared with those in fertilized embryos,and the reprogramming of nucleosome positioning was incomplete in SCNT embryos.Subsequently,we found that the gene expression levels of the inner cell mass(ICM)in SCNT embryos were positively correlated with the NDR scores around TSSs in donor cells.Furthermore,the aberrant nucleosome occupancy at enhancer regions in SCNT embryos was also associated with incorrect gene expression.These results indicated that the memory of nucleosome occupancy at promoter or enhancer regions in donor cells was a potential barrier for SCNT-mediated reprogramming.We further confirmed that the histone acetylation level in donor cells was related to the memory of NDRs in promoter regions.Finally,we predicted some important transcription factors(TFs)that were abnormal in SCNT embryos,including EGR1 and MLX,and their abnormal expression might lead to low SCNT efficiency.Our study provides insight into nucleosome reconfiguration during the preimplantation development of SCNT embryos.