Fertilized eggs yield stem cells

For the first time, scientists have successfully performed somatic-cell nuclear transfer (SCNT) using fertilized mouse eggs, producing stem cell lines and cloned animals, according to a study published this week in Nature.

The findings add preliminary support to the possibility researchers may be able to derive viable embryonic stem cells from non-viable human embryos left over from in vitro fertilization, the authors note.

Study author Kevin Eggan from the Harvard Stem Cell Institute in Cambridge, Mass., and his colleagues have obtained all necessary approvals from Harvard to begin SCNT experiments with donated human frozen zygotes. Eggan said he has already received private donations to conduct the work.

Eggan also received approval last year to use donated unfertilized eggs for SCNT experiments, but his group has not received one egg donation. Massachussetts legislation prevents compensation to women for the time and effort involved in donating ova for research.

"Based on this logistical challenge and the real inability to proceed, we turned to principles and considered why it was we needed unfertilized eggs to do SCNT experiments in the first place," Eggan told reporters during a phone-in press conference today (June 6).

In three additional studies, two in Nature and one in Cell Stem Cell, researchers refine a technique used to produce mouse embryonic stem cell-like cells by introducing four factors to a somatic cell, showing that, with more work, this technique may be used to generate embryonic stem cell lines from human somatic cells.

"I think [the four papers] are exciting in the sense that they further our understanding of nuclear reprogramming and [and its ability] to produce embryonic stem cell lines which we can use for therapeutic cloning," Vanessa Hall, from the Department of Experimental Medical Science at Lund University in Lund, Sweden, who was not involved with the work, told The Scientist.

Stem cell research "is going to be greatly assisted by looking at these mouse models to try to apply the principles to our human studies," said George Daley of the Children's Hospital Boston in Boston, Mass., who was not involved with the studies.

Conventional somatic-cell nuclear transfer (SCNT), during which scientists replace genetic material from an unfertilized egg with that of a somatic cell, works in mice, but has presented technical challenges and ethical difficulties when using human oocytes.

To investigate alternative techniques, a group led by Eggan generated abnormal mouse zygotes, similar to human embryos generated during in vitro fertilization that contain defects that render them non-viable. They removed the cells' mitotic chromosomes, replacing them with mitotic chromosomes from a donor mouse embryonic stem (ES) cell. After the group removed mitosis inhibitors, the cells divided and formed blastocysts.

The research "demonstrates that in fact you can use cells from very early fertilized embryos to do the same thing we thought only unfertilized eggs could do in the past," Eggan said.

From a total of 174 ES-cell-derived blastocysts and morulae transferred to foster mothers, nine resulted in living pups. Of these, seven died from breathing difficulties, one died from a developmental defect, and the last animal was rejected by its foster mother.

The three additional papers build on findings published last year in Cell from a group led by Shinya Yamanaka at Kyoto University in Kyoto, Japan. In last year's article, Yamanaka and colleagues showed that a combination of four factors, Oct4, Sox2, Klf and c-Myc, when introduced into mouse embryonic or adult fibroblasts, yielded pluripotent cells that closely resembled embryonic stem cells.

The studies, one of which is led by Yamanaka, show a refined strategy for identifying the new pluripotent cells induced by the four factors. To identify which cells had become pluripotent, the groups looked for expression of Nanog. This marker pinpointed pluripotent stem cell lines that, when comparing genetic and chromatin characteristics, more closely matched embryonic stem cells than the cells generated in the 2006 Cell article.

"I think initially we were quite surprised that it worked so easily. We did it, and the first time, it worked," Alex Meissner, co-author of the Nature study led by Rudolf Jaenisch from the Whitehead Institute for Biomedical Research in Cambridge, Mass., told The Scientist.

The findings "tell us now that we can understand the reprogramming process by understanding the way those four genes work." But there's a long way to go before these results inform work with human cells, researchers cautioned. "One of the downfalls [of the studies] is that if we want to take this to a therapeutic level, it's going to be very difficult to use genetic modification to induce embryonic stem cells from human somatic cells," Hall said.

Indeed, the studies revealed the inefficiency of converting somatic cells to pluripotent cells -- scientists successfully reprogrammed only 1 out of 10,000 cells exposed to the factors.

To optimize the method for application in human cells, Meissner and others plan to look for other safer ways to induce somatic cells to form pluripotent cells. He said it is possible that human somatic cells will require different or additional factors. "There's a little more work to do before we can translate it to humans. But it's a very important finding."

Kelly Rae Chi
mail@the-scientist.com

Links within this article

D. Egli, J. Rosains, G. Birkoff, and K. Eggan, "Developmental reprogramming after chromosome transfer into mitotic mouse zygotes," Nature, June 6, 2007.
http://www.nature.com

"Cracking cloning," The Scientist, June 1, 2007.
http://www.the-scientist.com/article/daily/53224

Kevin Eggan
http://www.mcb.harvard.edu/Eggan

N. Maherali, "Directly reprogrammed fibroblasts show global epigenetic remodeling and widespread tissue contribution," Cell Stem Cell, July 2007.
http://www.cellstemcell.com

T. Toma, "Genetic modification of mouse embryonic stem cells," The Scientist, Dec. 20, 2001.
http://www.the-scientist.com/article/display/20106

Vanessa Hall
http://www.nesu.mphy.lu.se/research/cv/vanessa_cv.html

George Daley
http://daley.med.harvard.edu

R. Lewis, "Stem cells...an emerging portrait," The Scientist, July 4, 2005.
http://www.the-scientist.com/article/display/15592

K. Takahashi and S. Yakanama, "Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors," Cell, Aug. 25, 2006.
http://www.the-scientist.com/pubmed/16904174

K. Okita, T. Ichisaka and S. Yamanaka, "Generation of germline-competent induced pluripotent stem cells," Nature, June 6, 2007.
http://www.nature.com

M. Wernig, et al., "In vitro reprogramming of fibroblasts into a pluripotent ES-cell-like state," Nature, June 6, 2007.
http://www.nature.com

Rudolf Jaenisch
http://www.wi.mit.edu/research/faculty/jaenisch.html