Genomic imprinting directs the allele‐specific expression of a subset of loci
according to their parental origin. Monoallelic expression of these genes is
regulated by imprinting control regions (ICRs) and is established in the
embryonic germ line through differential DNA methylation.
Differentiated cells can be reprogrammed to pluripotency by several strategies
including the ectopic expression of specific ‘inducers’ and by transfer of nuclei
into enucleated eggs. Cellular fusion of somatic cells with a pluripotent stem cell
partner can also lead to dominant pluripotent reprogramming. Although ES cells
(derived from the inner cell mass) and embryonic germ cells (EG, derived from
primordial germ cells) can both reprogram, EG cells are unique in being able to
erase genomic imprints from the somatic partner. In order to characterize the
earliest events in successful reprogramming, as well as EG‐specific DNA
demethylation, I generated experimental heterokaryons between B‐lymphocytes
and mouse stem cell lines. I showed that ES cells that lack Polycomb Repressor
Complex 2 (PRC2) failed to reprogram B cells and were unable to induce two
early events that characterise successful B cell reprogramming; a global
redistribution of HP1α and an increased serine 10 phosphorylation. In the second
part of my study I confirmed that EG cells were able to induce DNA demethylation
at several ICR in B cells following fusion. I present evidence that this
reprogramming of the somatic genome requires Tet1 and Tet2 and is
accomplished through a two‐step process involving both DNA synthesis and
conversion of 5methylcytosine into 5hydroxymethylcytosine.