Researchers hoping to replace defective genes with working copies may have something to learn from genes that naturally hop from region of the genome to another. New research finds that an enzyme used by these so-called transposons can work alongside the leading gene therapy technique to permanently alter cells--a feat that has so far proved elusive. The finding may increase the safety and efficacy of gene therapy.
Adenovirus is the leading tool in gene therapy because of its ability to insert DNA into a variety of cells. But it cannot integrate its DNA permanently into those cells. So repeated infection would be necessary for long-term treatment, and that would be costly and perhaps dangerous, even though the viruses used are stripped of genes that could cause an adverse reaction. Another potential tool is an enzyme that helps transposons jump around, the "Sleeping Beauty" transposase (ScienceNOW, 19 November 1997), which can shuttle genes from circular DNA into the chromosomes of liver cells. But since viruses carry linear DNA, there was no practical way to deliver genes for transposase to work on.
Now, a team led by geneticists Mark Kay and Stephen Yant of Stanford University in California has cleared this hurdle. The researchers permanently inserted the gene for factor IX--the defective protein in type B hemophilia--into mouse liver cells using two different andenoviruses. The first carried a good copy of the human factor IX gene. The second carried genes for Sleeping Beauty and an enzyme called Flp recombinase. When both viruses infect the same cell, Flp cuts the factor IX gene out of the virus and turns it into circular DNA, which Sleeping Beauty then inserts into the cell's genome. The team reports its results in the October issue of Nature Biotechnology.
The method's efficiency is still only a few percent--enough to produce substantial amounts of factor IX in mice, but not enough to treat other diseases and larger animals such as humans. But Kay says his team is already working on ways of improving that number by manipulating the amount of transposase the viruses produce.
"This is the first time that I'm aware of that an adenovirus has been used to get stable integration," says geneticist Marcus Grompe of Oregon Health Sciences University in Portland. "This is a major step forward."