Climbing Mount Efficiency--small steps, not giant leaps towards higher cloning success in farm animals.

Despite more than a decade of research efforts, farm animal cloning by somatic cell nuclear transfer (SCNT) is still frustratingly inefficient. Inefficiency manifests itself at different levels, which are currently not well integrated. At the molecular level, it leads to widespread genetic, epigenetic and transcriptional aberrations in cloned embryos. At the organismal level, these genome-wide abnormalities compromise development of cloned foetuses and offspring. Specific molecular defects need to be causally linked to specific cloned phenotypes, in order to design specific treatments to correct them. Cloning efficiency depends on the ability of the nuclear donor cell to be fully reprogrammed into an embryonic state and the ability of the enucleated recipient cell to carry out the reprogramming reactions. It has been postulated that reprogrammability of the somatic donor cell epigenome is influenced by its differentiation status. However, direct comparisons between cells of divergent differentiation status within several somatic lineages have found no conclusive evidence for this. Choosing somatic stem cells as donors has not improved cloning efficiency, indicating that donor cell type may be less critical for cloning success. Different recipient cells, on the other hand, vary in their reprogramming ability. In bovine, using zygotes instead of oocytes has increased cloning success. Other improvements in livestock cloning efficiency include better coordinating donor cell type with cell cycle stage and aggregating cloned embryos. In the future, it will be important to demonstrate if these small increases at every step are cumulative, adding up to an integrated cloning protocol with greatly improved efficiency.