Posted by admin at 8:02 PM Tuesday, March 30, 2010
The ethically fraught field of embryonic stem cell research received much attention in late 2007 when induced pluripotent cells stem cells (iPSCs) were derived from somatic cells manipulated with the Yamanaka factors– Oct3/4, Sox2, Klf4, c-Myc. These genes, which are highly expressed in embryonic stem cells, induce pluripotency and “embryonic stem cell-like” characteristics in human and mouse cells when overexpressed. Such cells hold promise for the field of regenerative medicine, and they dodge the controversy surrounding embryonic stem cells, since iPSCs can be derived from somatic cells, not embryos. Furthermore, they have demonstrated therapeutic benefit similar to that of embryonic stem cells. However, iPSCs are not free from drawbacks, and use could be limited in humans if viral transgenes are used in the induction process. This is especially true for oncogenes c-Myc and KLF4; reactivation of these in the host genome can lead to tumor formation. This has led researchers to examine the precise mechanisms of the Yamanaka factors and seek out combinations of 1-2 factors that are equally efficacious but pose less risk of tumorigenicity. Kim et al. (1) demonstrated in Feburary of this year that exogenous Oct4 expression was sufficient to reprogram adult mouse neural stem cells into iPSCs with capacity to differentiate into cells of endodermal, ectodermal, and mesodermal lineage. In a Nature Letter, Kim et al. showed that ectopic expression of Oct4 is sufficient to induce pluripotency in human fetal neural stem cells. In vitro, neural stem cells were retrovirally infected with human Oct4 and KLF (two factor) or Oct4 alone (one factor). Eight days after infection, cells were replated on feeder layers, and 10 weeks later, colonies with neural rosettes were observed. Within 5-6 days, the rosette could be removed from the colony, and the rest of the human ES-cell-like colony could be replated and cultured, with an overall reprogramming efficiency of 0.004% in the single factor colony. To confirm that the Oct4 reprogramming was sufficient, epigenetic analysis showed that the levels of methylation of the Oct4 promoter in the reprogrammed cells and embryonic stem cells were similar. The reprogrammed cells retained normal karyotypes, suggesting that single-factor reprogramming is not only feasible, but also not harmful to cell differentiation capacity and phenotype. It is possible that reprogramming of these neural stem cells is possible with only a single factor because their genetic and epigenetic profile is similar to that of embryonic stem cells. Future work is necessary to examine whether non-viral manipulation of cells can induce pluripotency with enough efficiency to create therapeutic cell lines, and which cell sources are ideal reprogramming candidates.
- Katie Ransohoff
[From September 3, 2009]