Retrotransposon expression as a defining event of genome reprograming in fertilized and cloned bovine embryos

Genome reprograming is the ability of a nucleus to modify its epigenetic characteristics and gene expression pattern when placed in a new environment. Low efficiency of mammalian cloning is attributed to the incomplete and aberrant nature of genome reprograming after somatic cell nuclear transfer (SCNT) in oocytes. To date, the aspects of genome reprograming critical for full-term development after SCNT remain poorly understood. To identify the key elements of this process, changes in gene expression during maternal-to-embryonic transition in normal bovine embryos and changes in gene expression between donor cells and SCNT embryos were compared using a new cDNA array dedicated to embryonic genome transcriptional activation in the bovine. Three groups of transcripts were mostly affected during somatic reprograming: endogenous terminal repeat (LTR) retrotransposons and mitochondrial transcripts were up-regulated, while genes encoding ribosomal proteins were downregulated. These unexpected data demonstrate specific categories of transcripts most sensitive to somatic reprograming and likely affecting viability of SCNT embryos. Importantly, massive transcriptional activation of LTR retrotransposons resulted in similar levels of their transcripts in SCNT and fertilized embryos. Taken together, these results open a new avenue in the quest to understand nuclear reprograming driven by oocyte cytoplasm.