Effect of nucleus transfer on gene expression in bovine embryos during early development

Background: Somatic cell nuclear transfer, commonly known as cloning is a powerful tool for the production of identical animals for productive, biomedical or conservation purposes. Today several species of mammals has been cloned using this technology. Nevertheless, the efficiency of the process is still low in terms of newborn animals. The causes for such inefficiencies are mainly the lack of knowledge about biological processes underlying this complex technology particularly nuclear reprogramming to allow the establishment of embryonic gene expression pattern. The abnormal phenotypes observed in clones during pregnancy and even after birth, have been associated with an aberrant gene expression during early embryo development. A better understanding about the molecular process that governs the early bovine embryo development could help to improve animal cloning efficiency. This paper reviewed the effect of somatic cell nuclear transfer on gene expression in bovine embryos at both pre and peri-implantation stages. Review: This paper reviewed the effect of somatic cell nuclear transfer on early bovine embryo development. During the early developmental period and prior to implantation, a bovine embryo passes through proliferation and differentiation stages in order to assure implantation and foetation. Two stages have been described: pre-implantation stage when the embryo reaches the blastocyst stage and peri-implantation also known as elongation stage. Those periods are the result of a coordinated and robustly regulated gene expression patterns. In cloned bovine embryos as well as in other species, several genes have been found deregulated not only during pre-implantation stages, but also during elongation. Many of those genes such as OCT4, SOX2, NANOG and FGF4 are related with maintain of cell pluripotence, while others such as INFtau, EOMES, CDX2 and TKDP1 are markers of trophoblastic function. Aberrant gene expression pattern is not morphologically evident at blastocyst stage. However a deregulation of the expression of trophoblastic markers during the peri-implantation period could be related with an incomplete morphological elongation. The pattern of gene expression established as consequence of the reprogramming of the donor nucleus could change depending on different factors inherent to the technique. Those factors include the donor cell, the cytoplast receptor and also the technical process associated to nuclear transfer such as enucleation, cell fusion, and activation protocol. Despite the multiples studies performed in order to improve cloned embryo competence in terms of gene expression pattern, today this topic is still the milestone of many research groups. Conclusion: Identification of deregulated genes may yield insights into mechanisms underlying the high mortality rate of cloned bovine embryos, and lead to strategies to reduce embryonic losses.