Will SCNT-ESCs Be Better than iPSCs for Personalized Regenerative Medicine?

When the first human induced Pluripotent Stem Cells (iPSCs) were generated (Takahashi et al., 2007xTakahashi, K., Tanabe, K., Ohnuki, M., Narita, M., Ichisaka, T., Tomoda, K., and Yamanaka, S. Cell. 2007; 131: 861–872Abstract | Full Text | Full Text PDF | PubMed | Scopus (7837)See all ReferencesTakahashi et al., 2007), the expectations surrounding the development of human SCNT methodologies for personalized medicine were rapidly wiped out. A recent report by Mitalipov and colleagues demonstrating the generation of human somatic cell nuclear transfer-derived embryonic stem cells (SCNT-ESCs) opens this long-standing debate once again, now with the added question of whether SCNT-ESCs will prove to be a better source than iPSCs for personalized regenerative medicine (Tachibana et al., 2013xTachibana, M., Amato, P., Sparman, M., Gutierrez, N.M., Tippner-Hedges, R., Ma, H., Kang, E., Fulati, A., Lee, H.S., Sritanaudomchai, H. et al. Cell. 2013; 153: 1228–1238Abstract | Full Text | Full Text PDF | PubMed | Scopus (259)See all ReferencesTachibana et al., 2013). To resolve this debate, researchers will need to address issues that, among others, include (1) the reproducibility, simplification, and expansion of SCNT technologies; (2) the potential of SCNT-ESCs for proper cell-type-specific differentiation and engraftment; (3) the role that maternal mitochondria might play in SCNT-ESCs; (4) the safety concerns inherently related to the use of pluripotent cells; and (5) the possibility that distinct molecular signatures underlie different pluripotent states.As is the case for iPSC derivation, somatic nuclear age, disease status, mutational load, and specific cell type may influence epigenetic plasticity and potentially compromise SCNT-ESC derivation and utilization. SCNT should thus be expanded to include comparative analyses of a variety of cell types from healthy, diseased, and aged individuals (the report by Mitalipov and colleagues focused exclusively on nuclei derived from fetal fibroblasts as well as those derived from a young patient with Leigh Syndrome). Regardless of the donor cell type, oocyte quality seems to have a determining role for successful SCNT. Molecular signatures defining oocyte quality, and accordingly their reprogramming potential, will need to be determined to enhance SCNT-ESC reproducibility, uniformity, and efficiency.It is not clear whether SCNT-ESCs will, similar to iPSCs, face issues in terms of altered differentiation potential and immature phenotypes (Robinton and Daley, 2012xRobinton, D.A. and Daley, G.Q. Nature. 2012; 481: 295–305Crossref | PubMed | Scopus (417)See all ReferencesRobinton and Daley, 2012). Abnormal migration and/or integration of differentiated cells derived from iPSCs into the target host tissue may eventually compromise functionality. These abnormalities could be due to incomplete resetting of the epigenome, acquisition of aberrant methylation patterns, and/or inefficient differentiation protocols. Since it is argued that SCNT-ESCs may display a more complete resetting of the epigenome, as SCNT reprogramming involves the establishment of embryonic imprinting patterns, it would be of enormous interest to determine whether and if so, how, SCNT-ESCs are functionally superior to iPSCs under the same differentiation conditions. Interestingly, a recent report by Hayashi and colleagues has indicated that both iPSCs and allogeneic ESCs derived from mice are capable of generating viable oocytes that result in normal offspring (Hayashi et al., 2012xHayashi, K., Ogushi, S., Kurimoto, K., Shimamoto, S., Ohta, H., and Saitou, M. Science. 2012; 338: 971–975Crossref | PubMed | Scopus (179)See all ReferencesHayashi et al., 2012). Whereas the findings by Hayashi and colleagues argue that both types of pluripotent cells present comparable differentiation potential, it is well accepted that murine and human PSCs present important differences that warrant further investigation.A potential concern related to SCNT-ESCs involves heteroplasmy, the presence of mitochondrial DNA (mtDNA) from dual origins. The mtDNA carried over during nuclear transfer may trigger immunogenicity, a controversial issue surrounding the use of iPSCs (Yamanaka, 2012xYamanaka, S. Cell Stem Cell. 2012; 10: 678–684Abstract | Full Text | Full Text PDF | PubMed | Scopus (248)See all ReferencesYamanaka, 2012). Although this issue has been debated, recent data support claims that murine SCNT-ESCs lack immunogenicity. For example, Craven and colleagues reported that donor mitochondria fall below the immune system’s detection level based on DNA amplification techniques (Craven et al., 2010xCraven, L., Tuppen, H.A., Greggains, G.D., Harbottle, S.J., Murphy, J.L., Cree, L.M., Murdoch, A.P., Chinnery, P.F., Taylor, R.W., Lightowlers, R.N. et al. Nature. 2010; 465: 82–85Crossref | PubMed | Scopus (158)See all ReferencesCraven et al., 2010). However, whether this will be the case when differentiated human SCNT-ESCs are exposed to actual immune cell recognition remains unknown. Moreover, due to the high degree of sequence variability of human mtDNA, changes may occur in the molecular interactions between nuclear and mitochondrial genomes and thus alter cellular function. If maternal mitochondria and reprogrammed somatic nuclei are able to reestablish proper interaction, SCNT-ESCs could constitute a new approach to treating mitochondrial diseases present in homoplasmy. Yet, since most mitochondrial diseases are heteroplasmic, other approaches, such as spindle transfer, isogenic correction in iPSCs, and induction of heteroplasmic shift, might be needed.Safety concerns constitute a major issue associated with the use of PSCs. With the continuous improvement of nonintegrative approaches for iPSC generation, the remaining safety issues relate to the potential transplantation of residual PSCs or the possibility of in vivo dedifferentiation. Interestingly, dedifferentiation has been shown to underlie both cell transformation and reprogramming to iPSCs, and the presence of “stem cell” signatures in cancer cells has been correlated with increased malignancy (Visvader and Lindeman, 2012xVisvader, J.E. and Lindeman, G.J. Cell Stem Cell. 2012; 10: 717–728Abstract | Full Text | Full Text PDF | PubMed | Scopus (328)See all ReferencesVisvader and Lindeman, 2012). Along this line, a major difference between both reprogramming processes is that, contrary to SCNT, iPSC generation is a long process influenced by cell proliferation. This extended timeframe opens the possibility that spontaneous mutations contributing to the dedifferentiation and/or enhanced proliferation are positively selected during reprogramming to iPSCs. If proven to be the case, selection during iPSC reprogramming would be analogous to what has been described during tumorigenesis and suggests that SCNT technologies may have the advantage of reducing the risk for oncogenic transformation. Additionally, reprogramming by SCNT or iPSC approaches will most probably generate differences at the epigenetic and/or genetic level, which may affect genetic instability and potential for malignant transformation. These questions still await experimental answers.Controversy remains as to whether iPSCs are molecularly identical to ESCs or if they represent a distinct pluripotent state (Yamanaka, 2012xYamanaka, S. Cell Stem Cell. 2012; 10: 678–684Abstract | Full Text | Full Text PDF | PubMed | Scopus (248)See all ReferencesYamanaka, 2012). To date, reprogramming has been described to occur by (1) factors present in the oocyte (SCNT), (2) factors typical of ESCs derived from the inner cells mass of the blastocyst (iPSCs), and more recently, (3) factors involved in lineage specification (iPSCs) (Shu et al., 2013xShu, J., Wu, C., Wu, Y., Li, Z., Shao, S., Zhao, W., Tang, X., Yang, H., Shen, L., Zuo, X. et al. Cell. 2013; 153: 963–975Abstract | Full Text | Full Text PDF | PubMed | Scopus (110)See all References, Montserrat et al., 2013xMontserrat, N., Nivet, E., Sancho-Martinez, I., Hishida, T., Kumar, S., Miquel, L., Cortina, C., Hishida, Y., Xia, Y., Esteban, C.R., and Izpisua Belmonte, J.C. Cell Stem Cell. 2013; 13Abstract | Full Text | Full Text PDF | PubMed | Scopus (60)See all References). This indicates that different paths to pluripotency can be used and extends the initial question about which cell source, SCNT-ESCs, iPSCs, or ESCs, would be most suitable for cell therapy. Noticeably, whereas ESCs from cloned embryos are virtually indistinguishable from those derived from fertilized ones by microarray analysis (Brambrink et al., 2006xBrambrink, T., Hochedlinger, K., Bell, G., and Jaenisch, R. Proc. Natl. Acad. Sci. USA. 2006; 103: 933–938Crossref | PubMed | Scopus (153)See all ReferencesBrambrink et al., 2006); higher resolution techniques and analysis of noncoding regions might highlight previously unexpected differences. Identification of different molecular signatures will contribute to the development of better reprogramming strategies and may lead to the elucidation of the best cell source for clinical applications.Despite some criticisms and unanswered questions on the practicality and utility of SNCT-ESCs, human SCNT technologies will probably contribute to the identification of novel reprogramming factors, which in turn may lead to more efficient generation and higher quality iPSCs. Paradoxically, identification of mechanisms facilitating iPSC reprogramming stemming from an increased understanding of human SCNT might eventually contribute to the disuse of SCNT-ESCs, which will always face ethical concerns that other reprogramming approaches avoid. Regardless of which source is deemed most useful for cell therapy, the testing and comparison of SCNT-ESCs with current PSCs will provide new vistas and opportunities for the study of the mechanisms underlying reprogramming. The hope is that these studies may eventually help the development of effective and safe stem-cell-based therapies.