EDITOR'S NOTE:The scientific reports by Hwang et al.1,2 that are described in this article were retracted by Science on January 12, 2006. See Snyder and Loring (N Engl J Med 2006; 354:321-4) for further information.
In June 2005, Hwang and coworkers at Seoul National University1reported that pluripotent human embryonic stem cells can efficientlybe generated by nuclear transfer from a wide variety of patients(Figure 1). The authors transferred somatic-cell nuclei fromeight male and three female donors, 2 to 56 years of age, intooocytes whose nuclear genomes had been removed. To underscorethe clinical relevance of their work, they used donors who hadconditions that are potentially amenable to stem-cell therapy:congenital hypogammaglobulinemia, spinal cord injury, and juvenilediabetes.
Figure 1. Derivation of Patient-Specific Therapeutic Cells through Nuclear Transfer.
Nuclear-transfer embryonic stem cells that carry the nuclear DNA of an existing person can be obtained by injecting a nucleus from a patient into an enucleated ovum. The nuclear-transfer construct is then activated, whereupon it undergoes cell division and forms a blastocyst. Explantation of the blastocyst to cell culture yields a line of cells that can differentiate into ectodermal, mesodermal, and endodermal cells (i.e., cell lines that have the potential to develop into any type of cell). A bottleneck in the procedure has been the low efficiency with which nuclear-transfer embryonic stem cells are derived from blastocysts. Hwang et al.1 recently reported a dramatic improvement in the efficiency of stem-cell derivation from blastocysts: from 5 percent to 35 percent. They went on to show that the stem cell lines could differentiate into somatic cells of the ectoderm, mesoderm, and endoderm lineages, including cells with characteristics of the skin, striated muscle, smooth muscle, neuroepithelium, cartilage, renal tissue, gastrointestinal epithelium, respiratory epithelium, colonic epithelium, and bone. The data in the blue boxes indicate the improvement in the efficiency of the specific process since 2004.
Cells containing nuclei from nine donors developed to the blastocyststage, whereas cells containing nuclei from the other two donorsfailed to do so. Blastocysts from each of the nine patientsyielded 1 or 2 embryonic stem cell lines (referred to here bythe generic term "nuclear-transfer embryonic stem cells" todenote that they were derived from nuclear-transfer constructs),for a total of 11 embryonic stem cell lines from 31 blastocysts.On average, one cell line was established per 16.8 oocytes,an efficiency of 6.0 percent. This reflects an increase in efficiencyby a factor of more than 14 as compared with the report lastyear by the same group,2 in which a single human nuclear-transferembryonic stem cell line was derived from 242 oocytes. Thisimprovement is attributed in part to the use of oocytes fromyounger donors in the present study.
With this improved efficiency, the line has been crossed betweenviewing the derivation of human nuclear-transfer embryonic stemcells as an experimental system and viewing it as a viable clinicalproposition. Scientifically speaking, this is a pedestrian crossing.The derivation of embryonic stem cells from the inner cell massof blastocyst-stage embryos was first achieved in mice nearly25 years ago, in primates 10 years ago, and in humans in 1998.Elements of this work showed that in general, it was possibleto maintain embryonic stem cells in culture for prolonged periodsin an undifferentiated state, although this has yet to be shownfor human nuclear-transfer embryonic stem cells. Like the humannuclear-transfer embryonic stem cells described by Hwang etal., undifferentiated embryonic stem cells from a variety ofspecies are pluripotent, in that they can give rise to eachof the three founding germ layers of an early embryo.
The next step in assessing the therapeutic potential of humannuclear-transfer embryonic stem cells will be to derive purepopulations of clinically relevant cells from them in vitro.It is now possible to induce mouse embryonic stem cells to differentiateinto many types of cells (though fewer than 10 percent of theestimated total types of cells), including pancreatic beta cells,cardiomyocytes, and specific subgroups of neurons. There isstill only one reported study in which such differentiationhas been shown to produce cells that have not been geneticallyaltered and yet are able to correct a deficit after being transplanted— in this case, midbrain dopaminergic neurons in a mousemodel of Parkinson's disease.3 Prescriptive differentiationinto cells with neuronal specifications has also been shownfor human nuclear-transfer embryonic stem cells4 with the firstreport of the behavior of neuronal precursors derived from suchcells after transplantation.5 The derivation of human nuclear-transferembryonic stem cells is a prelude to the arguably more difficultand time-consuming work ahead and yet brings us closer to theprospect of patient-specific cell therapy.
The importance of the report by Hwang et al. goes beyond this,for it gives the clearest indication yet that the United Stateshas lost the initiative in the human nuclear-transfer debate.In March, the majority of the Asian countries voting on a U.S.-backed,nonbinding statement by the United Nations calling for a banon all forms of human cloning this past spring rejected it;among them were Cambodia, China, India, North Korea, South Korea,Japan, Singapore, and Thailand. While the United States remainsrooted in atavism, Hwang and coworkers have shown that Asiais moving forward.
Source Information
From the Laboratory of Mammalian Molecular Embryology, RIKEN Center for Developmental Biology, Kobe, Japan.
References
Hwang WS, Roh SI, Lee BC, et al. Patient-specific embryonic stem cells derived from human SCNT blastocysts. Science (in press).
Hwang WS, Ryu YJ, Park JH, et al. Evidence of a pluripotent human embryonic stem cell line derived from a cloned blastocyst. Science 2004;303:1669-1674. [Free Full Text]
Barberi T, Klivenyi P, Calingasan NY, et al. Neural subtype specification of fertilization and nuclear transfer embryonic stem cells and application in parkinsonian mice. Nat Biotechnol 2003;21:1200-1207. [CrossRef][ISI][Medline]
Perrier AL, Tabar V, Barberi T, et al. Derivation of midbrain dopamine neurons from human embryonic stem cells. Proc Natl Acad Sci U S A 2004;101:12543-12548. [Free Full Text]
Tabar V, Panagiotakos G, Greenberg ED, et al. Migration and differentiation of neural precursors derived from human embryonic stem cells in the rat brain. Nat Biotechnol 2005;23:601-606. [CrossRef][ISI][Medline]
A correction has been published: N Engl J Med 2006;355(4):429.
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