All existing embryonic stem-cell lines, including those approved for federal funding, are contaminated with a non-human molecule that compromises their usefulness in studying treatments for human diseases, according to a study by researchers from UCSD School of Medicine and the Salk Institute.

In a study published Jan. 23 in Nature Magazine, researchers found that these stem-cell lines contain a sialic acid called Neu5Gc, a molecule that human cells are unable to produce. Their findings concluded that stem cells incorporate the molecule when researchers grow them in laboratory cultures derived from animal materials. All traditional methods of growing human stem cells in dish cultures use animal-derived materials, such as mouse and calf serum.

“With this discovery, the pre-existing general concern about using animal products for deriving human embryonic stem cells has become more specific,” the study’s senior author and professor of cellular and molecular medicine, Ajit Varki, stated in a university announcement.

Stem cells are specialized cells in human embryos that can develop into many different types of cells that form specific organs and tissues. Scientists hope to use these cells to develop treatments for a variety of diseases, including diabetes and Alzheimer’s.

However, because humans have natural antibodies against Neu5Gc, the study concluded that contaminated stem cells may be ineffective for possible therapeutic use in human subjects.

The team tried growing stem cells using methods that avoid animal contaminants, including using human-derived serum. When researchers grew cells in human serum with low amounts of Neu5Gc antibodies, it reduced the immune response, but did not eliminate it, the study stated. No known methods exist to remove Neu5Gc completely from contaminated cells.

“It would seem best to start over again with newly derived human embryonic stem cells that have never been exposed to any animal products,” Varki stated. “However, such an approach could not be pursued under existing rules for the use of federal grant dollars.”

In 2001, President George W. Bush restricted federal funding to stem-cell lines in existence at the time, citing moral and ethical concerns over the use of human embryos for harvesting the cells.

Proponents of human embryonic stem-cell research have used the new study to rally for expanded federal funding, according to the Coalition of Americans for Research Ethics, a group that opposes embryonic stem-cell research.

“The reality is, there is really not much to this report at all as a justification for expanding government funding policy to include new [stem-cell] lines,” the group’s Communications Director Gene Tarne stated in an e-mail. “Rather than relying on scare tactics, proponents … need to provide some really hard evidence that the effectiveness of [stem cells] to provide safe and reliable therapies for humans is anything more than pure speculation on a very distant and increasingly unlikely proposition.”

The National Institutes of Health, which control federal stem-cell research funding, did not return calls seeking comment.

C.A.R.E. also stated that there are still approved stem cells that have not been used for experiments and that have never been exposed to animal serum.