THE PROSPECTS for treating cystic fibrosis by gene therapy are looking brighter following successful tests with a safer type of 鈥渟huttle鈥� for ferrying replacement genes into a patient鈥檚 lungs. During the tests, a British-led research team inserted healthy copies of the cystic fibrosis gene into cells lining sufferers鈥� noses with the aid of microscopic droplets of fat called liposomes.
Other groups that have attempted to correct the genetic defect that causes cystic fibrosis have used viruses as 鈥渧ectors鈥� to carry the healthy gene into the target cells. Unfortunately, viruses can also cause inflammation in the lungs, an undesirable side effect in CF patients, whose lungs are already diseased. The new method for introducing corrective genes into the lungs of CF patients promises to be much safer.
Cystic fibrosis is caused by defects in a gene called CFTR, which plays a part in the transport of chloride ions out of the cells lining the lungs, airways and gut. Defects in this gene mean that cystic fibrosis patients have difficulty expelling mucus and inhaled microorganisms from their lungs. As a result, they are very susceptible to lung infections, and usually die at around 30 years old.
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As an alternative to the viral vector, the researchers, at the Royal Brompton and St Mary鈥檚 Hospitals in London, the Medical Research Council鈥檚 Human Genetics Unit in Edinburgh, and the University of Pittsburgh, linked loops of DNA containing healthy copies of the CFTR gene to microscopic liposomes. When the liposomes come in contact with a cell, they fuse with its outer membrane, and release the DNA into the cell鈥檚 interior. To test the system, the team sprayed the liposomes into the noses of volunteers with cystic fibrosis. 鈥淭he cells lining the nose are very similar to those lining the lung,鈥� explains David Porteous of the human genetics group in Edinburgh. But the cells in the nose are easier to monitor, and the risks are lower if any nasal cells are damaged.
In the event, none of the volunteers suffered any unpleasant side effects. And a single spraying partially corrected the cystic fibrosis defect. The researchers assessed the effectiveness of the treatment by measuring the voltage across the layer of cells lining the nose. This voltage is higher in cystic fibrosis sufferers than in healthy people. A single spraying reduced this difference by around 20 per cent. The researchers report in the first issue of Nature Medicine that the effect lasted for up to a week.
These results are 鈥渁t least as good鈥� as those achieved with viruses as gene vectors, says Duncan Geddes of the Royal Brompton Hospital. A group led by Ronald Crystal of the New York Hospital-Cornell Medical Center, for instance, has transported the CFTR gene into cystic fibrosis patients using a genetically engineered version of the adenovirus that causes the common cold. The healthy gene showed up in 14 per cent of cells lining the nose, where it could be detected for up to 10 days. But when the researchers sprayed the virus into the lungs, one of their four patients developed severe inflammation (This Week, 28 August 1993).
The British researchers have yet to test their spray in the lungs, but are confident that the liposomes will not cause inflammation. The question, however, is how effective the shuttle system is. In the nose, the researchers had to add more copies of the healthy CFTR gene to match the performance of the adenovirus. This is because adenoviruses carry genes directly to the cell鈥檚 nucleus, ensuring that the instructions they carry are read. Genes carried by liposomes are only guaranteed a ride into the cell鈥檚 cytoplasm, and fewer will find their way to the nucleus. Many will be broken down by the cell鈥檚 waste disposal system before they get there.
Gene therapy researchers warn that it will be years before doctors are routinely correcting the defective cystic fibrosis gene. The eventual goal is to compensate for the defect completely, with a treatment that is effective for much longer.
Bob Williamson, who heads the group at St Mary鈥檚 Hospital, says this will require much more efficient vectors. 鈥淧eople in 10 to 15 years will laugh at the crudity of the liposomes and viruses that we鈥檙e using today,鈥� he says. The aim is to make customised vectors which combine the best aspects of both systems, and include other genetic sequences to ensure that the information carried by the healthy CFTR gene is used by the target cells lining the lungs.