In my last post, I wrote about virologists using directed evolution to produce viruses that fight cancer. Another nice example of evolution being harnessed for medical use was published earlier this year. Katherine Excoffon and colleagues are working to develop effective gene therapy for cystic fibrosis.
Cystic fibrosis is caused by loss-of-function mutations in the gene for the cystic fibrosis transmembrane conductance regulator (CFTR). This protein sits on the surface of cells that line the lungs. It serves as a chloride channel, and it helps protect the lungs from the bacterium Pseudomonas aeruginosa. Among other symptoms, individuals who can’t make functional CFTR suffer chronic lung infections.
The basic idea in gene therapy for cystic fibrosis is introduce copies of the normal gene for CFTR into the cells that line the lungs. If these cells can make working CFTR from the new genes, then cystic fibrosis could be cured.
Excoffon, along with other researchers, is using a common virus called adeno-associated virus (AAV) to carry the CFTR gene into airway cells. One problem with this approach is that most strains of AAV have low infectivity. This may seem odd for a respiratory virus. Excoffon thinks AAV’s low infectivity is the result of an evolutionary compromise between transmission and virulence. The evolution of virulence is a topic for another time. But if Excoffon is right, it should be possible, under the right conditions, for AAV to evolve much higher infectivity.
Starting with a mixture of two strains of AAV, Excoffon boosted the amount of genetic variation by inducing mutations in copies of the viruses’ genomes and encouraging them to recombine. She then maintained them on cultured human airway cells for five dish-to-dish transfers. This should allow highly infective variants to become numerically predominant. It should, in other words, allow the viral population to evolve high infectivity.
Indeed, it did. The strain that evolved in Excoffon’s dishes, AAV2.5T, was more than 500 times more infective than the starting strain she compared it to. Furthermore, when loaded with functional copies of the CFTR gene, the new virus proved highly effective in introducing the gene into cultured airway cells from cystic fibrosis patients. The treated cells soon began making working CFTR in much higher concentrations than even normal airway cells.
As I said in the last post, it’s a long way from the lab to the clinic, but this looks to me like a promising development.
The full reference for the report by Excoffon and colleagues is: Excoffon, K. J. D. A., J. T. Koerber et al. 2009. Directed evolution of adeno-associated virus to an infectious respiratory virus. Proceedings of the National Academy of Sciences 106: 3865-3870.
For evidence that one of CFTR’s functions is to protect against infection with Pseudomonas aeruginosa see: Pier, G. B., M. Grout, and T. S. Zaidi. 1997. Cystic fibrosis transmembrane conductance regulator is an epithelial cell receptor for clearance of Pseudomonas aeruginosa from the lung. Proceedings of the National Academy of Sciences 94: 12088-12093.
For a review of research on treating cystic fibrosis with gene therapy see: Griesenbach, U. and E. W. F. W. Alton. 2009. Gene transfer to the lung: Lessons learned from more than 2 decades of CF gene therapy. Advanced Drug Delivery Reviews 61: 128–139.