Steve Connor: Even if gene therapy does work, how will a stretched NHS foot the bill?

Gene therapy is one of those medical "breakthroughs" that has unfortunately failed to live up to expectations. Several clinical trials, usually involving inherited diseases caused by defects in single genes, have ended in failure and, in at least one case, the death of patients.

For the past two decades, scientists have tried with varying degrees of success to insert, augment or in some way alter the damaged or missing genes that have caused a particular illness. This latest attempt is somewhat different, in that even a partial increase in gene function should result in a marked improvement in a patient's quality of life.

Even a 10 or 20 per cent improvement in the pumping efficiency of a failing heart, for instance, could allow a patient to climb the stairs without getting out of breath.

The two studies to start in Britain are part of "phase 2" clinical trials on a few dozen patients to primarily test safety, but also the efficacy of the treatment. It could take another two or three years before any meaningful results appear.

A further three or even five years are then necessary for the final, "phase 3" trials involving hundreds of patients. These should show beyond reasonable doubt whether the therapy really does significantly improve a patient's outlook.

But if gene therapy proves to work, the big question is whether the NHS will be able to afford yet another new treatment, costing several thousand pounds a shot.