Time course of transcriptional reprogramming in bovine nuclear transfer embryos

Somatic cell nuclear transfer (SCNT) into an enucleated oocyte can reprogram a differentiated nucleus to a totipotent state. However, the time course of transcriptional reprogramming has not been determined. To monitor the inactivation of somatic genes after SCNT in the bovine model system, we determined transcript levels of 159 genes highly expressed in fetal fibroblast nuclear donor cells, but not in metaphase II recipient oocytes. For 18 of these genes significantly higher transcript levels were found in four-cell SCNT embryos compared with four-cell embryos derived by in vitro fertilization (IVF), indicating incomplete silencing of somatic genes at this stage. RNA sequencing revealed that nearly 600 genes with no transcripts in oocytes were activated in eight-cell IVF embryos, in agreement with major embryonic genome activation (EGA). De novo transcription in SCNT embryos was delayed (16 genes before the 16-cell stage, 300 and >800 genes at the 16-cell and blastocyst stages). Intron-containing primary transcripts as another option to detect embryonic gene transcription revealed activation of >2,200 genes in IVF embryos at the eight-cell stage, while only 828 genes were activated in SCNT embryos. At the 16-cell stage, a higher number of activated genes was found in SCNT (1,917) than in IVF embryos (738). Self-organizing tree algorithm clustering confirmed that in SCNT embryos transcription of genes that are normally activated during major EGA is delayed. Our study provides detailed insight into the timing of genome activation in cloned embryos that is substantially different from IVF embryos in spite of similar developmental kinetics. The following cell stages were collected for NT embryos: embryos at the four-cell, eight-cell, 16-cell, and blastocyst stage. Three biological replicates of each stage were sequenced on an Illumina GAIIx. Differentially abundant mRNA-species, de novo transcription and global gene expression trends were compared to investigate the time course of embryonic reprogramming in NT embryos.