Nuclear Cloning and the Reprogramming of the Genome

A major issue raised some 50 years ago in seminal frog cloning experiments was the question of whether nuclei of terminally differentiated cells can be reprogrammed to generate animals after nuclear transfer. We have shown recently that monoclonal mice can be derived from mature B and T cells by nuclear transfer, demonstrating that the genome of terminally differentiated cells can be reprogrammed to direct development of a new animal. The efficiency is, however, very low, suggesting that most cloned animals may be derived from somatic stem cells rather than terminally differentiated cells as assumed. Nuclear transfer represents an unbiased experimental approach that allows distinguishing between epigenetic and genetic alterations of the genome. We have used this approach to derive animals from postmitotic neurons to investigate whether neural functions such as olfactory receptor choice or memory involve genetic in addition to epigenetic changes. In our most recent experiments we have derived cloned mice from nuclei of cancer cells in an effort to distinguish between epigenetic (=reversible) and genetic (=irreversible) changes that are involved in malignant transformation. Finally, we have performed a "proof of principle" experiment in a mouse disease model to provide evidence that therapeutic cloning combined with gene therapy represents a valid strategy for transplantation therapy.