FEATURE ARTICLE
Gene Therapy
Investigators have been searching for ways to add corrective genes to cells harboring defective genes. A better strategy might be to correct the defects
Eric Kmiec
Adenovirus and Others
Considering some of the safety issues surrounding the use of retroviral vectors, investigators have been casting about for other viruses that can deliver genes to cells without disrupting their normal chromosomal configuration. There has been much interest in the use of adenoviruses for this purpose. The bulk of the early work on adenoviral gene therapy was conducted by Ronald Crystal at Cornell Medical School and James Wilson at the University of Pennsylvania.
Like the retroviruses, adenoviruses deliver their genetic payload to the nucleus, but, except under rare circumstances, the genes do not integrate into the resident chromosomes. This, of course, relieves concern about random genetic integration, but it also means that the therapeutic gene is only transiently active. The adenoviral vectors have to be repeatedly administered in order to maintain a steady therapeutic dose.
Adenoviruses can infect a broad range of human cells, including those of the lung, liver, blood vessels and brain. In fact, brain tumors have been treated with adenoviral vectors carrying "suicide genes," whose expression leads to cell death only when its product interacts with a specific drug taken by the patient. These studies generated mixed results.
Adenoviral vectors have also been used in human trials to correct mutations in the cystic-fibrosis transmembrane receptor (CFTR) gene, which contributes to cystic fibrosis. The success of these trials, however, has been quite low. For one thing, the host's immune system registers the adenoviral vector as foreign and eliminates it from the system. In addition, some of the vectors cause an inflammatory response at the high levels required to achieve therapeutic doses.
One of the most promising vehicles to emerge from recent gene-therapy studies is adeno-associated virus (AAV). This virus infects a wide range of cells, including lung and muscle cells, and it integrates its genes within the host's. In addition, it can infect nondividing cells and does not elicit an immune response—both of which are important advantages over retroviral and adenoviral vectors. The work on this virus has been pioneered by three investigators: Kenneth Berns and Nicholas Muzyczka at the University of Florida and R. Jude Samulski at the University of North Carolina at Chapel Hill. Significant advances in the use of AAV for gene therapy have recently been reported by Mark Kay and colleagues at Stanford University and Kathryn High and coworkers at the University of Pennsylvania. Both of these research groups used a modified AAV vector to achieve long-term expression and correction in animals of a gene that contributes to hemophilia. This achievement required a detailed appreciation for the basic biology of AAV.
However, as expected, this virus has some drawbacks. First, it can carry only a small genetic payload, which considerably restricts its usefulness. Second, it, too, carries the risk of disrupting functioning genes by randomly inserting itself into the chromosomes. Finally, it is somewhat difficult to manufacture these vectors in sufficiently high quantities. Other viruses under study as potential vector candidates include Herpes simplex, Vaccinia and even the human immunodeficiency virus.
In addition to viral-based vectors, investigators are continuing to explore nonviral delivery systems. One system that holds some promise delivers drugs via liposomes, small vesicles artificially created from lipids that resemble those making up the membranes of mammalian cells. Because they are constructed of virtually identical materials, the liposomes can fuse with cell membranes and empty their contents—which can include drugs or corrective genes—inside the cell. Some of the DNA delivered by liposomes makes its way into the cell's nucleus.
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