Parkinson's disease, which affects about 120,000 people in Britain, is a degenerative disease of the brain and is at present incurable. It robs sufferers of the ability to walk and even eat. The disease develops when dopamine-producing nerve cells in the brain start to die. This cuts the level of dopamine, a chemical that allows messages to be sent to parts of the brain that coordinate movement.

If researchers can get stem cells to develop into dopamine-producing nerve cells, this could help science to understand better how the condition develops. Stem cells that are turned into dopamine-producers may also be used as a Parkinson’s therapy. If the new cells can be transplanted into the brain, they may halt or reverse the disease and overcome the symptoms by replacing the cells that have died. This may even provide a lifelong cure.

But the day when we see stem-cell based treatments offered to patients is still a considerable way off. “Before any successful human transplantation can happen, a number of hurdles have to be overcome,” explains Dr Kieran Breen, the director of research and development at the Parkinson’s Disease Society, which has spent £1.6 million funding stem-cell research since 2001.

First, scientists have to be sure how to make stem cells switch into nerve cells. Research has been done on this by scientists at Edinburgh University. Other research is being conducted at Bristol University and Imperial College London.

The next step is to transplant the converted stem cells into the brain. Laboratory experiments on animals show that this is not straightforward: most of the transplanted cells have died. In one study, some transplanted cells formed cancerous tumours.

To help to overcome this, a team of scientists led by Bristol University’s Dr Maeve Caldwell is exploring how to ensure that the cells stay alive once transplanted. To forestall any danger of unwittingly implanting tumour cells, scientists at Imperial College London, are working out how to spot and remove them from converted stem-cell populations.

But there is another potential obstacle with dopamine-cell transplantation – the risk of rejection. Because the stem cells that scientists have worked on have been taken from embryos, they are essentially foreign to the recipient. One potential answer is to take a stem cell from an embryo and replace its DNA with DNA from the transplant recipient, so that their body sees it as their own.

The DNA-swapping technique was recently tried successfully by researchers at the Memorial Sloan-Kettering Cancer Centre in New York. Scientists created a Parkinson’s-like condition in a mouse, then took DNA from the mouse’s tail cell, put it in a stem cell from an egg, made that into nerve cells and transplanted these cells into the animal’s brain.

In March the scientists reported in the scientific journal Nature Medicine, that the cell survival rate was good.

This use of mixed DNA cells “looks like the way forward for us”, Breen says. But he adds that any cure is still a considerable way off. “It’s impossible to estimate the timetable for stem-cell cures. It’s something that many patients ask me. All I can say is, if you look at five years ago, we did not know how to make stem cells into nerve cells, we had no transplantation studies and nothing about transferring DNA into stem cells. We’ve come a long way.”

But any stem-cell therapy is unlikely to be a cureall for Parkinson’s, Breen cautions. “In the 1980s there were studies in Sweden and Canada that transferred foetal bone tissue into the brains of people with Parkinson’s. Some people got better, some got worse and some had no change at all. The lesson is that not everyone with Parkinson’s will benefit from stem-cell therapy. In the people who got worse, their transplanted cells went out of control. Learning to keep them under control is another issue we have to address. There is still lots to be done.”

Parkinson’s Disease Society Freephone Helpline: 0808 800 0303