Reshaping the transcriptional frontier: Epigenetics and somatic cell nuclear transfer

Abstract

SUMMARY Somatic-cell nuclear transfer (SCNT) experiments have paved the way to the field of cellular reprogramming. The demonstrated ability to clone over 20 different species to date has proven that the technology is robust but very inefficient, and is prone to developmental anomalies. Yet, the offspring from cloned animals exhibit none of the abnormalities of their parents, suggesting the low efficiency and high developmental mortality are epigenetic in origin. The epigenetic barriers to reprogramming somatic cells into a totipotent embryo capable of developing into a viable offspring are significant and varied. Despite their intimate relationship, chromatin structure and transcription are often not uniformly reprogramed after nuclear transfer, and many cloned embryos develop gene expression profiles that are hybrids between the donor cell and an embryonic blastomere. Recent advances in cellular reprogramming suggest that alteration of donor-cell chromatin structure towards that found in an normal embryo is actually the rate-limiting step in successful development of SCNT embryos. Here we review the literature relevant to the transformation of a somatic-cell nucleus into an embryo capable of full-term development. Interestingly, while resetting somatic transcription and associated epigenetic marks are absolutely required for development of SCNT embryos, life does not demand perfection.

Artificial cloning of domestic animals

“…In fact, it is surprising that they are capable of achieving a sufficiently appropriate epigenome that allows full development. As remarked by other authors, nature allows a certain amount of flexibility in the epigenome and gene expression during growth and development (28). As might be anticipated, clones that fail during gestation and/or have physiological abnormalities have been found to have abnormal epigenetic patterns whereas those that thrive have a comparatively normal pattern (28,36).…”

“…As remarked by other authors, nature allows a certain amount of flexibility in the epigenome and gene expression during growth and development (28). As might be anticipated, clones that fail during gestation and/or have physiological abnormalities have been found to have abnormal epigenetic patterns whereas those that thrive have a comparatively normal pattern (28,36). It is also interesting to note that the gestational losses and abnormalities observed in SCNT were also noted during the development of in vitro embryo production and culture techniques in domestic species in the 1990s.…”

“…Due to the cost and extended generation times for domestic species, most of these studies, so far, have focused on early embryonic and fetal development. As with epigenetic patterns described above, many of these studies report either the normal expression of key genes after a reconstructed embryo passes a developmental checkpoint (e.g., blastocyst formation) or abnormal gene expression after failure to pass such checkpoints (28,36). These experiments focused mainly on how SCNT worked or did not work, and few use SCNT to explore questions regarding developmental regulation of genes.…”

Dnmt3l-knockout donor cells improve somatic cell nuclear transfer reprogramming efficiency

“…Improving SCNT with Dnmt3l-KO donor cells specific therapeutic applications (Nishikawa et al 2008, Cantone & Fisher 2013, Long et al 2014. However, successful reprogramming occurs only in a proportion of cloned embryos; therefore, improvements in developmental efficiency and our understanding of epigenetic reprogramming are essential.…”

“…Embryonic stem cells generated from NT-derived blastocysts (NT-ESCs) serve as a potential resource for regenerative medicine. However, the low efficiency and developmental abnormalities of cloned embryos remain important challenges in NT-associated research and applications , Rodriguez-Osorio et al 2012, Long et al 2014.…”