(L to R): Jochen Utikal, Matthias Stadtfeld, Nimet Maherali,
Konrad Hochedlinger, Sarah Eminli, and Katrin Arnold
Nuclear transfer experiments, such as the cloning of Dolly
the sheep, have demonstrated that the genome of differentiated adult cells can
be reset to an embryonic state, indicating that restrictions on a cell’s
developmental potency can be erased. Understanding the factors that mediate
this process has been a long sought-after goal in the field of nuclear
reprogramming, and a large step forward was taken by a Shinya Yamanaka’s group
in a landmark study published last year1. Yamanaka’s group
identified a suite of four transcription factors, Oct4, Sox2, c-Myc, and Klf4,
whose introduction into adult mouse fibroblasts enabled the generation of cells
that were similar to embryonic stem (ES) cells. However, these induced
pluripotent (iPS) had epigenetic and transcriptional states that seemed to lie
somewhere between fibroblasts and ES cells, indicated that reprogramming was
incomplete. Furthermore, unlike ES cells, iPS cells could not successfully
contribute to live-born chimeric mice.
Using the same set of genes identified by Yamanaka’s group
but an optimized strategy to select for ES-like cells, we and two other groups
have independently shown that these four transcription factors are indeed
sufficient to reprogram an adult mouse cell to an embryonic state that is
molecularly and functionally indistinguishable from that of ES cells. Our
findings have been published in the inaugural issue of Cell Stem Cell2,
in parallel with the other two studies which have been presented in Nature3,4.
To compare the pluripotent state of iPS cells with ES cells,
we conducted extensive molecular analyses to characterize the epigenetic
modifications and gene expression patterns in iPS cells. Analysis of DNA
methylation at promoter regions, X chromosome inactivation patterns, and
genome-wide histone modifications all indicated that iPS cells were identical
to ES cells. Further, the transcriptional profiles of iPS and ES cells were
identical. When injected into mouse embryos, iPS cells showed broad
contribution to all tissues including the germline, which is the gold standard
for high-quality ES cells.
Our finding that only four transcription factors are
sufficient to induce genome-wide epigenetic and transcriptional changes that
permit the conversion of a fibroblast into an ES-like cell is remarkable, and
the mechanisms that guide this process will surely yield fascinating insight
into the regulation of the pluripotent state. Direct reprogramming to an ES
cell state without the use of embryos or oocytes has profound therapeutic
implications if this process could be recapitulated in human cells.
References:
- Takahashi, K. & Yamanaka, S. Cell 126, 663–676
(2006) - Maherali, N. et al. Cell Stem Cell
doi:10.1016/j.stem.2007.05.014 (2007). - Okita, K., Ichisaka, T. & Yamanaka, S. Nature
doi:10.1038/nature05934 (2007). - Wernig, M. et al. Nature doi:10.1038/nature05944 (2007).
