Gene therapy research aims to provide a replacement gene or gene sequence to do the job that the faulty gene is not doing. For example in Cystic Fibrosis a pair of faulty genes, one on each copy of chromosome 7, means that the body cannot make fully functioning CFTR (Cystic Fibrosis Transmembrane Conductance Regulator), which transports salt and water in and out of the cells that line the lungs and digestive system. The result is that the mucus lining the lungs and digestive system is very thick and sticky, and builds up causing respiratory and digestive problems. Gene therapy research is focussing on providing working copies of the CF gene to the lungs, as respiratory failure is the greatest threat to CF patients. However, gene therapy is proving much harder to implement in practice than it may sound in theory. The research is also expensive.
However, some faulty genes are not inactive, they actually produce a problem. For example in Huntington's Disease, a fault on chromosome 4 causes a variant copy of the protein, Huntingtin, to be produced. This builds up in the body and is toxic. So research into possible treatments for this and similar conditions has focused on ways of ‘switching off’ the faulty gene. In January 2005 in Nature Medicine a report from Iowa University claimed to have successfully switched off the gene that causes a similar brain disease in mice. The researchers hope to go on to trial a similar technique in humans, but estimate they are at least 5 years away from a potential treatment for humans that could be offered therapeutically. This may well be an optimistic estimate.