Cloning, stem cells and epigenetic reprogramming after nuclear transfer

Abstract

The process by which a single totipotent cell becomes a complex organism is a unidirectional program, with each mitotic division generating new cells that gradually differentiate towards more specified fates and specialized functions. Nuclear transfer (NT) experiments have demonstrated the epigenetic nature of development and showed, that although differentiated cells have a very limited developmental potential, the nuclei of these cells retain the potency to direct embryogenesis after reintroduction into the unfertilized oocyte. Herein, we have used the mouse as a model system for understanding both the nature of epigenetic reprogramming that occurs after NT as well as the ramifications it has for the development of cloned animals. Specifically, we investigated how epigenetic states are reprogrammed after NT and demonstrated that the inactive X chromosome is reactivated in NT embryos, resulting in normal X inactivation in female clones. Additionally, investigations into the factors that influence the survival of cloned animals, indicate that there are considerable genetic influences on the cloning process. These genetic factors modify the survival of mice cloned from ES cells by influencing the developmental potential of the donor ES cells rather then the reprogramming process itself. This realization has subsequently led to the development of novel methods for the expedited production of complex mutant mice, which are also described. Finally, we have created cloned embryos by NT from both cortical and mature olfactory sensory neurons to address question of nuclear equivalence in the brain and to investigate whether generation of synaptic diversity or odorant receptor choice, are mediated by genetic as well as epigenetic events.