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Human ES Cells and a Blastocyst from One Embryo: Exciting Science but Conflicting Ethics?

2008; Elsevier BV; Volume: 2; Issue: 2 Linguagem: Inglês

10.1016/j.stem.2008.01.021

1934-5909

Martin H. Johnson,

CRISPR and Genetic Engineering

In this issue of Cell Stem Cell, Chung et al., 2008Chung Y. Klimanskaya I. Becker S. Li T. Maserati M. Lu S.-J. Zdravkovic T. Ilic D. Genbacev O. Fisher S. Krtolica A. Lanza R. Cell Stem Cell. 2008; 2 (this issue): 113-117Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar remove a single blastomere to generate a human embryonic stem cell (hESC) line without prejudicing the development of the biopsied embryo. Their method stimulates new ideas about hESC formation, but ethicopolitical concerns remain. In this issue of Cell Stem Cell, Chung et al., 2008Chung Y. Klimanskaya I. Becker S. Li T. Maserati M. Lu S.-J. Zdravkovic T. Ilic D. Genbacev O. Fisher S. Krtolica A. Lanza R. Cell Stem Cell. 2008; 2 (this issue): 113-117Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar remove a single blastomere to generate a human embryonic stem cell (hESC) line without prejudicing the development of the biopsied embryo. Their method stimulates new ideas about hESC formation, but ethicopolitical concerns remain. Previous experiments on mouse embryos showed that embryonic stem (ES) cell lines can be obtained from biopsied individual blastomeres and that the remaining embryo can produce a live offspring (Chung et al., 2006Chung Y. Klimanskaya I. Becker S. Marh J. Lu S.-J. Johnson J. Meisner L. Lanza R. Nature. 2006; 439: 216-219Crossref PubMed Scopus (302) Google Scholar, Teramura et al., 2007Teramura T. Takehara T. Kishi N. Mihara T. Kawata N. Takeuchi H. Takenoshita M. Matsumoto K. Saeki K. Iritani A. et al.Cloning Stem Cells. 2007; 9: 485-494Crossref PubMed Scopus (8) Google Scholar). Applying a similar approach to human embryos, Klimanskaya et al., 2006Klimanskaya I. Chung Y. Becker S. Lu S.-J. Lanza R. Nature. 2006; 444: 481-485Crossref PubMed Scopus (469) Google Scholar showed that isolated human blastomeres could indeed form hESCs at low frequency. However, their methodologies (embryo dispersal and destruction) and culture conditions (using carrier hESCs) raised concerns and precluded development of the biopsied embryo. Now the same laboratory has used more rigorous culture methods to demonstrate that a single blastomere isolated from an eight to ten cell embryo can form an hESC line and that the biopsied human embryo can develop to a blastocyst. This outcome provides the best demonstration of biopsied embryo normality short of uterine transfer. Given that the biopsy procedure utilized is similar to that employed during preimplantation genetic diagnosis (PIGD), the biopsied embryos are predicted to be functional. This latest report has generated political and ethical comment but little biological interest. This is a pity, because within it lie pointers to the molecular mechanisms underlying both blastocyst formation and ESC generation. Early attempts to derive ESCs from single blastomeres isolated from morulae failed, the usual outcome being a small cluster of trophoblast cells. This outcome was explained by the distinctive properties of early blastomeres. During embryogenesis, the six cleavage divisions leading from a fertilized egg to a blastocyst appear to be programmed such that key developmental changes occur in particular developmental cell cycles. In particular, during the eight cell stage (the point at which the developing embryos were biopsied in this study), blastomeres transform their phenotype from nonpolar to polarized. This polarization is followed by two rounds of division in which cells are allocated to different positions and phenotypes: differentiative divisions generate a nonpolar inner and a polar outer cell, while conservative divisions generate two polar outer cells (Johnson and McConnell, 2004Johnson M.H. McConnell J.M. Semin. Cell Dev. Biol. 2004; 15: 583-597Crossref PubMed Scopus (171) Google Scholar). Thus, the ratio of inner to outer cells depends critically on the relative numbers of each division type, which are affected by cell shape and cell interactions (Johnson and McConnell, 2004Johnson M.H. McConnell J.M. Semin. Cell Dev. Biol. 2004; 15: 583-597Crossref PubMed Scopus (171) Google Scholar). When starting with an isolated eight cell blastomere, most divisions seem to be conservative and too few inner cells form. It is these cells that are the precursors of the ESCs. So, in order to maximize the chance of a single eight cell blastomere yielding an ESC line, all or most cells derived from it must develop along the “inside, nonpolar” route. This might be achieved by suppressing (or reversing) polarization so that all cells become epiblast-like or, should polarization occur, by maximizing the number of differentiative divisions such that the ratio of nonpolar epiblast to polar trophoblast cells is maximized. In previous publications on the generation of ES cells from mouse single blastomeres, it has not been possible to understand which, if either, of these routes to epiblast was operating, nor how. Chung et al., 2008Chung Y. Klimanskaya I. Becker S. Li T. Maserati M. Lu S.-J. Zdravkovic T. Ilic D. Genbacev O. Fisher S. Krtolica A. Lanza R. Cell Stem Cell. 2008; 2 (this issue): 113-117Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar, however, suggest not only that the critical molecular additive is laminin but also that it acts by disrupting polarization—as evidenced by disordered tight junctions and the lack of polarized microvilli. This finding is exciting not only with respect to ESC derivation but also for the hints it provides for our basic understanding of early developmental mechanisms. Of note, laminin is the first extracellular protein to be synthesized in the mouse embryo, being present from around the eight cell stage (Cooper and MacQueen, 1983Cooper A.R. MacQueen H.A. Dev. Biol. 1983; 96: 467-471Crossref PubMed Scopus (191) Google Scholar). Moreover, laminin is known to influence the distribution of cadherins (Klaffky et al., 2006Klaffky E.J. Gonzáles I.M. Sutherland A.E. Dev. Biol. 2006; 292: 277-289Crossref PubMed Scopus (20) Google Scholar), and E-cadherin redistribution to the inner surface of blastomeres is a key early event in polarization (Johnson and McConnell, 2004Johnson M.H. McConnell J.M. Semin. Cell Dev. Biol. 2004; 15: 583-597Crossref PubMed Scopus (171) Google Scholar). Might, in the normal embryo, the deposition of laminin between blastomeres stimulate, through its asymmetric distribution, the redistribution of E-cadherin and so initiate the polarization of the blastomeres? And, by surrounding an isolated blastomere with laminin, might this asymmetric positional signaling be lost and so polarization absent? These are the exciting and fundamental questions raised almost serendipitously by this study. We now need to know more about the time course and nature (prevention or reversal?) of polarity disruption. In addition, laminin has at least 15 isoforms, with distinct expression and distribution patterns, including differences between ICM and trophoblast (Klaffky et al., 2006Klaffky E.J. Gonzáles I.M. Sutherland A.E. Dev. Biol. 2006; 292: 277-289Crossref PubMed Scopus (20) Google Scholar). The laminin used in this study is a mixture of isotypes and proteolytic laminin fragments (Wondimu et al., 2006Wondimu Z. Gorfu G. Kawataki T. Smirnov S. Yurchenco P. Tryggvason K. Patarroyo M. Matrix Biol. 2006; 25: 89-93Crossref PubMed Scopus (42) Google Scholar), and so a more accurate identification of the active component(s) is needed. But what about the clinical impact of the paper and its ethicopolitical implications? One must ask: why is this work being done? It is difficult to see any direct clinical value. One hopes it is not a prelude to clinics promoting IVF for fertile couples with the prospect of a cryobankable hESC line fully compatible with their child. Such an offer would be unethical given the complex nature and appreciable failure rates of each of the clinical steps involved. The treatment would require IVF, blastomere biopsy, temporary cryopreservation of biopised embryos, derivation of ESC lines and their cryopreservation, and then selective thawing and uterine transfer of embryos with genetically matching hESC lines. There is currently no demonstrated advantage over cord blood banking—a much less invasive and easier procedure. The political motivation is also problematic. In the United States, federal funding of hESC derivation is prohibited unless “no harm” is done to the embryo. It is unclear whether this new technique will satisfy the ethical criterion of no harm, and statements from the administration that followed online publication of the article were equivocal at best. Paradoxically, it is the same no harm stimulus that may scupper federal approval. Thus, the same political motivation is also driving the high investment in efforts to derive induced pluripotent stem cells (iPSCs) from skin cells. This technology is fraught with dangers, including oncogenesis, but its promotion as a potential alternative to the use of human embryos may provide a potent disincentive to approve use of this new technique. Perhaps it is time that consumer societies faced the fact that IVF, PIGD, and associated reproductive technologies (ARTs) all involve research on and destruction of human embryos. Attempts like those above, and others involving genetically disabling an embryo (Meissner and Jaenisch, 2006Meissner A. Jaenisch R. Nature. 2006; 439: 212-215Crossref PubMed Scopus (214) Google Scholar) or attempting rescue of living blastomeres from otherwise “moribund” human embryos (Lerou et al., 2008Lerou P.H. Yabuuchi A. Huo H. Takeuchi A. Shea J. Cimini T. Ginsburg E. Racowsky C. Daley G.Q. Nat. Biotechnol. 2008; https://doi.org/10.1038/nbt1378Crossref PubMed Scopus (83) Google Scholar), may have been ingenious responses to political pressures, but these approaches basically ignore these central facts about ART. If these new ARTs are acceptable ethically as well as clinically, then banning hESC derivation seems ethically inconsistent. Perhaps it would be more honest to think through, and even to question, the biological basis of the belief in the early embryo's status, which rests on the view that human identity is fundamentally genetic (Johnson, 2001Johnson M.H. Stud. History Philos. Biol. Biomed. Sci. 2001; 32: 601-617Crossref Scopus (7) Google Scholar). In the age of epigenetics, this view is becoming increasingly difficult to defend both scientifically and ethically.