Targeted DNA Insertion May Aid Gene Therapy

For decades, scientists have tried to insert genes into precise locations within the genetic material of laboratory test organisms. Such experiments help them better understand the function of the inserted genes and enable them to disrupt other genes. Gene therapy also relies on the insertion of corrective genes into the cells of living humans. With most current methods, the genes insert randomly, so scientists cannot control whether the genes will function in their new location. Such random insertions can even lead to potentially disease-causing mutations.

New research indicates that it is possible to insert genes into any desired location in the genetic material of a host organism. Dr. Alan Lambowitz at the University of Texas, Austin and his coworkers accomplished this by harnessing the targeting mechanism of a portion of DNA called an intron. Often called "junk DNA," introns normally do not code for proteins, but exist in the middle of genes that do. Some introns can recognize a sequence of DNA and insert themselves precisely into it. They do this by matching up a region of their own genetic sequence with the targeted sequence. To see whether it is possible to construct introns to target any desired gene, the researchers chose two clinically relevant sequences to target: one in a gene in HIV, the virus that causes AIDS, and another in a human gene called CCR5 that encodes a protein necessary for HIV infection. People who have mutations in their CCR5 genes are resistant to HIV infection and so some scientists believe that disrupting the CCR5 gene may be an effective anti-AIDS therapy. The scientists modified a type of intron found in bacteria to generate a variety of introns with different genetic sequences in their targeting regions. They found that more than a dozen of these introns inserted themselves into the intended positions in the target genes. Dr. Lambowitz's group experimented on human cells in a laboratory, but the scientists hope the technique ultimately can treat HIV infection in people.

If further tests are successful, the method could be used both to disrupt specific genes and to add new genes at desired sites. This could advance all sorts of biomedical research, ranging from studies on basic gene function to the development of antiviral and antibacterial drugs. The work may also enhance the delivery of genes for gene therapy. Currently, the specialized viruses usually used to deliver corrective genes insert their cargo at random sites in human DNA. If the new method could deliver therapeutic genes to specified sites, it could make gene therapy safer and more effective. The work is an excellent example of an unexpected outcome from basic biomedical research. "We were carrying out basic research on [these] introns and how they related to gene structure," said Dr. Lambowitz. "Any kind of practical application was the furthest thing from our minds."

REFERENCES

Guo H, Karberg M, Long M, Jones JP III, Sullenger B, and Lambowitz AM. Group II introns designed to insert into therapeutically relevant DNA target sites in human cells. Science 2000;289:452-7.

Strauss, E. Targeting Intron Insertion into DNA. Science 2000;289:374.

Reporters may call Alisa Zapp Machalek at (301) 496-7301 to obtain the name of a scientist in the NIGMS Division of Genetics and Developmental Biology who can comment on this work.