Inspired by the body’s own system for fixing genetic glitches, scientists at a company in California have made a molecular repair kit that corrects mutations in a cell’s DNA.
In experiments with human cells harboring mutations such as those that cause a fatal childhood disease, the new system fixed the broken genes in up to 20 percent of the tested cells. That is a level of efficiency hundreds of times better than previous experiments using other approaches, and one probably adequate to elicit a cure if the technique were to be used on an actual patient, the scientists reported yesterday.
The team is only now beginning to assess the technique in live animals, a prerequisite for the human testing the company hopes to begin next year. But if it works as well in mice as it does in laboratory dishes, experts said, the approach could become a serious competitor to conventional gene therapy.
That field, which attempts to force-feed healthy genes into cells hobbled by defective ones, has been plagued by failure for more than a decade and recently was found to cause leukemia in some patients.
By contrast, the new system simply rewrites the small stretch of “misspelled” genetic code that is typically the reason a gene has gone bad.
“This is the first time anyone has been able to precisely and with high efficiency edit the human genome,” said study leader Michael Holmes, director of the gene modification group at Sangamo BioSciences Inc., of Richmond, Calif., which funded the study.
More than 4,000 diseases are caused by tiny DNA code “misspellings” in one or another of the 30,000 or so genes tucked inside the body’s cells. Lacking any method for repairing such tiny errors, researchers have been injecting entirely new genes into patients’ cells — usually ferrying them, in Trojan horse fashion, inside viruses, which have a natural talent for entering and infecting cells.
That system has proved imperfect. For one thing, viruses inject their payloads at random within a cell’s tangled mass of DNA, sometimes disrupting normal genes. Even when new genes land in good locations, the molecular machinery that regulates their activity is often thrown off, leaving the healthy genes operating at a level too low to be helpful or functioning in wrong parts of the body.
Sangamo’s scientists took a different approach, opting to rewrite the small errors in bad genes instead of replacing them .
They focused on tiny proteins called zinc fingers, which are made naturally by virtually all living organisms. Zinc fingers come in many varieties, each with a strong penchant for attaching itself to a particular sequence of DNA code. Once attached, they act like little levers to ratchet up or down the activity of genes. At other times, they help cells repair damaged DNA.
The Sangamo team created an entire library of zinc fingers, each designed to attach to a particular DNA sequence known to cause a disease. Each was also custom-equipped with an enzyme that allows it to cut open the strands of DNA at that location, making the code accessible for editing. When fresh snippets of correctly spelled DNA were added to cells along with these engineered zinc fingers, the cells used those bits of DNA as templates to guide the rewriting of the mutated region.
The company’s prime goal is to remove blood cells from patients with genetic diseases, use zinc fingers and corrective DNA to edit errors, then reinfuse the repaired cells into the patients.
In the newest experiments on cells in culture dishes, described in yesterday’s online version of the journal Nature, the method corrected an error in the gene that causes X-linked severe combined immunodeficiency (SCID), a fatal disease of childhood.
Tests indicated that the zinc fingers degraded after a few days, leaving the cells permanently repaired
Other scientists said the approach looked promising but predicted it would end up struggling with problems of its own.
James M. Wilson, a gene therapy researcher at the University of Pennsylvania, noted that the first step of the new process calls for the zinc fingers to make a fresh break in the DNA to accommodate the insertion of a corrected sequence. “That’s the same kind of break you get with radiation,” which can lead to cancer, he said.
Wilson and others also noted that the published study was not designed to detect whether the system caused DNA breaks in unexpected places in the genome. “Even if it’s 99 percent specific, which would be rare in biology,” Wilson said, that could still lead to problems, because a large number of cells would be targeted.
Donald Kohn, a researcher at the University of Southern California who is using conventional gene therapy in an effort to cure children with SCID, said: “I think that gene correction, rather than gene addition as most gene therapy is currently approached, is theoretically a better way to go for many genetic diseases.” But he, too, warned that much work remained to prove the approach useful and safe.
