Cloned stem cells prove identical to fertilized stem cells

January 18, 2006

Scientists generally agree that all cloned animals are biologically flawed. But they don't agree about what that means for stem cells derived from cloned embryos, the basis for therapeutic cloning.

Also known as somatic cell nuclear transfer, therapeutic cloning is a promising approach to creating individually customized cellular therapies for treating certain disorders. Demonstrated in mice but not in humans, it begins with stem cells derived from a cloned embryo. But if cloned embryos can't produce normal organisms, how can they produce normal stem cells?

Analyzing the complete gene-expression profiles of both cloned and fertilization-derived stem cells in mice, scientists at MIT and the Whitehead Institute for Biomedical Research now have concluded that the two are, in fact, indistinguishable.

"This paper demonstrates clearly that it doesn't matter if a stem cell has been derived from a cloned embryo or from a fertilized embryo," says Whitehead member and MIT biology Professor Rudolf Jaenisch, senior author on a paper that will appear online the week of Jan. 16 in the Proceedings of the National Academy of Sciences. "Both can be equally good for therapy."

To create a clone, a scientist removes the nucleus from a donor cell, then places it into an egg from which the nucleus has been removed. The researcher then tricks the egg into thinking it's been fertilized. The egg develops into a blastocyst, an early-stage embryo consisting of no more than 100 or so cells. The scientist can then either remove the stem cells from this blastocyst, or place it into a uterus where it has the potential to develop into a fetus.

Here's where things get complicated. The original donated nucleus may have come from, say, a skin cell. For a viable fetus to develop, the egg needs to reprogram the genome of the skin cell, shutting off genes specific for skin tissue and turning on genes needed for embryonic development, genes that are normally dormant in tissue-specific cells. In other words, the egg needs to erase all tissue-specific memories from the skin cell and revert it into a genomic blank slate.

But this entire process is almost never perfect, and nearly all cells in a cloned blastocyst retain some memory of their original source. As a result, the developing fetus inevitably has some degree of genetic abnormality. Most clones, in fact, die in utero or at birth. The few clones that make it into adulthood are often plagued by bizarre health complications. This is one reason why scientists generally believe that attempting to clone a human being is morally reprehensible.

But are the cloned embryo's stem cells beleaguered by the same defects?

Studies have demonstrated that a small number of stem cells in the blastocyst appear to be spared this faulty reprogramming. When stem cells from a cloned blastocyst are removed and placed into a dish, most die. A few, however, survive and give rise to an embryonic stem cell line, and these appear to be thoroughly reprogrammed.

Researchers have tried to test the integrity of these surviving stem cells by transplanting them into fertilized blastocysts and then observing the overall health of the resulting animal. Although these animals generated entirely from cloned stem cells appear to be fine, many scientists don't accept this result as definitive.

Tobias Brambrink, a postdoctoral researcher in the Jaenisch lab, tried a different approach, comparing gene expression in cloned and fertilization-derived stem cells. With a series of microarray chips, Brambrink measured which genes were active and which were silent in both kinds of cells. To ensure the accuracy of his results, he compared five lines of cloned stem cells with five fertilization-derived stem cell lines.

"The results are very clear," says Brambrink. "If a gene is active in fertilized stem cells, it's also active in cloned stem cells, and at the same level of activity. The same goes for genes that are silent. There is really no significant molecular difference between both kinds of stem cells."

"In my opinion, these results solidify the argument that while a cloned animal is abnormal, a cloned stem cell is perfectly normal," says Jaenisch.

Source: MIT (by David Cameron)

Explore further: Researcher explores the current state of domestic animal cloning

Related Stories

Researchers successfully clone adult human stem cells

April 18, 2014

(Phys.org) —An international team of researchers, led by Robert Lanza, of Advanced Cell Technology, has announced that they have performed the first successful cloning of adult human skin cells into stem cells. A paper ...

Saving the snow leopard with stem cells

January 23, 2012

(PhysOrg.com) -- The survival of the endangered snow leopard is looking promising thanks to Monash University scientists who have, for the first time, produced embryonic stem-like cells from the tissue of an adult leopard.

Recommended for you

A cataclysmic event of a certain age

July 27, 2015

At the end of the Pleistocene period, approximately 12,800 years ago—give or take a few centuries—a cosmic impact triggered an abrupt cooling episode that earth scientists refer to as the Younger Dryas.

New blow for 'supersymmetry' physics theory

July 27, 2015

In a new blow for the futuristic "supersymmetry" theory of the universe's basic anatomy, experts reported fresh evidence Monday of subatomic activity consistent with the mainstream Standard Model of particle physics.

Dense star clusters shown to be binary black hole factories

July 29, 2015

The coalescence of two black holes—a very violent and exotic event—is one of the most sought-after observations of modern astronomy. But, as these mergers emit no light of any kind, finding such elusive events has been ...

Image: Hubble sees a dying star's final moments

July 31, 2015

A dying star's final moments are captured in this image from the NASA/ESA Hubble Space Telescope. The death throes of this star may only last mere moments on a cosmological timescale, but this star's demise is still quite ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.