Molecular reprogramming in bovine embryos after serial somatic cell chromatin transfer

Somatic Cell Nuclear Transfer (SCNT), commonly known as cloning, is the transfer of a somatic nucleus into an enucleated oocyte to produce a clone. The chromatin structure of somatic cells permits the expression of certain genes, while silencing the rest of the genome. The cytoplasm of oocytes can reprogram a somatic nucleus by reactivating the genes necessary for embryonic development and silencing the somatic genes. However, the low efficiency of SCNT indicates that successful nuclear reprogramming is a rare event. The objectives of this study were determine the extent of transcriptional reprogramming in bovine blastocysts produced by serial rounds of chromatin transfer (from first and fourth generations), using blastocysts produced by in vitro fertilization (IVF) as controls, to identify cumulative errors in the transcriptome profile. Differentially expressed genes were studied further to determine their function in embryonic development. We identified a set of transcripts consistently misregulated in cloned blastocyst, some of which had a more marked misregulation in the embryos produced by 4 successive rounds of cloning. Among the genes significantly upregulated in both CT groups compared to IVF blastocysts were both de novo DNA methylation enzymes DNMT3A and DNMT3B. Expression patterns, structural and functional analyses were performed for DNA methyltransferases. A high structural and functional conservation was observed for DNA methyltransferases among human, mouse, and bovine species. A set of genes that participate in early embryonic development, chromatin remodeling and DNA methylation were differentially regulated in cloned embryos and had not been fully annotated at the time of the analysis. We annotated those genes and submitted them to the Bovine Genome Sequencing Consortium database. These results have important implications for the selection of models for the study of DNA methylation during early development. The present study provides a valuable data set for identifying possible cumulative errors in somatic cell chromatin transfer that could hinder nuclear reprogramming shedding light on the epigenetic role in reprogramming and cell plasticity