Using morpholinos to induce exon skipping, researchers rescued some dystrophin function in dogs with the canine form of Duchenne muscular dystrophy. Scientists from Children's National Medical Center in Washington, DC, and the National Center of Neurology and Psychiatry in Tokyo restored dystrophin function to about 20 percent of normal levels. "[The] treatment has clearly shown efficacy in a large animal," says lead author Eric Hoffman from Children's, though he adds that the team's Annals of Neurology study was not blinded.
In both dogs and people with Duchenne muscular dystrophy, frameshift deletions in the dystrophin gene give rise to non-functional dystrophin protein, a membrane protein in muscle cells, and, in the patient, leads to progressive muscle weakness and wasting. Another form of muscular dystrophy, called Becker muscular dystrophy, has milder symptoms even though it results from a deletion in the same gene. In that case, the reading frame is kept intact and the protein is still viable. In the 1990s, the Japanese researchers came across a patient who had the genomic version of Duchenne, but phenotypically presented as having Becker — giving researchers the idea that they too could "turn Duchenne into Becker," Hoffman says.
In this study, researchers used a cocktail of morpholinos to skip exons and restore the reading frame in dogs. In the past, attempts to use anti-sense treatments ran into problems delivering the drug to affected cells. Treating Duchenne has an advantage there: one of the hallmarks of the disease is having unstable plasma membranes. Because of the holes in the cell membranes, morpholinos slip into the cells from the bloodstream. "Once it leaks in, it starts making de novo dystrophin and seals itself in," Hoffman says. "That's where we see this biggest bang for the buck as far as where we can actually do IV small molecule antisense and systemically modulate splicing."
Though they looked at Duchenne in dogs, Hoffman says their results — a rescuing of function beyond the critical 10 percent threshold and an apparent lack of toxicity — "bodes particularly well for human studies." The dog, he says, is a worst-case scenario since the deletion is close to a functional domain and to get the dystrophin gene back into frame, they had to skip over multiple exons. "With a lot of human patients, you can get by with just one," he says.
Hoffman is hopeful that their collaboration with AVI Biopharma, which holds the intellectual property rights for the human application of morpholinos, will soon enter clinical trials. Since Duchenne muscular dystrophy is an orphan disease, any treatment that shows efficacy could be fast-tracked for FDA approval.