FAULTY genes can now be replaced by 鈥渟pare parts鈥� using viruses that home in
on their targets with unprecedented precision. Experts say this new approach to
gene therapy could be far more successful than conventional gene therapy.
At the moment, gene therapy relies on viruses and other 鈥渧ectors鈥� to shuttle
DNA carrying healthy genes into patients鈥� chromosomes. But they insert the genes
at random points. So the new DNA can disrupt the way normal genes work, and the
new genes may not churn out proteins properly because they are not surrounded by
sequences that normally turn them on and off.
To solve these problems, researchers would ideally like to repair patients鈥�
genes, rather than adding new ones. Also, some diseases, such as Huntington鈥檚,
are caused by faulty dominant genes that need to be removed or turned
off鈥攕imply adding a new gene wouldn鈥檛 help.
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In the lab, researchers can alter a cell鈥檚 genes using a process called
homologous recombination. By applying an electric current to cells, they can
鈥渆lectroporate鈥� the membranes, creating microscopic pores. Then they introduce
engineered DNA that is similar to the target gene. Once in the cell, this piece
of DNA sometimes pairs up with the target gene and swaps places.
But this technique is no use to gene therapists. 鈥淥bviously you can鈥檛
electroporate a person,鈥� says David Russell at the University of Washington
School of Medicine in Seattle. Russell and his colleague Roli Hirata have
adapted the technique using an adeno-associated virus鈥攁 virus which gets
into cells with the help of adenoviruses. Thousands of copies of the
adeno-associated virus genome can safely accumulate in a cell nucleus. The
researchers started with human cell cultures engineered to have a faulty gene
for resistance to the antibiotic neomycin. Then they infected the cells with a
virus carrying a healthy gene. Homologous recombination left approximately 0.1
of the cells resistant to neomycin.
In another experiment, the researchers infected fibroblast cells, which make
tissues such as cartilage, with a virus containing a mutated gene for a certain
enzyme. Cells lacking the normal version of this enzyme resist a toxin called
6-thioguanine. Following homologous recombination, approximately 1 per cent of
the cells again became resistant, the team says in this month鈥檚 Nature
Genetics (vol 18, p 325).
鈥淚t surprised me that it worked that well,鈥� says Russell. He admits that the
method is not nearly efficient enough yet for gene therapy. But it is
considerably more effective than all other attempts to alter existing genes with
viruses.
Russell doesn鈥檛 fully understand why the experiment worked. 鈥淭he mechanism is
wide open,鈥� says Russell. He speculates that because the adeno-associated virus
has single-stranded DNA, the new gene was not already paired up and would be
more likely to interact with a similar gene. And with many copies of the gene
into the cell nucleus, the chances of gene swaps were high.
鈥淭his is an entirely new approach and the fact that it鈥檚 doable will be a
catalyst for more research,鈥� says Alan Bernstein of the University of Toronto.
鈥淭hey have actually put in a point mutation. That鈥檚 impressive.鈥�
