A global collaboration between scientists from the Burnham Institute for Medical Research, US, and the University of Oxford, UK, has also revealed that the positive effects can be boosted when the stem cells are administered along with a drug. The team claims that the research represented the first demonstration that stem cells can be implanted in certain diseases without needing to first suppress the immune system.

The research focussed on treating Sandhoff's disease - one of around 40 genetic disorders called lysosomal storage diseases, which affect 1 in 5000 people, typically children. There is no specific treatment and children with Sandhoff's "generally die by age three from respiratory infections", according to the US National Institute of Neurological Disorders and Stroke.

Sanhoff's is caused by a genetic mutation, which reduces the amount of hexosaminidase enzyme produced. Brain cells use this protein to metabolise excess lipids, which can build up in lysosomes and cause severe symptoms such as progressive deterioration of the central nervous system, early blindness and seizures.

"It's encouraging that this approach works in such a severe disorder," Dr Frances Platt of Oxford told DrugResearcher.com.

The scientists implanted neural stem cells into mice and discovered that they spread throughout the brain, both repairing damaged tissue and replacing damaged nerve cells.

The stem cells appear to exert their positive effects primarily through increased levels of hexosaminidase in the brain and reduced lipid storage. However, the team also claims that the research provided the first evidence that "stem cell-derived nerve cells may integrate electrically and functionally into a diseased brain".

With these effects combined, the scientists observed that the onset of symptoms was delayed, well-being and motor function was preserved, and lifespan was extended by over 70 per cent.

"Our studies suggest that functional neuronal replacement can be complemented and, under some conditions, eclipsed by a range of other stem cell actions that nevertheless exert a number of critical stabilising forces," said Dr Evan Snyder, director of Stem Cells and Regeneration at Burnham.

"In fact, our study offers the first evidence that stem cells employ multiple mechanisms -- not just cell replacement, which collaborate to benefit disease. These findings also raise the possibility - somewhat counter-intuitively - that stem cells may inherently exert an anti-inflammatory influence in degenerative diseases," he continued.

The team then investigated the effect of adding a drug to the therapy. Over the past ten years, the lab had been developing a drug to treat this class or disorders. Zavesca (miglustat) is now marketed by Actelion and is approved to treat Gaucher's disease - another disease caused by the build up of lipids. The drug is a so-called 'substrate reduction therapy', which works by reducing the amount of lipids formed.

They discovered that the beneficial effects of the stem cells increased and the life span of the mice doubled once Zavesca was used as well. Although the team say neither therapy would work on it's own, the study showed that "the most successful therapies, including those employing stem cells, will likely invoke the use of multiple strategies in concert."

"Although we understand what the drug is doing, we need to understand more about what exactly the stem cells do," said Dr Platt.

The team then moved onto testing human stem cells - isolated both directly from the central nervous system (CNS) and to those derived embryonic stem cells (called 'adult' stem cells to distinguish them from embryonic stem cells even though they are taken from developing brain tissue).

In the first head-to-head comparison of human embryonic and 'adult' stem cells in the same disease, both types of cell were showed to be as effective as each other and more effective than the mouse stem cells. The positive results have encouraged the team to explore the possibility of starting a clinical trial although Dr Platt said it was perhaps too early to think of a partnership with the pharma industry.

The research may also be applicable to other neurodegenerative disorders such as Alzheimer's and Parkinson's disease, explained Dr Platt.

The stem cell therapy did not cure the disorder permanently though - as the mice grow older, the production of more mutant cells eventually outstrips the positive effects of the stem cells. This might be because, once a critical threshold of storage is reached, the cells rapidly become dysfunctional, according to the scientists.