and Medical Aspects of Human Reproductive Cloning" -
Cells: Scientific Progress and Future Research Directions
The same technology—cloning—that could one day cure diseases like
Parkinson's, diabetes and even heart disease, could also be used to make a
copy of a human being.
This ScienCentral news story looks at the promise of human cloning, and the
danger of cloning humans.
Human Cloning vs. Cloning Humans
When scientists say "cloning,"
they mean somatic cell nuclear transfer.
To make a clone, scientists remove the nucleus from an unfertilized egg, and
replace it with a nucleus from an adult body cell such as a skin cell. In
doing so, the resulting cell somehow gets "reprogrammed"—turning
off the genes that made it behave like a skin cell, while turning on genes
that make it act like a developing embryo. The "pre-embryo" is grown
in the lab for a day or two, to just about 100 cells.
What happens next is what separates human cloning from cloning humans. In "reproductive
cloning," the pre-embryo is implanted into a female's uterus, where it
may develop into an exact copy of the adult animal that donated its body cell
the sheep. In so-called "therapeutic cloning," its development
is stopped and scientists can culture from it millions of embryonic stem (ES)
cells. Researchers are quickly learning how to coax ES cells into differentiating
into all sorts of body cells that could be used to replace cells and tissues
damaged by disease or aging.
Jaenisch, a leader in nuclear transfer research, has become an advocate
against attempting to clone humans. Research at his MIT
lab raises big concerns about the safety of reproductive cloning. Their
latest study, published in Proceedings
of the National Academy of Sciences, used high tech gene
chips to analyze whether normal-looking mouse clones have genes that function
"A year ago we only guessed, we only thought, they probably are not normal,"
Jaenisch says. "Now we know for sure, they are not normal."
Jaenisch says that makes it simple to oppose reproductive cloning, at least
for the foreseeable future. "From a scientific point of view, we know
with a high degree of certainty that any baby one would derive through this
nuclear transfer technology—well, most of them would die [before birth].
But the few which make it to birth will be abnormal. Some would be dying very
early, some may develop to let's say school age, but they will definitely
be abnormal," he says.
The majority of scientists agree. In January, the National Academy of Sciences
released a report on the scientific and medical aspects of human reproductive
cloning that concluded it would be so dangerous, the United States ought
to ban it.
No therapies... yet
Therapeutic cloning hasn't yielded any new therapies yet. But scientists are
so convinced it someday will, their
pleas persuaded President Bush to permit limited federal funding of ES
cell research for the first time in US history (see "Human
Cloning - The Ethics").
The first proof that therapeutic cloning can work in an animal model of human
disease came this year, when two MIT labs collaborated to create a mouse embryonic
stem cell line from a body cell of a mouse with an immune deficiency. "This
was a mouse model of the bubble boy disease," says George
Q. Daley whose lab collaborated with the Jaenisch lab on the research.
"Because that line was derived from the original mouse, it was genetically
identical to it, and so it carried the same genetic lesions that produced
the immune deficiency," says Daley. "In the petri dish, we were
able to correct those genetic defects... And then using methods we've just
recently developed, were able to direct the development of blood stem cells
from these ES cells, and put them back into the original mouse."
The therapy partially restored the mouse's immunity. "As with many firsts,
it was not 100 percent effective," Daley says. "But, it at least
convinced us that with the technology available today, we could put this into
As enlightening as it might be to create a new human ES cell line from a child
suffering from the disease, the scientists can't undertake this under the
Presidentâ€™s funding policy. The same huge majority of scientists that
favors banning reproductive cloning wishes the United States would support
But as Daley says, "it would require a change in the presidential policy
for the Federal government to support this work."