Therapy Orders Cells to Fix Their Own DNA

A new gene therapy technique that appears to harness a cell's own genetic repair mechanism to rewrite its DNA sequence has shown remarkable success in rats. The findings, in this month's Nature Medicine, could have a big impact on efforts to cure hemophilia and other incurable genetic diseases.

For more than a decade researchers have been trying to find a way to efficiently insert corrective DNA into cells. Clifford Steer and his team at the University of Minnesota Medical School in Minneapolis used a technique that previously has shown tantalizing results: so-called chimera molecules, loops that contain both DNA and RNA. Genes are made of DNA, and cells use RNA--a closely related molecule--to translate those genes into proteins. A chimera molecule is built to match a target gene nucleotide for nucleotide except for the bases researchers want to alter--in hemophilia, for instance, just a single nucleotide needs correcting.

Although nobody knows for sure how chimeras rewrite DNA sequences, researchers have suspected that when the chimera binds to its matching DNA region in a cell, the cell's DNA repair mechanism detects the mismatch and--like an overstrike key on a typewriter--deletes the mutant base in its own DNA and inserts the correct one. The RNA in the chimera resists enzymes that break down foreign DNA in the cell and helps the molecule bind tightly to the target gene.

Although previous efforts to use chimeras were done only with cultured cells--with mixed success--the new study is the first to test the approach in animals. Steer and his colleagues injected their molecule, coated with a polymer that allowed the chimera to enter liver cells, into the tail veins of rats. In this case, the chimeras were designed not to cure a disease, but to induce the mutation that causes hemophilia in people, which occurs when liver cells can't make a protein essential for blood clotting. Indeed, blood from treated rats did not coagulate as readily as those that received a saline injection. When the researchers inspected the animals' livers, they found that 40% of cells had the induced mutation. Changing the DNA in even 5% of liver cells would be enough to cure a hemophilia patient, Steer says. The next step, he says, will be to try the technique in reverse, this time to cure the hemophilia mutation in dogs.

One expert calls the experiment an "unbelievable" success. Gene therapy researcher Michael Strauss of Humboldt University in Berlin cautions that liver cells might be especially amenable to the treatment. However, "if the technique can be reproduced by others in other cell systems and other target genes," he says, "we would have a method that theoretically could provide a cure for genetic diseases."