|Stanford University, Stanford, Calif.|
|DNA mismatch repair|
Several decades ago, scientists discovered that bacterial cells (which are simpler than animal cells in several ways) had an amazing ability: They could detect and fix errors in their DNA by an efficient and precise process that came to be known as mismatch repair. The term reflects the principle that each of the four nucleic acids in DNA has only one other with which it can correctly “match,” or combine
Mismatches occur when the wrong nucleic acid is placed on one of the rungs of the ladderlike DNA structure. When bacterial repair systems detect a mismatch in any of the rungs, they move in and repair it.
Later, scientists recognized that more complex cells, including human cells, also have the ability to repair DNA mutations. However, it wasn’t until about 10 years ago that investigators began to consider using this mechanism as a therapy.
“Really, the question was, if a cell has the ability to correct mutations that occur during life, can we get the cell to correct inherited mutations?” says Tom Rando, an associate professor in the Department of Neurology and Neurological Sciences at Stanford School of Medicine. “It’s one thing to recognize a normal biological function. It’s quite another to harness that function and get it to do what you want.”
About 1995, he and others began to investigate that question seriously. By that time, Rando had earned doctoral degrees in cell and developmental biology and in medicine, both from Harvard, and had started studying electrophysiology, particularly the electricity-like activity that transmits signals in the nervous system.
He wasn’t particularly interested in muscle diseases until, as a young trainee in neurology at the University of California at San Francisco in the late 1980s, he was introduced to the MDA clinic. One genetic disease in particular — myotonic muscular dystrophy — captivated him.
Myotonic dystrophy involves both myotonia, the inability to relax muscles on command, which results from abnormalities in nerve signals, and dystrophy, involving degeneration of muscle.
Rando’s interest in myotonic dystrophy, he says, “was the transition between being interested in the electrical properties of the nervous system and getting into the muscular dystrophies.”
Mice, dogs and chimeras
The UCSF lab was using the mdx mouse, which has a point mutation that causes a premature stop codon in the dystrophin gene, as a model for studying muscular dystrophy. Rando recalls, “We thought maybe we should try and see if we could direct the cell’s own mismatch repair mechanisms to correct the mdx point mutation.”
Rando’s group and a group working with a dog model of Duchenne MD published papers in 2000 showing that gene repair of point mutations was possible in both types of animals, although it was very inefficient.
“This was all proof of principle, rather than looking at therapeutic efficacy,” Rando says. “We were just trying to see how it worked, how efficiently it worked, and what some of the hurdles might be.”
Rando’s original plan was to use molecules made of both DNA and RNA. These were known as chimeric molecules, a chimera being a beast in Greek mythology that combines parts of different animals.
Nowadays, his group uses molecules made solely of DNA, because these are much easier to make and use. He calls this method oligonucleotide-mediated gene repair. (Oligo means few, and there are only a small number of nucleotides in each repair molecule.)
Repairs that last
Rando believes the gene-repair approach to treating muscular dystrophy “avoids many disadvantages of other forms of gene therapy.” For one thing, the repair would likely be permanent, since it affects the genes in their natural place on the chromosome, while many forms of gene therapy insert a gene that stays outside the cell’s chromosomes and will likely eventually be lost. For another, it requires no viruses, which can have unpredictable effects.
“At the end of the therapy, you have a completely normal gene,” Rando says. “It’s truly a repair.”
Rando says the technique isn’t yet effective enough to be meaningful to patients, but he remains optimistic.
“We have some new generations of oligonucleotides that we’re trying, and what we’re looking for are better ways to deliver them and higher levels of efficiency. The question will be, as with all these therapies, how many of the hurdles can be overcome. None of them are impossible.”