Scientists are closing in on techniques that could let them safely repair almost any defective gene in a patient, opening the door for the first time to treatments for a range of genetic disorders that are now considered incurable.
The breakthrough, announced in the journal Nature in June, relies on so-called zinc fingers, named after wispy amino acid protuberances that emanate from a single zinc ion. When inserted into human cells, the fingers automatically bind to miscoded strands of DNA, spurring the body's innate repair mechanism to recode the problem area with the correct gene sequence.
A method for fixing miscoded DNA by injecting foreign genes into cells won headlines three years ago when doctors in France and Britain announced a handful of successful cures related to X-linked severe combined immunodeficiency disease, or SCID, also known as "bubble boy" disease. But that method was ultimately proven unsafe.
In a paper published earlier this month, scientists at California biotechnology company Sangamo BioSciences showed that zinc fingers can be used to erase targeted portions of DNA without risk of harmful side effects.
"This doesn't just deliver a foreign gene into the cell," said Nobel Prize winner and CalTech President David Baltimore, who with a Sangamo paper co-author Mathew Porteus proposed this method to cure genetic diseases. "It actually deletes the miscoded portion and fixes the problem."
At the heart of the breakthrough is the concept of "if it's broke, break it some more." Cells have a method of DNA repair called homologous recombination, which fixes breaks in the double helix of our chromosomes. But the process only repairs places where the DNA has been cut, not where genes have been miscoded.
Using a package of synthesized zinc fingers, cells can be tricked into doing nano-surgery on their own genes, Sangamo researchers found. The zinc fingers home in like a guided missile on the exact spot in the genome doctors are trying to target and then bind to it. DNA-devouring enzymes then cut through the double helix of DNA at the exact beginning and end of the targeted gene, and a template of donor DNA helps rebuild the deleted strand.
While such a therapy has been theorized for years by Baltimore and others, Sangamo scientists are the first to show test-tube results with human cells. In a paper published June 2, Sangamo researchers showed how they were able to correct the defective gene in 18 percent of the T-cells extracted from the body of an X-linked SCID patient.
That should be enough to cure the disease, as it only takes one corrected T-cell to repopulate a person's immune system with healthy cells, according to Sangamo.
If successful in trials, Sangamo's technology would be the first successful gene therapy, three decades after the concept of curing diseases by tinkering with the genome was first proposed. Most gene therapy trials have failed because the methods of inserting new genes into cells (usually with modified viruses as vectors) haven't proved to be effective enough.
One trial that did succeed, but then ended in tragedy, was a 2002 French X-linked SCID trial that used retroviruses to deliver a new gene into the patients. The new gene cured the disease in 12 patients, but went on to cause leukemia in three of them. It turned out the foreign gene, in addition to producing the protein that vanquishes X-linked SCID, had the unexpected side effect of sometimes turning on a cancer-causing gene.
Sangamo's technology overcomes that problem. Whereas the French viruses inserted the foreign gene randomly into the host cell's genome, the zinc fingers are highly specific and can land only at the targeted gene.
"They've certainly raised the bar for gene-therapy safety," said Scott Wolfe, a zinc-finger researcher at the University of Massachusetts Medical School in Worcester, Massachusetts. He points out that the early proof-of-principle work was highly toxic to the cells. The zinc fingers weren't specific enough and they created so many double-stranded breaks in the DNA that a lot of the cells chose to commit suicide rather than try to repair all the breaks. "They really seem to have solved the toxicity problem altogether."
Although X-linked SCID patients will probably be the first to try the therapy, the technology is extremely versatile for a host of human diseases. "Right now, its greatest weakness appears to be that it is optimized for very small patches of gene repair," said Baltimore. "If it's a long sequence of DNA that has to be fixed, this might not be the best way to do it."
Nevertheless, there are a lot of ways to attack diseases without replacing whole genes. Other potential targets for the therapy range from many types of cancer to cystic fibrosis and even AIDS. "If they can figure out how to optimize their zinc fingers for any spot on the genome, this could target any gene you want it to," said Wolfe.
