Cloned Stem Cells are “Normal”
Cloned Stem Cells are “Normal”
Researcher at the Whitehead Institute for Biomedical Research have determined that stem cells which have grown from a cloned embryo are identical to stem cells from a normally fertilized embryo, at least in mice. One of the concerns that is frequently cited about embryonic stem cell research is the potential for genetic abnormalities if used therapeutically, since cloned animals often have genetic abnormalities. A press release about the study is on Newswise, and the story has been picked up by several other media sources.
I’m going to quote extensively here from the release, because it gives a very cogent and well-written description of the cloning process:
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.
So there has been a lot of concern about what would happen if stem cells derived from a cloned embryo were used therapeutically. But the new study indicates that the stem cells do not have the genetic issues of the clone. The research compared gene expression in five different lines each of stem cells derived from clones and stem cells derived form fertilized eggs, and found no differences. The Scientist reports the researchers examined over 30,000 genetic transcriptions and “even sophisticated algorithms could not tell the clone-derived cells from the fertilized ones.”
The research was done by transferring nuclei from several types of cells into an egg, then taking cells from the inner mass of the blastocyst when it developed. It is of note that most of the cells derived from the clones died in culture, and only a few survived to generate these stem cells. One of the researchers has suggested that “the process of deriving a stem cell line selects only unusual, highly proliferative cells.” So I presume it is possible that defective cells were unable to continue growing in culture, which would be a sort of self-regulating mechanism against genetic flaws.
Although the study was done in mice and clearly cannot be said to demonstrate the same for humans, it does suggests that the SCNT procedure may have fewer genetic risks than has been surmised, which is good news for potential therapeutic cloning.