Researchers from the Memorial Sloan-Kettering Cancer Center have reported successfully treating Parkinson’s-like symptoms in mice using a therapeutic cloning technique. Published online March 23 in the journal Nature Medicine, the Sloan-Kettering study has been widely portrayed as a possible breakthrough in the fight against Parkinson’s disease.
However, even the study’s lead author says that the work is best seen as a general demonstration of the power of therapeutic cloning techniques. For a number of reasons, experts say, such techniques are unlikely to be very useful in treating Parkinson’s disease itself.
A history of promise
The transplantation of new cells into the brains of people with Parkinson’s disease to replace those killed by the disease has been studied for two decades. So far, about 200 people around the world have received implants of fetal brain cells, and some have experienced a partial abatement of their symptoms as a result.
Why has the technique not taken off? One reason is that it hasn’t worked for all people. Another is that researchers generally prefer not to rely on tissue from aborted pregnancies for their supplies of therapeutic cells. Transplants of brain cells into a patient from an unrelated adult donor are difficult because the new tissue would likely provoke an immune rejection response.
Cell-therapy researchers therefore have tried to develop cloning techniques that would allow a patient’s own cells to be used therapeutically.
The Sloan-Kettering study
The study reported in Nature Medicine was led by neurosurgeon Viviane Tabar and was based on a cloning technique she and her colleagues demonstrated in mice in 2001.
“We showed then that we can transfer the nucleus from a tail cell, put it in an oocyte (egg cell), make an embryonic stem cell line and then differentiate it into different types of cells,” Tabar says.
“What hadn’t been done [before the recent study] is the proof of principle that this technology can actually be used to cure a disease.”
In the March study, Tabar and her colleagues took 24 mice from two different strains, snipped off bits of their tails and shipped these tissue samples to the cloning laboratory of Japanese colleague Sayaka Wakayama, first author of the 2001 study.
Wakayama and his colleagues then cultured cells from these samples, extracted their nuclei and implanted these nuclei into oocytes from another mouse strain. The resulting cells grew as if they had just been “born” through conception, and for practical purposes became embryonic stem cells. Over time Wakayama’s group chemically induced these stem cells to “differentiate” into dopamine-producing neurons.
Back at Sloan-Kettering, researchers injected the 24 mice with a chemical meant to kill dopamine-producing neurons in their brains. Of these, six showed consistent parkinsonian signs after six months and received brain injections of the dopamine-producing neurons Wakayama’s team had derived from their tail-skin cells.
A large number of these cells “took” in the brains of the mice that received them and resulted in clear reductions in parkinsonian symptoms according to a series of standard measures. In addition, for a control group of mice that received genetically-mismatched cell transplants, most of the transplanted cells failed to survive, resulting in a very weak behavioral improvement.
“Is it at all feasible to claim that you can take a skin biopsy from an animal and then make dopamine neurons and give it back to them? That’s really what we proposed to do and what we achieved,” says Tabar. “And here we demonstrated even at the level of the lab mouse, which is very tolerant to immune mismatch, that there is an advantage in giving an autologous (self-derived) graft. All of this sounds rather intuitive, but actually it has never been done before.”
“I think the work that was done at Sloan-Kettering was magnificent,” says neurologist William Langston, who heads a private, California-based research center, the Parkinson’s Institute. “It’s great science, and it’s a really important step.”
The problem with Parkinson’s
Tabar emphasizes that she is a cloning researcher, not a Parkinson’s researcher. “In that paper, even though the focus was on Parkinson’s disease, it was really just sort of a platform to highlight the very considerable therapeutic potential of using embryonic stem cells. And as we learn how to differentiate embryonic stem cells into different [cell types], they will become available for different diseases. So some groups are working on making pancreatic islet cells for diabetes. Others are looking at blood cells, bone cells, etc., and the potential is huge.”
For Parkinson’s disease, however, the potential of therapeutic cloning may not be so huge.
“I’m afraid that we are still a very long way from seeing this as a meaningful therapy in Parkinson’s disease,” says Langston. Tabar acknowledges there are many hurdles to overcome.
Perhaps the most obvious of these hurdles is the lack of knowledge of what really happens when cells are transplanted into the human brain.
Langston notes that in the original fetal-cell studies in humans, many of the transplanted cells not only survived but formed new connections within the brains of people with Parkinson’s. “Yet the clinical outcome was not very impressive at all,” he says. “And we still don’t understand why.”
The fetal cell experiments in people with Parkinson’s disease were meant to supply the neurotransmitter dopamine to a region of the brain known as the striatum, which is relatively deprived of dopamine during the progress of the disease. In a healthy brain this dopamine is pumped up from a deeper brain region known as the substantia nigra, whose neurons connect to the striatum via long stalk-like processes. In the brains of people with Parkinson’s disease most of those nigral dopamine-producing neurons are dead or dying.
But transplanting dopamine-producing cells into the substantia nigra isn’t considered a viable option. No one knows how to make such cells regrow to connect to the distant striatum; moreover, the substantia nigra lies within the vital and delicate brainstem region. “If you get into trouble there, like a hemorrhage or something, it could be pretty catastrophic,” says Tabar.
Cell therapies aimed at treating Parkinson’s have therefore simply delivered new dopamine-producing cells to the striatum itself, instead of repairing the integrated, regulated, dopamine-delivery system of the substantia nigra.
This strategy may not work. The dopamine production of transplanted cells is generally “not as regulated as in a normal dopamine neuron,” says Xiaoxi Zhuang, a neurobiologist at the University of Chicago. Normal dopamine-producing cells are wired into networks of other cells, and within those networks are heavily regulated by connections to other cells. “We’ve done some research that indicates it may be a problem to have unregulated dopamine in the brain,” Zhuang says.
“Integration in the circuitry is one of the big questions hovering over the use of transplantation in Parkinson’s,” agrees Tabar.
What is known is that when dopamine-producing cells are transplanted to the striatum, they are liable to behave in unexpected ways. Tabar says that in one of the six mice her group treated with self-derived cells, the transplanted cells grew excessively. The overgrowth turned out, she said, to be “made of just primitive, young, neural stem-cell-like cells,” which apparently had slipped through the cell selection process along with dopamine-producing neurons.
Langston notes too that in the original fetal-cell transplant experiments, the grafted cells caused some patients to experience a side effect he calls “runaway dyskinesia”—a spontaneous twisting syndrome apparently caused by an excessive, uncontrolled supply of dopamine to movement-regulating cells in the striatum. “It was as if the graft went into overdrive,” he says. “And so it was a major problem. And we still don’t understand why those dyskinesias occurred.”
Perhaps the most important reason why neural cell therapy is unlikely soon to become a standard clinical approach to Parkinson’s is that it attempts to treat only the movement-related “parkinsonian” symptoms. These are only the best known—and most treatable—disabilities in a disease that also includes non-dopamine-related problems with digestion, gait and balance, speech and swallowing, smell and cognition.
“It’s like there’s a rising tide in this disease, and the nigrostriatal system is just part of it,” says Langston. For people with Parkinson’s disease who have been taking medicine for at least ten years, he adds, “the parkinsonism is no longer even among the top five causes of disability.”
“Ultimately the cell transplantation techniques are going to contribute,” but the bigger payoff is likely to come from therapies that treat more than the parkinsonian symptoms, Langston says. The biggest payoff of all, a drug that prevents Parkinson’s, “is going to require discovery of the cause of the disease,” he says.
Tabar now is working to refine the cloning technique so that it can be used in humans for a variety of diseases. She doesn’t currently plan to extend her parkinsonian mouse study into other animals or aim for clinical trials in humans.
“One of the things that people rightfully bring up is that doing therapeutic cloning the way we’re describing it requires the use of oocytes, and that’s going to be problematic in humans,” she says. A relatively new technique, pioneered by Yamanaka’s lab, involves the transfer of a few key genes into skin cells to reprogram them as embryonic-type stem cells, without the need for oocytes. “So that may practically be an easier method,” Tabar says, for therapeutic cloning in humans.