Armed with a zebrafish model of Duchenne muscular dystrophy (DMD) and a library of 1,200 chemicals already approved for human use, researchers at Children's Hospital Boston have identified a compound that reverses the loss of muscle structure and function associated with DMD, seemingly by compensating for the loss of a critical protein. The discovery, published on March 14 in the online Early Edition of the Proceedings of the National Academy of Sciences (PNAS), opens up new avenues for understanding the physiology of DMD as well as a host of new potential therapeutic options.
The team was led by Genri Kawahara, PhD, and Louis Kunkel, PhD, director of the Program in Genomics at Children's. In 1987 Kunkel discovered the primary biological cause of DMD: the loss of the protein dystrophin, caused by mutations in its encoding gene.
DMD is one of 10 muscular dystrophies, a group of devastating, progressive, and incurable genetic diseases almost exclusively affecting boys. In DMD, the combination of the lack of dystrophin and inflammatory reactions causes the muscles to weaken and waste over time.
Using a zebrafish model of DMD, Kunkel and his collaborators set out to screen 1,200 chemicals already approved for human use for any that might have a restorative effect on muscle tissue. Of these 1,200, seven had the desired effect. One in particular, aminophylline, had a significantly stronger effect on the fishes' muscle structure and survival.
"The compound is not replacing the lost dystrophin protein in the fishes' muscles," said Children's Kunkel, who is also a professor of pediatrics and of genomics at Harvard Medical School, "but rather is compensating for that loss."
Aminophylline is an anti-inflammatory and is already in clinical use to treat asthma. A non-selective phosphodiesterase (PDE) inhibitor, it blocks the same biochemical pathway as sildenafil citrate (Viagra®, Pfizer). When studied in the zebrafish model, sildenafil also had a strong positive effect, similar to aminophylline. "We know that other groups have had positive results with sildenafil citrate salt in a mouse model of DMD," Kunkel noted. "The effects of PDE inhibitors in these models indicate that muscle vasculature may play a larger role in DMD than previously thought."
"The zebrafish have turned out to be an ideal vehicle for this kind of small molecule screening in DMD," Kunkel continued. "Because we can grow and test so many at a time, we were able to screen this chemical library very quickly, and the fact that our results reflect those of other groups using other models validates our screening strategy. We are already screening a few thousand more chemicals, and because we're focusing on ones that have already been approved for human use, we are confident that we will be able to take candidates found in the fish quickly through mouse studies and into the clinic."
Children's Hospital Boston