MDA grantee James Ervasti and colleagues at the University of Minnesota-Twin Cities in Minneapolis have found that a protein known as utrophin, injected into mice lacking the dystrophin protein and showing a disease resembling Duchenne muscular dystrophy (DMD), conferred significant benefits.
The experiments Ervasti and colleagues describe online May 26, 2009, in PLoS Medicine, are the first to show benefit from the direct injection into DMD mice of utrophin protein, rather than utrophin genes or gene modifiers.
Dystrophin is the muscle protein missing in people with DMD and partially absent in people with Becker muscular dystrophy (BMD). The utrophin protein is very similar to dystrophin and is thought to partially compensate for dystrophin’s absence.
The advantage of utrophin-based therapies in DMD and BMD is that utrophin is highly unlikely to provoke an unwanted response from the immune system. Dystrophin can elicit an immune response from people whose immune systems haven't previously been exposed to it, such as those with DMD and some people with BMD. But, because people with DMD and BMD already make normal utrophin, their immune systems are more tolerant of it.
Utrophin therapies have been explored in dystrophin-deficient mice as a strategy to treat DMD or BMD for several years, and they've shown promise. Until now, most of the experiments have involved either transferring extra utrophin genes ("gene therapy") into the mice or boosting production of utrophin from their existing utrophin genes ("gene upregulation"). Both those strategies are viable and continue to be the subject of experimentation.
Ervasti and colleagues’ experiment involved neither gene therapy nor gene upregulation, but “protein therapy.”
The investigators injected miniaturized utrophin protein molecules (micro-utrophin) into the abdomens of DMD mice twice a week for three weeks, starting at 18 days after birth. They attached a cell-penetrating molecule called TAT to each utrophin protein molecule. (The investigators also tried using full-length utrophin molecules, but the micro-utrophins penetrated cells better.)
The TAT-micro-utrophin penetrated all the tissues the researchers examined. In addition, it aligned itself with the muscle-fiber membrane as part of a cluster of proteins the way dystrophin normally would. Loss of the integrity of this cluster, and therefore of the muscle-fiber membrane itself, is a hallmark of DMD and to a lesser extent of BMD.
When compared with untreated mice, the mice that received utrophin protein injections had lower levels of a muscle enzyme called creatine kinase (CK) in their blood, which told the researchers that the muscle fibers in the treated mice were more intact and prevented CK leakage out into the bloodstream. The treated mice also showed fewer cellular signs of muscle degeneration than did their untreated counterparts, as well as better force production by the muscles and less susceptibility to contraction-related drops in force.
Direct injections have pros and cons
Until recently, it's been believed by many experts that direct injections of muscle proteins like utrophin or dystrophin would not be effective because of the size of the molecules, their inability to penetrate cell membranes, and a concern that they wouldn't assume their proper cellular locations.
Ervasti says protein injections or infusions would have the disadvantage of having to be given more frequently than gene injections, but he doesn't see that as an insurmountable obstacle.
Noting that protein therapy has been successful in the metabolic muscle disorder Pompe disease (see FDA OKs Lifesaving Treatment), he said, "In my opinion, TAT-utrophin most directly and simply addresses the cause of DMD."
Ervasti acknowledged that direct injections of protein molecules as large as utrophin have not until now been considered a promising MD treatment approach.
However, he said, "The ultimate objective of scientific research is to make possible today what was impossible yesterday. Today we have another weapon to combat MD."