A new study carried out on rats shows that injections of human embryonic stem cells (ES cells) can relieve the symptoms of Parkinson's disease (PD) - but the treatment also produces potentially cancerous cells, say US researchers. A team based at Cornell University Medical College in New York has developed a new way of growing ES cells that makes them able to turn into the brain cells needed to treat PD. But the research, published in the journal Nature Medicine, also shows that the technique results in some cells that remain unspecialised, which could potentially grow into tumours.
People with PD are affected by tremors, stiff muscles and slow movements, caused by a gradual loss of nerve cells in an area of the brain controlling movement. The disease involves the specific loss of cells that produce the neurotransmitter dopamine, leading to hopes that it may be possible to develop new treatments for PD that replace these cells. In particular, scientists are trying to get human ES cells - the body's 'master' cells that can form any tissue - to grow into dopamine-producing brain cells in the laboratory.
However, efforts to develop new cell-based therapies for PD have so far been confounded by difficulties in producing enough of the right kind of dopamine-producing cell. In the latest study, the scientists overcame this problem by growing human ES cells alongside nerve-supporting 'glial' cells, isolated from fetal brain tissues. 'What we were really trying to do was to mimic the environment of the developing brain to increase the efficiency of dopamine-neuron generation, but also to bias the cells towards generating the type of dopamine neurons that we wanted', said team leader Steve Goldman.
When the scientists injected the new dopamine neurons into the brains of rats with symptoms of PD, the animals showed a practically complete recovery. 'The positive results were really strong', said Goldman, adding 'the animals exhibited almost a complete restoration of normal function'. However, each stem cell transplant also contained 'undifferentiated' cells that had failed to grow into neurons, which can potentially keep dividing and multiplying. These cells could eventually become cancerous, said Goldman, although the rats in the study were killed before they developed any such tumours.
To avoid this potentially lethal side effect, the cells used for transplants will need to be sorted so that any undifferentiated cells are removed before injection, says neurologist Olle Lindvall, of University Hospital in Lund, Sweden. He told Nature News that it will be several years before clinical trials of stem cell treatments for PD can begin.
The finding has implications for other experimental treatments that aim to use ES cells, such as a planned trial for spinal injuries that US company Geron hope to embark upon next year. Thomas Okarma, president of Geron, told the Washington Post newspaper that his company's cells have not produced any tumours in animal studies. But he said the US Food and Drug Administration has asked for additional data on this issue before it will give the trial final approval. 'What they worry about, and rightly so, is that there are rogue undifferentiated cells lurking in the cell population that we haven't detected', he said.