Researchers offer proof-of-concept
for Altered Nuclear Transfer
CAMBRIDGE, Mass. (Oct. 17, 2005) - Scientists at Whitehead
Institute for Biomedical Research have successfully
demonstrated that a theoretical-and controversial-technique
for generating embryonic stem cells is indeed possible,
at least in mice.
The theory, called altered nuclear transfer (ANT),
proposes that researchers first create genetically altered
embryos that are unable to implant in a uterus, and
then extract stem cells from these embryos. Because
the embryos cannot implant, they are by definition not
"potential" human lives. Some suggest that this would
quell the protests of critics who claim that embryonic
stem cell research necessitates the destruction of human
life. Scientists and ethicists have debated the merits
of this approach, but so far it has not been achieved.
"The success of this procedure in no way precludes
the need to pursue all forms of human embryonic
stem cell research," says Jaenisch, who is also
a professor of biology at MIT. |
"The purpose of our study was to provide a scientific
basis for the ethical debate," says Whitehead Member
Rudolf Jaenisch, lead author on the paper that will
be published in the October 16 online edition of the
journal Nature. "Our work is the first proof-of-principle
study to show that altered nuclear transfer not only
works but is extremely efficient."
First proposed by William Hurlbut, Stanford University
professor and member of the President's Council on Bioethics,
ANT has been described as an ethical alternative to
somatic cell nuclear transfer (SCNT), also known as
therapeutic cloning.
For SCNT, a donor nucleus, for example one taken from
a skin cell, is implanted into a donor egg cell from
which the nucleus had been removed. This egg cell is
then tricked into thinking it has been fertilized. That
causes it to grow into a blastocyst-a mass of about
100 cells-from which stem cells are removed. These embryonic
stem cells can divide and replicate themselves indefinitely,
and they can also form any type of tissue in the human
body. However, to cull these stem cells, the blastocyst
must be destroyed, which some critics insist is tantamount
to destroying a human life.
The procedure theorized by Hurlbut is similar to SCNT,
but with one crucial twist: Before the donor nucleus
is transferred into the egg cell, its DNA is altered
so that the resulting blastocyst has no chance of ever
becoming a viable embryo. As a result, a "potential
human being" is not destroyed once stem cells have been
extracted.
Jaenisch—a firm supporter of all forms of human
embryonic stem cell research—has shown that technical
concerns about this approach can be overcome.
Jaenisch and Alexander Meissner, a graduate student
in his lab, focused on a gene called Cdx2, which enables
an embryo to grow a placenta. In order to create a blastocyst
that cannot implant in a uterus, the researchers disabled
Cdx2 in mouse cells.
They accomplished this with a technique called RNA
interference, or RNAi. Here, short interfering RNA (siRNA)
molecules are designed to target an individual gene
and disrupt its ability to produce protein. In effect,
the gene is shut off. Jaenisch and Meissner designed
a particular form of siRNA that shut off this gene in
the donor nucleus and then incorporated itself into
all the cells comprising the blastocyst. As a result,
all of the resulting mouse blastocysts were incapable
of implantation.
However, once the stem cells had been extracted from
the blastocysts, Cdx2 was still disabled in each of
these new cells, something that needed to be repaired
in order for these cells to be useful. To correct this,
Meissner deleted the siRNA molecule by transferring
a plasmid into each cell. (A plasmid is a unit of DNA
that can replicate in a cell apart from the nucleus.
Plasmids are usually found in bacteria, and they are
a staple for recombinant DNA techniques.) The stem cells
resulting from this procedure proved to be just as robust
and versatile as stem cells procured in the more traditional
fashion.
"The success of this procedure in no way precludes
the need to pursue all forms of human embryonic stem
cell research," says Jaenisch, who is also a professor
of biology at MIT. "Human embryonic stem cells are extraordinarily
complicated. If we are ever to realize their therapeutic
potential, we must use all known tools and techniques
in order to explore the mechanisms that give these cells
such startling characteristics."
ANT, Jaenisch emphasizes, is a modification, but not
an alternative, to nuclear transfer, since the approach
requires additional manipulations of the donor cells.
He hopes that this modification may help resolve some
of the issues surrounding work with embryonic stem cells
and allow federal funding.
This research was supported by the National Institutes of Health/National Cancer Institute.
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