"The current stem cell lines are problematic because they accumulate mutations as they grow," said Dr. Bert Vogelstein, chairman of a key stem cell research committee of the National Research Council and Institute of Medicine. "Every time a cell divide in a tissue culture it accumulates mutations. In aggregate those mutations may cause certain problems."
Vogelstein and other medical experts testified before a Senate appropriations subcommittee.
Stem cells are unspecialized cells that can become any cell type with specialized functions. They can be gleaned from several sources, such as placenta, umbilical cords and adult cells, but scientists say those derived from human embryos are most useful for therapeutic and research applications.
Stem cells, if implanted in a patient, could grow cells afresh, offering cures for intractable diseases like Parkinson's disease, Alzheimer's and diabetes.
On August 9 President Bush announced that embryonic stem cell research should receive federal funding. But that decision permits federal funding only on the some 60 existing stem cell lines that have already been derived.
While stem cell providers the world over are looking to the National Institutes of Health to fund stem cell research on those existing lines, experts like Vogelstein warn of problems.
One issues is that many of the existing lines were "cultured with animal cells and serum" creating the fear that pathogens in mice or cows could be transmitted to humans if the stem cells are used for therapeutic applications.
Another problem is that patient's immune system may attack the newly introduced stem cells. One way of sidestepping the problem is through a process known as somatic cell nuclear transfer or SCNT. That procedure, also known as therapeutic cloning, involves taking DNA from a cell belonging to a patient and inserting it into an egg whose nucleus has been removed. When the cell divides, it produces stem cells that can be put in the patient without causing an immune response.
But some lawmakers would like to see so-called therapeutic cloning -- which, unlike reproductive cloning, will never result in a birth -- outlawed.
A proposed measure by Sen. Sam Brownback, R-Kan., would outlaw the act of therapeutically cloning a human embryo. It would also ban the importation of products derived from therapeutic cloning.
Vogelstein said such a ban could mean that "regenerative medical applications will never occur and people with terrible diseases won't be healed."
He also said that since much biomedical research is international, "it's likely most useful lines and reagents will be developed outside the U.S." Therefore, a ban on importation would also curtail important research.
Another witness, James Thompson, the cell biologist from the University of Wisconsin-Madison who discovered stem cells, said despite the President's Aug. 9 decision, "the existing human embryonic stem cell lines will not fulfill their promise unless the National Institutes of Health begins to aggressively fund this area of research."
Thompson point out that the NIH Human Embryonic Stem Cell registry, which is necessary to initiation federal funding, had not been completed.
Later in the hearing, Sen. Arlen Specter, D-Pa., asked another witness, Wendy Baldwin, deputy director for extramural research at the NIH, why the registry was still operating.
Baldwin said the project was behind schedule due to the terrorist strikes but promised that it would be up on the NIH website within a week.