Jumping Gene Offers Nonviral Alternative to Gene Therapy

A jumping gene first identified in a cabbageeating moth may provide a safer, targetspecific alternative to viruses for gene therapy, researchers say.

They compared the ability of the four bestcharacterized jumping genes, or transposons, to insert themselves into a cell’s DNA and produce a desired change, such as protecting the cell during radiation therapy.

They found the piggyBac transposon was five to 10 times better than the other circular pieces of DNA at making a home and a difference in several mammalian cell lines, including three human ones.

“If we want to add a therapeutic gene, we can put it in the transposon and use it to deliver the gene into the cell,” said Dr. Joseph M. Kaminski, an MCG radiation oncologist and a corresponding author on research published in September 2006 in the online Proceedings of the National Academy of Sciences Early Edition. “You can use these wherever retroviruses have been used.”

In addition to piggyBac, researchers looked at what was believed to be the most efficient transposon in mammalian cells, hyperactive Sleeping Beauty, first found “asleep” in fish. They also looked at Tol2, another fish transposon, and Mos1, found in insects.

The piggyBac transposon, which has close relatives in the human genome, is widely used to genetically modify insects. Sleeping Beauty has been used to correct hereditary diseases, including hemophilia, in a mouse model.

For this study, researchers used transposons to deliver an antibiotic resistant gene. They found that while piggyBac might be less efficient than a virus, it puts Sleeping Beauty to shame when it comes to making cells antibioticresistant.

“Sleeping Beauty has captured the field as far as transposons to be used in mammals,” said Dr. Stefan Moisyadi, a University of Hawaii molecular biologist and a corresponding author. “But by comparing different transposons, we showed Sleeping Beauty is far inferior to piggyBac.”

Scientists have used viruses to deliver genes for more than 20 years because of their adeptness at infiltrating cells and inserting themselves in DNA. But efficiency comes at a price. Major complications, including deaths, have plagued gene therapy trials.

“With viruses, you don’t have control,” said Dr. Kaminski. “People have tried to modify viruses for sitespecific integration and have not been very successful. Once they get into the cell, they can insert wherever they want.”

Dr. Kaminski’s previous work, published in 2002 in The FASEB Journal, hypothesized that the integration site for transposons can be selected. “Typically, viruses and transposons will integrate anywhere along the genome,” he says. “If they integrate anywhere, it can obviously cause harm. If we can target the integration, be able to insert the gene at a safe spot in the genome, that would be beneficial.” He confirmed that targeting integration is possible in a paper he coauthored in 2005, also in The FASEB Journal.

“We can do it in insects,” said Dr. Moisyadi. “I think it’s a short step to take it to a targeting mechanism we can use in humans.”

Transposons also are cheaper to produce and probably safer than viruses. Another plus: Unlike viruses, they can carry larger genes, such as the dystrophin gene to help correct muscular dystrophy. On the other hand, unlike retroviruses, transposons have to be coated with lipid to slip into cells.

Although piggyBac is not as successful as the virus at integrating into DNA, “we could potentially make ahyperactive version of piggyBac, like they did for Sleeping Beauty, which might be as good or better than retroviruses,” Dr. Kaminski said. “I think we’ll do it or somebody will. I think it’s a safer method. One of our next goals is to use transposons to deliver a radioprotective gene, called manganese superoxide dismutase, to potentially protect normal tissue from radiation damage.”

In cancer, he suspects gene therapy will focus on this type of modification of normal tissue for protective purposes as well as manipulating the immune response. However, it has broad applications for correcting single gene disorders, such as hemophilia, sickle cell disease and muscular dystrophy.