"Duchenne's muscular dystrophy is the most common inherited fatal neuromuscular disorder," said Tejvir Khurana, assistant professor in the department of physiology at the Pennsylvania Muscle Institute, part of the University of Pennsylvania. "There are a variety of muscular dystrophies. For the common person, they are probably most strongly associated with the Jerry Lewis telethon."
Khurana is senior author of an article in the Sept. 21, 2004, issue of the Proceedings of the National Academy of Sciences describing a new approach to treat the progressive degenerative disease. In "Heregulin ameliorates the dystrophic phenotype in mdx mice," University of Pennsylvania researchers and their colleagues at the University of Copenhagen and the Children's Hospital of Philadelphia reported on experiments showing the up-regulation of a related molecule, utrophin, by heregulin can to some extent compensate for the loss of dystrophin in an animal model of Duchenne's. The approach, which used injections rather than gene therapy to deliver the heregulin, is complementary to other successes that have been reported in recent years.
Duchenne's muscular dystrophy is characterized by a lack of the protein dystrophin, which normally is present just below the membrane of muscle cells. Its absence weakens the membrane - basically, "your muscle is progressively getting ripped apart," Khurana said. "If a normal muscle can lift 10 kgs 10 times without ripping, a Duchenne's muscle may start to rip on the fifth repetition."
Those damages lead to loss of strength by around age 5. By age 12, those afflicted often are wheelchair-bound, and few survive beyond their early 30s.
It already was known that the molecule utrophin can compensate to some extent for the muscle damage caused by dystrophin's absence. Utrophin normally is expressed at high levels early in development, and in fact, the mouse model of Duchenne's does not show muscle necrosis until those high levels have declined. Up-regulation of utrophin beyond its usual time of expression thus was seen to be a promising approach to ameliorating Duchenne's.
Heregulin is one protein known to up-regulate utrophin production in cultured muscle cells. In the research presented here, the researchers tested whether heregulin could up-regulate utrophin in vivo, and whether such up-regulation would have a positive effect on muscles in a model of Duchenne's, the mdx mouse.
The researchers regularly injected mdx mice, which lack dystrophin, with either heregulin or control solution for three months. In some experiments, the researchers also tested a group of double mutant mice, which lack both dystrophin and utrophin.
Heregulin injections led to up-regulated transcription and translation of utrophin. Protein levels were increased to a greater degree than RNA levels, which Khurana said is "consistent with what's been shown elsewhere for this particular transcript. The protein hangs on a lot longer than the RNA, so it's probably due to accumulation," though the same effect can be seen to a lesser degree in cell cultures, where relative longevity is less of an issue.
Next, the researchers checked whether that up-regulation had any effect on the muscle itself. The researchers tested muscle fibers from treated and control mice for a variety of morphological features. They found that heregulin treatment reduced inflammation and necrosis in muscles from mdx mice, as well as membrane permeability. As expected, mice lacking utrophin - where there is nothing to up-regulate by heregulin injections - showed no such improvements.
They also found that treated mice regenerate their muscles less. Usually in mdx mice, "because there is a problem with the muscle, the mouse is compensating by regenerating the muscle a lot," Khurana said. He interprets the reduced regeneration to mean that after heregulin injections, there is less muscle damage that necessitates such regeneration in the first place. "We are not at normal levels; there is still damage going on. But it is certainly an improvement," Khurana said.
Muscles then were dissected and tested for their ability to contract repeatedly in response to stimulation.
"When a muscle is stimulated, it will respond with a contraction of a certain force, but when it is repeatedly stimulated and stretched, the force will drop," Khurana said. "In Duchenne's muscle, that force drop is a lot."
Dissected mdx muscles showed a force drop of nearly 70 percent between the first and 10th stimulation. In contrast, mdx muscles from mice that had been treated with heregulin showed a force drop that was significantly reduced, though still above the force drop of between 40 percent and 50 percent shown by muscles from regular mice.
Khurana pointed out that in the experiments, heregulin treatment improved a number of the problems with the muscle membrane that are seen in Duchenne's muscles, but had no effect on muscle mass. Those findings complement results he and his colleagues published in Nature in 2002 on the effects of inhibiting myostatin, a regulatory protein that inhibits the formation of muscle mass. Inhibiting myostatin in mdx mice led to an increase in muscle mass and strength, but did not fix the problems with the membrane that heregulin now has been shown to address.
The researchers are planning to investigate the long-term effects of heregulin injections next. The experiments reported lasted for three months, but some pathologies in the mdx mice, notably damage to the diaphragm, take longer to become severe.
One advantage of the approach used by the authors is that the heregulin is effective when injected, which means that the approach would not need to wait for improvements in gene therapy to be pursued in the clinic. With reports of cures of immune system disorders followed later by reports of leukemia in some of those cured, it is fair to say that gene therapy has seen dramatic highs and lows in recent years, even taking into account that medical research is pretty much, by definition, a bumpy ride. Khurana said that the pharmacological approach might ultimately be used in tandem with gene-therapy approaches.
"If gene therapy can fix one component and this approach can fix another," he said, "then together they might give you an even better fix."