Interpretation of reprogramming to predict the success of somatic cell cloning

In the context of mammalian somatic cell cloning, the term reprogramming refers to the processes that enable a somatic cell nucleus to adopt the role of a zygotic nucleus. Gene re-expression is one measure of reprogramming if correlated with subsequent developmental potential. This paper describes several experiments utilizing pre-implantation gene expression to evaluate reprogramming and clone viability. We have established a direct correlation between Oct4 expression in mouse clones at the blastocyst stage and their potential to maintain pluripotent embryonic cells essential for post-implantation development. Furthermore, the quality of gene expression in clones dramatically improves when genetically identical clones are combined in clone-clone aggregate chimeras. Clone--clone aggregates exhibit a higher developmental potential than single clones both in vitro and in vivo. This could be mediated by complementation between blastomeres from epigenetically different clones within the aggregate rather than by the increase in cell number resulting from aggregation. We also discuss the use of tetraploid embryos as a model to evaluate reprogramming using gene expression and demonstrate that somatic cell nuclei can be reprogrammed by blastomeres to re-express embryonic specific genes but not to contribute to post-implantation development.