Scientists from the Salk Institute have, for the first time, directly converted human skin cells into transplantable white blood cells, which are the soldiers of the immune system that fight infections and invaders. This work could prompt the creation of new therapies that introduce new white blood cells into the body that can attack diseased or cancerous cells or augment immune responses for other conditions.
This work, which shows that only a small amount of genetic manipulation could prompt this direct conversion, was published in the journal Stem Cells.
“The process is quick and safe in mice,” says senior author Juan Carlos Izpisua Belmonte, who holds the Salk’s Roger Guillemin Chair. “It circumvents long-standing obstacles that have plagued the reprogramming of human cells for therapeutic and regenerative purposes.”
The problems that Izpisua Belmonte mentions, includes the long time (at least two months) numbingly tedious cell culture work it takes to produce, characterize and differentiate induced pluripotent stem (iPS) cells. Blood cells derived from iPSCs also have other obstacles: they engraft into organs or bone marrow poorly and can cause tumors.
The new method designed by Izpisua Belmonte and his team, however, only takes two weeks, does not produce tumors, and engrafts well.
“We tell skin cells to forget what they are and become what we tell them to be—in this case, white blood cells,” says one of the first authors and Salk researcher Ignacio Sancho-Martinez. “Only two biological molecules are needed to induce such cellular memory loss and to direct a new cell fate.”
This faster reprogramming technique developed by Belmonte’s team utilized a form of reprogramming that does not go through a pluripotency stage. Such techniques are called indirect lineage conversion or direct reprogramming. Belmonte’s group has demonstrated that such approaches can reprogram cells to form the cells that line blood vessels. Thus instead of de-differentiating cells into an embryonic stem cell-type stage, these cells are rewound just enough to instruct them to form the more than 200 cell types that constitute the human body.
Direct reprogramming used in this study uses a molecule called SOX2 to move the cells into a more plastic state. Then, the cells are transfected with a genetic factor called miRNA125b that drives the cells to become white blood cells. Belmonte and his group are presently conducting toxicology studies and cell transplantation proof-of-concept studies in advance of potential preclinical and clinical studies.
“It is fair to say that the promise of stem cell transplantation is now closer to realization,” Sancho-Martinez says.
Study co-authors include investigators from the Center of Regenerative Medicine in Barcelona, Spain, and the Centro de Investigacion Biomedica en Red de Enfermedades Raras in Madrid, Spain.
Making tissue-specific progenitor cells that possess the ability to survive, but have not passed through the pluripotency state is a highly desirable goal of regenerative medicine. The technique known as “direct reprogramming” uses various genetic tricks to transdifferentiate mature, adult cells into different cell types that can be used for regenerative treatments.
Juan Carlos Izpisua Belmonte and his colleagues from the Salk Institute for Biological Studies in La Jolla, California and his collaborators from Spain have used direct reprogramming to convert human skin cells into a type of white blood cells.
These experiments began with harvesting skin fibroblasts from human volunteers that were then forced to overexpress a gene called “Sox2.” The Sox2 gene is heavily expressed in mice whose bone marrow stem cells are being reconstituted with an infusion of new stem cells. Thus this gene might play a central role is the differentiation of bone marrow stem cells.
Sox2 overexpression in human skin fibroblasts cause the cells express a cell surface protein called CD34. Now this might seem so boring and unimportant, but it is actually really important because CD34 is expressed of the surfaces of hematopoietic stem cells. Hematopoietic stem cells make all the different types of white and red blood cells in our bodies. Therefore, the expression of these protein is not small potatoes.
In addition to the expression of CD34, other genes found in hematopoietic stem cells were also induced, but not strongly. Thus overexpression of SOX2 seems to induce an incipient hematopoietic stem cell‐like status on these fibroblasts. However, could these cells be pushed further?
Gene profiling of hematopoietic stem cells from Umbilical Cord Blood identified a small regulatory RNA known as miR-125b as a factor that pushes SOX2-generated CD34+ cells towards an immature hematopoietic stem cell-like progenitor cell that can be grafted into a laboratory animal.
When SOX2 and miR-125b were overexpressed in combination, the cells transdifferentiated into monocytic lineage progenitor cells.
What are monocytes? They are a type of white blood cells and are, in fact, the largest of all white blood cells. Monocytes compose 2% to 10% of all white blood cells in the human body. They play multiple roles in immune function, including phagocytosis (gobbling up bacteria and other stuff), antigen presentation (identifying and altering other cells to the presence of foreign substances), and cytokine production (small proteins that regulate the immune response).
Monocytes express a molecule on their cell surfaces called CD14, and when human fibroblasts overexpressed Sox2 and miR-125b, they became CD14-expressing cells that looked and acted like monocytes. These cells were able to gobble up bacteria and other foreign material, and when transplanted into a laboratory animal, these directly reprogrammed cells generated cells that established the monocytic/macrophage lineage.
Cancer patients, and other patients with bone marrow diseases can have trouble making sufficient white blood cells. A technique like this can generate transplantable monocytes (at least in laboratory animals) without many of the drawbacks associated with reprogramming human cells into hematopoietic stem cells that possess true clinical potential. Also because this technique skips the pluipotency stage, it is potentially safer.
In previous posts, we have examined the debate that surrounds a population of stem cells called “Very Small Embryonic-Like Stem Cells” or VSELs. Ratajczak and his colleagues have published several papers describing the isolation and characterization of VSELs from bone marrow. VSELs express some of the same genes as embryonic stem cells (Oct4, Nanog, and Sox2), and are also very small, and have the capacity to differentiate into a wide variety of cell types.
The debate over VSELs exists because although some scientists have been able to reproduce Ratajczak’s work, but several important scientists have not been able to do so. Therefore a debate rages as to whether or not VSELs exist.
On top of this debate comes findings by several laboratories of small stem cells in bone marrow that possess the ability to divide and differentiate into several different cell types. For example Shmilovici in 2007 and Charles Vacanti at Harvard University in 2001 reported the isolation of “spore-like cells” from human bone marrow. These cells were reportedly smaller than 6 micrometers in diameter and were activated by harsh conditions or injury. However, a paper by Danova-Alt and others in 2012 examined very small embryonic-like stem cells from umbilical cord blood and was unable to confirm that these cells possesses the properties of stem cells. Stout and others in 2007 isolated “blastomere-like stem cells” from pig skeletal muscle and showed that they were mobilized into circulating blood after trauma. Also, last year, the laboratory of Muhammad Ashraf at the University of Cincinnati Medical Center isolated small juvenile stem cells from aged human bone marrow. Therefore, even though prominent laboratories have not been able to confirm the existence of VSELs, it seems very likely that some sort of small-sized stem cells are present in bone marrow. However, the nature of these cells and their capacity to proliferate and differentiate remains uncertain to date.
A recent paper from researchers at StemBios Technologies has reported the isolation and characterization of small stem cells from bone marrow that they call “SB cells.” These SB cells are less than 6 micrometers in diameter and are present in human bone marrow and peripheral blood. Even more interestingly, these SB cells express several cell surface proteins not found in other bone marrow cells (Lgr5, for example). They also did not express several of the markers found on the surfaces of VSELs (i.e. CD133). Thus these SB cells seems to constitute a unique bone marrow-based stem cell population.
When differentiated in culture, SB cells formed liver cells, fat cells, and neuron-like cells. When injected into the tail veins of a mouse, SB cells spread throughout the mouse body and were found in the brain, liver, skeletal muscles. Further work established that the SB cells had in fact differentiated into liver cells, neurons, and skeletal muscles in the bodies of the host mice.
These cells represent a potential rich source of material for regenerative medicine. In fact, Stem Bios Technologies Inc have begun planning a human clinical trial using SB cells to determine their efficacy in treating human maladies. If this trial proves successful, then SB cells and their isolation and mobilization might play a remarkable role in future regenerative treatments.
Bioengineers from the laboratory of Song Li at UC Berkeley have used physical cues to help push mature cells to de-differentiate into embryonic-like cells known as induced pluripotent stem cells.
Essentially, Li and his coworkers grew skin fibroblasts from human skin and mouse ears on surfaces with parallel grooves 10 micrometers apart and 3 micrometers high, in a special culture medium. This procedure increased the efficiency of reprogramming of these mature cells four-fold when compared to cells grown on a flat surface. Growing cells in scaffolds of nanofilbers aligned in parallel had similar effects.
Li’s study could significantly advance the protocols for making induced pluripotent stem cells (iPSCs). Normally iPSCs are made by genetically engineering adult cells so that they overexpress four different genes: Oct-4, Sox-2, Klf-4, and c-Myc. To put these genes into the cells, genetically modified viruses are used, or plasmids (small circles of DNA). Initially, Shinya Yamanaka, the scientist who invented iPSCs, and his co-workers used retroviruses that contained these four genes. When fibroblasts were infected with these souped-up retroviruses, the viruses inserted their viral DNA into the genomes of the host cells and expressed these genes.
Shinya Yamanaka won the Nobel Prize for this work in Physiology or Medicine in 2012 for this work. Unfortunately, retroviruses and can cause insertional mutations when they integrate into the genome (Zheng W., et al., Gene. 2013 Apr 25;519(1):142-9), and for this reason they are not the preferred way of making iPSCs. There are other viral vectors that do not integrate into the genome of the host cell (e.g., Sendai virus; see Chen IP, et al., Cell Reprogram. 2013 Dec;15(6):503-13). There are also techniques that use plasmids, which encode the four genes but do not integrate into the genome of the host cell. Finally, synthetic messenger RNAs that encode these four genes have also been used to make iPSCs (Tavernier G,, et al., Biomaterials. 2012 Jan;33(2):412-7).
The use of physical cues to make iPSCs may replace the need for gene overexpression, just as the use of particular chemicals can replace the need for particular genes (Zhu, S. et al. Cell Stem Cell 7, 651–655 (2010); Li, Y. et al. Cell Res. 21, 196–204 (2011)). If physical cues can replace the need for the overexpression of particular genes, then this discovery could revolutionize iPSC derivation; especially since the overexpression of particular genes in mature cells tends to cause genome instability in cells (Doris Steinemann, Gudrun Göhring, and Brigitte Schlegelberger. Am J Stem Cells. 2013; 2(1): 39–51).
“Our study demonstrates for the first time that the physical features of biomaterials can replace some of these biochemical factors and regulate the memory of a cell’s identity,” said study principal investigator Song Li, UC Berkeley, Professor of bioengineering. “We show that biophysical signals can be converted into intracellular chemical signals that coax cells to change.”
To boost the efficiency of mature cell reprogramming, scientists also use a chemical called valproic acid, which dramatically affects global DNA structure and expression.
“The concern with current methods is the low efficiency at which cells actually reprogram and the unpredictable long-term effects of certain imposed genetic or chemical manipulations,” said the lead author of this study Timothy Downing. “For instance, valproic acid is a potent chemical that drastically alters the cell’s epigenetic state and can cause unintended changes inside the cell. Given this, many people have been looking at different ways to improve various aspects of the reprogramming process.”
This new study confirms and extends previous studies that showed that mechanical and physical cues can influence cell fate. Li’s group showed that physical and mechanical cues can not only affect cell fate, but also the epigenetic state and cell reprogramming.
“Cells elongate, or example, as they migrate throughout the body,” said Downing, who is a research associate in Li’s lab. “In the case of topography, where we control the elongation of a cell by controlling the physical microenvironment, we are able to more closely mimic what a cell would experience in its native physiological environment. In this regard, these physical cues are less invasive and artificial to the cell and therefore less likely to cause unintended side effects.”
Li and his colleagues are studying whether growing cells on grooved surfaces eventually replace valproic acid and even replace other chemical compounds in the reprogramming process.
“We are also studying whether biophysical factors could help reprogram cells into specific cell types, such as neurons,” said Jennifer Soto, a UC Berkeley graduate student in bioengineering who was also a co-author on this paper.
Timothy Downing, et al., Nature Materials 12, 1154–1162 (2013).
For the first time, stem cell scientists have reprogrammed cultured skin cells into induced pluripotent cells (iPSCs) with near-perfect efficiency.
Even several laboratories have examined protocols to increase the efficiency of cellular reprogramming, a research team at the Weizmann Institute of Science in Rehovot, Israel has managed to increase the conversion rate to almost 100%, ten times the rate normally achieved, by removing a single proteins called Mbd3. This discovery can potentially allow scientists to generate large volumes of stem cells on demand, which would accelerate the development of new treatments.
In 2006, scientists from the laboratory of Shinya Yamanaka showed that mature cells could be reprogrammed to act like embryonic stem cells (ESCs). These reprogrammed adult cells could grow in culture indefinitely and differentiate into any type of cell in the body. However the creation of iPSc lines was notoriously inefficient and labor-intensive. Low cell-conversion rates have slowed the study of the reprogramming process itself. It has also discouraged the development of protocols for producing iPSCs under GMP or “Good Manufacturing Practice” conditions for use in human patients.
However, in a series of experiments that were published in the journal Nature, Weizmann Institute stem-cell researcher Jacob Hanna and his team have reprogrammed cells with nearly 100% efficiency. Moreover, Hanna and his group showed that reprogrammed cells transition to pluripotency on a synchronized schedule.
“This is the first report showing that you can make reprogramming as efficient as anyone was hoping for,” says Konrad Hochedlinger, a stem-cell scientist at Harvard Medical School in Boston, Massachusetts. “It is really surprising that manipulating a single molecule is sufficient to make this switch, and make essentially every single cell pluripotent within a week.”
To make iPSCs from adult cells, scientists typically transfect them with a set of four genes. These genes turn on the cells’ own pluripotency program, which converts them into iPSCs. But even established techniques convert less than 1% of cultured cells. Many cells get stuck in a partially reprogrammed state, and some become pluripotent faster than others, which makes the whole reprogramming process difficult to monitor.
Hanna and his team investigated the potential roadblocks to reprogramming by working with a line of genetically-engineered mouse cells. In these cells, the reprogramming genes were already inserted into the genomes of the cells and could be activated with a small molecule. Such cells normally reprogram at rates below 10%. But when a gene responsible for producing the protein Mbd3 was repressed, reprogramming rates soared to nearly 100%.
Hanna says that the precise timing of embryonic development led him to wonder whether it is possible to “reprogram the reprogramming process.” Cells in an embryo do not remain pluripotent indefinitely, explained Hanna. Usually, Mbd3 represses the pluripotency program as an embryo develops, and mature cells maintain their expression of Mbd3. However, during cellular reprogramming, those proteins expressed from the inserted pluripotency genes induce Mbd3 to repress the cells’ own pluripotency genes.
This hamstrings reprogramming, says Hanna. “It creates a clash, and that’s why the process is random and stochastic. It’s trying to have the gas and brakes on at the same time.” Depleting the cells of Mbd3 allows reprogramming to proceed unhindered.
The team also reprogrammed cells from a human, using a method that does not require inserting extra genes. This technique usually requires daily doses of RNA over more than two weeks. With Mbd3 repressed, only two doses were required.
Stem cell researchers at the University of California, San Diego have designed a simple, reproducible, RNA-based method of generating human induced pluripotent stem cells (iPSCs). This new technique broad applications for the successful production of iPSCs for use in therapies and human stem cell studies.
Human iPSCs are made from adult cells by genetically engineering adult cells to overexpress four different genes (Oct4, Klf4, Sox2, and c-Myc). This overexpression drives the cells to de-differentiate into pluripotent stem cells that have many of the same characteristics as embryonic stem cells, which are made from embryos. However, because iPSCs are made from the patient’s own cells, the chances that the immune system of the patient will reject the implanted cells is low.
The problem comes with the overexpression of these four genes. Initially, retroviruses have been used to reprogram the adult cells. Unfortunately, retroviruses plop their DNA right into the genome of the host cell, and this change is permanent. If these genes get stuck in the middle of another gene, then that cell has suffered a mutation. Secondly, if these genes are stuck near another highly-expressed gene, then they too might be highly expressed, thus driving the cells to divide uncontrollably.
Several studies have shown that in order to reprogram these cells, these four genes only need to be overexpressed transiently. Therefore, laboratories have developed ways of reprogramming adult cells that do not use retroviruses. Plasmid-based systems have been used, adenovirus and Sendai virus-based systems, which do not integrate into the genome of the host cell, have also been used, and even RNA has been used (see Federico González, Stéphanie Boué & Juan Carlos Izpisúa Belmonte, Nature Reviews Genetics 12, 231-242).
The UC San Diego team led by Steven Dowdy has used Venezuelan equine virus (VEE) that they engineered to express the reprogramming genes required to make iPSCs from adult cells. Because this virus does not integrate into the host genome, and expresses RNA in the host cell only transiently, it seems to be a safe and effective way to make buckets of messenger RNA over a short period of time.
The results were impressive. The use of this souped-up VEE produced good-quality iPSCs very efficiently. Furthermore, it worked on old and young human cells, which is important, since those patients who will need regenerative medicine are more likely to be young patients than old patients. Also, changing the reprogramming factors is rather easy to do as well.
A Journal article in the August 9th edition of Science Magazine features work from the laboratories of Yang Zhao and Hongkui Deng, both of whom are from the College of Life Sciences and Peking-Tsinghua Center for Life Sciences at Peking University in Beijing, China. Zhao and Deng and colleagues used small molecules to transform adult cells into induced pluripotent stem cells.
To review, induced pluripotent stem cells are derived from adult cells by genetically engineering the adult cells to express a cocktail of four genes (OCT4, Klf4, Sox2, and c-Myc). To introduce these genes into cells, viruses are normally used, but other techniques are also available. The resultant cells look and act like embryonic stem cells, but they do not require the death of embryos.
In this paper, Deng and colleagues took mouse embryonic fibroblasts (skin cells cultured from mouse embryos) and used them to screen over 10,000 small molecules for their ability to substitute for the OCT4 gene in the production of iPSCs. If this sounds labor intensive, that’s because it is. To conduct the screen, they used mouse embryonic fibroblasts that were infected with viruses that expressed Sox2, Klf4, and c-Myc. These genes are not enough to convert adult cells into iPSCs. However, with these chemicals, these three genes could produce iPSCs from mouse embryonic fibroblasts (MEFs). They identified at least three molecules; Forskolin, 2-methyl-5-hydroxytryptamine and a synthetic molecule called D4476, that could substitute for OCT4.
Thus, by using chemicals, they could get away from using one of the genes required to de-differentiate adult cells into iPSCs. Could they whittle down the number of genes even further? Previously, Deng and Zhao published a paper in which a chemical cocktail was used to substitute for the other three genes so that conversion into iPSCs was achieved by introducing only the OCT4 gene into cells (Li, YQ et al., CELL RESEARCH 21(1): 196-204. They called this cocktail “VC6T.” Therefore, they used VC6T and Forskolin, on their MEFs and the beginnings of de-differentiation occurred, but not much else.
Could chemicals be identified that would take the cells the rest of the way to iPSCs? Another chemical screen examined this possibility. In this test, the MEFs were rigged so that they expressed OCT4 when the cells were treated with the antibiotic doxycycline. By giving the cells doxycycline for 4-8 days, and then testing chemicals to take the cells the rest of the way, they identified a slew of compounds that, when given to the OCT4-expressing MEFs, they became iPSCs.
Then came the real test – make iPSCs with just chemicals and no introduced genes. Could it be done? When they gave the MEFs some of the chemicals identified in the last screen (they called it DZNep), plus VC6T, the expression of OCT4 went up, but the cells simply did not look like iPSCs. So, they changed the culture medium to a “2i” culture system that inhibits some key regulatory proteins in the cells. When they used this same chemical cocktail in a 2i culture system, it worked and iPSCs were produced. Deng and Zhao called these stem cells “chemically induced pluripotent stem cells” or CiPSCs.
Next, they optimized the dosages of these chemicals in order to increase the efficiency of iPSC production. They were able to increase the efficiency of iPSC production to 5% (1 of every 20 colonies of cells), which is respectable. They also identified yet another small molecule that beefed up iPSC production by another 40-fold. Also, this chemical cocktail was able to make iPSCs from mouse adult fibroblasts, fat-derived stem cells, and fibroblasts from newly born mice.
When the CiPSC lines were characterized, they made all the right genes to be designated as pluripotent stem cells, and they had normal numbers of normal-looking chromosomes all the way through 13 passages.
When injected into mice with dysfunctional immune systems, the CiPSCs made tumors that were mixtures of tissues of all over the body. When they were transferred into early mouse embryos, they could contribute to the bodies of developing mice, and they could even contribute to the production of eggs and sperm, When baby mice were completely made from CiPSCs, those mice were fertile and had babies of their own. This is the ultimate test of pluripotency and the CiPSCs passed it with flying colors.
Other experiments in this paper examined why these chemicals induced pluripotency in adult cells, but these experiments, though interesting, are lost in the fact that this research group has generated iPSCs without using any viruses, or genetic engineering technology. These CiPSCs are true pluripotent stem cells and they were generated without killing any embryos or introducing genes that might drive cells to become abnormal.
If this can be replicated with human cells, it would be earth-shattering for regenerative medicine.
During liposuction patients lose a fat cells, fat-based mesenchymal stem cells, and now, according to new results from UCLA scientists, stress-enduring stem cells.
This new stem cell population has been called a Multi-lineage Stress-Enduring Adipose Tissue or Muse-AT stem cells. UCLA scientists found Muse-AT stem cells by accident when a particular machine in the laboratory malfunctioned, killing all the cells found in cells from human liposuction, with the exception on the Muse-AT stem cells.
Gregorio Chazenbalk from the UCLA Department of Obstetrics and Gynecology and his research team discovered, after further tests on Muse-AT stem cells, that they not only survive stress, but might be activated by it.
The removal of Muse-AT stem cells from the human body by means of liposuction revealed cells that express several embryonic stem cell-specific proteins (SSEA3, TR-1-60, Oct3/4, Nanog and Sox2). Furthermore, Muse-AT stem cells were able to differentiate into muscle, bone, fat, heart muscle, liver, and neuronal cells. Finally, when Chazenbalk and his group examined the properties of Muse-AT stem cells, they discovered that these stem cells could repair and regenerate tissues when transplanted back into the body after having been exposed to cellular stress.
“This population of cells lies dormant in the fat tissue until it is subjected to very harsh conditions. These cells can survive in conditions in which usually cancer cells can survive. Upon further investigation and clinical trials, these cells could prove a revolutionary treatment option for numerous diseases, including heart disease, stroke and for tissue damage and neural regeneration,” said Chazenbalk.
Purifying and isolating Muse-AT stem cells does not require the use of a cell sorter or other specialized, high-tech machinery. Muse-AT stem cell can grow in liquid suspension, where they grow as small spheres or as adherent cells that pile on top of each other to form aggregates, which is rather similar to embryonic stem cells and the embryoid bodies that they form.
We have been able to isolate these cells using a simple and efficient method that takes about six hours from the time the fat tissue is harvested,” said Chazenbalk. “This research offers a new and exciting source of fat stem cells with pluripotent characteristics, as well as a new method for quickly isolating them. These cells also appear to be more primitive than the average fat stem cells, making them potentially superior sources for regenerative medicine.”
Embryonic stem cells and induced pluripotent stem cells are the two main sources of pluripotent stem cells. However, both of these stem cells have an uncontrolled capacity for differentiation and proliferation, which leads to the formation of undesirable teratomas, which are benign tumors that can become teratocarcinomas, which are malignant tumors. According to Chazenbalk, little progress has been made in resolving this defect (I think he overstates this).
Muse-AT stem cells were discovered by a research group at Tokohu University in Japan and were isolated from skin and bone marrow rather than fat (see Tsuchiyama K, et al., J Invest Dermatol. 2013 Apr 5. doi: 10.1038/jid.2013.172). The Japanese group showed that Muse-AT stem cells do not form tumors in laboratory animals. The UCLA group was also unable to get Muse-AT stem cells to form tumors in laboratory animals, but more work is necessary to firmly establish that these neither form tumors nor enhance the formation of other tumors already present in the body.
Chazenbalk also thought that Muse-AT stem cells could provide an excellent model system for studying the effects of cellular stress and how cancer cells survive and withstand high levels of cellular stress.
Chazenbalk is understandable excited about his work, but other stem cells scientists remain skeptical that this stem cells population has the plasticity reported or that these cells are as easily isolated as Chazenbalk says. For a more skeptical take on this paper, see here.
Embryonic stem cells might provide the means to heal a variety of physical ailments. However the problem with embryonic stem cells is not necessarily in their use, but in their derivation. In order to make embryonic stem cell lines, human embryos are destroyed.
Now that federal funding is available to not only work with existing embryonic stem cell lines but to MAKE new lines, there is nothing to stop researchers from thawing and (I’m sorry to be so blunt) killing human embryos. Can we have our “cake and eat it too?” Can we have the benefits of embryonic stem cells and not destroy embryos? Perhaps we can.
In 2001, Masako Tada reported the fusion of embryonic stem cells with a connective tissue cell called a fibroblast. This fusion reprograms the fibroblasts so that they behave like embryonic stem cells (Current Biology 11, no. 9 (2001): 1553–8). This suggests that something within embryonic stem cells can redirect the machinery of somatic cells to become more like that of embryonic stem cells. In 2006 Kazutoshi Takahashi and Shinya Yamanaka were able to generate embryonic stem cell lines by introducing four specific genes into mouse skin fibroblasts. These “induced pluripotent stem cells” (iPSCs) shared many of the properties of embryonic stem cells derived from embryos, but when transplanted into mouse embryos, they were not able to participate in the formation of an adult mouse (Cell 126, no. 4 (2006): 663–76). This experiment showed that it is possible to convert adult cells into something that resembles an embryonic stem cell. Could we push adult cells further? In 2007, three different research groups used retroviruses to transfer four different genes (Oct3/4, Sox2, c-Myc and Klf4) into mouse skin fibroblasts and completely transformed them into cells that had all the features and behaviors of embryonic stem cells (Cell Stem Cell 1, no. 1 (2007): 55–70; Nature 448 (2007): 313–7; Nature 448 (2007): 318–24.).
These experiments drew a great deal of excitement, but there were several safety concerns that had to be addressed before iPSCs could be used in human clinical trials. Scientists used engineered retroviruses to introduce genes into adult cells in order to reprogram them into iPSCs (Current Topics in Microbiology and Immunology 261 (2002): 31-52). Retroviruses insert a DNA copy of their genome into the chromosomes of the host cell they have infected. If that viral DNA inserts into a gene, it can disrupt it and cause a mutation. This can have dire consequences (see Folia Biologia 46 (2000): 226-32; Science 302 (2003): 415-9). Fortunately this is not an intractable problem. The conversion of adult cells into iPSCs only requires the transient expression of the inserted genes. Secondly, scientists have created retroviruses that self-inactivate after their initial insertion (Journal of Virology 72 (1998): 8150-7; Virology 261, (1999). One laboratory has also discovered a way to make iPSCs with a virus that does not insert into host cell chromosomes (Science 322 (2008): 945-9). Other researchers have designed ingenious ways to move the necessary genes into adult cells without using viruses (Science 322 (2008): 949-53). Both procedures avoid the dangers associated with the use of retroviruses.
A second concern involves the genes used to convert re-program adult cells into iPSCs. One of these genes, c-Myc, is found in multiple copies in human and animal tumors. Thus increasing the number of copies of the c-Myc gene might predispose such cells to form tumors (Recent Patents on Anticancer Drug Discovery 1 (2006): 305-26; Seminars in Cancer Biology 16 (2006): 318-30). Indeed, the increased ability of iPSCs made by Yamanaka to cause tumors in laboratory animals underscore this concern (Hepatology 46, no 3 (2009): 1049-9). Several groups, however, have succeeded in making iPSCs from adult cells without the use of the c-Myc gene (Science 321 (2008): 699-702; Nature Biotechnology 26 (2008): 101-6; Science 318 (2007): 1917–20), although the conversion is much less efficient. Additionally, several groups have established that particular chemicals, in combination with the addition of a subset of the four genes originally used, can effectively transform particular cells into iPSCs (Cell Stem Cell 2 (2008): 525-8). Thus the larger safety concerns facing iPSCs have been largely solved.
Finally, patient-specific iPSCs have been made in several labs, even though they have not been used in clinical trials to date. Here is a short list of some of the diseases for which patient-specific iPSCs have been made: