Cloned human ES cells: a great leap forward, and still needed?

Abstract

The prospect of cloning human embryos by somatic cell nuclear transfer (SCNT) into oocytes has engendered a mix of fear, excitement, controversy and extremely polarized opinions in spite of the fact that it never worked robustly let alone efficiently, until now. In the May issue of Cell, Tachibana et al. (2013) reportedon theunprecedentedderivationof karyotypically normal embryonic stem cells (ESCs) from cloned human embryos. This was not the first report of ESCs generated after SCNT into human oocytes (Noggle et al., 2011), but it is the first case in which these cell lines are diploid and their nuclear genome is perfectly matched to the donors of the somatic nuclei. Not only that, but when one looks at the cases of SCNT in mammals reported to date, it becomes obvious that human SCNT in the hands of Tachibana et al. was also quite efficient in comparison with other species. For example, in their first round of human SCNT Tachibana et al. produced 10 cloned blastocysts from 42 oocytes (24%); of these blastocysts, 8 were used for derivation, yielding 4 ESC lines (50%); in 2 subsequent rounds of SCNTwith oocytes from different women, the rates of blastocyst formation were 27 and 60%, and the rates of ESC derivation were 25 and 33%. Cloned ESCs are the tools used for therapeutic cloning, which is designed to remedy disease, not as a means for reproduction. Therapeutic cloning was first accomplished in mice in 2002 (Rideout et al., 2002) after the demonstration 2 years earlier that mouse ESCs could be derived after SCNT (Munsie et al., 2000). Similar attempts in humans resulted in two reports of cloned ESCs, which were later revealed as being fabricated and were retracted (Kennedy, 2006). Except for a very few reports, rather a long period of silence ensued in human SCNT.This vacuumoccurredeven though the experimental protocols were advanced enough to allow the derivation of ESC lines from single human blastomeres (Chung et al., 2008). It seemed that, in humans, making immunocompatible cells to regenerate tissues or to treat disease would require something other than SCNT into oocytes. If 242 human oocytes did not suffice to produce a genuine cloned ESC line (Hwang et al., 2004), then one should consider that persevering in SCNTmight never pay off, suggesting that there may be some biological roadblock that first needs tobe removed.Researchon theputative roadblocks to human SCNT came to a standstill when, in 2006, the direct induction of pluripotency in somatic cells (iPSCs) provided an easier alternative to cloned ESCs (Takahashi and Yamanaka, 2006) without the technical and ethical limitations that are associated with the use of oocytes and the generation of totipotent embryos. iPSCs and ESCs are functionally interchangeable, although they are distinguishable on the molecular level (Kim et al., 2010; Polo et al., 2010; Ohi et al., 2011). In 2007 the report that non-human primate ESCs could be derived by SCNT (Byrne et al., 2007) revived the SCNTmethod, but it was not sufficient to overcome the mistrust which had followed the fabricated reports of cloned human ESCs. Paraphrasing Cibelli (2007) therapeutic cloning was on the death bed, although death has not been pronounced. Tachibana et al. have now raised human therapeutic cloning from the death bed, reporting that cloned human blastocysts support the derivation of ESCs. Cloned human blastocysts had been obtained before (French et al., 2008; Fan et al., 2011), but the derivation of ESCs was either not attempted, not successful or not reported.ToenableESCderivation, cloned human embryos have to be produced that not only develop to the morphological blastocyst stage, but also harbor a functional inner cell mass (ICM) capable of forming an epiblast. At face value this is the prime achievement of Tachibana et al. and it prompts the first question:What did the authors do differently to previous investigators? In fact, there is more to it, as I am going to illustrate below. It is difficult to predict the future based on one paper, but provided that the experiments are reproducible in the hands of others, the results of Tachibana et al. also prompt a second question: what does the achievement of cloned human ESCsmean for basic science and for biomedicine? I reflect on the pros and cons of the newachievement and I try to identify open issues that the revived field of human SCNT will likely have to confront.