http://www.reutershealth.com/archive/2002/01/10/eline/links/20020110elin004.html
Jan 10, 2002
A certain type of genetic defect
that has been seen in cloned animals may be due to the cells used in the
studies, and not the cloning procedure itself, researchers in Japan report.
To clone an animal, scientists remove
the nucleus from an egg and replace it with the nucleus of a cell from
the "donor" that is to be cloned. In many experiments in cloning mice,
researchers have gotten the donor nuclei from embryonic stem cells. In
other attempts at animal cloning, such as the creation of Dolly the sheep,
adult or fetal cells have been used.
Animal cloning is a complicated process,
and it only occasionally results in a birth. There have also been concerns
about genetic anomalies occurring in surviving clones. Just last week,
Dolly was reported to have arthritis, raising concerns that the cloning
process exacted a genetic defect.
Among the problems that can occur
are anomalies in imprinted genes, which play a key role in growth and development.
Imprinted genes differ from other genes in that their expression in the
body depends on whether they came from the father or mother. Some research
has shown that mice cloned from embryonic stem cells can have abnormalities
in their expression of imprinted genes.
In the new study, reported in the
January 11th issue of Science, scientists cloned mice using nuclei from
adult somatic cells--cells other than egg or sperm. The investigators found
that the embryos, fetuses and full-term animals they created were no more
likely than normal mice to have faulty imprinted genes.
The findings suggest that the cloning
procedure itself is not to blame for past imprinted-gene abnormalities
in mice, according to Atsuo Ogura of the National Institute of Infectious
Diseases in Tokyo, Japan, and colleagues.
Instead, Ogura told Reuters Health,
the problem may have arisen from mutations that accumulated in the donor
embryonic stem cells during the process of culturing them.
Cloning procedures have garnered
much interest because scientists hope to use them in treating disease.
Unlike reproductive cloning, such therapeutic cloning would involve creating
new cells and tissue to replace those damaged by disease. Scientists do
not foresee using embryonic stem cells as donors in therapeutic cloning,
Ogura's team points out.
Still, Ogura said, it is also possible
that other potential donor cells, when cultured for long periods, could
develop genetic abnormalities.
This might be avoided, the researcher
said, by being particularly careful about the cells' culturing conditions.
In this study, Ogura's team used "freshly prepared" donor cells.
However, the investigators looked
only at problems with imprinted genes. Ogura noted that scientists will
have to keep studying cloned animals to confirm that other genetic mechanisms
are normally maintained.
SOURCE: Science 2002;295:297.
Copyright © 2002 Reuters Limited
By Amy Norton
NEW YORK, (Reuters Health)