Mon Feb 2, 2009
By
Maggie Fox,
Health and Science Editor
WASHINGTON (Reuters) - Researchers who tried to use
mouse, cow and rabbit eggs to make human clones said on Monday the effort
failed to produce workable embryos but added that they showed human cloning
should work in principle.
Mixing human
and animal cells does not appear to program the egg properly, said Dr. Robert Lanza of Massachusetts-based Advanced Cell Technology.
But using
human cells did reprogram the egg cell or oocyte and
activate the genes needed to make a viable embryo, Lanza
and colleagues reported in the journal Cloning and Stem Cells.
Several teams
have tried to make animal-human hybrids as a source of embryonic stem cells,
the master cells of the body. Because human eggs are scarce -- it requires a
surgical procedure to get them from a woman -- some scientists came up with the
idea of using animal egg cells.
The cloning
technique is called somatic cell nuclear transfer. The nucleus is removed from
an egg cell and replaced with the nucleus from another type of cell from the
donor animal or person who is to be cloned.
Done right,
the process starts the egg growing and dividing as if it had been fertilized by
a sperm, but the resulting embryo carries mostly the DNA of the donor.
"The idea
was to simply to plunk a patient's DNA into an empty
cow or rabbit egg -- and presto -- you reprogram the DNA back into a stem
cell," Lanza said in a telephone interview.
But teams that
have tried to do this have always ended up with what looks like a cell dividing
over and over to become an embryo, but which eventually fizzles out.
"For the
last decade, we've carried out literally hundreds of experiments trying to
create patient-specific stem cells using animal eggs," Lanza
said.
BEAUTIFUL
HYBRIDS
"We got
beautiful little hybrid embryos, but it didn't work no matter how hard we
tried."
A mouse-human
hybrid petered out after just one division. The cow and rabbit human hybrids
went further, but stopped at the point when maternal DNA is supposed to kick in
and turn the ball of cells into a proper embryo, Lanza
said.
Lanza's team used a new method called global gene
expression analysis to see which genes were turned on and off as the eggs grew.
"We never
had the tools before to actually look inside the cell and see what's going
on," Lanza said. It appears that using the egg
of another species turns off the genes needed to make an embryo instead of
turning them on, he said.
But the
human-human clone did turn on the right genes, although it, too stopped
dividing before it could produce stem cells, Lanza
said.
"We see
exactly the same genes turned on in a normal embryo are actually turned on in a
human clone," he said.
Ian Wilmut of the
"This
very important paper suggests that livestock oocytes
are extremely unlikely to be suitable as recipients for use in human nuclear
transfer," Wilmut said in a statement.
But Lanza said it might be possible to use other methods to
create "banks" of stem cells that match the several hundred tissue
types found among humans.
This could
include cloning humans, using a single cell from growing embryos used for fertility
treatment, or a new method called induced pluripotent
stem cells, made by taking a sample of skin and reprogramming the cells to act
like embryonic stem cells, Lanza said.