Harvard University - Department of Molecular & Cellular Biology


by Konrad Hochedlinger and Nimet Maherali

June 11th, 2007

(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.


  1. Takahashi, K. & Yamanaka, S. Cell 126, 663–676 (2006)
  2. Maherali, N. et al. Cell Stem Cell doi:10.1016/j.stem.2007.05.014 (2007).
  3. Okita, K., Ichisaka, T. & Yamanaka, S. Nature doi:10.1038/nature05934 (2007).
  4. Wernig, M. et al. Nature doi:10.1038/nature05944 (2007).