Cloned, fertilized stem cells look the same
Mouse cells, whether from nuclear transfer or fertilization, show identical transcription profiles.
Researchers at the Whitehead Institute in Cambridge, Massachusetts, have found that cultured stem cells from cloned mouse embryos have transcription profiles that are indistinguishable from those from normally fertilized embryos. The results
, published in the January 24 issue of Proceedings of the National Academy of Sciences
, support the hope that damaged cells could one day be safely replaced with cells carrying a patient?s own DNA.
"We knew from the phenotypic evidence that [the stem cells] were normal, but we had never done a detailed molecular analysis,? said Tobias Brambrink, first author on the paper. "From a molecular or a transcriptional point of view, stem cell lines, whether cloned or fertilization-derived, are indistinguishable," Brambrink said.
Many embryos created by cloning carry fatal defects, often because the donor DNA has been modified to express only genes that were needed in the mature cell it was extracted from. For example, genes such as oct4
that are needed for embryonic development are usually not reactivated, while genes that were switched on later in development can stay "on" when a nucleus is transferred into an embryo. Some researchers have suggested that this "epigenetic memory" could also pass to stem cells derived from these embryos, making it risky to use them to treat patients.
To investigate, the team, led by Rudolf Jaenisch
, transferred nuclei from T-cells, B-cells, and fibroblasts into enucleated eggs. They grew these nuclear-transfer embryos to the blastocyst stage, and then tried to culture cells from the inner cell mass. Most of the extracted cells failed to thrive in their new, artificial environment, but a small fraction became proliferating lines of embryonic stem cells (ESCs).
The team used microarrays to monitor the expression of over 30,000 transcripts in five lines of nuclear-transfer ESCs, and compared this expression to ESC lines created by ordinary fertilization. They easily distinguished the transcription profiles of ESCs with different genetic background, but even sophisticated algorithms could not tell the clone-derived cells from the fertilized ones.
?After derivation of stem cells, these epigenetic errors, which we have seen in cloned mouse embryos or cloned bovine embryos, disappear,? said Miodrag Stojkovic
at the Príncipe Felipe Research Center in Valencia, Spain, who was not part of the research.
Jaenisch, who is also at the Massachusetts Institute of Technology, has suggested that the process of deriving a stem cell line selects only unusual, highly proliferative cells. Through mechanisms that remain unclear, these cells have been "reprogrammed" so that their expression is similar, although not identical, to that in an early embryo. His group had already shown that when these cells are added to blastocysts, the resulting mice developed normally, even when the blastocysts comprised tetraploid cells that can't develop properly on their own.
Stojkovic cautioned that there are huge differences between mouse and human ESCs, however, and much remains to be done. Earlier claims of human ESCs derived from cloned embryos, including patient-specific lines, have now been discredited
. Nonetheless, the new results suggest that such lines could be safer than some had feared.
The concern about abnormalities from cloning "is a huge one," agreed Carol Ware
of the University of Washington in Seattle, who was not involved in the work. Still, this paper, she said, "goes a long way to helping to resolve the issue."
Links within this article
Tobias Brambrink, Konrad Hochedlinger, George Bell, and Rudolf Jaenisch, "ES cells derived from cloned and fertilized blastocysts are transcriptionally and functionally indistinguishable," Proceedings of the National Academy of Sciences, January 24, 2006.
R. Jaenisch, "Human cloning - the science and ethics of nuclear transplantation." New England Journal of Medicine, December 30 2004.
I.Oransky, "All Hwang human cloning work fraudulent," The Scientist, January 10, 2006