Scientists at Whitehead Institute for Biomedical Research havesuccessfully demonstrated that a theoretical--andcontroversial--technique for generating embryonic stem cells is indeedpossible, at least in mice.
The theory, called altered nuclear transfer (ANT), proposes thatresearchers first create genetically altered embryos that are unable toimplant in a uterus, and then extract stem cells from these embryos.Because the embryos cannot implant, they are by definition not"potential" human lives. Some suggest that this would quell theprotests of critics who claim that embryonic stem cell researchnecessitates the destruction of human life. Scientists and ethicistshave debated the merits of this approach, but so far it has not beenachieved.
"The purpose of our study was to provide a scientific basis forthe ethical debate," says Whitehead Member Rudolf Jaenisch, lead authoron the paper that will be published in the October 16 online edition ofthe journal Nature. "Our work is the first proof-of-principle study toshow that altered nuclear transfer not only works but is extremelyefficient."
First proposed by William Hurlbut, Stanford Universityprofessor and member of the President's Council on Bioethics, ANT hasbeen described as an ethical alternative to somatic cell nucleartransfer (SCNT), also known as therapeutic cloning.
For SCNT, a donor nucleus, for example one taken from a skincell, is implanted into a donor egg cell from which the nucleus hadbeen removed. This egg cell is then tricked into thinking it has beenfertilized. That causes it to grow into a blastocyst--a mass of about100 cells--from which stem cells are removed. These embryonic stemcells can divide and replicate themselves indefinitely, and they canalso form any type of tissue in the human body. However, to cull thesestem cells, the blastocyst must be destroyed, which some critics insistis tantamount to destroying a human life.
The procedure theorized by Hurlbut is similar to SCNT, butwith one crucial twist: Before the donor nucleus is transferred intothe egg cell, its DNA is altered so that the resulting blastocyst hasno chance of ever becoming a viable embryo. As a result, a "potentialhuman being" is not destroyed once stem cells have been extracted.
Jaenisch--a firm supporter of all forms of human embryonicstem cell research--has shown that technical concerns about thisapproach can be overcome.
Jaenisch and Alexander Meissner, a graduate student in hislab, focused on a gene called Cdx2, which enables an embryo to grow aplacenta. In order to create a blastocyst that cannot implant in auterus, the researchers disabled Cdx2 in mouse cells.
They accomplished this with a technique called RNAinterference, or RNAi. Here, short interfering RNA (siRNA) moleculesare designed to target an individual gene and disrupt its ability toproduce protein. In effect, the gene is shut off. Jaenisch and Meissnerdesigned a particular form of siRNA that shut off this gene in thedonor nucleus and then incorporated itself into all the cellscomprising the blastocyst. As a result, all of the resulting mouseblastocysts were incapable of implantation.
However, once the stem cells had been extracted from theblastocysts, Cdx2 was still disabled in each of these new cells,something that needed to be repaired in order for these cells to beuseful. To correct this, Meissner deleted the siRNA molecule bytransferring a plasmid into each cell. (A plasmid is a unit of DNA thatcan replicate in a cell apart from the nucleus. Plasmids are usuallyfound in bacteria, and they are a staple for recombinant DNAtechniques.) The stem cells resulting from this procedure proved to bejust as robust and versatile as stem cells procured in the moretraditional fashion.
"The success of this procedure in no way precludes the need topursue all forms of human embryonic stem cell research," says Jaenisch,who is also a professor of biology at MIT. "Human embryonic stem cellsare extraordinarily complicated. If we are ever to realize theirtherapeutic potential, we must use all known tools and techniques inorder to explore the mechanisms that give these cells such startlingcharacteristics."
ANT, Jaenisch emphasizes, is a modification, but not analternative, to nuclear transfer, since the approach requiresadditional manipulations of the donor cells. He hopes that thismodification may help resolve some of the issues surrounding work withembryonic stem cells and allow federal funding.
This research was supported by the National Institutes of Health/National Cancer Institute.
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