Breakthrough of 2008 - Reprogramming
January 26, 2009 by Aaron Cheung
In December 2008, the scientific journal Science, named the incredible process of stem cell reprogramming breakthrough of the year 2008. Reprogramming is the reversal of the state of a somatic cell (i.e. a differentiated cell or a cell that has become a skin cell or kidney cell) back to a state reminiscent to that of a pluripotent state (i.e. an embryonic stem cell).
Long before Science named reprogramming breakthrough of the year, reprogramming had already been shown possible when pluripotent stem cells were made by other processes such as cell-cell fusion where a somatic cell was fused with an embryonic stem cell and the resulting “hybrid” exhibited embryonic traits.
Another process is somatic cell nuclear transfer (SCNT) where the nucleus of a somatic cell is transferred into an enucleated oocyte (oocyte that lacks a nucleus) and the resulting cell is able to become a pluripotent stem cell. In fact, SCNT was how the first mammal, Dolly the sheep, was cloned by Dr Ian Wilmut at the Roslin Institute in UK (reported in scientific journal Science in February 1997). As promising as these studies were, both these processes had severe limitation as cell-cell fusion created pluripotent stem cells that had the wrong number of chromosomes, and SCNT required the use of oocytes which led to ethical concerns.
However, the reprogramming that Science considered being a breakthrough was first brought to light by Dr. Shinya Yamanaka at the Kyoto University (reported in scientific journal Cell in August 2006). Dr. Yamanaka hypothesized that during cell-cell fusion or SCNT, there must be something in the environment of the pluripotent stem cell (in cell-cell fusion) or the oocyte (in SCNT) that causes the differentiated nucleus to return back to a pluripotent state. He hypothesized that the “something in the environment” are transcription factors, proteins that transcribe genes into transcripts.
For that reason, Dr. Yamanaka screened 24 transcription factors that he thought were important in pluripotent stem cells. To his surprise, when he introduced all 24 transcription factors into mouse fibroblasts, they reprogrammed to an embryonic state, which he called induced Pluripotent Stem (iPS) cells. He then very systematically narrowed the 24 transcription factors down to the 4 minimally needed transcription factors (Oct4, Sox2, Klf4, and c-Myc) also known as the “Yamanaka Factors” in the scientific community.
Little over a year later, Dr. Yamanaka repeated this incredible feat in the human system where he was able to generate human iPS cells from human fibroblasts by using the same original Yamanaka Factors (reported in scientific journal Cell in November 2007). This work took the stem cell community by storm as there is now a system that allows the generation of personalised pluripotent stem cell without the surrounding ethical issues related to the use of embryos (for human embryonic stem cells) and/or oocytes (for human pluripotent stem cells from SCNT – although this feat has not been achieved to date).
With the possibility to generate personalized iPS cells, the potential in regenerative medicine is huge. Personalized iPS cells will allow us to generate disease-specific iPS cells from patients to model human diseases which I have blogged in my last two blogs. Personalized iPS cells will allow one to perform drug screens that are specific for each patient. Finally, one day, personalised iPS cells can be used in cell therapy such that patient will be treated using their own cells such that immune rejection will not be an issue.
However, many obstacles still need to be overcome, for example, the current method of delivering the Yamanaka Factors require the use of viruses which integrate randomly into our genome which may cause oncogenic transformation leading to tumor formation. Scientists hypothesize that iPS cells will one day be able to be generated without the use of viruses such as by using chemicals. This has already been done in mouse iPS cells which I have blogged about previously. It will be a matter of time before this is achieved in human iPS cells too.
As a graduate student who started merely after a month on the achievement of human iPS cells, it was truly a blessing to have the opportunity to work in this exciting field and to see how fast this field has been growing since its birth. Not only has iPS cell field impacted the stem cell community; it is also being introduced in undergraduate studies as I am also a TA at the University of Toronto. For the first tutorial of an Introductory Genetics course; students were instructed to read about the breakthrough of the year article in Science.
Since the first human embryonic stem cells isolated back in 1998 by Dr. James Thomson at the University of Wisconsin (reported in scientific journal Science in November 1998), stem cell scientists have come a long way to generate personalized pluripotent stem cells for its use in regenerative medicine, with the discovery of cell-cell fusion, SCNT, and now reprogramming using the Yamanaka factors. Only time will tell where the iPS field will lead us to.
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Rett Syndrome and induced Pluripotent Stem cells goes “Green”
June 17, 2009 by Aaron Cheung
Stem cell researchers at The Hospital for Sick Children have now devised a method, reported in the scientific journal Nature Methods, that will facilitate the generation of induced Pluripotent Stem (iPS) cells. I have previously blogged about the promise of iPS cells (see related articles). iPS cells are cells which have been reprogrammed from a differentiated somatic cell (such as your skin cells) to a pluripotent stem cell (like an embryonic stem cell, only that you don’t need to use embryos!). The generation of iPS cells is inherently very rare which occurs at a frequency of about 0.01% meaning you can generate about one iPS cell line out of hundred thousand cells. Out of which, many lines fail to satisfy the different rigorous tests necessary to be given the prestigious name of “iPS cell”.
A group led by Dr. James Ellis, and lead author Dr. Akitsu Hotta, at The Hospital for Sick Children has generated a reporter, named EOS, that will facilitate the generation of iPS cells for both mouse and human. The reporter has DNA elements (called promoters) that will drive the expression of a fluorescent protein (GFP) and an antibiotic resistance gene (puromycin) that will only turn on in pluripotent stem cells, but remain off in differentiated cells. Therefore, scientists can now generate iPS cells from different sources and the iPS cells will literally “light up with a green colour” once reprogramming has been achieved. Furthermore, by adding puromycin to your culture, it will keep the iPS cells in a pluripotent state as iPS cells will normally spontaneously differentiate.
To demonstrate the applicability of EOS, Ellis also derived iPS cells from patients who have a neurodevelopmental disorder called Rett Syndrome. Rett Syndrome is an Autism Spectrum Disorder which affects almost exclusively females at an incidence of 1 in 10,000 and is one of the leading causes of mental retardation in girls. These Rett Syndrome iPS cells were green when placed under a fluorescent microscope and Ellis demonstrated that neurons can be generated from these Rett Syndrome iPS cells, the cell type affected in Rett Syndrome.
In conclusion, the EOS reporter will facilitate the generation of iPS and will be particularly useful for new labs going into this rapidly developing field. Furthermore, Rett Syndrome iPS cells that were also generated in this study will be invaluable for future drug screens that may be beneficial to those with Rett Syndrome.
Article:
Hotta, A. et al. Isolation of human iPS cells using EOS lentiviral vectors to select for pluripotency. Nature Methods 6, 370–376 (2009).
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[...] the incredible feat of iPS cells recently as the scientific journal Science has named it “Breakthrough of the year”. These iPS cells are thought to hold the same regenerative potential as human embryonic stem [...]
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