Genomic Imprinting, Nuclear Cloning and the Reprogramming of the Genome

Rudolf Jaenisch
Whitehead Institute for Biomedical Research

It has been well established that DNA methylation is essential for the maintenance of monoallelic expression of imprinted genes. Thus, inactivation of the maintenance DNA methyltransferase, Dnmt1, in ES cells results in the elimination of not only CpG methylation from the bulk of the genome but also from all imprinted genes. Importantly, upon reintroduction of a functional copy of Dnmt1 into mutant ES cells, methylation of the bulk of the genome, i.e. of all non-imprinted genes, is restored to normal levels. In contrast, no remethylation of the demethylated imprinted genes occurs indicating that the methylation status of imprinted genes is set only during gametogenesis and cannot be altered in the postzygotic embryo.

We are interested in the role of imprinting in mammalian development. Our goal is to establish an experimental system that would allow us to generate embryos which are essentially non-imprinted. The phenotype of such "imprint-free" embryos should be highly informative as to the intrinsic role of imprinting in mammalian development if there is any. To generate non -imprinted embryos we are using the timed inactivation and reactivation of the DNA Dnmt1 during gametogenesis and postzygotic development as well as nuclear cloning approaches.

One of the most interesting issues of nuclear cloning is the question of genomic reprogramming, i.e. the question whether the process involves the resetting of the epigenetic modifications which are characteristic for the adult donor nucleus to that of the embryo. To this end we are using molecular approaches to compare the methylation status of normal and cloned blastocysts.