Stem cells: The cost of perpetual youth

Epigenetic integrity of paternal imprints enhances the developmental potential of androgenetic haploid embryonic stem cells

The use of two inhibitors of Mek1/2 and Gsk3β (2i) promotes the generation of mouse diploid and haploid embryonic stem cells (ESCs) from the inner cell mass of biparental and uniparental blastocysts, respectively. However, a system enabling long-term maintenance of imprints in ESCs has proven challenging. Here, we report that the use of a two-step a2i (alternative two inhibitors of Src and Gsk3β, TSa2i) derivation/culture protocol results in the establishment of androgenetic haploid ESCs (AG-haESCs) with stable DNA methylation at paternal DMRs (differentially DNA methylated regions) up to passage 60 that can efficiently support generating mice upon oocyte injection. We also show coexistence of H3K9me3 marks and ZFP57 bindings with intact DMR methylations. Furthermore, we demonstrate that TSa2i-treated AG-haESCs are a heterogeneous cell population regarding paternal DMR methylation. Strikingly, AG-haESCs with late passages display increased paternal-DMR methylations and improved developmental potential compared to early-passage cells, in part through the enhanced proliferation of H19-DMR hypermethylated cells. Together, we establish AG-haESCs that can long-term maintain paternal imprints.

Effects of reprogramming on genomic imprinting and the application of pluripotent stem cells

Pluripotent stem cells are considered to be the ideal candidates for cell-based therapies in humans. In this regard, both nuclear transfer embryonic stem (ntES) cells and induced pluripotent stem (iPS) cells are particularly advantageous because patient-specific autologous ntES and iPS cells can avoid immunorejection and other side effects that may be present in the allogenic pluripotent stem cells derived from unrelated sources. However, they have been found to contain deleterious genetic and epigenetic changes that may hinder their therapeutic applications. Indeed, deregulation of genomic imprinting has been frequently observed in reprogrammed ntES and iPS cells. We will survey the recent studies on genomic imprinting in pluripotent stem cells, particularly in iPS cells. In a previous study published about six years ago, genomic imprinting was found to be variably lost in mouse iPS clones. Intriguingly, de novo DNA methylation also occurred at the previously unmethylated imprinting control regions (ICRs) in a high percentage of iPS clones. These unexpected results were confirmed by a recent independent study with a similar approach. Since dysregulation of genomic imprinting can cause many human diseases including cancer and neurological disorders, these recent findings on genomic imprinting in reprogramming may have some implications for therapeutic applications of pluripotent stem cells.

Papers of note in Nature (7666)

This week’s articles describe the consequences of preserving pluripotency in cultured stem cells, how glutamate metabolism in T cells contributes to autoimmune disease, and how inactivating BRAF mutations promote tumor growth.