Embryonic and adult stem-cell findings may re-fuel cloning debates
21 June 2002
US scientists have reversed the symptoms of Parkinson's disease in rats using stem cells from mouse embryos1. Another team has compelling evidence that they have isolated a stem cell from adult human bone marrow that can produce all the tissue types in the body, from blood to muscle to nerve2.
Stem cells from embryos were known to give rise to every type of cell. Those from adults were thought to have a more limited repertoire.
Researchers hope to use stem cells to repair or replace diseased or damaged organs, leading to new treatments for human disorders that are currently incurable, including diabetes, spinal-cord injury and brain diseases.
The new reports may re-fuel the debate in the US Senate over whether to permit the cloning of human embryos for medical research, which stalled earlier this week. US scientists are fighting to be able to harvest stem cells from human embryos. Opponents, such as anti-abortion groups, claim that such studies are unnecessary because adult stem cells are an equally versatile alternative.
Today's papers do not settle the adult-versus-embryo dispute: they suggest that both could yield promising therapies. Ultimately, different cell types might best treat different diseases, so most scientists advocate continued research on both types. "Parallel work is the efficient way to go," says stem-cell researcher Neil Theise of New York University.
Parkinson's disease affects up to 5 million people worldwide. It comes about when nerve cells in the brain that produce a chemical called dopamine die, causing movement and walking difficulties.
Scientists have previously struggled to make sufficient dopamine-producing cells for a transplant. "We nail that," says Ron McKay of the National Institutes of Health in Bethesda, Maryland. His team engineered a gene into embryonic stem cells that keeps them churning out the correct type of nerve cell.
The researchers transplanted the cells into the brains of rats with Parkinson's symptoms. The animals stopped running in circles and survived for 2-3 months - "the equivalent of forever", says McKay.
This is one of the first demonstrations that embryonic stem cells can help animals recover from disease. But the research has a long way to go before it can be tested in humans.
McKay is now trying the technique on monkey and human embryonic stem cells. In earlier efforts to treat Parkinson's, transplanted fetal nerve cells had trouble making the correct level of dopamine - this problem will also have to be tackled.
If it pans out, stem-cell transplants might be used to repair Parkinson's brain damage alongside drugs that prevent nerve cells from degenerating in the first place, predicts Ole Isacson of McLean Hospital and Harvard Medical School in Belmont, Massachusetts, who is carrying out similar studies: "I can't wait for it".
And the technique - using gene therapy to repair or alter stem cells - could be applied to treat many other diseases. Researchers are already working on treating diabetes by transplanting cells that secrete insulin into the pancreas. "It's a very exciting way to engineer cells that are therapeutically useful," says Isacson.
But embryonic stem cells still have disadvantages. First, transplanted cells sometimes grow into tumours. Second, the human embryonic stem cells that are available for research would be rejected by a patient's immune system.
Tissue-matched transplants could be made by either creating a bank of stem cells from more human embryos, or by 'cloning' a patient's DNA into exisiting stem cells to customize them. This is laborious and ethically contentious.
These problems could be overcome by using adult stem cells, taken from a patient, that are treated to repair problems and then put back. But until now some researchers were not convinced that adult stem cells could, like embryonic ones, make every tissue type.
Catherine Verfaillie of the University of Minnesota Medical School in Minneapolis and her team have all but settled that debate by identifying an adult cell that can grow into any other cell type. "It's an extraordinary thing," says stem-cell expert Austin Smith of the Centre for Genome Research in Edinburgh, UK, who has criticized some earlier studies.
Verfaillie's findings were first revealed in the press in January this year. Now they have undergone scrutiny by other scientists and the details have been published. The team isolated a rare cell in bone marrow from mouse, rat and human. They injected the mouse cells into mouse embryos.
The cells' descendants turned up in almost every tissue including blood, brain, muscle, lung and liver. "People didn't think such cells could be generated," says Stuart Orkin of Harvard Medical School in Boston, Massachusetts.
Groups arguing against human cloning have already quoted Verfaillie's results as evidence that there is no need to clone humans to make embryonic stem cells. But scientists suspect that there may be some subtle differences between the newly discovered adult cells and embryonic stem cells, which could affect how they act therapeutically.
"It's very exciting that we can now compare the two," says Smith. "Politicians
and lobbyists can say what they like, but it's clear you have to investigate
both. Nobody serious has ever suggested anything other than that."