Over the next few weeks, nine volunteers at the hospital will have normal genes sprayed into the cells lining their noses and lungs, to counter the effects of the faulty genes that they have inherited.
If successful, the new therapy could be offered routinely to CF patients within five years, according to Dr Duncan Geddes, director of respiratory medicine at the hospital.
Children would benefit most, he said, if they got the treatment early enough, before lung damage occurs. 'If we can give paediatricians evidence that gene therapy is safe and that it works, then they see no reason not to give it to children.'
But Dr Geddes cautioned that it would be wrong to talk in terms of a 'cure', because the gene transplants will have to be given repeatedly.
Cystic fibrosis is the most common inherited disease in Britain, affecting more than 7,000 children and young adults. Their lungs clog with thick sticky mucus which provides a perfect breeding ground for bacteria and leaves them open to recurrent infections. These scar the lungs and eventually lead to lung failure.
There is no cure, and people with CF used to die in early childhood. With modern antibiotics and intensive nursing, people with CF now have a much longer life expectancy.
In 1989, after more than a decade of effort by scientists around the world, researchers at the universities of Michigan, in the United States, and Toronto, in Canada, isolated the gene responsible for the disease. The defective CF gene fails to control the passage of salt and water in and out of the body's cells. In the lungs, this failure results in the sticky mucus. By replacing faulty CF genes with normal ones the defect can be corrected.
However, the US trials, which began in April, have hit a problem and have been stopped. The doctors used a genetically engineered version of the common cold virus to infect the lungs of CF sufferers: the virus enters the cells lining the lung and thus transfers the normal CF gene into them. But the procedure caused inflammation in the lungs of some patients, who were already sensitive to infections.
In contrast, Professor Bob Williamson of St Mary's Hospital Medical School believes that the technique he has devised for the British trials may be milder and less liable to side effects. Instead of employing a virus, he has found a way of using fat globules, called liposomes, to carry the genes. Cells have a fatty outer membrane, Professor Williamson explained yesterday, and the two will join just as 'two oil drops floating on water will fuse and form a single drop'.
The clinical trials announced yesterday are the first for CF to be held outside the US. The safety and efficacy of the procedure have been extensively tested, partly on mice with a defect analogous to CF, and the ethics have been examined by a government committee on gene therapy, under the chairmanship of Sir Cecil Clothier.
One concern was that, because no one can know the full consequences, new genes should not be passed down to future generations, and it was decided that initially only men should take part, since virtually all men with CF are infertile.