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Cellular Reprogramming in Pursuit of Immortality

M. Azim Surani1,2,* 1Wellcome Trust Cancer Research UK Gurdon Institute 2Wellcome Trust Medical Research Council Stem Cell Institute University of Cambridge, Cambridge CB2 1QN, UK *Correspondence: [email protected] http://dx.doi.org/10.1016/j.stem.2012.11.014

The discovery that phenotypic diversity among differentiated cells results from epigenetic and not genetic differences, and can be reset to restore pluripotency, promises revolutionary advances in medicine. I discuss how this and related seminal discoveries have brought us to an exciting future.

Immortality comes in many forms. For endeavor in the field also seeks to explore indeed be reprogrammed when trans- example, the creators of great works of and answer fundamental questions in planted into oocytes and acquire totipo- art that become ingrained in human con- biology, such as how cells acquire and tency (Campbell et al., 1996). Numerous sciousness achieve immortality through maintain their different states. If we can mammalian species such as pigs and collective appreciation. John Gurdon understand this regulation, and as a result cows have now been cloned, including and Shinya Yamanaka, who received the manipulate cellular states experimentally, and famously a ‘‘copy cat,’’ aptly called 2012 Nobel Prize for Physiology or Medi- we could unlock the potential to provide Cc (for carbon copy). This work clearly cine for their groundbreaking work on great advances in human medicine. showed that cellular differentiation in- ‘‘the discovery that mature cells can be Aldous Huxley’s 1932 science fiction volves epigenetic rather than genetic reprogrammed to become pluripotent,’’ book Brave New World depicts a future modifications to generate the myriad of have deservedly achieved immortality for with human clones of different classes differentiated cells that emerge from a another reason—their research—and the occupying diverse societal niches in totipotent zygote. The term ‘‘epigenetic’’ work they have been engaged in is a hierarchical order. Nearly 30 years later, has come into vogue more recently, nothing less than an attempt to turn the John Gurdon’s work on cloning in although Conrad Waddington used it developmental clock back to the time Xenopus brought reality a little closer to initially in 1954 in his famous and fre- when we start our individual journeys science fiction when he showed that, quently invoked ‘‘epigenetic landscape.’’ from a single cell. when transplanted into a Xenopus egg Importantly, epigenetic modifications, Development is unidirectional; indi- devoid of its own genetic material, a unlike genetic changes, can be erased, vidual cells do not return to the point differentiated intestinal cell nucleus can manipulated, and reinitiated, and logi- from where they start; instead they differ- acquire totipotency and develop into cally, therefore, one cell type could be entiate to terminal states and then ulti- a living individual. Using this approach, converted to another cell type. mately succumb to aging and disease. it is possible to generate an endless Why did it take so long to clone Throughout this complex process, cells number of genetically identical animals. a mammal? The difficulty was partly tech- by and large retain the same genetic infor- This simple but elegant experiment is nical, owing to the necessity of microma- mation they had at the start. This state- notable because it clarified an issue that nipulation of small and fragile mammalian ment sounds so obvious now, but this others, including Briggs and King in early oocytes, but another important compo- was not clear in 1962 when John Gurdon 1950s, were exploring, which was to nent, as in all things to do with develop- published his findings (Gurdon, 1962) establish whether cells as they differen- ment, was a matter of getting the timing and established the principle that cells tiate retain the complete genetic infor- right. The study on cloned sheep provided do not lose genetic information when mation they were endowed with at the the much needed impetus to pursue they differentiate. He showed that indi- beginning. the mechanism of reprogramming, which vidual cells can be reprogrammed back It was clear at the time that this funda- became the focus of intense interest and to the point from where they can recapitu- mental discovery in Xenopus should activity especially after Yanagimachi late their developmental history. This in theory apply to mammals, so it is established cloning in the experimentally founding principle was important for the surprising that despite strenuous efforts more tractable mice in 1998, which inspired work of Yamanaka in 2006, by many groups, including that of Steen generated many new ideas through the whose simple and elegant approach has Willadsen and Davor Solter, it took work of Jaenisch and others. Nonethe- made it possible to convert any adult another 35 years to move from theory to less, the mechanism of reprogramming mammalian cell into an induced pluripo- reality. It was not until 1996 that we had of somatic nuclei still remains to be fully tent stem cell (iPSC) (Takahashi and an unequivocal demonstration that the elucidated. Yamanaka, 2006), and has revolutionized same principle applies in mammals, Cloning in mammals induced some and accelerated research on stem cells in when Ian Wilmut, Keith Campbell, and strong reactions, as it raised the pros- a way that we could not have foreseen just their colleagues showed with Dolly the pect of human cloning and, for some, a few short years ago. The combined sheep that adult mammalian cells could an opportunity to seek ‘‘immortality’’ for

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themselves. To stem growing public have unexpected origins, and can be the and moved to center stage, with concerns, a nonbinding United Nations foundation of entire and important new increasing efforts being directed toward Declaration called for the ban of all forms fields of research. defining specific culture conditions, and of human cloning in 2005. So, is this the Despite the germ cell origin of at least identifying transcription factors, such as end of the matter? There was a time, some apparently pluripotent ECCs, Oct4, Sox2, and Nanog, that are essential unimaginable now, that the pursuit of ECCs themselves were never unequivo- for pluripotency. human in vitro fertilization (IVF) and IVF cally shown to give rise to germ cells. The work on cloning and ESCs babies, for which Robert Edward received However, the work on ECCs concen- occurred alongside other advances, a Nobel Prize in 2010, was viewed with trated attention on important questions, among which was the work of Hal intense hostility both by the public and and helped to establish basic approaches Weintraub and colleagues, who in 1987 by some prominent scientists because of and culture conditions that became demonstrated that expression of a single concerns that the procedure could result critical for the quest toward alternative transcription factor, MyoD, in fibroblasts in the birth of deformed babies. Yet, since ways of isolating pluripotent stem cells. could elicit a program of myogenesis the birth of Louise Brown, the first ‘‘test The work on ECCs in many ways set the and convert them into muscle cells (Davis tube baby’’ in 1978 (Steptoe and Ed- scene for what followed, which were et al., 1987). Different cell types re- wards, 1978), there are now 5 million IVF attempts to derive pluripotent stem cells sponded in this way to a greater or lesser babies worldwide. It is important to note by another route. extent, and MyoD was dubbed a ‘‘master that the derivation of human pluripotent The work of Martin Evans and Matt regulator.’’ This demonstration estab- stem cells was also helped by IVF, which Kaufmann in Cambridge, and Gail Martin lished the important principle that tran- was the source of blastocysts used for at UCSF in California, built on insights scription factors can force the conversion this work, and that IVF has also led to from the ECC studies. These researchers of one cell type into another. Preceding the great advances in prenatal genetic independently reported the derivation the work on MyoD was the demonstration diagnosis. Recent work has also raised of embryonic stem cells (ESCs) from by Peter Jones that treatment of 10T1/2 the possibility of dealing with the devas- the inner cell mass of blastocysts in cells with a DNA demethylating agent tating consequences of inheritance of 1981(Evans and Kaufman, 1981; Martin, called 5-azacytidine caused them to defective mitochondria by using cloning 1981). The cells they established showed differentiate into adipogenic, myogenic, techniques to transplant pronuclei from the properties of self-renewal and the and chondrogenic cells. This experiment patients with defective zygotes into potential to differentiate into all cell types, suggested the importance of epigenetic normal enucleated donor oocytes with including germ cells. The transmission of mechanisms in regulating cell fates, and normal mitochondria. A new program ESCs through the germline became the was important for the identification of along these lines funded by the Wellcome focus of major attention because they MyoD. The concept of transcription factor Trust at Newcastle University may poten- provided an important tool for gene master regulators was also taking hold tially eradicate defective mitochondria targeting, gene traps, and for generating through the work of Walter Gehring, who permanently from afflicted families. reporters, and thus for major advances showed in 1995 that the mammalian Parallel but unrelated studies that were in mammalian developmental biology Pax6 gene, which specifies eye develop- crucial building blocks for this year’s and, ultimately, to the creation of mouse ment, could also induce eye development Nobel Prize had unexpected and unusual models for human diseases. This advance in ectopic sites in Drosophila. origins. In the 1950s, Leroy Stevens found was recognized by the award of the Nobel The pioneering work of Henry Harris in that a certain mouse strain was prone Prize to Capecchi, Evans, and Smithies 1969 also provided important back- to development of enormous testicular in 2007. ground to cellular reprogramming. Harris tumors, which consisted of many differen- It took nearly 18 years after the estab- showed that malignant cancer cells fused tiated somatic cell types (Stevens and lishment of mouse ESCs before James to fibroblasts were no longer malignant Little, 1954). He subsequently showed Thomson first isolated and established due to gene repression (Harris et al., that these testicular teratomas could orig- human pluripotent stem cells (Thomson 1969). Helen Blau’s work in 1983–84 inate from primordial germ cells (PGCs), et al., 1998). While not absolutely iden- along similar lines showed that fusion of the precursors of sperm and eggs. These tical, mouse and human ESCs are by cells with skeletal muscle cells activated studies eventually led to the identification and large similar. The derivation of human muscle gene expression in nonmuscle of embryonal carcinoma cells (ECCs) from ESCs changed the perception of these cells, indicating the existence of transact- teratomas, with the demonstration that cells from being a tool for developmental ing factors capable of inducing transdif- a single ECC can form a teratoma; ECCs biologists to being cells with a huge ferentiation. Takashi and Masako Tada can therefore be thought of as cancer potential for advances in medicine, in 1997 in my lab and thereafter indepen- stem cells. Notably, like single cells of because it became possible to imagine dently in 2001 also showed that pluripo- the inner cell mass of blastocysts, an that ESCs could be used to generate tent stem cells fused with somatic cells ECC introduced into a host blastocyst any cell type for therapeutic purposes. could transform the latter into pluripotent is capable of differentiating into many The work on the biology and properties stem cells. This experiment showed that diverse cell types, illustrating its pluripo- of ESCs themselves was being pursued ESCs contain trans-acting factors that tent character. Human ECCs were also by relatively few scientists, notably and can convert somatic cells into pluripotent identified and were the subject of investi- particularly by Austin Smith. These stem cells without the need to transplant gations. Thus, significant discoveries can studies emerged from the background somatic nuclei into oocytes.

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Thus, the idea that ESCs must contain considered germ cells to be the ‘‘most ease states. Advances in this field might factors capable of converting somatic fascinating and deeply mysterious cells’’ even make it possible to generate cells to pluripotent stem cells was starting of them all. With her enthusiastic support, synthetic cells with desirable diverse to emerge. However, the low efficiency of we embarked on a quest to unravel the properties that do not have direct in vivo restoring totipotency to somatic nuclei mysteries of mammalian germ cells at counterparts. following transplantation into oocyte and the Gurdon Institute 12 years ago with Biological research has undergone a the poor understanding of the underlying Petra Hajkova, Mitinori Saitou, and dramatic transformation in recent years, mechanism led to the notion that a large Katsuhiko Hayashi, among others. The with genome-scale approaches and number of factors, perhaps even over recent work by Hayashi and Saitou on advances in systems biology becoming 100, might be needed to convert somatic the derivation of sperm and oocytes particularly prominent. Rather than being cells into pluripotent stem cells. The from mouse ESCs and iPSCs has raised a pursuit undertaken by small groups or differentiated cell state is also highly the possibility of breaking the ‘‘Weissman even individuals, it is now a multi- stable and maintained by DNA meth- Barrier’’ by transferring genetic informa- disciplinary, multigroup, and international ylation, an epigenetic modification that tion from the soma to the germline enterprise, and many of the current is difficult to erase. Nevertheless, through the conversion of somatic cells approaches to funding are structured Yamanaka took up this challenge, and in to gametes. The prospect of being able around supporting this type of endeavor. a relatively short time was able to report to generate the totipotent state from adult Looking back, however, it is evident that his groundbreaking achievement of in- somatic cells in this way brings us close to transformational advances can start with ducing pluripotency by introducing a the concept of immortality. small steps by individuals that may seem specific set of transcription factors (Taka- There is little doubt that the ability to of little consequence except to those hashi and Yamanaka, 2006). Other than generate iPSCs from somatic cells has who are driven simply by a desire to find the sheer reliability of the approach, it transformed the anticipated advances in out how it all works. In my view, it is crucial was also astonishing that only four regenerative medicine, in ways that have to provide support for individuals, some of factors, Oct4, Sox2, Klf4, and c-Myc, been articulated in numerous research whom may well be future Nobel laureates, were needed to induce pluripotency in and review articles. Just to take one who have an obsession to explore partic- somatic cells. Different factor combina- example, the ability to generate iPSCs ular areas of research for their own sake. tions and even noncoding RNAs have from patients with specific diseases What we can safely predict is that the since been reported to function in reprog- or mutations has opened up prospects future is unpredictable, and that we can ramming of mouse and human somatic for studying how such disease states look forward to many more currently cells, but in most cases only a few factors develop from the start, by following iPSCs unknowable twists that underpin quantum are required to induce the transformation. as they recapitulate all the steps toward leaps in science. This discovery, which has revolutionized differentiation of affected tissues, and the field of reprogramming and stem cells, how they function at a cellular level REFERENCES has been adopted on a truly phenomenal through the analysis of differentiated and industrial scale within a short space of cells. 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