Muscle Growth
Inhibition of myostatin
Myostatin is produced in muscle cells as an inactive protein consisting of 375 amino acids. After several steps of molecular rearrangements, it becomes biologically active. and then initiates a series of chemical reactions inside the cell, which lead to the downregulation of enzymes for the biosynthesis of new muscle proteins. Therefore by inactivating myostatin, the regeneration of the muscle fibers of Duchenne boys could possibly be stimulated so that they would not be destroyed as fast or might even increase in size. Non-dystrophic mice whose gene for myostatin had been knocked out by genetic methods, have up to three times larger skeletal muscles with significantly more fibers of larger than normal diameter. There are cattle, the Belgian Blue Breed, which are very muscular because their myostatin gene was inactivated by a mutation centuries ago.
And in Berlin, a now 7-year old boy was identified whose skeletal muscles are about twice as large as in a normal child. He is physically very strong. His mother was an Olympic runner, and several other relatives were also very strong. Because of a mutation in this family had changed the normal splicing of the three myostatin exons, the boy and probably his affected relatives, too, have a very low level of myostatin in their muscles. This is a strong indication that downregulation of myostatin would lead to an increase of muscle growth in Duchenne boys, too.
Myo 029. Kathryn Wagner of the Wellstone Muscular Dystrophy Center at the Johns Hopkins University in 11 Baltimore
reported that her research team had raised mdx mice which, in addition of not having dystrophin, also could not make any myostatin. Adult mice of these myostatin knock-out animals had more normal muscles, had less fibrosis, scar tissue, and they regenerated their muscles faster than "normal" mdx mice.
Together with Dr. Lee Sweeney, similar experiments will be performed on dystrophic dogs. The question was now whether the absence of myostatin would have similar effects on the heart. This would counteract a cardiomyopathy, but a hypertrophic, an enlarged, heart would be problematic in Duchenne boys. However, recent investigations with mdx mice showed that the blockade of myostatin had no effect on the heart. This means that the activity of myostatin seems to be restricted to skeletal muscles alone. In cooperation with the company Wyeth Pharmaceuticals, a clinical phase I/II trial with three different dosages of the potential drug Myo 029 was started with 36 adult muscular dystrophy patients, inclusive some Becker patients.
Myo 029 is a specific antibody which binds to myostatin and blocks its activity. It does not cause immune rejection because its protein structure is the human one, it is "humanized". It can be injected into the circulation or under the skin. If the trial should give encouraging results, Wyeth will intensify their efforts to bring Myo 029 to the clinic. In the meantime, parents should not buy any so-called myostatin inhibitors offered on the Internet. These compounds have not gone through clinical trials and therefore are probably ineffective or even dangerous.
Upregulation of insulin-like growth factor, IGF-I. IGF-I
is a protein with about 70 amino acids in one chain with three stabilizing bridges, thus with a similar shape as insulin. Six different forms can be produced in humans with slightly different structures, but resulting in the same IGF-I protein. IGF-I is very beneficial for muscle, because it helps to promote growth and strength. However, the effects of IGF-I are not limited to muscle. The satellite cells, when activated by injury or degradation, produce a specific receptor protein in their membranes, to which IGF-I binds. The consequence is a stimulation of the proliferation of the satellite cells and their further development to myotubes and muscle fibers. As this stimulated regeneration of muscle fibers would be important for maintaining dystrophic muscle tissues, IGF-I is of interest for a possible therapeutic use in Duchenne children. However, other tissues can also respond to IGF-I, and when there are high levels in the blood, there is increased risk of cancer.
Therefore, in order to establish IGF-I as a therapy for muscle disease, strategies must be developed to reduce the potential side effects in other tissues. The research team of Elisabeth Barton of the University of Pennsylvania in Philadelphia works with mice which were obtained by crossing mdx mice with trans_genic, i.e. genetically engineered, mice that produce high levels of the IGF-I in their muscles throughout their lifetime. These mdx-IGF-plus mice show an increased muscle growth with quite healthy-looking muscles and much less fibrosis than the usual mdx mice.
This work demonstrates the benefits the IGF-I could have for Duchenne children. But because this growth factor interferes with many signalling pathways in cells, potentially serious side effects cannot be excluded if higher dosages are used to optimize the effect on muscles. For this reason, a method was developed to "mask" the IGF-I by complexing it with the IGF binding protein-3 which is a naturally circulating protein in the bloodstream. This complex releases IGF-I only where and when it is needed, and helps to stabilize the protein in the circulation so that fewer injections are needed. A commercial formulation of this complex, called IPLEX™ is already approved by the FDA for the treatment of growth failure in children due to IGF-I deficiency. A first clinical trial with IPLEX is now being performed at University of Rochester with support from the NIH and MDA with 15 adult myotonic dystrophy patients. A trial to optimize the dosage will follow in 2007. This strategy could be very effective in getting IGF-I to the muscle without causing side effects in other tissues.
Another way to create higher levels of IGF-I in muscle tissue would be to transport its gene into the muscles by a vector like the adeno-associated virus (AAV) which would then instruct the muscles to make more IGF-I. First experiments in that direction have been done in Dr. Barton's laboratory which showed that only one of the two similar forms of IGF-I, namely IGF-IA, is effective in mdx mice at promoting hypertrophy, the enlargement of muscle fibers.
Current work with this technique succeeded in increasing the level of IGF-I 30 to 40 fold after intramuscular injection of the AAV vectors carrying the correct IGF-I gene. The newly synthesized IGF-I stayed in the muscle tissue, it did not leak out into the blood, thus side effects caused by activation of non-muscular tissues may be avoided. Viral gene therapy will take several years until it could be tried in Duchenne boys. However, this research will help to identify which form of IGF-I works best for muscular dystrophy.