Duchenne muscular dystrophy (DMD) is a severe inherited muscular dystrophy that causes progressive muscle degeneration which eventually leads to loss of the ability to use muscles and death, and every year tens thousands of children are born afflicted with the disease. It is caused by mutations in the DMD gene that encodes dystrophin, a protein vital to the maintenance of muscle cell structure and function. Not surprisingly there are several charities around the world dedicated to finding a cure, and it looks as if their persistance may soon pay off.
Earlier this week the NIH announced some exciting results from a study (1) of a cocktail of morpholinos, small artificial molecules also called antisense oligonucleotides which mimic DNA and bind to it, that in a process termed exon skipping act as patches to allow the production of dystrophin where it would otherwise fail due to a mutation in the DMD gene. The team lead by Dr. Eric Hoffman found that they when they injected this morpholino cocktail into the bloodstream of dogs that suffer from duchenne muscular dystrophy skeletal muscle degeneration stopped, though the degeneration of cardiac muscle continued. They chose dogs for this experiment because they accurately mimic the physiological effects of human DMD, so the researchers were able to tell if the exon-skipping approach could actually restore enough of the dystrophin function to halt the progression of the disease. Mouse models of DMD, particularly the Mdx mouse which has a mutation in exon 23 that prevents it from making dystrophin, have proved invaluable to research on exon-skipping and other approaches to treating DMD. Their drawback is that the mice only develop a relatively mild version of the disease, and so are not always ideal if you want the determine whether a “patched” dystrophin will actually prevent muscle deterioration. While the truncated dystrophin protein produced as a result of exon skipping does not function as well as normal dystrophin in this study on dogs, they did demostrate that enough dystrophin function was restored to halt deterioration and make a real difference to patients.
Where this work is an advance on previous research is that it uses an intravenous injection that then relied on the bloodstream to circulate the morpholinos to all muscle groups, rather than directly injecting the morpholinos into each of the muscle that need treatment. This is a significant improvement that will make the technique far more practical in the clinic. The use of a cocktail of morpholinos that each target different mutation sites in the DMD gene is also interesting, many DMD patients have several different mutations in their DMD genes and previous methods using antisense oligonucleotides have only been of potential benefit to a small proportion of patients, whereas the cocktail approach may benefit more that 90% of them.
As I mentioned above a serious drawback with the morpholino cocktail technique was that it failed to restore dystrophin function in the heart, but another recent research paper (2) suggests that this problem can be solved by attaching a cell-penetrating peptide to the morpholino. Using this approach Dr. Qi Lu and colleagues at the McColl-Lockwood Laboratory for Muscular Dystrophy were able to safely restore almost full dystrophin activity in both cardiac and sleletal muscles by intravenous injection of a cell-penetrating peptide linked to a morpholino that patches the exon 23 mutation in the Mdx mouse model of DMD. This is an excellent result, and if it can be combined with a cocktail approach has great potential for the future treatment of DMD.
All in all morpholinos are looking like an increasingly promising approach to treating DMD, and along with other approaches including the drug PTC124 that is currently in clinical trials* and stem cell transplantation, offer hope to the many thousands of DMD sufferers around the world.
*As you might expect the basic research that underpinned the dicsovery of PTC124 and the subsequent pre-clinical evaluation of its efficacy and safety relied heavily on mouse models of Duchenne muscular dystrophy and cystic fibrosis (3).
1) Yokota T. et al. “Efficacy of systemic morpholino exon-skipping in duchenne dystrophy dogs” Annals of Neurology Published Online: 13 Mar 2009, DOI:10.1002/ana.21627
2) Wu B. et al. “Effective rescue of dystrophin improves cardiac function in dystrophin-deficient mice by a modified morpholino oligomer” PNAS Volume 105(39), pages 14814-14819. DOI:10.1073/pnas.0805676105
3) Hirawat S. “Safety, tolerability, and pharmacokinetics of PTC124, a nonaminoglycoside nonsense mutation suppressor, following single- and multiple-dose administration to healthy male and female adult volunteers.” J Clin Pharmacol. Volume 47(4), pages 430-444 (2007) DOI:10.1177/0091270006297140